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The radiation chemistry of mixtures of ethane and hexafluoroethane in the gas phase

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Title:
The radiation chemistry of mixtures of ethane and hexafluoroethane in the gas phase
Creator:
Scanlon, Michael David, 1944-
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English
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x, 125 leaves : ill. ; 28 cm.

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Subjects / Keywords:
Atoms ( jstor )
Chemical mixtures ( jstor )
Dosage ( jstor )
Electrons ( jstor )
Fluorocarbons ( jstor )
Hydrocarbons ( jstor )
Hydrogen ( jstor )
Ions ( jstor )
Oxygen ( jstor )
Radiolysis ( jstor )
Chemistry thesis Ph. D
Dissertations, Academic -- Chemistry -- UF
Ethanes ( lcsh )
Hexafluoroethane ( lcsh )
Radiochemistry ( lcsh )
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bibliography ( marcgt )
non-fiction ( marcgt )

Notes

Thesis:
Thesis--University of Florida.
Bibliography:
Bibliography: leaves 118-124.
General Note:
Typescript.
General Note:
Vita.
Statement of Responsibility:
by Michael David Scanlon.

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THE RADIATION CHEMISTRY OF MIXTURES OF ETHANE
AND HEXAFLUOROETHANE IN THE GAS PHASE









By

MICHAEL DAVID SCANLON


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


1976















ACKNOWLEDGEMENTS


The author wishes to express his deep gratitude and appreciation

to Professor Robert J. Hanrahan, under whose direction this investigation was done, for his advise, guidance, and understanding. Special gratitude is also extended to his parents and family for their encouragement, patience, and sacrifice.

















TABLE OF CONTENTS


ACKNOWLEDGEMENTS ....... ................

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

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

ABSTRACT ...... ........................

CHAPTER

I. INTRODUCTION ...... ...............

II. EXPERIMENTAL METHODS AND EQUIPMENT . . .

A. Sources and Preparation of Materials B. Sample Preparation ... ..........

C. Sample Irradiation ... ..........

D. Product Analysis ...............

III. THE GAMMA RADIOLYSIS OF HEXAFLUOROETHANE

A. Experimental Results ... ........

B. Discussion ..... ..............

C. Summary ...... ................

IV. THE GAMMA RADIOLYSIS OF ETHANE .......

A. Experimental Results ...........

B. Discussion ..... ..............


C. Summary.


V. THE
AND

A.


GAMMA RADIOLYSIS OF MIXTURES OF HEXAFLUOROETHANE ETHANE ......... ......................

Experimental Results ... .................


Page

ii

v

vi ix


. . . . . 65











TABLE OF CONTENTS
(Continued)


Page

B. Discussion ............ ................. . 72

C. Summary .......... ........................ .116

REFERENCES ... ............. ...... ............... .118

BIOGRAPHICAL SKETCH .......... ........................ .125















LIST OF TABLES


Table Page

I-i A Comparison of the Mass Spectrum of Ethane with
Hexafluoroethane .......... ..................... 7

III-i Radiolysis Yields for Hexafluoroethane ............ ...33

IV-i G Values for the Radiolysis Products from Ethane ....... 44

IV-2 A Comparison of the G Values for the Radiolysis Products
from Ethane ........ ....................... . 52

IV-3 G Values for the Radiolysis Products from Ethane with Scavenger Added . . . . ..... .................. . 53

IV-4 Proposed Radiolysis Mechanism for Ethane ........... ...55

V-i Composition of Irradiated Mixtures ... ............ ... 67

V-2 G Values for the Major Radiolysis Products from EthaneHexafluoroethane Mixtures ..... .. ........... . 73

V-3 G Values for the Minor Radiolysis Products from EthaneHexafluoroethane Mixtures ..... ................ . 74

V-4 G Values for the Major Radiolysis Products from EthaneHexafluoroethane Mixtures with 10% added Oxygen Scavenger 75

V-5 G Values for the Minor Radiolysis Products from EthaneHexafluoroethane Mixtures with 10% added Oxygen Scavenger 76

V-6 Proposed Radiolysis Mechanism for the Mixtures of
Ethane and Hexafluoroethane ..... ............... ..111















LIST OF FIGURES


Figure

'I-i 11-2 11-3 11-4 11-5


11-6 11-7 11-8 11-9

II-10 III-1

111-2 111-3

IV-I


IV-2 IV-3 IV-4 IV-5 IV-6 IV- 7 IV-8


Gas chromatogram of irradiated ethane ....... Production of hydrogen as a function of dose . Production of CH4 as a function of dose .... Production of C 3 H8 as a function of dose Production of n-C4 H0 as a function of dose. Production of C2H4 as a function of dose . . . Production of C 2H2 as a function of dose.


Vacuum system .......... .......................

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

Nickel vacuum tight radiolysis vessel .... ...........

Cylindrical radiolysis vessel and holder ............

Dosimetry: hydrogen yield from ethylene as a function of irradiation time .......... ...... ............

Schematic diagram of the "Duplex Gas Chromatograph". Automatic Toepler pump . ...... ..................

Product trapping vessel ...... ...................

Spark discharge vessel ....... ..................

Titration curves for quantitative analysis of F ion Gas chromatogram of irradiated hexafluoroethane ....... Production of CF4 as a function of dose ... ..........

Production of C3F8 and C4FI0 as a function of dose . Gas chromatogram of the non-condensible products in the radiolysis of ethane ........ ..................


Page

10

14 15 16 17 19 20 24 26 28 30 31 32


40


. . . . . 42

. . . . . 45

. . . . . 46

. . . . . 47

. . . . . 48

. . . . . 49

. . . . . 50










LIST OF FIGURES
(Continued)


Figure Page

V-I Gas chromatogram of an irradiated equimolar mixture
of ethane and hexafluoroethane ... .............. . 69

V-2a Yields of H2, CH4, and C2H4 in the radiolysis of
C2H6 - C2F6 mixtures as a function of mixture composition ........... ...................... 77

V-2b Yields of n-C4HI0, C3H8, and CH2CF2 in the radiolysis
of C2H6 - C2F6 mixtures as a function of mixture
composition ........ ...................... . 78

V-2c Yields of HF, CF3H, C2F5H, and C2H2 in the radiolysis
of C2H6 - C2F6 mixtures as a function of mixture
composition ........ ...................... . 79

V-3a Yields of H2, C2H2, C2H4 in the radiolysis of
C2H6 - C2F6 mixtures with 10% 02 as a function of
mixture composition ...... .................. . 80

V-3b Yields of CH4, C3H8, and n-C4HI0 in the radiolysis
of C2H6 - C2F6 mixtures with 10% 02 as a function of
mixture composition ...... .................. . 81

V-3c Yields of HF, CF3H, and C2F5H in the radiolysis of
C2H6 - C2F6 mixture with 10% 02 as a function of
mixture composition ...... .................. . 82

V-4 Yields of H2 for various mixtures of C2H6 and C2F6
as a function of dose ..... ................. . 83

V-5 Yields of H2 for C2H6 - C2F6 mixtures with 10% 02
as a function of dose ..... ................. . 85

V-6 Yields of CH4 for various mixtures of C2H6 and C2F6
as a function of dose .... ...................... 87

V-7 Yields of CH4 for C2H6 - C2F6 mixtures with 10% 02
as a function of dose ..... ................. . 89

V-8 Yields of C3H8 for various mixtures of C2H6 and C2F6
as a function of dose ................ ......... 91

V-9 Yields of C3H8 for C2H6 - C2F6 mixtures with 10% 02
as a function of dose ...... ................. .. 93

V-10 Yields of n-C4HI0 for various mixtures of C2H6 and
C2F6 as a function of dose. . . ... ............ . 95










LIST OF FIGURES
(Continued)


Figure Page

V-i Yields of n-C4HI0 for C2H6 - C2F6 mixtures with
10% 02 as a function of dose ...... ............... 97

V-12 Yields of C2H4 for various mixtures of C2H6 and C2F6
as a function of dose ..... ........... ...... 99

V-13 Yields of C2H4 for C2H6 - C2F6 mixtures with 10% 02
as a function of dose ........ ... .......... . 101

V-14 Yields of CF3H for various mixtures of C2H6 and C2F6
as a function of dose ...... ................. . 103

V-15 Yields of CF3H for C2H6 - C2F6 mixtures with 10% 02
as a function of dose ...... ................. . 104

V-16 Yields of C2F5H for various mixtures of C2H6 and C2F6
as a function of dose ...... ................. . 105

V-17 Yields of C2F5H for C2H6 - C2F6 mixtures with 10% 02
as a function of dose ...... ................. . 106

V-18 Yields of CH2CF2 for various mixtures of C2H6 and
C2F6 as a function of dose ..... ............... ...107

V-19 Yields of HF for various mixtures of C2H6 and C2F6
as a function of dose . . . .... .............. . 108

V-20 Yields of HF for C2H6 - C2F6 mixtures with 10% 02
as a function of dose ...... ................. . 109


viii
















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 OF MIXTURES OF ETHANE
AND HEXAFLUOROETHANE IN THE GAS PHASE By

Michael David Scanlon

August, 1976

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

The gas phase radiolysis of mixtures of ethane and hexafluoroethane were studied at 50 torr pressure for systems containing 0 to 100% of the fluorocarbon, both pure and with 10% added oxygen. For the pure system, twenty-two products were identified, and the yields over the entire mixture range have been recorded. The major radiolytic products were H2, CH4, C3H8, n-C4H10, C2H4, C2H2, CF3H, C2F5H, CF2CH2, and HF. The minor products included i-C4 H 0 i-C5 H 2 n-C5 H12 C6 H14 C3H6, 1-C4H cis-2-C4H trans-2-C4H8 C5H 10 CH3CF39 CH3C2F and CF3C2H5. It was observed that the addition of 10% oxygen substantially reduced the saturated hydrocarbon products, while the unsaturated hydrocarbon products were enhanced. Among the mixed products, HF, CF 3H, and C2 F 5H were essentially unaffected by added oxygen, while CH 3CF CF 2CH2 CF3 C 2H 5, and CH3C2F5 were eliminated. The fact that pure fluorocarbon compounds were not formed in the mixtures, and that the hydrocarbon products were significantly enhanced over their ideal mixture lines










indicated that decomposition of ethane is sensitized by the presence of hexafluoroethane and that a major process in the radiolysis of the mixtures is the ability of the hexafluoroethane to transfer charge and/or excitation energy to ethane, which subsequently decomposes, forming the hydrocarbon products. Among the mixed products,CH3CF3, CH2CF CF3C2H5, and CH C F were attributed to radical combination reactions, while CF H
3 25 13 and C2 F 5H were attributed to hydride ion transfer reactions from ethane, with HFformed via a simple hydrogen atom abstraction reaction. The results of this system were compared with the cyclohexane-perfluorocyclohexane system and the cyclobutane-perfluorocyclobutane system. This comparison revealed that in the radiolysis of fluorocarbon-hydrocarbon mixtures, the products resemble the pure hydrocarbon component rather than the pure fluorocarbon and that, as the chain length is lowered, energy (and/or charge) transfer to the hydrocarbon becomes much more important than protection of hydrocarbon by electron capture.

















I. INTRODUCTION


In the past ten years there have been a number of research papers published concerning the radiation chemistry of fluorocarbon - hydrocarbon mixtures (1-10). The most extensively studied system has been the radiolysis of the mixtures of cyclohexane and perfluorocyclohexane (1-4, 6-8, 10). It was found that when a small amount of perfluorocyclohexane was added to cyclohexane, the hydrogen yield was sharply decreased from the value in pure cyclohexane, the bicyclohexy[ dimer yield was increased, the cyclohexene yield was decreased slightly, and c-C6 F H was formed. These results can be understood if we consider the following reaction scheme proposed for radiolytic decomposition of cyclohexane (3, 7, 10, 11):


c-C6H12 c-C6 H12 c-C6H2 c-C6 H12 c-C 6H *2 H- + c-C6H12

2 c-C6 H * 2 c-C6H11.


(c-C6H +e ) --c-C6H1
c-C6H12 6 12


--* c-C6H10 + H2

c-C6HII. + H.


----- H2 + c-C6HII"

- 12 H22

-- c-C6H10 + c-C6H12


Reactions (I-i) through (1-7) summarize the major steps in the radiolysis of pure cyclohexane. When cyclohexane is irradiated, the primary products are positive ions accompanied by an equivalent number


(i-1) (1-2)

(1-3) ('-4) (1-5) (1-6) (1-7)










of electrons as well as excited molecules. The ions and electrons can also combine to form excited molecules. The excited molecules then decompose to give either hydrogen and cyclohexene or hydrogen atoms and cyclohexyl radicals. The cyclohexyl radicals either combine to form bicyclohexyl or disproportionate to yield cyclohexene and cyclohexane.

Several reactions mechanisms have been proposed to explain the

effect of perfluorocyclohexane on cyclohexane. What is fairly clear is that perfluorocyclohexane, an excellent electron scavenger, interferes with Reaction (1-1) by capturing electrons, thereby preventing the charge neutralization process between the cyclohexyl ions and electrons. The most frequently mentioned mechanism is the following, suggested by Sagert (3):


c-C6F1 +e -+c-C6F2 (I-8)
CC6 F12 +e)C 6 F12

c-C6F + c-C6H +- )c-C6F + c-C6H + HF (1-9)
6 12 6 12 6 11 6 11


An alternative scheme was proposed by Rajbenbach and Kaldor (2), who suggested that Reaction (1-8) is followed by


cc-F +c-C - c-C6FI1 + HF + c-C6HII- (I-10) cc-F +c-CH 12+ c-C6FH + c-C6H (1-11)



The original Sagert mechanism appears unacceptable in view of the observation by Kennedy and Hanrahan (10) that added iodine does not reduce the yield of c-C6 F H in the radiolysis of cyclohexane-perfluorocyclohexane solutions; it is inferred that thermalized perfluorocyclohexyl radicals are not involved in the reaction. It was suggested that Reaction (1-9) might involve proton transfer to c-C6F12 giving an intimate ion pair or a charge-transfer complex, which dissociates to






-3-


HF and an excited c-C6FII- radical:


c-C6F12- + c-C6H 12+ (c-C6 F12 6 H+)


-- 6H1I + (c-C6F12-.... H+ (c-C 6 FII'*)* + HF


Since the resulting (c-C6FI.) * would be excited with a considerable fraction of the charge neutralization energy, it could react with the hydrocarbon substrate at an enhanced rate.

The Rajbenbach and Kaldor mechanism is attractive because it does not invoke participation of c-C6 F 1 radical. However, Reaction (I-10) is unacceptable if the electron capture energy associated with the formation of c-C6 F 12 has been dissipated to the medium. In that case, the process is at least 30 kcal/mole endoergic.

