The radiation chemistry and mass spectrometry of trifluoromethyl iodide and pentafluoroethyl iodide in the gas phase

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Title:
The radiation chemistry and mass spectrometry of trifluoromethyl iodide and pentafluoroethyl iodide in the gas phase
Physical Description:
xii, 131 leaves : ill. ; 28 cm.
Language:
English
Creator:
Hsieh, Tacheng, 1947-
Publication Date:

Subjects

Subjects / Keywords:
Iodides   ( lcsh )
Mass spectrometry   ( lcsh )
Radiation chemistry   ( lcsh )
Ionized gases   ( lcsh )
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bibliography   ( marcgt )
theses   ( marcgt )
non-fiction   ( marcgt )

Notes

Thesis:
Thesis--University of Florida.
Bibliography:
Includes bibliographical references (leaves 125-130).
Statement of Responsibility:
by Tacheng Hsieh.
General Note:
Typescript.
General Note:
Vita.

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 000161405
notis - AAS7745
oclc - 02672693
System ID:
AA00003925:00001

Full Text










THE RADIATION CHEMISTRY AND MASS SPECTROMETRY OF
TRIFLUOROMETHYL IODIDE AND PENTAFLUOROETHYL
IODIDE IN THE GAS PHASE










BY

TACHENG HSIEH


A DISSERTATION PREzErIED TO THE CRADE'ATE COUNCIL OF
THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE RElQIRE!,ITS FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY







UNIVERSITY OF FLORIDA


1976













ACKNOWLEDGMENTS


The author expresses his sincere appreciation to his research

director, Prof. Robert J. Hanrahan, for his advice and encouragement

throughout this work. He also thanks Dr. John R. Eyler for

providing access to the ICR experiments.

Special appreciation goes to his wife, Jinn-Hwei, for her

understanding and patience that has made this work possible.
















TABLE OF CONTENTS


Page

ACKNOWLEDGMENTS ......................... ...................... ii

LIST OF TABLES ....................... ........ ............ ... v

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

ABSTRACT ...................................................... x

I. INTRODUCTION ......... ....................... ........... 1

A. Foreword .............. .. ..... ...... ................. 1
B. Review of Previous Work ............................ 1

II. EXPERIMENTAL PROCEDURES AND APPARATUS ..................... 5

A. Reagents and Their Purification ..................... 5
B. Sample Preparation ................................. 6
C. Sample Irradiation .............. ............ .... 8
D. Dosimetry .......................................... 10
E. Analytical Equipment and Product Analysis ......... 12

III. ION-MOLECULE REACTIONS IN THE SYSTEMS TRIFLUOROMETHYL
IODIDE AND PENTAFLUORL,'ETHYL I:'DIDE .................... 22

A. Experimental Results ............................... 22
B. Discussion ..... ................................... 26
C. Summary ..................... ...... .............. 43

IV. THE GAMMA RADIOLYSIS OF TRIFLUOROMETHYL IODIDE .......... 45

A. Experimental Results ......................... ... 45
B. Discussion ................... ............ ......... 52
C. Summary ......................... .... ..... 63

V. THE GAMMA RADIOLYSIS OF PENTAFLUOROETHYL IODIDE .......... 64

A. Experimental Results ............................ 64
B. Discussion ....... .. ................................. 85
C. Summary .......................................... 95










TABLE OF CONTENTS (continued)


Page

APPENDIX I APPEARANCE POTENTIAL MEASUREMENTS ON C2F5 ......... 98

APPENDIX II IDENTIFICATION OF RADIOLYSIS PRODUCTS .............. 109

APPENDIX III RELATIVE FLAME IONIZATION DETECTOR RESPONSE OF
RADIOLYSIS PRODUCTS ............................... 123

REFERENCES .................................. ............. 125

BIOGRAPHICAL SKETCH .......................................... 131














LIST OF TABLES


Table Page

1 Rate Constants of Fragment Ions in the C2F5I System ...... 27

2 Rate Constants of Fragment Ions in the CF3I System ....... 34

3 Ion-Molecule Reactions in C2F5I .......................... 35

4 Ion-Molecule Reactions in CF I ......................... 41

5 Radiolysis Yields for CF3I ............................... 53

6 Radiolysis Mechanism in the CF3I System .................. 55

7 Comparison of Radiolysis Conditions in the CF3I System ... 60

8 G Values for Radiolysis Products from C2 F 5 at 50 Torr ... 83

9 Secondary Ionic Processes in the Radiolysis of C2F5I ..... 88

10 Neutral Secondary Processes in the Radiolysis of C2F5I ... 91

11 Selected Thermochemical Data for Fluorocarbon Species .... 102

12 Selected Thermochemical Data for Fluorocarbon Ions ....... 104

13 Selected Bond Dissociation Energies for Fluorocarbon
Species ........................ .... ....... ... ........ .. 105

14 Mass Spectra of Pentafluoroethyl Iodide Radiolysis
Product Nos. 1 and 2 ........................ ... ...... .. 114

15 Mass Spectra of Pentafluoroethyl Iodide Radiolysis
Product Nos. 3 and 4 ............................ ........ 115

16 Mass Spectra of Pentafluoroethyl Iodide Radiolysis
Product Nos. 5 and 6 ................................... 116

17 Mass Spectra of Pentafluoroethyl Iodide Radiolysis
Product Nos. 7 and 8 .................................... 117











LIST OF TABLES (continued)


Table Page

18 Mass Spectra of Pentafluoroethyl Iodide Radiolysis
Product Nos. 9, 10, and 11 ..... .................... .. 118

19 Mass Spectra of Pentafluoroethyl Iodide Radiolysis
Product Nos. 12, 13, and 14 ............................. 120

20 Mass Spectra of Pentafluoroethyl Iodide Radiclysis
Product Nos. 15, 16, and 17 ............. .............. 122

21 Relative Flame Ionization Detector Response on MicroTek
2000 Research Gas Chromatograph ........................ 124















LIST OF FIGURES


Figure Page

1 Annular radiolysis vessel and holder ..................... 9

2 Dosimetry: Hydrogen yield from ethylene as a function of
irradiation time .................. ..... ........... ....... 11

3 Gas chromatographic sample loops ......................... 13

4 Schematic of Bendix high pressure ion-molecule reaction
source ...... ......... .... ........... ... ....... ....... ... 18

5 Schematic of ICR cell used for ion-molecule studies ...... 20

6 Normalized CF CF3 C2F5', and CF2I ion intensities as
a function of delay time in the T.O.F. spectrum of C2F51
at 37.0 microns and 500 C ................................ 23

7 Normalized C2F4I and C2F I ion intensities as a function
of delay time in the T.O.F. spectrum of C2F5I at 37.0
microns and 50 C ....................................... 24

8 Normalized I 12 C2F 12 and (C2F5I)2 ion intensities
as a function of delay time in the T. .F. spectrum of
C2F5I at 37.0 microns and 500 C .......................... 25

9 Ion Cyclotron Double Resonance spectrum of C2F5I+ with
an irradiating field of 0.48 V ......................28

10 Ion Cyclotron Double Resonance spectrum of C2F4I with
an irradiating field of 0.48 V .......................... 28

11 Normalized CF I+, CF2I1, and CF II ion intensities
a function of pressure in the T.0.F. spectrum of CF3I
at 25 C .............................................. 29

12 Normalized I2 CF312 and (CF3I)2+ ion intensities as
a function of pressure in the T.O.F. spectrum of CF3I
at 250 C ........... ............................... ..... 330










LIST OF FIGURES (continued)

Figure Page

13 Normalized ion single-resonance intensities as a
function of pressure in the ICR spectrum of CF3I at
25 eV and 25 C ........................................ 31

.14 Normalized CF +, CF2I and CF3I+ ion intensities as
a function of delay time in the T.O.F. spectrum of CF3I
at 50.0 microns and 500 C .............................. 32

15 Normalized I+, 12, CF312 and (CF3I)2 ion intensities
as a function of delay time in the T.O.F. spectrum
of CF3I at 50.0 microns and 500 C ...................... 33

16 Production of 1I (pure, ; 5%o HI, 0 ), CF3H (5 HI,
o ), and H2 (5o HI, ) as a function of dose in the
CF3I system ..................... ..................... 46

17 Production of CF4 (pure, 4 ; Y5 HI, 0 ) as a function
of dose in the CF3I system ............................ 47

18 Production of C2F6 (pure, 0' ;Yo HI, 0 ) as a function
of dose in the CF3I system ........................... 48

19 Production of C2F4 (pure, i ;Y5 HI, [] ) and C3Fg
(pure, 0 ; 5% HI, 0 ) as a function of dose in the
CF3I system ............................................ 49

20 Production of C2F5I (pure, ( ; 5% HI, 0 ) as a
function of dose in the CF3I system ................ 50

21 Production of CF212 (pure, 0 ; 5% HI, 0 ) and CF2IH
(5o HI, [ ) as a function of dose in the CF3I system... 51
22 Production of 12 (pure, Y ; 5% HI, O ) as a function
of dose in the C2F5I system ........................... 69

23 Production of CF4 (pure, ( ; Y5 HI, 0 ) as a
function of dose in the C2F5I system ................... 70

24 Production of C2F6 (pure, 0 ; 5o HI, 0 ) as a
function of dose in the C2F5I system ................... 71

25 production of C2F4 (pure, 0 ; 5% HI, 0 ) as a
function of dose in the C2F5I system .................. 72


viii









LIST OF FIGURES (continued)


Figure Page
26 Production of C3F6 (pure, ; 5% HI, 0 ) as a
function of dose in the C2F5I system .....................73
27 Production of C3Fg (pure, 0 ) and n-C4F10 (pure, i )
as a function of dose in the C2F5I system ................74
28 Production of CF3I (pure, ; 5% HI, 0 ) as a
function of dose in the C2F5I system .....................75
29 Production of C2F3I (pure, ; 5% HI, 0 ) as a
function of dose in the C2F5I system .................... 76

30 Production of n-CF7I (pure, ; 5% HI, 0 ) and
i-C3F7I (pure, i ) as a function of dose in the
C2F5I system ............................................ 77
31 Production of n-C4F9I (pure, 0 ; 5% HI, 0 ) and
1-C3F 5 (pure, N ) as a function of dose in the
C2F5I system ......... ...... .... ...................... 78
32 Production of s-C4F9I (pure, ; 5% HI, 0 ) as a
function of dose in the C2F5I system .....................79

33 Production of CF212 (pure, ; 5% HI, 0 ) as a
function of dose in the C2F5I system .....................80

34 Production of CF2ICF2I (pure, ; 5% HI, 0 ) and
CF3CFI2 (pure, i ) as a function of dose in the
C25FI system ........................................... 81
35 Production of CF3H ( 0 ), C2F5H ( 0 ), CF2IH ( i ),
and H2 ( & ) as a function of dose in the 5% HI-added
C2F5I system ...................................... .82
36 R.P.D. curves for CF3+, C2F5+, and C2F5I+ from C2F5I;
N2 from N2 .......... .. .......... ... ........... 99

37 Gas chromatogram of irradiated pentafluoroethyl iodide ..110
















Abstract of Dissertation Presented to the Graduate Council
of the University of Florida in Partial Fulfillment of the Requirements
for the Degree of Doctor of Philosophy


THE RADIATION CHEMISTRY AND MASS SPECTROMETRY OF
TRIFLUOROMETHYL IODIDE AND PENTAFLUOROETHYL
IODIDE IN THE GAS PHASE

BY

Tacheng Hsieh

March, 1976
Chairman: Dr. R. J. Hanrahan
Major Department: Chemistry


The ion-molecule reactions in CF I and C2F5I have been investi-

gated using a Bendix Time-of-Flight mass spectrometer to obtain

kinetic data, and a Varian Ion Cyclotron Resonance mass spectrometer

to obtain information on the reaction pathways. To obtain kinetic

data, measurements of the variation of the ion intensities with

changes in delay time were carried out. The most characteristic

reaction is ion-molecule condensation leading to the formation of

dimers such as (CF I)2 in the CF3I system and (C2F5I)2+ in the C2F5I

system. Other reactions involving charge transfer, fluoride-ion

transfer, iodide-ion transfer, CF3 transfer, iodine-atom transfer,

and collision induced dissociation are also observed. Fluoride ion

transfer processes are observed only when the final products are

stable molecules, such as CF4 in the CF I system and CF4, C2F6, and









CF3I in the C2F5I system. The parent ion and major fragment ions

resulting from electron impact on perfluoroalkyl iodides are formed

with considerable internal excitation amounting to as much as 1.5 eV.

