Group Title: radiation chemistry and photochemistry of ethyl bromide in the gas phase / by Arthur Jesse Frank
Title: The radiation chemistry and photochemistry of ethyl bromide in the gas phase / by Arthur Jesse Frank
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Title: The radiation chemistry and photochemistry of ethyl bromide in the gas phase / by Arthur Jesse Frank
Physical Description: xiii, 202 leaves. : : ill. ; 28 cm.
Language: English
Creator: Frank, Arthur Jesse, 1945-
Publication Date: 1975
Copyright Date: 1975
 Subjects
Subject: Ethyl bromide   ( lcsh )
Radiochemistry   ( lcsh )
Photochemistry   ( lcsh )
Chemistry thesis Ph. D
Dissertations, Academic -- Chemistry -- UF
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
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Thesis: Thesis -- University of Florida.
Bibliography: Bibliography: leaves 196-201.
General Note: Typescript.
General Note: Vita.
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Bibliographic ID: UF00098929
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: alephbibnum - 000580759
oclc - 14081271
notis - ADA8864

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THE RADIATION CHEMISTRY AND PHOTOCHEMISTRY
OF ETHYL BROMIDE IN THE GAS PHASE












By

ARTHUR JESSE FRANK


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





UNIVERSITY OF FLORIDA
































The author is proud to dedicate this
dissertation to his wife, Abbey, who
has come on strong during the low
moments of this work and has also
shared in all of the highlights of it.


















ACKNOWLEDGMENTS


The author wishes to express his sincere appreciation to

Dr. R. J. Hanrahan who first suggested this research project and

who has always been willing to take the time to offer his advice and

encouragement throughout this work. Thanks are also extended to

Drs. M. L. Muga and P. M. Achey for their interest in this project.

Special appreciation is given to Dr. J. E. Fanning and A. R,

Ravishankara for their friendship and assistance, especially during

the final stages of this work. Again, thanks are due to T, Hsieh

for his assistance. The author also wishes to acknowledge the many

hours that A. Wendt spent doing the programming that produced the

Gould plots found in the dissertation. He is also grateful to Alexis

VanDenAbell who gallantly volunteered her time to type portions of

the first draft.

Thanks are also due to Nancy McDavid for typing the final

manuscript.

Finally he is especially grateful to his wife, Abbey, for her

unfailing encouragement and understanding that has made this work

possible.


















TABLE OF CONTENTS


Page

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

LIST OF TABLES . .. . . .......... vii

LIST OF FIGURES .. . . . . .... ....... ... ix

ABSTRACT. . . . . . .... .. ..... xii

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

A. Foreword . . . . . . . . 1
B. Review of Previous Work . . . . . . 2

II. EXPERIMENTAL PROCEDURES AND EQUIPMENT. . . .... 14

A. Reagents and Their Purification. . ,.... . 14
1. Ethyl bromide, . . . 14
2. Ethylene . .. ..... .. . . 15
3. Oxygen . . . . . . . . 15
4. Hydrogen bromide . . . . . .... 15
5. Chromatographic calibration standards and other
reagents . ... .. . . . ... . 16

B. Preliminary Remarks on Sample Preparation and
Analysis . . . . . . . . . . 16
1. Principle assumption underlying sample prepara-
tion and analysis. ........... . 16
2. Storage of sample . . . . . . .. 16
3. Treatment for cleaning radiolysis and photolysis
vessels. . . . . . ...... .. 16


C. Preparation of Samples
1. Vacuum system, .
2. Ethyl bromide .
3. Ethyl bromide with
4. Ethylene . . .


for Radiolysis.


added oxygen,


D. Preparation of Sample for Photolysis
1. Vacuum system . . .. .
2. Ethyl bromide . . .. . .
3. Ethyl bromide with added oxygen,
4. Hydrogen bromide .. .....













TABLE OF CONTENTS (continued)


II. (continued)


E. Sample Irradiation . . .
1. Radiation source, vessel and
ment . . .
2. Dosimetry . . . .


ancillary equip-


F. Sample Photolysis. . . . . . . . . 38
1. Photolysis lamp and vessels. . . .. . . 38
2. Actinometry. . . . . . ..... . 43

G. Preparation and Spark Discharge of Sample. . . ... 45


H. Analytical Equipment and End Product Analysis . .
1. Gas chromatography . . . . . ....
a. Principle instruments . . . .
b. Evaluation of column packing . . . .
c. Products non-condensible at -196C . ..
d. Organic products condensible at -1960C .
2. Gas chromatography-mass spectrometry-computer
system in the identification of organic products
3. High pressure mass spectrometry . . . .
a. Equipment for ion-molecule studies . .
b. Chemical ionization study, . . . .
4. Potentiometric titration of hydrogen bromide .
5. Spectrophotometric and chemical analysis of
bromine. . . . . . . . . .

III. EXPERIMENTAL RESULTS . . . . . . . ..

A. Qualitative Identification of Products . . ..
B. Quantitative Determination of Radiolysis Products.
C. Quantitative Determination of Photolysis Products .
D, Ion-Molecule Reactions . . . . . . .


. 48
S48
S48
S50
. 55
S57

S59
. 63
S63
.64
S65

.67

. 69

S69
S88
.110
.127


IV. DISCUSSION AND INTERPRETATION. . . .. .... .. .130


Introduction . . . . . . . .
Photolysis . . . . . . . . . .
Computer Simulation of the Photolysis Mechanism.
Ion-Molecule Reactions . . . . . .
Radiolysis . . . . . . . . . .
Comparison of Photolysis and Radiolysis. ..
Comparison of the Gas Phase Radiolysis of Ethyl
Chloride, Ethyl Bromide and Ethyl Iodide . .
Summary . . . . . . .


, .130
S. .130
, .144
. .151
S. .160
.. .174

S .177
186


APPENDIX .. . . . . . . . . . .


Page

. 30

. 30
. 35













TABLE OF CONTENTS (continued)

Page

REFERENCES . . ......... . . . . 196

BIOGRAPHICAL SKETCH. . . . . .. ... . . . .. 202

















LIST OF TABLES
Table Page

1 The Relative Intensity Distribution on a Logarithmic
Scale of the Emission Lines in the Spectral Range from
170.0 to 450.0 nm for the General Electric 15 Watt
Germicidal Lamp . . .... .... .. .... .. 41

2 Products in Spark Discharge of Ethyl Bromide. . ... 75

3 Mass Spectra of Ethyl Bromide Spark Discharge Product
Nos. 6, 7 and 8 . . . . .. ....... 82

4 Mass Spectra of Ethyl Bromide Spark Discharge Product
Nos. 11, 13, 14 and 15. . . . . . . . 84

5 Mass Spectra of Ethyl Bromide Spark Discharge Product
Nos. 19 and 24. ... ... . .. . . . . 86

6 G Values for Major and Semimajor Radiolysis Products
from Ethyl Bromide Vapor at 100 Torr Grouped by Shape
of Dose-Yield Plots in the Pure System, ... . . 89

7 G Values of Minor Radiolysis Products from Ethyl Bromide
Vapor at 100 Torr .... ........... . 96

8 Quantum Yields for Major and Semimajor Photolysis
Products from Ethyl Bromide Vapor at 100 Torr Grouped
By Shape of Dose-Yield Plots in the Pure System ... 1ll

9 Quantum Yields of Minor Photolysis Products at 253.7 nm
from Ethyl Bromide Vapor at 100 Torr, . . . ... 117

10 Photolysis Mechanism . . . . . .. . 132

11 General Photolysis Mechanism Used in Computer Simulations
A, B and C .. . . . . . . .. . . 146

12 Rate Constants Used in Computer Simulations A, B and C, 148

13 Radical Concentrations at Photolysis Time 5 Minutes for
Computer Simulations A, B and C . . . ... .. 152

14 Relative Product Distribution of the Major and Semimajor
Products from the Radiolysis and Photolysis of Ethyl
Bromide Vapor at 100 Torr, .............. 175











LIST OF TABLES (continued)

Table Page

15 Comparison of G Values in Radiolysis of Pure Ethyl
Chloride, Ethyl Bromide and Ethyl Iodide, . ... . 179

16 Bond and Activation Energies for the Ethyl Halides, 181

















LIST OF FIGURES


Figure

1 Radiolysis vacuum system . . . . . . . .

2 Sample analysis submanifold. . . . . . .

3 Photolysis vacuum system . . . . . . .

4 Cross section of cobalt-60 gamma ray source . . .


Page

21

22

27

32


Annular radiolysis vessel and holder .

Cylindrical radiolysis vessel and holder

Dosimetry: hydrogen yield from ethylene
of irradiation time . . . . .

Photolysis lamp ................

Photolysis vessels ............

Spark discharge vessel .. ......

1600 MicroTek gas chromatograph sampling

Gas sampling loops . . . . . .

Portion of product trapping assembly and
separator . . . .. . . .


as a function


module ....



jet molecular


14 Potentiometric titration curve of hydrogen bromide .

15 Gas chromatograms of irradiated ethyl bromide .....
a. Non-condensible hydrocarbons . . ... ... .
b. Condensible low molecular weight hydrocarbons, .
c. Condensible intermediate and high molecular weight
products . . . . . . . . . . .

16 Production of hydrogen bromide and hydrogen as a func-
tion of dose . . . . . . . . . . .

17 Production of bromine and 1,2-dibromoethane as a func-
tion of dose . . . . . . . . . . .












LIST OF FIGURES (continued)

Figure Page

18 Production of methane and acetylene as a function
of dose . . .. . . . .. . 104

19 Production of ethane as a function of dose. . . ... 105

20 Production of ethylene as a function of dose. .. , 106

21 Production of methyl bromide and vinyl bromide as a
function of dose . . . .. . . . 107

22 Production of 1,l-dibromoethane as a function of dose 108

23 Production of bromoform as a function of dose . 109

24 Production of hydrogen bromide as a function of
photolysis time . . . . . . . . 120

25 Production of bromine and 1,1-dibromoethane as a func-
tion of photolysis time ................ 121

26 Production of ethane as a function of photolysis time 122

27 Production of ethylene as a function of photolysis
time. . . . .. . . . . . . . 123

28 Production of methyl bromide and 1,1,2-tribromoethane
as a function of photolysis time. .... ...... 124

29 Production of methane and vinyl bromide as a function
of photolysis time. . .. . .. . . . .125

30 Production of 1,2-dibromoethane as a function of
photolysis time . ... . . . . . . . . 126

31 The relative intensities of CH58Br C2H5+, C2H3 ,
C2H6Br C4Ho1Br and C4Ho1Br as a function of
pressure . . . . . . . . . . . 129

32 Computer simulation of photolysis mechanism compared
with experimental data: Production of hydrogen bro-
mide as a function of photolysis time . .... 153

33 Computer simulation of photolysis mechanism: Produc-
tion of bromine as a function of photolysis time. .. 154

34 Computer simulation of photolysis mechanism compared
with experimental data: Production of ethane as a
function of photolysis time . . .... ...... 155












LIST OF FIGURES (continued)

Figure Page

35 Computer simulation of photolysis mechanism compared
with experimental data: Production of ethylene as a
function of photolysis time . . . . ... 156

36 Computer simulation of photolysis mechanism compared
with experimental data: Production of 1,1-dibromoethane
as a function of photolysis time. .. ... .. . . 157

37 Computer simulation of photolysis mechanism compared
with experimental data; Production of 1,2-dibromoethane
as a function of photolysis time. . . . .... 158

38 Computer simulation of photolysis mechanism compared
with experimental data: Production of vinyl bromide as
a function of photolysis time . . .. ..... .. 159












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 PHOTOCHEMISTRY
OF ETHYL BROMIDE IN THE GAS PHASE

By

Arthur Jesse Frank

March, 1975

Chairman: Dr. R. J. Hanrahan
Major Department: Chemistry

The primary and secondary decomposition modes of gamma irradiated

ethyl bromide in the gas phase at room temperature have been investi-

gated. Supplementary information on the system has been derived

from a parallel study of the 253.7 nm photolysis and the high pressure

mass spectrometry,

The G values and quantum yields of the major and minor products

both in the absence and in the presence of oxygen are reported. In

both the photolysis and radiolysis several products exhibit a well-

defined induction period. For the oxygen-free system in the dose

range from 1.0 x 1020 e.v./gram to 1.5 x 1020 e.v./gram the major

radiolytic products and their respective G values are hydrogen

bromide, 3.89; hydrogen, 1.39; ethane, 2.70; ethylene, 2,17;

acetylene, 0.31; methane, 0,0831; methyl bromide, 0.080; vinyl

bromide, 0.32; 1,l-dibromoethane, 0.88 and 1,2-dibromoethane, 0.12.

