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Infrared multiple photon dissociation spectra of gaseous ions

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Infrared multiple photon dissociation spectra of gaseous ions
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Peiris, Dilrukshi Manjalika Patuwathavithana
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Absorption spectra ( jstor )
Cations ( jstor )
Ethers ( jstor )
Gas spectroscopy ( jstor )
Ions ( jstor )
Irradiation ( jstor )
Laser spectroscopy ( jstor )
Lasers ( jstor )
Mass spectroscopy ( jstor )
Photolysis ( jstor )

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INFRARED MULTIPLE PHOTON DISSOCIATION
SPECTRA OF GASEOUS IONS














By

DILRUKSHI MANJALIKA PATUWATHAVITHANA PEIRIS





















A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 1994





























To my son, mother, and in loving memory of my father.













ACKNOWLEDGEMENTS


This dissertation is not the work of one person, but of many. Therefore, it is with great satisfaction that I share

credit with all those persons who have made this document possible.

I would like to thank many of the staff in the chemistry department, particularly Susan Ciccarone, Steve Miles, Larry Hartley, and Joe Shalosky for their always cordial and prompt help.

I also wish to extend my thanks to Dr. Jose Riveros for his guidance and expertise in the area of gas-phase solvated. ion chemistry, and the use of his laboratories at the

University of S~o Paulo, Brazil. I will always cherish

pleasant memories of the many good people I met during my visit.

My sincere thanks go to Drs. Timothy Anderson, Sam

Colgate, Willis Person, David Richardson, and Martin Vala. Each, through his active participation, offered valuable

suggestions that aided in the completion of this dissertation. I also appreciate their exemplary thoughtfulness, advice, and involvement in my academic career.

Heartfelt thanks are extended to all Eyler group members past and present. In particularly, Ragulan Ramanathan whose iii







help with experimentation, friendship and encouragement was a

relief from the everyday struggles of graduate school. A

special thank you goes out to all my relatives and friends in

Sri Lanka and in the States, who have selflessly provided diverse forms of support along the way.

I am indebted to my research advisor, Dr. John Eyler, for providing the opportunity and assistance I required in turning a distant dream into reality. I also wish to extend my

deepest appreciation for his financial support, patience, and helpful criticism throughout the experimental work, and superior editing skills for the preparation of this dissertation. He deserves a special thank you for funding my

trip to Brazil, which was a significant experience in the development of this research.

It goes without saying that this work would never have

been attempted, much less completed, without the support, sacrifices and encouragement of my parents and family. I

would not have reached the point I am today without the love of my late father Dr. Chandrarathna Patuwathavithana, mother

Dorothy, brother Chatura Vithana, sister-in-law Suram, husband Asoka Peiris and especially my son Eshal. I hope someday that Eshal will understand why Mommy could not be there more to watch him grow.







iv














TABLE OF CONTENTS




ACKNOWLEDGMENTS.............................. ..... iii

LIST OF FIGURES .. .. ............ .. ....... ..... vii



A.BSTRACT! .. ................................... Xii

CHAPTERS



2 THEORY AND INSTRUMENTATION .................... 12

FTICR Mass Spectrometry ............... 12
Development and Background ....... 12
Theory of Operation ............... 13
Theory of IRM'PD .............. ... ... .. 19
IRMPD/FTICR Mass Spectrometry ........ 26
Electron Ionization Experiments..........26 Electrospray Ionization Experiments .. 28
The FTICR Cell........ .......... .. .. .. .. 29
Multipass Process ............ 29
Modified White-Type Cell............... 31
Newly-Modified White-Type Cell...........33
FTICR Pulse Sequence......................... 42

3 INFRARED MULTIPLE PHOTON DISSOCIATION
SPECTRA OF GASEOUS IONS ................ 45


(P4e ~ump Technique............ 46

~i'and Two-Laser Experiments ...... 50
FTICR Pulse Sequence......................52
Results...................... ............ 54
Discussion ................ .......... 58





V








4 INFRARED MULTIPLE PHOTON DISSOCIATION SPECTRA
OF METHANOL SOLVATED ANIONS AND PROTON
BOUND METHANOL DIMER CATIONS ......... 72

Introduction ................... ........ .. 72
Experimental ........................ 75
Methanol and d-methanol Solvated

Proton (deuteron) Bound Methanol
(d-methanol) Diner Cations .....80o Results and Discussion......................81
Methanol Solvate of the Fluoride In81
Deuterated Methanol Solvate of the
Fluoride Ion .. .. ............... 82
Methanol Solvate of the Chloride Ion 83
Deuterated Methanol Solvate of the
Chloride Ion. .................. 84
Methanol Solvate of the Methoxy Ion .. 84
Proton Bound Methanol Diner Cation ... 91
Proton Bound Deuterated Methanol
Diner Cation.........................93
Deuteron Bound Deuterated Methanol
Diner Cation.........................94
conclusions ......................... 112

5 INFRARED MULTIPLE PHOTON DISSOCIATION OF
CROWN ETHER COMPLEXES......................... 114

Introduction.................................. 114
Experimental.................................. 117
ESI Source............................... 117
ESI/FTICR Mass Spectrometry..............118
ESI/FTICR Pulse Sequence........... 120

Discussion ......................... 129
conclusions ........................... 139

6 CONCLUSIONS AND FUTURE WORK........................140

conclusions .......................... 140
Future Work.................................. 144
REFERENCES ....... .. .. .. .. .. .............. 147

BIOGRAPHICAL. SKETCH .. .. .. .. .. ............... 157








vi











LIST OF FIGURES

Figr PG

2.1 An expanded three-dimensional view
of a typical z-axis elongated
FTICR analyzer cell ...................................... ... 16

2.2 A digitized time domain signal for
protonated biB (2-methoxydiethyl) ether
(diglyme) cation produced by electron
ionization ... .............................................. 20

2.3 The Fourier transformed ion cyclotron
resonance mass spectrum of diglyme
cations resulting from the time domain signal
observed in Figure 2.*2 ............. .................. .. .. 21

2.4 A schematic representation of the
energy diagram for the infrared multiple
photon absorption and dissociation
process ........... ....................................................... 24

2.5 A schematic representation of the
FTICR mass spectrometer equipped
with a 2 Tesla magnet ............................................. 27

2.6 An illustration of the set-up used
by White to obtain very long optical
paths for irradiation experiments ............... 30

2.7 An expanded three dimensional view
of the modified White-type FTICR analyzer cell used in experiments to obtain IRHPD spectra presented
in chapter 3 ......................................................... 32

2.8 An expanded three dimensional view
of the nevly modified White-type FTICR analyzer cell used in IRHPD experiments
presented in chapters 4 and 5 .......................... 35

2.9 The CO2 laser (pulsed or continuous)
beam pathway for double pass arrangement
before entering the vacuum chamber ............... 37

2.10 The CO2 laser (pulsed or continuous) beam
pathways for multipass arrangement before
entering the vacuum chamber.................................. 38


vii







2.11 IRMPD mass spectrum
(laser energy = 500 mJ pulse-)
of protonated diglyme cation obtained
with cw CO2 laser beam subjected to
(top) center pass, and (bottom) multipass
arrangements .................................... 39

2.12 IRMPD mass spectrum
(laser energy 750 mJ pulse-')
of protonated diglyme cation obtained
with cw CO2 laser beam subjected to
(top) center pass, and (bottom) multipass
arrangements .................................... 40

3.1 A schematic representation of the
two-laser (probe-pump) photodissociation
process ................................... 48

3.2 Schematic representaion of the cutaway
view of the modified White-type cell
and the two-laser beam pathways
inside the cell ................................. 51

3.3 Experimental pulse sequence employed in
the one- and two-laser probe-pump
techniques ...................................... 53

3.4 One-laser infrared multiple photon
dissociation spectrum of the protonated
molecular ion of diglyme ........................ 56

3.5 One-laser IRMPD spectrum of the positive
molecular ion of 3-bromopropene
(allyl bromide) ................................. 57

3.6 One-laser IRMPD spectrum of the negative
molecular ion of gallium
hexafluoroacetylacetonate Ga(hfac)3 ............. .... 59

3.7 Two-laser IRMPD spectrum of the protonated
molecular ion of diglyme ........................ 60

3.8 Two-laser IRMPD spectrum of the positive
molecular ion of 3-bromopropene ................. 61

3.9 Two-laser IRMPD spectrum of the negative
molecular ion of Ga(hfac)3 ....................... 62

3.10 Gas phase neutral infrared
spectrum of diglyme ............................. 63



viii







3.11 Gas phase neutral infrared
spectrum of 3-bromopropene ...................... 64

3.12 Gas phase neutral infrared
spectrum of Ga(hfac)3 . . . . . . . . . . . . . 65

4.1 A schematic representation of the
laser beam pathway (16 passes) inside
the newly modified White-type
FTICR cell ................. ... ................ 76

4.2 Vacuum line apparatus
(pressure ca. 104 Torr) used for the
synthesis of methyl nitrite ..................... 79

4.3 IRMPD spectrum of methanol solvated
fluoride ion (CH3OHF) .......................... 86

4.4 IRMPD spectrum of d-methanol solvated
fluoride ion (CH3ODF) .......................... 87

4.5 IRMPD spectrum of methanol solvated
chloride ion (CH3OHCl") ......................... 88

4.6 IRMPD spectrum of d-methanol solvated
chloride ion (CH3ODCl") ......................... 89

4.7 IRMPD spectrum of methanol solvated
methoxy anion (CH3OHOCH3") .......................... 90

4.8 IRMPD spectrum of proton bound methanol
dimer cation (CH3OH)2H. .......................... 97

4.9 IRMPD spectrum of proton bound d-methanol
dimer cation (CH3OD)2H. ......................... 98

4.10 IRMPD spectrum of deuteron bound d-methanol
dimer cation (CH3OD)2D. ......................... 99

4.11 Gas-phase neutral infrared spectrum
of methanol .............. .. ........................ 100

4.12 Gas-phase neutral infrared spectrum
of d-methanol ................................... 101

4.13 Overlaps between the CO2 laser lines
and the absorption spectrum.
(top) ions of low molecular
complexity;and (bottom) ions of
high molecular complexity ....................... 105


ix







5.1 The internal ESI/FTICR mass spectrometer ........ 119


5.2 ESI/FTICR pulse sequence used for
IRMPD experiments ................................ 121

5.3 Structures of the crown ethers and
complexes discussed in chapter 5 ................ 124

5.4 ESI/FTICR mass spectra obtained with
18-crown-6/NaCl/KCl in
50:50 methanol:water solution ................... 125

5.5 ESI/FTICR mass spectra obtained
with 18-crown-6/NaCl/KCl in 50:50
methanol:water, and with cw CO2
laser irradiation wavelength
of 10.60 pm .................................... 126

5.6 ESI/FTICR mass spectra obtained
with 18-crown-6/NaC1/KC1 in 50:50
methanol:water, and with cw CO2
laser irradiation wavelength
of 10.58 pm .................................... 127

5.7 ESI/FTICR mass spectra obtained
with 18-crown-6/NaCl/KC1 in 50:50
methanol:water, and with cw CO2
laser irradiation wavelength
of 9.58 pm ............ ..... .................. 128

5.8 ESI/FTICR mass spectra obtained
with 18-crown-6 in 49:49:2 methanol:water:acetic acid
solution: (top) without laser
irradiation, the insert shows isolated (18-crown-6)H30+ with
13C peak resolved; and (bottom) with
laser irradiation at 10.60 pm ................... 130

5.9 ESI/FTICR mass spectra obtained
with 15-crown-5/NaCl/KC1 in 50:50
methanol:water solution: (top) without
laser irradiation; and (bottom) with laser
irradiation at 10.60 pm ........................ 131

5.10 ESI/FTICR mass spectra obtained with
12-crown-4/NaCl/KCl in 50:50
methanol:water solution: (top) without
laser irradiation; and (bottom) with laser
irradiation at 10.60 pm ........................ 132


x















LIST OF TABLES


Table PAGE

2.1 Energy measurement for both pulsed
and cw C02 laser beam after reflecting from each mirror and the ZnSe window,
and the distances for each mirror and for the ZnSe window from the
laser head using the newly modified
White-type cell ................................. 43

3.1 Gaseous ion vibrational frequencies
from one- and two-laser studies, frequencies of the corresponding
gas phase neutrals, and the ion-neutral
peak shifts ..................................... 66

3.2 The C-F stretching frequencies of
A and E modes used in assigning
peak frequencies for the Ga(hfac)3
neutral and the anion ........................... 70

4.1 Gaseous ion vibrational frequencies
from IRMPD studies .............................. 102

4.2 Some calculated and experimental bond
lengths and vibrational frequencies
of CH30H and CH30HF ............................. 107
















xi
















Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy
INFRARED MULTIPLE PHOTON DISSOCIATION SPECTRA OF GASEOUS IONS

By

Dilrukshi Manjalika Patuwathavithana Peiris December 1994

Chairman: John R. Eyler
Major Department: Chemistry

This dissertation presents a series of studies elucidating the infrared multiple photon dissociation (IRMPD) spectra of gaseous ions stored in a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. An "indirect" method such as IRMPD often has to be utilized to obtain gasphase ion spectra. The ability of FTICR to trap ions easily under collision-free or collisional conditions for long periods has made it the instrument of choice for IRMPD studies.

A novel two-laser approach was developed to obtain IRMPD spectra and is presented in this dissertation. Use of a lowpower tunable CO2 (probe) laser, a second, more powerful CO2 (pump) laser, and a modified White-type multipass FTICR



xii







analyzer cell has helped to overcome some limitations of this technique, including limited tuning and/or low output power in one-laser experiments.

IRMPD spectra in the 934-1055 cm-4 range have been obtained for the protonated molecular ion of digyme, the positive molecular ion of 3-bromopropene, and the negative molecular ion of gallium hexafluoroacetylacetonate, using one and two lasers. Comparisons between the ion spectra and those of the corresponding neutral species are made.

Further modifications of the White-type FTICR cell dramatically enhanced the photodissociation effects and enabled IRMPD spectra for methanol solvated anions and proton bound methanol dimer cations to be obtained in the 920-1060 cm"I region. IRMPD pathways for proton bound methanol dimer cations were also observed. The neutral gas-phase spectra of methanol and d-methanol were compared with the corresponding IRMPD spectra.

Crown ether complexes formed in methanol/water solutions were subsequently transported into the gas phase using a homebuilt electrospray ionization source. Complexes of 18-crown6, 15-crown-5, and 12-crown-4 with Na+, K+ and H30+ were observed. The techniques of IRMPD were used to obtain fragmentation and binding information. Comparison of IRMPD mass spectra for crown ether complexes indicated a difference in the binding of H30+ to crowns compared to Na+ and K+, and that the Na+ is bound to the crown ether more strongly than K+.

xiii













CHAPTER 1
INTRODUCTION

Mass spectrometry is unique among the techniques

available for analyzing molecules. In mass spectrometry, molecules are ionized first and then these ions are

subsequently examined in detail. Mass detection principles and the instrumentation needed to perform experiments are very simple. Mass analysis in mass spectrometry is usually

accomplished with one of five basic types of instrument: magnetic sector, quadrupole, time-of flight, ion cyclotron resonance, and ion trap. These instruments and their

limitations have been discussed in detail (1). The uniqueness of the molecular structure information available from mass spectrometry is well known. One of the most important mass

spectrometric methods for additional structural information is high mass resolution. Fourier transform ion cyclotron

resonance (FTICR) mass spectrometry has already established its ability to give routinely higher mass accuracy and mass resolution than other mass spectrometric methods.

In addition to its high mass resolution capability, FTICR mass spectrometry benefits from the ability to trap ions for

very long periods (2), accurate mass measurements (3), and the capability of performing collisional activation at low







2

pressures (4). The invention of softer ionization sources, such as laser desorption (LD) (5,6), matrix assisted laser desorption (MALDI) (7,8), and electrospray ionization (ESI) (9, 10), combined with instrumental advances has expanded the utility of Fourier transform ion cyclotron resonance (FTICR) mass spectrometry as an analytical tool.

The ability of the FTICR to trap ions for a very long period makes it a suitable instrument for studying ionmolecule reactions. The trapped ions can participate in ionmolecule or photochemical reactions or be selectively ejected

using FTICR double resonance techniques. The ability to

select and eject ions from the cell permits positive

identification of complex reaction pathways that occur during the trapping period. Thus, much chemical information and/or physical quantities have been obtained from studies of ionmolecule reactions by FTICR. The determination of reaction rate constants (11), gas-phase acidity and basicity measurements (12), ionization potentials (IPs) (13), electron affinities (EAs) (14), structural determination of fragments

(15), and determination of competitive reaction pathways (16) has been presented in the literature.

Collision-induced dissociation (CID), also known as

collisionally-activated dissociation (CAD), is the most widely used technique for ion structure determination. Although

multiple sector or quadrupole mass spectrometers are most commonly used for this approach, Cody and Freiser demonstrated








3

that CID also can be performed in a FTICR instrument (17). The CID technique involves isolating an ion of interest, accelerating the ion into a target gas, and detecting the daughter ions produced from the parent ion. The ability to

perform CID over a range of collisional energies with high resolution mass analysis of the CID daughter ions and the capability of performing sequential HS-MS make the FTICR an excellent instrument for performing CID.

In spite of widespread application of CID, molecules of > 3,000 Da in molecular weight are seldom successfully fragmented (18). In recent years, surface-induced

dissociation (SID) has been explored as an alternative to CID. The application of SID in FTICR has been reported by Williams et al. (19), and Ijames and Wilkins (20). The SID experiment has now been performed in quadrupole, sector, time-of flight

and quadrupole ion trap mass spectrometers. In addition to CID and SID, Jacobson et al. (21) showed that sustained of fresonance irradiation (SORI) can be used to transfer small increments of internal energy into ions. Thus, SORI combined

with CID can be used as a selective probe f or determination of the lowest energy fragmentation pathways available for an ion

of interest. However, the SORI/CID method is still in its early stages of development compared to SID and CID methods for structural analysis.

An alternative to ion fragmentation by CID and SID is photodissociation (PD), which utilizes continuous and/or







4

pulsed lasers ranging from the ultraviolet (uv) to the

infrared (ir). CID of mass selected ions results in the deposition of a range of internal energies, where the average

excitation energy is dependent on the collision energy (22) and the number of collisions (23). The nondiscrete energy deposition may result in nonselective fragmentation of the ion of interest. Photodissociation of mass-selected ions results in an additional degree of selectivity. A discrete energy or

range of energies can be deposited into the mass-selected ions by photoexcitation. This discrete excitation results in both molecule-specific and bond-selective PD, especially when the molecular ions are produced with low internal energies.

Furthermore, high-mass ions are not efficiently fragmented by CID because of the increasing amount of energy that a large molecule can accommodate before dissociating (the number of vibrational modes increases) and the decreasing amount of energy that can be transferred during a collision (center of mass effect) (24).

The ability of FTICR to trap ions easily under collisionfree or collisional conditions for long time periods has made it possible to irradiate ions with light during these periods. Also, the combination of monochromatic light sources with the FTICR spectrometer has proven quite successful (25). In

addition, the ability of FTICR to use pulsed lasers to acquire a complete mass spectrum of photo fragments with high mass resolution from a single laser pulse makes it the instrument of choice for photodissociation studies.






5
During the past 20 years the technique of photodissociation has been applied to different kinds of problems in gas-phase ion chemistry. Dunbar (26) first

introduced the application of technique to study CH3Cl+ in an ICR mass spectrometer. Moreover, studies of photodetachment of electrons from negative ions by Meyer and coworkers and coworkers (27) have yielded electron affinities and related thermochemical data important in describing the intrinsic acidity of organic and inorganic molecules. Until 1982, PD studies of trapped ions were limited to ions in ICR cells. In 1982, Hughes et al. (28) and Louris et al. (29) showed that photodissociation could be coupled with quadrupole ion storage and ion trap mass spectrometry, respectively.

Photodissociation can be used in many different ways to obtain: (a) photon-induced fragment mass spectra, by comparing mass spectra obtained with and without irradiation; (b) a photodissociation spectrum parent ion decay or fragmentation efficiency is monitored as a function of irradiation wavelength; (c) isomeric differentiation, ion fractions in mixtures of isomeric ions can be determined by variation of the trapping/irradiation time; and (d) dissociation of ions with photons having an energy below the dissociation threshold photodissociation by absorption of two visible photons, or by sequential absorption of infrared and visible photons, or by absorption of multiple infrared photons (IRMPD).








6

The measurement of photofragment mass spectra is comparable to classical methods in mass spectrometry such as CID and fragmentation studies as a function of internal energy. In contrast, photodissociation spectra add

essentially new information through measurement of a physical property of the stable nonfragmenting ions: the photon induced decay as a function of optical wavelength. The ion decay is

determined by the absorption spectrum of the ion and the fragmentation efficiency as a function of the internal energy of the excited ion. Since the rate of photodissociation of the ions is primarily determined by the rate of photon absorption by the ions, this method has much in common with

conventional optical absorption spectroscopy, and provides an interesting bridge between mass spectrometry and optical absorption spectroscopy.

Spectroscopic studies of molecular ions have become a challenging and an interesting area of study in recent years (30, 31). Unlike neutral molecules and ions in solution or in the solid state, for which IR, UV-VIS and NMR spectroscopies

give useful structural information, there is no general method to obtain structural information for gaseous ions. Although infrared spectroscopy has been used on very small ions (32,33), the applications for complex species have yet to be realized. This limitation arises primarily from the

difficulty of obtaining a high enough density of ions of a known mass in a small volume to yield a measurable absorbance,








7

thus leading to reliable spectra. However, when the energy of irradiation is enough to cause to dissociation, an indirect method, such as ion photodissociation, can be utilized to obtain reliable spectra and structural information for gaseous ions.

Infrared multiple photon absorption leading to

dissociation has been shown to be a widespread phenomenon since it was first observed by Isenor and Richardson (34). It was subsequently demonstrated that the sequential absorption of many photons by a single molecule can occur, even in the

absence of collisions, and that this process occurs in a large number of molecules and ions (35). Since then the realm of infrared spectroscopy of gas-phase molecular ions has become

an area of considerable interest for obtaining the vibrational frequencies of gaseous ions (36,37), through the information available from IPMP dissociation. Although a large number of

cations have now been studied (37), only a few studies have been performed to extend the technique to include molecular anions (38).

Woodin et al. demonstrated the feasibility of IRMPD with a relatively low-power CO2 laser (39). In addition, Wight and Beauchamp used a low-power (20 W/cm2) continuous wave (cw) CO2 laser to obtain infrared multiphoton induced electron

detachment spectra to distinguish between three C7H7_ isomers

(40). In 1983, Honovich and Dunbar obtained photodissociation spectra with both an infrared (IR) laser and a visible laser








8

(41). Irradiating the ions with two lasers simultaneously, or with an IR laser followed by the visible laser, has many similarities with IRMPD, but the large increment of energy deposited by the visible photon makes it possible to effect

dissociations which would be difficult with infrared light alone. Use of this novel approach gave the possibility of obtaining infrared spectra of several ions which are not accessible with IR lasers alone.

Even though infrared spectroscopy of ions by the IRMPD approach is a promising area of study, it is constrained by

the lack of powerful, broadly tunable IR lasers. A novel twolaser approach known as "probe-pump" technique developed in our laboratory enabled us to overcome some of these limitations and obtain reproducible spectra for several gasphase cations and anions. In these experiments, a low-power tunable CO2 laser (probe) and a more powerful CO2 laser (pump) were used. At the low pressures used in FTICR (07- 10-8 Torr) there is not a sufficient number of collisions to deactivate vibrationally excited ions formed by the low-power tunable CO2 laser. Thus, in two-laser experiments, the first laser is used to vibrationally excite them and promote the ions into the quasicontinuum. Once the ions have appreciable vibrational excitation, then a second CO2 laser is used to drive the ions to the dissociation limit. IRI4PD spectra of

the protonated molecular ion of bis methoxy diethyl ether (diglyme), and the molecular ions of 3-bromopropene and







9

gallium hexafluoroacetylacetonate are presented in chapter 3. Spectra were obtained using one and two lasers and compared with neutral spectra. For all the ions studied in this dissertation the decomposition pathways are invariant to change in laser wavelength, but the photodissociation yield does depend on the wavelength. All photodissociation spectra presented are limited to the tuning range of CO2 lasers, 920 1060 cm"i.

In the past decade many chemists and physicists have devoted themselves to the study of cluster properties, structure and reactivites. In cluster chemistry it is important to understand the transition from isolated atoms and molecules to bulk materials. The study of successively larger cluster ions is analogous to the process of solvation, one solvent molecule at a time. A number of gas-phase studies have investigated ions solvated with single neutral molecules to begin following the transition from gas to solution phase ionic behavior (42-44). In particular, solvated halide ions have been studied due to their structural simplicity and their existence in solution. In chapter 4, IRMPD spectra of methanol and deuterated methanol solvated fluoride and

chloride anions, and the methanol solvated methoxy anion are presented. In addition, IRMPD spectra of proton bound methanol and deuterated methanol dimer cations are presented. The shifts in the IRMPD peak frequencies are compared with the respective neutral peaks.







10

In the work reported in chapters 3 and 4 ions were produced using conventional electron ionization (EI) methods. One of the requirements for EI is that the sample should have

at least moderate vapor pressure. Some solid and liquid samples can be heated to produce the required partial pressure needed to use EI. However, this method is limited due to excessive fragmentation of the parent ion. To circumvent problems with EIj softer ionization techniques such as laser

desorption, matrix assisted laser desorption and ESI were introduced.

These ionization methods and their limitations have been appeared in several review papers (45). These reports

indicate that electrospray has emerged as the method of choice for ionizing less volatile molecules. The multiple charging

process of ESI allows high molecular weight peptides and proteins to be detected by conventional mass spectrometers in

the m/z 500 to 2000 amu range. The ions are produced in solution and subsequently transferred into gas phase for mass spectrometric analysis.

ESI/MS was first performed on polystyrene ions by Dole et al. in 1964 (46) but they did not pursue the work because of the limited instrumentation. Several years later, Yamashita and Fenn performed ESI/MS experiments (47) with more modern instrumentation. Fenn's demonstration prompted several research groups to couple ESI with different mass spectrometric analyzes. In our laboratory we have coupled an








11

ESI source to a 2 Tesla FTICR mass spectrometer. ESI allows multiply charged ions to be probed using IRMPD techniques to obtain spectra, which are inaccessible with other ionization techniques.

In chapter 5, ESI was used to produce several host-guest crown ether complexes such as [crown-Na]+, [crown-K]+, and [crown-H30]+ (where crown is 18-crown-6, 15-crown-5, and 12crown-4). Crown ethers are a special class of macrocylic molecules which are recognized for their ability to form hostguest complexes (48). Gas-phase IRMPD studies of these complexes are presented in this chapter. Finally, conclusions of the studies and future work are presented in chapter 6.













CHAPTER 2

THEORY AND INSTRUMENTATION


FTICR Mass S~ectrometrv


Development and Background


The fundamental principle of ion cyclotron resonance (ICR) mass spectrometry was described by Lawrence and Levingston (49). In 1949, Sommer et al. demonstrated the omegatron (50) based on Lawrence's principle. The omegatron

was used for the analysis of very light ions under high-vacuum conditions, but its poor mass resolving power limited its use as a more general analytical instrument. In 1965, Wobschall

described an ICR instrument which used a variable magnetic field that allowed ions of various masses to be brought into resonance sequentially (51). Since the ions drifted through the analyzer cell from one end to the other during the analysis, this ICR instrument also lacked high mass

resolution. In 1973, Mclver used a trapped-ion ICR instrument

(52), in which ions were formed, trapped and detected in a rectangular cell. Although this instrument offered great performance improvements over the previous ICR instruments, limited mass range, mass resolution, and slow scanning speeds were still major drawbacks.


12







13

In 1974 Comisarow and Marshall applied the principles of Fourier transformation to ICR (53), which made it much more suitable for solving analytical problems. The new name

Fourier transform ion cyclotron resonance (FTICR) mass spectrometry was given to this technique. Since the release of commercial FTICR mass spectrometers in the early 1980s, analytical applicability of the technique has fully blossomed. These analytical applications rely on many advantages of FTICR including high mass resolution, high mass measurement

accuracy, capabilities of positive/negative ion detection, and the ability of interfacing with a variety of ionization techniques such as fast atom bombardment (FAB), secondary ion mass spectrometry (SIMS), glow discharge (GD), field

desorption (FD), electrospray (ESI) matrix assisted laser desorption (MALDI), and electron ionization (El).



Theory of operation



The basic theory of FTICR mass spectrometry has recently been explained in detail by Marshall and Grosshans (54). For

an ion to achieve a stable circular orbit, the centrifugal force (Fl) and the Lorentz force (F2) acting on the ion should be equal in magnitude. Equations 2.1 and 2.2 define the

forces F1 and F2, respectively.



F1 mv2/Ir (2.1)







14



F2 = qVB (2.2)



In Equations 2.1 and 2.2 q is the ion charge, v is the ion velocity, r is the radius of gyration of the ion, and B the magnetic field strength. The result of combining Equations

2.1 and 2.2 is given in Equation 2.3.



qB = mv/r (2.3)



Rearranging Equation 2.3 gives Equation 2.4.



vi.r = q~im (2.4)



The radial motion of the ion perpendicular to the magnetic field is shown in Equation 2.5.



WC gB/r (2.5)



where w,, (=v/r) is cyclotron frequency of the particle in radians per second.

The ions formed within the cell or the ions transported from an external ion source undergo circular motion perpendicular to the magnetic-field (z-axis) Ions in the cell are held in the radial direction (x and y axes) by the magnetic field and are constrained along the Z-axis by







15

applying a charge to the trapping-plates of the same sign (positive or negative) as the ions to be trapped. The

equation of motion in the presence of both electric (E) and magnetic (B) fields is governed by Eq. 2.6:



mdvldt q[E + v x BI (2.6)



When E=O, the solution of Equation 2.6 is given by Equation 2.5. Presence of a trapping electric field along with the magnetic field produces two independent motions in addition to circular cyclotron motion (g.). They are harmonic trapping

motion in the z-direction between the trap plates and a circular magnetron motion in the radial direction (54). Since trapping oscillation and magnetron motion occur with periods much longer than the cyclotron motion, the previous two motions are ignored in most of the mathematical treatments of ion motion for simplicity. Cyclotron frequency shifts caused by the trapping potential and a modified FTICR cell to reduce cyclotron frequency shifts have been investigated by Wang and Marshall (55), among others

For the experiments presented in chapters 3 and 4, ions

were formed, detected and analyzed within a single analyzer cell located in a vacuum chamber, nominally at 10-8 10-9 Torr, within the solenoid of a 2 Tesla superconducting magnet. A typical z-axis elongated FTICR cell is shown schematically in Figure 2.1. The two opposing "trap" plates









MACOR Excite plate Front trap plate
Back trap plate Receive plate


lID!








FILAMENT Receive plate
MACOR

Excite plate


Figure 2.1. An expanded three-dimensional view of a typical z-axis elongated FTICR
analyzer cell.







17

are perpendicular to the magnetic field (B), and the pairs of opposing "transmitter" and "receiver" plates are parallel to the magnetic field. These three pairs of plates are

electrically isolated. They are used in a repetitive sequence of events that are separated in time to obtain a mass spectrum.