The fragmentation of C2F5I under electron bombardment was studied

using a Bendix Time-of-Flight mass spectrometer. Using the Fox

Retarding Potential Difference technique, measurements were made of

the appearance potentials of parent ion as well as major ionic

fragments including C2F5If (10.66 eV), C2F5+ (11.71 eV), and CF3+

(13.73 eV). From these results it is found that AH0 for the parent

molecule C2F5I is > -236.4 kcal/mole, the (CF3CF2-I) bond dissociation

energy is < 47.0 kcal/mole and the (CF3-CF2I) bond dissociation

energy is ( 73.6 kcal/mole.

The gamma-radiolysis of gaseous C F 5I was studied at 50 Torr

and 240 C, both pure and with added HI. In all, 17 products were

formed in the radiolysis. For the pure system the major radiolytic

products and their respective G values are 12, 0.91; C2F6, 0.28;

C2F4, 0.78; C3F8, 0.15; n-C4F0, 0.42; CF I, 0.18; CF22, 0.18;

CF2ICF21, 0.11; and CF3CFI2, 0.052. It was observed that the addition
of approximately 5% HI dramatically increased the G values of I2

(from 0.91 to 2.84) and the initial G value of CF3I (from 0.18 to

2.50), while decreasing other product yields by 50 to 100%. Tn

addition, CFJP, C2FH, CF2IH, and H2 were also formed in the HI-added

system. Results are discussed in terms of ionic fragmentation and

ion-molecule chemistry of C2F5I, as observed in this investigation,

as well as postulated bond-rupture processes of neutral excited

species. The observed low overall yield is due to the back reactions









between 12 and C2F5 radicals.

The gamms-radiolysis of gaseous CF3I was also studied at 25

Torr and 240 C, both pure and with added HI. The major radiolytic

products and their corresponding G values in the pure system are
12, 0.50; CF4, 0.55; and C2F6, 0.11. With added HI scavenger, the

additional products CF3H, CF2IH and H2 were observed. In general,

the results can be interpreted in terms of known ion fragmentation

and ion-molecule chemistry of CF3I, investigated as part of the

present work, as well as neutral fragmentation processes and radical

reactions observed during photolysis. However, a comparison of the

present work with results from other laboratories shows that CF3I

can break down under different radiolysis conditions to give C2F6

plus 12 (Cooper, et al.), CF4 plus CF2I2 (McAlpine, et al.), or

CF4 + 12 (CF2)n (present work). Some suggestions are presented

concerning the factors which control branching of the reaction

pathways during radiolysis of this compound.














I. INTRODUCTION


A. Foreword


Investigations of the gamma radiolyses of trifluoromethyl

iodide and pentafluoroethyl iodide in the gas phase were undertaken

to study the primary and secondary processes leading to their

decomposition. Studies of ion-molecule reactions in both systems

provided supplementary information on the decomposition mechanism.

This work was undertaken both for purposes of comparison with

the radiolysis (1, 2) and mass spectrometry (3, 4, 5) of the

hydrocarbon analogs, and also because of continuing interest in the

Kasper-Pimental iodine atom laser based on photolysis of CF I

(6, 7, 8). Although the radiolysis of gaseous CF3I has been

investigated in two laboratories, there are discrepancies in the

obtained results. No previous work has been reported on the

radiolysis of C2F5I, or on the ion-molecule reaction chemistry of

either compound.



B. Review of Previous Work


More than two decades ago, Dacey (9) studied the gas phase

photolysis of CF3I using a 2537 R mercury resonance lamp. At low

pressure only C2F6 and 12 were formed, both with low quantum yields.

However, at pressures above 10 Torr, small amounts of C2F4 polymer








were also present. More recently, Skorobogatov and Smirnov(7) made

a mass spectrometric analysis of the volatile product residues after

the pulsing of CF3I and C3F I lasers. From the results of these

analyses, Skorobogatov concluded that: (1) In the CF3I system, the

electronically excited I(2P /2) atoms promote the conversion of CF3I

into C2F4 and C2F6, while the nonexcited (2 P3/2) atoms promote the

conversion of CF I into CF4 and (2) In the C3F7I system, the I(2P1/2)

atoms are responsible for the formation of C2F4, C3F6, and C2F5I,

while the I( 2P/2) atoms account for the conversion of C F7I into

CF4, C2F6, and polymers.

McAlpine and Sutcliffe (10) investigated the.gamma radiolysis

of CF I at a dose rate of 4.6 X 1019 eV/g-hr. These authors give

the following account of the radiolysis mechanism in this system.

In the absorbed dose range of 1.19 to 44.5 X 1021 eV, the primary

processes were:


CF3I vvvx- CF3I' + e 1

n'A CF + I+ + e 2

V^XW- CF + + I + e 3

qw+ CF I (excited molecule) 4


The main fate of the parent ions is neutralization to give e -ited

molecules and ultimately CF3 radicals

+ ** *
CFI + e --> CFI ----> CF + I 5


The consequence of the detailed primary processes listed in Equations









1 to 5 is summarized by Equation 6. Secondary processes postulated

by McAlpine are given in Equations 7 to 10:

*
CF3I '~AWv+ CF + I' 6

CF + CF3I > CF2I. + CF4 7

M + CF2I- + I. > CF2I2 + M 8

CF 2I + CF I -- CF2I2 + CF3. 9

M + CF3. + I- CF3I + M 10


McAlpine also stated that no C2F6 was formed when the glass vessel

surface was conditioned. (The irradiation vessel was preconditioned

by heating in vacuum for 0.5 hr followed by repeated irradiation of

samples of trifluoromethyl iodide until consistent results were

obtained.) However, C2F6 was formed in unconditioned vessels.

At about the same time, Cooper and coworkers (11) also studied
20
the gamma radiolysis of CF3I at dose rates of 1.3 to 2.6 X 1020 eV/g-hr.

Their observations were in contradiction with McAlpine's work. Cooper

proposed the following reaction scheme:

+ +
CF I '1AAvA CF I + e > CF + I* + e 11

--- CF. + I + e- 12

CF3I 3 vv\.+ CF2 + F. + I. 13

I + CF3I > CF 3I + 1- 14

I- + CF3I > CF3. + I2 15

CF 3 + CF* > C2F6 16
3 3 26 1










CF3 + F -- CF 17

CF2 + CF2 C2F and/or polymers 18

CF3 + I -- CF3I 19

CF 3 + 12 CF3I + I. 20

There were several disagreements in the yields reported by

McAlpine and by Cooper; the respective values are G(C2F4) = 0

(McAlpine) or 0.89 (Cooper); G(C2F6) = 0 (McAlpine) or 3.0 (Cooper);

G(CF212) = 0.82 (McAlpine) or 0 (Cooper); and G(I2) = 0.13 (McAlpine)

or 4.08 (Cooper).














II. EXPERIMENTAL PROCEDURES AND APPARATUS


A. Reagents and Their Purification


Trifluoromethyl iodide


PCR, Incorporated trifluoromethyl iodide was purified using

preparative gas chromatography. A 10-foot-long stainless steel

column packed with 60/80 mesh silica gel was used, and was operated

at 400 C with a helium flow rate of 60 ml/min. The collected

trifluoromethyl iodide was then transferred through a barium oxide

drying tube to a U loop on the vacuum line. It was degassed by

several freeze-pump-thaw cycles and stored at -1960 C in a vessel

attached to the vacuum line.


Pentafluoroethyl iodide


PCR, Incorporated pentafluoroethyl iodide was purified and

stored using the same method as that for trifluoromethyl iodide as

described above.


Ethylene


Matheson Company C. P. grade ethylene (99% minimum purity)

was passed through a barium oxide drying tube into a storage vessel

on the vacuum line and degassed by the freeze-pump-thaw method.









Hydrogen iodide


Hydrogen iodide was prepared by dehydrating Matheson Coleman

& Bell reagent grade hydriodic acid (57%) with P205. The acid was

frozen in a round bottom flask with liquid nitrogen, and P205 was

added on top of it. The flask containing the frozen mixture was

attached to the vacuum line and allowed to melt very slowly. The

HI that was released when the acid reacted with the P205 was

transferred through a P205 drying tube to another flask attached to

the vacuum line. Several freeze-pump-thaw cycles were performed to

separate HI from the iodine which was also produced. The HI was

kept frozen at liquid nitrogen temperature until used.


Chromatographic calibration standards


The gas calibration standards were used as received. (These

gases as well as liquid samples were obtained from PCR, Inc.) The

liquid standards were degassed by the freeze-pump-thaw method and

transferred to a gas chromatographic sample loop via a metering

vessel of known volume.



B. Sample Preparation


Vacuum system


All samples for radiolysis were prepared on a vacuum line.

The pumping system was a Welch Duo-Seal mechanical pump connected

through a liquid nitrogen trap to a two stage mercury diffusion

pump. These pumps were connected to the main manifold through a








second liquid nitrogen trap and a stopcock. Attached to the main

manifold were a mercury manometer, two metering vessels of known

volume (26.0 ml and 335.1 ml respectively), a thermocouple vacuum

gauge, storage vessels for CF3I, C2F5I, and HI, several Teflon-plug

stopcocks with 0-ring joints, a Toepler pump-McLeod gauge apparatus,

and a submanifold used for transferring radiolysis products to a

sample loop for analysis following irradiation. The submanifold

utilized only Teflon-plug stopcocks and had a separate thermocouple

vacuum gauge for monitoring sample transfer operations.


Treatment for cleaning radiolysis vessels


The vessels used for radiolysis were rinsed with distilled

water and annealed at 5650 C to remove any organic residues. They

were then attached to the vacuum line and pumped on for at least

5 hours before filling them with samples.


Metering and filling of radiolysis vessels


Before sample preparation, the manifold was isolated from the

pumps and the C2F5I vapor was allowed to expand into the 335.1 ml

metering vessel until a desired pressure was reached. The valve to

the metering vessel was closed and the excess material in the main

manifold was condensed back into the storage vessel using liquid

nitrogen. When HI was added, this was metered in by using the 26.0 ml

standard vessel and was introduced after the C2F5I transfer. After

samples were metered in, the entire metered amount was then vacuum

transferred into a radiolysis vessel while continuously monitoring






8

the pressure with the thermocouple gauge. Although this transfer

was completed within 5 minutes, 20 minutes were allowed for this

process. After all materials were transferred into the radiolysis

vessel, the vacuum line was opened to the pumps for another 10

minutes before the radiolysis vessel was sealed off with a natural

gas-oxygen flame.

The same procedures were employed in the case of CF I.



C. Sample Irradiation


Radiation source and vessels

Irradiations were carried out at room temperature (240 C) in

a Cobalt-60 gamma ray source which has been described in detail

elsewhere (12).

The annular radiolysis vessel used in this work is shown in

Fig. 1. It was made of Pyrex and equipped with a breakseal, a cold

finger and a 10 cm path length Supersil quartz optical cell (#S18-

260; Pyrocell Manufacturing Co.) connected by a quartz to Pyrex

graded seal. The two annular vessels used in these investigations

had volumes of 370.7 ml and 382.4 ml, respectively.