In the photolysis of the pure system between 60 and 90 seconds at an ab-

sorbed light intensity of 7.7 x 1015 quanta/sec, the major photolytic











products and their respective quantum yields are hydrogen bromide,

0.26; ethane, 0.40; ethylene, 0.028; methane, 0.00052; methyl bromide,

0.00091; vinyl bromide, 0.009; 1,l-dibromoethane, 0.102 and 1,2-

dibromoethane, 0.0092.

Carbon-halogen bond rupture is the major primary event in each

system. There is also substantial evidence for HBr elimination. In

addition, all secondary processes in the photolysis apparently occur

in the radiolysis as well.

A numerical integration method capable of handling steady state

assumptions has been used to calculate the product distribution based

on the proposed photolysis mechanism. The predicted and experimental

product distributions are found to be in reasonable agreement.

A comparison has been made of the radiation chemistry of ethyl

bromide with that of ethyl chloride and ethyl iodide which have

already been studied. The product distributions of the three systems

differ substantially. The C2 dihalogenated compounds in the ethyl

bromide system are produced in a substantially larger amount than in

either ethyl chloride or ethyl iodide. Differences in the chemical

kinetics of the three systems are explained primarily on the basis

of energetic arguments.

















I, INTRODUCTION


A. Foreword


The investigation of the gamma radiolysis of ethyl bromide in

the gas phase was undertaken to study the primary and secondary

processes leading to its decomposition. Parallel studies of the

253.7 nm photolysis and high pressure mass spectrometry of the

system provided supplementary information on the decomposition

mechanism. In both the radiolysis and photolysis, oxygen was added

to identify free radical intermediates and to determine their

contribution to the observed stable products.

An additional goal of this investigation was to compare the

radiation chemistry of ethyl bromide with that of other ethyl

halides since the chloride and the iodide systems have already

been studied.

No extensive work on the gas phase radiolysis or the room

temperature photolysis of ethyl bromide is reported in the literature.

A survey of other investigations relevant to the present work

follows in Sec. I-B.











B. Review of Previous Work


Roof and Daniels (1) investigated the 313 nm photolysis of

acetaldehyde and ethyl bromide at about 310C. Progress of the

reaction was followed by pressure measurements and by end-product

analysis of CO from acetyaldehyde and C2H4 from ethyl bromide. The

authors reported that the decomposition of ethyl bromide gave rise to

free radicals which sensitized the decomposition of acetaldehyde,


C2H5Br --+ C2H5' + Br' 1


Friedman, Bernstein and Gunning (2) studied the gas phase

photolysis of ethyl bromide in the presence of mercury and a ten-

fold excess of cyclopentane to measure the C13 isotope effect. Over

the temperature range measured (30 to 250C) the isotope effect,

as determined mass spectrometrically, was constant, The cyclopentane

and mercury eliminated the back reaction involving ethyl radicals

and bromine atoms.


C2H5' + C5H10 -- C2H6 + C5H9 2


Hg + Br' ----- 1/2Hg2Br2 3


Ethane was formed with a quantum yield of nearly unity over the entire

temperature range investigated; only a small amount of ethylene

was detected. These results suggested that in the spectral band of

their lamp, 210 to 260 nm, the major primary event was C-Br bond

scission.












Barker and Maccoll (3) photolyzed ethyl bromide in the gas

phase with 253.7 nm radiation between 150 and 300C, The progress

of the reaction was monitored continuously by pressure measurements

and checked by HBr titration. The following observations were

noted:

(1) The pressure-time plots exhibited well-defined induction

periods.

(2) Small amounts of the free radical scavenger propene inhibited

the reaction and prolonged the induction period.

(3) Added hydrogen bromide accelerated the reaction rate.

(4) Above 500 torr the kinetic equation was first order in

ethyl bromide with an overall activation energy of 10.5

kcal/mol.

(5) Also above 500 torr and at 2930C the quantum yield for the

reaction was quite large, about 500,

(6) Below 50 torr the rate equation became second order.

(7) The stoichiometry of the overall reaction corresponded to


C2H5Br ---- C2H4 + HBr 4


A radical chain mechanism was proposed to account for these observa-

tions.


hv + C2H5Br --- C2H5' + Br' 5


Br' + C2H5Br --- HBr + C2H4Br' 6


C2H4Br' -- C2H4 + Br'












C2H4Br' + Br' ---- Chain-ending 8


C2H5' + Br --- Chain-ending 9


Br' + Br' + M -- Chain-ending 10


Br' + Wall -- Chain-ending 11


Steps 5, 6, 7 and 11 lead to the low pressure second order kinetic

equation and steps 5, 6, 7 and, for instance, 8 satisfied the first

order kinetic equation.

From pyrolysis studies, Semenov (4) postulated that the radical

formed in step 6 can take one of two forms:


Br' + C2H5Br ---- HBr + 'CH2CH2Br 6a


Br' + C2H Br --- HBr + CH3CHBr 6b


Reaction 6a was estimated to be 3 kcal/mol more exoergic than reaction

6b. Only the radical 'CH2CH2Br can decay to C2H4 and Br* in a

unimolecular process and so propagate the chain. In contrast, the

radical CH3CHBr is relatively inert to unimolecular decay and can

only recombine or react as in step 12.


CH3CHBr + CH3CH2Br ---- CH3CH2Br + 'CH2CH2Br 12


Benson and O'Neal (5) reinterpreted the data of Barker and

Maccoll (3) and proposed the following mechanism for the low

pressure region.












hv + C2H5Br -- C2H5' + Br' 5

Br' + C2H5Br --- HBr + CHCH2CHBr 6a

'CH2CH2Br + HBr C2H5Br + Br' 6a'

Br' + C2H5Br -- HBr + CH3CHBr 6b

CH3CHBr + HBr ----- C2H5Br + Br" 6b'

CH2CH2Br + M ---- C2H4 + Br' + 1 13

Br' + CH3CHBr -- CH3CHBr2 14

In contrast to the Barker and Maccoll (3) investigation as well as

the present work, Gurman, Dubinskii and Kovalev (6) made no mention of

the presence of hydrogen bromide in the 253.7 nm photolysis of liquid

phase ethyl bromide at room temperature. As determined by gas chroma-

tography and spectrophotometry the major products reported were ethane,

dibromoethane and bromine as well as a small amount of ethylene, less

than 3% of the total product yield. The production of the organic

products increased linearly with dose up to about 1020 quanta/ml. while

the bromine plateaued at about 1018 quanta/ml, Addition of hydrogen

bromide prior to photolysis caused the bromine production to increase

and to become more nearly linear while at the same time decreasing

the dibromoethane formation. The mechanism suggested to account for

these observations was

hv + C2H5Br ---- C2H5 + Br' 15


C2H5' + C2H5Br -- C2H6 + C2H4Br'











Br' + Br' -- Br2 17


(C2H4Br + Br')--- (C2H4Br2) 18


C2H4Br' + Br2 -- C2H4Br2 + Br' 19


The asterisk denotes translationally "hot" species and the brackets

indicate a solvent cage. Since butyl bromide was not observed the

authors suggested that thermalized ethyl radicals played only a

minor role. Furthermore, the decrease in the dibromoethane yield

with decreasing temperature was interpreted as support for the "cage"

effect.

Donovan and Husain (7) obtained negative results in a search for

electronically excited Br(42 P/2) atoms in the vacuum ultraviolet

flash photolysis of ethyl bromide using kinetic spectroscopy, The

failure to detect Br(42 P/2) atoms was attributed to the rapid

collisional deactivation of the excited state by ethyl bromide

molecules.

Prior to the advent of gas chromatography, Schuler and Hamill (8)

studied the fast electron and X-ray decomposition of liquid phase

ethyl bromide. Hydrogen bromide and bromine were the only products

reported. Addition of triphenylmethane enhanced the G value for

hydrogen bromide. Bromine atoms were presumed to abstract hydrogen

from triphenylmethane rather than to back react.


Br' + (C6H5)3CH --- HBr + (C6H )3C'












There has been no other investigation of the radiolysis of

ethyl bromide; however, Neddenriep and Willard (9) studied the gamma

radiolysis of degassed liquid n-propyl bromide as a function of dose,

temperature and concentration of additives, In the pure system,

hydrogen bromide, propane, 1,2-dibromopropane and 2-bromopropane

were the major products reported. Hydrogen bromide was determined by

spectrophotometry and the organic compounds were measured by gas

chromatography. Lesser amounts of hydrogen, C1- and C2-brominated

and unbrominated hydrocarbons as well as a telomeric compound were

also reported. As evidenced by the nature of the products, the major

primary event is rupture of the C-Br bond. In the additive-free

system at room temperature, no bromine was detected by spectropho-

tometry and the net rate of HBr production decreased to zero with

increasing dose. One or more of the elementary steps producing HBr

was speculated to have an appreciable activation energy since the

G value for HBr formation increased from 0.06 at -78C to 10.5 at

50C. Other products also exhibited a temperature sensitivity,

These observations suggested that an important step in the radical

chain process leading to the decomposition of n-propyl bromide was

hydrogen abstraction to form H3r,


Br' + CH3CH2CH2Br ----- HBr + CH3CHCH2Br 21


In the presence of additives (HBr and 02) bromine was produced in

a substantial amount, a fact attributed to either the reactions between

peroxy radicals formed and HBr, or to the prevention of reactions












between organic radicals and bromine. At high dose with added HBr

and 02, the G value for bromine decreased with increasing dose and

in some cases even became negative. This indicated that Br2 was

competing with HBr or 02 for some reaction intermediate. Furthermore,

it was found that when bromine was added to the system prior to irradi-

ation, the bromine concentration decreased with dose in agreement

with the above observation.

Schindler (10) investigated the gas phase decomposition of ethyl

chloride induced by 2.8 Mev electrons. The effects of various addi-

tives indicated that roughly half of all primary events were molecular.

Ethylene, acetylene, vinyl chloride and hydrogen were assumed to arise

in part from molecular processes. In contrast to the alkyl bromides,

C-H rather than C-C1 bond rupture was the major primary event, The

probability of single bond rupture in the primary event was approxi-

mately represented by the ratio C2H4C1-H ) C2H5-C1 : CH3-CH2C1 = 1

0.75 : 0.15. The monochloroethyl and ethyl radicals were the main

radicals produced in the primary event. Steps 22 to 26 were charac-

terized as their major reactions.