Kennedy and Hanrahan (10) suggested that the Rajenbach and Kaldor mechanism is acceptable provided that Reaction (1-8) is followed very rapidly by (I-lOa), so that the reactant c-C F still contains its
6 12
electron capture energy:


(c-C6F 12)* + c-CH - c-C6FII + HF + c-C6HII.


(I-10a)


Alternatively, Reactions (1-8) and (I-10) could occur as a single concerted process (10):


e + c-C6F12 + c-C6H12 - c-C6FII + HF + c-C6HII.


(1-12)


Reaction (I-11) would follow in any event.

All investigators, who have considered the problem, have ruled out dissociative electron capture as a major process:


e + c-CF - F + c-C6F (II-3


(I-9a) (I-9b)


(1-13)










Since the appearance potential for F from perfluorocyclohexane is

1.8 eV (12), the process is 41 kcal/mole endoergic. Both the modified Sagert mechanism and the modified Rajbenbach and Kaldor mechanism are compatible with all previous observations in the radiolysis of cyclohexane-perfluorocyclohexane solutions.

Unlike the cyclohexane-perfluorocyclohexane systems where protection of the decomposition of the hydrocarbon by added fluorocarbon is the major process, experimental results in the radiolysis of the mixtures of cyclobutane and perfluorocyclobutane indicate the decomposition of cyclobutane to hydrocarbon products is sensitized by the presence of perfluorocyclobutane. Heckel and Hanrahan (9) found that the yields of ethane, propane, ethylene, butene, and several of the other cyclobutane radiolysis products were higher than would be expected for an ideal mixture. In such a mixture, the yields of the hydrocarbon products would decrease linearly with the fraction of energy absorbed by the cyclobutane. They attributed these increased hydrocarbon yields to energy transfer from the excited perfluorocyclobutane to cyclobutane, leading to the decomposition of the cyclobutane (1-14). The decrease in the perfluoroethylene yield and the


(C4F8 + C4H8 --- 4H8 + C4F8 (1-14)



absence of higher fluorocarbon products which were seen in the radiolysis of pure perfluorocyclobutane also indicate that this perfluorocyclobutane is protected from radiolytic decomposition by cyclobutane. There still seems to be some tendency toward decomposition of the perfluorocyclobutane as suggested by the abrupt rise in the hydrogen fluoride yield at low concentrations of added perfluorocyclobutane. This result was also










observed in all the previous studies of fluorocarbon-hydrocarbon mixtures (1-11). Also, the fact that the hydrogen yields in the radiolysis of cyclobutane-perfluorocyclobutane mixtures is essentially linear with respect to the energy absorbed in cyclobutane suggests there is a tradeoff between a tendency for protection by the perfluorocyclobutane (via the electron capture process) and a rather noticeable tendency toward energy transfer from the perfluorocyclobutane to the cyclobutane, leading to formation of products other than hydrogen.

If we consider the fact that both the Sagert mechanism and the

Rajbenbach and Kaldor mechanism proposed to explain the results in the perfluorocyclohexane-cyclohexane system depend on electron capture by the fluorocarbon, it follows that there should be less protection by perfluorocyclobutane than by perfluorocyclohexane as the lifetime of the perfluorocyclobutyl negative ion is about a hundred times shorter than that of the perfluorocyclohexyl negative ion (13).

With these results in mind, it was decided to investigate a somewhat simpler system, the mixtures of ethane and perfluordethane, to establish the radiolytic behavior in this system and also to further elucidate the radiolysis mechanism of the mixtures of hydrocarbons and fluorocarbons in general.

Both the perfluorocyclohexyl and perfluorocyclobutyl negative ions have been observed (13) and as mentioned earlier are believed to play a prominent role in the radiolysis mechanism in their mixtures with hydrocarbons. The perfluoroethyl ion is postulated (14), but has not been seen (14, 15). Apparently, if it exists it has a lifetime of less than one microsecond. This implies that, in the mixtures of ethane and hexafluoroethane, protection of the hydrocarbon by the perfluorocarbon (via










electron capture) plays a more limited role than in previous studies and that other processes such as energy transfer should be more important.

As with the other analogous hydrocarbon-fluorocarbon systems, the

ethane-hexafluoroethane system is interesting because of the considerable differences in bond strengths, electron affinities, ionization potentials, thermal stabilities, and mass spectra of the two gases. Table I-I compares the mass spectra of ethane (16) and hexafluoroethane (17) and the dissimilarities are quite striking. For example, there is virtually none of the parent ion in the hexafluoroethane, as compared to 26% for ethane. Also in hexafluoroethane, the most abundant ion is the perfluoromethyl ion, whereas in ethane the methyl ion is approximately 5%, thereby indicating that carbon-carbon bond rupture is much more prevalent in the fluorocarbon than in ethane. One immediately observes that in ethane, there is a much greater tendency for the carbon-hydrogen bond to break than there is for the carbon-fluoride bond to break in hexafluoroethane.

Although there has been no previous work done on the mixtures of

ethane and hexafluoroethane, there havebeen a few papers concerning the radiolytic behavior of pure hexafluoroethane and numerous papers on ethane. These results will be presented in the Discussion Sections of Chapter III and Chapter IV respectively.






-7-


TABLE I-i

A Comparison of the Mass Spectrum of Ethane (left side)
with Hexafluoroethane (right side)



ION ABUNDANCE ION


C2H6+ 26.2 -0.15 C2 F6+
++
C2H5+ 21.7 41.3 C 2F5+ 'C2 H4 +100.0 0.55 C2 F4 +
+ + C 2 H3 +33.3 0.12 C 2F 3+ C 2H 23.0 0.13 C F 242 242 C2H+ 4.1 0.42 C2 F+
+ �


CH 3 +4.6 100.0 CF 3+
+ + CH2 3.3 10.1 CF2 CH+ 1.2 18.3 CF+


H+ 2.6 1.2 F+

















II. EXPERIMENTAL METHODS AND EQUIPMENT


A. Source and Preparation of Materials


Hexafluoroethane: Peninsular Chemresearch hexafluoroethane was purified by preparative gas chromatography using a 2.5 meter column packed with Analab 100-110 mesh silica gel, followed by several freezepump-thaw cycles, and stored in a vessel attached to the vacuum line. Gas chromatographic analysis showed that the C2F6 was free of any impurities.

Ethane: Matheson Company C.P. grade ethane was also purified by

preparative gas chromatography using a 5 meter column packed with silica gel of 60-200 mesh (W.H. Curtin), followed by several freeze-pump-thaw cycles, and stored in a vessel attached to the vacuum line. Gas chromatographic analysis showed that the ethane was free of any impurities.

Ethylene: Matheson Company C.P. grade ethylene was passed through BaO into a storage vessel on the vacuum line. Possible air contamination was then removed by several freeze-pump-thaw cycles.

Oxygen: Matheson Company research grade oxygen was used as received from the manufacturer.

Other Reagents: Chemicals used for chromatographic calibration standards and miscellaneous experiments were used as received.


B. Sample Preparation


Vacuum System: The vacuum system used is shown in Fig. II-1. The
















Fig. II-i. Vacuum system.



1 - 15 Inlet ports

G Thermocouple gauge

S3 - S7 Ground glass stopcocks

C Gas mixing chamber

M Mercury manometer

V1 ,V2 Storage reservoir (1000 cc.) V V Storage vessels

T2 Liquid nitrogen trap









-11-


pumping system consisted of a Welch Duo-Seal fore pump connected through a liquid nitrogen cold trap to a two stage mercury diffusion pump. These pumps were connected to the main manifold through a second liquid nitrogen trap, T2 and stopcock S3 (Fig. II-1). Connected to the main manifold were a submanifold attached at stopcock S V a vacuum thermocouple gauge, G, and three inlet ports, I!, I, and I Inlet ports 1' 2' 3
I and I were used chiefly for introducing samples, whereas a 500 ml
1 2
storage vessel was attached to 13. Attached to the submanifold were a manometer, M, storage vessels V and V2, a gas mixing chamber, C, and inlet ports 14 and 1 The irradiation vessels were usually attached to inlet ports 14 and 15. Ffscher-Porter 4 mm, 0-ring sealed Teflonglass valves and ground glass valves lubricated with Halocarbon 25-5S grease were the two types of stopcocks used in the vacuum system. All inlets, I - 1 , were equipped with No. 5 O-ring joints.

Hexafluoroethane: Before each sample preparation, hexafluoroethane, stored in storage reservoir VI, was deaerated by repeated freeze-pumpthaw cycles until the thermocouple gauge indicated the absence of air. Valves4 was then closed and the sample was allowed to expand into the known volume of the radiolysis vessel (Fig. 11-3), which was attached to inlet 14' During the course of this operation, the pressure was monitored with a mercury manometer, M. At the desired pressure, the radiolys:is vessel was closed and the excess material was condensed back into the storage vessel with liquid nitrogen. The ideal gas law was used to calculate the amount of sample in the radiolysis vessel.

Hexafluoroethane with added oxygen:* A known amount of hexafluoroethane was condensed into storage vessel V of the mixing chamber, C,
4
and the valve to V closed. Then the desired amount of oxygen was added







-12-


to the mixing chamber. With stopcock S5 closed, the hexafluoroethane in vessel V4 was released into the mixing chamber, and the sample was mixed for ten minutes, using a magnetic circulating pump. After mixing was complete, the desired amount of the mixture was allowed to expand into the previously evacuated radiolysis vessel, which was attached to inlet

1 In all irradiations with oxygen, oxygen constituted 10% of the total pressure.

Ethane and ethane with oxygen: The preparation of ethane for radiolysis followed the same procedure as that used for hexafluoroethane. Likewise, samples of ethane and added oxygen were prepared using the same procedures as that for hexafluoroethane with oxygen.

Mixtures of hexafluoroethane and ethane: A known amount of hexafluoroethane, stored in V1, was condensed into vessel V4 of the mixing chamber, C, and the valve to V4 closed. Similarly a known amount of ethane, stored in V25 was expanded into the mixing chamber, and then condensed into V Enough of. the mixture of known composition was stored in V4 for several radiolysis experiments. The mixture was then allowed to expand into the known volume of the radiolysis vessel. After closing off the radiolysis vessel, the excess material was condensed back into storage vessel V4.

Mixtures with added oxygen: A known amount of the mixture of

hexafluoroethane and ethane that was stored in vessel V4 of the mixing chamber was condensed into vessel V3. Then the desired amount of oxygen was added to the mixing chamber. With stopcock S closed, the mixture
5
was released into the mixing chamber, and the sample was mixed for approximately 10 minutes. After mixing was complete, the desired amount of the sample was allowed to expand into the previously evacuated radiolysis vessel, which was attached to inlet port 14*






-13-


C. Sample Irradiation


Radiation source and vessels: All irradiations were performed

using a cobalt-60 gamma irradiator which has been described previously

(18). Figure 11-2 is a cross-sectional view of the irradiator.

The helium arc weld nickel vessel used in most of the irradiations is shown in Fig. 11-3. Two vessels were made, having volume of 103.5 and 105 cc , respectively. The vessels were made of pure nickel, having Hoke monel diaphragm valves (4611N4M). A rigid aluminum sample holder was used to assure reproducible positioning of the radiolysis vessels relative to the Co-60 source.

Glass vessels were used to determine the amount of hydrogen fluoride formed in irradiated mixtures of ethane and hexafluoroethane. Figure 11-4 shows the Pyrex vessel with its holder. Three vessels were used having volumes of 29.9 cc , 30.0 cc , and 30.2 cc.

Dosimetry: Ethylene dosimetry (19, 20) was used to determine the absorbed dose rate of samples in the various radiolysis vessels. Ethylene was irradiated from 5 to 44 hours at a pressure of 50 torr. After irradiation, the radiolysis vessel was connected to a special duplex gas chromatograph, which is described in the next section, and non-condensible gases were separated from the condensible gases (see Section II-D). The non-condensible gases were passed through the molecular sieve column and the hydrogen yield was calculated. From the slope of the plot of hydrogen yield vs. time (see Fig. 11-5) and from the known G value of hydrogen in ethylene (G = 1.2) (19, 20), the dose rate was determined. The absorbed dose rate for ethylene was 3.15 x 1019 19
eV/gram-hr. on Feb. 3, 1972 for the nickel vessels and 2.72 x 10 eV/ gram-hr. on Jan. 10, 1973 for the glass vessels. These values were






-14-


Legend: (A) counterweight; (B) upper support; (C) control rod handle;
(D) extra top shielding; (E) storage turret; (F) 400 curie
Co60; (G) shutter shown open; (H) rear wall; (I) door;
(J) downward shielding; (K) door carriage; (L) door crank;
(M) door frame.

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


Fig. 11-2. Cross section of cobalt-60 gamma ray source.












Hoke Monel Valve


Cylinder Wall Thickness 3/64"


O-Ring Seal, 3/8" I.D., 3/32" Wall


End Wall Thickness 1/32"


Heliarc Weld


Heliarc Weld


Nickel vacuum tight radiolysis vessel.


1 17/32"


Fig. 11-3.







-16-


Fig. 11-4. Cylindrical radiolysis vessel and holder.
















0.2





0l)
0
0



I
-40.1











0.0 I I

0 5 10 15 20 25 30 35 40 45 Irradiation time, hours Fig. 11-5. Dosimetry: hydrogen yield from ethylene as a function of irradiation time.






-18-


corrected for Co-60 decay during subsequent irradiations.

Energy absorption in ethane, hexafluoroethane and in the mixtures was calculated relative to the ethylene result. Application of the Bragg-Gray cavity principle (21-23) as well as the calculation of mass stopping powers are described fully elsewhere (24). The dose rates obtained were 3.35 x 1019 eV/gram-hr. for ethane and 2.46 x 1019 eV/gramhr. for hexafluoroethane for the nickel vessels and 2.89 x 1019 eV/gramhr. for ethane and 2.12 x 1019 eV/gram-hr. for hexafluoroethane for the glass vessels. For mixtures of ethane and hexafluoroethane it was assumed that each gas absorbed energy independently in proportion to the mass (more precisely, the electron stopping power fraction) of that gas present.



D. Product Analysis


Gas chromatograph instrument: The gas chromatograph used for the qualitative and quantitative analyses of the irradiated samples was constructed by Dr. Edgar Heckel (25) and is diagrammed in Fig. 11-6. It was possible to analyze all condensible and non-condensible products, except hydrogen fluoride, using this chromatograph. It consisted of two independent gas chromatograph units - one with a flame ionization' detector, the other with a thermal conductivity detector - connected by a common inlet system, which could be evacuated when desired. The inlet system contained two inlet ports, a U-trap, a gas sampling loop, a thermocouple gauge, and an automatic Toepler pump.