If a radio frequency (rf) electric f ield has the same frequency as the cyclotron frequency of the trapped ion, the

ion will absorb energy and its orbital radius and velocity will increase without changing its cyclotron frequency. This

rf excitation is applied differentially across the opposing transmitter plates of the cell. After the rf excitation pulse, ions will have increased kinetic energy and increased orbital radii. In addition, the rf pulse causes the ions which were out of phase with each other to move together coherently. The average ion kinetic energy (KE) in the absence of ion-molecule collision can be expressed using Eq.

2.7:



KE q2E2t2/8md2 (2.7)



where E is the amplitude of the rf excitation, t is the rf pulse width and d is the distance between the excitation plates of the FTICR cell.

An ion of interest can be isolated using the ejection capabilities available in the FTICR mass spectrometer (56).







is

These include single frequency ejection, swept frequency

ejection and swift ejection (56b) During the ejection pulse, an oscillating voltage with a frequency corresponding to an

ion or range of ions is applied to the "excite" plates of the cell. The ions absorb energy at their characteristic

cyclotron frequencies and move to larger cyclotron radii, eventually striking one of the cell plates where they are neutralized. Thus, ion ejection is governed by the average kinetic energy that an ion possesses. Therefore, as shown in Equation 2.7, ion ejection is dependent upon the duration of the rf pulse (t), and/or the amplitude of the rf pulse (E).

A broad band mass spectrum is obtained by applying (for a 3T magnetic field) a 20 kHz -2.66 MHz rf pulse, referred to as an 'Irf-chirp", for about 1 ms to the excite plates of the cell. This pulse excites all of the ions in a 17-2300 amu mass range into coherent motion, producing an image current consisting of all of the superimposed ion frequencies in the

mass range. Once the ions are moving coherently, the "packet" of ions attracts electrons to whichever "receiver" plate it is approaching. Thus, an image current is created on the opposed plates (57).

This image current, containing frequencies of all the ions present in the cell, is digitized to give a time-domain signal. Subsequent Fourier transformation of the digitized image current yields a frequency-domain spectrum of the ions present in the FTICR cell. A reference compound (most often







19

perfluorotributyl amn, (PFTBAJ), of a known composition is introduced into the FTICR cell, and a table is produced that matches the measured frequencies of the ions to their known,

exact masses. The resulting calibration table is then used to convert the frequency domain spectrum for all other ions which may be subsequently studied. As an example, the time-domain

spectrum (digitized image current) of ions formed following 50 eV electron ionization of diglyme (1.0 x 10-6 Torr) is shown in Figure 2.2, and the corresponding mass spectrum is shown in Figure 2.3.

Ultra high resolution, particularly at low mass, is readily achieved with FTICR mass spectrometry. The limiting, f actor in achieving high resolution measurements is damping of the coherent motions of the excited ion packets due to collisions with neutral species in the cell. For example, when the pressure in the cell is ca. 1.0 x 10-6 Torr, the time required for the intensity of the time-domain signal to fall to baseline is ca. 8 ms (Figure 2.2). If the pressure in the

cell is in the low 10-9 Torr, the time-domain signal lasts for several seconds. All the experiments presented in this

dissertation were performed under low 10-7 _108 Torr pressure conditions. Since high mass resolution was not required in

these experiments, the time-domain signals were collected for < 8 ins.

Theory of IRMPD

It is well known that absorption of infrared light causes vibrational transitions within a molecule. It is also

























O0










C-)C

0 2 4 6 8
Time (milliseconds)




Figure 2.2. A digitized time domain signal (transient) for protonated
bis(2-methoxydiethyl) ether (diglyme) cation prodpeced electron ionization
(50 eV) in a diglyme sample of pressure 1.0 x 10- Torr.
















100












50












0

20 60 100 140
M/Z


Figure 2.3. The Fourier transformed (frequency domain) ion cyclotron resonance mass
spectrum of diglyme cations resulting from the time domain signal observed in
Figure 2.2.







22

well understood that a molecule with sufficient vibrational excitation can undergo bond dissociation or rearrangement reactions (58). However, the energy provided by a single infrared photon (3 kcal mol-1) is not sufficient to achieve bond dissociation (requiring 50-100 kcal mol-1). Therefore, bond dissociation reactions require absorption of many infrared photons.

Isenor and Richardson (34) were the first to demonstrate

that bond dissociation reactions could be performed using infrared lasers. The most common source of infrared light is from C02 lasers. The C02 laser is the most versatile gas laser, and can operate in either pulsed or continuous mode. In addition, it can produce the highest continuous power of any gas laser. Laser action involves the rotational lines of two vibrational transitions within the C02 molecule. C02 is

a linear, triatomic molecule that has three normal vibrational modes: symmetric stretching mode (vj), the bending mode (V2), and the asymmetric mode (V3)'

The principles of laser action in C02 lasers have been discussed in detail (59). Briefly, the lasing medium usually consists of a mixture Of C02, N2, and He, and each gas serves a specific role. A typical gas ratio in the mixture is 1:1:8

C02:N2:He. Both N2 and C02 absorb energy from electrons in the discharge. The most significant excitation mechanism appears to be the direct excitation of N2 by the electrons from the

plasma. The energy is transferred from N2 to ground-state C02








23

molecules by collisions. The excitation of C02 molecules leads to a large population present in the vibrational state

from which the laser transition begins. Helium helps to increase the depopulation of the lower states of the laser transition.

The lasing principles are the same for both pulsed and cw C02 lasers. The pulsed C02 laser is capable of generating high-powered pulses (MW range) of short duration (As range).

The beam diameter of pulsed lasers ranges between 2 and 5 cm2 r and the laser f luence is therefore 1-2 J cm-2. The cw laser produces continuous output at lower power than the pulsed laser. The power output of a cw laser is proportional to the

length of the laser tube (usually 1-2 meters), and output powers range between 30 and 50 W. The beam diameters usually
2
range between 0.5 and 2 cm

The multiple photon process involves sequential

absorption of photons from either pulsed or CW C02 lasers through three distinct regimes: the discrete level regime; the quasicontinuum regime; and the dissociation threshold level. A schematic energy diagram for IRMP absorption and dissociation processes is shown in Figure 2.4. The discrete level regime consists of the individual spectroscopic states

of the molecules at low energies. Excitation by C02 laser requires that the laser frequency be in resonance with the energy separation of any two states. As the internal energy of the ion increases, the density of states increases until





24






Dissociation Threshold





Quasicontinuum



t
V-4
J-o

Discrete V-2 j J 0
Level
Regime v-1 J 0


.................. ............................. J 1
V-0 J-0


Figure 2.4. A schematic representaion of the energy
diagram for the infrared multiple photon
(IRMP) absorption and dissociation process.








25

eventually the separation of states is less than the laser bandwidth. This stage is defined as the quasicontinuum. In

the quasicontinuuu dissociation radiation of any frequency can be readily absorbed. Above this regime the dissociation rate

constant is expected to increase with increasing internal energy and the ion will dissociate.

Both pulsed and cw CO2 lasers were used in this dissertation. The pulsed laser was a L1uonics Model TE 860 CO2 laser (60) with a rectangular bean profile of 2 cm, x 2.75 cm, and a resolution of ca. 2 cm-1. The pulsed laser wasline tunable over a wavelength range of 9.10-10.92 Am. The cw

laser was an Apollo Model 570 CO2 (61) laser which could be

line tuned over a wavelength range of 9.0-11.3 Am, with a beam prof ile of 0. 8 cm-2, and a resolution of ca. 2 cm-1. Beam profiles were measured in front of the laser head. It was

necessary to measure the wavelength accurately when obtaining the IRMPD spectra. This was done by using an Optical

Engineering Model 16A CO2 laser spectrum analyzer (62) equipped with an infrared grating which can be obtained over a wavelength range of 9.1-11.3 Am. It was necessary to keep the laser energy or power constant throughout the wavelength scan of the IRMPD experiments. To achieve this, a Scientech Model 365 power meter along with a thermopile detector (63) was used. This detector had a calibrated head with an absorbing surface for infrared light. Heating of the absorbing surface by the laser produced an electrical output








26

to the meter which gave the energy (or power) output of the laser beam in the range of 0.01 mJ 30 J.



IRMPD/FTICR Mass Spectrometrv



Electron Ionization Experiments



All experiments were performed using a home-built FTICR mass spectrometer equipped with either a Nicolet FTMS 1000 (64a) or an Ion Spec (64b) data station and a prototype 2 Tesla superconducting magnet. A schematic representation of the FTICR mass spectrometer used in chapters 3 and 4 is shown in Figure 2.5. The high vacuum chamber was positioned inside the 20 cm bore of the magnet. This high vacuum chamber was pumped by two oil diffusion pumps (65) with a combined pumping speed of 1000 L/s. The background pressure of the system was maintained at 5.0 x 10-9 Torr. A third 300 L/s oil diffusion pump was mainly used to evacuate the inlet system. All

chemicals were introduced into the high vacuum chamber by using the inlet system heated to ca. 100 0C. Purity was

confirmed by broadband mass spectra and the samples were used without further purification except for removal of dissolved gases by multiple freeze-pump-thaw cycles. This system was equipped with three laser windows on the 8" flange to which the FTICR analyzer cell was mounted.







Ionization gauge 2 Tesla superconducting
GaslUquid inlet

1064 nm laser I
beam I Solids probe 4 Laser
F window






FL-J





LaserFTICR Cell
window Quartz lens Graphite plug

Chamber diffusion pump Inlet diffusion pump (300 I/s)
(7001/s) Chamber mechanical pump (M Is)

Figure 2.5. A schematic representation of the FTICR mass spectrometer equipped with a 2
Tesla superconducting magnet.







28

ElectrosDrav Ionization Experiments


In chapter 5, crown ethers formed by ESI were studied using the IRMPD technique. Figure 5.1 illustrates the

ESI/FTICR mass spectrometer used in this series of experiments and a detailed discussion of the ESI source and the instrumentation will be presented in chapter 5. This type of "internal" electrospray ionization source was first demonstrated by Hofstadler and Laude (66) and we followed Laude's idea and modified the mass spectrometer shown in Figure 2.6 to accommodate the ESI source. The modifications and assembly are explained in detail elsewhere (67). Briefly, the vacuum chamber for the ESI consisted of five concentric chambers of increasing diameter with four differentially pumped regions. A more detailed description of these pumping regions will be given in chapter 5.

A known mass of commercially available crown ether compound was dissolved in 50:50 methanol:water to obtain a sample of 1.0 x 10-5 M concentration. Sodium and potassium complexes of crown ethers were formed by adding 1-3 drops of 1.0 x 10-5 M sodium chloride (NaCl) or potassium chloride (KCl), respectively. Sample solutions were delivered to the electrospray needle through a 6' long 22-gauge teflon tube. Operating parameters for the ESI source are discussed in chapter 5.







29

The FTICR Cell



The first FTICR experiments was done using a cubic cell

(52), and the method was later expanded to several other ion

trap geometries including orthorhombic (68), cylindrical (69), hyperbolic (70), and open (71). For the experiments presented in this dissertation a standard z-axis elongated rectangular

FTICR cell (Figure 2. 1) was modif ied f or IRMPD experiments using White's demonstration (72), to obtain long optical paths within a small volume.



Multipass Process


A schematic illustration of the set-up used by White to increase the optical path length for irradiation experiments

is shown in Figure 2.6. The essential parts of the set-up are three spherical, concave mirrors, each of which has the same

radius of curvature. Light enters through a slit close to one end of mirror B, then it passes to mirror A, from mirror A to mirror B, then to mirror A', then back to mirror BF and then

again to mirror A. Alternatively, the light reflects back and forth between mirrors B, A', and A. The most important

adjustment is the separation of the centers of curvature of the mirrors A and A'. The ratio of the length of mirror B to

the separation of centers of curvature of mirrors A and A' determines the number of times the light goes through the cell. This can be either f our times f or one image on B, eight











Laser beam in







Mirror 1
Mirror 3 Mirror 2





Laser beam out Figure 2. 6. An illustration of the set-up used by White to obtain very long optical paths
for irradiation experiments.







31

times for three images, twelve times for five images, and sixteen times for seven images.



Modified White-Type Cell



The z-axis elongated cell was modified in our laboratory for IRMPD by incorporating White's multipass optical arrangement. Experiments in chapter 3 were performed by using

the modified White-type FTICR cell as shown in Figure 2.7. The cell dimensions are 2.5 cm, 2.5 cm, and 6.5 cm along the x, y, and z magnetic field axes, respectively. Three

spherical, concave, and well-polished brass mirrors were incorporated as the receive plates. All three mirrors had the same radius of curvature of 3.12 cm. One of the mirrors was 3.13 cm in length, 2.34 cm in width, and 0.73 cm in thickness. The two opposite mirrors, which are identical in size, were 2.34 cm in length and 2.5 cm in width with a 0.82 cm thickness. The solid excite plates were replaced with stainless steel mesh. A 2.0 cm hole was machined into one of

the trap plates and 90% transparent mesh covered the hole. The first laser beam was turned by the gold-plated mirror (2.5 cm x 2.5 cm ) into the cell through a 2.0 cm o.d. hole in the receive plate, and thus began the multipass reflection process. In this set-up, laser light reflected inside the cell eight times, thus giving six more passes than a regular








screened excite plate
(fits above mirrors)

trap p Macori~. "receive plate

electron
bea ............... ,, _ '':'=:. .. ..... ...;;k"M a
Macor


......... non-resonant
...laser


turning mirror brass mirrors resonant laser

Figure 2.7. An expanded three dimensional view of the modified White-type FTICR analyzer
cell used for one- and two-laser experiments to obtain IRMPD spectra, presented in chapter 3. The three spherical mirrors used to create
multipasses were incorporated in to the stainless steel receive plates. The dotted-lines illustrate both multipasses (eight passes) and the center pass
of the laser beam.







33

cell. A second laser beam entered through the mesh trap plate and traversed the cell twice. Laser beam pathways are shown in Figure 2.7 (denoted by dotted-lines). A detailed description of irradiation pathways for the one and two-laser experiments, including schematics will be further presented in chapter 3.



Newly-Modified White-Type Cell



The White-type cell used in chapter 3 was limited to only

8 passes of the laser beam due to its limited ratio of the length of mirror 3 to the separation of centers of curvature of mirrors I and 2. In addition, the laser beam alignment on

the brass mirrors was a difficult task because the brass mirrors were very flexible Therefore, many problems were encountered when doing the multipass alignment with a heliumneon (He-Ne) laser before inserting the cell into the vacuum chamber. Moreover, inhomogeneity of the electric field lines

inside the cell created by the brass mirrors required a trapping potential between 3-5 Volts to contain ions during irradiation. To circumvent the above mentioned problems the White-type cell was further modified. In this arrangement the ratio between the length of mirror 3 and the distance between center of curvature of mirrors 1 and 2 was increased. Also, the brass mirrors were replaced by well-polished stainlesssteel mirrors. The new design enabled us to obtain 16 laser passes inside the cell.







34

Figure 2.8 shows the newly-modified White-type FTICR cell which was used for the IRMPD experiments in chapters 4 and 5. The cell dimensions were 2.5, 3.5, and 6.5 cm along the x, y, and z magnetic field axes, respectively. Three spherical, concave well-polished stainless steel mirrors were incorporated into the receive plates. One of the mirrors was 4.5 cm in length and 0.95 cm in width with a 3.1 cm radius of curvature. The two opposite mirrors, which were identical in size, had 0.63 cm diameter and 3.1 cm radius of curvature. The three mirrors were positioned in such a manner to allowed the laser beam to pass 16 times inside the cell. The solid excite plates were replaced with stainless steel mesh to facilitate the laser alignment. A gold-coated turning mirror (2.5 cm x 2.5 cm) was attached to one of the MACOR (73) spacers holding the cell plates. The turning mirror allows the laser light to reflect into the cell through a 1.5 cm o.d. hole in one of the MACOR spacers holding the cell plates. Then the laser beam was subjected to multipasses inside the cell, as shown in Figure 2.8.

To demonstrate the photodissociation efficiencies of the newly-modified White-type FTICR cell (sixteen vs. two passes), protonated diglyme ion was formed using EI, isolated, and photodissociated. Both multipass and double pass arrangements were used and the photodissociation mass spectra were obtained at 10.60 gm output of the cw CO2 laser by keeping the energy constant at 500 mJ. For the double pass laser experiments,






screened excite plate receive plate


back trap plt N
MA,. front trap plate








....... laser beam

stainless steel mirrors I MACOR
receive plate Au coated turning mirror
Figure 2.8. An expanded three dimensional view of the newly-modified White-type FTICR
cell used to obtain IRMPD spectra presented in chapters 4 and 5. All
experiments were perfomed using the multipass arrangement (16 passes) and the
laser path is illustrated by the dotted-lines.







36

the laser beam entered the vacuum chamber through the center of a ZnSe window mounted on a three-window flange as shown in Figure 2.9. Then, the laser entered from the front trap plate (facing the window) and reflected from the back trap plate to traverse the cell a second time. Next, similar IRMPD

experiments were performed using multipass laser irradiation. As shown in Figure 2.10, the laser light was reflected from three gold-coated mirrors before entering the ZnSe window. After entering the ZnSe window, the beam was reflected once more from another gold-coated mirror before starting the multipass process. Figures 2.11(a) and 2.11(b) show the IRMPD mass spectra obtained with a double pass and 16 passes, respectively. To further demonstrate the photodissociation efficiencies, experiments with protonated diglyme cation were carried out in a similar manner but increasing the laser energy to 750 mJ. IRMPD mass spectra from double pass and multipass arrangements are shown in Figures 2.12(a) and

2.12(b), respectively.

The calculations for the percent photodissociation of protonated diglyme cation will be discussed in detail in chapter 3. The peak intensities of the parent (m/z 135) and photofragment ions (m/z 103 and 59) (figures 2.11 and 2.12) were used to calculate the photodissociation efficiencies. The results obtained for the multipass IRMPD experiment (Figure 2.11(b)) indicate that the amount of photodissociation is increased by ca. 60% when compared with that of double pass












PuMirro (1)e







Mirror ()





Cell uppor rodZnSe window


_____ _____Mirror (3)





Figure 2.9. The C02 laser (pulsed or continuous (cw)) beam pathway for double pass
arrangement before entering the vacuum chamber. The beam pathways are illustrated only for the cw COg laser, but the pulsed CO2 laser could be used to obtain the double pass inside the FTICR cell, using mirror 1' instead of
mirror 1.














Mirror (1)
ow laser



Spherical mirror

Turning mirror M _irror (2)0
Cell support rod ZnSe window
p )oft rod)




Figure 2.10. The CO2 laser (pulsed or continuous (cw)) beam pathway for multipass
arrangement before entering the vacuum chamber. The beam pathways are
illustrated only for the cw CO2 laser, but a pulsed CO2 laser could be used to obtain the multipasses inside the FTICR cell, using mirror 1' instead of
mirror 1.









39




o



O



>

UJ z












01 I I
50 .100. 150 200 250
MASS IN A.M. U









>

Zm I


4












20 40 60 80 100 120 140
MASS IN A.M.U.


Figure 2.11. IRMPD mass spectrum of protonated diglyme
cation obtained with cw CO, laser. The lser energy was kept constant aE 500 mJ pulse- at 10.60 pm irradiation wavelength. The laser beam was subjected to (top) center pass, and
(bottom) multipass arrangements.
7S








40



























50 100 150 200 250
MASS IN A.M.U. I















U,
z t.J
0

















-J
W





















Figure I .... .... I... ..I..
20 40 60 80 100 120 140
MASS -N A.M.U


Figure 2.12. IRMPD mass spectrum obtained for the
protonated diglyme cation under the same conditions used to obtain Figure 2.10 but cw
CO2 laser energy was 750 mJ pulse- The
laser beam was subjected to (top) center pass,
and (bottom) multipass arrangements.







41

experiment (Figure 2.11(a)). Although the amount of photodissociation for both laser irradiation was increased when 750 mJ pulse-1 laser energy was used (Figure 2.12(a) and 2.12(b)), the calculated percent photodissociation revealed that amount of photodissociation with the multipass experiment was still ca. 60% greater when compared with that of doublepass experiment.

To decide which laser was to be used in one-laser IRMPD experiments presented in chapters 4 and 5, laser power measurements were obtained at several mirrors outside the vacuum chamber using both pulsed and cw CO2 lasers. The cell was mounted on the flange holder and the 8" flange was mounted on the table. The beam was subjected to multipasses inside the cell as shown in Figure 2.10. First, laser-burns were taken, using thermal paper at all three mirrors and the ZnSe window. Next, the He-Ne laser was aligned on all three burns on the mirrors and through the ZnSe window. Then, the He-Ne laser was centered on the gold-coated turning mirror, and the multipass reflections were optimized. Dry-ice or cigarette smoke was used to observe sixteen passes inside the FTICR cell.

The CO2 laser light was reflected through the same pathway, using the aligned He-Ne beam as a guide-line. This method allowed the energy of the CO2 laser beams to be measured after each mirror. The receive plate that contained the larger mirror was carefully removed without disturbing the








42

multipasses. This enabled the laser energy at the curved mirror (after the first reflection) inside the cell to be monitored. The measured laser energy after each reflection by the mirrors, and the distance from the laser head to each mirror are shown in Table 2.1. The energy measurements were

taken several times and the average value was tabulated. Results indicated that cw laser energy was higher than that of the pulsed laser after the first pass in the multipass

reflection process. In addition, the observation of the laser beam burn-patterns at each mirror proved that a more collimated beam could be obtained from the cw laser. These findings led to selection of the cw C02 laser as the laser of choice for one-laser IRMPD studies.



FTICR Pulse Seauence



The pulse sequences f or one and two-laser IRMPD experiments are discussed with figures and presented in chapters 3, 4, and 5. Briefly, first, a one ms quench pulse

removed all ions from the cell when +15 and -15 V were applied to the trap plates (during the quench pulse Ion Spec electronics provided 15 V on one of the trap plates). Then,

ions were formed by electron impact during the 50-100 ms ionization period, or transfered from the ESI source for 1 s.

Next, a "thermal ization" delay allowed time for ion/neutral collisions to occur and relax the ions. A series of ejection sweeps were used to eject all ions except the ion of interest














Table 2.1. Energy measurement for both pulsed and cw CO2 laser beam outputs after
reflecting from each mirror and the ZnSe window, using the newly-modified FTICR cell. The distances for each mirror and for the ZnSe window from the
laser head are also presented.







Energy (mJ pulse-') Distance (inches)

Position pulsed CO, cw COL pulsed CO2 cw CO2

Laser Head 853 850

Mirror 1 722 750 27 47.5
Mirror 2 518 624 107 90
Mirror 3 507 568 170.5 133
ZnSe Window 348 398 183.5 146

Spherical mirror 103 290 216 179
(mirror 4) T I ____ _







44

from the FTICR cell. Six ejection pulses are available from

the Nicolet FTMS 1000 (the Ion Spec provides more than 25 ejection pulses) data station used in these experiments. Then the pulsed CO2 laser was triggered, or the cw CO2 laser was gated on for variable length irradiation periods of 50-500 ms at a constant laser energy. Immediately after the laser was fired the ions were excited and detected utilizing the standard frequency chirp excitation method. For each of the line-tuned laser wavelengths a broad band (10 kcHz 2.66 MHz) time domain signal consisting 16,384 data points averaged for

50 100 repetitions was collected. Thess averaged time

domain data was then apodized using a three term Blackman Harris window function (74) and zero filled once prior to Fourier transformation.













CHAPTER 3
INFRARED MULTIPLE PHOTON DISSOCIATION SPECTRA OF GASEOUS IONS


Introduction


Many different kinds of ions are observed in mass spectrometry, and these are the result of a variety of

ionization and fragmentation processes. These processes have been studied experimentally for many years, but for most of the ions little is known of the structures and energy states. optical absorption spectroscopy is a very powerful technique for obtaining such information in the gas phase. However it

is not practicable, except in very few cases of simple di- and triatomic species to obtain a direct absorption spectrum for molecular ions (75,76). One is most often forced, then, to utilize an indirect method, such as ion photodissociation, to obtain spectra and structural information for gaseous ions.

In ion photodissociation, an ion absorbs one or more photons until it gains sufficient energy to dissociate into fragments. The disappearance of parent ion or the appearance of the fragment ions as a function of irradiation wavelength can then lead to a photodissociation spectrum of the parent ion under favorable conditions (77). This approach has exhibited reliable results for interpretation of energy


45







46

levels, dissociation thresholds, and structures of gaseous ions, when they are subjected to UV-visible irradiation, quite often in Fourier transform ion cyclotron resonance (FTICR) mass spectrometers (78).

The availability of tunable CO2 lasers and the ability to perform mass analysis on charged particles have made infrared photodissociation spectroscopy a very powerful spectroscopic probe for elucidating the gas phase ion structure, energetics, and dynamics. The technique of infrared multiple photon dissociation of ions has been used by several researchers (7982) to obtain spectra of gaseous ions in order to assist in determining ion structure. Zhao et. al. demonstrated the feasibility of IRMPD of molecules in molecular beams (83) and obtained structural information as well as thermal decomposition data for polyatomic molecules. However, due to both the limited tunability of standard ir laser sources (CO2, CO, NO), and partly to the low power of these lasers, there has been no systematic attempt to use IRMPD to obtain gaseous ion spectra and structural information.


Probe-umD Techniaue


Our laboratory has been heavily involved (84-86) in coupling UV-visible and both continuous wave (cw) and pulsed CO2 lasers to FTICR mass spectrometers for several years. As a result of these investigations structural information about







47

isomeric ions (87,88) and some crude spectra for diol ions

(89) have been obtained, but the above mentioned limitations precluded acquisition of high quality IRMPD spectra. However, a novel two-laser approach developed in our laboratory (89,90) has demonstrated that reproducible spectroscopic data for gaseous ions can be obtained, and this technique promises to overcome many of the limitations imposed by the use of CO2 and other lasers.

An ion with a sufficiently high density of vibrational and rotational levels is excited to the "quasicontinuum" by resonant multiphoton absorption from a low-power tunable pulsed CO2 (probe) laser. In the quasicontinuum the ion can readily absorb additional photons regardless of their energy. These additional photons, provided by a more powerful cw CO2 (pump) laser, impart sufficient internal energy to the ion to dissociate it into fragments, which are observed using standard FTICR detection techniques. The "probe-pump" twolaser photodissociation process is shown schematically in Figure 3.1.

The applicability of our recently developed method for obtaining spectra of gaseous ions, using a low power tunable pulsed CO2 laser for probing the resonant absorption spectrum and a more powerful non-resonant cw CO2 laser to complete the dissociation, is demonstrated in the work reported here. In particular, one- and tvo-laser spectra for the protonated bis(2-methoxyethyl) ether (diglyme) cation, the 3-bromopropene







FRAGMENT
ION(S) Dissociation
____Threshold


PUMP VIBRATIONALLY Quasi
LASER EXCITED ION _____ continuum


V-4
V-3 J -0
V=2 Ij J0O
V=l J-O

PR B..-TAPPED V- .
LA S E R "P IO N v o ........................... J- 1
....... J ., 1

Figure 3.1. Schematic representation of the two-laser (probe-pump) photodissociation
process.







49

(allyl bromide) cation, and the gallium hexafluoroacetylacetonate anion ([Ga(hfac)3]-) have been obtained.



Experimental



All IRMPD experiments were performed on a home-built FTICR mass spectrometer equipped with a 2T superconducting magnet, and controlled by a Nicolet FTMS 1000 data station. Chapter 2 gives more details about the home-built vacuum system, including a figure. The background pressure was maintained below 2 x 10-9 Torr and samples were leaked in up to a pressure of 5.5 x 10-8 Torr. For each spectrum 16,384 data points were acquired by signal averaging 50 ion transient response signals.

To enhance photodissociation effects the FTICR analyzer cell used in these experiments was modified to increase the irradiation path length by a multipass arrangement as shown in Figure 2.7, and discussed in chapter 2. This White-type ICR cell was first demonstrated in our laboratory for IRMPD experiments using a single laser, and was shown to enhance dramatically ion photodissociation effects. In the present study the White-type cell was used to obtain spectra for both one and two laser experiments.

As shown in Figure 2.7 the turning mirror attached to one end of one receive plate reflected the resonant pulsed laser light into the cell to give eight passes. In two-laser experiments








50

the non-resonant cw laser entered via one of the trap plates,

which was modified by addition of a coarse stainless steel mesh.



One- and Two-laser EXperiments



Single-laser experiments were performed using the cw CO2 laser. This laser was gated on for variable length irradiation periods (ranging from 50 to 500 ins) by the FTICR data station at a constant energy of 1 J pulse-1. Two-laser experiments were performed with the pulsed CO2 laser as the resonant probe laser and the cw CO2 laser as the non-resonant pump laser. The probe laser energy was kept constant at 10015 mJ pulse-. Triggering of this laser was also controlled by the FTICR data station. For two-laser experiments the pump laser was operated at a fixed wavelength of 10.58 Am and a constant energy of 1 J pulse-1. All

energies measured here were obtained in front of the laser head and it is estimated that ca. 70% of this beam entered the FTICR cell.

The laser beams entered the vacuum chamber through two ZnSe windows mounted on a three window flange. As shown in Figure 3. 2, the pulsed CO2 (resonant) laser beam was reflected by the turning mirror into the cell, and subsequently reflected from the spherical mirrors to create eight passes inside the cell. The CW CO2 laser (non-resonant) entered the vacuum chamber through a second window, passed into the cell








Au MIRROR




OW C02 LASER RECEIVE PLATE


ELECTRON L
BEAM

TRAP PLATE PLT




Au TURNING MIRROR Au MIRROR


Figure 3.2. Schematic representation of the cutaway view of the modified White-type cell
and the two-laser beam pathways.







52

through one of the trapping plates, and reflected from the other trapping plate, thus giving a double pass inside the cell.



FTICR Pulse Secruence



Ions were formed by electron ionization. The electron beam voltage was varied from 10-30 volts for the cations and

the anion was formed by electron attachment using a low energy electron beam (0.5 V) The molecular ion of interest was isolated by ejecting all other ions from the cell. Ions were then allowed to undergo several collisions with the neutrals at a pressure of 5.5 x 10-8 Torr for approximately is. No difference in photodissociation was observed as this collision time was varied over the (limited) range from 1.5-0.75 s. Subsequent to thermalization the probe laser was fired and another series of ion ejections was used to remove any

unwanted adducts and fragments formed by the probe laser. These ejections were carried out with a minimum time delay (<30 ins) before the pump laser irradiation period. Then the pump laser was gated on for 30 ins. All ions present after this time were excited and detected, and the extent of

photodissociation was obtained by measuring the intensities of the parent and fragment ions, as given by their mass spectral peak areas. The pulse sequences for both one-laser and twolaser (probe-pump) approaches are depicted in Figure 3.3.













.r 0C CI Ca 00
C.) 4,0 0 ~ *404
CS o -,-O

N M (M t5
0 M U) Eq EU)
C30 .2 iFJ X4
















Time


Figure 3.3. Experimental pulse sequence employed in the two-laser probe-pump technique.
The ejections after the probe laser and the pump laser gating steps were
eliminated when only one-laser experiments were performed.







54

A positive trapping potential of 3 to 4 V was used for diglyme and 3-bromopropene cations, and a potential of -2 V to

-3 V was used for [Ga(hfac)3]-. These higher than usual trapping potentials were necessary when using the White-type cell, because of the distortion of the electric field lines in the analyzer cell due to the curved receive plates.
To obtain gas phase spectra of the neutral molecules a 10 cm quartz cell was used in a Nicolet 740 (approximate pressure for diglyme 1.9x102 Torr) or a Perkin Elmer 1600 (approximate pressure for allyl bromide 2.6x102 Torr) FTIR spectrometer. The [Ga(hfac)3]- spectrum was obtained in the solid state (KBr pellet) with the latter instrument.
All samples were obtained from commercial sources. The purity was confirmed by broadband mass spectra and the samples were used without further purification.