The sample holder (Fig. 1) allowed reproducible positioning of

the radiolysis vessel during irradiations. The vessel fits onto a

metal post; the height of this metal post allowed the Cobalt-60

source to rest in the center of the vessel.
















































0


r-7
0I)

Pi
0


H
4-3



0 b
0)

z





ry)0
o rr




CI)
(D-
Cd



H ON
.z4d
tuo
m
~c\Z







"4-
rx.~


^=7H









D. Dosimetry


It has been reported that the hydrogen yield in ethylene under

gamma radiolysis in the pressure range of 150 to 1000 Torr at room

temperature is independent of absorbed dose (13). Furthermore, the

G value for hydrogen production in ethylene has been established to

be 1.2 (14).

The absorbed dose rate in the ethylene system was determined

by measuring the hydrogen yield at room temperature at irradiation

times between 5 and 24 hours and at a pressure of 200 Torr. Following

irradiation the amount of hydrogen (along with small amounts of methane

and ethylene) was determined using the Toepler-McLeod apparatus.

Knowing the total pressure of the mixture and the quantities of

methane and ethylene (determined by gas chromatographic analyses),

the hydrogen yield could be calculated. The amount of H2 plotted

against the irradiation time gave a straight line, as shown in Fig. 2.

From the slope of the plot in Fig. 2 and the accepted G value for

hydrogen, the absorbed dose rate in ethylene was calculated to be

4.29 X 1019 eV/g-hr on June 26, 1975.

Assuming that the radiolysis vessels used approximated a

Bragg-Gray cavity (15), the rate of energy deposition in ethylene

can be correlated with that in C2F5I and CF3I. Since the application

of the Bragg-Gray principle is justified (16), the ratio of the

energy deposited per unit mass in the sample to that in the dosimeter

can be determined by the ratio of their mass stopping powers (17).

The final form of the dosimetry equations used to calculate









11









*r


0N 0


Cd


4
F-H
\ C


o
CN 0



C,d
\ O




z r-C

0 0



0 Z )
0 r-.:


C\ 1 0
O --




S-1 r-1




N-I -I-
000

00







0



Clo
\ -H



*** rz
Nd 3(i\


HZiD JO u-o E/sai-ouioxoTuI -H









the absorbed dose rates in C2F5I and CF3I are


Dose (C2F51) = 0.666 Dose (C24)

Dose (CF I) = 0.636 Dose (C2H4)


in units of eV/g-hr. Therefore, the absorbed dose rates on June 26,

1975 were 2.86 X 1019 eV/g-hr for C2F5I and 2.73 X 1019 eV/g-hr for

CF I. During subsequent irradiations, the absorbed dose rates were

corrected for the decay of Cobalt-60. Sample weights used in both

vessels were 0.088 g for CF3I and 0.222 g for C2F5I. (Actual sample

pressures varied slightly from the normal values due to slight

difference in the volume of the vessels used.)



E. Analytical Equipment and Product Analysis


Gas chromatograph

A MicroTek model 2000 research gas chromatograph equipped with

a flame ionization detector and a thermal conductivity detector was

used for the quantitative analysis of organic products. In this

instrument, the gas chromatographic column is enclosed in an oven whose

temperature is controlled by a multifunction temperature programmer.

The output of the gas chromatographic detector system was fed to a

1 mV Westronics recorder. All products were transferred on : vacuum

line submanifold to one of the sample loops shown in Fig. 3 for

subsequent injection into the gas chromatograph.


a. Products noncondensible at -1960 C

In the determination of the noncondensibles, the radiolysis






13














L1















L2


Fig. 3 Gas chromatographic sample loops.









vessel was attached to a submanifold leading to the Toepler pump-

McLeod gauge apparatus through a breakseal. After a good vacuum had

been reached, a "zero" pressure reading was taken with an Ealing

cathetometer. The system was isolated from the vacuum pumps and the

breakseal was broken open. The products were passed through a liquid

nitrogen U-trap. The noncondensible fraction was collected and

transferred in 12 Toepler pump cycles to the McLeod gauge for

measurement. It was then transferred to sample loop L1 for flame

ionization gas chromatographic analysis. This procedure allowed

determination of small quantities of methane and ethylene that

contributed to the pressure measurement. Hydrogen yields were

determined by difference.


b. Organic products condensible at -1960 C

The organic condensibles were transferred to smaple loop L2 on

the submanifold by breaking the breakseal. All products were separated

on a 9 ft, 0.25" O.D. X 0.020" wall stainless steel column packed with

60/80 mesh silica gel with a helium carrier gas flow rate of 40 ml/min.

The column was operated at 350 C until n-C4F10 eluted, after which

the temperature was increased at a rate of 20 C per minute to 150 C.

At the end of each analysis, the column was cinditioned at 2000 C for

several hours. A 5 m, 0.25" 0.D. X 0.020" wall stainless steel column

packed with 30% SE-30 on 60/80 mesh acid washed Chromosorb P was used

to investigate possible product decomposition on the silica gel column

described above. Products measured using the SE-30 column were in

agreement with those measured using the silica gel column, indicating







15


that there was no significant decomposition of products on the silica

gel column.

Prior to each analysis, the relative response of the flame

ionization detector to a typical fluorocarbon compound was determined

using perfluoropropane.

Since CF4 has an extremely small molar response in a flame

ionization detector (18, 19), it was measured using a thermal

conductivity detector.


Gas chromatograph-mass spectrometer-computer system

A detailed description of the gas chromatograph-Bindix model

14-107 mass spectrometer-General Automation SPC-12 minicomputer system

has been given elsewhere (20, 21). The gas chromatograph was equipped

with a stream splitter at the column exit. A Hoke "Milli-Mite"

metering valve diverted approximately 1/3 of the column effluent to

the detector of the gas chromatograph and the remaining fraction to

the ion source of the Bendix mass spectrometer through a single stage

jet molecular separator. The effluent splitter system allows the gas

chromatograph to operate at stmospheric pressure while running the

mass spectrometer under vacuum (10-6 Torr). Helium is used as the

carrier gas since it is easily skimmed off by the molecular separator;

furthermore, it provides minimum interference with the mass spectra

of other species. The mass spectral data acquisition is accomplished

by a General Automation SPC-12 minicomputer. During a gas chromato-

grap-mass spectrometer run, data are stored on magnetic tape (PEC 9-

track magnetic tape unit) and are retrieved and reduced at a later

time.









Spectrophotometry

A Beckman DU spectrophotometer with a Gillford model 222

photometer and power supply was used. Two readings were necessary

for each determination. First, the sample was condensed into the

cold finger of the radiolysis vessel and the cell absorbance determined.

Then the sample was volatilized and its optical density was measured

at 800 C. The difference between the two optical density readings

was taken to be proportional to the concentration of iodine. The

amount of iodine present was determined taking the extinction
-1 -1
coefficient to be 820 liter mole cm at 520 nm (22, 23). The

uncertainty in individual 12 measurements may be as much as 20% in

the worst case, although the standard deviation of the 12 dose-yield

plot was only 6%.


Equipment for ion molecule studies


a. High pressure mass spectrometry

The Bendix mass spectrometer could easily be changed from the

analytical to the ion-molecule mode (24). The high pressure ion

source was constructed as described by Futrell and coworkers (25).

A slight modification was made so that the filament is shielded to

prevent electrons from entering the ion focus region.

The ion source is housed inside a stainless steel "cross" which

is connected directly to the drift tube of the mass spectrometer.

Besides the regular pumping system supplied with the Bendix instrument,

a CVC Type D4 oil diffusion pump backed by a Welch Duo-seal forepump,

acting as an auxiliary fast pumping system was used. This system was






17
attached directly to the ion source by means of metallic bellows in

order to maintain a pressure differential of about 1000 : 1 between

the interior of the source and its surrounding region inside the

"cross". With this setup, ion-molecule reactions could be studied

at a pressure as high as 0.5 Torr.

Fig. 4 shows a schematic diagram of the ion-molecule reaction

source. The ion-molecule reaction chamber is a rectangular block

with a length of 0.222 inch in the direction parallel to the flight

tube; 0.25 inch in the direction parallel to the electron beam; and

1 inch in the vertical direction. The electron beam was pulsed at a

rate of 10 kHz (i.e. 100 microseconds for each complete cycle). A

small repulsive d.c. potential was applied to the backing plate R to

achieve the repeller field strength which is required to extract ions

from the source.

At some time interval, variable from zero to about 17 microseconds

after the electron beam was shut off, a +25 volt pulse was applied to

grid G1 and a synchronous pulse of -150 volts was applied to grid G2.

The pulse on Gl (blocking pulse) prevents any additional ions from

entering the focus region and the pulse on G2 (focus pulse) gates all

ions within the focus region between G1 and G2 into the acceleration

region. These ions are transmitted down the drift tube and appear at

the detector as several peaks in an order characterized by their ion

masses. This design allows data to be obtained as a function of

reaction time as well as source pressure.

During the off-cycle of the focus pulse, ions are continuously

coming out of the source. If they were allowed to reach the detector,

it would lead to a large continuous background current. The split













)H

e 0 0)

r 0 ET
( O 4
-P 3
H 0 0

bD o Cd
to -l a) a) iH
cd H Hl H
GAMMA


- 0 4-
, -p


,-i
o bD -
o k i F-
SH 0 &
& o
o ho o o.
0 d3 0 -


H-i NQ


LL-


(~L~


Oo


0

Ik
I---- '*----


o o
u\








plate structure of Grid G2 prevents this from happening (26). During

the off-cycle, a negative bias of about 150 volts is applied to the

lower half-plate. Since the upper half is at ground potential,

positive ions which diffuse past the grid (attached to the upper

half-plate) are accelerated against the lower half-plate and lost to

the walls. Therefore, only during the focus pulse are the ions in

the focus region swept into the acceleration region by G1 (+25 volts)

and G2 (-150 volts, both upper and lower half-plate).


b. Ion Cyclotron Resonance mass spectrometry

Ion Cyclotron Resonance (ICR) mass spectrometry is now well

established as a technique for the study of ion-molecule reactions (27).

In a typical apparatus used in such studies, a uniform magnetic field

B is oriented along the Z axis (Fig. 5) and a d.c. electric field E
s
(in the source region) is present in the Y direction. An ion of mass

to charge ratio m/e in crossed d.c. electric and magnetic fields will

drift in the X direction following a cycloidal trajectory with a

characteristic frequence of revolution w If an r.f. electric field

Er of frequency w1 is applied perpendicular to B, the ions will absorb

energy from the r.f. electric field and be accelerated when wl = wc.

Single resonance spectra are obtained by scanning B and measuring the

power absorption from a fixed frequency marginal oscillator.

A Varian V-5900 instrument equipped with the standard three

section flat cell was used (Fig. 5). The drift plate separation in

all sections was 1.1 cm, the length of the source region from the

filament location to the end of the source drift plates was 2.54 cm




















0
-A
I-
0r


t









0




On y,
0o


0




uL


2


wx


m -.
0p0
or *



"- p o
O OH
0 0 -0 H



0ol 0
0 0)

1 'd H H
0n3 d0) -





(1 0 z 0o

'd C 0) z
3o o



'dr-t c -t
-0 I 0 4- C\0


< C )




o00
S0 C 0
'd 4 A rl P ,
o -d o -f




*t i (1)rl ( P m
3 Il n7 ;1 Pe

( Co 0

-H 0) Hi 0 H 0)
S4 C l
0r )d ( -
H* rA .
0 -H
r -0 Cd -
I M CH 0 ()
004C Cd
H0 0\0 )-





CH C H H
2 -0+ (Q
D CM 0)


0)0 H0 (U 0

O 0 c *H
o 0), 1

M 0 r +3

0 O3 p
0 z d
0 cH 0 -i l

EN 0 p :S
6 )t-d 0) W) 0
H r-H H )
r C0Ha 0
HC 3H 0'-




i4-
EeH
00 OM


1-






cL
Q
--2--





Ot

c/1








and the length of the analyzer section was 6.35 cm. A single trapping

plate was used on each side of both the source and the analyzer

sections; separation was 2.54 cm. The level of the observing

oscillator frequency was set as low as possible in order to minimize

ion loss to the wall in the analyzer region. The emission current

was kept below 0.5 PA.