Cl' + C2H5C1 ---- HC1 + C2H4C1' 22


C2H5' + HC1 ---- C2H6 + C1 23


2C 2H5' --- C4H10 24


C2H5' + C2H4C1' ----4- C4HgC1 25


2C2H4C' ---- C4H8C12












Five years after Schindler's work, Tiernan and Hughes (11)

reported on the role of positive ions in the X-radiolysis of gaseous

ethyl chloride. The participation of ionic intermediates in the

radiolytic mechanism was related, with the aid of various additives,

to the ionic fragmentation scheme from their high pressure mass

spectrometer study. Their results on the far ultraviolet photolytic

decomposition of excited ethyl chloride molecules were correlated

with the data from the other studies to derive a radiolytic mechanism,

The data suggested that the major radiolytic reaction mode in the

system was neutral unimolecular decomposition rather than ionic

reactions.

The gas phase decomposition of ethyl iodide with 2,8 Mev

electrons was investigated by Schindler and Wijnen (12). The products

formed were hydrogen, methane, ethane, ethylene, acetylene, methyl

iodide, vinyl iodide, diiodomethane, l,l-diiodoethane and a negligible

amount of 1,2-diiodoethane. Nonradical processes accounted for all

of the ethylene as in the case of ethyl chloride, and 70% of the

hydrogen. The probability for single bond rupture in the primary

event was roughly represented by the ratio C2H5-I : C2H4I-H

CH3-CH21 = 1 : >0.06 : 0.01. The primary processes postulated

to account for the observed products were similar to those in ethyl

chloride (10) except for step 32.


C2H51 -iA- C2H5' + I 27


-a-w+ C2H4 + HI












-sVt C2H2 + H2 + HI 29


-'kIv C2H3I + H2 30


-v+ C2H41' + H' 31


-vW CH4 + 'CHI' 32


--,+ CH3' + CH2I' 33


The wavy arrows in reactions 27 through 33 read "yields under the

influence of ionizing radiation" and do not necessarily imply a one-

step process. About one third of all the primary events were at-

tributed to molecular processes. Reactions of thermalized ethyl

radicals with HI accounted for most of the ethane produced; the

contribution of "hot" ethyl radicals was found to be unimportant.

Irsa (13) studied the unimolecular ionic dissociation of ethyl

bromide induced by electron impact, On the basis of appearance

potential measurements derived from the vanishing current or initial

break method and other energetic arguments (bond energies, ioniza-

tion potentials and electron affinity data) the following fragmenta-

tion processes were proposed:


C2H5Br ----- C2H5Br+ + e 34

-- C2H + Br + e 35


-- + CH2Br + CH3 + e- 36


(or CH2Br+ + CH + H2 + e












Evidence for the formation of Br", C2H and C2H2" was also presented

in this study,

Tsuda and Hamill (14) reexamined in more detail the appearance

potentials of C2H5 and C2H Br by the retarding potential difference

method on a Bendix Time-of-Flight mass spectrometer, From the

structure in the ionization efficiency curves for C2H5+ and C2H5Br

they were able to assign excited states to these ions. Direct

measurements of negative ions were also made using a magnetic instru-

ment. Their results indicated that ion-pairing processes always

occurred at onset


C2H5Br C2H5 + Br 37


where several excited electronic states were associated with the

C2H5 ion. At higher energies, the ion-neutral process took place.


C2H5Br --- C2H + r + e 35


Their data suggested that the ion-pair and ion-neutral processes

preceded from a common pre-ionized state.

In 1959 Pottie and Hamill (15) reported the first examples of

persistent collision complexes between ions and molecules, In a

mass spectrometry study of the alkyl halides the bimolecular process

was observed.


C2HgBr + C2H5Br --- C4H oBr 38











A more detailed exploration of the ion-molecule reactions occurring

in ethyl bromide was carried out by Theard and Hamill (16) using

high pressure mass spectrometry,

C2H5Br+ + C2H5Br --- C410Br 38

--- C4H10Br + Br 39


--- C2H6Br2 + C2H4 40

--- C4H Br+ + HBr 41


CH2Br + C2H Br CHBr2 + C2Hg 42

Beauchamp and coworkers (17) reinvestigated the ion-molecule

reactions in ethyl bromide using an ion cyclotron resonance spec-

trometer. In contrast to the results of Hamill and coworkers (15,

16) the ionic dimer was not observed, The primary ions C2H5Br+,

C2H3, C2H, Br and CH2Br+ were found to undergo reactions 39 and

43 through 47.

C2HBr+ + CHBr --- C211 Br+C2H5 + Br 39


CH2Br+ + C2H5Br --- C2H + CH2Br2 43

Br+ + C21HiBr --- C2H5Br+ + Br 44


C2 H + C215Br --- C2H5rH + C2H2 45


C2H+ + C H Br --- C Br+H + C2H 46
2 5g 2 5 2 5 2 4 CHq 4


---- C2H5Br+C215 + HBr 47


C2H5Br' H + C2H5Br












Their double resonance experiments indicated that the bromine atom

in the diethylbromonium ion originated with equal probability from

the ionic and neutral fragment.

Recently Sieck and Gordon (18) used photoionization mass spec-

trometry to confirm the observations of Hamill and coworkers (15,

16) regarding the formation of the dimer ion in ethyl bromide. The

lower limit for the dissociative lifetime of the ion-molecule colli-

sion complex (C2H5Br)- was estimated to be 5.4 microseconds,

Negative ions are also important at low electron energies in

the alkyl bromides. Christodoulides and Christophorou (19) and

Christophorou and coworkers (20) employed the electron swarm beam

method to study the mechanism of dissociative electron attachment to

several alkyl bromides including ethyl bromide. The dissociative

electron attachment rate yielding Br- for ethyl bromide was a maximum

at a mean electron energy of 0.76 e.v. in the energy range of about

0.05 to 2.2 e.v. The process leading to Br proceeded through a

shortlived (<10-13 sec.) compound negative ion state, Although

Br was the most abundant ion in this study, their data suggested

that a second longer lived (10-i0 to 10-6 sec.) electron attachment

process was occurring. Also in this study the mean autoionization

lifetime of 55 x 10-14 sec. was determined.

Bansal and Fessenden (21) redetermined the thermal electron

attachment rate for ethyl bromide using the microwave conductivity-

pulse radiolysis technique. The maximum rate of electron attachment

occurred at an electron energy of 0.76 e.v. in agreement with the

results of Christodoulides and Christophorou (19).

















II. EXPERIMENTAL PROCEDURES AND EQUIPMENT


A. Reagents and Their Purification


1. Ethyl bromide


Baker analyzed reagent grade ethyl bromide was dried overnight

with Drierite and then fractionally distilled through a 4 foot

glass-helix packed Todd still. The still was operated under total

reflux for two hours and then at a reflux ratio of 50 to 1. The

middle cut which boils at 38.3 + 0.1C was retained. It was then

degassed on the vacuum line and vacuum distilled through a 25 cm.

column of barium oxide to storage vessel S3 (Fig. 1). The barium

oxide served not only as a drying reagent but also to remove possible

traces of sulfuric acids utilized in the commercial preparation of

ethyl bromide. It was then deoxygenated on the vacuum line by a

series of freeze-pump thaw cycles.

During the course of this research, several bottles of Baker

reagent grade ethyl bromide (Lot No. 39119) were tested for purity

with the gas chromatograph-mass spectrometer system. Although

alcohol-free as assayed on the bottle's label, several impurities

such as methyl bromide, 1,2-dibromoethane and bromoform as well as

some other mono-, di-, and tribrominated compounds were present in

total amounts on the order of 0.1% (cf 0.0001% analysis on the












bottle's label). In any event, following the distillation procedure

no impurities were detected with the flame ionization gas chromato-

graph. Periodically the purity of the sample in the storage vessel

was rechecked on the gas chromatograph.


2. Ethylene


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

passed through a Pyrex drying tube of barium oxide into storage

vessel S1 (Fig. 1) on the vacuum line. Oxygen was then removed

by the freeze-thaw method,


3. Oxygen


Matheson Company research grade oxygen was bled into the

vacuum system through a 1 in. drying column of 60-200 mesh reagent

grade silica gel.


4. Hydrogen bromide


Matheson Company hydrogen bromide (99.8% minimum purity)

was admitted through 1/4 in. copper tubing into a Pyrex drying

tube filled with P205 between copper filings. The gas was then

stored in a standard volume vessel on the vacuum line (Fig, 3),

The sample was deaerated by several cycles of freezing, pumping

and melting.












5. Chromatographic calibration standards
and other reagents


The gas calibration standards and other miscellaneous reagents

were used as received from their vendors,



B. Preliminary Remarks on Sample Preparation and Analysis


1. Principle assumption underlying sample
preparation and analysis


In sample preparations and analyses the amount of sample irradi-

ated or photolyzed as well as the corresponding product yields was

determined by assuming the applicability of the ideal gas law,


2. Storage of sample


Ethyl bromide was stored in the dark at -196C because of the

possibility of photodecomposition. Also the vacuum line was never

flamed to remove adsorbed gas molecules, even under a high vacuum,

because it was found that this would lead to the formation of ther-

molysis products which would contaminate the unirradiated sample.

In addition to this precaution, at no time was a Tesla coil applied

to the vacuum system while a sample was on the line because of the

possibility that it would initiate sample decomposition,


3. Treatment for cleaning radiolysis and
photolysis vessels


The vessels used for radiolysis were rinsed at least six times

with distilled water and then annealed at 565C to pyrolyze any












remaining organic residues. The photolysis vessels were cleaned in

a similar manner, except that a rinse with nitric acid preceded the

distilled water step, The nitric acid rinse was done to remove

possible mercury contamination arising from the vessel coming into

contact with mercury during the post-photolysis procedures.

The apparent effectiveness of this treatment in eliminating

traces of organic and inorganic bromide residues was supported by

the observation that water wetted the vessel walls uniformly- and

that no bromide ions were detected by potentiometric titration.

In the case of ethyl bromide-oxygen mixtures, because of the sub-

stantial amount of bromine formed, a rinse with sodium thiosulfate

was used as an initial step in the cleaning procedure in some of the

later experiments.



C. Preparation of Samples for Radiolysis


1. Vacuum system


The vacuum system used for sample preparation for radiolysis

experiments is shown in Figs. 1 and 2, It consisted of four

sections: a pumping section capable of reaching pressures of 0,1

to 0.5 microns, a section for pre-radiolysis sample preparation,

another for post-radiolysis sample preparation and analysis and a

main manifold interconnecting the three sections.

The pumping station was the conventional high vacuum type. It

consisted of a two-stage mercury diffusion pump, P, backed by a











Welch Duo-Seal Model 140OB forepump, and two liquid nitrogen traps,

T1 and T2. Two 8 mm, bore two-way ground glass stopcocks, V1 and

V2, could be rotated 180 degrees to allow pumping by the diffusion

and mechanical pump in union, or the mechanical pump alone, These

glass valves as well as the two glass valves, V8 and V9, in the

sample analysis section were lubricated with Dow Corning high vacuum

grease; the halocarbon greases proved unsatisfactory as ethyl bromide

tended to dissolve in them, All other stopcocks, with the exception

of a single 6 mm. West valve, V3, were Fischer-Porter 4 mm. 0-ring

sealed Teflon-glass valves.

The pumping station communicated with the main manifold through

the West stopcock, V3. Attached to the main manifold were a mercury

manometer, Ml, two one liter reservoirs, Sl and'S2, for reagent

storage, three inlets, I, 12 and 13, for introducing samples and a

vacuum thermocouple gauge, Gl, Strategically located, this vacuum

gauge as well as gauges G2 and G3 on the submanifolds, allowed the

pressure to be monitored independently through a multiposition

thermocouple meter.