Results


The first series of experiments was performed using the White-type multipass FTICR cell and one tunable higher power cw laser. The total energy per pulse was kept constant as the laser was tuned to various photodissociation wavelengths. The percent photodissociation (calculated by dividing the relative intensities of the photofragments by the relative intensities of all ions detected) as a function of laser wavelength was obtained for each of the three compounds. The experiment was repeated fifteen times for a given wavelength and the average percent photodissociation was calculated. All the error limits represent the standard deviation of the mean.







55

When the cw laser was tuned through the wavelength range 10.49-10.71 pm (953-934 cm-1), the positive ion C6H1503 (protonated diglyme cation, m/z=135), underwent IRMPD to form two fragment ions at m/z =103 (C5H1102+) and m/z=59 (C3H7O+) according to the reactions:


C6H15OH + nhvir C5H1102+ + CH40

C3H70+ + C3H802


The photodissociation spectrum obtained is shown in Figure
3.4.
A limited wavelength IRMPD study of the 3-bromopropene positive ion has been previously reported (91). Results were only reported for the five strongest CO2 laser lines at 9.28, 9.54, 10.25, 10.59, and 10.67 pm. The ion undergoes IRMPD yielding exclusively C3H5+ via loss of Br*.



C3H5Br+* + nhuir C3H5+ + Br'


The single laser IRMPD spectrum obtained for this ion is shown in Figure 3.5 for CO2 laser wavelengths from 10.18 to 10.70 pm (982-934 cm-1).
The Ga(hfac)3 anion (m/z=691) was formed by low energy electron attachment. When the CO2 laser was tuned from 9.49 to 9.62 pm (1054-1039 cm-1) the negative ion photodissociated by losing a negatively charged ligand (hfac, m/z=207).






60


6 50
z
400
O 30(I) U)
0 20I
0
n 1001
934 936 938 940 942 944 946 948 950 952 954 WAVENUMBER / cm-1
Figure 3.4. One-laser infrared multiple photon dissociation spectrum of protonated
molecular ion bis (2-methoxyethyl) ether (diglyme) at a probe laser energy of
1 j pulse-1. Error estimates are 95% confidence limits.






70


S60
z
0
0 500
O 40U) U)
030


O
0
a.. 2010 f I I
930 940 950 960 970 980 990
WAVENUMBER / cm-1
Figure 3.5. One-laser infrared multiple photon dissociation spectrum of positive
molecular ion of allyl bromide (3-bromopropene) at a probe laser energy of 1
J pulse-1. Error estimates are 95% confidence limits.







58



Ga(hfac)3 + nhUir (hfac) + Ga(hfac)2




The photodissociation spectrum obtained is shown in Figure

3.6.

In a second series of experiments, useful two laser photodissociation spectra were obtained using the same

compounds as discussed above and the probe-pump technique. Figures 3.7, 3.8, and 3.9 present the spectra of protonated diglyme, allyl bromide and gallium hexafluoroacetylacetonate ions, respectively.

For comparison, gas phase neutral IR spectra were obtained for all three compounds used in this study. They are shown in Figures 3.10, 3.11, and 3.12.

Table 3.1 summarizes IR peak frequencies for the three ions obtained from Figs. 3.4-3.9 (one- and two-laser experiments gave identical results) and also includes the corresponding neutral IR peak frequencies for the relevant vibrations from Fig. 3.10-3.12, and the frequency shifts.



Discussion



Previous studies of IRMPD behavior of the hexafluoropropene cation (C3F6+) (89) demonstrated that a relatively low-power probe laser source (ca. 100 mJ pulse-'), while incapable of inducing photodissociation by itself, could






30


'X 25

z
0 20
I

0 15C/)
U)
01

0
015

0
1038 1040 1042 1044 1046 1048 1050 1052 1054 WAVENUMBER / cm-i

Figure 3. 6. one-laser infrared multiple photo dissociation spectrum of the negative
molecular ion of Ga(hfac)3 at a probe laser energy of 1 J pulse'1. Error
estimates are 95% confidence limits.






22

0 21o 200 19
0 Ul)
0 18

0 o
I- 17

S16

15
934 936 938 940 942 944 946 948 950 952 954
WAVENUMBER / cm-1
Figure 3.7. Two-laser infrared multiple photon dissociation spectrum of protonated
molecular ion of diglyme at a probe laser energy of 100 mJ pulse-1 and a pump
laser energy of 1 J. Error estimates are 95% confidence limits.






25


-. 20

z 0

~o

0 0 F
0
5

0~
90 940 950 960 970 980 990

WAVENUMBER cm-I
Figure 3.8. Two-laser infrared multiple photon dissociation spectrum of positive
molecular ion of 3-bromopropene at a probe laser energy of 100 mJ pulse-1 and
a pump laser energy of 1 J. Error estimates are 95% confidence limits.






14

~12

z
o 10 < 8

0 Cl)
u) 65
40
IL 2-.

1038 1640 1042 1044 1046 1048 1050 1052 1054

WAVENUMBER / cm-1

Figure 3.9. Two-laser infrared multiple photon dissociation spectrum of negative
molecular ion of Ga(hfac)3 at a probe laser energy of 100 mJ pulse-1 and a
pump laser energy of 1 J. Error estimates are 95% confidence limits.












O L Z

<
S0.2






0.0
O







850 950 1050 1150 1250 1350
WAVENUMBER / cm-1
Figure 3.10. Gas phase neutral infrared spectrum of diglyme (pressure =0.45 Torr).







0.6


0.5


W 0.4z


0 C')
0o
< 0.2,


0.1


0,
800 850 900 950 1000 1050 1100
WAVENUMBER / cm-1
Figure 3.11. Gas phase neutral infrared spectrum of 3-bromopropene (pressure = 0.45 Torr)







3.31


2.91

U 2.51
0 z
S2.11
0
CO
< 1.71


1.31


0.91
500 600 700 800 900 1000 1100 1200 1300 1400 WAVENUMBER / cm-1

Figure 3.12. Gas phase neutral infrared spectrum of Ga(hfac)3 (pressure = 0.45 Torr).









Table 3.1. Gaseous ion vibrational frequencies from one- and two-laser studies,
frequencies of the corresponding gas phase neutrals, and the ion-neutral peak
shifts.



Peak frequency (cm-1) Vibrational one- and
Molecule mode two-laser neutral shift (cm1)

Diglyme C-a-C 940.5 (18%)b 1030 ca. 90a
Allyl bromide C-Br 944.2 (20%) 920 ca. 24
951.2 (15%) 930 ca. 21
972.0 (1%) 972
975.9 (8%) 981 ca. 5
979.7 (6%) 985
a
Ga(hfac), C-F 1043.2 (20%) 1151
a
1052.0 (25%) 1240


a Neutral-ion peak shifts cannot be determined since the ion peaks cannot be assigned unambiguously. See text. b One-laser percent photodissociation.








67

be used in conjunction with a higher power pump laser source

to obtain photodissociation spectra in a similar manner to higher power single laser experiments. Similar results were found for the two-laser studies of the three compounds examined here.

The one- and two-laser spectra are quite similar, as both approaches use the cw CO2 laser to up-pump the population in

high, dense vibrational states to the quasicontinuum. This similarity does lend credence to the assertion that resonant

photon absorption is the "bottleneck" to dissociation in each process. Examination of Figures 3.4-3.6 and 3.7-3.9 reveals

that the maximum percent photodissociation obtained for the one laser experiments was greater than that for the two laser spectra. This difference in dissociation can be explained by the much lower energy per pulse of the resonant laser in the two- versus one-laser experiments. As mentioned in the

experimental section, the one-laser experiments were carried

out at 1 J total irradiation energy, whereas for the two-laser experiments the resonant laser energy was kept constant at 100 uJ pulse-1. The maximum per cent photodissociation was decreased only by a factor of 3-5 for all two-laser experiments performed, even though there was a tenfold decrease in total energy per pulse of the resonant laser.

Since the IR spectra of neutral diglyme (Figure 3.10) shows a number of features in the 950-1250 cm-1 region, exact assignment of the IRMPD peak is difficult. We have







68

tentatively related the ion peak to the somewhat structured neutral peak between 960 and 1050 cm"I, rather than to any of the stronger bands between 1100 and 1250 cm"1, primarily because the absorbance of the former band is similar to that of the allyl bromide neutral band (Figure 3.11) and the IRMPD spectra of the corresponding ions show almost the same extent of photodissociation. It is not possible to assign the ion peak unambiguously to specific lines in the P or R branches of the neutral or to the sharp Q branch of approximately the same width.

Comparison of the IRMPD and neutral IR spectra reveals very little difference for the C-O-C stretch of diglyme and the C-Br stretch of 3-bromopropene. A possible explanation for the similarity in diglyme spectra is that while formation of protonated diglyme involves addition of H+ onto the 0 atom [C-O+(H)-C], there is little change in the nature of the bonding orbitals controlling the force constant for C-O-C stretch. Similarity in 3-bromopropene spectra can be attributed to the fact that electron removed upon formation of the positive ion is in a bromine non-bonding orbital and thus there is again negligible change in the bonding orbitals controlling the force constant for the C-Br stretch upon ionization.

In contrast, the IRMPD spectrum of Ga(hfac)3- shows a much larger shift when compared to the IR spectrum of its neutral precursor. The IR spectra of fluorine-substituted compounds







69

have C-F stretching absorptions in the 1350-1000 cm range, with the exact position depending on the nature and the degree of fluorination (92). As the complexity of the molecule increases the accurate assignment of any one peak to a particular normal mode becomes complicated. This is partly due to Fermi resonance (93) and partly due to bonds having similar vibrational frequencies.

Assignment of the observed C-F stretching frequencies in the Ga(hfac)3 anion spectrum (Figure 3.9) and the neutral IR spectrum (Figure 3.12) is made by comparison with previous studies (94-96) in Table 3.2. For CF3, CF3+, and C2F6 the degenerate C-F stretching mode has a higher frequency than the nondegenerate C-F stretching mode. We assume a similar assignment for Ga(hfac)3, with the degenerate C-F stretching mode assigned a higher frequency (ca. 1240 cm-1) and the nondegenerate C-F stretching frequency assigned a lower frequency (1151 cm1l). Two peaks are observed for the degenerate C-F stretching mode at 1217 cm" and 1262 cm1; presumably the splitting is due to Fermi resonance involving either a combination or an overtone band of one or two of the peaks observed at lower frequencies in Figure 3.11.

Shin and Beauchamp have obtained (97) IRMPD spectra of some organometallic compounds containing CF3 ligands. Two peaks in the CF3Mn(CO), neutral spectrum were attributed to C-F stretching modes of A and E symmetry respectively, and shifts to lower frequency of each peak were seen in both







70











Table 3.2. The C-F stretching frequencies of A and E
modes used in assigning peak frequencies for
the Ga(hfac)3 neutral and the anion.



Molecule A1 (cm-1) E (cm-1) Reference

CF3" 1084 1252 39, 40
CF3 + 1125 1667 40
C2F6 1116 1250 41
Ga(hfac)3 1151 1240
Ga(hfac)3- a a


a Cannot be assigned unambiguously; see explanation in text.







71

CF34n (CO) 3(NO) -and CF3Mn (CO) 4. Explanation for these shifts involved increase of the electron density in the carbon adonor orbital of the CF3 group in the anionic species. Also, the spectra apparently show that the frequency of the

degenerate C-F stretching mode is more sensitive to changes in the net charge of the molecule and ligand substituents,

leading to a higher frequency shift when compared with the nondegenerate C-F stretching mode (cf. the results for CF3+ vs. CF3, in Table 3.2.

We have assumed a similar trend in A and E mode shifts in assigning the peak(s) observed for [Ga(hfac)3J- in Figure 3.9 to the asymmetric C-F stretch. The electron added in

formation of the anion occupies an orbital with strong antibonding character localized on one or more of the hfac ligands. This will definitely lead to a lowering of the C-F stretching frequency in the anion when compared to the

neutral. Given the relatively narrow wavelength range spanned by Figure 3.9, it is not possible to assign unambiguously the peaks seen there. Both the symmetric and asymmetric C-F

stretching modes may have been reduced by ca. 100 cm71 and 200 cm1 respectively, leading to the two peaks seen in the spectrum. or, more likely, the spectrum may be due to a single peak split by a Fermi resonance interaction of either

the symmetric or the asymmetric stretching mode with one (overtone) or two (combination) modes of the correct symmetry of lower frequency which cannot be observed given the limited wavelength range of the CO2 laser.













CHAPTER 4
INFRARED MULTIPHOTON DISSOCIATION SPECTRA OF METHANOL
SOLVATED ANIONS AND PROTON BOUND METHANOL DIMER CATIONS Introduction


The study of clusters has become a very active area in

chemistry (98). Cluster ions are often the energetically preferred form of ions in relatively cool gas media, and both positive and negative cluster ions are known to be important in the ion-molecule chemistry of the upper atmosphere. Much

of the interest in studying loosely bound aggregates of atoms, molecules and ions arises from the fact that such systems exhibit properties intermediate between the gas and condensed phases. Within the broad topics of cluster structure and dynamics, the fundamental phenomenon of salvation is of

central importance (99,100), and various thermodynamic and structural studies of solvated species, particularly ions, have appeared in the literature (101,102). The study of

successively larger cluster ions is analogous to the process of salvation, one solvent molecule at a time. Therefore,

solvated ions afford a particularly interesting collection of systems for study because they bridge the gap between bare isolated ions and ionic solids and electrolyte solutions.


72







73

one approach for investigating ion-solvent interactions has been to study solvated ions in the solution-phase (103). In solution, solvated ions govern a host of phenomena including solvent dependent variations in the reactivity and spectral characteristics of anions and cations. Gas-phase investigations of ion-solvent chemistry, utilizing mass

spectrometric techniques (104,105), provide a unique way to study intermolecular interactions. In such studies the

properties of the ions can be studied as solvent molecules are added one by one to the ion, in the absence of complicating effects due to solvent and counter ion interactions.

Even though a large number of solvated cations have been studied, it is only recently that mass spectrometric techniques have been extended to study spectroscopy of solvated anions. The simplicity of obtaining gas-phase

solvated anions using low pressure ion-molecule reactions allows the capabilities of FTICR to be utilized to study the

chemistry of such ions (106). Briefly, the reaction mechanism involves an elimination-type reaction initiated by attack of

a strong gas-phase base (A-) on a species with an acidic site (HBC), as shown in reaction (4.1):



A- + HBC __+ AHB_ + C (4.1)



Although mass spectrometric studies give a partial picture about the stabilities and structure of solvated ions,








74

spectroscopic probes are required to obtain a more concrete

picture of cluster structure and dynamics. Furthermore,

photodissociation studies have been an important technique for elucidating such characteristics of complexes in the gas-phase (107). Also, photodissociation spectra of gas phase ions, when compared to comparable spectra obtained in solution, provide insight into solvation effects. The photodissociation of weakly bound -complexes, such as solvated ions, following excitation with infrared photons has attracted much interest

in recent years (108, 109). From interpretation of these spectra useful information on the lifetime and the structure

of solvated ions has been obtained. The method has been

further extended to identify molecules or reaction products in large cluster environments (42). Methanol serves as a good solvent candidate for such photodissociation experiments

because the C-0 stretch band of methanol at v = 1034 cm-1 overlaps the CO2 laser spectrum. From a theoretical point of view, methanol solvated ions have attracted much interest for a long time due their structural simplicity. The

intramolecular potential (110a) and the binding energies

(110b) for methanol dimers have been calculated. Recently, energetics of the gas-phase proton transfer between methanol and the fluoride ion were calculated (111).

The techniques of IRMPD, utilizing a cw C02 laser, are further extended in this chapter to obtain spectra for methanol and deuterated methanol solvates of the anions and







75

proton bound methanol dimer cations. IRMPD spectra of

methanol solvated fluoride ion (CH3OHF-), d-methanol solvated fluoride ion (CHIODF-), methanol solvated chloride ion (CH3OHCl-), d-methanol solvated chloride ion (CH3ODCl-),

methanol solvated methoxy anion (CH3OHOCH3 ), proton bound methanol dimer cation ([CH3OH]2H+), proton bound d-methanol dimer cation ([CH3OD]2H+), and deuteron bound d-methanol dimer cation ([CH3OD]2D+), in the 920 -1060 cm-1 region (10.60 9.60 gm P and R branches of the CO2 laser) are presented in this chapter. In addition, some interesting multiple photon dissociation pathways of methanol dimer cations are discussed. The neutral gas-phase spectra of methanol and d-methanol were obtained and compared with the corresponding IRMPD spectra.


Experimental




The newly modified White-type FTICR analyzer cell, as discussed in chapter 2 (Figure 2.9), and the cw CO2 laser were used for all the IRMPD experiments presented in this chapter. The laser was reflected from the turning mirror into the cell and subsequently reflected from the spherical mirrors to create multipasses inside the cell. Careful alignment

facilitated 16 passes of the laser light inside the cell. A schematic representation of the multipass process inside the cell is shown in Figure 4.1. The new cell set-up demonstrated









Mirror 1
Back trap plate Mirror 2

iIi I I
J; L ;;I,

















Laser beam MACOR spacer Mirror 3
Figure 4.1. A schematic representaion of the laser beam pathway (16 passes) inside the
newly-modified White-type FTICR analyzer cell.








77

fewer alignment problems because the three mirrors were fixed permanently into the receive plates. Furthermore, trapping and photodissociation efficiencies were enhanced compared to

the White-type cell used in chapter 3. The photodissociation effects have been described in chapter 2, and trapping efficiencies will be discussed later in this chapter.

All experiments were performed using the FTICR mass spectrometer discussed in chapter 2 (Figure 2.5). Briefly, the background pressure was maintained around 5 x 10-9 Torr, for each spectrum 16,384 data points were acquired by signal averaging 100 ion transient response signals, and the experiment was repeated several times under the same conditions but with and without laser irradiation.



Methanol and d-methanol Solvated Anions



The primary negative ions were produced via electron capture using 2-3 eV electrons (beam pulse = 25-30 ins) and a trapping voltage of 2 V. The distortion of the electric field lines in the cell was minimized by using smaller mirrors (less curved) as the receive plates, and satisfactory trapping could be obtained with a lower voltage.

The solvated halide ions were generated by sequential ion-molecule reactions (vide infra, Eqs. 4.2, 4.5, 4.8, 4.11, 4.14, and 4.18). The primary reaction that generated

CH3OHOCH3- was different from the others. First, 5 ml isoamyl







78

nitrite ([CH3]2CHCH2CH2ONO, MW = 117 g mo1-1) was reacted with excess methanol (10 ml) by using the vacuum apparatus (background pressure ca. 10-4 Torr) shown in Figure 4.2. Then, after ca. 10-12 min. reaction time, the neutral product methyl nitrite (CH3ONO, MW = 61 g mo1-1) was collected (reaction 4.17). Using the methyl nitrite, the conditions to form CH3OHOCH3- were simlar to those for the formation of other anions, and are shown in reactions 4.18 and 4.19.

Pressure of CF4 (carbon tetrafluoride) or S02F2 (sulfuryl fluoride) or ClCO2C2H5 (ethyl chloroacetate) or CH3ONO (methyl nitrite) was maintained at 5.5 x 10-8 Torr and CH30H or CH30D was leaked in up to a pressure of 2.5 x 10-7 Torr.

The anion of interest was isolated by using a series of ejection pulses. Ions were then allowed to undergo several collisons with the neutrals at a pressure of 2.5 x 10-7 Torr for ca. 1-1.3 sec. No difference in photodissociation was observed as collision time was varied over a range of 0.85-1.5 sec. Immediately after the "cooling-period", the cw laser was gated for variable length irradiation periods ranging from 75
- 350 ms by the FTICR data station. The energy was kept constant at 500 mJ pulse-1. The molecular anion, the only ion present in the cell after laser irradiation (vide infra) was excited and detected. The extent of photodissociation was obtained by measuring the intensities of the molecular anion before and after irradiation, as given by their mass spectral peak areas.








To a To a Baratron gauge
thermocouple Tt
gauge
ge System manifold









Vent




Product Reactants Trap
Oil diffusion pump To a mechanical pump

Figure 4.2. Vacuum line apparatus used in the synthesis of methyl nitrite (CH3ONO).







80

All reagents except isoamyl alcohol were obtained from commercial sources and were used without further purification.


Proton (deuteron) Bound Methanol (d-methanol) Dimer Cation


The proton bound methanol dimer ion (CH3OH)2H+, was generated by using a mixture of 17:2:3 (v:v:v) H20:CH3OH: C2H4CI2 (1,1-dichloroethane), introduced up to a pressure of 9 x 10-7 Torr. Similarly, (CH3OD)2H+ and (CH3OD)2D+ were formed by 12:1:2 (v:v:v) D20:CH3OD:C2H4C12, leaked in up to 6 x 10-7 Torr in pressure.

Primary cations were formed by electron ionization, using a 25-30 ms pulse of electrons, a beam voltage of 35 V, and a trapping voltage of 2 V. Then, ca. 250 ms delay time was given to form the proton-bound dimer cations via ion-molecule reactions. Next, the molecular ion was isolated using a series of ejection pulses and ions were allowed to thermalize for 1.25-1.35 sec at pressure of 9 x 10 -7 Torr and 6 x 10 -7 Torr for (CH3OD)2H+ and (CH3OD)2D+, respectively. The isolated cation of interest was irradiated in a similar manner to the anions discussed in the previous section. All the ions present in the cell were excited and detected immediately after the laser was gated. The extent of photodissociation was obtained by measuring the intensities of the parent and the fragment ions, as given by their mass spectral peak areas.

All reagents except H20 were obtained from commercial suppliers. Sample purity was confirmed by broadband mass







81

spectral analysis. No additional sample purification was required.

The gas-phase reduced pressure (0.40 Torr) spectra of neutral methanol and deuterated methanol were obtained in a Nicolet 740 FTIR. The quartz cell used was discussed in chapter 3.



Results and Discussion





The first series of experiments was performed with methanol and d-methanol solvated anions. The sequence of formation reactions for the solvated anions is



Methanol Solvate of the Fluoride Ion



(i) Electron beam pulse



CF4 + e- F + CF3 (4.2)




(ii) Reaction



F + HCO2CH3 CH3OHF + CO (4.3)







82

(iii) Laser pulse



CH3OHF- + nhvir F- + CH3OH (4.4)



To further check these results, CH3OHF and CH30DF- were formed in an alternative manner.

(i) Electron beam pulse



SO2F2 + e- F- + SO2F (4.5)



(ii) Reaction



F + HCO2CH3 CH3OHF- + CO (4.6)



(iii) Laser pulse



CH3OHF- + nhvir F- + CH3OH (4.7)



Deuterated Methanol Solvate of the Fluoride Ion




(i) Electron beam pulse



CF4 + e- F + CF3 (4.8)







83

(ii) Reaction



F + DCO2CH3 CH3ODF- + CO (4.9)



(iii) Laser pulse



CH3ODF- + nhvir F- + CH3OD (4.10)



Methanol Solvate of the Chloride Ion



(i) Electron beam pulse



CICO2C2H5 + e- CICO2- + C2H5 (4.11)



(ii) Reaction



CiCO2- + CH3OH CH3OHCl- + CO2 (4.12)



(iii) Laser pulse



CH3OHCl- + nhvir Cl- + CH3OH (4.13)







84
Deuterated Methanol Solvate of the Chloride Ion


(i) Electron beam pulse



CIC02C2H5 + e- CICO2 + C2H5 (4.14)



(ii) Reaction


CICO2 + CH30D CH3ODC1- + CO2 (4.15)


(iii) Laser pulse


CH3ODCl- + nhvir Cl- + CH3OD (4.16)


Methanol Solvate of the Methoxy Ion


C5H11N02 + CH30H CH3ONO + C5H110H (4.17)


(i) electron beam pulse


CH3ONO + e- CH30 + NO (4.18)

(ii) Reaction


CH30- + CH30H CH3OHOCH3- (4.19)







85

(iii) laser pulse



CH3OHOCH3- + nhvir CH30- + CH30H (4.20)



The total energy per pulse was kept constant as the cw laser was line-tuned to various wavelengths. The

photoproducts X- or OCH3- were ejected during the laser pulse in order to prevent them from reacting with excess solvent molecules present in the cell. The percent photodissociation was calculated by dividing the difference in intensities of the parent ion (laser off minus laser on) by the intensity of the parent ion when the laser was off. Thus, the depletion of the parent ion as a function of laser wavelength was obtained. The experiment was repeated ten times (both laser on and laser off) for a given wavelength and the average percent photodissociation was calculated. All the error limits represent the standard deviation of the mean. The IRMPD

spectra of CH3OHF-, CH3ODF-, CH3OHCI-, CH3ODCl-, and CH3OHOCH3are depicted in Figures 4.3, 4.4, 4.5, 4.6, and 4.7, respectively.

A second series of experiments was performed using the methanol proton-bound dimer cations. This study also details mechanisms for generating the proton and deuteron bound dimer cations via a sequence of bimolecular reactions, and multiple photon dissociation routes even though the major objective was to obtain IRMPD spectra. Although other studies have reported







45
CH3OHF" + nhvir ----- > F" + CH3OH 40- 1046.85


0 979.73
30
o I I
U 970.56
0
25

S 20?
0 G
CL 1510

5

0 a I a 1 I
920 940 960 980 1000 1020 1040 1060
WAVENUMBER (cm-)

Figure 4.3. IRMPD spectrum of methanol solvated fluoride ion (CH3OH-). Error estimates
are 95% confidence limits.








50
CH3ODFf nhvir ----> F" + CH3OD1046.85 45
40 979.73 40
Z
0 35- 970.56

1043.16 U 300
(A
0 25O
20

15

10

5

0
920 940 960 980 1000 1020 1040 1060
WAVENUMBER (cm1)

Figure 4.4. IRMPD spectrum of deuterated methanol solvated fluoride ion (CH3ODF-) Error
estimates are 95% confidence limits.




Full Text
CHAPTER 6
CONCLUSIONS AND FUTURE WORK
Conclusions
The major focus of this dissertation was to elucidate the
infrared spectra of gas-phase ions as an aid in structural
identification, through the use of Fourier transform ion
cyclotron resonance mass spectrometry. The technique of
infrared multiple photon dissociation spectroscopy was
utilized to indirectly determine the optical absorption
spectrum of an ion, observing the wavelength dependent
disappearance of the parent ion or the appearance of fragment
ions of interest by dissociation following infrared photon
absorption. The major advantage of using a FTICR mass
spectrometer for such photodissociation studies is the ability
to trap ions for extended periods of time, which permits an
ensemble of ions to be exposed to a large number of photons
during these periods. In other words, the ion/photon
interaction time can be lengthened appreciably and less
intense light sources can be used.
Unfortunately, obtaining infrared spectra of ions is a
rather difficult task. Part of the problem is the lack of
intense, broadly tunable light sources; another part is
related to the small amount of energy contained in an IR
140


Pulsed laser
Figure 2.9. The C02 laser (pulsed or continuous (cw)) beam pathway for double pass
arrangement before entering the vacuum chamber. The beam pathways are
illustrated only for the cw CO^ laser, but the pulsed C02 laser could be used
to obtain the double pass inside the FTICR cell, using mirror 1' instead of
mirror 1.


PHOTODISSOCIATION
Figure 4.3.
IRMPD spectrum of methanol solvated fluoride ion (CH3OH_) Error estimates
are 95% confidence limits.


LIST OF TABLES
Table PAGE
2.1 Energy measurement for both pulsed
and cw C02 laser beam after reflecting
from each mirror and the ZnSe window,
and the distances for each mirror
and for the ZnSe window from the
laser head using the newly modified
White-type cell 43
3.1 Gaseous ion vibrational frequencies
from one- and two-laser studies,
frequencies of the corresponding
gas phase neutrals, and the ion-neutral
peak shifts 66
3.2 The C-F stretching frequencies of
A and E modes used in assigning
peak frequencies for the Ga(hfac)3
neutral and the anion 70
4.1 Gaseous ion vibrational frequencies
from IRMPD studies 102
4.2 Some calculated and experimental bond
lengths and vibrational frequencies
Of CHjOH and CHjOHF" 107
xi


31
times for three images, twelve times for five images, and
sixteen times for seven images.
Modified White-Type Cell
The z-axis elongated cell was modified in our laboratory
for IRMPD by incorporating White's multipass optical
arrangement. Experiments in chapter 3 were performed by using
the modified White-type FTICR cell as shown in Figure 2.7.
The cell dimensions are 2.5 cm, 2.5 cm, and 6.5 cm along the
x, y, and z magnetic field axes, respectively. Three
spherical, concave, and well-polished brass mirrors were
incorporated as the receive plates. All three mirrors had the
same radius of curvature of 3.12 cm. One of the mirrors was
3.13 cm in length, 2.34 cm in width, and 0.73 cm in thickness.
The two opposite mirrors, which are identical in size, were
2.34 cm in length and 2.5 cm in width with a 0.82 cm
thickness. The solid excite plates were replaced with
stainless steel mesh. A 2.0 cm hole was machined into one of
the trap plates and 90% transparent mesh covered the hole.
The first laser beam was turned by the gold-plated mirror (2.5
cm x 2.5 cm ) into the cell through a 2.0 cm o.d. hole in the
receive plate, and thus began the multipass reflection
process. In this set-up, laser light reflected inside the
cell eight times, thus giving six more passes than a regular


122
FTICR cell by opening the shutter mounted on the first
conductance plate. While the shutter was open, argon gas was
pulsed in, with the pressure rapidly rising to ca. 4 x 10-4
Torr to thermalize and to assist in trapping the ions. The
pressure fell after closure of the pulsed valve approximately
as shown in the figure. The shutter was opened one second for
ion injection and then closed. Following the ion injection a
3 s delay allowed the argon gas to be pumped away from the
cell region. The ion of interest was isolated by several
ejection pulses and then thermalized during this 3 s delay.
Also, during this delay period, a cw C02 laser was gated on
for variable length irradiation periods (75 to 100 ms) at a
constant laser energy of 500 mJ pulse-1. Immediately after
the end of the laser gating period, all the ions present were
excited by the standard frequency chirp excitation method and
64k time-domain points were acquired during broad-band
detection (15-900 amu). For each wavelength studied, 100
scans were averaged to enhance the signal-to-noise ratio.
First, a mass spectrum without laser irradiation was
recorded, and then a mass spectrum under the same conditions
but with laser irradiation was obtained. Comparison of the
two mass spectra with and without irradiation at a given
wavelength allowed determination of the fragment ions due to
IRMPD of the ions produced by ESI.
All crown ethers and alkali metal salts were obtained
from commercial sources and used without further purification.