The automatic Toepler pump system, shown in Fig. 11-7, includes the Toepler pump itself, an Asco three-way solenoid valve, and a twenty-four position Guardian Electric stepping relay, which was activiated through










Thermoconductivity
Detector \




Nitrogen


Gas Chromatograph


Flame Ionization


Gas
Sampling
Loop



Mercury over Glass Frit


Check Valve


Schematic diagram of the "Duplex Gas Chromatograph."


Fig. 11-6.
















Solenoid Valve


110 Volts AC


1 Amp


Fig. 11-7. Automatic Toepler pump.


Toepler P ump







-21-


the electrical contacts built into the Toepler pump. The stepping relay advanced after each cycle of the Toepler pump, in response to closure of its contacts, automatically causing the next pump cycle to occur.

The column-oven-detector system used for permanent gas analysis

usually contained a molecular sieve column, and the parallel section for organic gas analysis employed a silica gel column. Heating wire was wrapped around all the connecting tubing that came into contact with the condensible products. All valves in the chromatograph unit were obtained from Republic Manufacturing Co. Valves 1 - 5 were Teflon-plug models (2-way, No. A-330, three-way, No. A-310) and valve 6 wasa bellows-sealed type (No. B-139).

The output of both the thermal conductivity detector and the flame ionization detector were fed into a Photovolt Microcord recording potentiometer Model 44 with 10 in chart paper, full scale sensitivities of 0.5, 1, 2, 5, and 10 mv , and chart speeds of 0.5 and 2 in /min.

Analysis of non-condensible products: After the sample had been irradiated, the radiolysis vessel was attached to the inlet system of the gas chromatograph by means of a silicone O-ring and a horseshoeclamp. The entire inlet system was evacuated by putting valve 1 in position D and opening valve 6. After a good vacuum had been attained, valve 6 was closed, thus isolating the system from the pump. Valve 7 was then opened allowing all the condensible organic products to be collected in the trap, which was cooled in a dewar of liquid nitrogen. The automatic Toepler pump switch was then turned on, allowing the noncondensible gases to be drawn from the radiolysis vessel and inlet system into the gas sampling loop. The Toepler pump was operated automatically through ten cycles, while the pressure was monitored using the






-22-


thermocouple gauge. At the end of the tenth cycle, the solenoid valve system was automatically deactiviated and the mercury was allowed to rise against the glass frit, concentrating all the non-condensible into the gas sampling loop. Then valve 2 was closed, isolating the condensible products in the liquid nitrogen trap, and valve 5 was opened allowing the carrier gas to sweep the non-condensible gases into the molecular sieve column (Fig. 11-6) of the thermal conductivity chromatograph system. This chromatograph was equipped with a Gow Mac thermal conductivity detector and a 3.5 m x 0.25 in O.D. column of molecular sieve (5A). Hydrogen and methane were easily separated by using nitrogen carrier gas at a flow rate of 20 ml/min at 110'C (see Fig. IV-2).

Analysis of condensible organic products: After the non-condensibles were analyzed, the liquid nitrogen Dewar was removed from the trap, and the trap was heated by electrical resistance wire, which was wrapped around it. Valve 3 was then opened allowing the nitrogen carrier gas to sweep the condensible products into the silica gel column of the flame-ionization chromatograph unit. This chromatograph was equipped with an Aerograph flame ionization detector, a 30 cm pre-column of 100-110 mesh Analabs silica gel, a 5 m x 0.25 in O.D. column of 60-200 mesh silica gel (W.H. Curtin), and an F & M Scientific Corporation Model 40 Linear Temperature Programmer.

The pre-column, which was outside the oven compartment, was wrapped with heating wire and could readily be heated to 200'C within about two minutes.

The irradiated products of hexafluoroethane, ethane, and their mixtures were analyzed using the above arrangement. The only difference in the analyses of these various samples was in the initiation time of the






-23-


temperature programming. For hexafluoroethane samples, programming was started 15 minutes after injection of the sample onto the column. For ethane the time was 20 minutes after injection, and for the mixtures it was 30 minutes. The pre-column was heated up at the same time the temperature programmer was started. The oven compartment was programmed from 250C to 2000C at a rate of 30 per minute. The pre-column reached 2000C in a matter of minutes. Some typical chromatograms are shown in the next chapter.

The sensitivity of the flame ionization detector to the radiolysis products was calibrated relative to that for hexafluoroethane.

The radiolysis products were identified by their retention times and also by trapping the individual product via a splitting valve into a liquid nitrogen-cooled U-tube. The contents of the tube were then analyzed in the Bendix Time of Flight mass spectrometer (see next section).

Mass spectrometric product identification: The gas chromatograph described above was also used to trap individual products for mass spectrometric analysis. This was accomplished by placing a stream splitting valve in front of the flame ionization detector (see Fig. 11-6).

As the product mixture was fractionated by the gas chromatograph,

the major portion of each effluent peak was diverted to a liquid nitrogencooled trap attached to the outlet of the splitter valve; the remaining portion of each peak went to the chromatographic detector as usual. The trap consisted of a U-tube to which two Fischer-Porter 4 mm, O-ring sealed Teflon-glass valves were attached (see Fig. 11-8). The U-trap was packed with Pyrex helices. The trapping procedure was as follows: the irradiated sample was analyzed in the usual manner. When the flame







-24-


Fig. 11-8. Product trapping vessel.






-25-


ionization detector started to register the peak of the radiolysis prodnct to be trapped, the U-tube was attached to the outlet of the splitting valve and then immediately submerged in a Dewar containing liquid nitrogen. After passage of the desired component, both Teflon-glass valves were closed and the sample was stored in liquid nitrogen. The sample was then attached to the vacuum line and the nitrogen carrier gas and condensed oxygen in the trap were pumped out. The trap was connected to the "semi-direct" inlet of the Bendix Model 14-107 Time of Flight mass spectrometer for analysis. A General Automation SPC-12 minicomputer had been interfaced with the mass spectrometer and programmed to acquire, analyze, and print out reduced data giving mass numbers and normalized intensities (26).

To prepare the large amounts of reaction products needed for mass

spectrometric analysis, the "spark discharge technique" was employed (5). A 500 ml, round-bottomed flask provided with two stainless steel electrodes about an inch apart, equipped with a Fischer-Porter Teflon plug needle valve, served as the spark discharge vessel (see Fig. 11-9). The vessel was filled with from 5 to 10 torr of gas in the usual manner and removed from the vacuum line. One electrode was grounded to a cold water pipe while the other was in contact with a Tesla coil, which was powered through a Variac. The lowest setting on the variac that would maintain a discharge was used (about 30 volts) and the sample was sparked from 5 to 10 minutes.

Hydrogen fluoride analysis: After the sample had been irradiated, the radiolysis vessel was cooled in liquid nitrogen, and 2 ml of 95 percent ethanol-5 percent water solution was added to the solid sample through a breakseal fitting. The vessel was removed from the liquid







-26-


Fig. 11-9. Spark discharge vessel.






-27-


nitrogen and was temporarily sealed with a rubber cap. While the sample was warming up to room temperature, it was shaken for 15 minutes to help recover the hydrogen fluoride absorbed on the vessel walls. The ethanolic solution was transferred into a 5 ml polyethylene beaker. The radiolysis vessel was then rinsed out with another 2 ml of 95% ethanol, which was added to the beaker. This solution was then titrated using a procedure

-3
described by Heckel and Marsh (27). A solution of 1 x 10 M lanthanum nitrate (Fisher Scientific Co.) in a 10 microliter gas tight Hamilton syringe was used as a titrant. The electrode potential between the Orion Model (#94-09) fluoride ion activity electrode and a Beckman Model 39270 fiber junction calomel electrode was monitored by a high input impedance (10 mega ohms) Hickok digital volt-ohm meter. During the titration, the solution was stirred with a Teflon coated magnetic stirrer. A titration curve for standardizing the electrode and a typical titration curve for an irradiated sample is shown in Fig. II-10.









-28-


CO





.H
o 20











4-1








UJ - 20
--4
aH












-40














-60

0 10 20 30 40 La(NO3 3' nanoequivalents Fig. II-10. Titration curves for quantitative analysis of F ion.

*, known solution, 2.4 x 10-8 moles NaF; 0, radiolysis

sample.
















III. THE GAMMA RADIOLYSIS OF HEXAFLUOROETHANE


A. Experimental Results


Hexafluoroethane was irradiated at room temperature over the absorbed dose range of 0.738 x 1020 to 9.84 x 1020 eV/gram. Most radiolyses were carried out at 50 torr, however experiments at pressures of 20 and 100 torr were also performed. The product G values were independent of pressure over this range. Figure III-I shows a typical product chromatogram. The three major products are tetrafluoromethane, octafluoropropane, and decafluorobutane, and the two minor products are perfluoropentane and perfluorohexane. No unsaturated fluorocarbon compounds were found. The major products and the minor product C6F14 were identified by their retention times and their mass spectral cracking patterns. C F was
5 12
only identified by mass spectrometry. Figures 111-2 and 111-3 show the yields of the various products are linear when plotted as a function of absorbed dose. From these plots are obtained the G values, the number of molecules formed per 100 eV of energy absorbed by the hexafluoroethane. These yields are listed in Table III-1.

Table III-1 also shows the effect of adding oxygen to the hexafluoroethane prior to radiolysis. The G value for tetrafluoromethane was reduced by approximately 75%, while the other radiolysis products were eliminated completely.


-29-



















C2 6 C F
C~3F8
0
CCF
4 10


4
CF 512



x2 x5,000 x32 x16 x4


I I I I I I I I I
4 8 12 16 20 24 28 32 36 40 44 46 48

Elution Time (minutes)


Gas chromatogram of irradiated hexafluoroethane.


Fig. III-l.






-31-


1.0








0
00
4



-0 .5 -10% O2 '0
02















0.0
0 2 4 6 8 10

Dose, eV/g x 1020


Fig. 111-2. Production of CF4 (pure, 0 ; 10% 02, EJ ) as a function of
dose.















0.2


-4)
0



0 Q.)






0


C4 F10






0.0
0 1 2 3 4 5 6 7 8 9 Dose, eV/g x 1-2 Fig. 111-3. Production of C 3F 8(pure,O0 ) and C 4F 10 (pure,O3 ) as a function of dose.











TABLE III-i Radiolysis Yields for Hexafluoroethane.


G Values

Kevana Cooperb This Work Product
Pure 1% 02 Pure 5% Br2 Pure 10% 02


2.45 1.3 0.40 0 0.15 0


0 0


2.50


0.31 0.10


0


0


0


0


0


2.27 0.23 0.09


0.015 0.009


0.61


0


0


0


0


aL. Kevan, Reference 32. bR. Cooper and H.R. Haysom, Reference 30.


CF4 C3F C4FI C4 F10

C5F2 C5 F12 C6 F14






-34-


B. Discussion


There have been radiolysis studies of hexafluoroethane in both the gas pahse (28-30) and liquid phase (31). In addition Kevan and coworkers did the rare gas-sensitized radiolysis of both phases (32, 33). Table III-1 compares the results of this work with that of Cooper and Haysom (30) and also Kevan (32). In the pure system there seems to be reasonable agreement in the yields of the major products. Kevan reported the formation of C5F12 and C6 F 14, but did not give any yield data on them. Cooper and Haysom reported seeing higher molecular weight products, but were not able to identify these products with certainty. In all three studies, the material balance is not particularly good with a fluorine to carbon ratio of 3.35 to 1 for Kevan's results, 3.52 to 1 for Cooper and Haysom's results, and 3.56 to 1 for this work.

It is seen in all three studies that when a radical scavenger is added to C2F6, the higher molecular weight products disappear. There is some disagreement with respect to the effect of scavengers on the formation of tetrafluoromethane. When Cooper added 5% chlorine to C2 F6 the G value of CF4 was found to be 0.40. When 5% hydrogen chloride was added, the G value was reduced to 0.26, and when 5% bromine was added, the yield of CF4 was undetectable. Kevan added 1% oxygen to C2F6 and found CF4 had a G value of 1.3. In the present investigation, 10% 02 was added and the G value of CF4 was found to be 0.61. It is quite possible that 1% oxygen is not sufficient to completely scavenge the CF3. radical. Cooper and Haysom reported that in all their scavenging experiments it took at least 3% scavenger to completely eliminate the radical processes. The unscavengeable yield of CF4 will be discussed later in this section.







-35-


In light of the radiolysis results and scavenger studies, it seems clear that the major products in the radiolysis of hexafluoroethane arise chiefly through radical reactions. Based on the above results, we can propose the following mechanism: C2F6 + e- * 2F6" + e- (III-1) C2F2 - 2CF3. (111-2) C 2F 5. + F. (111-3)

- 2 F4 + F2 (111-4) F- + CF. -* CF4 (iII-5) F. + C F - + C2 F (111-6) F + CF. - C2F6 (111-7) CF + CF -* C2 F6 (111-8) CF3" + C2F - C3F7" (111-9) F- + C3F --+ C3F8 (111-10) CF3" + C2F5" C 3F8 (111-11) C2F5" + C2F *-+ C4FI0 (111-12) C2F5" + C3F 7* C5F12 (111-13) C3F7' + C3F 7-' C6F14 (111-14)


Primary excitation (III-1) is probably followed most frequently by carbon-carbon bond rupture (111-2) as the dissociation of the C-C bond requires approximately 25 kcal/mole less energy than C-F bond rupture (111-3) (14, 15). Reaction (111-4) is proposed because C2F4 appears to be an essential precursor to account for formation of the higher molecular weight products. Net formation of C2F4 is not observed, and the strongly endothermic reaction (II-4) is probably a minor process.

Evidence from photochemical studies indicate that unlike hydrocarbons,







-36-


fluorocarbon radicals do not disproportionate (34-36) and fluorine atoms do not abstract fluorine from fluorocarbons (37-39). Therefore Reactions (111-5) through (111-14) would be the most probable radical reactions to explain the experimental results as they all have zero or small activation energies (40, 41).

Instead of postulating Reaction (111-4) in order to account for the higher molecular weight products, another possibility may be that the C 2F 5 radicals combine to form excited C4FI0, which then can collisionally deactivate (Reaction (III-12b)) or decompose to two C2 F 5 radicals (Reaction (III-12c)) or to CF and C3 F radicals (Reaction (III-12d)).