A valve, V4, connected the center of the main manifold with the

submanifold which was used mainly for pre-radiolysis sample prepara-

tion. Ethyl bromide was stored in a detachable 500 cc. vessel, S3,

connected to this submanifold through an 0-ring joint, DeDending on

whether irradiation was to be carried out in a large or a small

radiolysis vessel (Figs. 5 and 6), the sample was metered to the

30.39 cc. standard volume vessel, Wl, or to the 324,7 cc, standard











volume vessel, W2, The amount of sample was measured with the

mercury manometer, M2, Attached to this submanifold through valve

V7 was a tubulation connected to the radiolysis vessel, R, shown in

Figs. 1, 5 and 6. The 0-ring joint between the stopcock and the

radiolysis vessel served both as an entry port for glass blowing and

as a means for attaching a 29.76 cc, standard volume vessel used in

the oxygen scavenger experiments.

The second submanifold in Figs. 1 and 2, which was used for

sample analysis following irradiation, communicated with the main

manifold through two valves, V5 and V6, The tubulation on this

submanifold isolated by valve Vl2 was for attaching the irradiated

sample. Valve V13 opened to the gas loop, L1 (Figs. 1 and 12),

employed in the gas chromatographic end product analysis, Also

connected to this manifold through values V10 and V11 was a Toepler

pump-McLeod gauge combination, C, with an intermediary glass-helix

packed U-trap, T3. The U-trap was used to prevent passage of con-

densibles into the combined Toepler oump-McLeod gauge during analysis

of non-condensibles. The three-way ground glass stopcock, V8, was

generally kept at a high vacuum by pumping through valve V6. Valve

V8 channelled the non-condensible gases from the radiolysis vessel

to the McLeod gauge, D, or to the gas loop, L3 (Figs. 1 and 12), or

from the McLeod gauge to the gas loop, L3,


2. Ethyl bromide


Ethyl bromide in storage reservoir S3 was deaerated by repeated

cycles of freezing, pumping and melting until the thermocouple



































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vacuum meter indicated the absence of air, Valve V4 was then closed

and the sample was allowed to expand into standard volume vessel W1

or 1W2 while the pressure was monitored with mercury manometer M2'

The final pressure was read with an Ealing cathetometer, At the

desired pressure required in the radiolysis vessel, the valve to the

standard volume vessel was closed and the excess material was con-

densed back into the cold finger on the storage vessel with liquid

nitrogen. Following reevacuation of the submanifold, value V4 was

again closed and the material in the standard volume vessel was

vacuum transferred to the radiolysis vessel by means of liquid

nitrogen. Although the transfer was essentially complete within

3 to 4 minutes as indicated on the thermocouple meter, 20 minutes

was allotted for this process, The radiolysis vessel was then

sealed off from the vacuum line with an oxygen-methane torch at a

reproducible mark, Recalibration of the vessel volume at a later time

confirmed that its volume never deviated more than 1% from its original

calibrated volume. After the glass cooled, the liquid nitrogen was

removed from under the vessel and it was allowed to warm to room

temperature before irradiation,

All irradiations were carried out at a pressure of 100 torr

except those intended for the analysis of hydrogen bromide, in which

case, the irradiation was at 300 torr.


3. Ethyl bromide with added oxygen


Under the condition that the radiolysis vessel was almost

entirely at -196'C, it was estimated that the amount of oxygen











transferred could be calculated by the ideal gas law with a correc-

tion factor of 4 corresponding to the difference in temperature between

ambient and -196C. However, such a temperature correction is

inadequate because of the uncertain extent of localized heating as

the vessel is sealed off using a hand torch. The actual quantity of

oxygen transferred to the vessel was measured with the Toepler-

McLeod gauge assembly. Knowing the actual quantity of oxygen trans-

ferred in a given case, an empirical correction was determined. Thus,

the quantity of oxygen introduced into the radiolysis vessel is

stated mathematically as

PV K "TR { "PL
I( N2



or, in words, that the pressure desired in the radiolysis vessel, PV,

equals an empirical constant, B(=0.8), times the ratio of room

temperature to liquid nitrogen temperature, TR/TN2,, times the oxygen

pressure in the vacuum line, PL' Although it is perhaps evident, it

should, nevertheless, be pointed out that for complete vacuum trans-

fer of the ethyl bromide, its transfer should precede that of oxygen.

In all irradiations with oxygen, the oxygen constituted 5%

of the total pressure except in the case of samples intended for HBr

determination, where it was 12% of the total pressure. The correspond-

ing ethyl bromide pressures were 100 and 300 torr respectively.












4. Ethylene


The preparation of ethylene differed from that of ethyl bromide

(Sec. II-C-2) only in that the former originated from storage vessel

S1 or S2 rather than from S3,



D. Preparation of Sample for Photolysis


1. Vacuum system


To avoid mercury sensitized reactions in photolysis experiments,

a mercury-free vacuum system was utilized for sample preparation.

This system is illustrated in abbreviated form in Fig, 3. The

pumping station consisted of the Labglass LG-10980 two stage oil dif-

fusion pump, P2, heated by a 50 ml, Glas-Col heating mantle, backed

by a Welch Ouo-Seal Model 1400B forepump, and two liquid nitrogen

traps, T4 and T5, The oil diffusion pump and the trapswere attached

to the vacuum line by means of 0-ring joints, Three 10 mm, West

0-ring sealed Teflon-glass valves, V14, V15 and V16, allowed pumping

by the diffusion and mechanical pump together or the mechanical

pump alone. The pumping station and the submanifold each communi-

cated with the main manifold through the 10 mm. West valves, V17

and V18. All other valves on the vacuum line were Fischer-Porter

4 mm, 0-ring sealed Teflon-glass valves.

Preparation of samples for photolysis was conducted on the sub-

manifold, All pressure measurements were determined with a Wallace

and Tiernan, Model 62-075 series 1000 differential pressure gauge

































































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G4, except those for actinometry (Sec. II-F-2) which were made on a

Kollsman 10 in. absolute mercury pressure gauge, G5. The extent of

vacuum was monitored through a thermocouple vacuum gauge G6, Ethyl

bromide was stored in a detachable 120 cc, vessel, S4, connected to

the submanifold by an 0-ring joint. The amount of sample delivered

to the photolysis vessel was determined by PV measurements using the

standardized 223 cc. volume submanifold in conjunction with the

Wallace and Tiernan gauge. The standardized volume refers to that

portion of the submanifold resulting when valves V22 and V24 are

opened and all other valves are closed.


2. Ethyl bromide


The experimental procedure for preparing ethyl bromide for

photolysis is analogous to that for radiolysis. Ethyl bromide in

storage reservoir S4 was deaerated by repeated freezing, pumping and

melting cycles until the thermouple vacuum meter registered no change.

Next, with valves V20, V22, and V24 opened and the rest of the valves

on the submanifold closed, valve V26 to the storage vessel, S4, was

cracked open to admit ambient ethyl bromide to the submanifold while

the pressure was monitored on the Wallace and Tiernan gauge. When

the desired pressure was reached, valves V20 and V26 were closed and

then valve V25 was opened to the photolysis vessel, F. The material

in the standard volume submanifold was then condensed into the

photolysis vessel and frozen at -196C using a Dewar of liquid

nitrogen over a period of 20 minutes. After pumping on the frozen












residue for 3 minutes, the vessel was sealed with an oxygen-methane

torch and then allowed to warm to room temperature after the heated

glass had cooled. As with the radiolysis vessel, the photolysis

vessel was sealed at a reproducible position to minimize the devia-

tion from its original calibrated volume.

A pressure of 100 torr was used in all photolyses of the pure

sample.


3. Ethyl bromide with added oxygen


The general experimental procedure described in Sec, IIC-3 was

followed. In all photolyses, the pressure of the ethyl bromide was

kept at 100 torr and the oxygen was 5% of the total pressure,


4. Hydrogen bromide


Because of the corrosiveness of the hydrogen halide, the

pressure gauge G5 was employed in place of the 'allace and Tiernan

gauge, G4, used in the earlier sample preparations.


The hydrogen bromide was bled into the submanifold which was

open to a standard volume vessel, as detailed in Sec, II-A-4, until

a pressure of about one atmosphere was read on gauge G5. The hydrogen

bromide was then condensed into the standard volume vessel with

liquid nitrogen and the residue gas pumped off. Utilizing the volume

of the submanifold, except for the section beyond valve V22, the

sample was degassed by the freeze-thaw method, Next the degassed

sample was expanded into the submanifold and valve V18 was cracked












open. When the desired pressure was registered on the gauge G5,

the valve to the standard volume vessel was closed and the remain-

ing material pumped off. After evacuation, the hydrogen bromide

was vacuum transferred to the photolysis vessel by means of liquid

nitrogen and the vessel was then sealed off with the torch.

Depending on whether the small or the large photolysis

vessel (Fig. 9) was being used, the standard volume vessel

described above was either 29.76 cc. or 120.5 cc. All actinometry

experiments with hydrogen bromide were carried out at a pressure

of 129.7 torr.


E. Sample Irradiation


1. Radiation source, vessel and
ancillary equipment


The cobalt-60 source employed in this investigation has been

detailed elsewhere (22). Fig. 4 shows a cross-sectional view of

the irradiator.

Two types of radiation vessels (Figs. 5 and 6) were used in

this investigation. Each was made of Pyrex and equipped with a

single breakseal. The large annular vessel (Fig. 5) with the 10 cm,

quartz optical cell was used principally for the spectrophotometric

analysis of bromine (Sec. II-H-5) and for the unscavenged hydrogen

determination. The annular portion of the vessel efficiently utilized

the radiation flux from the source, which could be positioned in its











center, and the vessel's optical cell made spectrophotometric analysis

simple. However, because of its large volume and shape, this type of

vessel was less convenient than the smaller cylindrical ones (Fig. 6)

for gas chromatographic (Sec. II-H-1) and hydrogen bromide (Sec. II-

H-4) analyses.

The volumes of the two annular vessels and the three cylindrical

vessels are listed below.


Annular Vessels

Vessel No. Volume

2 341.2 cc.

3 368.1 cc,

Cylindrical Vessels

Vessel No. Volume

8 29,92 cc.

9 29,96 cc,

10 30.24 cc.


The sample holders (Figs, 5 and 6) allowed reproducible position-

ing of the radiolysis vessels during irradiation. The annular vessels

fit snugly on a Teflon cap on a metal post; the height of this combi-

nation insured that the cobalt-60 source was situated in the center

of the vessel. After frequent exposure to the radiation flux for

about a year, however, the Teflon cap disintegrated and had to be

replaced.







32


































(G) shutter shown open- (H) rear wall; (1) door; (J) downward
I-i -
s F strg t F 44uiCU






shielding; (K) door carriage; (L) door crank; () door frame.








Emergency 6 foot tube in ground, under source, is not shown.
Fig. 4 Cross section of cobalt-60 gamma ray source
Legend=(A) counterweight; (B) upper support; (C) control rod handle;
(D) extra top shielding; (E) storage turret; (F) 400 curie C060;
(G) shutter shown open; (H) rear wall; (I) door; (J) downward
shielding; (K) door carriage; (L) door crank; ([1) door frame.

Emergency 6 foot tube in ground, under source, is not shown.











Fig. 4 Cross section of cobalt-60 gamma ray source





















































L
C)
-C

C)
-z















C)
-C







In

C))






















































Fig. 6 Cylindrical radiolysis vessel and holder












The vessel holder (Fig. 6) for the smaller vessels was an

aluminum block with a 0.630 in, diameter center hole for the source

and five peripheral holes of 0.787 in. diameters for the 0.786 in.

diameter vessels. Each peripheral hole was equally spaced on a

circle of 0.727 in. radius. A metal wall of 0.018 in. thickness

was left between the center hole and each of the peripheral holes.

The depth of the center hole was 3.125 in. and 4.25 in, for each of the

peripheral holes. The small cylindrical radiolysis vessel was,

therefore, positioned securely within the vessel holder and cobalt-60

wafers midway along its length.