. Quench
3 Pulse valve tri
Shutter opened
Shutter closed
(1 s)
(3 s)
n
Q)
Figure 5.2.
ESI/FTICR pulse sequence used for the IRMPD experiments.
Detection


2.11 IRMPD mass spectrum
(laser energy = 500 mJ pulse*1)
of protonated diglyme cation obtained
with cw C02 laser beam subjected to
(top) center pass, and (bottom) multipass
arrangements 39
2.12 IRMPD mass spectrum
(laser energy 750 mJ pulse*1)
of protonated diglyme cation obtained
with cw C02 laser beam subjected to
(top) center pass, and (bottom) multipass
arrangements 40
3.1 A schematic representation of the
two-laser (probe-pump) photodissociation
process 48
3.2 Schematic representaion of the cutaway
view of the modified White-type cell
and the two-laser beam pathways
inside the cell 51
3.3 Experimental pulse sequence employed in
the one- and two-laser probe-pump
techniques 53
3.4 One-laser infrared multiple photon
dissociation spectrum of the protonated
molecular ion of diglyme 56
3.5 One-laser IRMPD spectrum of the positive
molecular ion of 3-bromopropene
(allyl bromide) 57
3.6 One-laser IRMPD spectrum of the negative
molecular ion of gallium
hexafluoroacetylacetonate Ga(hfac)3 59
3.7 Two-laser IRMPD spectrum of the protonated
molecular ion of diglyme 60
3.8 Two-laser IRMPD spectrum of the positive
molecular ion of 3-bromopropene 61
3.9 Two-laser IRMPD spectrum of the negative
molecular ion of Ga(hfac)3 62
3.10 Gas phase neutral infrared
spectrum of diglyme 63
viii


Laser beam in
Mirror 1
Mirror 2
Laser beam out
u>
o
Figure 2.6.
An illustration of the set-up used by White to obtain very long optical paths
for irradiation experiments.


8
(41) Irradiating the ions with two lasers simultaneously, or
with an IR laser followed by the visible laser, has many
similarities with IRMPD, but the large increment of energy
deposited by the visible photon makes it possible to effect
dissociations which would be difficult with infrared light
alone. Use of this novel approach gave the possibility of
obtaining infrared spectra of several ions which are not
accessible with IR lasers alone.
Even though infrared spectroscopy of ions by the IRMPD
approach is a promising area of study, it is constrained by
the lack of powerful, broadly tunable IR lasers. A novel two-
laser approach known as "probe-pump" technique developed in
our laboratory enabled us to overcome some of these
limitations and obtain reproducible spectra for several gas-
phase cations and anions. In these experiments, a low-power
tunable C02 laser (probe) and a more powerful C02 laser (pump)
were used. At the low pressures used in FTICR (10-7 10"8
Torr) there is not a sufficient number of collisions to
deactivate vibrationally excited ions formed by the low-power
tunable C02 laser. Thus, in two-laser experiments, the first
laser is used to vibrationally excite them and promote the
ions into the quasicontinuum. Once the ions have appreciable
vibrational excitation, then a second C02 laser is used to
drive the ions to the dissociation limit. IRMPD spectra of
the protonated molecular ion of bis methoxy diethyl ether
(diglyme), and the molecular ions of 3-bromopropene and


2
pressures (4). The invention of softer ionization sources,
such as laser desorption (LD) (5,6), matrix assisted laser
desorption (MALDI) (7,8), and electrospray ionization (ESI)
(9,10), combined with instrumental advances has expanded the
utility of Fourier transform ion cyclotron resonance (FTICR)
mass spectrometry as an analytical tool.
The ability of the FTICR to trap ions for a very long
period makes it a suitable instrument for studying ion-
molecule reactions. The trapped ions can participate in ion-
molecule or photochemical reactions or be selectively ejected
using FTICR double resonance techniques. The ability to
select and eject ions from the cell permits positive
identification of complex reaction pathways that occur during
the trapping period. Thus, much chemical information and/or
physical quantities have been obtained from studies of ion-
molecule reactions by FTICR. The determination of reaction
rate constants (11), gas-phase acidity and basicity
measurements (12), ionization potentials (IPs) (13), electron
affinities (EAs) (14), structural determination of fragments
(15), and determination of competitive reaction pathways (16)
has been presented in the literature.
Collision-induced dissociation (CID), also known as
collisionally-activated dissociation (CAD) is the most widely
used technique for ion structure determination. Although
multiple sector or quadrupole mass spectrometers are most
commonly used for this approach, Cody and Freiser demonstrated


REFERENCES
1. Busch, K. L.; Glish, G. L.; McLuckey, S. A. Mass Spec
trometry/Mass Spectrometry; VCH Publishers: New York,
1988.
2. Francl, T.; Sherman, M. G.; Hunter, R. L.; Locke, M. J.;
Bowers, W. D.; Mclver, R. T.; Jr. Int. J. Mass Spectrom.
Ion Processes, 1983, 54, 189.
3. Ledford, E. Jr.; Rempel, D. L.; Gross, M. L. Anal. Chem. ,
1983, 55,339.
4. Bricker, D. L.; Adams, T. A. Jr.; Russell, D. H. Anal.
Chem., 1983, 55, 2417.
5. Brenna, J. T.; Creasy, W. R. Appl. Spectrosc. 1991, 45,
80.
6. Nuwaysir, L. M.; Wilkins, C. L.; Simonsick, W. J. Jr. J.
Am. Soc. Mass Spectrom., 1990, 1, 66.
7. Castoro, J. A.; Koster, C.; Wilkins, C. L. Rapid Commun.
Mass Spectrom. 1992, 6, 239.
8. Hettich, R. L.; Buchanan, M. V. J. Am. Soc. Mass Spect
rom, 1991, 2, 22.
9. Hofstadler, S. A.; Bruce, J. E. ; Rockwood, A. L. ;
Anderson, G. A.; Winger, B. E.; Smith, R. D. Int. J. Mass
Spectrom. Ion Processes, 1994, 132, 109.
10. Henry, K. D.; Williams, E. R.; Wang, B. H.; Mclafferty,
F. W.; Shabanowitz, J.; Hunt, D. F. Proc. Natl. Acad.
Sci., 1989, 86, 9075.
11. Byrd, G. D.; Fresier, B. S. J. Am. Chem. Soc. 1982,104,
5944.
12. Kleingeld, J. C.; Nibbering, N. M. M. Org. Mass Spectr
om., 1982, 17, 136.
13. Ramanathan, R.; Zimmerman, J. A.; Eyler, J.R. J. Chem.
Phys., 1993, 98, 7838.
147


47
isomeric ions (87,88) and some crude spectra for diol ions
(89) have been obtained, but the above mentioned limitations
precluded acquisition of high quality IRMPD spectra. However,
a novel two-laser approach developed in our laboratory (89,90)
has demonstrated that reproducible spectroscopic data for
gaseous ions can be obtained, and this technique promises to
overcome many of the limitations imposed by the use of C02 and
other lasers.
An ion with a sufficiently high density of vibrational
and rotational levels is excited to the "quasicontinuum" by
resonant multiphoton absorption from a low-power tunable
pulsed C02 (probe) laser. In the quasicontinuum the ion can
readily absorb additional photons regardless of their energy.
These additional photons, provided by a more powerful cw C02
(pump) laser, impart sufficient internal energy to the ion to
dissociate it into fragments, which are observed using
standard FTICR detection techniques. The "probe-pump" two-
laser photodissociation process is shown schematically in
Figure 3.1.
The applicability of our recently developed method for
obtaining spectra of gaseous ions, using a low power tunable
pulsed C02 laser for probing the resonant absorption spectrum
and a more powerful non-resonant cw C02 laser to complete the
dissociation, is demonstrated in the work reported here. In
particular, one- and two-laser spectra for the protonated
bis(2-methoxyethyl) ether (diglyme) cation, the 3-bromopropene


104
resolution is 0.5 cm*1. The gaseous ions undergo at most one
collision during the photodissociation period, and thus their
spectra will not be collisionally broadened, as the gas-phase
neutral spectra expected to be. Thus, ions of low molecular
complexity such as methanol solvated anions will exhibit sharp
absorption peaks with a spacing which is, in general, not the
same as that between the C02 laser lines used for irradiation.
Figure 4.13a illustrate this case. Peaks seen in IRMPD
spectra will be the result of coincidences between the sharp
absorption bands and the sharp laser lines. In contrast,
methanol proton bound dimer cations correspond to "large"
molecule limit, so a broader absorption spectrum should be
obtained. For these, the IRMPD spectra should more closely
approach the ion (and corresponding neutral) spectra. This
case is shown schematically in Figure 4.13b. However, the
lack of sharp spectral features reduces the chance of a
complete interpretation of the spectrum. Unfortunately, there
is no spectroscopic information available for the anion
species except two isolated studies for CH30HF*, one
theoretical (112) and one experimental (116). In the
experimental study, the relative photodissociation cross-
section was found to vary with wavelength, but no distinct
absorption peaks were observed in the IRMPD spectrum. The
authors used NF3 to form the primary negative ion, F*, and
CH3OHF* was produced with excess internal energy (24-
kcal/mole). The ions which were formed with excess internal


134
shown in Figures 5.5-5.7. At 10.60 /xm laser irradiation of
(18-crown-6)K+ and (18-crown-6)H30+ produced K+ (m/z 39),
C4H902+ (m/z 89) and C7H123+ (m/z 144) However, (18-crown-
6)Na+ was not observed to dissociate to give Na+ at m/z 23.
In addition, the relative intensity of the peak at m/z 287 did
not change by any considerable amount. Similar IRMPD results
were obtained for 10.58 /xm laser irradiation (Figure 5.6) In
contrast, as shown in Figure 5.7, C4H902+ was the only fragment
observed at 9.58 /xm. Absence of Na+ and K+ and the unaffected
relative intensities of (18-crown-6)Na+ and (18-crown-6)K+
indicated that only (18-crown-6)H30+ dissociated at this
wavelength.
To confirm the formation of (18-crown-6)H30+, 18-crown-6
was dissolved in 49:49:2 CH30H:H20:CH3C00H and electrosprayed
under the same conditions. As shown in Figure 5.8a, peaks due
to (18-crown-6)H30+ and (18-crown-6) H+ were observed in the
mass spectrum. IRMPD of these ions at 10.60 /xm (Fig. 5.8b)
produced fragments with the probable composition C4H902+ and
C2H50+. This suggests that complexation of 18-crown-6 with
H30+ and H+ results in very different types of interactions
when compared to complexation with metal ions. Izatt et al.
(144) showed that a stable complex between H30+ and
dicyclohexyl-18-crown-6 can be formed in the condensed phase.
Following these studies Sharma and Kebarle (147) investigated
the complexes of H30+ and CH3OH2+ with crown ethers in the gas-
phase. Results from Kebarle's studies indicated that


Relative Intensity
m/z
Figure 5.5. ESI/FTICR mass spectrum obtained with 18-crown-6/NaCl/KCl in 50:50
methanol:water solution and with cw C02 laser irradiation at 10.60 pm
wavelength.
126



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5.1 The internal ESI/FTICR mass spectrometer 119
5.2 ESI/FTICR pulse sequence used for
IRMPD experiments 121
5.3 Structures of the crown ethers and
complexes discussed in chapter 5 124
5.4 ESI/FTICR mass spectra obtained with
18-crown-6/NaCl/KCl in
50:50 methanol:water solution 125
5.5 ESI/FTICR mass spectra obtained
with 18-crown-6/NaCl/KCl in 50:50
methanol:water, and with cw C02
laser irradiation wavelength
of 10.60 jra 126
5.6 ESI/FTICR mass spectra obtained
with 18-crown-6/NaCl/KCl in 50:50
methanol:water, and with cw C02
laser irradiation wavelength
of 10.58 /xm 127
5.7 ESI/FTICR mass spectra obtained
with 18-crown-6/NaCl/KCl in 50:50
methanol:water, and with cw C02
laser irradiation wavelength
of 9.58 /m 128
5.8 ESI/FTICR mass spectra obtained
with 18-crown-6 in 49:49:2
methanol:water:acetic acid
solution: (top) without laser
irradiation, the insert shows
isolated (18-crown-6)H30+ with
13C peak resolved; and (bottom) with
laser irradiation at 10.60 jtxm 130
5.9 ESI/FTICR mass spectra obtained
with 15-crown-5/NaCl/KCl in 50:50
methanol:water solution: (top) without
laser irradiation; and (bottom) with laser
irradiation at 10.60 /urn 131
5.10 ESI/FTICR mass spectra obtained with
12-crown-4/NaCl/KCl in 50:50
methanol:water solution: (top) without
laser irradiation; and (bottom) with laser
irradiation at 10.60 im 132
x


Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
INFRARED MULTIPLE PHOTON DISSOCIATION
SPECTRA OF GASEOUS IONS
By
Dilrukshi Manjalika Patuwathavithana Peiris
December 1994
Chairman: John R. Eyler
Major Department: Chemistry
This dissertation presents a series of studies
elucidating the infrared multiple photon dissociation (IRMPD)
spectra of gaseous ions stored in a Fourier transform ion
cyclotron resonance (FTICR) mass spectrometer. An "indirect"
method such as IRMPD often has to be utilized to obtain gas-
phase ion spectra. The ability of FTICR to trap ions easily
under collision-free or collisional conditions for long
periods has made it the instrument of choice for IRMPD
studies.
A novel two-laser approach was developed to obtain IRMPD
spectra and is presented in this dissertation. Use of a low-
power tunable C02 (probe) laser, a second, more powerful C02
(pump) laser, and a modified White-type multipass FTICR
xii


129
the maximum energy outputs in the 10.6-9.6 /xm region (P and R
branches), from the cw C02 laser. The ESI mass spectrum
obtained following electrospraying the CH30H/H20/CH3C00H
solution containing (18-crown-6)H30+ is shown in Figure 5.8a,
and results from photodissociation of (18-crown-6)H30+ at
10.60 /urn are shown in Figure 5.8b. IRMPD experiments carried
out with the 15-crown-5/NaCl/KCl solution and mass spectra
obtained with laser off and laser on at 10.60 /xm irradiation
wavelength are shown in Figures 5.9a and 5.9b, respectively.
In one series of experiments an equimolar mixture of 15-crown-
5/12-crown-4/NaCl/KCl in 50:50 CH30H:H20 solution was
electrosprayed. Figures 5.10a and 5.10b illustrate the
results with no irradiation and with laser irradiation at
10.60 /xm, respectively.
Discussion
The host-guest interactions between the crown ethers and
the metal ions have been explained by three separate concepts:
(a) structural or preorganizational flexibility the
flexibility of the crown ethers to undergo conformational
changes under different conditions (140); (b) optimal fit or
best fit-enhanced stability of the complex-observed when the
cation radius closely matches that of the crown cavity (141);
and (c) the macrocyclic effect-increased stability of metal
ion binding observed with the crown complexes as opposed to


74
spectroscopic probes are required to obtain a more concrete
picture of cluster structure and dynamics. Furthermore,
photodissociation studies have been an important technique for
elucidating such characteristics of complexes in the gas-phase
(107). Also, photodissociation spectra of gas phase ions,
when compared to comparable spectra obtained in solution,
provide insight into solvation effects. The photodissociation
of weakly bound complexes, such as solvated ions, following
excitation with infrared photons has attracted much interest
in recent years (108,109). From interpretation of these
spectra useful information on the lifetime and the structure
of solvated ions has been obtained. The method has been
further extended to identify molecules or reaction products in
large cluster environments (42). Methanol serves as a good
solvent candidate for such photodissociation experiments
because the C-0 stretch band of methanol at v = 1034 cm-1
overlaps the C02 laser spectrum. From a theoretical point of
view, methanol solvated ions have attracted much interest for
a long time due their structural simplicity. The
intramolecular potential (110a) and the binding energies
(110b) for methanol dimers have been calculated. Recently,
energetics of the gas-phase proton transfer between methanol
and the fluoride ion were calculated (111).
The techniques of IRMPD, utilizing a cw C02 laser, are
further extended in this chapter to obtain spectra for
methanol and deuterated methanol solvates of the anions and


70
60
50
40
30
20
10
9
One-!
mole<
J pu!
940 950 960 970 980 990
WAVENUMBER / cm-1
aser
se
-l
infrared multiple photon dissociation spectrum of positive
ion of allyl bromide (3-bromopropene) at a probe laser energy of 1
Error estimates are 95% confidence limits.


INFRARED MULTIPLE PHOTON DISSOCIATION
SPECTRA OF GASEOUS IONS
By
DILRUKSHI MANJALIKA PATUWATHAVITHANA PEIRIS
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
1994


To a
thermocouple
gauge
To a Baratron gauge
System manifold
Product
Reactants
Oil diffusion pump
To a mechanical
pump
VO
Figure 4.2.
Vacuum line apparatus used in the synthesis of methyl nitrite (CH3ONO).


Table 3.1.
Gaseous ion vibrational frequencies from one- and two-laser studies,
frequencies of the corresponding gas phase neutrals, and the ion-neutral peak
shifts.
Molecule
Vibrational
mode
Peak frequency (cm*1)
shift (cm'1)
one-
two-
and
laser
neutral
Diglyme
C-O-C
940.5
(18 %) b
1030
ca. 90
Allyl bromide
C-Br
944.2
(20%)
920
ca. 24
951.2
(15%)
930
ca. 21
972.0
(1%)
972
975.9
(8%)
981
ca. 5
979.7
(6%)
985
Ga (hfac)
C-F
1043.2
(20%)
1151
a
1052.0
(25%)
1240
a
a Neutral-ion peak shifts cannot be determined since the ion peaks cannot be assigned
unambiguously. See text.
b One-laser percent photodissociation.


84
Deuterated Methanol Solvate of the Chloride Ion
(i) Electron beam pulse
C1C02C2H5 + e" C1C02" + C2H5 (4.14)
(ii) Reaction
C1C02~ + CH3OD CH3ODCI" + C02 (4.15)
(iii) Laser pulse
CH30DC1 + nhvir -* Cl" + CH3OD (4.16)
Methanol Solvate of the Methoxv Ion
C5HnN02 + CH3OH -* CH3ONO + C5HnOH (4.17)
(i) electron beam pulse
CH3ONO + e" CH30" + NO (4.18)
(ii) Reaction
CH30" + CH3OH
CH3OHOCH3"
(4.19)


14
F2 = qvB (2.2)
In Equations 2.1 and 2.2 g is the ion charge, v is the ion
velocity, r is the radius of gyration of the ion, and B the
magnetic field strength. The result of combining Equations
2.1 and 2.2 is given in Equation 2.3.
gB = mv/r (2.3)
Rearranging Equation 2.3 gives Equation 2.4.
v/r = gB/m (2.4)
The radial motion of the ion perpendicular to the magnetic
field is shown in Equation 2.5.
coc = gB/m (2.5)
where ac (=v/r) is cyclotron frequency of the particle in
radians per second.
The ions formed within the cell or the ions transported
from an external ion source undergo circular motion
perpendicular to the magnetic-field (z-axis) Ions in the
cell are held in the radial direction (x and y axes) by the
magnetic field and are constrained along the Z-axis by


80
All reagents except isoamyl alcohol were obtained from
commercial sources and were used without further purification.
Proton (deuteron) Bound Methanol id-methanol) Dimer Cation
The proton bound methanol dimer ion (CH3OH)2H+, was
generated by using a mixture of 17:2:3 (v:v:v) H20:CH3OH:
C2H4C12 (1,1-dichloroethane), introduced up to a pressure of
9 x 10"7 Torr. Similarly, (CH3OD)2H+ and (CH3OD)2D+ were
formed by 12:1:2 (v:v:v) D20:CH30D:C2H4C12, leaked in up to 6
x 10"7 Torr in pressure.
Primary cations were formed by electron ionization, using
a 25-30 ms pulse of electrons, a beam voltage of 35 V, and a
trapping voltage of 2 V. Then, ca. 250 ms delay time was
given to form the proton-bound dimer cations via ion-molecule
reactions. Next, the molecular ion was isolated using a
series of ejection pulses and ions were allowed to thermalize
for 1.25-1.35 sec at pressure of 9 x 10 ~7 Torr and 6 x 10 ~7
Torr for (CH3OD)2H+ and (CH3OD)2D+, respectively. The isolated
cation of interest was irradiated in a similar manner to the
anions discussed in the previous section. All the ions
present in the cell were excited and detected immediately
after the laser was gated. The extent of photodissociation
was obtained by measuring the intensities of the parent and
the fragment ions, as given by their mass spectral peak areas.
All reagents except H20 were obtained from commercial
suppliers. Sample purity was confirmed by broadband mass


ABSORBANCE
Figure 3.11.
Gas phase neutral infrared spectrum of 3-bromopropene (pressure =0.45 Torr).


133
chain analogues (142). In this study, explanations will be
limited to the best fit or the optimal fit concept.
As shown in Figure 5.4, ESI of 18-crown-6/NaCl/KCl in
CH30H/H20 produced peaks due to (18-crown-6)Na+ (m/z 287) ,
(18-crown-6)K+ (m/z 303), and (18-crown-6)H30+ (m/z 283) in
order of decreasing intensity. However, complexes of the type
(18-crown-6)2M+ (where M = Na, K and H30) were absent from the
spectrum. The absence of sandwich complexes (18-crown-6) 2M+
(M = Na or K) for 18-crown-6 has been documented in several
studies (143). 18-crown-6 has a cavity with a radius between
1.34 and 1.55 (144), which is large enough for the crown
ether to wrap around the cation to form a three dimensional
cavity with all oxygen atoms coordinated to the cation (radius
of Na+ = 1.02 and K+ = 1.38 ) (145). On the other hand,
ions with larger radii than Na+ and K+, such as Rb+ (1.52 )
and Cs+ (1.67 ) (144), cannot fit within the cavity of the
18-crown-6. Since the first crown ether is not able to
completely coordinate the metal ion, a second crown ether
completes the coordination requirements and forms a sandwich-
type complex. The formation of (18-crown-6) 2Rb+ and (18-
crown-6)2Cs+ has been documented in the gas-phase (143).
IRMPD experiments of crown ether complexes formed with Rb+ and
Cs+ using the ESI source are in progress to further confirm
our results.
Results from the IRMPD of (18-crown-6)Na+, (18-crown-
6)K+, and (18-crown-6)H30+ at 10.60, 10.58, and 9.58 /Ltm are


143
sharp absorption bands. However, comparisons with gas-phase
spectra of pertinent neutrals and consideration of changes in
bonding and anti-bonding orbitals due to ionization allows a
reasonable interpretation of the vibrational frequencies
present in the IRMPD spectra.
A secondary goal of this dissertation was to investigate
the IRMPD dissociation pathways of electrosprayed crown ether
complexes. The IRMPD technique can be applied to differen
tiate the bond dissociation of crown ether complexes, by
observing either the photodissociation wavelength dependence
or the products of the photodissociation. Unlike CID experi
ments, where extensive fragmentation is observed, IRMPD
proceeds through lowest energy fragmentation pathways. In
addition, bond and molecule specific dissociation properties
can be used as a tool to identify different fragmentation
patterns within a molecule or different bond activations and
binding interactions for a wide variety of molecules.
IRMP dissociation for both H30+- and H+-crown ether
complexes occurs via losses of (C2H40) structural units.
Crown ether complexation with H30+ results in a different type
of bond dissociation than alkali-metal-crown ether complexes.
Finally, Na+ binds to the crown ether cavity more strongly
than K+.


50
45
40
35
30
25
20
15
10
5
0
00
00
940
960 980 1000 1020 1040 1060
WAVENUMBER (cm1)
spectrum of methanol solvated chloride ion (CH30HC1 ) Error estimates
i% confidence limits.


Ill
addition, Lisy and co-workers observed that transitions in the
(CH3OD) 2 spectrum are blue-shifted (higher frequencies) when
compared with (CH3OH)2 spectrum. A similar trend was observed
in this study. However, the IRMPD spectrum of (CH3OH)2H+
resulted in only one peak. Since all peaks are blue shifted
when compared with that of the (CH3OH)2H+, the unobservable
transition in this spectrum is very likely to be in the 980 -
1038 cm'1 region of the IRMPD spectrum which is unobtainable by
the cw C02 laser. The two-peak structure for the dimers could
be attributed to two nonequivalent CH3OH molecules in the
dimer. Thus, the C-0 stretch in each subunit would experience
a different environment with respect to the other, and result
in a different absorption frequency.
The most important point obtained from this study is that
all IRMPD spectra have a strong absorption band (or two) in
the 9.6 /urn region, which is similar to the neutral IR spectra
of methanol and d-methanol. This vibrational transition is
assigned to the C-0 stretching mode in methanol. The results
from this study imply that the chromophoric character could be
preserved in the IRMPD spectra and that IR spectra of
structurally similar neutral molecules can provide insight as
to whether an ion will absorb in the C02 laser region of the
spectrum.


PHOTODISSOCIATION / %
25
Figure 3.8
0 1 1 1 1 i
930 940 950 960 970 980 990
WAVENUMBER / cm-1
Two-laser infrared multiple photon dissociation spectrum of positive
molecular ion of 3-bromopropene at a probe laser energy of 100 mJ pulse'1 and
a pump laser energy of 1 J. Error estimates are 95% confidence limits.


39
Figure 2.11. IRMPD mass spectrum of protonated diglyme
cation obtained with cw COo laser. The laser
energy was kept constant at 500 mJ pulse at
10.60 pm irradiation wavelength. The laser
beam was subjected to (top) center pass, and
(bottom) multipass arrangements.
o


17
are perpendicular to the magnetic field (B), and the pairs of
opposing "transmitter" and "receiver" plates are parallel to
the magnetic field. These three pairs of plates are
electrically isolated. They are used in a repetitive seguence
of events that are separated in time to obtain a mass
spectrum.
If a radio frequency (rf) electric field has the same
frequency as the cyclotron frequency of the trapped ion, the
ion will absorb energy and its orbital radius and velocity
will increase without changing its cyclotron frequency. This
rf excitation is applied differentially across the opposing
transmitter plates of the cell. After the rf excitation
pulse, ions will have increased kinetic energy and increased
orbital radii. In addition, the rf pulse causes the ions
which were out of phase with each other to move together
coherently. The average ion kinetic energy (KE) in the
absence of ion-molecule collision can be expressed using Eq.
2.7:
KE = qzE2t2/8md2 (2.7)
where E is the amplitude of the rf excitation, t is the rf
pulse width and d is the distance between the excitation
plates of the FTICR cell.
An ion of interest can be isolated using the ejection
capabilities available in the FTICR mass spectrometer (56).


52
through one of the trapping plates, and reflected from the
other trapping plate, thus giving a double pass inside the
cell.
FTICR Pulse Sequence
Ions were formed by electron ionization. The electron
beam voltage was varied from 10-30 volts for the cations and
the anion was formed by electron attachment using a low energy
electron beam (0.5 V). The molecular ion of interest was
isolated by ejecting all other ions from the cell. Ions were
then allowed to undergo several collisions with the neutrals
at a pressure of 5.5 x 10~8 Torr for approximately Is. No
difference in photodissociation was observed as this collision
time was varied over the (limited) range from 1.5-0.75 s.
Subsequent to thermalization the probe laser was fired and
another series of ion ejections was used to remove any
unwanted adducts and fragments formed by the probe laser.
These ejections were carried out with a minimum time delay
(<30 ms) before the pump laser irradiation period. Then the
pump laser was gated on for 30 ms. All ions present after
this time were excited and detected, and the extent of
photodissociation was obtained by measuring the intensities of
the parent and fragment ions, as given by their mass spectral
peak areas. The pulse sequences for both one-laser and two-
laser (probe-pump) approaches are depicted in Figure 3.3.


LIST OF FIGURES
Figure PAGE
2.1 An expanded three-dimensional view
of a typical z-axis elongated
FTICR analyzer cell 16
2.2 A digitized time domain signal for
protonated bis(2-methoxydiethyl) ether
(diglyme) cation produced by electron
ionization 20
2.3 The Fourier transformed ion cyclotron
resonance mass spectrum of diglyme
cations resulting from the time domain signal
observed in Figure 2.2 21
2.4 A schematic representation of the
energy diagram for the infrared multiple
photon absorption and dissociation
process 24
2.5 A schematic representation of the
FTICR mass spectrometer equipped
with a 2 Tesla magnet 27
2.6 An illustration of the set-up used
by White to obtain very long optical
paths for irradiation experiments 30
2.7 An expanded three dimensional view
of the modified White-type FTICR
analyzer cell used in experiments
to obtain IRMPD spectra presented
in chapter 3 32
2.8 An expanded three dimensional view
of the newly modified White-type FTICR
analyzer cell used in IRMPD experiments
presented in chapters 4 and 5 35
2.9 The C02 laser (pulsed or continuous)
beam pathway for double pass arrangement
before entering the vacuum chamber 37
2.10 The C02 laser (pulsed or continuous) beam
pathways for multipass arrangement before
entering the vacuum chamber 38
vii


85
(iii) laser pulse
CH3OHOCH3" + nhvir CH30" + CH3OH (4.20)
The total energy per pulse was kept constant as the cw
laser was line-tuned to various wavelengths. The
photoproducts X- or OCH3~ were ejected during the laser pulse
in order to prevent them from reacting with excess solvent
molecules present in the cell. The percent photodissociation
was calculated by dividing the difference in intensities of
the parent ion (laser off minus laser on) by the intensity of
the parent ion when the laser was off. Thus, the depletion of
the parent ion as a function of laser wavelength was obtained.
The experiment was repeated ten times (both laser on and laser
off) for a given wavelength and the average percent
photodissociation was calculated. All the error limits
represent the standard deviation of the mean. The IRMPD
spectra of CH3OHF", CH30DF", CH30HC1~, CH30DC1", and CH3OHOCH3"
are depicted in Figures 4.3, 4.4, 4.5, 4.6, and 4.7,
respectively.
A second series of experiments was performed using the
methanol proton-bound dimer cations. This study also details
mechanisms for generating the proton and deuteron bound dimer
cations via a sequence of bimolecular reactions, and multiple
photon dissociation routes even though the major objective was
to obtain IRMPD spectra. Although other studies have reported


50
the non-resonant cw laser entered via one of the trap plates,
which was modified by addition of a coarse stainless steel
mesh.
One- and Two-laser Experiments
Single-laser experiments were performed using the cw C02
laser. This laser was gated on for variable length
irradiation periods (ranging from 50 to 500 ms) by the FTICR
data station at a constant energy of 1 J pulse-1. Two-laser
experiments were performed with the pulsed C02 laser as the
resonant probe laser and the cw C02 laser as the non-resonant
pump laser. The probe laser energy was kept constant at
100115 mJ pulse-1. Triggering of this laser was also
controlled by the FTICR data station. For two-laser
experiments the pump laser was operated at a fixed wavelength
of 10.58 /nm and a constant energy of 1 J pulse-1. All
energies measured here were obtained in front of the laser
head and it is estimated that ca. 70% of this beam entered the
FTICR cell.
The laser beams entered the vacuum chamber through two
ZnSe windows mounted on a three window flange. As shown in
Figure 3.2, the pulsed C02 (resonant) laser beam was reflected
by the turning mirror into the cell, and subsequently
reflected from the spherical mirrors to create eight passes
inside the cell. The cw C02 laser (non-resonant) entered the
vacuum chamber through a second window, passed into the cell


Figure 4.12. Gas-pase neutral infrared spectrum of partially deuterated methanol (CH3OD).
101


105
Overlap between the C02 laser lines and the
IRMPD spectra. (top) ions of low molecular
complexity; and (bottom) ions of high
molecular complexity.
Figure 4.13


81
spectral analysis. No additional sample purification was
required.
The gas-phase reduced pressure (0.40 Torr) spectra of
neutral methanol and deuterated methanol were obtained in a
Nicolet 740 FTIR. The quartz cell used was discussed in
chapter 3.
Results and Discussion
The first series of experiments was performed with
methanol and d-methanol solvated anions. The sequence of
formation reactions for the solvated anions is
Methanol Solvate of the Fluoride Ion
(i) Electron beam pulse
CF4 + e F + CF3 (4.2)
(ii) Reaction
F- + HC02CH3
CH30HF + CO
(4.3)


4
pulsed lasers ranging from the ultraviolet (uv) to the
infrared (ir) CID of mass selected ions results in the
deposition of a range of internal energies, where the average
excitation energy is dependent on the collision energy (22)
and the number of collisions (23) The nondiscrete energy
deposition may result in nonselective fragmentation of the ion
of interest. Photodissociation of mass-selected ions results
in an additional degree of selectivity. A discrete energy or
range of energies can be deposited into the mass-selected ions
by photoexcitation. This discrete excitation results in both
molecule-specific and bond-selective PD, especially when the
molecular ions are produced with low internal energies.
Furthermore, high-mass ions are not efficiently fragmented by
CID because of the increasing amount of energy that a large
molecule can accommodate before dissociating (the number of
vibrational modes increases) and the decreasing amount of
energy that can be transferred during a collision (center of
mass effect) (24).
The ability of FTICR to trap ions easily under collision-
free or collisional conditions for long time periods has made
it possible to irradiate ions with light during these periods.
Also, the combination of monochromatic light sources with the
FTICR spectrometer has proven quite successful (25). In
addition, the ability of FTICR to use pulsed lasers to acquire
a complete mass spectrum of photo fragments with high mass
resolution from a single laser pulse makes it the instrument
of choice for photodissociation studies.