C2F5' + C2F5" (C4F 0)* (III-12a) (C4F1)* + M - C4FI0 + M (III-12b) (C4F10) -+ C2 F + C2 F (III-12c) (C4FI0) -1 CF 3 + C3 F (III-12d)



Kevan and Hamlet (28) postulated that the unscavengeable CF4 was due to either molecular dissociation of excited C2 F6 (111-15)


C2 F6 CF4 + CF2 (111-15)



or to one of the following ion-molecule reactions:


CF3+ + C2F6 - CF4 + C2F5+ (111-16) C2F5+ + C2F6 -+ CF4 + C3F7+ (111-17)



As mentioned in the Introduction, CF3+ and C2F5+ are the main ions in the mass spectrum of C2F6, so one might expect that either or both of their reactions could be important. Reaction (111-17) can be eliminated







-37-


+
as the C F ion has never been observed in the mass spectrometric
3 7
investigations of C2F6 (42-45). In fact, it has been shown that C2 F5+ produced either as primary fragments or as reaction products are essentially unreactive with C2F6 (43-46).

Although Reaction (111-16) is thought to be endothermic, Marcotte and Tiernan (43) observed it using a tandem mass spectrometer and suggested that this reaction occurred because in their experiments CF3+ had as much as 2.9 eV excess internal energy. More recently Ausloos and co-workers (45) investigated this reaction using a photoionization mass spectrometer, in which it was shown that CF3 ions having no internal energy undergo Reaction (111-16), with a rate constant of 4 x l0-ll
3
cm /molecule-second at pressures below 0.01 torr. Although this reaction is slow, it does appear that it is the major source of the unscavenged CF4 when oxygen is used as a scavenger. Cooper and Haysom's results which show only small amounts of CF in the presence of chlorine and
4
hydrogen chloride, and no CF4 in the presence of bromine, seem to imply that there is some type of ionic process involved when these scavengers are present which would interfere with Reaction (111-16). They suggest this possibility particularly when HCI is the scavenger. Amphlett and Whittle (47) report that photochemically generated CF 3radicals react with HCI to give CF 3H exclusively, whereas Cooper and Haysom found that when C2F6 was irradiated in the presence of 5% HCI, not only CF3H (G = 1.6), but also CF3C1 (C = 1.0) was formed. Since Reaction (111-16) is fairly slow, it probably could be suppressed in the presence of reactive additives which remove CF3

Reaction (111-15) is still a possibility, but Cooper and Haysom's results with added bromine seem to cast doubt on it, as there is no







-38-


reason to believe that small amounts of bromine would interfere with the primary decomposition of excited C2 F6



C. Summary


The gamma-radiolysis of gaseous C2F6 was investigated at 50 torr,

both pure and with 10% oxygen added. For the pure system, the radiolytic products and their respective G values were CF 2.27; C3 F 0.23; C4F10, 0.09; C5F12, 0.015; and C6 F14 0.009. All radiolysis products except for CF4 (G = 0.61) were eliminated when 10% 02 was added as scavenger. The results were discussed mainly in terms of excited C2 F6 decomposing into free radicals, which can then combine. The unscavenged CF4 was accounted for by the following ion-molecule reaction:


CF3 + C2F 6--CF4 + C2F5

















IV. THE GAMMA RADIOLYSIS OF ETHANE


A. Experimental Results


Ethane was irradiated at room temperature over the absorbed dose
120 121
range of 1.26 x 10 to 1.34 x 10 eV/gram. Radiolyses were carried out at 20 and 50 torr. There seem to be little, if any difference in product G values at these two pressures. Figure IV-I shows a typical chromatogram of the non-condensible products and Fig. IV-2 shows a chromatogram of the condensible organic products. In all 17 organic products were identified. The major products are hydrogen, methane, ethylene, propane, acetylene, and n-butane. Figures IV-3 through IV-8 show the yields of the major products are essentially linear when plotted as a function of absorbed dose. The results are summarized in Table IV-I. The minor products are propylene, i-pentane, n-pentane, 1-butene, cis-2-butene, trans-2-butene, hexane, and pentene; their G values are listed in Table IV-I. Both major and minor products were identified by their retention times and their mass spectral cracking patterns. In the case of pentene and hexane, however, only the empirical formulas could be established. Due to similarities in both G.C. retention times and mass spectral cracking patterns of the several isomers of each compound, it could not be determined with certainty which isomers were produced.

Table IV-l also summarizes the effect of 02 on the system. When 02 was added to the ethane, it is noticed that among the major products the hydrogen yield was 'reduced by approximately 50%, the n-butane yield by


-39-





















0










CH4







2 4 6 Elution time (minutes) Fig. IV-. Gas chromatogram of the non-condensible products in the radiolysis of ethane.












Fig. IV-2. Gas chromatogram of irradiated ethane (nitrogen gas flow rate of 30 ml /min
on a 5 m x 0.25" O.D. glass column packed with 60-200 mesh silica gel).

Peak Identification

1. CH4 10. n-C5 H2 2. C 2H6 11. 1-C4 H8

3. C 2H4 12. trans-2-C4 H8

4. C3H8 13. cis-2-C4 H8

5 C2H2 14. C6H14 6 4 i-C4H10 15. C6H14 7 4 n-C4H0 16. C5HI0 8 C3H6 17. C5HI0

9. i-C5H12

















1 3


0

C%
U)
a) 4
0




x16 xl0,000 x64 x16


-- I I 1 I
5 10 15 20 25 30 Elution time (minutes)


x128


35 40 45


Fig. IV-2a.


8


































60 65 70 75 80 85 90 95 Elution time (minutes) Fig. IV-2b.





-44-


G Values for the


TABLE IV-i

Radiolysis Products from Ethane


Product H2

CH4

C3H8

n-C4 H0





i-C4 H10 C3H6




i-C5 H2 n-C5 H12 I-C4 H8 trans-2-C4 H8 cis-2-C4 H8 C6H14 (2 peaks) C5HI0 (2 peaks)


Pure System

7.24 0.53 0.49 2.13 0.12 0.05 0.016 0.008 0.013 0.004 0.007 0.004 0.013 0.020 0.009


10% Oxygen

3.12 0.24 0.14 0.27 1.52 0.29

0

0.032

0 0

0.011 0.007 0.019

0 0





-45-


1.5








W) 0
0
0Pure
0









U10 0 i.02
U













0 .0
r-I









0.5- 0













0.0 ,
0 2 4 6 8 10 12

Dose, eV/g x 10


g. IV-3. Production of hydrogen (pure, 0 ; 10% 02, ) as a function
of dose.


Fi




-46-


0.08









0.06

0
E
0






. 0.04










0.02


0.00


Fig. IV-4.


2 4 6 8 10 12

Dose, eV/g x i20


Production of CH4 (pure, 0 ; 10% 02, El ) as a function of dose.





-47-


2 4 6 8 10


Dose, eV/g x


1 -20


Fig. IV-5. Production of C3H8 (pure,
dose.


0 ; 10% 02, 0 ) as a function of


0.08 0.06 0.04









0.02


0.00





-48-


0.3


0.0


0




Fig. IV-6.


2 4 6 8 10 12


Production of of dose.


Dose, eV/g x 10-20 n-C4HIO (pure, 0 ; 10% 02, ) as a function





-49-


0 2 4 6 8 10 12


Dose, eV/g x 1o20


Fig. IV-7.


Production dose.


of C2H4 (pure, 0 ; 10% 029 El ) as a function of


0.3





-50-


10% 02


0.04









0.03 0.02 0.01









0.001


2 4 6 8 10 12


Dose, eV/g x


10- 20


Fig. IV-8.


Production of C2H2 (pure, 0 ; 10% dose.


O ,E ) as a function of


Pure





-51-


90%, the propane yield by 70%, and the methane yield by 50%. On the other hand, the major unsaturated products were greatly enhanced. For example, the ethylene yield increased by a factor of ten, and the acetylene yield increased by a factor of 6. Among the minor products, added oxygen essentially eliminated the saturated products such as ibutane, i-pentane, n-pentane, and hexane. All the unsaturated product yields, except for the pentenes, were increased.



B. Discussion


Table IV-2 compares this work with the results of several other ethane radiolysis investigations. One notices quite a variation in results. Peterson and co-workers (48), Heckel and Niessner (49a,b), and Holland and Stone (50) did their experiments at very low doses, while Yang and Manno (51), von Binau (52) and this laboratory performed their investigations at higher doses. One notices at higher doses the yields of hydrogen and especially the unsaturated compounds are lower than for the low dose work, while the yields of higher molecular weight compounds are higher. This probably illustrates the influence which the build-up of radiolysis products can have on the observed product yields. In all of the above studies, the material balance is not very good with a hydrogen to carbon ratio of 3.9:1 for Yang and Manno, 3.6:1 for von BUnau, 3.52:1 for Peterson and co-workers, 3.44:1 for Heckel and Niessner,

3.65:1 for Holland and Stone and 3.72:1 for this work.

Table IV-3 shows the effect of scavenger on the radiolysis of ethane reported by avrious investigators. While there seems to be discrepancies among the several investigations, probably due to differences in experimental conditions, such as dose, pressure, etc., certain general trends








TABLE IV-2

A Comparison of the G Values for the Radiolysis Products from Ethane Product This Lab Yang and von Binau (52) Peterson (48) Heckel and Holland and Manno (51) Niessner (49a,b) Stone (50) 112 7.24 6.8 6.0 8.3 8.08 8.7 CH4 0.53 0.61 0.78 0.5 0.4 0.75 C3H8 0.49 0.54 0.80 0.75 1.1 0.75 n-C4 H0 2.13 1.1 1.8 2.4 2.3 2.5 C2H4 0.12 0.05 1.5 2 1.2 C2H2 0.05 0 0.43 i-C4H10 0.016 0.034 0.11 C3H6 0.008 0

C5H12 0.017 0.1 C4H8 0.024 0 %.1 C6H14 0.020 0.09 C5H10 0.009 0.54

C7H16 0.03














G Values for the


TABLE IV-3 Radiolysis Products from


Ethane with Scavenger Added


Ausloos (58)


4% NO


Yang (51)


5% NO



4.05 0.34 0.14 0.27 1.36 0.27 0.03

0.014


Peterson (48)


3% NO


0.4 2.0


Ausloos (61)


10% 02


0.28 0.04 0.12

0.93 0.25 0.02

0.001


5% NO


0.28 0. 04 0.14 1.76 0.32

0.024 0.002


Heckel and Niessner (49a,b)

5% 02



3.5 0.15

0.2 0.37

2.2 0.6


This Work 10% 02



3.12 0.24 0.14 0.27 1.52 0.29 0.032 0.037


Product


H2 CH4 3H8 n-C4 H0




C4 10
C2H4 C2H2 C3H6 CH8


0.43 0.071

0.19 2.0

0.28 0.04




-54-


emerge. By comparing Table IV-2 with IV-3, it is noticed that when scavenger is added, the hydrogen yield is reduced by more than half, the lower molecular weight saturated compounds such as methane, propane, and butane are drastically reduced, the higher molecular weight saturates are eliminated, while the unsaturated compounds are increased. (In the case of the higher dose studies, they are increased quite substantially.)

As is evident from Tables IV-2 and IV-3, there have been numerous publications concerning the various aspects of the radiolysis of ethane (48-61). Also the vacuum ultraviolet photochemistry (62-71) and the ion-molecule reactions (72-80) of ethane have been extensively researched. From the information obtained from these investigations, it is possible to propose a set of reactions to help explain the products formed in the radiolysis of ethane. Table IV-4 summarizes the overall radiolysis mechanism.

When ethane is irradiated, both excited and ionized molecules are formed (Reaction IV-I). The excited ethane can decompose to give excited ethylene and hydrogen, an excited ethyl radical and hydrogen atom, methane and methylene, or two methyl radicals. Reactions (IV-2) to (IV-5) have been well established through photochemical studies (62-71), indicating that while Reactions (IV-2) and (IV-3) are the dominate processes, Reactions (IV-4) and (IV-5) play an increasingly important role at higher energies.

Through some very simple but elegant experiments using mixtures of C2H6 and C2 D as well as CH 3CD , Okabe and McNesby (62) showed that in all the vacuum ultraviolet photochemical irradiations, only a minor amount of HD was formed, thereby indicating that the excited ethane





-55-


TABLE IV-4

Proposed Radiolysis Mechanism for Ethane I. Primary Physical Processes: Energy Absorption


C2H6


-v (C2H6)* or (C2H6+ + e-)


II. Primary Chemical Processes: Neutral Fragmentation


(C2H6)*







(CH 3-CH)* (CH2-CH2) + (CH 2-CH2 (CH2-CH2 (C2 H 5)* + M (C2 H5*


M


(CH 3-CH)* + H2

(C2H5")* + H"

-+ CH + CH
4 * 2
-+ CH + CH3"

(CH2-CH2)*

-+ C2H4 + M

CH + H2


-- CH5. + M


-+ C2H4 + H.