2. Dosimetry

In the pressure range of 150 to 1000 torr at room temperature,

the hydrogen yield in ethylene under gamma radiolysis has been re-

ported (23) to be independent of absorbed dose up to about 5% con-

version. Furthermore, the G value for hydrogen production in ethylene

has been established (23, 24) as 1.2 molecules/100 e.v.

The absorbed dose rate of the ethyl bromide system was deter-

mined using ethylene dosimetry. The irradiation of ethylene was

carried out at 23 + 2C from 4 to 24 hours between the pressures of

200 and 600 torr in the vessels shown in Figs. 5 and 6. Following

irradiation the total pressure of hydrogen along with a small quantity

of methane and ethylene was determined using the Toepler-McLeod

apparatus, C (Fig. 1). Knowledge of the total pressure of the mixture

and the quantitative contribution of methane and ethylene allowed

the hydrogen yield to be calculated. From the slope of the plot in











Fig. 7 and the accepted G value for hydrogen formation, the energy

absorbed by the system was calculated. For the two types of vessels

used and the geometries in which the irradiations were carried out,

the absorbed dose rate of ethylene was 2.06 x 10 e.v./gram-hr. on

March 7, 1972, for the annular vessels and 2.72 x 1019e.v./gram-hr. on

January 10, 1973, for the small cylindrical vessels.

To the extent that the radiolysis vessels approximate the Bragg-

Gray cavity (25, 26), the rate of energy deposition in ethylene can

be correlated to that in ethyl bromide. Since the application of

the Bragg-Gray principle is justified (27), the ratio of the energy

deposited per unit mass in the sample and in the dosimeter can be

determined by the ratio of their mass stopping power as detailed

elsewhere (28).

The final form of the equation used to calculate the absorbed

dose rate in ethyl bromide is


Dose(C2H5Br) = 0.729 Dose(C2114)


in units of e.v./gram-hr. The absorbed dose rate, therefore, of

ethyl bromide on March 7, 1972, for the large annular vessel was

1.50 x 1019e.v./gram-hr. and on January 10, 1973, for the smaller

radiolysis vessel was 1,98 x 1019e,v./gram-hr. During subsequent

irradiations, the absorbed dose rates were corrected for cobalt-60

decay.



































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F. Sample Photolysis


1. Photolysis lamp and vessels


The photolysis lamp employed in this study was a General

Electric 15 watt Germicidal lamp which is illustrated in Fig. 8.

The Germicidal lamp is essentially a low pressure mercury lamp in a

quartz envelope.

The emission spectrum of this lamp was examined with a McPherson

Model 218 Vacuum Ultraviolet monochromator with double-beam attachment

and ratio-recording electronics. The McPherson monochromator was

operated with an entrance and an exit slit of 10 microns and at a

scan rate of 5 nm/min. The window on the monochromator was 3 mm,

thick Far-UV Silica.

In the spectral range of 170.0 to 450.0 nm, the principal lines

are 253.7 nm, 296.7 nm, 313.0 nm, 365.0 nm, 404.6 nm and 435.8 nm.

It should be noted (Table 1) that the output of the lamp is rich in

the 253.7 nm resonance line and that the 184.9 nm line is unobserved.

Fig. 9 shows the two types of photolysis vessels used in these

experiments. The smaller vessel was essentially a 10 cm. quartz cell

attached to a Pyrex cold finger and break seal. The long cylindrical

vessels were constructed of 20 mm. O.D. x 18 mm. I.D. General Elec-

tric Type 204 clear fused quartz with a Pyrex breakseal, The

General Electric quartz was later found to be inferior to several

other brands, for instance, that of the American Quartz Company, A

number of pieces of quartz received from G.E. had striations that












marred their optical quality and so care had to be exercised in the

selection of the pieces used. Also glass blowing on this material

proved difficult because slight overheating of the glass caused

bubbles to form in it.

The smaller vessel was used only for the spectrophotometric

analyses of bromine. All other product yields were determined using

the large vessels.

The volumes of the photolysis vessels are listed below.


Small Vessel No. Volume

5 34.2 cc.


Large Vessel No. Volume

2 88,22 cc.

3 91.81 cc.

4 93.68 cc.


The lamp fixture was a standard desk lamp designed to hold two

fluorescent lamps. In the original experimental arrangement, two

Germicidal lamps were used together with the large photolysis vessel

positioned between them. Their total output, however, led to too

rapid photodecomposition of the HBr actinometer. One of the lamps

was then removed and replaced by three Pyrex tubes that aided in the

positioning of the long photolysis vessel, The vessel was positioned

longitudinally and parallel to the single lamp and at a perpendicular

distance of about 2 cm. Marks on the glass tubing allowed the

vessels to be positioned reproducibly. The vessel's length allowed it












































































C,
U-

















Table 1

The Relative Intensity Distribution on a Logarithmic Scale of the
Emission Lines in the Spectral Range from 170.0 to 450.0 nm for the General
Electric 15 Watt Germicidal Lamp



Wavelength (nm) Log Relative Intensity

253.7 1 00

296.7 0.05

313.0 0.14

365.0 0.06

404.6 0.48

435.8 0.56

























































0 10 cm,


Fig. 9 Photolysis vessels











to be situated between the lamp's filaments, and air was passed

between the vessel and the lamp to avoid thermal effects and to

remove possible ozone formed. The latter has a strong absorption

band at 253.7 nm (29). Prior to each photolysis, the lamp was turned

on for a period of five minutes to allow it to stabilize, In addi-

tion, to minimize voltage fluctuations from the power line, a Sola

transformer was employed. The geometry of the arrangement and the

lamp's output allowed the photolysis to be carried out in periods of

less than 10 minutes.


2. Actinometry


Gaseous hydrogen bromide was employed for actinometry with the

Germicidal lamp. The near ultraviolet absorption spectrum of hydro-

gen bromide resembles closely that of ethyl bromide as would be

expected if the electronic transition originates from an orbital on

the bromine atom. The replacement of a hydrogen atom by an ethyl

group shifts the absorption maximum to the red. The hydrogen bromide

absorption band is rather broad with a maximum at about 185 nm and

a long wavelength tail extending to 299 nm (30), while the absorption

band of ethyl bromide reaches a broad maximum at 203 nm and tails to

approximately 300 nm (31). At longer wavelengths extending out to at

least 400 nm both molecules are optically transparent. The lower

energy lines of the Germicidal lamp above 253,7 nm should, therefore,

contribute insignificantly to the photolytic decomposition of these

gases; and hydrogen bromide, it may be concluded, was an excellent

choice as an actinometer for this investigation.











Hydrogen bromide was photolyzed at a pressure of 129.7 torr

and a temperature of 22.5 + 0.5C in a mercury-free system.

The quantum yield for hydrogen production with less than 1% conver-

sion has been taken as unity (32, 33). The hydrogen yield corre-

sponded to an absorbed intensity of 8,5 + 0.1 x 1013 quanta/cc.-

sec. (7.7 + 0.2 x 1015 quanta/sec.) for the large vessels and 8.0 +

0.1 x 1013 quanta/cc.-sec. (2.7 + 0.1 x 1015 quanta/sec.) for the

small photolysis vessel. The experimental conditions of the acti-

nometry were designed so that the absorbance of the hydrogen bromide

actinometer was the same as that of the ethyl bromide. Combination

of the deal gas law with the Beer-Lambert law for equal absorbance

of the hydrogen bromide and ethyl bromide yielded the expression


P(HBr)= a(C2H5Br) P(C2H5Br)
Sa(HBr)



-5 -1 -1
= .82 x 105 torr cm. 100 torr
6.03 x 10 torr cm.

where the extinction coefficients at 253.7 nm, a, for hydrogen bromide

and ethyl bromide respectively were interpolated from the graphical

data of Porret and Goodeve (30) and Goodeve and Taylor (31) and the

pressure of ethyl bromide, P(C2H5Br), was that employed in this

study.

The configuration of the photolysis apparatus (Fig, 8) pre-

cluded straightforward determination of the incident light

intensity, Io, because the path length of light through the photolysis











vessel was not easily ascertained. However, since actinometry ex-

periments were carried out at several pressures it was possible, as

a matter of interest, to determine this path length, Using the
P 15
absorbed light intensities I at 129.7 torr (7.7 x 105 quanta/sec.)
a
and I at 389.5 torr (20.5 x 1015 quanta/sec.) of the large

vessels, the path length was calculated through successive iter-

ations on a computer with the equation



P /abP
I l/ 1-e abP
Ia /Io l-e-a
a o


The path length, b, was found to be 16.0 cm., in contrast to the

2 cm. diameter of the photolysis vessel. Apparently, a major portion

of the light traversed the tube obliquely. Considerable internal

scatter from the cylindrical glass walls may also have occurred.

Insertion of the value of b into the Beer-Lambert law gives an

incident light intensity of 6.5 x 1016 quanta/sec,



G. Preparation and Spark Discharge of Sample


The technique of using a Tesla discharge to produce good

yields of the same kind of products formed by radiolysis has been

employed before in this laboratory (34).

The spark discharge vessel (Fig. 10) was a 500 ml, round-

bottom flask provided with two stainless steel electrodes opposite

each other, about 1 in. apart, and with a Fischer-Porter Teflon plug
























































Fig. 10 Spark discharge vessel











needle valve. The incorporation of the second Fischer-Porter Teflon

needle valve to the cold finger at the bottom of the vessel for the

sparking of gaseous mixtures followed the design by D. R. Johnson in

this laboratory.

In the vacuum preparation of ethyl bromide with oxygen, the

ethyl bromide was first admitted to the vessel from storage reser-

voir, S3 (Fig. 1). After the desired pressure was read on the

cathetometer, the valve to the discharge vessel was closed, the

sample was condensed into the cold finger and the valve to the

cold finger closed. Following reevacuation of the submanifold,

valve V4 was closed and the valve to the discharge vessel reopened.

A determined amount of oxygen was then admitted and the valve to the

vessel again closed. After removing the vessel from the submani-

fold, the condensed sample was warmed to room temperature and the

valve to the cold finger opened to allow the two gases to mix by

diffusion.

In the spark discharge, one electrode was grounded to a cold

water pipe and the other was in contact with a Tesla coil. Power to

the Tesla coil was supplied through a Variac set at about 30 volts.

During the discharge, a pale violet-blue color surrounded the

electrode ends; however, the low voltage setting caused no dis-

tinguishable arcing.

For the purpose of column evaluation (Sec. II-H-1-b), the spark

discharge was carried out at an ethyl bromide pressure between 4 and

6 torr for a period of about 5 minutes. Approximately 12 torr












sparked for 12 minutes'provided sufficient products for product

identification on the gas chromatograph mass-spectrometer system

(Sec. II-H-2). When oxygen was added, it comprised about 5% of

the total pressure.



H. Analytical Equipment and End Product Analysis


1. Gas chromatography


a. Principle instruments

A modified MicroTek 1600 gas chromatograph with a hydrogen

flame ionization detector was the principal instrument used in the

quantitative analysis of the organic products. The output of its

detection system was fed to the 1 mv, pen of a Minneapolis-Honeywell

double pen, 1 mv. and 10 mv, full scale, recorder. The instrument

had an output attenuator with a range of 1, 2, 4, 8, . 128

and an input attenuator marked X1 (most sensitive), X100 and X10000.

It was found experimentally, however, that the input attenuator

ranges were Xl, X573 and X57300,

The flow system for this instrument is illustrated in Fig. 11.