50
5-
0 i 1 1 1 1 1 1
920 940 960 980 1000 1020 1040 1060
WAVENUMBER fcm'1)
Figure 4.4.
IRMPD spectrum of deuterated methanol solvated fluoride ion (CH30DF_) Error
estimates are 95% confidence limits.


106
energy might be thermally activated prior to laser
irradiation. Such ions, with excess vibrational energy,
dissociate more readily than thermally relaxed ions (116).
Therefore, IRMPD spectra of chemically activated ions appear
to be somewhat broadened relative to those of relaxed ions.
Hence, distinguishing the exact absorption frequencies could
be rather difficult.
In a recent theroretical study by Wladkowski et al.
(112), the geometric structure and vibrational frequencies of
CHjOHF were calculated using several methods. In one of these
methods CH3OHF was subjected to quantitative vibrational
analysis by means of the scaled quantum mechanical (SQM) force
field procedure calibrated on four isotopomers of methanol.
In this investigation reference electronic wave functions were
determined by the restricted Hartree Fock (RHF) self
consistent field method. Some of the values obtained by those
calculations, and some experimental values for the methanol
molecule are tabulated in Table 4.2. As shown in Table 4.2,
when the CH3OHF complex is formed, the C-0 distance is
predicted to shorten by 0.02 A and the 0-H distance is
elongated by 0.05 A compared to those in the free CH30H
molecule. This results from a change in the nature of bonding
orbitals, which also control the force constants for the C-0
and the O-H stretch. Thus, vibrational frequencies of CH3OHF'
system should differ from those of the parent methanol,
primarily as a result of pertubations to force constants


117
The photodissociation studies of weakly bound complexes
can offer insights into their structure, energetics and
dynamics. IRMPD dissociation pathways and spectra of methanol
solvated anions and methanol dimer cations were presented in
chapter 4. These results were obtained using a C02 laser,
whose spectrum overlaps the C-0 stretching band of methanol.
Because crown ethers have several C-0 stretching bands, these
compounds are good candidates for infrared photodissociation
experiments using C02 lasers. This paper describes our
ESI/IRMPD/FTICR MS work on 18-crown-6 (1,4,7,10,13,16 hexaoxa-
cyclooctadecane), 15-crown-5 (1,4,7,10,13 pentaoxacyclo
tridecane), and 12-crown-4 (1,4,7,10 tetraoxacyclododecane)
complexes with sodium, potassium, and H30+ ions.
Experimental
ESI source
A complete description of the electrospray source
developed in our laboratory appears elsewhere (139) Briefly,
ionization takes place in a PEEK (polyether ether ketone)
cavity, which allows a stainless steel capillary to be heated
to above 175 C without softening or melting the spray
assembly. A 1.73 cm o.d. x 15 cm PEEK rod was machined to
accommodate a 0.150 mm i.d. x 6.4 cm stainless steel spray


45
40
35
30
25
20
15
10
5
0
9
MPD
(CH3OH)2H+ + nhv|r
>
(CH3OH)H+
-> c2h7o-
VO
940 960 980 1000 1020 1040 1060
WAVENUMBER (cm1)
spectrum of proton bound methanol dimer cation (CH3OH) 2H Error
ites are 95% confidence limits.


33
cell. A second laser beam entered through the mesh trap plate
and traversed the cell twice. Laser beam pathways are shown
in Figure 2.7 (denoted by dotted-lines). A detailed
description of irradiation pathways for the one and two-laser
experiments, including schematics will be further presented in
chapter 3.
Newly-Modified White-Type Cell
The White-type cell used in chapter 3 was limited to only
8 passes of the laser beam due to its limited ratio of the
length of mirror 3 to the separation of centers of curvature
of mirrors 1 and 2. In addition, the laser beam alignment on
the brass mirrors was a difficult task because the brass
mirrors were very flexible Therefore, many problems were
encountered when doing the multipass alignment with a helium-
neon (He-Ne) laser before inserting the cell into the vacuum
chamber. Moreover, inhomogeneity of the electric field lines
inside the cell created by the brass mirrors required a
trapping potential between 3-5 Volts to contain ions during
irradiation. To circumvent the above mentioned problems the
White-type cell was further modified. In this arrangement the
ratio between the length of mirror 3 and the distance between
center of curvature of mirrors 1 and 2 was increased. Also,
the brass mirrors were replaced by well-polished stainless-
steel mirrors. The new design enabled us to obtain 16 laser
passes inside the cell.


151
60. Lumonics, Ltd., 105 Schneider Rd., Kanata, ON K2K 1Y3,
Canada.
61. Apollo Laser Inc., 9201 Independence Ave., Chatsworth, CA
91311.
62. Optical Engineering Inc., 2495 Bluebell Dr., Santa Rosa,
CA 91561.
63. Scientech Inc., 5649 Arapahoe Ave., Boulder, CO 80303.
64. (a) Now sold by Waters/Extrel FTMS, P. O. Box 4508,
Madison, WI 53711. (b) Ion Spec Corporation, 17951
Skypark Circle, Irvine, CA 92715.
65. Alcatel Vacuum Products, 40 Pondpark Rd., Hingham, MA
02043.
66. Hofstadler, S. A.; Laude, D. A. Anal. Chem., 1992, 64,
572.
67. (a) Ramanathan, R.; Shalosky, J. A.; Eyler, J. R. in
Proceedings of the 41st ASMS Conference on Mass Spec
trometry and Allied Topics; San Francisco, CA, 1993, p.
752. (b) Ramanathan, R.; Dejsupa, C.; Eyler, J. R. in
Proceedings of the 42nd ASMS Conference on Mass Spectrom
etry and Allied Topics; Chicago, IL, 1994, p. 543.
68. Comisarow, M. B.; Marshall, A. G. Chem. Phys. Lett. 1974,
26, 489.
69. Kofel, P.; Allemann, M.; Kellerhals, H. P.; Wanczek, K.
P. Int. J. Mass Spectrom. Ion Process, 1986, 14, 1.
70. Yin, W. W.; Wang, M. ,* Marshall, A. G.; Ledford, E. B. Jr.
J. Am. Soc. Mass Spectrom., 1992,3, 188.
71. Beu, S. C.; Laude, D. A. Int. J. Mass Spectrom. Ion
Proc., 1992, 112, 215.
72. White, J. U. J. Opt. Soc. Am., 1942, 32, 285.
73. MACOR is a machinable glass ceramic. 6100 Fulton
Industrial Blvd. Atlanta, GA 30336.
74. Harris, F. J. Proc. IEEE., 1978, 66, 51.
75. (a) Saykally, R. J.; Woods, R. C. Ann. Rev. Phys. Chem.,
1981, 32, 403. (b) Schafer, E.; Begemann, C. S.;
Gudeman, C. S.; Saykally, R. J. J. Chem. Phys., 1983, 79,
3159.


ABSORBANCE
Figure 3.12.
Gas phase neutral infrared spectrum of Ga(hfac)3 (pressure = 0.45 Torr).


RELATIVE INTENSITY
40
Figure 2.12. IRMPD mass spectrum obtained for the
protonated diglyme cation under the same
conditions used to obtain Figure 2.10, but cw
CC>2 laser energy was 750 mJ pulse-1. The
laser beam was subjected to (top) center pass,
and (bottom) multipass arrangements.


77
fewer alignment problems because the three mirrors were fixed
permanently into the receive plates. Furthermore, trapping
and photodissociation efficiencies were enhanced compared to
the White-type cell used in chapter 3. The photodissociation
effects have been described in chapter 2, and trapping
efficiencies will be discussed later in this chapter.
All experiments were performed using the FTICR mass
spectrometer discussed in chapter 2 (Figure 2.5). Briefly,
the background pressure was maintained around 5 x 10~9 Torr,
for each spectrum 16,384 data points were acquired by signal
averaging 100 ion transient response signals, and the
experiment was repeated several times under the same
conditions but with and without laser irradiation.
Methanol and d-methanol Solvated Anions
The primary negative ions were produced via electron
capture using 2-3 eV electrons (beam pulse = 25-30 ms) and a
trapping voltage of 2 V. The distortion of the electric field
lines in the cell was minimized by using smaller mirrors (less
curved) as the receive plates, and satisfactory trapping could
be obtained with a lower voltage.
The solvated halide ions were generated by sequential
ion-molecule reactions (vide infra, Eqs. 4.2, 4.5, 4.8, 4.11,
4.14, and 4.18). The primary reaction that generated
CH3OHOCH3- was different from the others. First, 5 ml isoamyl


1
Figure 5.1. The internal ESI/FTICR mass spectrometer. (1)superconducting magnet (2T),
(2) FTICR cell, (3) mechanical pumps, (4) 700 L/s diffusion pump, (5) 300 L/s
diffusion pumps, (6) 800 L/s cryo pump, (7) ion gauge, (8) gas/liquid inlet,
(9) teflon tubing (22-gauge), (10) 3/4" stainless steel tubing, (11) PEEK
spray cavity, (12) skimmer, (13) 2nd conductance limit, (14) 1st conductance
limit, (15) heated capillary, (16) syringe needle, (17) laser windows, (18)
tube lens, and (19) shutter.
119


75
proton bound methanol dimer cations. IRMPD spectra of
methanol solvated fluoride ion (CH3OHF~), d-methand solvated
fluoride ion (CH3ODF), methanol solvated chloride ion
(CH30HC1-) d-methanol solvated chloride ion (CH30DC1~) ,
methanol solvated methoxy anion (CH3OHOCH3), proton bound
methanol dimer cation ([CH3OH]2H+), proton bound d-methanol
dimer cation ([CH3OD]2H+), and deuteron bound d-methanol dimer
cation ([CH3OD]2D+), in the 920 -1060 cm-1 region (10.60 9.60
ixm P and R branches of the C02 laser) are presented in this
chapter. In addition, some interesting multiple photon
dissociation pathways of methanol dimer cations are discussed.
The neutral gas-phase spectra of methanol and d-methanol were
obtained and compared with the corresponding IRMPD spectra.
Experimental
The newly modified White-type FTICR analyzer cell, as
discussed in chapter 2 (Figure 2.9), and the cw C02 laser were
used for all the IRMPD experiments presented in this chapter.
The laser was reflected from the turning mirror into the cell
and subsequently reflected from the spherical mirrors to
create multipasses inside the cell. Careful alignment
facilitated 16 passes of the laser light inside the cell. A
schematic representation of the multipass process inside the
cell is shown in Figure 4.1. The new cell set-up demonstrated


103
(denotated by ?) in the IRMPD spectra are regions either where
the C02 laser had insufficient energy for these experiments or
produced no laser output at all.
IRMPD spectral features will depend on the facileness of
multiphoton excitation: in other words, the ease of populating
the quasicontinuum. Woodin and co-workers extensively studied
the multiple photon absorption mechanisms of initial
excitation of ions to the quasicontinuum (39). Several
molecules with varying degrees of molecular complexity were
studied and divided into two categories which depend on the
number of photons required to reach the quasicontinuum.
Furthermore, in a subsequent study Woodin et al. (39)
categorized the proton bound dimer of diethyl ether as being
in the large molecule category. By the same token methanol
proton bound dimer cations could be classified as "large"
molecules in which the cations will be expected to be in the
quasi-continuum after absorption of one IR photon photon.
Rosenfeld et al. studied IRMPD of methanol solvated fluoride,
and suggested that CH3OHF~ must absorb at least nine C02 photons
to dissociate (115).
Comparison of IRMPD and neutral IR spectra must be done
carefully due to limitations in resolution and spectral
coverage in the IRMPD spectrum because the spacing between the
sharp C02 laser lines and the absorption peaks in the IRMPD
spectra will not be the same. The frequency resolution for
the IRMPD spectra is 2 cm'1, whereas gas-phase neutral IR


40
35
30
25
20
15
10
5
0
9
(CH3OD)2D+ + nhvir
>
(CH3OD)D+
-> c2h6od
VO
VO
940 960 980 1000 1020 1040 1060
WAVENUMBER (cm1)
RMPD spectrum of deuteron bound deuterated methanol dimer cation (CH3OD)2D+,
rror estimates are 95% confidence limits.


91
bimolecular reaction sequences which lead to formation of
proton bound dimers (112) this is the first instance where a
sequence has been explained in detail which results in the
production of methanol and d-methanol dimers. The sequence of
events for the formation of ions is as follows.
Proton Bound Methanol Dimer Cation
(i) electron beam pulse
H20 + e' -* H20+ (4.21)
(ii) reactions (mechanism)
H20+ + H20 -* H30+ + OH (4.22a)
H30+ + CH3CHC12 (CH3CHC12)H+(H20) ] (unstable) (4.22a)
[ (CH3CHC12)H+(H20) ] (H20) +CHC1CH3 + HC1 (4.22c)
(H20)+CHC1CH3 + H20 -* (H20)H+(H20) + CH2CHC1 (4.22d)
(H20)2H+ + CH3OH -* (H20) (CH3OH)H+ + H20 (4.22e)
(H20) (CH3OH)H+ + CH3OH
(CH3OH)2H+ + H20 (4.22f)


absorbance
Figure 4.11. Gas-phase neutral infrared spectrum of methanol (CH3OH). (top curve)
resolution 2 cm-1, and (bottom curve) resolution 0.5 cm-1.
100


73
One approach for investigating ion-solvent interactions
has been to study solvated ions in the solution-phase (103).
In solution, solvated ions govern a host of phenomena
including solvent dependent variations in the reactivity and
spectral characteristics of anions and cations. Gas-phase
investigations of ion-solvent chemistry, utilizing mass
spectrometric techniques (104,105), provide a unique way to
study intermolecular interactions. In such studies the
properties of the ions can be studied as solvent molecules are
added one by one to the ion, in the absence of complicating
effects due to solvent and counter ion interactions.
Even though a large number of solvated cations have been
studied, it is only recently that mass spectrometric
techniques have been extended to study spectroscopy of
solvated anions. The simplicity of obtaining gas-phase
solvated anions using low pressure ion-molecule reactions
allows the capabilities of FTICR to be utilized to study the
chemistry of such ions (106) Briefly, the reaction mechanism
involves an elimination-type reaction initiated by attack of
a strong gas-phase base (A-) on a species with an acidic site
(HBC), as shown in reaction (4.1):
A" + HBC AHB~ + C (4.1)
Although mass spectrometric studies give a partial
picture about the stabilities and structure of solvated ions,


92
(iii) Laser pulse
(CH3OH)2H+ + nhvir -* (CH3OH)H+ + CH3OH (4.23a)
(CH3)2OH+ + H20 (4.23b)
The results from the above reactions (4.23a and 4.23b)
demonstrated that two photodissociation pathways could be
obtained. The most abundant peak was protonated methanol
(reaction 4.23a), and the only other plausible route of
decomposition is shown in reaction 4.23b. Caserio and
Beauchamp investigated the ion-molecule reactions of several
proton-bound alcohol dimers (112). One of the conclusions was
that the proton bound dimers rapidly eliminate water to yield
the corresponding protonated dialkyl ether. This is
consistent with the higher proton affinity of diethyl ether
(210 kcal/mol) compaired to that of water (166 kcal/mol)
(113). To investigate whether (CH3)2OH+ is a secondary
fragmentation product from (CH30H)H+, the photo-product
(CH3OH)H+ was continuously ejected from the cell during the
laser-induced time period. The laser enhanced signal
intensity due to (CH3)2OH+ remained the same with or without
the ejection of (CH30H)H+. This confirms that reaction 4.23b
is a primary IRMPD pathway.


o
o
18-crown-6 15-crown-5 12-crown-4
[18-crown-6]M+
M=Na, K or HaO
[15-crown-5]2M+
M=Na, K or HaO
Figure 5.3.
The structures of the crown ethers and complexes discussed in this study.
124


78
nitrite ([CH3]2CHCH2CH2ONO, MW = 117 g mol-1) was reacted with
excess methanol (10 ml) by using the vacuum apparatus
(background pressure ca. 10-4 Torr) shown in Figure 4.2.
Then, after ca. 10-12 min. reaction time, the neutral product
methyl nitrite (CH3ONO, MW = 61 g mol-1) was collected
(reaction 4.17). Using the methyl nitrite, the conditions to
form CH3OHOCH3- were simlar to those for the formation of
other anions, and are shown in reactions 4.18 and 4.19.
Pressure of CF4 (carbon tetrafluoride) or S02F2 (sulfuryl
fluoride) or C1C02C2H5 (ethyl chloroacetate) or CH3ONO (methyl
nitrite) was maintained at 5.5 x 10-8 Torr and CH3OH or CH3OD
was leaked in up to a pressure of 2.5 x 10-7 Torr.
The anion of interest was isolated by using a series of
ejection pulses. Ions were then allowed to undergo several
collisons with the neutrals at a pressure of 2.5 x 10-7 Torr
for ca. 1-1.3 sec. No difference in photodissociation was
observed as collision time was varied over a range of 0.85-1.5
sec. Immediately after the "cooling-period", the cw laser was
gated for variable length irradiation periods ranging from 75
- 350 ms by the FTICR data station. The energy was kept
constant at 500 mJ pulse-1. The molecular anion, the only ion
present in the cell after laser irradiation (vide infra) was
excited and detected. The extent of photodissociation was
obtained by measuring the intensities of the molecular anion
before and after irradiation, as given by their mass spectral
peak areas.


54
A positive trapping potential of 3 to 4 V was used for
diglyme and 3-bromopropene cations, and a potential of -2 V to
-3 V was used for [Ga(hfac)3]~. These higher than usual
trapping potentials were necessary when using the White-type
cell, because of the distortion of the electric field lines in
the analyzer cell due to the curved receive plates.
To obtain gas phase spectra of the neutral molecules a 10
cm quartz cell was used in a Nicolet 740 (approximate pressure
for diglyme 1.9xl02 Torr) or a Perkin Elmer 1600 (approximate
pressure for allyl bromide 2.6xl02 Torr) FTIR spectrometer.
The [Ga(hfac)3]- spectrum was obtained in the solid state (KBr
pellet) with the latter instrument.
All samples were obtained from commercial sources. The
purity was confirmed by broadband mass spectra and the samples
were used without further purification.
Results
The first series of experiments was performed using the
White-type multipass FTICR cell and one tunable higher power
cw laser. The total energy per pulse was kept constant as the
laser was tuned to various photodissociation wavelengths. The
percent photodissociation (calculated by dividing the relative
intensities of the photofragments by the relative intensities
of all ions detected) as a function of laser wavelength was
obtained for each of the three compounds. The experiment was
repeated fifteen times for a given wavelength and the average
percent photodissociation was calculated. All the error
limits represent the standard deviation of the mean.


49
(allyl bromide) cation, and the gallium hexafluoroacetyl-
acetonate anion ([Ga(hfac)3]-) have been obtained.
Experimental
All IRMPD experiments were performed on a home-built FTICR
mass spectrometer equipped with a 2T superconducting magnet,
and controlled by a Nicolet FTMS 1000 data station. Chapter
2 gives more details about the home-built vacuum system,
including a figure. The background pressure was maintained
below 2 x 10-9 Torr and samples were leaked in up to a
pressure of 5.5 x 10~8 Torr. For each spectrum 16,384 data
points were acquired by signal averaging 50 ion transient
response signals.
To enhance photodissociation effects the FTICR analyzer cell
used in these experiments was modified to increase the
irradiation path length by a multipass arrangement as shown in
Figure 2.7, and discussed in chapter 2. This White-type ICR
cell was first demonstrated in our laboratory for IRMPD
experiments using a single laser, and was shown to enhance
dramatically ion photodissociation effects. In the present
study the White-type cell was used to obtain spectra for both
one and two laser experiments.
As shown in Figure 2.7 the turning mirror attached to one end
of one receive plate reflected the resonant pulsed laser light
into the cell to give eight passes. In two-laser experiments


PHOTODISSOCIATION / %
1038 1040 1042 1044 1046 1048 1050 1052 1054
WAVENUMBER / cm-1
One-laser infrared multiple photo dissociation spectrum of the negative
molecular ion of Ga(hfac)3 at a probe laser energy of 1 J pulse-1. Error
estimates are 95% confidence limits.
Figure 3.6.


42
multipasses. This enabled the laser energy at the curved
mirror (after the first reflection) inside the cell to be
monitored. The measured laser energy after each reflection by
the mirrors, and the distance from the laser head to each
mirror are shown in Table 2.1. The energy measurements were
taken several times and the average value was tabulated.
Results indicated that cw laser energy was higher than that of
the pulsed laser after the first pass in the multipass
reflection process. In addition, the observation of the laser
beam burn-patterns at each mirror proved that a more
collimated beam could be obtained from the cw laser. These
findings led to selection of the cw C02 laser as the laser of
choice for one-laser IRMPD studies.
FTICR Pulse Sequence
The pulse sequences for one and two-laser IRMPD
experiments are discussed with figures and presented in
chapters 3, 4, and 5. Briefly, first, a one ms quench pulse
removed all ions from the cell when +15 and -15 V were applied
to the trap plates (during the quench pulse Ion Spec
electronics provided 15 V on one of the trap plates) Then,
ions were formed by electron impact during the 50-100 ms
ionization period, or transfered from the ESI source for 1 s.
Next, a "thermalization" delay allowed time for ion/neutral
collisions to occur and relax the ions. A series of ejection
sweeps were used to eject all ions except the ion of interest


58
Ga(hfac)3 + nhuir (hfac) + Ga(hfac)2
The photodissociation spectrum obtained is shown in Figure
3.6.
In a second series of experiments, useful two laser
photodissociation spectra were obtained using the same
compounds as discussed above and the probe-pump technique.
Figures 3.7, 3.8, and 3.9 present the spectra of protonated
diglyme, allyl bromide and gallium hexafluoroacetylacetonate
ions, respectively.
For comparison, gas phase neutral IR spectra were
obtained for all three compounds used in this study. They are
shown in Figures 3.10, 3.11, and 3.12.
Table 3.1 summarizes IR peak frequencies for the three
ions obtained from Figs. 3.4-3.9 (one- and two-laser
experiments gave identical results) and also includes the
corresponding neutral IR peak frequencies for the relevant
vibrations from Fig. 3.10-3.12, and the frequency shifts.
Discussion
Previous studies of IRMPD behavior of the
hexafluoropropene cation (C3F6+) (89) demonstrated that a
relatively low-power probe laser source (ca. 100 mJ pulse-1) ,
while incapable of inducing photodissociation by itself, could


137
Maleknia and Brodbelt used high energy CID to investigate the
fragmentation pathways of K+-crown ether complexes (151) The
potassium ion predominates in the CID spectra of the K+-crown
ether adducts. However, additional dissociation routes due to
homolytic cleavage of the carbon-carbon and carbon-oxygen
bonds were observed for all ether adducts.
In a previous LD study (136) it was observed that 15-
crown-5 and 12-crown-4 form cation bound sandwich complexes
with M+ (heterogenous complexes). Similar metal ion bound
dimer complexes have been documented in condensed phase
studies (152) and gas-phase studies (143). These studies have
shown that sandwich-type complex formation is governed by the
relative sizes of the cations and the crown ether cavities.
To build upon these earlier studies, we electrosprayed an
equimolar mixture of 15-crown-5/12-crown-4/NaCl/KCl in 49:49:2
CH3OH:H20:CH3COOH solution. As shown in Figure 5.10a, both
homogenous and heterogeneous sandwich complexes were observed
in the mass spectrum. As mentioned earlier, CH3COOH was added
to enhance the formation of H30+ and H+ complexes. Ions
observed include (15-crown-5)2K+ (m/z 479), (15-crown-5)2H30+
(m/z 459), (l5-crown-5)(12-crown-4)Na+ (m/z 419), (15-crown-
5)Na+ (m/z 243), (15-crown-5)2H+ (m/z 441), (15-crown-5)H30+
(m/z 463), (15-crown-5)2Na+ (m/z 463), (12-crown-4)2Na+ (m/z
375) and (15-crown-5)H+ (m/z 221) in order of decreasing
intensity. The irradiation of these ions by C02 laser output
at 10.60 fxm resulted in photodissociation (Figure 5.10b)


154
107. (a) Castleman, Jr., A. W.; Hunton, D. E.; Hoffmann, M.;
Lindeman, T. G.; Lindsay, D. N. Int. J. Mass Spectrom.
Ion Phys., 1983, 47, 199. (b) Levinger, N. E.; Ray, D.;
Alexander, M. L.; Lineberger, W. C. J. Chem. Phys., 1988,
89, 5654.
108. Ivanco, M.; Evans, D. K.; McAlpine, R. D. J. Phys. Chem.,
1989, 93, 2383.
109. Miller, R. M. J. Phys. Chem., 1986, 90, 5561.
110. (a) Del Bene, J. E. J. Chem. Phys., 1971, 55, 4633. (b)
Jorgensen, W. J. J. Chem. Phys., 1979, 71, 5034.
111. Wladkowski, B. D.; East, A. L. L.; Mihalick, E.; Wesley,
A. D.; Brauman, J. Chem. Phys., 1994, 100, 2058.
112. Beauchamp, J. L.; Caserio, M. J. J. Am. Chem. Soc., 1972,
94, 2638.
113. Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J.
L.; Levin, R. D.; Mallard, W. G. J. Phys. and Chem. Ref.
Data 1988, 17.
114. Bomse, D. S.; Woodin, R. L.; Beauchamp, J. L. J. Am.
Chem. Soc., 1979, 62, 5503.
115.
Rosenfeld, R. N.;
Chem. Soc., 1979,
Jasinski, J. M.;
101, 3999.
Brauman,
J. I.
J.
Am.
116.
Rosenfeld, R. N.;
Chem. Soc., 1982,
Jasinski, J. M.;
104, 658.
Brauman,
J. I.
J.
Am.
117. Dewar, M. J. S. Hyperconjugation; Ronald: New York, 1962.
118. Jasinski, J. M.; Brauman, J. I. J. Chem. Phys., 1980, 73,
6191.
119. Bomse, D. S.; Beauchamp, J. L. J. Am. Chem. Soc., 1981,
104, 658.
120. LaCosse, J. P.; Lisy, J. M. J. Phys. Chem., 1990, 94,
4398.
121. Huskien, F. ; Stemmier, M. J. Chem. Phys., 1988, 144, 391.
122. Katta, V.; Chowdhury, S. K.; Chait, B. T. J. Am. Chem.
Soc., 1990, 112, 5348.
123. Smith, R. D.; Loo, J. A.; Edmonds, c. G.; Udseth, H. R.
Anal. Chem., 1990, 62, 693.


28
Electrosprav Ionization Experiments
In chapter 5, crown ethers formed by ESI were studied
using the IRMPD technique. Figure 5.1 illustrates the
ESI/FTICR mass spectrometer used in this series of experiments
and a detailed discussion of the ESI source and the
instrumentation will be presented in chapter 5. This type of
"internal" electrospray ionization source was first
demonstrated by Hofstadler and Laude (66) and we followed
Laude*s idea and modified the mass spectrometer shown in
Figure 2.6 to accommodate the ESI source. The modifications
and assembly are explained in detail elsewhere (67). Briefly,
the vacuum chamber for the ESI consisted of five concentric
chambers of increasing diameter with four differentially
pumped regions. A more detailed description of these pumping
regions will be given in chapter 5.
A known mass of commercially available crown ether
compound was dissolved in 50:50 methanol:water to obtain a
sample of 1.0 x 10-5 M concentration. Sodium and potassium
complexes of crown ethers were formed by adding 1-3 drops of
1.0 x 10-5 M sodium chloride (NaCl) or potassium chloride
(KC1), respectively. Sample solutions were delivered to the
electrospray needle through a 6* long 22-gauge teflon tube.
Operating parameters for the ESI source are discussed in
chapter 5.


ABSORBANCE
Figure 3.10.
CT>
OJ


149
29. Louris, J. N.; Brodbelt, J. S.; Cooks, R. G. Int.
J. Mass Spectrom. Ion Proc. 1987, 75, 345.
30. Miller, T. A.; Bondybey, V. A. Molecular Ions: Spec
troscopy, Structure, and Chemistry; North-Holland
Publishing: Amsterdam, 1983.
31. Ashfold, M. N. R.; Baggott, J. E. Molecular Photodisso
ciation Dynamics; The Royal Society of Chemistry: London,
1987.
32. Kawaguchi, K.; Hirota, E. J. Chem. Phys., 1987, 87, 6838.
33. (a) Rosenbaum, N. H.; Owrutsky, J. C.; Tack, L. M. ;
Saykally, R. J. J. Chem. Phys., 1986, 10, 5308. (b)
Owrutsky, J. C.; Keim, E. R. ; Coe, J. V.; Saykally, R. J.
J. Phys. Chem., 1989, 93, 5960.
34. Isenor, N. R.; Richardson, C. Appl. Phys. Lett., 1971,
18, 224.
35. Rosenfeld, R. N.; Jasinski, J. M.; Brauman, J. I. J. Am.
Chem. Soc., 1979, 101, 3999.
36. (a) Buck, U.; Gu, X.; Lauenstein, C.; Rudolph, A.; J.
Phys. Chem., 1988, 92, 5561. (b) Shen, M.H.; Farrar, J.
M. J. Phys. Chem., 1989, 93, 4386.
37. Thorne, L. R.; Beauchamp, J. L. In Gas Phase Ion Chem
istry; Bowers, M. T., Ed.; Academic: New York, 1984, vol.
3, Chapter 81.
38. Drzaic, P. S.; Marks, J.; Brauman, J. I. In Gas Phase Ion
Chemistry; Bowers, M. T., Ed.; Academic: New York, 1984,
vol. 3, Chapter 21.
39. Woodin, R. L.; Bomse, D. S.; Beauchamp, J. Chemical and
Biochemical Applications of Lasers; Moore, C. B. Ed.;
Academic Press: New York, 1977, vol. iv.
40. Wight, C. A.; Beauchamp, J. L. J. Chem. Phys., 1984, 88,
4426.
41. (a) Honovich, J. P.; Dunbar, R. C. J. Am. Chem. Soc.,
1982, 104, 6220. (b) Honovich, J. P.; Dunbar, R. C. J.
Phys. Chem., 1983, 87, 3755.
42. Draves, J. A.; Schulten-luthey, Z.; Wen-Long, L.; Lisy,
J. M. J. Chem. Phys. 1990, 93(7), 4589.
43. Wei, S.; Tzeng, W. B.; Castleman, A. W. J. Chem. Phys.,
1990, 92(1), 332.