Double-resonance spectra were obtained by sweeping the double-

resonance frequency 2 while the analyzer was set at the magnetic

field required to observe a secondary ion of interest with the marginal

oscillator set at wl. The change in product ion intensity caused by

changing the kinetic energy of the primary ion was thus directly

observed. The double resonance r.f. field was applied to the source

region and the irradiating field strength was kept as low as possible

to prevent sweep-out effect (28).

Pressure was monitored by the Vac-ion pump control unit and the

rate constant for a well-established reaction (29)

+ +
CH 4 + CH4 > CH5 + CH


was measured as a reference under the same experimental conditions as

the reactions being studied, in order to back calculaLe the actual

pressure and thus calibrate the readings from the Vac-ion pump.

Due to the uncertainties in the measurements of ion tra it

times, the absolute pressures, and the complexities of the reactions

involved in the systems studied, no attempt was made to determine the

absolute reaction rate constants. The primary objective was to use

the double-resonance technique to identify reaction channels.














SIII. ION-MOLECULE REACTIONS IN THE SYSTEMS TRIFLUOROMETHYL
IODIDE AND PENTAFLUOROETHYL IODIDE


A. Experimental Results


C2FI system

Figs. 6, 7, and 8 show the variation of normalized and diffusion

corrected (17, 25) ion intensities as a function of delay time for

the high pressure Time-of-Flight mass spectrum of C2F5I at 37.0 microns

and 500 C. Intensities of CF+, CF3 and I+ decrease rather rapidly

with delay time; these ions disappear at 4, 9, and 12 psec, respec-

tively. Intensities of C2F5 CF2I C2FI+ and C2F5 + ions increase

with increasing reaction time. At 4 psec, C2F5+ ion intensity reaches

maximan while that of CF2I+ ion becomes constant. After 4 psec, the

abundance of C2F5+ ion drops off sharply while C2F I+ and C2F5I+ ions

continue to grow. As the reaction time increases, three new ions

having masses greater than the parent appear in the spectrum (Fig. 8)

and have been identified to be the species I2 C2F5 2 and (C2F5I)2

In the experiment illustrated in Fig. 7, the intensity of C2F5I

reaches a constant value beyond 9 psec. Under other experim- tal

conditions, however, the intensity of C2FI+ decreased as the heavier

ions (C2F5I)2+ and C2F512+ increased after approximately 6 psec.

Semilogarithmic plots of corrected intensities for the formation

and/or loss of ions against delay time gave straight lines; resulting















3 -




0 0
H *H




E 3
O 0







0 0

1-P d
u+ 0




0o r o


+ rl
0 0
rZ4 + *
a 0 c r





+ ) + 0
+ CH





00

0 0

+
+
0 0

0 *)
N

o *H

\O

Co

0N 0 0








24







00

r,.

a)









t o

0 o
4L)
NN ctO


H O
m


z 0



N~*HH
0 p 00



c d C\]
\+ 0

o 0
0 CO



o o o o o





\) \k- *H 7
H






a +0
9 \NH










t m


i-I
X4 v













o a
O O o O

r? o N H
0 /|x/./I
















Sd
0O

0 o

+ \

0
0 O 0



I- c i
HH






o0 -
ad

SO o|

N 00
H CH 0C





co 0
+"

z 0 NC
*H rd




0 -S 0
NH






0 o


ul
o -
COl
000 0 0








+ /
N0



H






cC

E O
0





co


00 &




(/W 0()









rate constants are summarized in Table 1. The semilogarithmic

treatment for the formation of ions is plotted as ln((A P)/Ao)

versus delay time for the ions I2 C2F51 +, and (C2F5I)2+, or

plotted as In((Po P)/Po) versus delay time for the ions C2 F5

CF2I+, C2FI+ and C2F5I where P is the corrected ion intensity,

A is the corrected initial intensity for the reactant ion, and P
0 o
is the corrected final intensity for the product ion.

Figs. 9 and 10 show typical Ion Cyclotron Double Resonance

(ICDR) spectra taken to identify reaction channels leading to the

formation of C2F5I+ and C2FI+ ions, respectively.


CF I system

Figs. 11 through 15 show the pressure and time dependence of

the ion intensities in the high pressure Time-of-Flight mass spectro-

metry as well as in the Ion Cyclotron Resonance study of the CF3I

system. Heavy ions such as I2 CF3I2 and (CF3I)2 were also formed

in this system. The semilogarithmic treatment as performed in the

C2F5I system gave good straight lines which led to the rate constants

for the formation or loss of fragment ions as listed in Table 2.



B. Discussion


C2FI system

The reaction pathways established on the basis of Ion Cyclotron

Double Resonance spectroscopy or high pressure Time-of-Flight mass

spectrometry are listed in Table 3. These measurements appear to











Table 1
Rate Constants of Fragment Ions in the C2F5I System

ions k, cm molecule- sec- X 100

CF+ dec. (0-4 psec) 13.4
CF + dec. (2-9 psec) 2.58

C2F5+ inc. (1-3 usec) 1.26
C2F5+ dec. (4-14 psec) 1.46
CF2I+ inc. (1-4 sec) 2.69
C2F4I+ inc. (2-12 psec) 2.11
C2F5I+ inc. (3-9 usec) 3.58
I+ dec. (2.5-8 4sec) 2.26

I2+ inc. (2.5-8 psec) 1.26
C2F5 I+ inc. (7-14 usec) 0.13
(C2F5I)2+ inc. (7-14 wsec) 0.15




















177 1k,? 119 100


m/e irradiated
Fig. 9 Ion Cyclotron Double Resonance spectrum of C2F5I+
with an irradiating field of 0.48 V.







*r-I
C|


177 119 69
m/e irradiated
Fig. 10 Ion Cyclotron Double Resonance spectrum of C2F4I+
with an irradiating field of 0.48 V.


L. -I


177


127 119 100












80.0





70.0

CF3



60 0 o


040.0
0
CF I
CF 50.0 C-


30.0


H V
2 40.0


20.0


30.0


l10.0 Lf-. CF 3I+


I

0.0
20.0 40.0 60.0 80.0

Pressure in microns

Fig. 11 Normalized CF + I+, CF2I+ and CF I3 ion intensities
as a function of pressure in the T.0.F. spectrum of
CF I at 25 oC.





























(CF3I)2 +













+


CF I2 +
!


60.0


80.0


100.0


Pressure in microns


Fig. 12 Normalized
a function
at 25 oC.


I2 CF3I2+, and (CF3I)2+ ion intensities as
of pressure in the T.0.F. spectrum of CF3I
3


20.0


10.0


20.0


40.0




















CF I+
3


CF3
3


CF2I
2


i I i


4.0


6.0


Pressure, Torr X 105


Fig. 13 Normalized ion single-resonance
function of pressure in the ICR
at 25 eV and 250 C.


intensities as a
spectrum of CF I
3


50.0


40.0 I-


30.0 1-


20.0


10.0 -


0.0 I
0.


0


2.0


8.0


I I I















60.0 -



CCFF




++
40.0 0.0




CF I++
2 c
20.0 0.0
O H




20.0
O





0.0




0.0 I I I
0.0 2.0 4.0 6.0 8.0
Delay in microsecs

Fig. 14 Normalized CF3, CF2I and CF I+ ion intensities as a
function of delay time in the T.O.F. spectrum of CF I
at 50.0 microns and 50 C.

















10.0




8.0 0-

(D 30 D
(cF I)2+
6.0 -
I I+



S4.0




2.0



CF 3I2
0.0 I I
0.0 2.0 4.0 6.0 8.0
Delay time in microsecs

Fig. 15 Normalized I I2, CF3I2 and (CF3I)2+ ion.intensities as
a function of delay time in the T.O.F. spectrum of CF I at
50.0 microns and 50 0C.











Table 2
SRate Constants of Fragment Ions in the CF3I System

ions k, cm molecule- sec- X 100

CF + inc. (1-3 psec) 1.20
CF+ dec. (3.5-6 psec) 2.46
CF + dec. (6-9 psec) 1.24
CF2I+ inc. (3.5-7 psec) 1.68
CF I+ dec. (1-3 psec) 1.36
CF I+ inc. (3.5-7 psec) 1.97
I+ dec. (2-6 psec) 2.85

2+ inc. (3-6 psec) 0.79
CF 12+ inc. (4-8 psec) 0.034
(CF3I)2+ inc. (3-8 psec) 0.22












II 0
II



11 0 4.3 4 3 +3 P P z I z
II + 0 0 o o o 0 0 0 0 0-
II II E I
II 0
II I

II *H
II r7 6-1
II _0 -C N0 -N -\-
li e -i- '0 (N -3- C- 0' C'- N c:_ 0'. 1- 0)

ll Cd3 cN c( Nj r-i H N to
ii + + + + I + + i + +a,
II *
II H Rl
II H

SII O

SII (
r II C \ 0 cO


N II C '. N
II N N O
S 11 O C.
0 II 0 pq (

- II a ,-
O II o r.
+0 II D 1 0 0



O II 0 --I H H H ( c-4 C
0, II C r )B
a II H .
O II + + M M "
SII 0 0
0 II+ Q F
E II + + + + + + + N0H H

o II + r H
SII N a N N N N N N H P.c
II 0 00
E-< 11 i Cl 0 i
c- II p o
a II ( 0 0 +
O II H F-

II H O 4-
E- +H 0 H

II H H H H H H H H -H H H H H H a,
0i C 4-d
E II n3 pq r




11 pl I 3 -P
011 r- Fr CT, _r P4 IP Od





II + + .J 1 1
II + + + + + H 0
11 HH+ + H HH -H H H '^i tr> a, ,; CH 6
II + + C1 e rx 4 H O


"- Fze [N l CN N N+ + Nx rz4 (l N N 0 0 z-
II 0 0 0 0 0 0 0 0 0 0 0 a 0
II 04 ko
+ 0+




c a 0 a)
1 H N ( n -+ U? \o o oC 0 H N1 O 0 E-i H 0
1r-I -| r-. -l H -
II 4 -13 0









be straightforward and in good agreement with each other. Wherever

possible, rate constants and heats of reaction for individual

processes are also listed in the table.

.Reactions of CF : Fig. 6 shows that the intensity of CF ion

decreases very rapidly and becomes zero after only 4 psec. In the

same time region, the intensity of C2F5+ ion increases drastically,

and that of CF2I ion increases to a nearly constant value. These

observations indicate that the primary reaction channels for the CF

ion are the formation of C25+ and CF2I+ ions. The formation of

C2F 5 ion could proceed either by iodide ion transfer,


CF+ + C2F5I > CF ++ CFI III-1


or by a dissociative charge transfer.


CF+ + C2F5I -- > C2F5 + + C I. III-la


Thermodynamic calculations indicate both reactions are endothermic.

However, Reaction III-1 is more favorable, since it would require only

4.4 kcal/mole in excess of that supplied by the reactants in their

ground states. Similar thermodynamic arguments suggest that the

favorable process for the formation of CF2I+ ion would be CF3 ion

transfer from parent molecule to CF+ ion. The CF,- ion transfer

reaction has been reported (17) in the ion-molecule reactions of

hexafluoroacetone.


CF + C2F5I ---- > CF2I + CF3CF III-2


The endothermicity of Reaction III-2 is 25.6 kcal/mole. Marcotte






37

and Tiernan (30) have previously reported the participation of excited

reactant ions during ion-molecule reactions of fluorocarbon species.