An L-206-6 Loenco multiport valve with Viton-A 0-rings (valve A)

was arranged so that in a two-column series, the down-stream column

could be switched out of the flow system, while a Hoke "Milli-

Mite" needle valve introduced into the bypass compensated for the

flow restriction of the switched-out column. This arrangement was

designed to prevent the proportionately large quantity of












unreacted ethyl bromide from the irradiated or photolyzed sample

from flooding the detector. In practice, however, it was found that

the flooding of the column with the excess parent compound was more

important than the saturation of the detector. Also incorporated

into the flow system was an Air Products tri-tube flow meter unit

to adjust and monitor the flow rates of the carrier gas, hydrogen

and air. Accurate control of the carrier gas flow rate is necessary

to reproduce elution times, and of substantial importance in repro-

ducing flame sensitivity. The critical factor in control of the

flame sensitivity, however, is the hydrogen flow rate, Control of

the air flow rate is not crucial and could be done at the air

tank pressure regulator.

As a result of the replumbing of the flow system, a heating

cartridge and blower were installed into the MicroTek sampling

compartment to prevent the high boiling components of a sample from

distilling out of the carrier gas onto the relatively cool metal

surfaces of the sliding valve and stainless steel tubing enroute to

the detector.

The instrument provided for the vacuum transfer of a sample

from an externally attached vessel to a sample loop attached to the

front of the chromatograph by Swagelok fittings. This sample

module, which was installed by J. M. Donovan (35), is similar to a

design by Marcotte (36).

In the present work, however, all samples were transferred

on the vacuum line submanifold (Figs. 1 and 2) to one of the gas












loops shown in Fig. 12 and these introduced the sample through the

sample loop ports on the front of the chromatograph instrument.

Three types of gas sampling loops (Fig. 12) were employed in

this work. Differences in the design of loops L1 and L2 were

to accommodate the instruments on which they were used. The

former was used on both a 550 Tracor instrument and a Loenco gas

chromatograph, the latter solely on the 1600 MicroTek, In the

analysis of gases non-condensible at liquid nitrogen temperature, a

gas sampling loop similar to one designed by J. W. Buchanan (37) was

employed. This loop, L3, was attached directly above the Toepler

pump as indicated in Fig. 1 by means of a 10/30 tapered ground

glass joint. Loops L1 and L2 were attached through a single inlet

arm to the vacuum submanifold shown in Figs. 1 and 2 via a 1/4 in.

stainless steel Swagelok union or elbow joint assembled with two

Viton-A 0-rings as a front ferrule and with the back ferrule re-

versed from normal.


b. Evaluation of column packing

The radiolytic and photolytic decomposition of a compound is

generally limited to much less than 1% to prevent secondary and higher

order product formation. Experimentally this means that the products

are formed in submicromolar quantities and that the parent compound

is essentially unconsumed, Accordingly, since the preponderance of

sample is the parent compound, a very desirable and perhaps essential

characteristic is a rapid unloading of the column to determine the

compounds eluting after the parent. Furthermore, as is the case















Injection Port


SSliding
Valve A












eater






>wer





Sliding Valve:
In ---
Out -


Fig. 11 1600 MicroTek gas chromatograph sampling module



































~-0_


Fig, 12 Gas sampling loops.












in the ethyl bromide system, the number of products formed may

exceed 25 and cover a boiling point range of 4000C, Such a wide

boiling range generally requires temperature programming and this

may limit the nature of the column material employed, especially if

it is to be used with a flame ionization detector. The upper

temperature limit of a column given in the literature is usually

determined for a thermal conductivity detector. For a flame ioniza-

tion detector, the operating temperature must be kept substantially

lower than the maximum temperature if noise and rising baseline

resulting from column bleed are to be avoided, There is also a lower

temperature limit determined by the viscosity of the phase. The

less viscous phases at the analysis temperature give the most.

efficient separation.

The properties of a number of solid adsorbents and stationary

phases were evaluated under conditions that simulated those in this

study (Secs. II-H-1-c and -d). The gaseous mixture was obtained

by spark discharge (Sec. II-G), Most of the stationary phases

examined were methyl silicones which had low selectivity according

to the McReynoldsconstants (38). All of the liquid phases, including

Porapak Q,separate a homologous series of compounds according to

their boiling points. Only the solid adsorbent, silica gel, was

capable of separating all of the C2 hydrocarbons. Ethylene and acety-

lene were unresolved in all other cases.

A brief comparison of some of the column materials examined

follows below:











(1) 10% Diethylene glycol adipate and 90% GE SF-96 (28.6%

liquid phase) provided the most symmetrical peaks and unloaded the

most rapidly of the liquid phases; however, its thermal stability

was much poorer as bleeding occurred at a lower temperature.

(2) 30% SE-30 GC Grade behaved similar to 30% OV-101. It

unloaded fairly well; however, its thermal properties were not as

good.

(3) 30% OV-101 showed some tailing of peaks. A slight base-

line rise began at 160'C but did not become excessive until 190 to

200C. 10% OV-101 did not separate as well as the 30% loaded column

although it unloaded more rapidly as expected.

(4) Porapak Q (polystyrene-type porous polymer) recovered

relatively slowly from overloading and the alkyl bromides tailed

considerably unless rapid temperature programming was used.

(5) Baker reagent grade 60-200 mesh and Analabs' Anasorb 40-

50 mesh silica gel were comparable in their separation of the C2

hydrocarbons when the former was used at room temperature and the

latter at 85C. The bromides did not easily elute off of either

material.

All of the liquid phases described were coated on acid washed

Chromosorb P. Column dimensions in each case were about 3.1 m. x

0.25 in. O.D. except for Porapak Q in which they were 1 m. x 0.25

in. O.D. Several different temperature programming rates and flow

rates were also explored to determine the conditions which gave

the optimum separation of the compounds. Of the columns evaluated,












the 40-50 mesh silica gel was chosen for the determination of the low

molecular weight C2 hydrocarbons and the 30% OV-101 on acid washed

Chromosorb P was selected for the analysis of the intermediate and

high molecular weight organic compounds. Typical gas chromatograms

from the analysis of irradiated ethyl bromide are given in Fig. 15.


c. Products non-condensible at -196C

Immediately after radiolysis or photolysis, the vessel was

immersed in a Dewar of liquid nitrogen to reduce the possibility of

post-irradiation and post-photochemical effects (Sec. III-B),

In the determination of the non-condensibles and the condensible

organic products, the vessel was attached to the submanifold of the

mercury vacuum line (Fig. 2) by means of the breakseal fitting,

After a good vacuum had been reached, the U-trap, T3, was cooled in

liquid nitrogen for 20 minutes and then valve V5 was closed. The

hammer above the breakseal was then released and the gases non-

condensible at -196C were collected in two stages of Toeplering. In

the first stage, they were collected in 12 Toepler pump cycles at one

minute intervals. Next, valve V10 was closed and the condensible

material was vacuum transferred to loop L2 with liquid nitrogen over

a 20 minute period. The valve to the loop was then closed and valve

V10 on the U-trap reopened. Another series of 12 Toepler cycles

were carried out to complete the transfer of the non-condensibles.

By the sixth or seventh cycle the transfer was, in general, com-

plete as monitored through the thermocouple vacuum gauge, G

directly above the sample vessel. The main procedural differences











for the dosimetry and the actinometry were that the condensible

material was not transferred to the gas loop L2 and the Dewar of

liquid nitrogen was kept under the U-trap during the second stage

of Toeplering.

Hydrogen was determined by two methods. In ethylene dosimetry

and in the radiolysis and photolysis of pure ethyl bromide, the non-

condensibles were Toeplered to the McLeod gauge, D, for a PV

measurement and then to the non-condensible gas loop, L3, for gas

chromatographic analysis by flame ionization of the small quantity

of organic products that contributed to the pressure measurement

as described below in the discussion of methane. The hydrogen was

then determined by difference. In actinometry and in the oxygen

scavenged radiolysis, the non-condensible gases were Toeplered to

the gas loop L3 for analysis on the Tracor Model 550 gas chromato-

graph equipped in this laboratory by A. R, Ravishankara with a Gow-

Mac Model 10-285 thermal conductivity detector using WX filaments. The

hydrogen was separated on a 20 ft. x 0.25 in. O.D. copper column of

molecular sieve with nitrogen carrier gas at room temperature,

Direct analysis of hydrogen with thermal conductivity gas

chromatography could be employed only if one lambda or more of

hydrogen was being measured. When applicable, however, this method

left less doubt about the presence of air and was more expedient

than the PV measurement. When the McLeod gauge is used, it is

necessary to make an empirical correction for the small amount of

air present in the apparatus; otherwise, there will be an apparent

finite yield of hydrogen at zero radiation dose.











In the radiolysis and photolysis, methane was in sufficiently

small yield that essentially all of it could be Toeplered to the non-

condensible gas loop for analysis on the MicroTek Model 1600 gas

chromatograph (Sec. II-H-1-a). Methane was separated from the C2

hydrocarbons isothermally at 85C on a 3.1 m. x 0.25 in. O.D.

stainless steel column packed with Anasorb 40-50 mesh silica gel

with a nitrogen carrier gas at a flow rate of 95 cc./min. In dosime-

try only ethylene was present while in ethyl bromide radiolysis and

photolysis, both ethane and ethylene contributed to the non-

condensibles. In each instance, the components were quantitatively

determined relative to three methane standards injected with a

Unimetric 50 1l gas-tight syringe immediately prior to the actual

analysis. In 16 consecutive radiolysis experiments over a three-

week period, the maximum variation of the detector's response to

methane was 11% with a relative standard deviation of 3,3%, The peak

areas were determined by approximation of the peaks with several

triangles which gave more consistent values than the method of

single triangulation commonly referred to in the literature.


d. Organic products condensible at -196C

The organic condensibles were transferred to the gas loop L2

on the submanifold of the mercury vacuum line as described in

Sec. II-H-1-c. The low molecular weight C2 products ethane,

ethylene and acetylene were determined in separate experiments from

the intermediate and high molecular weight products on the 1600

MicroTek gas chromatograph.











The low molecular weight hydrocarbon analysis was carried out

under the same conditions described in Section II-H-1-c in the

discussion of methane. The C2 saturates and unsaturates were

identified by retention times and confirmed on the gas chromatograph-

mass spectrometer-computer system (Sec. II-H-2). After each analysis

the column was conditioned at 250C for several hours to elute ethyl

bromide and other adsorbed alkyl bromides.

The intermediate and high molecular weight products were

separated on a 4.2 m. x 1/4 in. 0.D. stainless steel column packed

with 30% OV-101 on 60-80 mesh acid washed Chromosorb P with a nitro-

gen carrier gas flow rate of 32 cc,/min. The gas chromatograph was

ballistically programmed from room temperature to 2000C according to

the following Variac schedule:


Variac Time Temperature
(volts) (min.) (dg C)

0 0-45 ambient

45 45-67 ambient-80

50 67-127 80-133

increment Iv. 127-210 133-200
every 5 min.


At the end of each analysis, the column was conditioned at 250C

for several hours. The products were identified on the gas chromato-

graph-mass spectrometer-computer system (Sec. II-H-2) and confirmed

when possible (Table 2) on the 1600 MicroTek gas chromatograph.

The response of the flame ionization detector to the products

was calibrated relative to the mean of three standard propane











injections made immediately prior to each analysis. All gas standards

were injected with a Unimetric 50X gas-tight syringe and liquid

standards with a Unimetric 10X liquid syringe. Reproducibility of

the standards based on 4 or 5 injections typically had a relative

standard deviation of much better than 1%.


2. Gas chromatography-mass spectrometry-computer
system in the identification of organic products


A detailed description of the gas chromatograph-Bendix Model

14-107 system has been given elsewhere (39). Recently, however, the

instrument has been interfaced with a single stage jet molecular

separator designed by R. J. Hanrahan and J. E. Prusaczyk in this

laboratory. The critical dimensions, which were suggested by an

earlier design of Ryhage (40), are jet and skimmer orfices of 0,010 in,

separated by a gap of 0.020 in. The separator was installed in an

existing U-trap of the gas chromatograph-output trapping manifold

(39) without interfering with the original design of the unit

(Fig. 13). Since the molecular separator allows the effluent from

the gas chromatograph column to be continuously diverted into the

ion source of the Bendix mass spectrometer, use of this modification

is more convenient and efficient than the old trapping system.