95
D30+ + CH3CHC12 -* CH3CHC10D2+ + DC1 (4.28b)
CH3CHC10D2+ + D20 [D20]2H+ + CH2CHC1 (4.28c)
[D20]2H + + CH3OD - CH3OD(D2HO)+ + D20 (4.283)
- (CH3OD) (D30)+ + HDO (4.28e)
CH3OD(D2HO)+ + CH3OD (CH3OD)2D+ + HDO (4.28f)
(CH3OD) (D30)+ + CH3OD (CH3OD)2D+ + D20 (4.28g)
(iii) Laser pulse
(CH3OD)2D+ + nhvir (CH3OD)D+ + CH3OD (4.29a)
- (CH3)2OD+ + D20 (4.29b)
Similar to IRMPD in (CH3OH)2H+ (reactions 4.23a and
4.23b), only two sets of photo-products were observed for the
deuteron bound d-methanol dimer. The most dominant peak was


Figure 2.5.
A schematic representaion of the FTICR mass spectrometer equipped with a 2
Tesla superconducting magnet.


3
that CID also can be performed in a FTICR instrument (17) .
The CID technique involves isolating an ion of interest,
accelerating the ion into a target gas, and detecting the
daughter ions produced from the parent ion. The ability to
perform CID over a range of collisional energies with high
resolution mass analysis of the CID daughter ions and the
capability of performing sequential MS-MS make the FTICR an
excellent instrument for performing CID.
In spite of widespread application of CID, molecules of
> 3,000 Da in molecular weight are seldom successfully
fragmented (18) In recent years, surface-induced
dissociation (SID) has been explored as an alternative to CID.
The application of SID in FTICR has been reported by Williams
et al. (19), and Ijames and Wilkins (20). The SID experiment
has now been performed in quadrupole, sector, time-of flight
and quadrupole ion trap mass spectrometers. In addition to CID
and SID, Jacobson et al. (21) showed that sustained off-
resonance irradiation (SORI) can be used to transfer small
increments of internal energy into ions. Thus, SORI combined
with CID can be used as a selective probe for determination of
the lowest energy fragmentation pathways available for an ion
of interest. However, the SORI/CID method is still in its
early stages of development compared to SID and CID methods
for structural analysis.
An alternative to ion fragmentation by CID and SID is
photodissociation (PD), which utilizes continuous and/or


4
INFRARED MULTIPLE PHOTON DISSOCIATION SPECTRA
OF METHANOL SOLVATED ANIONS AND PROTON
BOUND METHANOL DIMER CATIONS 72
Introduction 72
Experimental 75
Methanol and d-methanol Solvated
Anions 77
Proton (deuteron) Bound Methanol
(d-methanol) Dimer Cations 80
Results and Discussion 81
Methanol Solvate of the Fluoride Ion .. 81
Deuterated Methanol Solvate of the
Fluoride Ion 82
Methanol Solvate of the Chloride Ion .. 83
Deuterated Methanol Solvate of the
Chloride Ion 84
Methanol Solvate of the Methoxy Ion ... 84
Proton Bound Methanol Dimer Cation .... 91
Proton Bound Deuterated Methanol
Dimer Cation 93
Deuteron Bound Deuterated Methanol
Dimer Cation 94
Conclusions 112
5 INFRARED MULTIPLE PHOTON DISSOCIATION OF
CROWN ETHER COMPLEXES 114
Introduction 114
Experimental 117
ESI Source 117
ESI/FTICR Mass Spectrometry 118
ESI/FTICR Pulse Sequence 120
Results 123
Discussion 129
Conclusions 139
6 CONCLUSIONS AND FUTURE WORK 140
Conclusions 140
Future Work 144
REFERENCES 147
BIOGRAPHICAL SKETCH 157


146
It is also worthwile to point out some interesting
classes of molecules that could be used to obtain spectra for
gas-phase ions. (1) IRMPD spectra could be obtained for
alkali metal M+-crown (where M = Na, Rb, Cs, Cu, Ca etc., NH4+
and H30+) formed by ESI. The spectra could be used to compare
the shifts for different complexes. (2) IRMPD spectra of
solvated ions such as XY+ (where X = Na, K, Ca, Sr etc. and Y
= H20, NH3, and CH3OH) could be formed by either El or ESI
sources and the corresponding IRMPD shifts could be obtained
and compared for different cations and/or the solvents. A
further extension of this study could be done, using the
capabilites of the ESI source to produce solvated ions with
more than one solvent molecule attached (XYn+) [where n> 1].
Thus vibrational frequencies of previously unobtainable
cluster ions could be obtained. (3) Electrosprayed ions with
a varietry of ligand substituents of organometallic complexes
could be formed (138) and IRMPD spectra could be obtained.
Infrared chromophores such as CF3, NH3, and CO groups bonded
to a transition metal center are ideal complexes to
investigate the infrared photochemistry of organometallic
compounds.


55
When the cw laser was tuned through the wavelength range
10.49-10.71 |im (953-934 cm-1), the positive ion C6H1503+
(protonated diglyme cation, m/2=135), underwent IRMPD to form
two fragment ions at m/z =103 (C5H1102+) and m/z=59 (C3H70+)
according to the reactions:
CgH^jOH^ + nhvir * t CH4O
- C3H70+ + c3h8o2
The photodissociation spectrum obtained is shown in Figure
3.4.
A limited wavelength IRMPD study of the 3-bromopropene
positive ion has been previously reported (91). Results were
only reported for the five strongest C02 laser lines at 9.28,
9.54, 10.25, 10.59, and 10.67 un. The ion undergoes IRMPD
yielding exclusively C3H5+ via loss of Br* .
C3H5Br+ + nhuir C3H5+ + Br *
The single laser IRMPD spectrum obtained for this ion is shown
in Figure 3.5 for C02 laser wavelengths from 10.18 to 10.70 /xm
(982-934 cm-1) .
The Ga(hfac)3 anion (m/z=691) was formed by low energy
electron attachment. When the C02 laser was tuned from 9.49
to 9.62 jLim (1054-1039 cm-1) the negative ion photodissociated
by losing a negatively charged ligand (hfac, m/z=207).


100
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Fiaure 5.6. ESI/FTICR mass spectrum obtained with 18-crown-6/NaCl/KCl in 50:50
methanol¡water solution and with cw C02 laser irradiation at 10.58 pm
wavelength.
127


Dissociation
Threshold
i
V = 4
J-1
V o
u u
_ i n
i
i w
v- i
i '
J-1
V o J-o
Quasi -
continuum
co
Schematic representation of the two-laser (probe-pump) photodissociation
process.
Figure 3.1.


148
14. Bach, B. H.; Bruce, J. E.; Ramanathan, R.; Watson, C. H.;
Zimmerman, J. A.; Eyler, J. R. On Clusters and
Clustering, From Atoms to Fractals; Reynolds, P. J., Ed.;
Elsevier Science Publishers B. V.: Amsterdam 1993.
15.
Wight, C. A.; Beauchamp, J. L. J. Am. Chem.
103, 6499.
SOC., 1981,
16.
Bomse, D. S.; Beauchamp, J. L. J. Am. Chem.
103, 3292.
SOC., 1981,
17.
Cody, R. B.; Freiser, B. s. Int. J. Mass Spectrom. Ion
Phys., 1982, 41, 199.
18.
Yang, C. L. C.; Wilkins, C. L. Org. Mass.
1989, 24, 409.
Spectrom.,
19.
Williams, E. R.; Henry, K. D.; McLafferty, F. W.;
Shabanowitz, J.; Hunt, D. F. J. Am. Soc. Mass Spectrom.,
1990, 1, 413.
20.
Ijames, C. F.; Wilkins, C. L. Anal. Chem.
1295.
, 1990, 62,
21.
Gauthier, J. W.; Trautman, T. R.; Jacobson,
Chim. Acta, 1991, 246, 211.
D. B. Anal.
22.
Kim, M. S.; McLafferty, F. W. J. Am. Chem.
100, 3279.
Soc., 1978,
23.
Griffiths, I. W.; Mukhtar, E. S.; March, R.
F.M.; Beynon, J. H. Int. J. Mass Spectrom.
1981, 39, 125.
E.; Harris,
Ion Phys.,
24.
Amster, I. J.; Baldwin, M. A.; Cheng, M. T.;
J.; McLafferty, F. W. J. Am. Chem. Soc.,
1654.
Procter, C.
1983, 105,
25.
Brauman, J. I.; Smythe, K. C. J. Am. Chem.
91, 7778.
SOC., 1969,
26.
Dunbar, R. C. In Gas Phase Ion Chemistry; Bowers,
M. T. Ed.; Academic: New York, 1979, volume 3,
Chapter 20.
27. Meyer, F. K; Jasinski, J. M.; Rosenfeld, R. N.;
Brauman, J. I. J. Am. Chem. Soc., 1982, 104, 663.
28. Hughes, R. J. ; March, R. E. ; Young, A. B. Int. J. Mass
Spectrom. Ion Phys. 1982, 42, 255.


o
i i i i I i i i i | i i i i I i i i i | i i i i I i i i i | i i i i I i i i i | i i i rpi i i [i-i-n-f-ri "i i | i i i i | i i i i | i i i i | i i i i | i
2 4 6 8
Time (milliseconds)
A digitized time domain signal (transient) for protonated
bis(2-methoxydiethyl) ether (diglyme) cation produced electron ionization
(50 eV) in a diglyme sample of pressure 1.0 x 10-6 Torr.
Figure 2.2.


24
Quasicontinuum
~K
Discrete
Level
Regime
V = 4
V = 3
J-1
J = 0
V
2
V
V-0
J = 0
J-0
J1
J 0
A schematic representaion of the energy
diagram for the infrared multiple photon
(IRMP) absorption and dissociation process.
Figure 2.4.


144
Future Work
Recent emergence of the IRMPD process as a dissociation
technique has enabled researchers to elucidate spectra of
gaseous ions. Infrared spectra of several gas-phase cations
as well as anions were obtained and shifts in vibrational
frequencies were compared with the corresponding gas-phase
neutral ions. However, there are some major areas in this
technique which need to be refined in order to obtain
reliable, high resolution infrared spectra for a wide range of
molecules.
A major drawback of the IRMPD technique (other than the
low tunability and/or low laser output) is the misleading
spectral features obtained due to the coincidences of the
sharp absorption bands and the sharp laser lines. Use of an
optical parametric oscillator (0P0), capable of operating in
the infrared region could be used to overcome this problem.
An 0P0 is a device which produces intense optical radiation
possesing a high degree of spatial coherence, and continuous
tunability over a large spectral range (0.865- 4.6 jum) .
Another alternative approach which could be utilized to
overcome the above mentioned problem is the use of an infrared
extension package (IRP). An IRP package has been installed
already in our laboratory, and it generates wavelengths in the
1.54-4.5 /zm region by difference frequency mixing of the
output of a Nd:YAG pumped dye laser and the residual of the
1.06 /urn Nd: YAG laser beam.


118
needle and a 0.585 mm i.d. x 15 cm long stainless steel
desolvating capillary. The ESI source is inserted inside a
1.88 cm o.d. stainless steel probe and a vacuum seal is made
with a Viton 0-ring. The capillary protrudes 2 cm beyond the
end of the 1.88 cm probe.
ESI/FTICR Mass Spectrometer
All experiments were performed using an Ion Spec data
station (64b), a prototype 2.0 Tesla superconducting magnet,
and a home-built/assembled vacuum system with four
differentially pumped stages. The ESI/FTICR instrumentation
used in these experiments is shown in Figure 5.1. The 1.88 cm
o.d. probe with the PEEK spray assembly was inserted inside a
4.0 cm o.d. vacuum chamber which terminates with a blunt-ended
0.20 mm copper skimmer. The pressure in this first pumping
stage was maintained at 2.3 Torr by two 25 L/s mechanical
roughing pumps. A tube lens is positioned 4 mm from the
skimmer assisted in focusing ions exiting from the capillary.
The capillary and the skimmer were separated by an adjustable
distance of 5-10 mm. The distance was optimized to give
maximum ion current on the shutter head (vide infra) and
minimum pressure in the cell region. The 4 cm o.d. vacuum
chamber was inserted into a 10 cm o.d. vacuum chamber which
terminated with 4 mm conductance limit. The pressure in this
region was maintained at 10s Torr by a 800 L/s cryo pump. A


Mirror 1


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| I I I I p I I I | I I |r~p-|~TTT~| I I I I | I I I I
500
m i | mi
700
TrTT1T
m/z
900
Figure 5.7. ESI/FTICR mass spectrum obtained with 18-crown-6/NaCl/KCl in 50:50
methanol-.water solution and with cw C02 laser irradiation at 9.58 pm
wavelength.
128


23
molecules by collisions. The excitation of C02 molecules
leads to a large population present in the vibrational state
from which the laser transition begins. Helium helps to
increase the depopulation of the lower states of the laser
transition.
The lasing principles are the same for both pulsed and cw
C02 lasers. The pulsed C02 laser is capable of generating
high-powered pulses (MW range) of short duration (fj.s range) .
The beam diameter of pulsed lasers ranges between 2 and 5 cm2,
and the laser fluence is therefore 1-2 J cm-2. The cw laser
produces continuous output at lower power than the pulsed
laser. The power output of a cw laser is proportional to the
length of the laser tube (usually 1-2 meters), and output
powers range between 30 and 50 W. The beam diameters usually
range between 0.5 and 2 cm2.
The multiple photon process involves sequential
absorption of photons from either pulsed or cw C02 lasers
through three distinct regimes: the discrete level regime; the
quasicontinuum regime; and the dissociation threshold level.
A schematic energy diagram for IRMP absorption and
dissociation processes is shown in Figure 2.4. The discrete
level regime consists of the individual spectroscopic states
of the molecules at low energies. Excitation by C02 laser
requires that the laser frequency be in resonance with the
energy separation of any two states. As the internal energy
of the ion increases, the density of states increases until


22
well understood that a molecule with sufficient vibrational
excitation can undergo bond dissociation or rearrangement
reactions (58). However, the energy provided by a single
infrared photon (3 kcal mol-1) is not sufficient to achieve
bond dissociation (requiring 50-100 kcal mol-1) Therefore,
bond dissociation reactions require absorption of many
infrared photons.
Iseor and Richardson (34) were the first to demonstrate
that bond dissociation reactions could be performed using
infrared lasers. The most common source of infrared light is
from C02 lasers. The C02 laser is the most versatile gas
laser, and can operate in either pulsed or continuous mode.
In addition, it can produce the highest continuous power of
any gas laser. Laser action involves the rotational lines of
two vibrational transitions within the C02 molecule. C02 is
a linear, triatomic molecule that has three normal vibrational
modes: symmetric stretching mode (v1), the bending mode (v2) ,
and the asymmetric mode (v3) .
The principles of laser action in C02 lasers have been
discussed in detail (59). Briefly, the lasing medium usually
consists of a mixture of C02, N2, and He, and each gas serves
a specific role. A typical gas ratio in the mixture is 1:1:8
C02:N2:He. Both N2 and C02 absorb energy from electrons in the
discharge. The most significant excitation mechanism appears
to be the direct excitation of N2 by the electrons from the
plasma. The energy is transferred from N2 to ground-state C02


109
region and three peaks in the 930- 966 cm'1 region, and they
have nearly identical vibrational frequencies. Therefore, the
overall appearance of the IRMPD spectra is very similar and
thus did not demostrate absorption selectivity in the
wavelength region studied.
On the other hand, there are two distinct, readily
identified features in the IRMPD spectra of the anions
studied: (i) spectra of CH3OXY' and CH3OHOCH3' were quite
different in their absorption pattern in the 920 980 cm'1
region. There is only one peak observed in the IRMPD spectrum
of CH3OHOCH3' at ca. 978 cm'1 as opposed to CH3OXY' where at least
two or more peaks were observed. The CH3OHOCH3' spectral shift
could be due to the substituted anion OCH3*. A possible
explanation can be given by using the concept of negative
hyperconjugation (117). In this case, the excess negative
charge (118) from the high-lying oxygen lone pair is
redistributed into an empty manifold of degenarate C-H
antibonding orbitals; (ii) spectra of CH30HX' and CH30DX' were
different in the 9.6 jum (1030 1060 cm'1) region. The
spectrum of CH30HX' has only one sharp absorption peak as
opposed to the CH3ODX spectrum. In contrast the spectra of d-
methanol solvated anions have either two peaks or one peak
split into two. Therefore, isotopic substitution introduces
shifts in the absorption spectrum, and IRMPD spectra are
isotopically selective. Limitations in IRMPD spectral
resolution made further verifications of these observations
impossible.


46
levels, dissociation thresholds, and structures of gaseous
ions, when they are subjected to UV-visible irradiation, quite
often in Fourier transform ion cyclotron resonance (FTICR)
mass spectrometers (78).
The availability of tunable C02 lasers and the ability to
perform mass analysis on charged particles have made infrared
photodissociation spectroscopy a very powerful spectroscopic
probe for elucidating the gas phase ion structure, energetics,
and dynamics. The technique of infrared multiple photon
dissociation of ions has been used by several researchers (79-
82) to obtain spectra of gaseous ions in order to assist in
determining ion structure. Zhao et. al. demonstrated the
feasibility of IRMPD of molecules in molecular beams (83) and
obtained structural information as well as thermal
decomposition data for polyatomic molecules. However, due to
both the limited tunability of standard ir laser sources (C02,
CO, NO), and partly to the low power of these lasers, there
has been no systematic attempt to use IRMPD to obtain gaseous
ion spectra and structural information.
Probe-Pump Technique
Our laboratory has been heavily involved (84-86) in
coupling UV-visible and both continuous wave (cw) and pulsed
C02 lasers to FTICR mass spectrometers for several years. As
a result of these investigations structural information about


18
These include single frequency ejection, swept frequency
ejection and swift ejection (56b). During the ejection pulse,
an oscillating voltage with a frequency corresponding to an
ion or range of ions is applied to the "excite" plates of the
cell. The ions absorb energy at their characteristic
cyclotron frequencies and move to larger cyclotron radii,
eventually striking one of the cell plates where they are
neutralized. Thus, ion ejection is governed by the average
kinetic energy that an ion possesses. Therefore, as shown in
Equation 2.7, ion ejection is dependent upon the duration of
the rf pulse (t), and/or the amplitude of the rf pulse (E) .
A broad band mass spectrum is obtained by applying (for
a 3T magnetic field) a 20 kHz -2.66 MHz rf pulse, referred to
as an "rf-chirp", for about 1 ms to the excite plates of the
cell. This pulse excites all of the ions in a 17-2300 amu
mass range into coherent motion, producing an image current
consisting of all of the superimposed ion frequencies in the
mass range. Once the ions are moving coherently, the "packet"
of ions attracts electrons to whichever "receiver" plate it is
approaching. Thus, an image current is created on the opposed
plates (57).
This image current, containing frequencies of all the
ions present in the cell, is digitized to give a time-domain
signal. Subsequent Fourier transformation of the digitized
image current yields a frequency-domain spectrum of the ions
present in the FTICR cell. A reference compound (most often


142
the anion occupies an orbital with strong antibonding
character which is localized on the hfac ligands.
The technique of IRMPD was further extended to obtain
spectra for methanol solvated anions and proton bound methanol
dimer cations. Several interesting facts were obtained from
this study. The nature of the substituted anion could
facilitate spectral shifts in the methanol solvated anion
IRMPD spectra. Spectral shifts were also observed when d-
methanol was used as the solvent instead of methanol.
Therefore, IRMPD spectra are isotopically selective.
All IRMPD spectra have a strong absorption peak (or two)
similar to neutral IR spectra of methanol or d-methanol near
the 9.6 nm region (C-0 stretching mode in methanol). These
results imply that chromophoric character may be preserved in
the IRMPD spectra. Therefore, IR spectra of structurally
similar neutral molecules may provide insight as to whether an
ion will absorb in the C02 laser region of the spectrum.
In summary, infrared multiphoton dissociation represents
one of the very few techniques available for obtaining
spectral information on gas-phase ions. Analysis of these
photodissociation spectra is not as straightforward as for uv-
vis photodissociation spectra because the infrared studies
have been limited to the C02 laser wavelength range (920-1060
cm-1) and to a resolution of only ca. 2 cm-1. Also, the IRMPD
spectral peaks, especially of ions of low molecular complexi
ty, arise from coincidences between sharp laser lines and


150
44. Okumura, M.; Yeh, L. I.; Myers, J. D.; Lee, Y. T. J.
Phys. Chem., 1990, 94, 3416.
45. (a) Burlingame, A. L.; Millington, D. S.; Norwood, D. L.;
Russell, D. H. Anal. Chem., 1990, 62, 268R. (b) Burling
ame, A. L.; Baillie, T. A.; Russell, D. H. Anal. Chem.,
1992, 64, 467R.
46. Dole, M; Mack, L. L.; Hines, R. L.; Mobley, R. C.
Ferguson, L. D.; Alice, M. B. J. Chem. Phys., 1968, 49,
2240.
47. Yamashita, M.; Fenn, J. B. J. Phys. Chem., 1984, 88,
4451.
48. Gokel, G. W. Crown Ethers and Cryptands, Royal Society of
Chemistry: Cambridge, 1991.
49. Lawrence, E. O.; Levingston, M. S. Phys. Rev., 1932, 40,
19.
50. (a) Sommer, H.; Thomas, H. A.; Hippie, J. A. Phys. Rev.,
1949, 76, 1877; (b) Phys. Rev., 1951, 82, 697.
51. Wobschall, D. Rev. Sci. Instrum., 1965, 36, 466.
52. Mclver, R. T. Jr. Rev. Sci. Instrum., 1973, 44, 1071.
53. Comisarow, M. B.; Marshall, A. G. Chem. Phys. Lett. 1974,
25, 282.
54. Marshall, A. G.; Grosshans, P. B. Anal. Chem., 1991, 63,
215A.
55. Wang, M.; Marshall, A. G. Anal. Chem., 1989, 61, 1288.
56. (a) Comisarow, M. B.; Grassi, V.; Parisod, G. Chem. Phys.
Lett., 1978, 57, 413. (b) Marshall, A. G.; Wang, T. C.;
Ricca, T. L. J. Am. Chem. Soc., 1985, 107, 7893.
57. Comisarow, M. B. J. Chem. Phys., 1978, 69, 4097.
58. (a) Ambartzumanian, Rl. V.; Letokhov, V. S. Chemical and
Biochemical Applications of Lasers; Moore, C. B. Ed.;
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A.; Subdo, S. A.; Krajnovich, D. J.; Kwok, Y. R.; Shen,
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York, 1988.


116
during the relatively short collision duration. Thus, while
CID often gives a number of fragment ions, these ions are not
obtained via the pathway of lowest activation energy.
Since their discovery by Pedersen (131), crown ethers
have been used in several analytical applications. The
ability of crown ethers to complex with metal ions and organic
molecules has generated wide-spread interest in host-guest
chemistry (132). Desorption methods of ionization, such as
252 Cf plasma desorption (133), fast atom bombardment (FAB)
(134), field desorption (FD) (135), laser desorption (LD)
(136), and secondary ion (SI) (137) have greatly facilitated
the gas-phase study of crown ethers. Unlike the desorption
ionization techniques, ESI allows preformed ions to be
extracted directly from solution by an applied electrostatic
field. A high electrical potential is used to overcome
interionic (coulombic) forces and to separate the crown
ether/cation complexes from their counter ions. Formation of
ions in solution suggests that the gas-phase mass spectrum
should reflect the solution chemistry. This is further
supported by recent gas-phase studies which reflected the
conformations of proteins in solution (138). In addition this
technique imparts little or no excess internal energy to the
molecule of interest. Unlike, MALDI, LD, and FAB, where fresh
samples have to be introduced after the sample on the probe-
tip has been desorbed or bombarded, ESI ions are produced
continuously.


Figure 3.2
en
Schematic representation of the cutaway view of the modified White-type cell
and the two-laser beam pathways.


CHAPTER 4
INFRARED MULTIPHOTON DISSOCIATION SPECTRA OF METHANOL
SOLVATED ANIONS AND PROTON BOUND METHANOL DIMER CATIONS
Introduction
The study of clusters has become a very active area in
chemistry (98). Cluster ions are often the energetically
preferred form of ions in relatively cool gas media, and both
positive and negative cluster ions are known to be important
in the ion-molecule chemistry of the upper atmosphere. Much
of the interest in studying loosely bound aggregates of atoms,
molecules and ions arises from the fact that such systems
exhibit properties intermediate between the gas and condensed
phases. Within the broad topics of cluster structure and
dynamics, the fundamental phenomenon of solvation is of
central importance (99,100), and various thermodynamic and
structural studies of solvated species, particularly ions,
have appeared in the literature (101,102). The study of
successively larger cluster ions is analogous to the process
of solvation, one solvent molecule at a time. Therefore,
solvated ions afford a particularly interesting collection of
systems for study because they bridge the gap between bare
isolated ions and ionic solids and electrolyte solutions.
72


69
have C-F stretching absorptions in the 1350-1000 cm'1 range,
with the exact position depending on the nature and the degree
of fluorination (92). As the complexity of the molecule
increases the accurate assignment of any one peak to a
particular normal mode becomes complicated. This is partly
due to Fermi resonance (93) and partly due to bonds having
similar vibrational frequencies.
Assignment of the observed C-F stretching frequencies in
the Ga(hfac)3 anion spectrum (Figure 3.9) and the neutral IR
spectrum (Figure 3.12) is made by comparison with previous
studies (94-96) in Table 3.2. For CF3/ CF3+, and C2F6 the
degenerate C-F stretching mode has a higher frequency than the
nondegenerate C-F stretching mode. We assume a similar
assignment for Ga(hfac)3, with the degenerate C-F stretching
mode assigned a higher frequency (ca. 1240 cm'1) and the
nondegenerate C-F stretching frequency assigned a lower
frequency (1151 cm'1) Two peaks are observed for the
degenerate C-F stretching mode at 1217 cm'1 and 1262 cm'1;
presumably the splitting is due to Fermi resonance involving
either a combination or an overtone band of one or two of the
peaks observed at lower frequencies in Figure 3.11.
Shin and Beauchamp have obtained (97) IRMPD spectra of
some organometallic compounds containing CF3 ligands. Two
peaks in the CF3Mn(CO)5 neutral spectrum were attributed to C-F
stretching modes of Aj and E symmetry respectively, and shifts
to lower frequency of each peak were seen in both


0
sz
c
o

c
0)
0
CO
C
o
c
0
4<
0
N
o
o
00
9 05
o
o
n
c
0
2 CD
0
0
O
LET
mmmm
CL=
LLI
0
0
Q.
CL o>
cn
LJ
Time
Figure 3.3.
Experimental pulse sequence employed in the two-laser probe-pump technique.
The ejections after the probe laser and the pump laser gating steps were
eliminated when only one-laser experiments were performed.
Excitation
Detection


50
45
40
35
30
25
20
15
10
5
9
PD
CH3ODCI* + nhv,r -~> Cl + CH3OD
979.73
1046.85
1041.27

¡i
00
<£>
940 960 980 1000 1020 1040 1060
WAVENUMBER (cm ')
spectrum of deuterated methanol solvated chloride ion (CH3ODCl~) Error
tes are 95% confidence limits.


68
tentatively related the ion peak to the somewhat structured
neutral peak between 960 and 1050 cm'1, rather than to any of
the stronger bands between 1100 and 1250 cm'1, primarily
because the absorbance of the former band is similar to that
of the allyl bromide neutral band (Figure 3.11) and the IRMPD
spectra of the corresponding ions show almost the same extent
of photodissociation. It is not possible to assign the ion
peak unambiguously to specific lines in the P or R branches of
the neutral or to the sharp Q branch of approximately the same
width.
Comparison of the IRMPD and neutral IR spectra reveals
very little difference for the C-O-C stretch of diglyme and
the C-Br stretch of 3-bromopropene. A possible explanation
for the similarity in diglyme spectra is that while formation
of protonated diglyme involves addition of H+ onto the O atom
[C-0+(H)-C], there is little change in the nature of the
bonding orbitals controlling the force constant for C-O-C
stretch. Similarity in 3-bromopropene spectra can be
attributed to the fact that electron removed upon formation of
the positive ion is in a bromine non-bonding orbital and thus
there is again negligible change in the bonding orbitals
controlling the force constant for the C-Br stretch upon
ionization.
In contrast, the IRMPD spectrum of Ga(hfac)3" shows a much
larger shift when compared to the IR spectrum of its neutral
precursor. The IR spectra of fluorine-substituted compounds


6
The measurement of photofragment mass spectra is
comparable to classical methods in mass spectrometry such as
CID and fragmentation studies as a function of internal
energy. In contrast, photodissociation spectra add
essentially new information through measurement of a physical
property of the stable nonfragmenting ions: the photon induced
decay as a function of optical wavelength. The ion decay is
determined by the absorption spectrum of the ion and the
fragmentation efficiency as a function of the internal energy
of the excited ion. Since the rate of photodissociation of
the ions is primarily determined by the rate of photon
absorption by the ions, this method has much in common with
conventional optical absorption spectroscopy, and provides an
interesting bridge between mass spectrometry and optical
absorption spectroscopy.
Spectroscopic studies of molecular ions have become a
challenging and an interesting area of study in recent years
(30,31). Unlike neutral molecules and ions in solution or in
the solid state, for which IR, UV-VIS and NMR spectroscopies
give useful structural information, there is no general method
to obtain structural information for gaseous ions. Although
infrared spectroscopy has been used on very small ions
(32,33), the applications for complex species have yet to be
realized. This limitation arises primarily from the
difficulty of obtaining a high enough density of ions of a
known mass in a small volume to yield a measurable absorbance,


13
In 1974 Comisarow and Marshall applied the principles of
Fourier transformation to ICR (53), which made it much more
suitable for solving analytical problems. The new name
Fourier transform ion cyclotron resonance (FTICR) mass
spectrometry was given to this technique. Since the release
of commercial FTICR mass spectrometers in the early 1980s,
analytical applicability of the technique has fully blossomed.
These analytical applications rely on many advantages of FTICR
including high mass resolution, high mass measurement
accuracy, capabilities of positive/negative ion detection,
and the ability of interfacing with a variety of ionization
techniques such as fast atom bombardment (FAB), secondary ion
mass spectrometry (SIMS), glow discharge (GD), field
desorption (FD), electrospray (ESI), matrix assisted laser
desorption (MALDI), and electron ionization (El).
Theory of Operation
The basic theory of FTICR mass spectrometry has recently
been explained in detail by Marshall and Grosshans (54). For
an ion to achieve a stable circular orbit, the centrifugal
force (Fx) and the Lorentz force (F2) acting on the ion should
be equal in magnitude. Equations 2.1 and 2.2 define the
forces Fi and F2, respectively.
Fi
mv2/r
(2.1)


141
photon, which requires many photons to be absorbed to bring
about IR photodissociation. However, a novel approach
developed in our laboratory, a "probe-pump'1 technique, using
a low power tunable pulsed C02 laser for probing the resonant
absorption spectrum and a more powerful nonresonant cw C02
laser to complete the dissociation has shown definite promise
for overcoming some limitations of the IRMPD technique.
The irradiation path inside the FTICR analyzer cell was
modified by using a well known multipass arrangement which was
first described by White. Subsequent modifications of the
cell led to fewer alignment problems. The newly-modified
multipass White-type FTICR cell dramatically enhanced the
photodissociation effects. The applicability of the two-laser
method and the use of the modified cell enabled reliable
spectra for cations of diglyme and 3-bromopropene and the
anion of Ga(hfac)3 to be obtained.
The corresponding gas-phase neutral spectra were ob
tained, and vibrational frequencies of both the ion and
neutral species were compared. Relatively small shifts in
frequencies were observed for both protonated diglyme and 3-
bromopropene ions, compared to their corresponding neutrals.
This can be attributed to the fact that upon formation of the
ion there is negligible change in the bonding orbitals that
govern the force constant for the C-O-C (diglyme) or C-Br (3-
bromopropene) stretch. On the other hand, Ga(hfac)3_ shows a
much larger shift because the electron added in formation of


30
25
20
15
10
5
0
(CH30D),H + nhv, -> (CHjOH)D
32 -> (CH,OD)D+
.> c2H70+
> CjHgOD*
940 960 980 1000
1020 1040 1060
WAVENUMBER (cm1)'
VO
CD
spectrum of proton bound deuterated methanol dimer cation (CH3OD)2H+.
estimates are 95% confidence limits.