These authors pointed out that a large fraction of the CF+ species

formed by electron impact of C2F6 at 70 eV have internal energies

approaching 1.5 eV and the CF3+ ions in the tandem instrument have

as much as 2.9 eV of internal excitation. (It should be noted that

electrons of 100 eV energy were used in the high pressure Time-of-

Flight work and 25 eV in the ICR work. A tendency to form excited

ions under conditions of excess energy bombardment does not necessarily

invalidate appearance potential measurements taken at onset. This

point is discussed further in Appendix I.)

The rate constant for Reaction III-2 is 2.69 X 10-10 cm3
-1 -1
molecule sec Since the rate constant for the disappearance of

the CF+ ion is 13.4 X 10- cm3 molecule- sec- (Table ), the rate

constant for Reaction III-1 is estimated to be 10.7 X 10-10 cm3
-1 -1
molecule sec

Reactions of CF + and C2F .: From Figs. 6, 7, and 8 showing

the region between 4 and 10 psec, the combined decrease of the
intensities of CF+
intensities of CF and C2F5 is roughly equal to the combined increase

of the intensities of C2F4I and C2F 5I. The most important reactions

involving these species are as follows:


CF3+ + C2FI ---> C2F5I + CF3. III-3

CF3 + C2F5I > C2F4I + CF4 III-4

C2F5 + C2F5I -- > C2F5I+ + C2F.5 III-5

C2F5++ C2FI -- > CF4I+ + C2F III-6








All four reaction channels were identified by the ICDR technique. In

addition, the same set of reactions is strongly suggested by the high

pressure Time-of-Flight results shown in Figs. 6, 7, and 8. Reactions

II-3 and III-5 are charge transfer from CF3+ and C2F5+ ions to the

neutral parent molecules. However, thermodynamic calculations based

on ground state enthalpies of formation show that both are endothermic

reactions. The energy deficit is as high as 34.24 kcal/mole in

Reaction III-3. Hence, the reactants must be internally excited for

the reaction to occur; the reactant ions could be vibrationally and/or

electronically excited.

Reactions III-4 and III-6 are fluoride ion transfers from

neutral parent molecules to fragment ions CF + and C2 F5 for which

the exothermicities are 2.46 and 7.9 kcal/mole, respectively. The

possible dissociative charge transfer reactions have been ruled out,

since these reactions are endoergic by 114 kcal/mole.

The total rate constant for the decrease of CF+ ion (k + k)
3 4
is 2.58 X 010 cm molecule- sec- and for the disappearing of

C2F5+ ion (k5 + k ) is 1.46 X -10 cm3 molecule1 sec-1 (Table 1).

Since complicated processes are involved in the corresponding

product ions, no attempt was made to calculate individual rate

constant.

Reaction of CF +: There is no clear indication that t'

charge transfer from C2F ion to parent molecule occurs in the high

pressure mass spectrometry work, but this reaction is indeed seen

in the ICDR spectrum and requires 12.7 kcal/mole.








C2F4 + C2F5I > C2F51 + C2F III-7

This reaction is probably a very slow process since it is not seen

in the high pressure mass spectrometer.

Reactions of I : I+ ions are involved in the following

reactions:


I+ +C2F5I > C2F5I+ + I. III-8

I+ +C2F5I 12 + C2F5 III-9


Reaction III-8 is a simple charge transfer and Reaction III-9 is

an iodine atom abstraction reaction. The rate constant for the
s 2010 3 -1 -1
disappearance of reactant ion I+ is 2.26 X -10 cm molecule- sec.

The rate constant for the formation of I 2 is 1.26 X 10 cm
-1 -1
molecule sec Therefore, the rate constant for Reaction III-8
-10 3 -1 -1
can be estimated to be 1.00 X 10 cm molecule sec The

former is 4.24 kcal/mole endothermic and the latter is 16.6 kcal/mole

exothermic.

Reactions of CFI +: ICDR shows that two processes involve this

species as follows:


CF2I + C2F5I -- C2F5I + CF2I. III-10

CF2I + C2F5I -- CF4I+ + CF I III-11


This is essentially the same reaction pair which occurs with CF3

and C2F+ ions (charge transfer and fluoride ion transfer reactions).

An energy input of 1.4 kcal/mole is required in Reaction III-11.

Reliable thermochemical data on CF2I are not available, but from






40

ICDR experiments and rough estimation, Reaction III-10 is expected

to be endoergic by as much as 25 kcal/mole. Hence, Reactions III-10

and III-11 are not seen in the high pressure Time-of-Flight spectrum;

these two reactions are probably slow processes.

Reactions of C F I: The following reaction pathways are
21-5-
observed for parent ion C2F5 +:


C2FI+ + C2F5I -- C2F5I 2 + C2F5. III-12

C2F5I++ C25I --- (C2F5)2 III-13


Reactions III-12 and III-13 can be compared with results obtained

some years ago by Hamill and coworkers (3) on the corresponding

compound in the hydrocarbon series, ethyl iodide. In that investigation

the formation of simple dimer (C2H5I)2+ was assumed to occur via a

"sticky collision" process. In our system the corresponding

ion (C2F5I)2+ is presumably formed in the same way. Fragmentation

to give C2F5 2 is possible if sufficient energy is available. The

rate constants for the formation of C2F512+ and (C2F5)2+ ions are

0.13 X 010 and 0.15 X 0-10 cm3 molecule sec -, respectively.


CFI system

Ion-molecule reactions in the CF3I system are very similar to'

those observed in the C2F5I system. The following set of reactions

is compatible with the data of Table 4:


CF3 + CF3I > CF3 I + CF3. I1-14

CF3+ + CF I > CF2I + CF4 III-15

















O r
0
E--



0 0 0 0 0 0 0







0 0 0
H H -I E-H E-4






(N


+ i + i +







C0 +- 0 (\









SH+
** *
0 H CM 0 '- 0 0











0 H H 0 H 0

+ + + + + +


(NJ
+ + + C 'H
++ H

H H H + H C(
n cN r' + c C( r=4
0 0 U H 0 0

I Al IA





H H H H H H H
rX4 (4 4 r rX 5. r74
0 0 0 0 0 0 0

+ + + + + + +


+ + +
+ + H H H

0 0 H1 H 0 0 0


H H C\ O -


H
x
,-i



pt







H


0
c1










H


C)
4-)
-d













r- i H
r(

o





O C

0 0
0)

H
-I e
o



H *r.4
0 0
H o
O 0

E E3
C? 0
U 0)


4h





0




9 -
4i- 0)







*H *







Cm a
Cod

4, 4,




o o


-0) +
00) r













S4-l 0
00 0
0 O
0HI H
H
m a)
04' 4'
O 4
0 z
p 0

H w
Ea 0)
OH =
. -) I


0 0
0 4

0( 0
QC 0

030 B3


0O
o




H
0



0
4
I
I










0)





Co
r)



+
0
0




Co
0)
EP


a











0



0
Gp





*r
-P

IB
(D









I + CFI ---- CF I + I. 111-16

I + CF I -> 12 + CF III1-17

CF3I + CF I > CF + I. + CF I III-18

CF +1+ + CFI -> CF I2 + CF3 III-19

CF3I+ + CFI ---- > (CF I)2+ III-20


Reactions III-14 and III-16 are charge transfer from CF,+ and I ions

to parent molecules. Reaction III-15 is fluoride ion transfer from

parent molecule to CF + ion forming the stable CF molecule with a

rate constant of 1.68 X 010 cm molecule- sec as measured by

formation of CF2I+ ion. Reaction III-17 is iodine atom abstraction

by an iodide ion to form the 12 species. This reaction has a rate
-10 3 -1 -1
constant of 0.79 X 10 cm molecule sec The total rate

constant for the disappearance of I ion is 2.85 X 10-10 cm3 molecule-
-i
sec ; therefore the rate constant for Reaction III-16 becomes

2.06 X -10 cm3 molecule- sec .

From Fig. 13, there is a sharp increase of the intensity of
+ ~-5 -
CF + ion in the lower pressure region (0.2 X 10-5 to 2.5 X 10-5 Torr),

and a clearly correlated decrease in the intensity of CF3I+ over the

same region. This observation strongly suggests a reaction channel

in which CF I+ disappears and CF + is formed. It is proposed that

a collisionally induced dissociation of CF 1+ must occur. It is

necessary to assume that the CF I is internally excited to a

considerable degree, so that the dissociation process would be

energetically possible. Tiernan and Kevan (31) found that the

collision induced dissociation process is very common among








perfluoro compounds.

Reaction III-19 is iodine atom transfer from substrate to

parent ion CF 1 forming CF 12+ with a rate constant of 0.03 X 10-10
3 -1 -1
cm3 molecule sec The ion-molecule condensation reaction III-20

is also observed in this system, as shown in Fig. 15. This reaction

has a rate constant of 0.22 X 010 cm molecule- sec- for the

formation of dimer (CF3I)2 .

It is worthwhile mentioning here that the ether-type ions

CH H3 I + and C2H5IC5+ observed by Harill (3) and by Beauchamp (5)

in the methyl and ethyl iodide systems are not observed in either

the CF I or the C2F5I system; the corresponding ions CF ICF and

C2F5IC2F + are entirely absent under all conditions investigated.

The ions C2F5I 2, (C2F5I)2+, CF312, and (CF I)2+ are beyond the mass

range of the ICR instrument used in this work, although all these

ions were seen in the Bendix instrument.



C. Summary


Reactions such as charge transfer, fluoride ion transfer,

iodide ion transfer, CF ion transfer, iodine atom transfer,

collision induced dissociation, and ion-molecule condensation play

very important roles in the ion-molecule reactions of perfluoroalkyl

iodide systems. The fluoride ion transfer process apparently occurs

only when the final products are stable molecules such as CF4 in the

CF3I system and CF C2F and CF3I in the C2F5I system.

The parent ion and several major fragment ions resulting from

electron impact on perfluoroalkyl iodides are formed with large amounts






44

of internal energy, amounting to as much as 1.5 eV. It appears that

attempts to calculate bond energies in fluorocarbon systems, using

the assumption that all observable ion-molecule reactions must be

exothermic or thermalneutral, are of doubtful validity.
*<~ .















IV. THE GAMMA RADIOLYSIS OF TRIFLUOROMETHYL IODIDE


A. Experimental Results


The radiolysis of CF I was carried out at 25 Torr and room

temperature over the absorbed dose range of 0.453 X 1019 to

4.18 X 1019 eV. Three major products, iodine, tetrafluoromethane,

and hexafluoroethane are shown in Figs. 16, 17 and 18. Apparently,

their yields are all linear with respect to dose absorbed. The G

values for iodine, tetrafluoromethane, and hexafluoroethane are

0.50, 0.55, and 0.11, respectively. The amount of CF212 produced

(Fig. 21) is a linear function of dose from 0.45 X 1019 to about

1.5 X 1019 eV with a corresponding G value of 0.016. After this

dose, the G value for the production of CF2 I is reduced to 0.0092.

Other products such as C2F4, C3F8, and C2F5I (Figs. 19 and 20) were

also found in the radiolysis of the CF I system. However, their G

values were relatively small compared to those products mentioned

above.

In the HI scavenged system, most G values were substantially

reduced but the iodine yield was increased from 0.50 to 3.05. The

100 eV yields of CF4 and CF2I2 were reduced to 0.26 and 0.0081,

respectively. Other yields were reduced to very small residual


a G value is the number of molecules changed for each 100 electron
volts of energy absorbed.























2.0


1.0


0.0


2.0 3.0
Dose, eV X 10-19


Fig. 16 Production
(5% HI, [
of dose in


of 12 (pure, ; 5% HI, O), CF3H
) and H2 (5% HI, U ) as a function
the CF I system.






