In principle, the molecular separator compresses and accelerates

the gas chromatograph effluent into a narrow jet from which the

light helium atoms (carrier gas) are scattered and the heavy organic

molecular weight molecules are retained. In practice, it allows the

gas chromatograph to operate at atmospheric pressure and the mass

























































L






U 0
EQ
0 r

00r
0)


S
S0
0 0





'" 1Q


r0 U

0) 0


SL CL 0-


u. o~
rr E E ^ *
0 =0 C/C0



E L U.. o
E LL 0
0 0 00
0- LEO
EJ 00
00
0


L

4-,


OU
0
vl
4-4
00
1-












spectrometer at 10-5 torr. Helium is used as the carrier gas be,

cause it is easily skimmed off by the separator; fortunately, it

also provides minimum interference with the mass spectral cracking

pattern of other species.

A Hoke "Milli-Mite" metering valve diverted about 1/4 of the

column effluent to the detector of the gas chromatograph and the

remaining fraction to the ion source of the Bendix mass spectrometer.

The gas chromatograph separated the components while the mass

spectrometer provided information on the empirical formula and the

structure of the compounds. Data acquisition of the mass spectra

was accomplished under semi-automatic control of a General Automa-

tion SPC-12 minicomputer. The SPC-12 computer is capable of making

about 500 complete digitizations per second and taking 20 digiti-

zations per peak. The mass spectrometer software (41) includes

a conversational executive system which permits the operator to

communicate with the computer via an ASR-38 Teletype in 2 letter

mnemonics. A PEC 9-track IBM magnetic tape unit facilitates the

transfer of information to and from the computer. In particular,

during a gas chromatograph-mass spectrometer run, data are collected

and stored on magnetic tape and at a later time are retrieved and

reduced.

At a scan rate of 9 on the Bendix mass spectrometer, 20 sec.

is required to cover a mass spectral range of 14 to 400. At a more

rapid scan speed, resolution is sometimes sacrificed, while at a

slower speed there may not be sufficient time to scan the fragments











in the heavy mass range. Normally, both the gas chromatogram on

the chart recorder and the oscilloscope screen on the Bendix are

visually monitored, and when a peak is observed the spectral scan is

manually initiated from the mass spectrometer. Frequently, it is

possible to detect samples in small quantities that appear neither

on the chromatogram nor on the oscilloscope screen by repeated

scans and comparisons of the number of peaks in the spectra that are

printed out on the Teletype,

One problem encountered in optimizing experimental conditions

is the adjustment of the stream splitter to properly partition the

column effluent between the flame ionization detector and the ion

source. If insufficient carrier gas is proportioned to the gas

chromatograph detector, not only the column effluent but also air

and hydrogen from the ionization detector are pumped into the mass

spectrometer. In most operations, a supplemental carrier gas flow

is added between the column exit and the stream splitter to circum-

vent this effect (39).

Another problem related to flow control is the possibility

of lag time between the sample reaching the flame ionization detector

and the mass spectrometer. This presents a particular problem when

the column is heated during temperature programming. As the oven

temperature rises the sample tends to reach the mass spectrometer

before the gas chromatograph detector. The delay time may be as

great as 30 sec. at 1500C if the experiment was begun at room

temperature and signals were then being received concurrently. The











difficulty is resolved somewhat by proportioning additional carrier

gas to the gas chromatograph detector while the experiment is in

progress.

Column bleeding from the 30% OV-101 on Chromosorb P caused

only minor interference with the cracking pattern of the unknown

compounds until temperatures exceeded 1600C.


3. High pressure mass spectrometry


a. Equipment for ion molecule studies

The Bendix mass spectrometer could be readily modified from

the analytical to the ion-molecule mode (42). The high pressure

source and accessories were patterned after a design described in

detail by Futrell and coworkers (43). The ions were generated in a

continuous mode with 100 ev. electrons. With the exception of

reducing the pulse height signal on grid No. 1 from +25 volts to +23

volts, the ion source potentials were the same as those reported by

Futrell and coworkers (43). The horizontal and vertical deflection

control settings including the ion focus control were varied to

optimize resolution which would change as a function of the source

pressure. Because the relative ion intensity in the mass spectra

would change with slight variation of these controls, it was neces-

sary to visually monitor the mass spectra on an oscilloscope while

making these changes to insure that the ion intensities throughout

the whole spectrum would change uniformly.











b. Chemical ionization study

Chemi-ionization mass spectrometry is similar to electron

impact mass spectrometry except that in the former, the ionization

of a substance is effected by the reactions between the molecules of

the substance and chemical species other than electrons. The

chemical species are reactant ions or electronically excited meta-

stable neutrals formed by electron impact and ion-molecule reactions.

Only a cursory study was undertaken using this method for the

identification of products formed in the spark discharge of ethyl

bromide (Sec. II-G). Field (44) employed this method and found that

identification of a compound in methane was drastically simplified

over electron impact mass spectrometry.

The experimental arrangement used in the present study was

modeled after the set up in Sec, II-H-2 except that the high pressure

source replaced the analytical source. Helium from the gas chromato-

graph served as the ionizing reactant and the pressure in the source

was 0.5 torr.

Helium produced substantially more fragmentation than electron

impact making identification more complex, a result which suggests

that helium is more effective than 100 e.v. electrons in funneling

energy into the molecules. Such an explanation is not unreasonable

since the existence of states with excitation energy up to helium's

first ionization potential, 24.6 ev. (45), and above have been

known for some time. In the rare gases, some of these states

participate in Penning ionization, Hornbeck-Molnar reactions or

charge exchange.











4. Potentiometric titration of hydrogen bromide


The hydrogen bromide yield was determined electrochemically.

Immediately after irradiation and photolysis, the reaction vessel

was cooled to liquid nitrogen temperature, slightly acidified

distilled water was added to the solid sample through the breakseal

fitting. The vessel was removed from liquid nitrogen and the con-

tents were shaken until the solid melted, The hydrobromic acid

solution was then poured into a beaker and the vessel walls were

rinsed several more times with small quantities of the acidified

water to extract the remaining acid, About 30 ml. in total were

used in the extraction of the hydrogen bromide.

The Br- ion was titrated using either 0.02 or 0,05 M AgNO3

in a 50A gas-tight Hamilton syringe with a platinum needle, Differ-

ences in the EMF between an Orion Model 94-35A bromide ion specific

electrode and a double junction calomel electrode were monitored

using a high input impedance (10 megohms) Hickok digital volt-ohm

meter having a minimum voltage range from 0 to 199 mv. The refer-

ence electrode was in fact a Beckman Model 39270 fiber junction

calomel electrode in a slightly cracked test tube containing a 1

molar solution of KNO3. To insure adequate mixing, the solution

was continuously stirred with a Teflon coated magnetic stirrer.

A representative titration curve is reproduced in Fig. 14.

The equivalence points were always easily identifiable. For

example, when a 0.0200 molar KBr solution was titrated with a

0.0200 M AgNO3 solution, the equivalence point agreed to within














.240





.220




.200




.180




.160





.140




.120





.00 8 12 16 20 24 28 32 36 40 44

Volume of titrant, 0,02 M AgN03, added (microliters)

Fig. 14 Potentiometric titration curve of hydrogen bromide
Fig. 14 Potentiometric titration curve of hydrogen bromide











+ 0.0002 H1. In the actual determination, the uncertainty should be

within + 1%.


5. Spectrophotometric and chemical
analysis of bromine


The determination of bromine in the pure and scavenged radioly-

sis sample was made spectrophotometrically at room temperature in

the vapor state. For the analysis, the annular radiolysis vessel

with the attached optical cell (Fig. 5) was used. Absorbance

measurements at 416 nm were carried out between successive irradi-

ations on the Beckman DU spectrophotometer with the Gilford Model 222

photometer and power supply. In a typical analysis, the sample was

condensed into the cold finger of the reaction vessel and the cell

absorbance determined. An optical measurement was then made of the

thawed sample and the difference in the optical density was taken

to be proportional to the concentration of bromine. The amount of

bromine present was determined using the extinction coefficient
-1-l
170 -1 cm.- (46). The uncertainty in measurement may be as large

as 20%.

The same method was employed in the determination of bromine

in the photolysis.

In addition to the direct spectrophotometric measurement for

bromine in the pure radiolysis, a color test using dithizone (47)

was also tried. An irradiated sample was vacuum transferred to a

cold finger on the vacuum submanifold (Fig. 2) and the neck of the

tubulation was sealed off with a torch. The sealed sample was then










broken open under anhydrous carbon tetrachloride and dithizone

(diphenylthiocarbazone) was added dropwise. A change in color from

yellow to red would have indicated the presence of bromine. To

facilitate breaking open the sealed sample, the glass had been scored

with a file.

Another test carried out for bromine in the pure ethyl bromide

radiolysis involved breaking the sealed sample under a strip of

filter paper saturated with a solution of fluorescein in a 1:1

methanol-water mixture. The development of a red spot on the filter

paper would have pointed to the presence of bromine, This test

detects as little as 2 gamma (ly = 10-6g,) of bromine (48),

Also, after each radiolysis and photolysis the sample was

cooled to -1960C and inspected for yellow spots or colorations

testifying to the existence of bromine in the sample,

















III. EXPERIMENTAL RESULTS


A. Qualitative Identification of Products


The inorganic products formed from the radiolysis of ethyl bromide

were identified as hydrogen, hydrogen bromide and in the oxygen-scavenged

system, bromine. In the photolysis the same inorganic products

were likewise detected except that no hydrogen was observed. These

compounds were identified using thermal conductivity gas chromatography,

potentiometry and absorption spectroscopy respectively.

The organic products were identified by their gas chromatographic

retention times and when possible confirmed by their mass spectral

cracking patterns. The array of peaks present in a gas chromatogram

for a 24 hour radiolysis of the pure sample is shown in Fig. 15.

The low molecular weight hydrocarbons methane, ethane, ethylene and

acetylene were identified by retention times on a silica gel column

and later confirmed by mass spectrometry. Identification of a number

of the heavier products on the Bendix mass spectrometer was made

possible by the largeryields produced in the spark discharge of

ethyl bromide. N-butane, methyl bromide, vinyl bromide, methylene

bromide, 1,l-dibromoethane, cis-1,2-dibromoethylene, 1,2-dibromoethane,

bromoform, and 1,1 ,2-tribromoethane were identified using this means.

The remaining products were identified only by their gas chromatographic




















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0.- C
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LA0 C-


C-)

0


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40
C-
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0 CL-)

C0 o
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CM C- C CM0
CO 0- CM CO




I e- -u
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41








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0 C- LA CA =1:
u ol 0- CM i-L CQ r C) nC'



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C) 2: CM. C CM
C- CM C- C -
CQ 0 CO CO
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C- C- 1 0 U3
LA LA n0 CM 2: C- 2:
0)t CM 0) CM CM .0 C')
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) i- cM C') 0- iI) (0 1-.


