9
gallium hexafluoroacetylacetonate are presented in chapter 3.
Spectra were obtained using one and two lasers and compared
with neutral spectra. For all the ions studied in this
dissertation the decomposition pathways are invariant to
change in laser wavelength, but the photodissociation yield
does depend on the wavelength. All photodissociation spectra
presented are limited to the tuning range of C02 lasers, 920 -
1060 cm'1.
In the past decade many chemists and physicists have
devoted themselves to the study of cluster properties,
structure and reactivites. In cluster chemistry it is
important to understand the transition from isolated atoms and
molecules to bulk materials. The study of successively larger
cluster ions is analogous to the process of solvation, one
solvent molecule at a time. A number of gas-phase studies
have investigated ions solvated with single neutral molecules
to begin following the transition from gas to solution phase
ionic behavior (42-44). In particular, solvated halide ions
have been studied due to their structural simplicity and their
existence in solution. In chapter 4, IRMPD spectra of
methanol and deuterated methanol solvated fluoride and
chloride anions, and the methanol solvated methoxy anion are
presented. In addition, IRMPD spectra of proton bound
methanol and deuterated methanol dimer cations are presented.
The shifts in the IRMPD peak frequencies are compared with the
respective neutral peaks.


94
(CH3)2OD+ + HDO (4.26c)
(CH3)2OH+ + D20 (4.26c!)
In this case photodissociation proceeds via four
decomposition pathways for (CH3OD)2H+. Protonated d-methanol
((CH3OD) H+, was the major fragment followed by the
deuteronated d-methanol [(CH3OD)D+], deuteronated dimethyl
ether [(CH3)2OD+], and protonated dimethyl ether [(CH3)2OH+].
Further confirmation, by ejecting the photoproducts as
discussed in the previous paragraph, suggested that the four
reaction pathways were independent of each other.
Deuteron Bound Deuterated Methanol Dimer Cation
(i) Electron beam pulse
D20 + e
(4.27)
(ii) Reactions
d2o+ + d2o
(4.28a)


Table 3.2.
The C-F stretching frequencies of A and E
inodes used in assigning peak frequencies for
the Ga(hfac)3 neutral and the anion.
Molecule
(cm x)
E (cm-1)
Reference
cf3-
1084
1252
39, 40
cf3+
1125
1667
40
c2f6
1116
1250
41
Ga(hfac)3
1151
1240
Ga(hfac) 3~
a
a
a
Cannot be assigned unambiguously; see explanation in text.


PHOTODISSOCIATION / %
Figure 3.7.
Two-laser infrared multiple photon dissociation spectrum of protonated
molecular ion of diglyme at a probe laser energy of 100 mJ pulse1 and a pump
laser energy of 1 J. Error estimates are 95% confidence limits.


82
(iii) Laser pulse
CH3OHF" + nhvir F~ + CH3OH (4.4)
To further check these results, CH3OHF and CH3ODF- were formed
in an alternative manner.
(i)Electron beam pulse
S02F2 + e" -* F" + S02F (4.5)
(ii)Reaction
F" + HC02CH3 -* CH3OHF" + CO (4.6)
(iii)Laser pulse
CH3OHF + nhvir F" + CH3OH (4.7)
Deuterated Methanol Solvate of the Fluoride Ion
(i) Electron beam pulse
CF
CF3
+
e
F"
+
(4.8)


Table 2.1.
Energy measurement for both pulsed and cw CQ laser beam outputs after
reflecting from each mirror and the ZnSe window, using the newly-modified
FTICR cell. The distances for each mirror and for the ZnSe window from the
laser head are also presented.
Position
Energy (mJ pulse"1)
Distance (inches)
pulsed C02
cw C02
pulsed C02
cw C02
Laser Head
853
850
-
-
Mirror 1
722
750
27
47.5
Mirror 2
518
624
107
90
Mirror 3
507
568
170.5
133
ZnSe Window
348
398
183.5
146
Spherical mirror
(mirror 4)
103
290
216
179


Conclusions
We have demonstrated that ESI/FTICR mass spectrometry can
be used successfully to study host-guest chemistry. Unlike
CID experiments, where fragmentation of the crown ether is
observed, IRMPD proceeds through lowest energy fragmentation
pathways, which most often involve loss of an alkali metal
ion. In addition, the bond specific dissociation nature of
the IRMPD technique could be used as a tool to identify
different fragmentation patterns within a molecule.
Photodissociation of H30+-crown ether complexes follows
a similar pathway to the protonated crown ether. The fragmen
tation occurs via losses of (C2H40) structural units. This
indicates that crown ether complexation with H30+ instead of
an alkali-metal ion results in very different types of bond
activation and binding interactions. Comparison of IRMPD
results for Na+-crown ether and K+-crown ether complexes
suggests that Na+ is bound to the crown ether more strongly
than K+.


7
thus leading to reliable spectra. However, when the energy of
irradiation is enough to cause to dissociation, an indirect
method, such as ion photodissociation, can be utilized to
obtain reliable spectra and structural information for gaseous
ions.
Infrared multiple photon absorption leading to
dissociation has been shown to be a widespread phenomenon
since it was first observed by Isenor and Richardson (34) It
was subsequently demonstrated that the sequential absorption
of many photons by a single molecule can occur, even in the
absence of collisions, and that this process occurs in a large
number of molecules and ions (35). Since then the realm of
infrared spectroscopy of gas-phase molecular ions has become
an area of considerable interest for obtaining the vibrational
frequencies of gaseous ions (36,37), through the information
available from IRMP dissociation. Although a large number of
cations have now been studied (37), only a few studies have
been performed to extend the technique to include molecular
anions (38).
Woodin et al. demonstrated the feasibility of IRMPD with
a relatively low-power C02 laser (39). In addition, Wight and
Beauchamp used a low-power (20 W/cm2) continuous wave (cw) C02
laser to obtain infrared multiphoton induced electron
detachment spectra to distinguish between three C7H7 isomers
(40) In 1983, Honovich and Dunbar obtained photodissociation
spectra with both an infrared (IR) laser and a visible laser


Relative Intensity Relative Intensity
132
(15-crown-5)2K+
ESI/FTICR mass spectra obtained with equimolar
mixture of 15-crown-5/12-crown-4/NaCl/KCl in
49:49:2 methanol:water:acetic acid solution,
(top) without laser irradiation, and (bottom)
with laser irradiation at 10.60 pm.
Figure 5.10.


TABLE OF CONTENTS
page
ACKNOWLEDGMENTS .. iii
LIST OF FIGURES vii
LIST OF TABLES X
ABSTRACT xii
CHAPTERS
1 INTRODUCTION 1
2 THEORY AND INSTRUMENTATION 12
FTICR Mass Spectrometry 12
Development and Background 12
Theory of Operation 13
Theory of IRMPD 19
IRMPD/FTICR Mass Spectrometry 26
Electron Ionization Experiments 26
Electrospray Ionization Experiments ... 28
The FTICR Cell 29
Multipass Process 29
Modified White-Type Cell 31
Newly-Modified White-Type Cell 33
FTICR Pulse Sequence 42
3 INFRARED MULTIPLE PHOTON DISSOCIATION
SPECTRA OF GASEOUS IONS 45
Introduction 45
Pi^be-Pump Technique 46
Expeimnenyal 49
One^and Two-Laser Experiments 50
FTICR Pulse Sequence 52
Results 54
Discussion 58
v


60
Figure 3.
50
40-
O
h-
<
O
o
C/)
cn
Q
O
h-
o
30-
20-
q. 10
0
Ln
Ch
934 936 938 940 942 944 946 948 950 952 954
WAVENUMBER / cm-1
One-laser infrared multiple photon dissociation spectrum of protonated
molecular ion bis (2-methoxyethyl) ether (diglyme) at a probe laser energy of
1 j pulse-1. Error estimates are 95% confidence limits.


CHAPTER 1
INTRODUCTION
Mass spectrometry is unique among the techniques
available for analyzing molecules. In mass spectrometry,
molecules are ionized first and then these ions are
subsequently examined in detail. Mass detection principles
and the instrumentation needed to perform experiments are very
simple. Mass analysis in mass spectrometry is usually
accomplished with one of five basic types of instrument:
magnetic sector, quadrupole, time-of flight, ion cyclotron
resonance, and ion trap. These instruments and their
limitations have been discussed in detail (1) The uniqueness
of the molecular structure information available from mass
spectrometry is well known. One of the most important mass
spectrometric methods for additional structural information is
high mass resolution. Fourier transform ion cyclotron
resonance (FTICR) mass spectrometry has already established
its ability to give routinely higher mass accuracy and mass
resolution than other mass spectrometric methods.
In addition to its high mass resolution capability, FTICR
mass spectrometry benefits from the ability to trap ions for
very long periods (2) accurate mass measurements (3) and the
capability of performing collisional activation at low
1


19
perfluorotributyl amine, [PFTBA]), of a known composition is
introduced into the FTICR cell, and a table is produced that
matches the measured frequencies of the ions to their known,
exact masses. The resulting calibration table is then used to
convert the frequency domain spectrum for all other ions which
may be subsequently studied. As an example, the time-domain
spectrum (digitized image current) of ions formed following 50
eV electron ionization of diglyme (1.0 x 10~6 Torr) is shown
in Figure 2.2, and the corresponding mass spectrum is shown in
Figure 2.3.
Ultra high resolution, particularly at low mass, is
readily achieved with FTICR mass spectrometry. The limiting
factor in achieving high resolution measurements is damping of
the coherent motions of the excited ion packets due to
collisions with neutral species in the cell. For example,
when the pressure in the cell is ca. 1.0 x 10-6 Torr, the time
required for the intensity of the time-domain signal to fall
to baseline is ca. 8 ms (Figure 2.2). If the pressure in the
cell is in the low 10~9 Torr, the time-domain signal lasts for
several seconds. All the experiments presented in this
dissertation were performed under low 10-7 -10-8 Torr pressure
conditions. Since high mass resolution was not required in
these experiments, the time-domain signals were collected for
< 8 ms.
Theory of IRMPD
It is well known that absorption of infrared light causes
vibrational transitions within a molecule. It is also


Figure 2.1
MACOR
Excite plate
Front trap plate
FILAMENT
Back trap plate Receive plate
Receive plate
Excite plate
An expanded three-dimensional view of a typical z-axis elongated FTICR
analyzer cell.


110
The IKMPD spectra of methanol proton-bound dimer cations
also showed some interesting features. Addition of a second
methanol molecule to form the proton bound dimer cation
distributes the charge between both methanol moities. Upon
protonation, rehybridization at oxygen to add p-character to
the newly formed 0-H+-0 bond in proton bound dimer or 0-D+-0
in deuteron bound dimer results in increased s-character in
the C-0 bond. This will definitely result in shifts in C-0
stretching frequencies in the IRMPD spectrum. The spectrum of
(CH3OH) 2H+ has only one intense absorption peak at 1039 cm1.
However, both (CH3OD)2H+ and (CH3OD)2D+ have two peaks in the
1040 1060 cm'1 region. Peak frequencies are 1039 cm'1, 1045
cm1 for (CH3OD)2H+, and 1045 cm1, 1050 cm1 for (CH3OD)2D+. It
is clear that in both deuterated spectra there is a shift to
higher transition compared to that of fully hydrogenated
dimer cation.
IRMPD spectra for alcohol proton bound dimer cations have
not been previously reported except for one study. Bomse and
Beauchamp obtained narrow band (900 1100 cm'1) IRMPD spectra
for n- (C3H7OH) 2H+, and i-(C3H7OH)2H+ (119), but negative results
were obtained from Beauchamp's study because neither species
showed any change in the overall appearance of the spectra.
On the other hand, LaCosse and Lisy (120) and Huskien and
Stemmier (121) obtained IR spectra for methanol and d-methanol
neutral clusters utilizing molecular beam techniques. Both
experiments revealed two transitions for the dimers. In


CHAPTER 5
INFRARED MULTIPLE PHOTON DISSOCIATION
OF CROWN ETHER COMPLEXES
Introduction
Electrospray ionization (ESI) mass spectrometry has
undergone tremendous growth since its introduction by Dole et
al. (46). In 1984, Yamashita and Fenn revived (47) Dole's
idea and showed its applicability when coupled to a quadrupole
mass analyzer. This "soft ionization technique has been
applied to a variety of compounds ranging from organometallic
complexes (122) to high molecular weight peptides (123). The
unique ability of the ESI process to produce ions with high
charges distinguishes the technique from other ionization
methods. This multiple charging process allows high molecular
weight ions to be detected in the range of m/z 500 to 2000.
The coupling of electrospray ionization with Fourier
transform ion cyclotron resonance (FTICR) mass spectrometry
can exploit the high mass resolution and high mass accuracy of
this technique. The high mass resolution capability of
ESI/FTICR mass spectrometry has been demonstrated by Winger et
al. (124) and Beu et al. (125), and the high mass accuracy by
Watson et al. (126). ESI/FTICR mass spectrometers used for
their studies were based upon an external source design. In
114


115
the external source ESI/FTICR ions are formed outside the
magnetic field and guided with quadrupole rods or
electrostatic lenses into an analyzer cell. Hofstadler and
Laude (66) demonstrated an alternative approach, which
positions the ESI source inside the high magnetic field of the
FTICR mass spectrometer. The ESI source used in this
experiment was built in our laboratory based on Laude's design
(67) .
In addition to features such as high mass resolution and
mass accuracy, the ability to perform collision-induced
dissociation (CID) (127), seguential MS-MS (128), and
photodissociation (129) of large molecules enhances the
analytical utility of FTICR mass spectrometry.
Coupling ESI with IRMPD makes available a new
fragmentation tool for multiply-charged high molecular weight
ions. Unlike CID, which is used widely to obtain gas-phase
structural information for ions, IRMPD eliminates the
requirement of a collision gas in the FTICR cell and improves
mass resolution due to the lower pressures involved. In
addition, photodissociation involves the ions which are
axialized and in the center of the cell. Thus, lowest energy
fragmentation pathways are obtained with photodissociation
experiments compared to CID experiments (37,130). The
relatively slow excitation process in IRMPD experiments leads
to dissociation via the channel of lowest activation energy.
In CID, a range of internal energies is imparted to an ion


Table 4.1.
Gaseous ion vibrational frequencies from IRMPD studies.
IRMPD Spectrum
Peak Frequencies (cm-1)
CH30HF'
971 980 1047
CH3ODF-
949 971 977 1043 1047
CH30HCr
948 971 977 1041
CH30DC1'
953 971 980 1041 1047
CH3OHOCH3
979 1047
(CH3OH) 2H'
1039
(ch3od) 2h*
1039 1045
(CH3OD) 2d*
1045 1050
102


34
Figure 2.8 shows the newly-modified White-type FTICR cell
which was used for the IRMPD experiments in chapters 4 and 5.
The cell dimensions were 2.5, 3.5, and 6.5 cm along the x, y,
and z magnetic field axes, respectively. Three spherical,
concave well-polished stainless steel mirrors were
incorporated into the receive plates. One of the mirrors was
4.5 cm in length and 0.95 cm in width with a 3.1 cm radius of
curvature. The two opposite mirrors, which were identical in
size, had 0.63 cm diameter and 3.1 cm radius of curvature.
The three mirrors were positioned in such a manner to allowed
the laser beam to pass 16 times inside the cell. The solid
excite plates were replaced with stainless steel mesh to
facilitate the laser alignment. A gold-coated turning mirror
(2.5 cm x 2.5 cm) was attached to one of the MACOR (73)
spacers holding the cell plates. The turning mirror allows
the laser light to reflect into the cell through a 1.5 cm o.d.
hole in one of the MACOR spacers holding the cell plates.
Then the laser beam was subjected to multipasses inside the
cell, as shown in Figure 2.8.
To demonstrate the photodissociation efficiencies of the
newly-modified White-type FTICR cell (sixteen vs. two passes) ,
protonated diglyme ion was formed using El, isolated, and
photodissociated. Both multipass and double pass arrangements
were used and the photodissociation mass spectra were obtained
at 10.60 jum output of the cw C02 laser by keeping the energy
constant at 500 mJ. For the double pass laser experiments,


help with experimentation, friendship and encouragement was a
relief from the everyday struggles of graduate school. A
special thank you goes out to all my relatives and friends in
Sri Lanka and in the States, who have selflessly provided
diverse forms of support along the way.
I am indebted to my research advisor, Dr. John Eyler, for
providing the opportunity and assistance I required in turning
a distant dream into reality. I also wish to extend my
deepest appreciation for his financial support, patience, and
helpful criticism throughout the experimental work, and
superior editing skills for the preparation of this
dissertation. He deserves a special thank you for funding my
trip to Brazil, which was a significant experience in the
development of this research.
It goes without saying that this work would never have
been attempted, much less completed, without the support,
sacrifices and encouragement of my parents and family. I
would not have reached the point I am today without the love
of my late father Dr. Chandrarathna Patuwathavithana, mother
Dorothy, brother Cha tur a Vithana, sister-in-law Sur am, husband
Asoka Peiris and especially my son Eshal. I hope someday that
Eshal will understand why Mommy could not be there more to
watch him grow.
iv


29
The FTICR Cell
The first FTICR experiments was done using a cubic cell
(52), and the method was later expanded to several other ion
trap geometries including orthorhombic (68) cylindrical (69) ,
hyperbolic (70), and open (71). For the experiments presented
in this dissertation a standard z-axis elongated rectangular
FTICR cell (Figure 2.1) was modified for IRMPD experiments
using White's demonstration (72), to obtain long optical
paths within a small volume.
Multipass Process
A schematic illustration of the set-up used by White to
increase the optical path length for irradiation experiments
is shown in Figure 2.6. The essential parts of the set-up are
three spherical, concave mirrors, each of which has the same
radius of curvature. Light enters through a slit close to one
end of mirror B, then it passes to mirror A, from mirror A to
mirror B, then to mirror A', then back to mirror B, and then
again to mirror A. Alternatively, the light reflects back and
forth between mirrors B, A', and A. The most important
adjustment is the separation of the centers of curvature of
the mirrors A and A'. The ratio of the length of mirror B to
the separation of centers of curvature of mirrors A and A'
determines the number of times the light goes through the
cell. This can be either four times for one image on B, eight


5
During the past 20 years the technique of
photodissociation has been applied to different kinds of
problems in gas-phase ion chemistry. Dunbar (26) first
introduced the application of technique to study CH3C1+ in an
ICR mass spectrometer. Moreover, studies of photodetachment
of electrons from negative ions by Meyer and coworkers and
coworkers (27) have yielded electron affinities and related
thermochemical data important in describing the intrinsic
acidity of organic and inorganic molecules. Until 1982, PD
studies of trapped ions were limited to ions in ICR cells. In
1982, Hughes et al. (28) and Louris et al. (29) showed that
photodissociation could be coupled with quadrupole ion storage
and ion trap mass spectrometry, respectively.
Photodissociation can be used in many different ways to
obtain: (a) photon-induced fragment mass spectra, by comparing
mass spectra obtained with and without irradiation; (b) a
photodissociation spectrum parent ion decay or fragmentation
efficiency is monitored as a function of irradiation
wavelength; (c) isomeric differentiation, ion fractions in
mixtures of isomeric ions can be determined by variation of
the trapping/irradiation time; and (d) dissociation of ions
with photons having an energy below the dissociation threshold
photodissociation by absorption of two visible photons, or by
sequential absorption of infrared and visible photons, or by
absorption of multiple infrared photons (IRMPD).


138
producing (15-crown-5)Na+ (m/z 243), (15-crown-5)K+ (m/z 259),
C4H902+, C2H502+, K+, and (12-crown-4 )2Na+. The crown ether
complexes bound to H30+ and H+ dissociated to give fragments
due to (C2H40) nH+ units (where n = 1 and 2) All sandwich-type
complexes (except [12-crown-4]2Na+) dissociated to give the
respective alkali-metal bound monomer complexes. The alkali-
metal bound crown ether complex dissociated primarily via the
loss of the metal from the complex.
The loss of Na+ from the Na+-crown ether complexes was
not observed under infrared multiphoton dissociation condi
tions. In contrast, formation of K+ was observed with all
crown ether complexes. This implies that the binding of Na+
to the crown ether cavity is stronger than that of K+. Gas-
phase equilibrium constants for transfer from 18-crown-6 to
21-crown-7 have been determined for a number of metal cations,
by Chu et al. (153) These investigations indicate that Na+
is more strongly bound to 21-crown-7 than K+. In addition,
interactions of Na+ and K+ with crown ethers have been
investigated by several theoretical studies (154). These
theoretical studies found K+-crown ether complexes to be less
stable than Na+-crown ether complexes. This was explained by
the metal-oxygen bonds present in crown ether complexes. When
the size of the metal ion increases, the metal-oxygen bond
lengths increase and weaken the binding interactions. Thus,
K+ is dissociated from the crown cavity much easier than the
Na+ (154a).


25
eventually the separation of states is less than the laser
bandwidth. This stage is defined as the quasicontinuum. In
the quasicontinuum dissociation radiation of any frequency can
be readily absorbed. Above this regime the dissociation rate
constant is expected to increase with increasing internal
energy and the ion will dissociate.
Both pulsed and cw C02 lasers were used in this
dissertation. The pulsed laser was a Lumonics Model TE 860
C02 laser (60) with a rectangular beam profile of 2 cm x 2.75
cm, and a resolution of ca. 2 cm-1. The pulsed laser wasline
tunable over a wavelength range of 9.10-10.92 /urn. The cw
laser was an Apollo Model 570 C02 (61) laser which could be
line tuned over a wavelength range of 9.0-11.3 /xm, with a beam
profile of 0.8 cm-2, and a resolution of ca. 2 cm-1. Beam
profiles were measured in front of the laser head. It was
necessary to measure the wavelength accurately when obtaining
the IRMPD spectra. This was done by using an Optical
Engineering Model 16A C02 laser spectrum analyzer (62)
equipped with an infrared grating which can be obtained over
a wavelength range of 9.1-11.3 /Ltm. It was necessary to keep
the laser energy or power constant throughout the wavelength
scan of the IRMPD experiments. To achieve this, a Scientech
Model 365 power meter along with a thermopile detector (63)
was used. This detector had a calibrated head with an
absorbing surface for infrared light. Heating of the
absorbing surface by the laser produced an electrical output


136
for protonated 18-crown-6, 15-crown-5, and l2-crown-4 (150).
In our IRMPD studies of (18-crown-6)M+ (M = Na, K or H30+), a
fragment ion due to (C2H40)2H+ was observed in IRMPD mass
spectra at all three wavelengths. In addition, C7H1203H+ was
observed at 10.60 and 10.58 /urn laser irradiation. The absence
of (18-crown-6)H30+ after laser irradiation suggests that
(C2H40)2H+ is formed from (18-crown-6)H30+. In contrast,
irradiation of the alkali-metal bound crown ether complexes
results in dissociation of the bare alkali-metal ion from the
complex, suggesting that the crown ethers retain their cyclic
structure after the laser irradiation. The observation that
all three wavelengths do not result in the same fragmentaion
pattern indicates that IRMPD dissociation is wavelength
dependent. Unlike CID and SID methods which are not bond
specific, the IRMPD technique can be used to selectively
dissociate bonds.
To further clarify our speculations as to the dissocia
tion pathways of alkali-metal/crown ether complexes, 15-crown-
5 was electrosprayed with NaCl and KC1. All ions were ejected
from the FTICR analyzer cell except (15-crown-5)Na+ and (15-
crown- 5) K+ (Figure 5.9a). IRMPD at 10.60 /m resulted in the
dissociation of (15-crown-5)K+ to give K+. Absence of
(C2H40)2H+ from the IRMPD spectrum further confirms that
dissociation of alkali-metal/crown ether complexes proceeds
through loss of the metal ion from the complex and with the
crown ethers most likely retaining their cyclic structures.


157
153. Chu, I. H.; Zhang, H.; Dearden, D. V. J. Am. Chem. Soc.,
1993, 215, 5736.
154. (a) Michaux, G.; Relsse, J. J. J. Am. Chem. Soc., 1982,
104, 6895. (b) Mazor, M. H.; McCammon, J. A.; Lybrand,
T. P. J. Am. Soc., 1990, 112, 4411. (c) Wipff, G. ;
Weiner, P.; Koliman, P. J. Am. Chem. Soc., 1982, 104,
3249.


Relative Intensity Relative Intensity
131
CH "
fScT
306
500
m/z
T' 1 |
700
900
100-,
50-
+
+
rt
Z
in
c
3
o
u
u
in
+
in
c
$
o
u
o
I
in
A* #'* /
' 10
300
. r'-'-rr,p-.[ I" 1" " I'
500
m/z
700
I
900
ESI/FTICR mass spectra obtained with 15-crown-
5/NaCl/KCl in 50:50 methanol:water solution,
(top) without laser irradiation, and (bottom)
With laser irradiation at 10.60 pm.
Figure 5.9.


Figure
illustrated only for the cw C02 laser, but a pulsed C02 laser could be used
to obtain the multipasses inside the FTICR cell, using mirror 1' instead of
mirror 1.


83
(ii) Reaction
F" + DC02CH3 CH3ODF~ + CO (4.9)
(iii) Laser pulse
CH3ODF" + nhvir F" + CH3OD (4.10)
Methanol Solvate of the Chloride Ion
(i) Electron beam pulse
C1C02C2H5 + e" C1C02~ + C2H5 (4.11)
(ii) Reaction
C1C02_ + CH30H -* CH30HC1" + C02 (4.12)
(iii) Laser pulse
CH30HC1~ + nhvir -* Cl~ + CH3OH (4.13)


trap plat
electron
beam
screened excite plate
(fits above mirrors)
1/ receive plate
Macor
turning mirror
brass mirrors
non-resonant
laser
resonant laser
u>
K)
Figure 2.7. An expanded three dimensional view of the modified White-type FTICR analyzer
cell used for one- and two-laser experiments to obtain IRMPD spectra,
presented in chapter 3. The three spherical mirrors used to create
multipasses were incorporated in to the stainless steel receive plates. The
dotted-lines illustrate both multipasses (eight passes) and the center pass
of the laser beam.


96
deuteron bound d-methanol diner. The most dominant peak was
deuteronated d-methanol [(CH3OD)D+] and both dissociation
pathways (reactions 4.28a and 4.28b) were independent from
each other.
All three dimer cations yielded more than one set of
products from multiple photon dissociation. These could have
been observed due to a rearrangement in the transition state
with a sufficient energy release so that more than one
reaction channel might be accessable. Previously, multiphoton
dissociation of several proton bound dimers of alcohols and
ethers have been studied and more than one set of dissociation
products were obtained (31,114). Unfortunately, proton bound
methanol dimers were not studied, and direct comparison of
products is not possible. The observed branching ratios are
invariant to change in wavelength (920 -1060 cm'1), but a
systematic study of the effects was not attempted.
The total energy was kept constant as the laser was tuned
to various wavelengths. The percent photodissociation
(calculated as discussed in chapter 3) as a function of laser
wavelength was obtained for each of the three cations. The
IRMPD spectra of (CH3OH)2H+, (CH3OD)2H+, and (CH3OD)2D+ are
depicted in Figures 4.8, 4.9, and 4.10, respectively.
For comparison, gas-phase neutral IR spectra were
obtained for methanol and d-methanol and are shown in Figures
4.11 and 4.12, respectively. Table 4.1 summarizes the IRMPD
peak frequencies for all the ions obtained from Figures 4.3 -
4.10. The gaps between 958 966 cm'1 and 980 1038 cm'1


analyzer cell has helped to overcome some limitations of this
technique, including limited tuning and/or low output power in
one-laser experiments.
IRMPD spectra in the 934-1055 cm'1 range have been
obtained for the protonated molecular ion of digyme, the
positive molecular ion of 3-bromopropene, and the negative
molecular ion of gallium hexaf luoroacetylacetonate, using one
and two lasers. Comparisons between the ion spectra and those
of the corresponding neutral species are made.
Further modifications of the White-type FTICR cell
dramatically enhanced the photodissociation effects and
enabled IRMPD spectra for methanol solvated anions and proton
bound methanol dimer cations to be obtained in the 920-1060
cm'1 region. IRMPD pathways for proton bound methanol dimer
cations were also observed. The neutral gas-phase spectra of
methanol and d-methanol were compared with the corresponding
IRMPD spectra.
Crown ether complexes formed in methanol/water solutions
were subsequently transported into the gas phase using a home-
built electrospray ionization source. Complexes of 18-crown-
6, 15-crown-5, and 12-crown-4 with Na+, K+ and H30+ were
observed. The techniques of IRMPD were used to obtain
fragmentation and binding information. Comparison of IRMPD
mass spectra for crown ether complexes indicated a difference
in the binding of H30+ to crowns compared to Na+ and K+, and
that the Na+ is bound to the crown ether more strongly than
K+.
xiii


120
shutter mounted on this plate reduced the background pressure
to 3.0 x 10"8 Torr during the excitation and detection events
(67b). A TTL pulse from the Ion Spec data station triggered
the shutter to be open for ion injection from the source into
the cell. The pressure in the cell region was 2 x 10'7 Torr
during this period.
Finally, the entire assembly was inserted into a 15 cm
o.d. vacuum chamber which was separated from the FTICR cell by
another 4 mm conductance limit. The ESI source side of the
second conductance plate was pumped by two 300 L/s diffusion
pumps and the pressure in this region was maintained at 10'7
Torr. The cell side of the second conductance plate was
pumped by a 700 L/s diffusion pump and the pressure was
maintained at 3.0 x 10'* Torr during electrospray operation.
When the 1.88 cm probe was inserted into the strong magnetic
field, the electrospray ionization process occured 24 cm from
the FTICR cell.
For all IRMPD experiments presented in this chapter the
modified FTICR cell depicted in Figure 2.9 was used. As shown
in Figure 4.1 the cw C02 laser beam was turned into the cell
by the turning mirror and underwent to 16 passes inside the
cell by the three spherical mirrors.
ESI/FTICR Pulse Sequence
The pulse sequence utilized in these experiments is
illustrated in Figure 5.2. A quench pulse was initially
applied to the trap plates to eject any ions present in the
FTICR analyzer cell. Then the ions were allowed to enter the


145
The major drawbak for both of these approaches is the low
energies per pulse (especially the IRP package) associated
with them. However, with the use of the probe-pump technique
dicussed in chapter 3, the pulsed C02 probe laser could be
replaced by either the 0P0 or the IRP package. Even though
the energies are low, satisfactory results could be obtained
given two-laser photodissociation produced in that study. Use
of this probe laser should permit us to obtain infrared
spectra of gaseous ions which contain 0-H, N-H, and C-H
stretching modes.
Another crucial feature is the alignment of the laser
beam in order to obtain maximum photodissociation
efficiencies. In particularly, the position that the laser
beam hits on the turning mirror attached to the cell, will
mostly govern the efficency of the multipass process, and is
very critical. Also, the alignment of the laser beam for this
process was a rather difficult and a time-consuming process.
Therefore, an extremely important extension of this work is to
modify the alignment procedure to enhance the
photodissociation efficiencies. An infrared optical fiber
enclosed within a stainless tube could be inserted into the
vacuum chamber, next to the turning mirror on the FTICR cell,
and the laser beam could be focussed efficiently from the
other end of the fiber to circumvent the problems associated
with the alignment.