--+ C2H3*+ H2


III. Secondary Chemical Processes: Carbene Insertion


CH2 + C2 H6 (C3H8) + M (C H8)


(C3H8)* C3H8 + M CH + C2 H5


(IV-i)


(IV-2) (IV-3) (IV-4) (IV-5) (IV-6) (IV-7) (IV-8) (IV-9) (IV-10)

(IV-il) (IV-12)


(IV-13) (IV-14) (IV-15)




-56-


TABLE IV-4 (Continued)


IV. Primary Chemical Processes:


+ +
C2H ---+ C2H5
+ +
CRH -+ C2H3
+ +
C2H6 2 C2H4
++
C2H -H-+ C2H2+


V. Secondary Chemical


Ionic Fragmentation


+ H + H2 + H2 + H 2


Processes: Ion-Molecule Reactions


C2H6+ + C2H - (C 2


2


+ + C2H5 +C2H ---- C4HI C2H+ + C2H6 C 4H11+








+ + C2H3 + C2H - C4H
2 H4 2 H6 C4 H10









+ +


C2H3 + C2H6 -----+ C4H9 C2 H2 ++C2 H6 )C4H8+


C4HII+ + H"




+
C4H9 + H2



+
C3H7 + CH4




C23H5+ + C2H6 C4H9+ + H3




C43H8+ + H4 C32H75+ + C 2H3" C34H69+ + CH4 C 4 H8+ + H 2
+



CR3H + CH4
+



CH5 + C2H4 CR4H7 + H CR3H5 + CH3" C25H+ + C2H4
C4H7+ +H
CRH +1CH1C249++C


(IV-16) (IV-17) (IV-18) (IV-19)


(IV-20) (IV-21) (IV-22) (IV-23) (IV-24) (IV-25) (IV-26) (IV-27) (IV-28) (IV-29) (IV-30) (IV-31) (IV-32) (IV-33) (IV-34) (IV-35) (IV-36)


H6)'


H 6)





-57-


TABLE IV-4 (Continued)



V. C4Hl1 + A - 4H10 + AH+
+
C3H9 + A --+ C3H8 +AH+


VI. Secondary Chemical Processes: C2H6+ + e-(or X-)
M+(saturated) + e-(or X-)
+
S(unsaturated) + e (or X)


R+(carbonium ion) + e-(or X-) VII. Secondary Chemical Processes:


H- + C2 H6 CH + C2 H6 H" + C2 H4 CH + C2 H4 CH 3- + C 2H 5" 3 5 2 4 C25 + C2 H C2H5 + C25"









C3H7 + C3H7"


Neutralization


- C2 H 6
- M* - N*

(R.)*


Radical Reactions


-+ CH 5* + H2

-+ C2 H + CH4
- 2H5 4
-+ C4 H9



-+ C4 H





-- C2 H6 + C 2 H4
- C5H12

-- C2 H6 + C 3 H6
- 3H8 + C2H4 + C6 H14

-+ C3H8 + C3 H6


(IV-37) (IV-38)


(IV-39) (IV-40) (IV-41) (IV-42)


(IV-43) (IV-44) (IV-45) (IV-46) (IV-47) (IV-48) (IV-49) (IV-50) (IV-51) (IV-52) (IV-53) (IV-54) (IV-55)





-58



TABLE IV-4
(Continued)



VII. C4 H + C2 H - C6 H14 (IV-56)

- - H10 + C2H4 (IV-57)

-- C2H6 + C4H8 (IV-58) R- + C4 2 R-CH2-CH- C2H polymer (IV-59)




-59-


eliminates molecular hydrogen from a terminal crabon (Reaction (IV-2)). This leaves an excited ethylene species which rearranged (Reaction (IV-6)), and then can collisionally stabilize to ethylene (Reaction (IV-7)), decompose to acetylene and hydrogen (Reaction (IV-8)), or decompose to C2 H + H. (Reaction (IV-9)). The excited ethyl radical is usually assumed to immediately decompose to ethylene and a hydrogen atom (Reaction (IV-II)) (61), but it is possible for some to either collisionally stabilize or decompose to C2 H and H . The methylene radical can insert into ethane to form excited propane (Reaction (IV-13)), which can either collisionally stabilize or undergo decomposition by C-C bond cleavage to form methyl and ethyl radicals.

From the various scavenging studies in the radiolysis of ethane, it is clear that ionic reactions also play a very definite role. The mass spectrum of ethane (see Table I-1) shows that the most abundant ion is C 2H4 followed by CH2H6 C2H3 C2H2 and C2 H5. The reaction scheme which accounts for the major fragments of the ethane ion is shown by Reactions (IV-16) through (IV-19). The parent ethane ion can fragment
toeterCH+ CH+,
to either C H or C H 4 and both of these may possess enough energy to undergo further decomposition. Therefore one might expect to find radiolysis products formed through the reactions of C2 H6 C2 H5 C2 H4 C 2H and C2 H 2. The reactions which these ions undergo with ethane have been extensively investigated by high pressure mass spectrometric (72-78) and ion cyclotron resonance studies (79, 80). Reactions (IV-20) through (IV-38) list the major and many of the minor ion-molecule reactions thought to play a part in the radiolysis of ethane.

The ethane ion can react with ethane and form ethane dimer ion

(Reaction (IV-20)), which dissociates leading to the formation of C4 H ,+









C 4H9+ C3 H9 and C3 H 8+. Field and co-workers (77) showed the yield of (C 2H6)2 was fairly low in the high pressure (up to 5 torr) mass spectrometric studies, so only very minor amounts of the unscavenged radiolysis products ara due to these reactions. The addition of ethyl ion to ethane is much more significant and would result in the formation of C 4IH 1 which can dissociate mainly to C4 H 9 but also forms some C3 H7
+
and C 2H5 (61, 75). The ethylene ion can react with ethane to form C 4H 0, which has been shown to chiefly dissociate to C3 H and C3 H 6 but also some C4 H9 C4 H 8 and C3 H have been observed (77). The
+
vinyl ion can add to ethane to form the C4H9 ion, which decomposes to C 3H with C4 H and C2 H being minor reactions (75, 79). Finally the acetylene ion can react with ethane to form C 4H8 which can dissociate to form C 4H7, C 3H5, and C 21 . This also appears to be a minor process (61, 77). It should be mentioned that the addition of the major (262+' + +
ions to ethane resulting in the formation of (C H C H C4 H0
4 +
C4H9 and C 4H8 respectively have been observed in high pressure mass spectrometric studies (61, 74, 75, 79). These ions can dissociate, but in the radiolysis experiments some of these ions should collisionally stabilize considering the higher pressures of the radiolysis experiments.

Ausloos and co-workers (61) found that when they irradiated 1:1

mixtures of C H and C D in the presence of nitric oxide scavenger over
2 6 2 6
70% of the partially deuterated butane formed was C4D5 H indicating the main reactions were


C25+ + CD --- C4H5D6+ (IV-25a)


2 5 2 6 45
C H5D + A . C4D5H5 AD+ (IV-37a)
+ +
C2D +-i C2 - ... C:D 1-' (IV-25b)

C+ H - A .... -+ C, D � + AH+ (iV-37'b)
4 5 5





-61-


where A is a proton acceptor. This certainly indicates that Reaction (IV-25) followed by (IV-37) is an important ionic process in the radiolysis of ethane. By using similar deuterium labeling techniques, they
+
were also able to give strong evidence that the C3H9 from Reaction (IV-22) undergoes a similar proton transfer reaction (Reaction (IV-38)).

Following these various ion-molecule reactions, charge neutralization of the positive ions by either free electrons or negative ions will take place. Charge neutralization usually yields neutral excited species since an amount of energy equivalent to the ionization energy of the ionic species may be available if a free electron is involved in the neutralization. Unfortunately, at the present time little is known about the chemical changes which may result from the neutralization process, as it is very difficult to know with certainty exactly what ions are undergoing neutralization and then how they fragment (81). Reactions (IV-39) through (IV-43) represent the various types of charge neutralization processes which are probably occurring. The parent ethane ion can be neutralized by either an electron or a negative species X (for example, a hydride ion) forming an excited ethane molecule, which is either collisionally stabilized or can undergo reactions mentioned earlier (Reactions (IV-2) through (IV-5)). M+, a saturated species, or N+, an unsaturated species, can be neutralized to form an excited species, which will undergo similar reactions to those of excited ethane or ethylene. R+, a carbonium ion, can react with an electron or a negative ion to form an excited radical, which then can fragment to smaller species.

Subsequent to neutralization, reactions involving the neutral free radicals will occur (Reactions (IV-43) through (IV-59)). Gevantman and





-62-


Williams (53) used iodine to identify and estimate the radicals formed in the radiolysis of ethane. Principal radical products detected by the appearance of the corresponding iodides were H', C2 H 5. CH 3-, and in smaller amounts, n-C3H7, n-C and CH2.


It is well known from pyrolyses and photochemical studies that hydrocarbon radicals can undergo hydrogen abstraction reactions, combination reactions, and disproportionation reactions (82-84). Reactions (IV-43) through (IV-59) would be the most probable radical reactions to explain the experimental results. Radical combination reactions are more likely to occur for simple hydrocarbon radicals than disproportionation reactions or hydrogen abstraction reactions, as they have zero or very low activiation energies and high A factors, with associated rate constants close to collision rates, based on simple collision theory

(83). That hydrogen and hydrocarbon radicals are internally scavenged by the ethylene formed in the radiolysis seems clear as the yield of ethylene dramatically increases in the presence of a radical scavenger.

The above discussion does demonstrate that the radiolysis of ethane is fairly complex. It is noticed that there is a variety of pathways to the different radiolysis products. The unscavenged yield of hydrogen is formed chiefly from the molecular elimination of hydrogen from excited ethane (Reaction (IV-2)), with other sources being decomposition of either excited ethylene (Reaction (IV-8)) or excited ethane ion (Reaction (IV-18)). Small amounts might also arise from the hydrogen abstraction Reaction (IV-39), in which the hydrogen has excess energy (i.e., hot) or from some of the numerous ion-molecule reactions mentioned. The scavengeable hydrogen originates from a combination of hydrogen atoms or from hydrogen abstraction from substrate (Reaction (IV-43)). There





-63-


are several processes which led to the for[iation of hydrogen atoms, including the decomposition of excited molecules (Reactions (IV-3), (IV-9), and (IV-I)) or ions (Reaction (IV-16)), and also some from the various ion-molecule reactions.

The nonradical methane (i.e., unscavenged) is known to originate from the decomposition of excited ethane to methane and methylene (58), and various ion-molecule reactions such as Reactions (IV-25), (IV-32), and (IV-30) (61). The radical methane yield can be accounted for by hydrogen abstraction (Reaction (IV-43)) by the methyl radical. The methyl radicals are produced by several ways such as decomposition of excited C2H6 (Reaction (IV-5)) and excited propane (Reaction (IV-15)), and by various ion-molecule reactions such as (IV-22) and to a lesser extent (IV-29) and (IV-35).

Most of the propane is formed by methylene insertion into ethane (Reaction (IV-13)) and by the combination of methyl and ethyl radicals (Reaction (IV-47)), with minor amounts probably coming from various disproportionation reactions such as (IV-53) and (IV-55). The unscavenged propane can be accounted for by the proton transfer reaction (lV-38) of the C3H9+ ion formed in Reaction (IV-22).

A combination of ethyl radicals (Reaction (IV-49)) is the major

source of normal butane. The ethyl radicals are produced by decomposition of excited ethane (Reaction (IV-3)), hydrogen abstraction reactions (IV-43) and (IV-44), with minor amounts also coming from Reaction (IV-15) and neutralization of the C H formed in various ionic reactions mentioned. The nonradical butane is thought to be due to the proton
+
transfer reaction (IV-37) oi the C4H I ion formed in Reaction (IV-24).

When Ausloos and co-workers (58) irradiated mixtures of C2D6 and





-64-


C 2H6 in the presence of nitric onide, the ethylene consisted of mostly C2D4 and C 21H with only minor amounts cf C2 H3 D and C2D3H, implying that the decomposition of excited ethane to ethylene and hydrogen is the major source of ethylene. Other sources may be the various disproportionation reactions such as Reaction (IV-50), (IV-51), and (IV-57) or charge neturalization in some of the ionic processes. That ethylene is being consumed in radical reactions such as Reactions (IV-45), (lV-48), and (IV-59) seems fairly evident by the fact that the yield of ethylene is substantially enhanced in the presence of oxygen. Ethylene itself is known to be an excellent scavenger of thermal hydrogen atoms (85).

The main source of acetylene appears to be the decomposition of

excited ethylene (Reaction IV-8). A minor amount may be produced in the charge neutralization of the various ionic processes. The fact that the yield of acetylene is enhanced in the presence of oxygen indicated that the acetylene is probably also scavenging hydrogen atoms.

Using (C2D5)2CDCD3 as an ion interceptor, Ausloos and co-workers

(61) showed that the C3H5+ ion, which is produced in Reactions V-25) and (IV-32), can form propylene by a hydride transfer reaction (Reaction (IV-60)). Also some of the propylene yield may be produced by disproportionation reactions such as (IV-52) and (IV-55).

TI� +
C3B5+ + (C 2D)2CDCD3 ---+ C3H5D + C6 D 3 (IV-60)


+
The C4H7 ion formed in Reactions (IV-31) and (IV-34) should be

able to undergo a similar hydride transfer reaction to form butene. The

fact that the pentanes, hexares, and pentenes are eliminated when oxygen is present implies that they were formed by radical combination reactions.






-65-


C. Summary


The gamma-radiolysis of gaseous ethane was studied at 50 torr, both pure and with 10% oxygen added. For the pure system, the major radiolytic products and their respective G values were H2, 7.24; CH 0.53; C3H8, 0.49; n-C4HI0, 2.13; C2H4, 0.12; and C2H2, 0.05. The minor products included i-CC4H0, C3H8, C4H8, C5H12' C5HI0, and C6H W It was observed that the addition of 10% oxygen reduced the hydrogen and methane yields by approximately 50%, the n-butane yield by 90%, and the propane yield by 70%, while increasing the ethylene yield by a factor of ten and the acetylene yield by a factor of six. Among the minor products, added oxygen essentially eliminated the saturated products such as i-C4HI0, C5H12' and C6 H 14. All the unsaturated minor products yields, except for C 5H0, were slightly enhanced. The results were discussed in terms of both excited and ionized ethane. The excited ethane decomposed chiefly to excited ethylene and hydrogen, but some C2 H and H-, CH4 and CH2, or two methyl radicals were also produced. The ionized ethane can fragment into ions (C2 H5 C2 H3 C2 H4 C2 H 2) and hydrogen. The fragment ions then undergo ion molecule reactions with ethane. The non-scavengeable products were accounted for primarily by the decomposition of ethane to excited ethylene and hydrogen and by various ion-molecule reactions, while the scavengeable products were attributed to hydrogen abstraction reactions and radical combination reactions.
















V. THE GAMMA RADIOLYSIS OF MIXTURES OF HEXAFLUOROETHANE AND ETHANE


A. Experimental Results


A series of mixtures whose compositions ranged from pure hexafluoroethane to pure ethane were irradiated at a pressure of 50 torr. The exact compositions of the mixtures are shown in Table V-1 in units of mole fraction of ethane, partial pressures, and fraction of the total energy absorbed in ethane. This last quantity is just the fraction of the total electron stopping power of the mixture that is due to ethane. For example, in an equimolar mixture of hexafluoroethane and ethane, only 23% of the energy is absorbed by the ethane.

Figure V-1 shows a typical chromatogram from the radiolysis of an equimolar mixture of ethane and hexafluoroethane. Twenty-two products were identified. The major products were hydrogen, methane, ethylene, 1,1-difluoroethylene, trifluoromethane, propane, acetylene, n-butane, and propylene. Additionally, small amounts of pentafluoroethane, 1,1,1-trifluoroethane, 1,1,1,2,2-pentafluoropropane, 1,1,1-trifluoropropane, pentaneS, butenes, hexanes, and pentenes were formed. It is interesting to note that no pure fluorocarbon compounds were detected in the irradiated mixtures. All major products were identified by their retention times and by their mass spectral cracking patterns. Standards were available for all the reaction products except for the mixed propanes.

The yields of all the major radiolysis products of the mixtures of


-66-





-67-


TABLE V-I

Composition of Irradiated Mixtures


Mole Fraction
C2H6


0

0.125

0.25 0.50 0.75

0.90 0.95 1.00


P (C2H6)
torr


0

6.25 12.5 25 37.5 45

47.5 50


P(C2F6)
torr 50

43.75 37.5 25

12.5

5

2.5

0


Fraction of Energy Absorbed in C2 H6


0

0.041

0.09 0.23 0.47 0.73 0.85 1.00












Fig. V-i. Gas chromatogram of an irradiated equimolar mixture of ethane and hexafluoroethane (nitrogen gas flow rate of 30 ml /min on a 5 m x 0.25" O.D. glass
column packed with 60-200 mesh silica gel).