0





0

.1-
0





0








o .
0



































rC-
H
03

























0

0








OtD
[x.o


-1 N H-
0 0 0 0
~ n r
o o o ;


(soq-omOzoa3TO) 'pTajp_ 1jo







48











0.10





S 0.08

r4
0







0.04 -





0.02 -
0.o I0
02Fg /5% HI


0.00
0.0 1.0 2.0 3.0 4.0

Dose, eV X 10-19


Fig. 18 Production of C2F6 (pure, ; % HI, 0 ) as a function
of dose in the CF I system.






49








0.008 -







0.006

C3F8





0.004
M -








C2F
0

















C2Fn /5% HI

0.00 I
0.0 1.0 2.0 3.0 4.0

Dose, eV X 10-19

Fig. 19 Production of C2F4 (pure, a ; 5% HI, D ) and C3F
(pure, 0 ; 5% HI, O ) as a function of dose in the
CF3I system.















0.012





0.010




2 5
---/ C2F5I
0 0.008 .
o
0




0.006
H


U
0.004





0.002 /
/ C2F51 /5.HI



0.00oo I I I
0.0 1.0 2.0 3.0 4.0

Dose, eV X 10-19

Fig. 20 Production of C2F5I (pure, 5 ; 5% HI, O ) as a
function of dose in the CF3I system.









(saetouroJOTu) 'PT9ap HIZdo
Uri C !



I I










o r
H H0


\ e\





















H *
N








N 0

O


S0









SO rd
40 0 41
z
H z
\ CO]



Cl]
U i







H
o 0





\ \\ C



\ \i id u






0 0 0 0 0
0 0 0 0
*
0 0 0 0


(s3910mwoao-M) 'pTGTSX zjzq[)










values, below 0.005. Three additional products were also found in

the HI-added system, including two new organic products, identified

as CF3H and CF2IH, with 100 eV yields of 0.76 and 0.12, as well as

H2 formed with a yield of 1.67.

All product yields are plotted as a function of absorbed dose

in Figs. 16 through 21. All G values are listed on Table 5, together

with previous results (10, 11) on the CF3I system. Material balance

is reasonably satisfactory in this system. The ratio of C/F/I is

0.797/2.88/1. Kevan and Hamlet (32) reported that irradiation of

fluorinated compounds in Pyrex glass vessels resulted in large yields

of carbon dioxide and silicon tetrafluoride. This could contribute

to the shortages of carbon and fluorine. It is also possible that

polymer is formed in this system.



B. Discussion


The previous studies on the photolysis (9) and radiolysis

(10, 11) indicate that -the main primary event in the gas phase

radiolysis of CF I is the rupture of the C-I bond.


CF3I vvvCA CF3I --> CF 3 + I. IV-1


It is evident (33, 34) that one or both of the radicals CF3. and

I. may be excited.

The mass spectrum of trifluoromethyl iodide shows that the

most abundant ion is CF I+ (100) followed by I+ (95.6), CF + (77.4),

and CF2I+ (31.2). The initial absorption of ionizing radiation may

accordingly give rise to the reactions:




















\0 r-H r- C\2
,cO cC) 0
\O00 CH 0
o 0 0 0 \0 C C'-
0 0 0 0 Ls- H- \0O
0 0 0 0















0.I
0

V


Cfl\ 0

0







0


CNM H
SO H CO V H
Nx NC NX 5M N
N U N N r O N U F4 O N
H 0 U 0 0 03 0 03 0


9
r--
r.1








0 0
H0


<

0 0
0




H


O-









r C
1- n
ft X
i- C
^ .
S 0r
*

M ft
ri ^=


0

0 C\I 0
C( 0 0


0 0


NO
0
H -
H


N
0 0
0


CO
0











CM


E



ul


O
C



0
U
N
0




CY-
0






rZ-
fO
Iu
45






54

CF3I vvvl- CF + e IV-2

vvWW* CF + + I1 + e IV-3

'V\ W+ I + CF 3 + e IV-4
C2+
F'VV+* CF2I + F" + e IV-5


The electron affinity of iodine atom (3.07 eV) (35) is quite

large, therefore dissociative electron attachment leading to the

formation of I is a very feasible process.


e + CF3I --- CF3. + I IV-6


All primary events as well as the kinetic scheme postulated

for the gas phase radiolysis of CF3I, are listed in Table 6.

Ion-molecule reactions in Processes TV-7 through IV-10 are

observed in the high pressure Time-of-Flight mass spectrometry and

Ion Cyclotron Resonance spectrometry. Step IV-7 is fluoride ion

transfer from substrate to CF3+ forming the stable molecule CF4 and

the CFI+ ion. Reactions IV-8 and IV-9 are iodine atom transfers

from parent molecule to I+ and parent ion CFI + leading to the

formation of I2+ and CF 31+ ions and CF3* radicals. Process IV-10

is an ion-molecule condensation reaction. These reactions are

discussed in detail in the Ion-Molecule Reactions section of this

dissertation.

Positive ions formed in the primary process as well as in the

ion-molecule reactions undergo neutralization with iodide ions to

form excited species as shown in Steps IV-11 to IV-16. These excited

species will undergo further decomposition and produce more radicals










Table 6

Radiolysis Mechanism in The CF3I System


SCF I










e +

CF +

I +

CF I+ +

CF I +



CF3

CF2I+

12+

CF I2 +

(CF3I)2+

ID + ID

CF
CF 31

CF2I
CF +


'VVW,+


CFI+



CF 3I

CF I
CF3I
CF I
F3I

+ I

+ I


+ I

+ I
+ I

+


+ M

+ 12



CF I
3FI


CF3I

CF I+

CF

I +

CF2I+


2>
----->








^^----->
---->










----->
.---->













------>
----->


--> CF + I.
3
+ e

+ I* + e

CF + e

+ F- + e

CF + I

CF4 + CF2I+

12 + CF 3

CFI2+ + CF .

(CF3I)2

(complex) -- > neutral fragments

CFI ---> neutral fragments


CF22 > neutral fragments

31-
S3I
(complex) -- > neutral fragments

(complex) -- > neutral fragments

12 + M

CF I + I.

CF2I2 + I.

CF4 + CF2I'

C2F + I


IV-1

IV-2

IV-3

IV-4

IV-5

IV-6

IV-?

IV-8

IV-9

IV-10

IV-11

IV-12


IV-13

IV-14

IV-15

iv-16

IV-17

IV-18

IV-19

IV-20

IV -21









which will be different for each species (mainly CF 3, I', and CF2I.

radicals, and the carbene species CF2). These intermediates will

take part in secondary reactions as postulated below.

The low overall fields in the pure system and the high yield of

iodine observed in the presence of radical scavengers indicate that

the back reaction IV-18 plays a significant role in the radiolysis

of pure CF I. It has been reported previously (36, 37) that there is

no activation energy for Reaction IV-18 and the corresponding rate
-12 3 -1 -1
constant is 4.32 X 10 cm molecule sec These observations

indicate that the gas phase radiolysis of CF3I is somewhat similar to

the gas phase radiolysis of alkyl iodides. The non-productive cycle


CF3I > CF 3 + 1 IV-1

I' + I--> 12 IV-17

CF3 + I2--- > CF3I + I* IV-18


takes place unless there is another process to remove CF 3 radicals.

In the radiolysis and photolysis of CC13Br investigated by Young

and Willard (38), the main product is CC14. They reported that the

reaction CC13' + CC13Br > CC14 + CC12Br* is responsible for

CC14 formation and that CC1 3 radical could be either thermal or

hot. It is reasonable to postulate a similar reaction in th4 system

as shown in Equation IV-20. Thermochemical considerations also favor

this reaction with a small potential barrier. The analogous hydrocarbon

reaction CH + CHI --- > CH4 + CH2I* has an activation energy

of less than 8 kcal/mole (39). Although not much information is










available about fluorine atom abstraction reactions, it can easily be

shown that in analogous hydrogen atom abstractions about 88-90% of

the energy of the bond formed is available to aid the bond breaking

process (40). The energy of the C-F bond in CFq has been given as

128 kcal/mole (41), while calculations from published appearance

potential data (42) show that the C-F bonds in CF I have an energy of

115 kcal/mole. If it can be assumed that the energy-availability

factor of 88% 90o applies to Reaction IV-20, then the corresponding

activation energy is less than 3 kcal/mole.

The photochemical reactions caused by the external pumping flash

lamps in the CF3I laser system have been investigated previously

(6, 7, 8, 43). It has been reported that the lasing process can

persist a substantial time(several microseconds) after termination of

the pumping flash. Reaction IV-21 has been invoked to explain the

growth in the concentration of excited iodine atoms which cause the

late-time lasing. This reaction is also used to explain (8) the fact

that adding more CF3 radicals can increase the concentration of

excited iodine atoms and contribute to the output of the photochemical

iodine laser. Consumption of CF3 radicals in Reaction IV-21 allows

accumulation of 12 by combination of the iodine atom released in

Process IV-1.

Consistent with the reaction scheme presented, the data in

Table 5 indicate that about 50% of the CF4 and CFI2 yields are due to

thermal radical reactions. Tetrafluoromethane is formed in Reaction IV-7

(non-scavengeable) and IV-20 (scavengeable), respectively. Reaction

IV-19 accounts for the scavengeable portion of the CF2I2 yield, while









thermalization of a small fraction of the yield of excited CF2I2

intermediate in Process IV-13 can account for the non-scavengeable

yield of this product. Since Reaction IV-21 is the only suffested

route to formation of C2F6, this product should be entirely scavenge-

able. In fact, Table 5 shows that this yield is 97% scavengeable.

The 3% residue of unscavenged C2F6 may be due to a small extent of

participation by "hot" CF3' radicals in Reaction IV-21.

Additional reactions are postulated to account for the minor

products observed in this system. It has been reported (44, 45) that

C2z5H radicals are produced in small yield in the radiolysis of liquid

methyl iodide. The precursor to this species could easily be the

simple carbene CH2, although this was not suffested by the original
*
authors. The formation of excited CCl26 due to direct insertion

of CC12 into CC14 appears to be an efficient process in the photolysis

of CC14 (46). It is suffested that insertion of CF2 into CF3I accounts

for formation of several minor products in the present system, including

C3F8, C2F5I, and C2 F4


CF2 + CF I > C2F5I IV-22

C2F5I > C2F5' + I* IV-23


C2F5I C2F + IF IV-24

C2F5I --- CF3. + CF2I* IV-25

C2F5 + 2 >- C2F5I + I. IV-26

C2F5* + CF3I C3F8 + I. IV-27


Since one bond is broken but two strong bonds are formed by the








insertion of CF2 into substrate (Reaction IV-22), the resulting C F5I

should be excited to the extent of ca. 75 kcal. Subsequent fragmen-

tation Processes IV-23, IV-24, and IV-25 can ensue, as discussed

later in connection with the radiolysis of C F I. Reaction IV-24

provides an explanation of the minor product C2F The C2F5' radicals

formed in Reaction IV-23 can undergo the Reactions IV-26 and IV-27,

accounting for production of C2F5I and C3F Reaction IV-27 is postu-

lated by analogy with Reaction IV-21, discussed above. Reaction IV-25

probably occurs but is unobservable, since the CF3 and CF2I. fragments

are already present in the system.

The combination of two CF2 fragments is also possible but this

process will not compete if Reaction IV-22 has a reasonable rate

constant, since the latter process involves substrate. Additionally,

CF2 combination might fail to compete with scavenging of CF2 by

product I2.