Figure 2.3. The Fourier transformed (frequency domain) ion cyclotron resonance mass
spectrum of diglyme cations resulting from the time domain signal observed in
Figure 2.2.


135
complexes of H30+ to 18-crown-6 are favored by -12 kcal/mol
relative to 15-crown-5 complexes but the dissociation patterns
of (18-crown-6)H30+ and (15-crown-5)H30+ were not documented.
Dearden et al. (148) obtained gas-phase equilibrium constants
for transfer of some ammonium ions and H30+ from 18-crown-6 to
21-crown-7, utilizing FTICR techniques. The results indicated
that, as the steric bulkiness of the ammonium ions increases,
transfer from 18-crown-6 to 21-crown-7 decreases and NH4+ has
the highest equilibrium constant (greatest tendency for trans
fer) However, the sterically smallest guest, H30+, contra
dicted the observations, and had the lowest equilibrium
constant. The reason for the unusual behavior was explained
by the type of bonding interactions involved in the (18-crown-
6)H30+ complex. Both ligand and H30+ have C3 symmetry axes,
which allow the formation of three linear hydrogen bonds.
These findings support our results that the (18-crown-6)H30~
dissociates in a different manner when compared with (18-
crown-6)K+ or (18-crown-6)Na+, indicating that H30+ binding
interactions to the crown ethers are different than those of
the alkali-metal ions.
Lee and Allison studied the protonated complexes of 21-
crown-7, 18-crown-6, 15-crown-5, and 12-crown-4 using electron
ionization (149). The mass spectra contained (C2H40)4H+ (only
for 21-crown-7), (C2H40)3H+, (C2H40)2H+, and (C2H40)H+, corre
sponding to the successive loss of (C2H40)n (where n = 1-3)
units from the crown ethers. Similar results were obtained


CHAPTER 3
INFRARED MULTIPLE PHOTON DISSOCIATION
SPECTRA OF GASEOUS IONS
Introduction
Many different kinds of ions are observed in mass
spectrometry, and these are the result of a variety of
ionization and fragmentation processes. These processes have
been studied experimentally for many years, but for most of
the ions little is known of the structures and energy states.
Optical absorption spectroscopy is a very powerful technique
for obtaining such information in the gas phase. However it
is not practicable, except in very few cases of simple di- and
triatomic species to obtain a direct absorption spectrum for
molecular ions (75,76). One is most often forced, then, to
utilize an indirect method, such as ion photodissociation, to
obtain spectra and structural information for gaseous ions.
In ion photodissociation, an ion absorbs one or more
photons until it gains sufficient energy to dissociate into
fragments. The disappearance of parent ion or the appearance
of the fragment ions as a function of irradiation wavelength
can then lead to a photodissociation spectrum of the parent
ion under favorable conditions (77). This approach has
exhibited reliable results for interpretation of energy
45


108
introduced by the negative charge at oxygen and secondly from
the effect of increased mass.
As shown in Figure 4.3 and Table 4.1, absorption peaks
from the IRMPD spectrum of CH3OHF were observed at ca. 970,
979, and 1046 cm1. However, vibrational frequencies in the
900 cm'1 region were not predicted in Wladkowski's study. As
listed in Table 4.2, the C-0 stretch of CH3OHF is calculated
to be at 1100 cm 1, which is 64 cm1 blue shifted from that of
neutral CH30H, and the OHF linear bend is predicted to lie at
1226 cm1. In addition, the 0-H stretch in CH3OHF' is red
shifted by ca. 1800 cm'1 from that of the CH3OH. Since the O-H
stretch has been predicted to have big shift, it is possible
that other frequencies in the CH30HF are in error. By the
same token, it is possible that the C-O stretch and the OHF'
bend in the CH3OHF could be observed at lower frequencies than
their calculated value. Therefore, peak frequencies observed
in the IRMPD spectrum (Figure 4.3) at 1046 and 990 cm'1
respectively, could correspond to the C-0 stretch and OHF' bend
calculated by Wladkowski and co-workers at 1110 and 1226 cm'1,
respectively.
Observation of the IRMPD spectra for CH3OHF and CH30HC1'
indicates that there are no apparent changes in the overall
appearance of the spectra except in one region. In the 920-
960 cm'1 region, CH3OHF has no absorption whereas CH30HC1' has
one absorption peak at ca. 947 cm'1. Also, the IRMPD spectra
of both CH30DF and CH3ODCl' have two peaks in the 1038-1060 cm'1


11
ESI source to a 2 Tesla FTICR mass spectrometer. ESI allows
multiply charged ions to be probed using IRMPD techniques to
obtain spectra, which are inaccessible with other ionization
techniques.
In chapter 5, ESI was used to produce several host-guest
crown ether complexes such as [crown-Na]+, [crown-K]+, and
[crown-H30]+ (where crown is 18-crown-6, 15-crown-5, and 12-
crown-4). Crown ethers are a special class of macrocylic
molecules which are recognized for their ability to form host-
guest complexes (48). Gas-phase IRMPD studies of these
complexes are presented in this chapter. Finally, conclusions
of the studies and future work are presented in chapter 6.


Figure 3.9. Two-laser infrared multiple photon dissociation spectrum of negative
molecular ion of Ga(hfac)3 at a probe laser energy of 100 mJ pulse-1 and a
pump laser energy of 1 J. Error estimates are 95% confidence limits.


15
applying a charge to the trapping-plates of the same sign
(positive or negative) as the ions to be trapped. The
equation of motion in the presence of both electric (E) and
magnetic (B) fields is governed by Eq. 2.6:
mdv/dt = q[E + v x B] (2.6)
When E=0, the solution of Equation 2.6 is given by Equation
2.5. Presence of a trapping electric field along with the
magnetic field produces two independent motions in addition to
circular cyclotron motion (coc) They are harmonic trapping
motion in the z-direction between the trap plates and a
circular magnetron motion in the radial direction (54). Since
trapping oscillation and magnetron motion occur with periods
much longer than the cyclotron motion, the previous two
motions are ignored in most of the mathematical treatments of
ion motion for simplicity. Cyclotron frequency shifts caused
by the trapping potential and a modified FTICR cell to reduce
cyclotron frequency shifts have been investigated by Wang and
Marshall (55), among others
For the experiments presented in chapters 3 and 4, ions
were formed, detected and analyzed within a single analyzer
cell located in a vacuum chamber, nominally at 10"8 10~9
Torr, within the solenoid of a 2 Tesla superconducting magnet.
A typical z-axis elongated FTICR cell is shown schematically
in Figure 2.1. The two opposing "trap" plates


PHOTODISSOCIATION
Figure 4.7
WAVENUMBER (cm1)
P95tPcon£enoeeimints.SOlVated methXy l0n (CH3OHCH3') Error estimates


I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality, as
a dissertation for the degree of Doctor of Philosophy.
L
Johi&/ R. Eyler, Chairman
Professor of Chemistry
I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality, as
a dissertation for the degree of Doctor of Philosophy.
'^THjdku
Martin T. Vala
Professor of Chemistry
I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality, as
a dissertation for the degree of Doctor of Philosophy.
Willis B. Person
Professor of Chemistry
I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality, as
a dissertation for the degree of Doctor of Philosophy.
David E. Richardson
Professor of Chemistry


Table 4.2 : Some Bond Lengths and Vibrational Frequencies of CH3OH and CH3OHF~
Species
Bond Length ()
Vibrational Frequencies (cm-1)
C-0
O-H
C-0 Stretch
0-H Stretch
OHF" Bend
CH30Ha
1.4161
0.9526
1033
3681
CH30Hb
1.421
0.963
1033
3681
CH30HF
1.3872
1.0432
1100
1909
1226
a Calculated values from Reference 15.
b Experimental values from reference 15 and references therein.
107


To my son, mother, and in loving memory of my father.


screened excite plate receive plate
back trap plat
MACOR
front trap plate
laser beam
stainless steel mirrors
MACOR
receive plate Au turning mirror
u>
Figure 2.8. An expanded three dimensional view of the newly-modified White-type FT ICR
cell used to obtain IRMPD spectra presented in chapters 4 and 5. All
experiments were perfomed using the multipass arrangement (16 passes) and the
laser path is illustrated by the dotted-lines.


113
spectra with the respective neutrals. Theoretical
calculations of the vibrational frequencies of methanol
solvated anions and methanol proton bound dimer cations will
assist in unambiguous assignment of these peaks and hopefully
reveal many details about the structures of these ions.


36
the laser beam entered the vacuum chamber through the center
of a ZnSe window mounted on a three-window flange as shown in
Figure 2.9. Then, the laser entered from the front trap plate
(facing the window) and reflected from the back trap plate to
traverse the cell a second time. Next, similar IRMPD
experiments were performed using multipass laser irradiation.
As shown in Figure 2.10, the laser light was reflected from
three gold-coated mirrors before entering the ZnSe window.
After entering the ZnSe window, the beam was reflected once
more from another gold-coated mirror before starting the
multipass process. Figures 2.11(a) and 2.11(b) show the IRMPD
mass spectra obtained with a double pass and 16 passes,
respectively. To further demonstrate the photodissociation
efficiencies, experiments with protonated diglyme cation were
carried out in a similar manner but increasing the laser
energy to 750 mJ. IRMPD mass spectra from double pass and
multipass arrangements are shown in Figures 2.12(a) and
2.12(b), respectively.
The calculations for the percent photodissociation of
protonated diglyme cation will be discussed in detail in
chapter 3. The peak intensities of the parent (m/z 135) and
photofragment ions (m/z 103 and 59) (figures 2.11 and 2.12)
were used to calculate the photodissociation efficiencies.
The results obtained for the multipass IRMPD experiment
(Figure 2.11(b)) indicate that the amount of photodissociation
is increased by ca. 60% when compared with that of double pass


123
Series of experiments were performed using equimolar (1 X 10~5
M) mixtures of crown ethers and alkali-metal salts: 18-crown-
6/NaCl/KCl in 50:50 (v:v) CH30H :H20; 18-crown-6 in 49:49:2
CH30H:H20:CH3COOH; 15-crown-5/NaCl/KCl in 50:50 CH30H:H20; and
15-crown-5/12-crown-4/NaCl/KCl in 49:49:2 CH30H:H20:CH3COOH.
The solution was delivered to the syringe needle through
Teflon tubing with a syringe pump. The syringe pump was
operated at a rate of 2-4 /Ltl/min. The spray needle was biased
at 3.7 kV to obtain maximum ion current on the shutter head
(7-15 pA). The desolvating capillary was resistively heated
to above 175 C by a current of 3.5 A and the capillary was
maintained at a potential of 28 V. The tube lens, skimmer,
first conductance plate, second conductance plate, and trap
plates were held at 35 V, 6 V, 0 V, 0 V, and 5 V,
respectively.
Results
Structures of the crown ethers and alkali metal crown
ether complexes studied in this experiment are presented in
Figure 5.3. Figure 5.4 shows a typical ESI/FTICR mass
spectrum obtained with 18-crown-6/NaCl/KCl in CH30H/H20
solution. Figures 5.5-5.7 show the IRMPD results for (18-
crown-6)Na+, (18-crown-6)K+ and (18-crown-6)H30+ upon
irradiation at 10.60, 10.58, and 9.58 /xm, respectively, by the
cw C02 laser. These wavelengths were chosen because they give


71
CF3Mn(CO)3(NO)" and CFjMniCO)/. Explanation for these shifts
involved increase of the electron density in the carbon a-
donor orbital of the CF3 group in the anionic species. Also,
the spectra apparently show that the frequency of the
degenerate C-F stretching mode is more sensitive to changes in
the net charge of the molecule and ligand substituents,
leading to a higher frequency shift when compared with the
nondegenerate C-F stretching mode (cf. the results for CF3+
vs. CF3- in Table 3.2.
We have assumed a similar trend in A and E mode shifts in
assigning the peak(s) observed for [Ga(hfac)3]' in Figure 3.9
to the asymmetric C-F stretch. The electron added in
formation of the anion occupies an orbital with strong anti
bonding character localized on one or more of the hfac
ligands. This will definitely lead to a lowering of the C-F
stretching frequency in the anion when compared to the
neutral. Given the relatively narrow wavelength range spanned
by Figure 3.9, it is not possible to assign unambiguously
the peaks seen there. Both the symmetric and asymmetric C-F
stretching modes may have been reduced by ca. 100 cm*1 and 200
cm*1, respectively, leading to the two peaks seen in the
spectrum. Or, more likely, the spectrum may be due to a
single peak split by a Fermi resonance interaction of either
the symmetric or the asymmetric stretching mode with one
(overtone) or two (combination) modes of the correct symmetry
of lower frequency which cannot be observed given the limited
wavelength range of the C02 laser.


153
91. Gaumann, T; Riveros, J. M.; Zhu, Z. Helv. Chim. Acta.
1990, 73, 1215.
92. Bellamy, L. J. The Infrared Spectra of Complex Molecules;
John Wiley and Sons Inc.: New York, 1975; Chapter 19.
93. Graybeal, J. Molecular Spectroscopy; McGraw-Hill Inc.:
New York, 1988; Chapter 17.
94. Berney, C. V. Spectrochim. Acta. 1964, 20, 1437.
95. Prochaska, F. T.; Andrews, L. J. Am. Chem. Soc., 1978,
100, 2102.
96. Mills, M.; Person, W. B.; Crawford, Jr., B. J.
Chem. Phys., 1958, 28, 851.
97. (a) Shin, S. K.; Beauchamp, J. L. J. Am. Chem. Soc. 1990,
112, 2066. (b) Shin, S. K.; Beauchamp, J. L. J. Am.
Chem. Soc. 1990, 112, 2057.
98. (a) Castleman, Jr., A. W.; Keese, R. G.; Chem. Rev.,
1986, 86, 589. (b) Coe, J. V.; Sondgrass, J. T.; Freid-
hoff, C. B.; McHugh, K. M.; Bowen, K. H. J. Chem. Phys,.
1987, 87(8), 4302.
99. Brooker, M. H. In Chemical Physics of Solvation; North
Holland: Amsterdam, 1988, vol. B.
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1990, 93, 6102.
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1975, 33 362. (b) Briscese, S. M. J.; Riveros, J. M. J.
Am. Chem. Soc., 1975, 97, 230. (c) Riveros, J. M. J.
Chem. Soc., Chem. Commun., 1990, 773.


41
experiment (Figure 2.11(a)). Although the amount of
photodissociation for both laser irradiation was increased
when 750 mJ pulse-1 laser energy was used (Figure 2.12(a) and
2.12(b)), the calculated percent photodissociation revealed
that amount of photodissociation with the multipass experiment
was still ca. 60% greater when compared with that of double
pass experiment.
To decide which laser was to be used in one-laser IRMPD
experiments presented in chapters 4 and 5, laser power
measurements were obtained at several mirrors outside the
vacuum chamber using both pulsed and cw C02 lasers. The cell
was mounted on the flange holder and the 8" flange was mounted
on the table. The beam was subjected to multipasses inside
the cell as shown in Figure 2.10. First, laser-burns were
taken, using thermal paper at all three mirrors and the ZnSe
window. Next, the He-Ne laser was aligned on all three burns
on the mirrors and through the ZnSe window. Then, the He-Ne
laser was centered on the gold-coated turning mirror, and the
multipass reflections were optimized. Dry-ice or cigarette
smoke was used to observe sixteen passes inside the FTICR
cell.
The C02 laser light was reflected through the same
pathway, using the aligned He-Ne beam as a guide-line. This
method allowed the energy of the C02 laser beams to be
measured after each mirror. The receive plate that contained
the larger mirror was carefully removed without disturbing the


26
to the meter which gave the energy (or power) output of the
laser beam in the range of 0.01 mJ 30 J.
IRMPD/FTICR Mass Spectrometry
Electron Ionization Experiments
All experiments were performed using a home-built FTICR
mass spectrometer equipped with either a Nicolet FTMS 1000
(64a) or an Ion Spec (64b) data station and a prototype 2
Tesla superconducting magnet. A schematic representation of
the FTICR mass spectrometer used in chapters 3 and 4 is shown
in Figure 2.5. The high vacuum chamber was positioned inside
the 20 cm bore of the magnet. This high vacuum chamber was
pumped by two oil diffusion pumps (65) with a combined pumping
speed of 1000 L/s. The background pressure of the system was
maintained at 5.0 x 10-9 Torr. A third 300 L/s oil diffusion
pump was mainly used to evacuate the inlet system. All
chemicals were introduced into the high vacuum chamber by
using the inlet system heated to ca. 100 C. Purity was
confirmed by broadband mass spectra and the samples were used
without further purification except for removal of dissolved
gases by multiple freeze-pump-thaw cycles. This system was
equipped with three laser windows on the 8" flange to which
the FTICR analyzer cell was mounted.


BIOGRAPHICAL SKETCH
Dilrukshi M. Patuwathavithana Peiris was born and raised
in Colombo, Sri Lanka, and is the only daughter of Dr.
Chandrarathna and Dorothy Patuwathavithana. She received her
high school education from Visaka Vidyalaya, in Colombo, Sri
Lanka. She left the country and came to United States in 1983
in order to continue her higher studies. She went on to Iowa
State University where she completed her Bachelor of Science
degree in chemistry, and graduated with distinstion in May
1988.
She went to back to Sri Lanka after her grdauation and
spent several months with her family. In August 1990, she
moved to Gainesville, Florida, with her husband and son, to
pursue her doctoral studies in physical chemistry. In the
same month, she had to leave to Sri Lanka due to her father's
tragic death. She came back in January 1991 and joined Dr.
John Eyler's research group to study infrared laser spectros
copy of gas-phase ions. She had the great fortune of being
able to visit the University of Sao Paulo in Brazil where she
participated in a collaborative project between Drs. John R.
Eyler and Jose Riveros. This work experience helped her to
understand gas-phase solvated ion chemistry. She completed her
graduate studies and received a Doctor of Philosopy degree in
December 1994.
158


m/z
Figure 5.4.
ESI/FTICR mass spectrum obtained with 18-crown-6/NaCl/KCl in 50.50
methanol:water solution without laser irradiation.
125


152
76. Coxon, J. A.; Foster, J. A. J. Mol. Spectrosc. 1984, 103,
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77. Van der Hart, W. J. Mass Spectrometry Reviews, 1989, 8,
237.
78. (a) Morgenthaler, L. N.; Eyler, J. R. J. Chem. Phys.
1979, 71, 1486. (b) Morgenthaler, L. N.; Eyler, J. R.
J.Chem. Phys. 1981, 74, 4356. (c) Morgenthaler, L. N.;
Eyler J.R. Int. J. Mass Spectrom. Ion Phys. 1981, 37,
153.
79. (a) Woodin, R. L.; Bomse, D. S.; Beauchamp, J. L. J. Am.
Chem. Soc. 1978, 100, 3248. (b) Shin, S. K.; Beauchamp,
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Chemistry; Bowers, M. T., Ed.; Academic Press: New York,
1984; Vol
. 3.

H
00
Bensimon,
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M.; Rapin, J.; Gaumann, T.
Ion Processes, 1986, 72, 125.
Int.
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CM
CO
Young A.
1985, 63,
B.; March R. E. ; Hughes J. R.
2324.
Can.
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83.
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Phys.
1988, 88,
801.
84. Watson, C. H.; Baykut, G.; Eyler, J. R. In Fourier
Transform Mass Spectrometry; Buchanan, M. V., Ed.; ACS
Symposium Series 359; American Chemical Society: Wash
ington, DC, 1987: pp 140-154.
85. Moini, M.; Eyler, J.R. Int. J. Mass Spectrom. Ion Proc.
1987, 76, 47.
86. Zimmerman, J. A.; Bach, B. H.; Watson, C. H.; Eyler, J.
R. J. Phys. Chem. 1991, 95, 98.
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Am. Chem. Soc. 1985, 107, 8036.
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Conf. Mass Spectrom.; San Diego, CA, May 26-27, 1986, p.
337.
89. Watson, C. H.; Zimmerman, J. A.; Bruce, J. E.; Eyler, J.
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Spectrom.; Nashville, TN, May 19-24, 1991, p. 1509.
90.


CHAPTER 2
THEORY AND INSTRUMENTATION
FTICR Mass Spectrometry
Development and Background
The fundamental principle of ion cyclotron resonance
(ICR) mass spectrometry was described by Lawrence and
Levingston (49). In 1949, Sommer et al. demonstrated the
omegatron (50) based on Lawrence's principle. The omegatron
was used for the analysis of very light ions under high-vacuum
conditions, but its poor mass resolving power limited its use
as a more general analytical instrument. In 1965, Wobschall
described an ICR instrument which used a variable magnetic
field that allowed ions of various masses to be brought into
resonance sequentially (51). Since the ions drifted through
the analyzer cell from one end to the other during the
analysis, this ICR instrument also lacked high mass
resolution. In 1973, Mclver used a trapped-ion ICR instrument
(52) in which ions were formed, trapped and detected in a
rectangular cell. Although this instrument offered great
performance improvements over the previous ICR instruments,
limited mass range, mass resolution, and slow scanning speeds
were still major drawbacks.
12


Conclusions
In this study, we have further extended techniques of
IRMPD in conjunction with FTICR to obtain gas-phase spectra of
solvated anions as well as cations. These spectra have been
demonstrated to be a source of previously unobtainable
vibrational frequencies of solvated anions and proton bound
cations of methanol. Even though there are major limtations
due to resolution and spectral coverage in the IRMPD spectra,
some structural information was obtained and is summarized:
(i) IRMPD spectra can be used to distinguish methanol and d-
methanol solvated ions; (ii) one or more dissociation channels
which are invariant to the laser wavelength were obtained by
IRMP dissociation of methanol proton bound dimer cations;
(iii) spectral shifts observed for CH3OHX' and CH3OHOCH3'
indicated that IRMPD spectra of solvated anions are selective
to the substituted anion; (iv) The two peak structure is
observed in the IRMPD spectra of d-methanol proton bound dimer
cations due to two nonequivalent CH3OH molecules in the dimer,
and the transitions are blue shifted when compared to the
fully hydrogenated dimer; and (v) photodissociation spectra of
solvated anions and cations indicate that the chromophoric
character of neutral IR absorption spectra can be preserved,
to some extent, in the IRMPD spectra.
At this point, it is difficult to discuss a direct
comparison of the C-0 stretching frequency shifts in the IRMPD


10
In the work reported in chapters 3 and 4 ions were
produced using conventional electron ionization (El) methods.
One of the requirements for El is that the sample should have
at least moderate vapor pressure. Some solid and liquid
samples can be heated to produce the required partial pressure
needed to use El. However, this method is limited due to
excessive fragmentation of the parent ion. To circumvent
problems with El, softer ionization techniques such as laser
desorption, matrix assisted laser desorption and ESI were
introduced.
These ionization methods and their limitations have been
appeared in several review papers (45). These reports
indicate that electrospray has emerged as the method of choice
for ionizing less volatile molecules. The multiple charging
process of ESI allows high molecular weight peptides and
proteins to be detected by conventional mass spectrometers in
the m/z 500 to 2000 amu range. The ions are produced in
solution and subsequently transferred into gas phase for mass
spectrometric analysis.
ESI/MS was first performed on polystyrene ions by Dole et
al. in 1964 (46) but they did not pursue the work because of
the limited instrumentation. Several years later, Yamashita
and Fenn performed ESI/MS experiments (47) with more modern
instrumentation. Fenn's demonstration prompted several
research groups to couple ESI with different mass
spectrometric analyzes. In our laboratory we have coupled an


156
139. Ramanathan, R. Ph.D Dissertation, University of Florida,
1994.
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Rev., 1974, 74, 351.
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91, 6540.
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Chem. Soc., 1991, 113, 7451.
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Soc., Chem. Commun., 1972, 1308.
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Crystallogr., 1976, 32, 751.
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pounds; Izatt, R. M.; Christensen, J. J. Ed.; Academic
Press: Orlando, 1978.
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3913.
148. Dearden, D. V.; Chu, I.H.; Yu, X. in Proceedings of the
41stASMS Conference on Mass Spectrometry and Allied
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trom., 1992, 6, 376.
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Trans., 1972, 2, 1818. (b) Bajaj, A. V.; Poonia, N. S.;
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3249.


44
from the FTICR cell. Six ejection pulses are available from
the Nicolet FTMS 1000 (the Ion Spec provides more than 25
ejection pulses) data station used in these experiments. Then
the pulsed C02 laser was triggered, or the cw C02 laser was
gated on for variable length irradiation periods of 50-500 ms
at a constant laser energy. Immediately after the laser was
fired the ions were excited and detected utilizing the
standard frequency chirp excitation method. For each of the
line-tuned laser wavelengths a broad band (10 kHz 2.66 MHz)
time domain signal consisting 16,384 data points averaged for
50 100 repetitions was collected. Thess averaged time
domain data was then apodized using a three term Blackman
Harris window function (74) and zero filled once prior to
Fourier transformation.


67
be used in conjunction with a higher power pump laser source
to obtain photodissociation spectra in a similar manner to
higher power single laser experiments. Similar results were
found for the two-laser studies of the three compounds
examined here.
The one- and two-laser spectra are quite similar, as both
approaches use the cw C02 laser to up-pump the population in
high, dense vibrational states to the quasicontinuum. This
similarity does lend credence to the assertion that resonant
photon absorption is the "bottleneck" to dissociation in each
process. Examination of Figures 3.4-3.6 and 3.7-3.9 reveals
that the maximum percent photodissociation obtained for the
one laser experiments was greater than that for the two laser
spectra. This difference in dissociation can be explained by
the much lower energy per pulse of the resonant laser in the
two- versus one-laser experiments. As mentioned in the
experimental section, the one-laser experiments were carried
out at 1 J total irradiation energy, whereas for the two-laser
experiments the resonant laser energy was kept constant at 100
mJ pulse-1. The maximum per cent photodissociation was
decreased only by a factor of 3-5 for all two-laser
experiments performed, even though there was a tenfold
decrease in total energy per pulse of the resonant laser.
Since the IR spectra of neutral diglyme (Figure 3.10)
shows a number of features in the 950-1250 cm-1 region, exact
assignment of the IRMPD peak is difficult. We have


3.11Gas phase neutral infrared
spectrum of 3-bromopropene
64
3.12Gas phase neutral infrared
spectrum of Ga(hfac)3 65
4.1 A schematic representation of the
laser beam pathway (16 passes) inside
the newly modified White-type
FTICR cell 76
4.2 Vacuum line apparatus
(pressure ca. 10"* Torr) used for the
synthesis of methyl nitrite 79
4.3 IRMPD spectrum of methanol solvated
fluoride ion (CH3OHF) 86
4.4 IRMPD spectrum of d-methanol solvated
fluoride ion (CH3ODF~) 87
4.5 IRMPD spectrum of methanol solvated
chloride ion (CH30HC1) 88
4.6 IRMPD spectrum of d-methanol solvated
chloride ion (CH30DC1) 89
4.7 IRMPD spectrum of methanol solvated
methoxy anion (CH3OHOCH3) 90
4.8 IRMPD spectrum of proton bound methanol
dimer cation (CH3OH)2H+ 97
4.9 IRMPD spectrum of proton bound d-methanol
dimer cation (CH3OD)2H+ 98
4.10 IRMPD spectrum of deuteron bound d-methanol
dimer cation (CH3OD)2D+ 99
4.11 Gas-phase neutral infrared spectrum
of methanol 100
4.12 Gas-phase neutral infrared spectrum
of d-methanol 101
4.13 Overlaps between the C02 laser lines
and the absorption spectrum.
(top) ions of low molecular
complexity;and (bottom) ions of
high molecular complexity 105
ix


I certify that I have read this study and that in my
opinion it conforms to acceptable standards of scholarly
presentation and is fully adequate, in scope and quality, as
a dissertation for the degree of Doctor of Philosophy.
This dissertation was submitted to the Graduate Faculty
of the Department of Chemistry in the College of Liberal Arts
and Sciences and to the Graduate School and was accepted as
partial fulfillment of the requirements for the degree of
Doctor of Philosophy.
December 1994
Dean, Graduate School


93
Proton Bound Deuterated Methanol Dimer Cation
(i) Electron beam pulse
D20 + e D20+ (4.24)
(ii) Reactions (mechanism)
D20+ + D20 -* D30+ + OD (4.25a)
D30+ + CH3CHC12 -* CH3CHC10D2+ + DC1 (4.23d)
CH3CHC10D2+ + D20 [D20]2H+ + CH2CHC1 (4.25c)
[D20]2H+ + CH3OD -* CH3OD(D2HO)+ + D20 (4.251)
CH3OD(D2HO)+ + CH3OD -> (CH3OD)2H+ + D20 (4.25e)
(iii) Laser pulse
(CH3OD)2H+ + nhvir (CH3OD)H+ + CH3OD (4.26a)
- (CH3OD)D+ + CH3OH (4.23d)


ACKNOWLEDGEMENTS
This dissertation is not the work of one person, but of
many. Therefore, it is with great satisfaction that I share
credit with all those persons who have made this document
possible.
I would like to thank many of the staff in the chemistry
department, particularly Susan Ciccarone, Steve Miles, Larry
Hartley, and Joe Shalosky for their always cordial and prompt
help.
I also wish to extend my thanks to Dr. Jose Riveros for
his guidance and expertise in the area of gas-phase solvated
ion chemistry, and the use of his laboratories at the
University of Sao Paulo, Brazil. I will always cherish
pleasant memories of the many good people I met during my
visit.
My sincere thanks go to Drs. Timothy Anderson, Sam
Colgate, Willis Person, David Richardson, and Martin Vala.
Each, through his active participation, offered valuable
suggestions that aided in the completion of this dissertation.
I also appreciate their exemplary thoughtfulness, advice, and
involvement in my academic career.
Heartfelt thanks are extended to all Eyler group members
past and present. In particularly, Ragulan Ramanathan whose
iii


155
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Relative Intensity Relative Intensity
130
m/z
100
50-
0-1
+
o
LO
31 as
IN
U
+
in
O
ffi
I
u
.i
+
O
m
X
Cp
c
5
o
L.
u
I
00
Too
3
T1" I
500
m/z
700
900
Figure 5.8. ESI/FTICR mass spectra obtained with 18-crown-
6 in 49:49:2 methanol:water:acetic acid
solution. (top) without laser irradiation;
the insert shows the isolated XH30+ with C
peak resolved, and (bottom) with laser
irradiation at 10.60 pm-