Peak Identification

1. CH4 13. i-C5 H2 2. C2H6 and C2F6 14. n-C5H12 3. C2H4 15. CH3C2F5

4. CH CF 16. 1-C H 2 2 4 8
5. CF3H 17. CF3C2H5

6. C3-H8 18. trans-2-C4 H8

7. C2H2 19. cis-2-C4 H8

8. i-C4H10 20. C6H14 9. n-C4H10 21. C6H14 10. C2F5H 22. C5H o 11. CH3CF3 23. C5H10


12. C3H6















9





Cl)1

w 7
i 0 1 2
0 10 1 12
o



x 32 x10,000 x64 x32 x16 x64 x16


I II IIIII
0 5 10 15 20 25 30 35 40 45 50 Elution Time (minutes)


Fig. V-la.


















20 21

13 16 18 19 23
14 15 17 2


95 100


Elution Time (minutes)


Fig. V-lb.


105


110






-71-


the pure and oxygen scavenged systems are plotted as a function of absorbed dose in Figs. V-4 through V-19. The main purpose of these yield graphs is threefold: (1) to record and preserve the original data; (2) to show the effect of composition of mixtures on the linearity and slopes of the yields of various products with respect to energy absorbed; and (3) to show the relative amount of scatter in the data.

These graphs show that for any given mixture composition, the individual product yield increases with increasing absorbed dose. One might expect that as the amount of ethane in the mixture is decreased, the yields of the hydrocarbon radiolysis products should decrease also. Looking at Figs. V-4 through V-19, one notices this is not the case. It should be pointed out that as the amount of ethane is decreased in the mixture, the amount of hexafluoroethane is increased, causing the absorbed dose in eV/gram to decrease, as hexafluoroethane is over four times as heavy as ethane. Another factor strongly influencing these yield graphs is the effect of energy transfer from the hexafluoroethane to the ethane. This effect is clearly demonstrated in Figs. V-2 and V-3, which are summary graphs of the yields of the major radiolysis products as a function of mixture composition. In an ideal mixture, the yields of hydrogen and the hydrocarbon radiolysis products would decrease linearly with the fraction of energy absorbed by the ethane. Figures V-2 and V-3 show however that the yields of hydrogen, propane, n-butane, ethylene and acetylene are substantially enhanced over their ideal mixture lines. These figures also show that the yields of CF 3H, C2 F 5H, CF 2CH and HF increase with increasing amounts of hexafluoroethane. The G values for the major products at all mixture compositions in the unscavenged system are listed in Table V-2. Table V-3 lists all the minor radiolysis products.






-72-


The yield graphs for the oxygen scavenged system (see Figs. V-5

through V-20) demonstrate the same general trends as for the oxygen free system. The yields of all the major radiolysis products as a function of mixture composition are compiled in Fig. V-3. As in the oxygen free system, the yields of hydrogen and all the hydrocarbon radiolysis products, except for methane, are considerably higher than would be expected for an ideal mixture. In comparing the oxygen scavenged mixture system (see Table V-4) with the unscavenged mixture system (Table V-2), one notices that oxygen has the same effect on the hydrocarbon radiolysis products in the mixtures as it does in pure ethane (see Table IV-I) the saturated products are significantly reduced, while the unsaturated compounds are greatly increased. In the oxygen scavenged mixture system, the hydrogen yield was decreased by approximately 60%, the n-butane yield by 90%, the propane yield by 70%, and the methane yield by 50%. On the other hand, the ethylene yield increased by a factor of ten, and the acetylene yield by a factor of six. The yields of CF3 H C2 F 5H and HF are essentially unaffected by the addition of oxygen.

The minor products in the oxygen scavenged system are listed in

Table V-5. It is noticed that the higher molecular weight products and mixed products were eliminated, while the unsaturated products were enhanced in the presence of oxygen.


B. Discussion


As mentioned in the previous section, no pure fluorocarbon compounds were detected in the irradiated mixtures, but virtually all the hydrocarbon products were. These results are consistent with the generalization made initially by Fallgatter and Hanrahan (7) concerning the





-73-


TABLE V-2

G Values for the Major Radiolysis Products
Ethane-Hexafluoroethane Mixtures


Fraction of 0.09 0.23


0.91 0.05

0.18 0.68 0.06 0.01 0.44 0.24

0.27 0.70


2.43 0.21 0.82

2.44 0.12 0.10

0.28

0.16 0.17 0.59


Energy Absorbed

0.47


5.12 0.30 0.61

2.19 0.17 0.08 0.20 0.12 0.16 0.41


from


0.85


1.00


Product


H2 CH4 C3H8 n-C4 HI nC4 H10 C2H4 C242 CFH CF 3H

C2F5 CF 2CH2


HF


in C2 H6

0.73


6.51 0.38 0.54 2.21

0.19 0.09 0.12

0.07 0.09

0.27


7.03 0.43 0.50 2.20 0.15 0.06 0.06

0.03 0.04

0.13


7.24

0.53 0.44 2.13 0.12 0.05

0

0

0

0





-74-


TABLE V-3

G Values for the Minor Radiolysis Products from Ethane-Hexafluoroethane Mixtures



Fraction of Energy Absorbed in C2 H6

Product 0.09 0.23 0.47 0.73 0.85 1.00 i-C4H10 0.007 0.022 0.020 0.020 0.019 0.016 i-C5H12 0.003 0.010 0.011 0.013 0.014 0.013 n-C5H12 <.001 0.004 0.004 0.005 0.005 0.004 C6H14 0.005 0.014 0.015 0.018 0.020 0.020 C 3H6 0.005 0.024 0.020 0.015 0.012 0.008 1-C4H8 0.002 0.011 0.009 0.008 0.009 0.007 cis-2-C4H8 0.004 0.016 0.016 0.015 0.013 0.013 trans-2-C4H8 <.001 0.003 0.003 0.004 0.004 0.004 C5H10 0.003 0.011 0.010 0.011 0.010 0.009 CH 3CF3 0.092 0.040 0.021 0.009 0.005 0 CH3C2F5 0.049 0.040 0.019 0.010 0.003 0 CF3C2H5 0.062 0.053 0.032 0.013 0.004 0






-75-


TABLE V-4

G Values for the Major Radiolysis Products from
Ethane-Hexafluoroethane Mixtures
with 10% added Oxygen Scavenger


Fraction of 0.09 0.23


0.67 0.03 0.08 0.24 0.85

0.16 0.45 0.20

0

0.61


1.33 0.05

0.20 0.32 1.10 0.26 0.35 0.14

0

0.48


Energy Absorbed

0.47


1.98 0.09

0.19 0.30

1.20 0.26 0.22 0.09

0

0.31


Product H2

CH4 C3H8 n-C 4H 10 4104 C2H2 CFH CF 3H C 2F 5H 2F5H CF 2CH2 HF


in C2 H6

0.73


2.50 0.11

0.19 0. 30 1.24 0.28 0.15

0.05

0

0.21


0.85


2.83 0.15

0.17 0.29 1.31 0.28

0.07 0.02

0

0.08


1.00 3.12 0.24 0.14

0.27 1.52 0.29

0

0

0

0






-76-


TABLE V-5

G Values for the Minor Radiolysis Products from Ethane-Hexafluoroethane Mixtures
with 10% added Oxygen Scavenger



Fraction of Energy Absorbed in C2 H6

Product 0.09 0.23 0.47 0.73 0.85 1.00 i-C4H10 0 0 0 0 0 0 i-C5H2 0 0 0 0 0 0 n-C5HI2 0 0 0 0 0 0 C6H14 0 0 0 0 0 0 C3H6 0.015 0.036 0.033 0.032 0.032 0.031 1-C4H8 0.006 0.012 0.011 0.012 0.010 0.011 cis-2-C4H8 0.008 0.018 0.021 0.018 0.020 0.019 trans-2-C4H8 0.003 0.005 0.006 0.007 0.007 0.007 C5H10 0 0 0 0 0 0 CH3CF3 0 0 0 0 0 0 CH3C2F5 0 0 0 0 0 0 CF3C2H 50 0 0 0 0 0

















H6
H2





0.8- 4


0.6


0.4 CH 2
r 0.4-4 24



0.0 0
0.0 0.2 0.4 0.6 0.8 1.0 Fraction of Energy Absorbed in C2 H6 Fig. V-2a. Yields of H2, CH4, and C2H4 in the radiolysis of C2H6 - C2F6 mixtures as
a function of mixture composition. Read right-hand scale for H2 yield.





-78-


0.2 0.4 0.6 0.8 1.0


Fraction of Energy Absorbed in C2 H6


Fig. V-2b.


Yields of n-C4HI0, C3H8, and CH2CF2 in the radiolysis of C H - C F mixtures as a function of mixture composition.
2 6 2 6


2.6


2.4 2.2


2.0 1.8 1.6


1.4 1.2 1.0 0.8 0.6


0.4 0.2


0.0






-79-


0.4 0.6


0.8


Fraction of Energy Absorbed in C2 H6


Fig. V-2c.


Yields of HF, CF3H, C2F5H, and C2H2 in the radiolysis of C 2H 6- C2F6 mixtures as a function of mixture composition.


0.7 0.6 0.5 0.4 0.3 0.2


0.0 V
0.0


0.2


1.0



































0.2 0.4 0.6 0.8

Fraction of Energy Absorbed in C2 H6


0



0.2





0.1





0.0
1.0


Fig. V-3a.


Yields of H2, C2H2, and C2H4 in the radiolysis of C2H6- C2F6 mixtures with 10% 02 as a function of mixture composition. Read right-hand scale for C2H2 yield.


0.0
0.0





-81-


0.2 0.4 0.6 0.8

Fraction of Energy Agsorbed in C2 H6


Fig. V-3b.


Yields of CH4, C3H8, and n-C4HI0 in the radiolysis of C2H6 - C2F6 mixtures with 10% 02 as a function of mixture composition.


0.4 0.3 > 0.2






0.1


0.0





-82-


0.2 0.4 0.6 0.8

Fraction of Energy of Absorbed in C2 H6


Fig. V-3c.


Yields of HF, CF3H, and C2F5H in the radiolysis of C2H6 - C2F6 mixtures with 10% 02 as a function of mixture composition.


0.6 0.5 0.4


0.2 0.1





0.0





-83-


1.6 1.4 1.2


1.0 0.8


0.6 0.4


0.0


Fig. V-4a.


100%


2 4 6 8 10 12
Dose, eV/g x 1o20

Yields of H2 for various mixtures of C2H6 and C2F6 as a function of dose. Percent of energy absorbed by C2H6: 9%, A ; 73%, 0 ; 100%, 0 .





-84-


2 4 6 8 10
Dose, eV/g x 10-20


Fig. V-4b.


Yields for H2 for function of dose. 23%, A ; 47%, U ;


various mixtures of C2H6 and C2F6 as a Percent of energy absorbed in CA: 85%, 0.


1.0




0.8


23%






-85-


2 4 6 8 10


Dose, eV/g x 102


Fig. V-5a.


Yields for H2 for function of dose. 47%, N ; 73%, E0 ;


C2H6 - C2F6 mixtures with 10% 02 as a Percent of energy absorbed in C2H6: 100%, 0


0.4 0.3





-86-


0 2 4 6 8 10 12


Dose, eV/g x


Fig. V-5b.


Yields for H2 for C2H6 - C2F6 mixtures with 10% 02 as a function of dose. Percent of energy absorbed in C2H6: 9%, A ; 23%, A ; 85%, * .


0.6 0.5 0.4 0.3 0.2 0.4




0.0





-87-


2 4 6 8

Dose, eV/g x 10-20


Fig. V-6a.


Yields of CH4 for various mixtures of C2H6 and C2F6 as a function of dose. Percent of energy absorbed in C2H6: 9%, A ; 47%, U ; 73%, 0 .


0.08









0.06


0.04


0.02 0.00






-88-


2 4 6 8 10 12


Dose, eV/g x 10-20


Fig. V-6b.


Yields of CH4 for function of dose. 23%, A ; 85%, * ;


various mixtures of C2H6 and C2F6 as a Percent of energy absorbed by C2H6: 100%, 0 �


0.08









0.06 0.04 0.02


0.00






-89-


2 4 6 8


Dose eV/g x


Fig. V-7a.


Yields of CH4 for function of dose.
23%, ni ; 85%, 0 ;


C2H6 - C2F6 mixtures with 10% 02 as a Percent of energy absorbed in C2H6: 100%, 0.


0.02


0.01 0.00






-90-


2 4 6 8

Dose, eV/g x lo20


Fig. V-7b.


Yields of CH4 for C2H6 - C2F mixtures with 10% 02 as a function of dose. Percent o energy absorbed in C2H6: 9%, A ; 47%, E ; 73%, E3 .