The radiolysis of CF I has been investigated by two previous

research teams (10, 11), with results which differ with each other and

with the present study. Since the explanation of the diverse results

must lie in differences in the experimental conditions employed, a

detailed summary of the conditions used by each group is given in

Table 7. The predominant stoichiometric pathways observed by

Sutcliffe and McAlpine, by Shah, Stranks, and Cooper, and in the

present work are as follows:

McAlpine 2CF I = CF4 + CF2I2 IV-28

Cooper 2CF3I = C2F6 + 12 IV-29

This work 2CF I = CF4 + 2 +(CF2)n IV-30
3^ 2 (CF2) n








60




II
II
II
II C'- CM
II 0N r-H 00 F

II H-l
11 X

7II cO N 0
II h H H H* N

X1 > +
0I H0 0 qHrl
U 0 P1 H H-" -.- lX


cc N 0
II H X X I I +
11I C- M -
ll 1 I oo c^ o

II H 0
11 *1 in O CO N O

SC11




H l
>1 II
ii I H H +
113 00 00 0 N



r 110 H H N 54
\0 11

II ( N N 0 0

IIl N CC1 -1 N .t cc
C'- d II *
SII 0 r- 0 0


S 11
3 U) II
I 11 -
II
0 II HI
II O 0N 0 N



0 |I X HI 0 MHC)
*4 lc N ) 0O C- 0 CO H N
11 r H H H H
i l D rOl *
00-P 0 II
c r-fl | II
Q II


H II-I H 0 X < N +
SII N r -
0 II *1 I I I

.r II C- H ON
II 4 H
PC< II OO H H-
SII O

0 II O
C.) 1*I


II
11

II ?1-
11 -*
II ao
II
IO 11
II I
0
II -H 0 o


II ) I U) H i tO
II U) U 3 ) C ( *r *

II 0) F1. 0 0









Examination of Table 7 indicates that there are variations in

vessel size, sample pressure, dose rate, and the total dose delivered

to the sample. Of these parameters, it is suggested that the most

important factors are the sample pressure and the dose rate. The

former affects thermalization of excited intermediates as well as

diffusion rates; the latter determines the steady-state concentration

of intermediates. It is suggested that a high dose rate may promote

net Reaction IV-29, since a sufficiently high concentration of CF3*

radicals could lead to direct CF3' CF combination. Consistent

with this suggestion, Cooper used the highest dose rate, and observed

the stoichiometric pattern given by Equation IV-29. (Since Cooper

used a very small vessel and a moderate pressure, it is possible that

CF combination was wall-catalyzed.) At lower dose rates as used

by McAlpine and by this laboratory., the CF 3 combination reaction

appears to be negligible; this species disappears by reaction with

substrate:


CF + CF I ---> CF + CF2I. IV-20

-- > C2F6 + I IV-21


McAlpine's results as well as the present work suggest that Reaction

IV-20 is the predominate fate of the CF radical. Under the high

pressure conditions used by McAlpine, CF2I. apparently undergoes a

homogeneous gas phase reaction with 12.


CF2I + I2 > CF2I2 + I. IV-19


In the present work, the vessel was six times smaller than McAlpine's,








and the pressure was 30 times lower. It suggested that CF2I reaches

the wall and decomposes to give various organic products, releasing

iodine:


CF2I- + wall --6> (CF)wall + I. IV-31


This suggestion accounts for a nearly stoichiometric ratio of CF4

and I2 in the present work, coupled with a deficit of additional

organic products.

Taking a W value of 26.2 eV/ion-pair for CF3I (47), the initial

value for loss of CF I should be somewhere around 3.8. Assuming

Reactions IV-3 and IV-4 are equally important, the initial G value

for CF and I' radicals would also be around 3.8. This value is

somewhat more consistent with the product yields reported by Cooper

at higher dose rates and with the results of present work on the

HI-added system than with the values observed by McAlpine in the lower

dose rate experiments. These observations indicate that the removal

of 12 by Reaction IV-18 is predominate over other processes as soon

as the concentration of iodine molecule starts to build up.

In the HI-added system the following reactions should be

considered:


CF- + HI > CF3H + I- IV-32

CF2I + HI ---> CFIH + I. -33


Whittle (37) pointed out that an activation energy of only 0.5 kcal/mole

is sufficient for Reaction IV-32, with a rate constant of 3.85 X 10-13
m3 molecule-1 -1at room temperature. The formation of hydrogen
cm molecule sec at room temperature. The formation of hydrogen






63

due to the decomposition of added scavenger HI has been reported

previously by several workers (48, 49, 50). Since the ionization

potential of CF I (10.6 eV) is considerably higher than that of HI

(10.4 eV) (42), charge transfer from CF3I+ to HI should be an efficient

process.


CF3I + HI > HI + CF I IV-34


The concomitant interference with subsequent reactions of CF I+ will

cause a decrease in almost all product yields, even those not affected

by HI as a radical scavenger. Furthermore, formation of HI+ in

Reaction IV-34 provides a reasonable explanation of sensitized

formation of H2 in the presence of 5% added HI, as discussed later in

connection with the radiolysis of C2F5I.



C. Summary


The gamma-radiolysis of gaseous CF3I was studied at 25 Torr

pressure and 240 C, both pure and with added HI. The radiolytic

products and their corresponding G values in the pure system are

12, 0.50; CF4, 0.55; C2F 0.11; CF2I2, 0.016; C F, 0.012; and C2F5 I,

0.0014. CF H, CF IH and H were observed in the 5% HI-added system

with corresponding G values of 0.76, 0.12, and.1.67. The results are

discussed in terms of reactions of both ions and neutral species.

Under condition of present work, stoichiometric considerations show

that the overall reaction in the radiolysis of gaseous CF I can be

summarized as


2CF I -- > CF4 + 12 + (CF2)n














V. THE GAMMA FADIOLYSIS OF PENTAFLUOROETHYL IODIDE


A. Experimental Results


The radiolysis of C2F5I was carried out at 50 Torr pressure

and room temperature over the absorbed dose range of 1.27 X 1019

to 13.9 X 1019 eV. The product yields are plotted as a function of

dose absorbed in Figs. 22 through 35 at the end of this section.

Iodine: Fig. 22 shows that the amount of iodine produced is

independent of absorbed dose over the entire dose range. The G value

for the production of iodine in the pure and scavenged systems are

0.91 and 2.84, respectively.

Tetrafluoromethane (CF4): Fig. 23 shows that in both pure and

HI scavenged systems the CF4 yield is linear over the absorbed dose

range investigated. The corresponding G values for this compound

are 0.035 molecules /100 eV and 0.018 molecules /100 eV for pure and

HI scavenged systems, respectively.

Hexafluoroethane (C2F6): In the unscavenged system the G value

for C2F6 is 0.28 throughout the absorbed dose region. The 100 eV

yield is reduced to 0.16 with 51 HI-added to the system (Fig. ').

Tetrafluoroethylene (C2F4): Tetrafluoroethylene production is

shown in Fig. 25. The G values are 0.78 for the pure system and 0.54

for the scavenged system in the low dose region; however, the yield

increases back to approximately 0.78 after the absorbed dose reaches

6.0 X 1019 eV.








Hexafluoropropene (C3F6): The C3F6 production data shown in

Fig. 26 indicate that it is a minor product in this system. The

corresponding G value for this compound in the pure system is 0.0034;

the, yield is reduced to,the residual value of 0.0001 in the HI

scavenged system.

Octafluoropropane (C3F8): This compound's dose-yield plot is

shown in Fig. 27. The 100 eV yield is 0.15 in the unscavenged

system and is completely eliminated in the HI-added system.

n-Decafluorobutane (n-C4F10): Fig. 27 also shows that production
19
of n-CF10 is linear with absorbed dose up to 3.5 X 10 9eV, and then

levels off to a constant value. The G value in the-initial linear

region is 0.42 molecules/100 eV. This product is completely absent

in the scavenged system.

Trifluoromethyl iodide (CF3I): Fig. 28 shows that the production

of CF I is linear in the absorbed dose range studied, with a G value

of 0.18. In the HI scavenged system, the initial yield of CF3I

surprisingly increases to 2.50 and then falls back to about the same

value as in the pure system when the absorbed dose reaches 3 X 1019 eV.

Iodotrifluoroethylene (C2F3I): The data for this compound are

rather scattered because it eluted on the tail of the parent peak.

The 100 eV yield is estimated to be 0.015 in the pure system. In the

scavenged system, the initial yield is reduced to 0.0044 but .reases

to 0.020 when the absorbed dose reaches 4.0 X 1019 eV (Fig. 29).

n-Heptafluoropropyl iodide (n-C3F7 ): Fig. 30 shows that the

G value of n-C F I is 0.014 between 0 and 4.0 X 1019 eV but increases

to 0.086 after a dose of 4.0 X 1019 eV in the pure system. In the









HI-added system, the 100 eV yield reduces to a residue value of 0.0012.

i-Heptafluoropropyl iodide (i-C3F7I): Fig. 30 also shows that

the G value of i-C F7I is 0.0028 in the pure system and that this

compound is completely scavengeable in the HI-added system.

I-Iodo-Pentafluoropropene (1-C3F5I): Fig. 31 shows that the

yield of 1-C F5I is 0.0040 up to an absorbed dose of 4.0 X 1019 eV

in the pure system. In the scavenged system, the G value reduces

to zero.

n-Perfluorobutyl iodide (n-C4F9I): Fig. 31 also shows that the

amount of n-C4F9I produced is a linear function of dose with a

corresponding G value of 0.011. The effect of HI is to reduce the

initial G value to 0.0046; the net rate of production of n-CF 9I is

zero beyond 2.0 X 1019 eV.

s-Perfluorobutyl iodide (s-C4F9 ): As shown in Fig. 32 the

amount of s-C F I in both the HI free and scavenged systems is a

linear function of absorbed dose. The G avlue for the unscavenged

system is 0.026 and for the scavenged system is reduced to 0.0072.

Diiododifluoromethane (CF2 2): The amount of CF2I2 produced is

a linear function of dose from 1.0 X 1019 to 5.0 X 10 9 eV with a

corresponding G value of 0.18. After this dose, the G value for the

production of CF2 I starts leveling off. The effect of added HI is

to reduce the initial 100 eV yield to 0.068. As with the pure system,

the net G value for the production of additional CF2I2 is zero beyond

8.0 X 1019 eV (Fig. 33).

1,2-Diiodotetrafluoroethane (CF2ICF2I): As shown in Fig. 34

the low dose 100 eV yields of CF2ICF2I in the pure and HI-added systems








are 0.11 and 0.080 respectively. The high dose G value for the

production of additional CF2ICF2I is 0.23 for both pure and scavenged

systems.

1.1-Diiodotetrafluoroethane (CF3CFI2): Fig. 34 also shows that

the initial G value of CF CFI2 is 0.052 between the dose of 1.0 X 1019

and 6.0 X 1019 eV. Thereafter, the net G value for the additional

production of CF CFI2 is essentially zero. In the scavenged system,

the added HI effectively blocks CF CFI2 production.

In addition to the products mentioned above, CF3H, C2FH, CF2IH,

and H2 were also found in the 5% HI-added system. As shown in Fig. 35

the initial G value of CF H is 0.35 between an absorbed dose of

2.0 X 1019 and 4.0 X 1019 eV. The net G value reduces to 0.10 with an

absorbed dose beyond 6.0 X 1019 eV. Fig. 35 also shows that the G

value for the production of C2F5 is 1.94 between a dose of 2.0 X 1019

and 12.0 X 1019 eV. In the case of CF IH, the initial G value is 0.19

before the absorbed dose reaches 3.0 X 1019 eV. Thereafter, the net

production of additional CF2IH is reduced to zero. Fig. 35 also shows

that the initial H2 yield is 1.23 and reduces to 0.086 after the

absorbed dose reaches 3.0 X 1019 eV. All G values are listed in

Table 8; only products having a G value greater than 0.03 are discussed

in the following section.

From the G values listed in Table 8, the major stoichiometric

patterns for the radiolysis of gaseous C2F2I are as follows:


2CF5I = n-CFo + 12 i

2C2F5I = c3 F + CF2I2 ii








2C2F5I = C2F6 + C2F + 2 iii

2C2F5I = C2F6 + CF2ICF2I iv

2C2F5I = C2F4 + 2CF3I v

C2F5I = C2F + IF v.