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Detector Response




X CO
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74






Detector Response
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C) C









CO




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0 r-

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40 i











Table 2

Products in Spark Discharge of Ethyl Bromide

S Method of
Organic Compoundsa Boiling Points (0C) Identification

CH4 -161.5 1,2

C2H6 88.6 1,2

C2H4 -103,7 1,2

C2H2 84 1,2

C3H8 42,1 1,2

C4H10 0,5 1,2

CH3Br 3.5 1,2

C2H3Br 15,8 1,2

C2H5Br 38,4 1,2

2-C3H Br 59,4 1

1-C3H Br 71,0 1

2-C4H9Br 91.3 1

1,1-C2H2Br2 92d 2

CH2Br2 97 1,2

1-C4H9Br 101.6 1

1,1-C2H4Br2 108-10 1,2

trans-1,2-C2H2Br2 108 1,2












Table 2 (continued)


Organic Compoundsa

cis-1,2-C2H2Br2


1,2-C2H4Br2


Boiling Points (C)b

112.5e

131,7


Method of
Identification

1,2

1,2


C3-dibrominated at 120C

1,2-C H Br2

CHBr3

C3 or C4 brominated at 128C

meso-2,3-C4H8Br2

1,3-C3H6Br2 and/or
racemic-2,3-C4H8Br2

1,3-C4H8Br2

C2HBr3

C3 or C4 brominated at 140C


1,1,2-C2H3Br3

1,4-C4H8Br2

Unknown at 170C


141 .6

149.6




157.4f


167.3/160.6


174-5

163-4




188.9

197-8


aMethane through acetylene were separated under the conditions described
in Fig. 15 a and b, while the remaining products were separated accord-
ing to the conditions in Fig. 15c
bBoiling point data are from reference (49) unless otherwise indicated.


CNumbers represent:


1-Mass spectral cracking pattern,
2-Comparison of gas chromatographic retention
times with standard samples.
3-Gas chromatographic retention time and boiling
point data--see Sec. III-A.







77



Table 2 (continued)

dReference (50).

eReference (51).

fReference (52),

gReference (53).













retention times since their yields were too small to be amenable

to mass spectrometric analysis. The list of products identified

in the spark discharge is given in Table 2. Not all of these

products appeared in the radiolysis, for instance, trans-1,2-

dibromoethylene and tribromoethylene were not observed.

Mass spectral tables for the brominated compounds appear-

ing in the radiolysis are collected together for convenience

at the end of this section. Air and water background has been

subtracted out of the spectra. The bleeding of the methyl

silicone column contributed a number of minor peaks that did

not correspond to any hydrocarbon fragments in this system

such as 45,47, 90 and 208 as well as several that did correspond

such as 92, 93, 107 and 121. The former type of peak has also

been omitted from the spectra, These contaminants were only

minor, less than 1% of the base peak, and could be observed to

grow during temperature programming of the column.

The mass spectra of brominated compounds exhibit several

distinguishing features. The most prominent one arises from

the existence of two bromine isotopes which are two mass units

apart and in approximately equal abundance. Application of the

binomial expansion to the natural abundance of the two

isotopes allows their contribution to a compound to be esti-

mated (54). Furthermore, the molecular C1 and C2 brominated













ions are quite stable and their presence in the spectrum im-

mediately identifies the compound,

The following discussion will consider the mass spectrum

of each compound (Tables 3, 4, and 5) in the order that it elutes

from the gas chromatograph column (Fig. 15) into the mass

spectrometer.

Peak No. 6, CH3Br: Peaks 96 and 94 clearly correspond

to the molecular ion. Loss of Br, HBr and H2Br produces the peaks

15, 14 and 13. A comparison of the relative intensities of

the CH3+, CH2 and CH+ fragments with those of the Br+, HBr

and H2Br+ fragments indicates that during bond fission the

ionization of the hydrocarbon fragment is more important rela-

tive to the complementary bromine fragment, presumably due to

the greater electron affinity of the Br atom. With the excep-

tion of the large 93 peak whose origin is partially in doubt,

the remaining fragments agree with the methyl bromide assign-

ment.

Peak No. 7, C2H3Br: Peaks 106 and 108 correspond to

the molecular ion. Loss of Br, HBr and H2Br leaves relatively

intense 27, 26 and 25 peaks. The CBr+ fragment corresponds

to a C-C fission. The mass spectrum is that of vinyl bro-

mide.

Peak No. 8, CLHBr: The peak is the parent compound,

ethyl bromide.












Peak No, 11, CH2Br2; The molecular ion identifies the

compound as methylene bromide. Supportive evidence is provided

by the CH2Br CH and CH24 ions resulting from the loss of Br,

HBr2and Br2. In general, the loss of HBr2 rather than Br2

is more probable in the heavier di- and tri-brominated com-

pounds. The relative abundances of the three peaks 176: 174:

172 are in reasonable agreement with the expected 1:2:1 ratio

considering that the amount of sample eluting from the column

is continuously changing during the mass spectral scans.

Peak No. 13, 1,1-C2HBr2: Mass peaks 190, 188 and 186

correspond to the empirical formula C2H4Br2. The presence of the

CHBr2 ion as mass numbers 175, 173 and 171 and the absence

or at least insignificant intensity of the CH2Br fragments

at 92 and 94 establishes the compound as 1,l-dibromoethane

rather than 1,2-dibromoethane.

Peak No. 14, cis-1,2-C2H2Br2; The mass spectrum corre-

sponds to a compound with the empirical formula C2H2Br2,

The retention time data indicate the compound to be cis-

1,2-dibromoethylene rather than trans-1,2-dibromoethylene or

1,l-dibromoethylene.

Peak No. 15, 1,2-C2H4Br2: Absence of peaks at 171, 173
'I ' - 2!-4-'2 '
and 175 and comparison with the mass spectrum of Peak No. 13

identifies the compound as 1,2-dibromoethane.











Peak No. 19, CHBr3: The ratio of the relative intensities and

mass numbers of the peaks at 256, 254, 252 and 250 corresponds to

CHBr3. Loss of Br, HBr2 and Br2 yields the CHBr2 CBr and CHBr+

fragments.

Peak No. 24, 1,1,2-C2H3Br3; The largest mass numbers at 270,

268, 266 and 264 identify C2H3Br3 Loss of Br and HBr2 results in the

formation of the C2H3Br2+ and C2H2Br+ ions and C-C bond rupture yields

the CHBr2 fragment.

Four minor compounds which were never identified appeared in

the radiolysis. These correspond to the peak numbers 16, 18, 23 and

26 in Fig. 15. The position of these peaks in the gas chromatogram

reveals some information about them. The OV-101 column separates a

homologous series of compounds by their boiling points (Table 2),

and for a given number of carbon and bromine atoms the most unsaturated

compound elutes first. Peak No. 16 occurs before the simplesttri-

brominated compound, CHBr3, but after all the dibrominated C2 com-

pounds. Furthermore, it elutes before 1,2-C3H6Br2 whose boiling point

is 141.60C. The compound may be 1,1-C3H6Br2 which boils at 1300C

or an unsaturated dibrominated C3 compound. Peak No. 18 is bracketed

between the two highest boiling saturated dibrominated C3 compounds,

1,2-C3H6Br2 and 1,3-C3H6Br2 so it is probably either 1,3-C3H4Br2 or

a dibrominated C4 compound. Peak No. 23 may be a tribrominated C3

or C4 compound having a boiling point somewhere between C2HBr3

(Table 2) which boils at 163-4C and 1,1,2-C2H3Br3 which boils at

188-9C%. Peak No. 26 must be a compound with a boiling point











Table 3

Mass Spectra of Ethyl Bromide Spark Discharge
Product Nos, 6, 7 and 8

Relative Intensitiesa
m/e Assigned Formulasb No. 6 No. 7 No. 8

12 C 2.08 0.72 0.35

13 CH 4.33 1.38 0,92

14 CH2 9.03 1.81 0,76

15 CHI 78.52 2,57


3
C2


C2H
C2H2

C2H3

C2H4

C2H5
Br
81 Br

CBr

CHBr

CH2Br,C81Br

CH3Br,CH81 Br

CH281 Br

CH381Br

C2Br

C2HBr

C2H3Br,C2H81Br


2.88

13,73

32.18

100.00


29.15

28.31

10.49

4.08

43.38

100.00


87.93


14.09

13.70

2.26



1 .74


1.38

7.23

41 .52

99.97

12.48

100.00

15.13

16.32

1.52


5.85



3.72


0.92

1,87

37.36


0,82

1 19












Table 3 (continued)


m/e Assigned Formulasb No. 6 No. 7 No.8

108 C2H5Br,C2H381 Br 31.79 50.06

110 C2H 581Br 44.39


aBackground has been subtracted out of the mass spectra.

bBR means 79Br.


RF~l~ti~lp Tntpncitipza











Table 4

Mass Spectra of Ethyl Bromide Spark Discharge
Product Nos. 11, 13, 14 and 15

Relative Intensitiesa


m/e Assigned Formulasb

12 C

13 CH

14 CH2

15 CH3

24 C2

25 C2H

26 C2H2

27 C2H3

28 C2H4

39 C3H3

41 C3H15

79 Br

81 81Br

91 CBr

92 CHBr

93 CH2Br,C 1Br

94 CH1 Br

95 CH281Br

104 C2HBr

105 C2H2Br,C281Br

107 C2H4Br,C2H281Br


No. 11 No. 13 No. 14 No. 15


0.29


1 .42

4.69

9.19


5.50

1.75 27,22

3.42 100.00

3.32


2.60

6.78

48.16

45.39

21.26



100.00



67.24


21.20

20.97

1.72



0.88


2.67

3.34

64.46


1.11



4.57

22,11

59,12

1,01







40.02

38,52

3.99

2.28

4.71

1.53


100.00

50.49


0.41

0.80

1,92

0,32

1 .08

6,51

36.89

96.98

8.18





22,11

23,83

5.64



14.79



6.95



44.32

80.02












Table 4 (continued)


h


Relative Intensitiesa
No. 11 No. 13 No. 14 No. 15

55.98 100.00


1.06

1.76


2.60

4.44


1 .90


Assigned Formulas

C2H481Br

Br2

Br81Br
81Br2

CHBr2

CH2Br2,CBr 8Br

CHBr 8Br

CH2Br81Br,C81Br2

CH81Br2

CH281Br2

C2H2Br2,C2Br81Br

C2H4Br2,C2H2Br81Br,

C281Br2

C2H4Br81 Br,C2H281Br2

C2H4 81 Br2


11.04

18.55

8.62


35.98



61,48

29.30


7,55

12.15

5.72


spectra.


aBackground has been subtracted out of the mass

bBr means 79Br,


46.56



80.90



36.52


















m


Table 5

Mass Spectra of Ethyl Bromide Spark Discharge
Product Nos, 19 and 24

Relative Intensitiesa
/e Assigned Formulasb No, 19 No. 24

12 C 3.56 0.87

13 CH 9.20 1.67

14 CH2 2.29

24 C2 2.61

25 C2H 17.99

26 C2H2 73.19

27 C2H3 67.70

79 Br 67,31 43.59

81 81Br 62.73 43.65

91 CBr 59,68 6.06


92 CHBr

93 CH2Br,C81Br

94 CH3Br,CH81Br

105 C2H2Br,C281Br

107 C2H4Br,C2H281Br

158 BrBr

160 Br81Br

171 CHBr2

173 CHBr81Br

175 CH81 Br2

185 C2H3Br2,C2HBr81 Br


16,04

51,16

18.01


5.03

6.61

62.71

100.00

46.47


8,70


100,00

34.75



8.94

1.97

3.11

1 .43

85.21












Table 5 (continued)


b
Assigned Formulas

C2 H3Br8BrC2H81Br2

C2H381Br2
CHBr3

CHBr281Br

CHBr8Br2

CH Br3

C2H3Br,3,C2HBr281Br

C2H3Br281BrC2HB r8Br2
C2H3Br81Br2,C2H81Br3

C2H381Br3


Relative Intensitiesa
No. 19 No. 24

37.83

39.10


8.87

13.78

13.10

3.90


4,50

8.31

7.93

2.19


aBackground has been subtracted out of the mass spectra,
Br means 79
Br means Br.




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