0.03


0.02 0.01


0.00




Full Text

PAGE 1

THE RADIATION CHEMISTRY OF MIXTURES OF ETHANE AND HEXAFLUOROETHANE IN THE GAS PHASE By MICHAEL DAVID SCANLON 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 1976

PAGE 2

ACKNOWLEDGEMENTS The author wishes to express his deep gratitude and appreciation to Professor Robert J. Hanrahan, under whose direction this investigation was done, for his advise, guidance, and understanding. Special gratitude is also extended to his parents and family for their encouragement, patience, and sacrifice. 11

PAGE 3

TABLE OF CONTENTS Page ACKNOWLEDGEMENTS ii LIST OF TABLES v LIST OF FIGURES vi ABSTRACT ix CHAPTER I. INTRODUCTION 1 II. EXPERIMENTAL METHODS AND EQUIPMENT 8 A. Sources and Preparation of Materials 8 B. Sample Preparation . 8 C. Sample Irradiation 13 D. Product Analysis 18 III. THE GAMMA RADIOLYSIS OF HEXAFLUOROETHANE 29 A. Experimental Results 29 B. Discussion 34 C. Summary . 38 IV. THE GAMMA RADIOLYSIS OF ETHANE 39 A. Experimental Results , 39 B. Discussion 51 C. Summary 65 V. THE GAMMA RADIOLYSIS OF MIXTURES OF HEXAFLUOROETHANE AND ETHANE 66 A. Experimental Results 66 iii

PAGE 4

TABLE OF CONTENTS (Continued) Page B. Discussion 72 C. Summary 116 REFERENCES 118 BIOGRAPHICAL SKETCH 125 I iv

PAGE 5

LIST OF TABLES Table Page 1-1 A Comparison of the Mass Spectrum of Ethane with Hexaf luoroethane 7 III-l Radiolysis Yields for Hexaf luoroethane 33 IV1 G Values for the Radiolysis Products from Ethane 44 IV-2 A Comparison of the G Values for the Radiolysis Products from Ethane 52 IV-3 G Values for the Radiolysis Products from Ethane with Scavenger Added . . 53 IV-4 Proposed Radiolysis Mechanism for Ethane 55 V-l Composition of Irradiated Mixtures 67 V-2 G Values for the Major Radiolysis Products from EthaneHexaf luoroethane Mixtures 73 V-3 G Values for the Minor Radiolysis Products from EthaneHexaf luoroethane Mixtures 74 V-4 G Values for the Major Radiolysis Products from EthaneHexaf luoroethane Mixtures with 10% added Oxygen Scavenger 75 V-5 G Values for the Minor Radiolysis Products from EthaneHexaf luoroethane Mixtures with 10% added Oxygen Scavenger 76 V-6 Proposed Radiolysis Mechanism for the Mixtures of Ethane and Hexaf luoroethane Ill v

PAGE 6

LIST OF FIGURES Figure Page II-l Vacuum system 10 II-2 Cross section of cobalt-60 gamma ray source 14 11-3 Nickel vacuum tight radiolysis vessel 15 II-4 Cylindrical radiolysis vessel and holder 16 II5 Dosimetry: hydrogen yield from ethylene as a function of irradiation time 17 II-6 Schematic diagram of the "Duplex Gas Chromatograph". . . 19 II7 Automatic Toepler pump 20 II-8 Product trapping vessel 24 II-9 Spark discharge vessel 26 11-10 Titration curves for quantitative analysis of F ion . . 28 III-l Gas chromatogram of irradiated hexafluoroethane 30 III-2 Production of CF. as a function of dose 31 4 III-3 Production of C„F n and C.F__ as a function of dose ... 32 3 8 4 10 IV1 Gas chromatogram of the non-condensible products in the radiolysis of ethane 40 IV2 Gas chromatogram of irradiated ethane 42 IV3 Production of hydrogen as a function of dose 45 IV-4 Production of CH. as a function of dose 46 4 IV-5 Production of C H 0 as a function of dose 47 J o IV-6 Production of n-C,H, _ as a function of dose 48 4 10 IV7 Production of C„H, as a function of dose 49 z 4 IV8 Production of as a function of dose 50 vi

PAGE 7

LIST OF FIGURES (Continued) Figure Page V-l Gas chromatogram of an irradiated equimolar mixture of ethane and hexafluoroethane. 69 V2 a Yields of H2, CH^, and C2H4 in the radiolysis of C2Hg C2F6 mixtures as a function of mixture composition. 77 V2 b Yields of n-C^H^g, CgHg, and CH2CF2 in the radiolysis of C2Hg C2Fg mixtures as a function of mixture composition 78 V2 c Yields of HF, CF3H, C2F5H, and C2H2 in the radiolysis of C2Hg C2Fg mixtures as a function of mixture composition . 79 V3 a Yields of H2, C2H2, C2H4 in the radiolysis of C2H6 C2F5 mixtures with 10% O2 as a function of mixture composition 80 V3 b Yields of CH^, C3H3, and n-C^H 3 q in the radiolysis of C2H5 C2F5 mixtures with 10% O2 as a function of mixture composition 81 V3 c Yields of HF, CF3H, and C2F3H in the radiolysis of c 2 h 6 ~ c 2 f 6 mixture with 10% O2 as a function of mixture composition 82 V -4 Yields of H2 for various mixtures of C2H5 and C2F5 as a function of dose 83 V -5 Yields of H£ for C2H3 C2Fg mixtures with 10 % O2 as a function of dose 85 V-6 Yields of CH4 for various mixtures of C2Hg and as a function of dose . 87 V -7 Yields of CH4 for C2Hg C2Fg mixtures with 10 % O2 as a function of dose 89 V-8 Yields of C3Hg for various mixtures of C2H5 and C2Fg as a function of dose 91 V -9 Yields of C3H8 for C2Hg C2Fg mixtures with 10 % O2 as a function of dose 93 V -10 Yields of n-C^H^g for various mixtures of C2Hg and C2Fg as a function of dose. . 95 vii

PAGE 8

LIST OF FIGURES (Continued) Figure Page V-ll Yields of n-C^H^g for ^ 2^6 ~ *-'2^6 mixtures with 10% O 2 as a function of dose . 97 V-12 Yields of C 2 H 4 for various mixtures of C 2 H 5 and C 2 F 5 as a function of dose 99 V-13 Yields of C 2 H 4 for C 2 H 5 6 mixtures with 10% O 2 as a function of dose 101 V-14 Yields of CF 3 H for various mixtures of and as a function of dose 103 V-15 Yields of CF 3 H for C 2 H 5 C 2 F 5 mixtures with 10% O 2 as a function of dose 104 V-16 Yields of C 2 F 5 H for various mixtures of C 2 Hg and C 2 Fg as a function of dose 105 V-17 Yields of C 2 F 5 H for C 2 H 6 O 2 F 5 mixtures with 10% O 2 as a function of dose 106 V-18 Yields of CH 2 CF 2 for various mixtures of C 2 Hg and C 2 Fg as a function of dose 107 V-19 Yields of HF for various mixtures of C 2 Hg and C 2 Fg as a function of dose 108 V-20 Yields of HF for C 2 Hg C 2 Fg mixtures with 10% O 2 as a function of dose 109 • r I viii

PAGE 9

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 OF MIXTURES OF ETHANE AND HEXAFLUOROETHANE IN THE GAS PHASE By Michael David Scanlon August, 1976 Chairman: Dr. R.J. Hanrahan Major Department: Chemistry The gas phase radiolysis of mixtures of ethane and hexaf luoroethane were studied at 50 torr pressure for systems containing 0 to 100% of the fluorocarbon, both pure and with 10% added oxygen. For the pure system, twenty-two products were identified, and the yields over the entire mixture range have been recorded. The major radiolytic products were H^ CH^ C^g, n-^H^, C^, C^ , CFgH, C^H, CF^H^ and HF. The minor products included i-C.H 10 , i-C,.H 10 , n-C c H,„, C,H, . , C H, , 4 1U j 1Z b 1Z o 14 d o l-C 4 H g , cis-2-C^Hg, trans-2-C^Hg , ^H^, CHgCFg, CHgC^, and CF^H^ It was observed that the addition of 10% oxygen substantially reduced the saturated hydrocarbon products, while the unsaturated hydrocarbon products were enhanced. Among the mixed products, HF, CF^H, and C^F^H were essentially unaffected by added oxygen, while CH^CF^, CF 2 CH 2> CFgC 2 H 3 , and CHgC 2 F 3 were eliminated. The fact that pure fluorocarbon compounds were not formed in the mixtures, and that the hydrocarbon products were significantly enhanced over their ideal mixture lines ix

PAGE 10

indicated that decomposition of ethane is sensitized by the presence of hexaf luoroethane and that a major process in the radiolysis of the mixtures is the ability of the hexaf luoroethane to transfer charge and/or excitation energy to ethane, which subsequently decomposes, forming the hydrocarbon products. Among the mixed products, CH^CF^, CH^CF,,, CF^C 2 H^, and CH^C 2 F^ were attributed to radical combination reactions, while CF^H and C^F^H were attributed to hydride ion transfer reactions from ethane, with HF formed via a simple hydrogen atom abstraction reaction. The results of this system were compared with the cyclohexane-perf luorocyclohexane system and the cyclobutane-perf luorocyclobutane system. This comparison revealed that in the radiolysis of fluorocarbon-hydrocarbon mixtures, the products resemble the pure hydrocarbon component rather than the pure fluorocarbon and that, as the chain length is lowered, energy (and/or charge) transfer to the hydrocarbon becomes much more important than protection of hydrocarbon by electron capture. x

PAGE 11

I. INTRODUCTION In the past ten years there have been a number of research papers published concerning the radiation chemistry of fluorocarbon hydrocarbon mixtures (1-10). The most extensively studied system has been the radiolysis of the mixtures of cyclohexane and perf luorocyclohexane (1-4, 6-8, 10). It was found that when a small amount of perfluorocyclohexane was added to cyclohexane, the hydrogen yield was sharply decreased from the value in pure cyclohexane, the bicyclohexyl dimer yield was increased, the cyclohexene yield was decreased slightly, and c-C^F^H was formed. These results can be understood if we consider the following reaction scheme proposed for radiolytic decomposition of cyclohexane (3, 7, 10, 11): C_C 6 H 12 — ' n b r V/W->(cC 6 H i2 + e ) v c -C 6 H 12 * (I-l) C C 6 H 12 r \j f \j\/\j~y C C 6 H 12* (1-2) C C 6 H 12* c-C 6 H 10 + H 2 (1-3) C C 6 H 12* c-C,H_ • + Hb 11 (1-4) H+ c-C H 0 H 2 + C-C 6 H 11 " (1-5) 12 2 C C 6 H n y C 12 H 22 (1-6) 2 c ~ C 6 H 11 ' y C ' C 6 H 10 + C C 6 H 12 (1-7) Reactions (I-l) through (1-7) summarize the major steps in the radiolysis of pure cyclohexane. When cyclohexane is irradiated, the primary products are positive ions accompanied by an equivalent number 1 -

PAGE 12

2 of electrons as well as excited molecules. The ions and electrons can also combine to form excited molecules. The excited molecules then decompose to give either hydrogen and cyclohexene or hydrogen atoms and cyclohexyl radicals. The cyclohexyl radicals either combine to form bicyclohexyl or disproportionate to yield cyclohexene and cyclohexane. Several reactions mechanisms have been proposed to explain the effect of perf luorocyclohexane on cyclohexane. What is fairly clear is that perf luorocyclohexane, an excellent electron scavenger, interferes with Reaction ( I— 1 ) by capturing electrons, thereby preventing the charge neutralization process between the cyclohexyl ions and electrons. The most frequently mentioned mechanism >is the following, suggested by Sagert (3): c-C,F + e 6 12 C ~ C 6 F 12 C C 6 F 12 + c C 6 H 12 c-C,F • + c-C,H • + HF 6 11 6 11 ( 1 8 ) (1-9) An alternative scheme was proposed by Rajbenbach and Kaldor (2), who suggested that Reaction (1-8) is followed by C C 6 F 12 + C C 6 H 12 C C 6 F 11 + C C 6 H 12 cC 6 F h +HF + c-C 6 H 11 . cC 6 F h H + c-C 6 H n . ( 1 10 ) (1-11) The original Sagert mechanism appears unacceptable in view of the observation by Kennedy and Hanrahan (10) that added iodine does not reduce the yield of c-C^F H in the radiolysis of cyclohexane-perf luorocyclohexane solutions; it is inferred that thermalized perfluorocyclohexyl radicals are not involved in the reaction. It was suggested that Reaction (1-9) might involve proton transfer to c-C^F £ Saving an intimate ion pair or a charge-transfer complex, which dissociates to

PAGE 13

-3HF and an excited c-C,F 11 ‘ o 11 radical: C C 6 F 12 _ + C C 6 H 12 + ( c-C 6 F 12~ * * * H+ ) c-C 6 H ll * + (c C 6 F 12"-"H+) (c-C 6 F 11 *)* + HF (I-9a) ( I-9b) Since the resulting (c-C,F.. •)* would be excited with a considerable o 11 fraction of the charge neutralization energy, it could react with the hydrocarbon substrate at an enhanced rate. The Rajbenbach and Kaldor mechanism is attractive because it does not invoke participation of c-C^F • radical. However, Reaction ( I— 10) is unacceptable if the electron capture energy associated with the formation of c-C F.^ has been dissipated to the medium. In that case, the process is at least 30 kcal/mole endoergic. Kennedy and Hanrahan (10) suggested that the Rajenbach and Kaldor mechanism is acceptable provided that Reaction (1-8) is followed very rapidly by (I-10a), so that the reactant c-C,F. „ still contains its biz electron capture energy: (c C 6 F 12~>‘ + c C 6 H 12 * c C 6 F ll" + HF + c C 6 H ll ' (I 10a) Alternatively, Reactions (1-8) and (1-10) could occur as a single concerted process (10) : S ' + C “ C 6 F 12 + C C 6 H 12 * c C 6 F h’ + HF + C C 6 H H' (I 12) Reaction (1-11) would follow in any event. All investigators, who have considered the problem, have ruled out dissociative electron capture as a major process: e +C C 6 F 12— " F +c C 6 F n' (1-13)

PAGE 14

-4Since the appearance potential for F from perf luorocyclohexane is 1.8 eV (12), the process is 41 kcal/mole endoergic. Both the modified Sagert mechanism and the modified Rajbenbach and Kaldor mechanism are compatible with all previous observations in the radiolysis of cyclohexane-perf luorocyclohexane solutions . Unlike the cyclohexane-perf luorocyclohexane systems where protection of the decomposition of the hydrocarbon by added fluorocarbon is the major process, experimental results in the radiolysis of the mixtures of cyclobutane and perf luorocyclobutane indicate the decomposition of cyclobutane to hydrocarbon products is sensitized by the presence of perf luorocyclobutane . Heckel and Hanrahan (9) foupd that the, yields of ethane, propane, ethylene, butene, and several of the other cyclobutane radiolysis products were higher than would be expected for an ideal mixture. In such a mixture, the yields of the hydrocarbon products would decrease linearly with the fraction of energy absorbed by the cyclobutane. They attributed these increased hydrocarbon yields to energy transfer from the excited perfluorocyclobutane to cyclobutane, leading to the decomposition of the cyclobutane (1-14). The decrease in the perf luoroethylene yield and the ( c -C 4 F 8 ) * + c-C 4 H 8 (c-C 4 H 8 )* + c-C 4 F 8 1 (1-14) absence of higher fluorocarbon products which were seen in the radiolysis of pure perfluorocyclobutane also indicate that this perfluorocyclobutane is protected from radiolytic decomposition by cyclobutane. There still seems to be some tendency toward decomposition of the perfluorocyclobutane as suggested by the abrupt rise in the hydrogen fluoride yield at low concentrations of added perfluorocyclobutane. This result was also