Since the G value for n-C F10 is 0.42, the 100 eV yield for the loss
of C2F5I due to Equation i is 0.84. The G values for the products

C3F8 and CF2I2 in stoichiometric Equation ii are 0.15 and 0.18,
respectively; using the average of these two values, the G(-C2F5I)
due to Equation ii is 0.33. Taking G(CF2ICF2I) and G(CF3I) as 0.11

and 0.18, the G(-C2F5I) values for Equation iv and v are 0.22 and
0.18, respectively. The G value for C2F6 is 0.28. Since part of it
is due to Equation iv (0.112), the G(-C2 5I) for Equation iii is 0.34.
Similar arguments lead to a G(-C2FI1) for Equation vi of 0.52. From
the above figures, the overall 100 eV loss of C2F I in the pure
system is 2.42 which is close to the sum of the scavenged radical
yields (2.48). Since the reported W value for C2F5I is 27.7 eV/ion-pair

(47), the initial G(-C2F5I) should be around 3.6. Neither of the
values quoted above is close to 3.6; this may be due either to the
formation of polymer or to the importance of back reaction, as
considered in the Discussion.


a G(C2F4 in Eq. vi) = G(C2F4 overall) G(C2F4 in Eq. iii)
G(C2F4 in Eq. v)
= 0.78 0.17 0.09 = 0.52






69







6.0




5.0-




2 4.0 15% HI
O




0 3.





2.0







O 12
1.0





2 .0 3.0 6.0 9.o 12.0
f1 .0








Dose, eV X 10-19

Fig. 22 Production of 12 (pure, ;5% HI, O ) as a function
of dose in the C2F5I system.




















0
C















0
Crl



0'



oH






rt
0


o
0
p^


ON
O 0 0 0 0
0 0 0 0 0


(saomoioToui) 'P[apq. 17o


4)


1-


0

0




n



4
o






0








t-












Or=
CM,


0
IPz












































































co 0 0 0
o O o O o


N


(
o .
o


(seoioao~xoyn) 'pTqoL 9,J


0
.C




0

0
Cd


*H








4









0







0
c+















0 ul
H



























,0 r4
(H
MO


'-I
o
O-
X




o
a















H
I-.





C
rj


c)

0

*r-1
0


0




























o


o
to
'-4



































4 -
0




Om













pq
0
0




z



c-4
0


a


I I I I u
0
ND C~c
a-I 14 0 0 0


(se~anomo-Fioi) 'pTayX tr2








73


(saTouoczo.nu) IH 9%/ 09,D

O o O
o O o O
o o o o
0 0 0 0


o0 a




\4rI

or





0
H











0
Q NO




0 'A












m
O
H


















0 0 0
\0
o1






4t-






*0H
N


r4-






^ \ **-1 >
\ I -p (f
\ o \ 3


O -t N
0 0 0 0
0 0 0 0
O O o O


(selomoaui) 'PTGTX 9je














0.30







n-C F10
4 io


0.20





SC3F8



0.10










0.00 I
0.0 2.0 4.0 6.0 8.0

Dose, eV X 10-19

Fig. 27 Production of C3F8 (pure, 0 ) and n-C4F10 (pure, I ) as
a function of dose in the C2F5I system. (In both cases
the yield with 5% added HI is zero.)
























C
c,



0
Id
4-)
0
r.
0














H
4-3





(d
(/I




H














r4
c-N




H


a)
0
-4-*
oe

U,
oCa



0


co

0 r-
CO



0


(sabomo.ioTm) 'peaTs Ido




















o
C






\H o
0





,-I
0

H 0







oo d

O
0

O
\o -



S0 0




OO
H






S4-\ 4
0 \ 0










NN




0
04r
o












\\ H
0 0 0



(s o *0 d
0. 0 a 0
















0.006 0.12






0
o n-C F I

0
S3 7



S 0.004 0.08 o










C



00 3-Fn-c3 7 /% HI







0.00 I I I I .00
0.0 2.0 4.0 6.0 8.0

Dose, eV X 10-19

Fig. 30 Production of n-C3F7I (pure, ; 5% HI, 0 ) and
i-C3F7I (pure, U ) as a function of dose in the
C2F5I system. (Production of i-C3F71 is eliminated
with 5% added HI.)
0.0 ^j /^
M. y. /06. .

















































































0 CO $
r-H H 0 0 0
o 0 0 0 0
o O 0 0 0


(saeomo o.uop) 'PTaTX


78





0


1




C)
cii
cd







00
H

I 0

-l
a)







0
.H
P4
o'3
I 0
F-

4-H




















O
4-4











10
*H
O a)






CH

0 -=,


0 -i-b


0* 0 (1)


*ri A






*r
fc



















































































co 0o o
0 0 o 0 0
o o o o o


(sa-eowoaOTu) 'p[@TX I6,[-s


0










H





I-l


0
00 ,
O (




0
ar
0





0 -


0)
.C




0





H.
or









i-N
0















*H







0
o
+
o



































i- U
H




u






(0



0
OG















0.20






0.15


0.10






0.05


0.00


2.0 4.0 '6.0 8.0

Dose, eV X 10-19


Fig. 33 Production of CF2I2 (pure, *
function of dose in the C2F5I


; 5% HI, O ) as a
system.


0.0













0.25


0.20





0- CF ICF I
S 0.15 2 2

O / /o




) 0.10
H
0













0.05 / B
CF 3CFI2





0.00
0.0 2.0 4.0 6.0 8.0

Dose, eV X 10-19

Fig. 34 Production of CF2ICF2I (pure, O ; ef HI, O ) and
CF3CFI2 (pure, 0 ) as a function of dose in the C2F1I
system. (Yield of CF3CFI2 is eliminated with 5% added
HI.)











(sa[omoa~oTu) 'pTGTX H i



Sc\ Ho
0
o 0 0 0 o






\ V ca
Cp


o o
N 4-






O H H






H 4




'4
N 0 4


00








rM4


S0 N 0
0 0 0




crx
oo n













O4
rx



o 0



p 0

on
0 O O O O

*'Ih


(saTomowomi) 'spTaT ZH puWe HI AD 'HI3D

































0

0
xr-


H




0
CM




0) 'd
0
pI







0
o






.-i





H
C*
1 o
1-1


0
0 H
r-H ON
0 0
ON
r1 \
0
X



U;
m0 0
0




0
W I-
Ol 0


N
H
r-1
0
0
0






N
H
0
0
0 0






r--
0
0
0 0


co
CO
V (\ %r) \0 C\i
CO cO cOO CO r-i 0
0 No C- 0 r r 0 0 0








co C 0 o H co C0
9 C O0 0 O -Z 0 r r- 9 O 0
0 0 0 0 0 0 0 0 0 0






Cr O0 C 0 O r 0
0 CN C' r-!0 -t- r- 0 0 0
0 0 0 0 0 0 0 0 0 0
tr v o o o o o


o0 H o C- C-

0 .- rx4 F=4 r4 c Cr 0 0
p CN Rix N NM c c C I r= C I
P.. H 0 0 0 0 0 0 0 -


o o
SCO 0 o 0 0,
O C H NO CO O C N l
H O H rH 0




X


r-1
SCO 0 0
) V) -.t rH LO I O OD N
1 COO H O i O

S0 0 H 0


0
U2

co o o0
C0 r-l 0
3 0 r-I 0 Vi 0

0 C O O o C
0 OOON
















CM
O 0 O 00

0 c0 O 0 dH 0








o 0c 0 oO 0\ 0
00 0 00 0 0 0


0
0
00


C\l
EC- c0 0
0 \CO
0 0 0 0


rH N
0 r


0 Hr- \
-H H- CN c
o o o C
0 0 C) 0 0 0






SC H000000 V

S00


-O- H \0 CN




0 0il CM 00 0i V

S00 0 0 0 0





H
H H \H N CN
n o,0 o0 P H
cr I H H H 0,
enl N3- cr- 9- 1-4 o ;
H- C U2 0 0 0 0 0 0 S








B. Discussion


The mass spectrum of pentafluoroethyl iodide shows that the

most abundant ion is C2F5I+ (100%) followed by CF3+ (89%), C2F5 (85.),

I+ (604), CF+ (52%), C2F4 (36%), CF2I (35%), and C 2F4 (28%). The

initial absorption of high energy radiation may therefore give rise

to the following reactions as a result of primary ionization events:


C2F5I nV'+ C2F5I + e V-1

%Wv -+ CF + + CF2I- + e- V-2

'Vnnvv\ C2F + + I. + e- V-3

+ I+ + C2F5 + e" V-4

v + CF+ + neutral fragments + e V-5

CV2+ C2F + neutral fragments + e V-6

CVF+V CF2I + CF 3 + e V-7

VVVV- C2F4I + F' + e V-8


In addition, it is known that high energy radiation gives rise

to primary excitation processes (35). Since excited molecules

produced in this way usually decompose by the lowest energy routes

available, the following reactions are proposed:


C2F5I + e 'C+ F C2F5I + e V-9

C2FI -- C 2F5 + I- V-10

-- > CF + CF2I V-11








C 2FI > CF 3 + CF .+ I. V-12

> C2F4 + IF V-13


Primary excitation (V-9) is probably followed most frequently by

C-I bond rupture (V-10), since this is the weakest bond in the system.

Since C2F5I does not absorb light in the visible region of the

spectrum, the lowest available electronic state probably possesses

75 to 100 kcal/mole of internal energy. One or both of the radicals

C2F5- and I- formed in Process V-10 may be excited, since deposition

of just 47 kcal/mole could result in rupture of the C-I bond.

Rupture of the C-C bond (Reaction V-ll) is proposed as a minor but

still significant process. If either the C2F5. fragment formed in

V-10 or the CF,2I fragment in V-ll still possesses substantial

excitation energy, further fragmentation to give CF2 can occur; the

net process is then summarized in V-12.

Whereas C2F4 is not found in the radiolysis of perfluoroalkanes

(32, 51, 52) it is a moderately important product in the C2F5I system.

Although there are ionic routes to this product (see Reactions V-19

and V-20 below), it is appropriate to inquire whether it can be formed

in neutral processes as well. Formation by radical-radical

disproportionation, a significant process in hydrocarbon radiolysis

(53), is not possible in the present case since perfluoroalkyl

radicals do not undergo disproportionation reactions (54). One

possible direct route to C2F formation is suggested in Reaction V-13.

Although F2 elimination is not seen in the radiolysis of perfluoroalkanes

(32, 51, 52), thermochemical considerations still suggest the possibility








of IF elimination in this system. This is because the C-I bond

(47 kcal/mole) is much weaker than the C-F bond (115 kcal/mole) and

the I-F bond (67 kcal/mole) formed in this system is much stronger

than the F-F bond (37 kcal/mole). The IF product postulated in

Reaction V-13 would presumably be a reactive scavenger. Radicals

would probably abstract fluorine rather than iodine, due to the

greater strength of the bond formed. Consequently, accumulation of

IF, even as a minor product, is not likely.

Formation of negative ions by capture of slow electrons is also

postulated in this system:


e + C2F5I > C2F5. + I V-14

-> C2F5 + I' V-15


Since the electron affinity of the iodine atom (3.07 eV) is larger

than the electron affinity of the C2F5 radical (2.30 eV) (55), Process

V-14 is probably more important than Process V-15. Non-dissociative

electron capture to give C2F5I is not likely, since the electron

affinity of iodine exceeds the C-I bond strength.

Ion-molecule reactions observed in high pressure mass spectro-

metry can also occur in the gas phase radiolysis of this system, as

listed in Table 9. Reactions V-16, V-17, and V-18 are fluoride ion

transfers from parent molecule to the ions CF C2F5 and CF2I

leading to the formation of stable products CF4, C2F6, and CF3I,

respectively. Process V-19 is charge transfer from C2F4+ ion to

parent molecule forming C F5I+ ions and product C2F4. The transient