PAGE 15

-5observed in all the previous studies of fluorocarbon-hydrocarbon mixtures (1-11). Also, the fact that the hydrogen yields in the radiolysis of cyclobutane-perf luorocyclobutane mixtures is essentially linear with respect to the energy absorbed in cyclobutane suggests there is a tradeoff between a tendency for protection by the perf luorocyclobutane (via the electron capture process) and a rather noticeable tendency toward energy transfer from the perf luorocyclobutane to the cyclobutane, leading to formation of products other than hydrogen. If we consider the fact that both the Sagert mechanism and the Rajbenbach and Kaldor mechanism proposed to explain the results in the perf luorocyclohexane-cyclohexane system depend on electron capture by the fluorocarbon, it follows that there should be less protection by perf luorocyclobutane than by perf luorocyclohexane as the lifetime of the perf luorocyclobutyl negative ion is about a hundred times shorter than that of the perf luorocyclohexyl negative ion (13). With these results in mind, it was decided to investigate a somewhat simpler system, the mixtures of ethane and perf luoroethane , to establish the radiolytic behavior in this system and also to further elucidate the radiolysis mechanism of the mixtures of hydrocarbons and fluorocarbons in general. Both the perf luorocyclohexyl and perf luorocyclobutyl negative ions have been observed (13) and as mentioned earlier are believed to play a prominent role in the radiolysis mechanism in their mixtures with hydrocarbons. The perf luoroethyl ion is postulated (14), but has not been seen (14, 15). Apparently, if it exists it has a lifetime of less than one microsecond. This implies that, in the mixtures of ethane and hexaf luoroethane, protection of the hydrocarbon by the perfluorocarbon (via

PAGE 16

6 electron capture) plays a more limited role than in previous studies and that other processes such as energy transfer should be more important. As with the other analogous hydrocarbon-fluorocarbon systems, the ethane-hexaf luoroethane system is interesting because of the considerable differences in bond strengths, electron affinities, ionization potentials, thermal stabilities, and mass spectra of the two gases. Table 1-1 compares the mass spectra of ethane (16) and hexaf luoroethane (17) and the dissimilarities are quite striking. For example, there is virtually none of the parent ion in the hexaf luoroethane , as compared to 26% for ethane. Also in hexaf luoroethane , the most abundant ion is the perf luoromethyl ion, whereas in ethane the methyl ion is approximately 5%, thereby indicating that carbon-carbon bond rupture is much more prevalent in the fluorocarbon than in ethane. One immediately observes that in ethane, there is a much greater tendency for the carbon-hydrogen bond to break than there is for the carbon-fluoride bond to break in hexaf luoroethane. Although there has been no previous work done on the mixtures of ethane and hexaf luoroethane, there have been a few papers concerning the radiolytic behavior of pure hexaf luoroethane and numerous papers on ethane. These results will be presented in the Discussion Sections of Chapter III and Chapter IV respectively.

PAGE 17

-7TABLE 1-1 A Comparison of the Mass Spectrum of Ethane (left side) with Hexafluoroethane (right side) ION ABUNDANCE ION C 2 H 6 + 26.2 0.15 C F + 2 6 C 2 H 5 + 21.7 41.3 C F + 2 5 C 2 H 4 + 100.0 0.55 C F + 2 4 C 2 H 3 + 33.3 0.12 C 2 F 3 + c 2 h 2 + 23.0 0.13 C 2 F 2 + c 2 h + 4.1 0.42 C 2 F+ CH 3 + 4.6 100.0 CF 3 + CH 2 + 3.3 10.1 CF 2 + CH + 1.2 18.3 CF + H + 2.6 1.2 F +

PAGE 18

II. EXPERIMENTAL METHODS AND EQUIPMENT A. Source and Preparation of Material s Hexaf luoroethane : Peninsular Chemresearch hexaf luoroethane was purified by preparative gas chromatography using a 2.5 meter column packed with Analab 100-110 mesh silica gel, followed by several freezepump-thaw cycles, and stored in a vessel attached to the vacuum line. Gas chromatographic analysis showed that the was free of any impurities . Ethane : Matheson Company C.P. grade ethane was also purified by preparative gas chromatography using a 5 meter column packed with silica gel of 60-200 mesh (W.H. Curtin), followed by several freeze-pump-thaw cycles, and stored in a vessel attached to the vacuum line. Gas chromatographic analysis showed that the ethane was free of any impurities. Ethylene : Matheson Company C.P. grade ethylene was passed through BaO into a storage vessel on the vacuum line. Possible air contamination was then removed by several freeze-pump-thaw cycles. Oxygen : Matheson Company research grade oxygen was used as received from the manufacturer. Other Reagents : Chemicals used for chromatographic calibration standards and miscellaneous experiments were used as received. B. Sample Preparation Vacuum System : The vacuum system used is shown in Fig. II-l. The

PAGE 19

CJ cu o h CO o CO o >. a rH a CO o 'w' cd CD o Vj u B 00 Ph 0) p U 4-J P o rQ QJ •H P cd 4-J 6 4J O CO P u 00 CO cd CD > rH (U Cd X 6 CD 00 > 0) CO CJ o •H r— I o o 00 •H C 1 a cj X Po CD cu M o XI •H p 00 00 X M n e C B P ' cd cd •H a; u D o H u P • •H CD O CO p o o cr 00 C rC cd cd 4-J 4-* •H -H (h M H o O £ CO CO hJ m M co CN O > > 1 1 CJ £ CN * H rH CO i — 1 CO M CO > >

PAGE 20

10 -

PAGE 21

11 pumping system consisted of a Welch Duo-Seal fore pump connected through a liquid nitrogen cold trap to a two stage mercury diffusion pump. These pumps were connected to the main manifold through a second liquid nitrogen trap, and stopcock S 3 (Fig. II-l). Connected to the main manifold were a submanifold attached at stopcock S. , a vacuum thermo— 4 couple gauge, G, and three inlet ports, 1^, 1 ^, and I . Inlet ports 1^ and I were used chiefly for introducing samples, whereas a 500 ml storage vessel was attached to 1^. Attached to the submanifold were a manometer, M, storage vessels and V , a gas mixing chamber, C, and inlet ports 1^ and 1^. The irradiation vessels were usually attached to inlet ports 1^ and 1^. Fischer-Porter 4 mm, 0-ring sealed Teflonglass valves and ground glass valves lubricated with Halocarbon 25-5S grease were the two types of stopcocks used in the vacuum system. All inlets, 1^ 1^, were equipped with No. 5 0-ring joints. Hexaf luoroethane : Before each sample preparation, hexaf luoroethane , stored in storage reservoir V , was deaerated by repeated freeze-pumpthaw cycles until the thermocouple gauge indicated the absence of air. Valve was then closed and the sample was allowed to expand into the known volume of the radiolysis vessel (Fig. II-3) , which was attached to inlet 1^. During the course of this operation, the pressure was monitored with a mercury manometer, M. At the desired pressure, the radiolysis vessel was closed and the excess material was condensed back into the storage vessel with liquid nitrogen. The ideal gas law was used to calculate the amount of sample in the radiolysis vessel. Hexaf luoroethane with added oxygen : A known amount of hexafluoroethane was condensed into storage vessel of the mixing chamber, C, and the valve to closed. Then the desired amount of oxygen was added

PAGE 22

12 to the mixing chamber. With stopcock S,_ closed, the hexaf luoroethane in vessel was released into the mixing chamber, and the sample was mixed for ten minutes, using a magnetic circulating pump. After mixing was complete, the desired amount of the mixture was allowed to expand into the previously evacuated radiolysis vessel, which was attached to inlet I 4 • In all irradiations with oxygen, oxygen constituted 10 % of the total pressure. Ethane and ethane with oxygen : The preparation of ethane for radiolysis followed the same procedure as that used for hexafluoroethane. Likewise, samples of ethane and added oxygen were prepared using the same procedures as that for hexafluoroethane with oxygen. Mixtures of hexafluoroethane and ethane : A known amount of hexafluoroethane, stored in V , was condensed into vessel V. of the mixing 1 4 chamber, C, and the valve to closed. Similarly a known amount of ethane, stored in V , was expanded into the mixing chamber, and then condensed into V^. Enough of. the mixture of known composition was stored in for several radiolysis experiments. The mixture was then allowed to expand into the known volume of the radiolysis vfessel. After closing off the radiolysis vessel, the excess material was condensed back into storage vessel V, . 4 i Mixtures with added oxygen : A known amount of the mixture of hexafluoroethane and ethane that was stored in vessel of the mixing chamber was condensed into vessel V^. Then the desired amount of oxygen was added to the mixing chamber. With stopcock closed, the mixture was released into the mixing chamber, and the sample was mixed for approximately 10 minutes. After mixing was complete, the desired amount of the sample was allowed to expand into the previously evacuated radiolysis vessel, which was attached to inlet port I .

PAGE 23

-13C. Sample Irradiation Radiation source and vessels : All irradiations were performed using a cobalt-60 gamma irradiator which has been described previously (18). Figure II-2 is a cross-sectional view of the irradiator. The helium arc weld nickel vessel used in most of the irradiations is shown in Fig. II-3. Two vessels were made, having volume of 103.5 and 105 cc , respectively. The vessels were made of pure nickel, having Hoke monel diaphragm valves (4611N4M) . A rigid aluminum sample holder was used to assure reproducible positioning of the radiolysis vessels relative to the Co-60 source. * 1 • . • t Glass vessels were used to determine the amount of hydrogen fluoride formed in irradiated mixtures of ethane and hexaf luoroethane . Figure II— 4 shows the Pyrex vessel with its holder. Three vessels were used having volumes of 29.9 cc , 30.0 cc , and 30.2 cc. Dosimetry : Ethylene dosimetry (19, 20) was used to determine the absorbed dose rate of samples in the various radiolysis vessels. Ethylene was irradiated from 5 to 44 hours at a pressure of 50 torr. After irradiation, the radiolysis vessel was connected to a special i duplex gas chromatograph, which is described in the next section, and non-condensible gases were separated from the condensible gases (see Section II-D) . The non-condensible gases were passed through the molecular sieve column and the hydrogen yield was calculated. From the slope of the plot of hydrogen yield vs. time (see Fig. II-5) and from the known G value of hydrogen in ethylene (G = 1.2) (19, 20), the dose rate 1 9 was determined. The absorbed dose rate for ethylene was 3.15 x 10 eV/gram-hr. on Feb. 3, 1972 for the nickel vessels and 2.72 x 10 19 eV/ gram-hr. on Jan. 10, 1973 for the glass vessels. These values were

PAGE 24

-14Legend: (A) counterweight; (B) upper support; (C) control rod handle; (D) extra top shielding; (E) storage turret; (F) 400 curie Co 6 ; (G) shutter shown open; (H) rear wall; (I) door; (J) downward shielding; (K) door carriage; (L) door crank; (M) door frame. Emergency 6 foot tube in ground, under source, is not shown. FigH-2. Cross section of cobalt-60 gamma ray source.

PAGE 25

Hoke Monel Valve -15FigH-3. Nickel vacuum tight radiolysis vessel.

PAGE 26

16 -

PAGE 27

17 saxoinojoTni ‘PT9TX Z H H 5. Dosimetry: hydrogen yield from ethylene as a function of irradiation time.

PAGE 28

-18corrected for Co-60 decay during subsequent irradiations. Energy absorption in ethane, hexaf luoroethane and in the mixtures was calculated relative to the ethylene result. Application of the Bragg-Gray cavity principle (21-23) as well as the calculation of mass stopping powers are described fully elsewhere (24). The dose rates 19 19 obtained were 3.35 x 10 eV/gram-hr. for ethane and 2.46 x 10 eV/gram19 hr. for hexaf luoroethane for the nickel vessels and 2.89 x 10 eV/gram19 hr. for ethane and 2.12 x 10 eV/gram-hr. for hexaf luoroethane for the glass vessels. For mixtures of ethane and hexaf luoroethane it was assumed that each gas absorbed energy independently in proportion to the mass (more precisely, the electron stopping power fraction) of that gas present. D. Product Analysis Gas chromatograph instrument : The gas chromatograph used for the qualitative and quantitative analyses of the irradiated samples was constructed by Dr. Edgar Heckel (25) and is diagrammed in Fig. II-6. It was possible to analyze all condensible and non-condensible products, except hydrogen fluoride, using this chromatograph. It consisted of two independent gas chromatograph units one with a flame ionizdtion' detector, the other with a thermal conductivity detector connected by a common inlet system, which could be evacuated when desired. The inlet system contained two inlet ports, a U-trap, a gas sampling loop, a thermocouple gauge, and an automatic Toepler pump. The automatic Toepler pump system, shown in Fig. II-7, includes the Toepler pump itself, an Asco three-way solenoid valve, and a twenty-four position Guardian Electric stepping relay, which was activiated through

PAGE 29

19 c o •H U O N H •HO 00 C -U C O U *H MO 4J 4J 4J (DO -H £ Q M CU Cu O o jfi H Fig. II-6 . Schematic diagram of the "Duplex Gas Chromatograph.

PAGE 30

20 ih aj r— I CU Ph S QJ 3 O PL, H CO a e < I hH M OX) •H Automatic Toepler pump.

PAGE 31

21 the electrical contacts built into the Toepler pump. The stepping relay advanced after each cycle of the Toepler pump, in response to closure of its contacts, automatically causing the next pump cycle to occur. The column-oven-detector system used for permanent gas analysis usually contained a molecular sieve column, and the parallel section for organic gas analysis employed a silica gel column. Heating wire was wrapped around all the connecting tubing that came into contact with the condensible products. All valves in the chromatograph unit were obtained from Republic Manufacturing Co. Valves 1-5 were Teflon-plug models (2-way, No. A-330, three-way, No. A-310) and valve 6 was a bellows-sealed type (No . B-139) . The output of both the thermal conductivity detector and the flame ionization detector were fed into a Photovolt Microcord recording potentiometer Model 44 with 10 in chart paper, full scale sensitivities of 0.5, 1, 2, 5, and 10 mv , and chart speeds of 0.5 and 2 in /min. Analysis of non-condensible products : After the sample had been irradiated, the radiolysis vessel was attached to the inlet system of the gas chromatograph by means of a silicone 0-ring and a horseshoeclamp. The entire inlet system was evacuated by putting valve 1 in position D and opening valve 6. After a good vacuum had been attained, valve 6 was closed, thus isolating the system from the pump. Valve 7 was then opened allowing all the condensible organic products to be collected in the trap, which was cooled in a dewar of liquid nitrogen. The automatic Toepler pump switch was then turned on, allowing the noncondensible gases to be drawn from the radiolysis vessel and inlet system into the gas sampling loop. The Toepler pump was operated automatically through ten cycles, while the pressure was monitored using the

PAGE 32

22 thermocouple gauge. At the end of the tenth cycle, the solenoid valve system was automatically deactiviated and the mercury was allowed to rise against the glass frit, concentrating all the non-condensible into the gas sampling loop. Then valve 2 was closed, isolating the condensible products in the liquid nitrogen trap, and valve 5 was opened allowing the carrier gas to sweep the non-condensible gases into the molecular sieve column (Fig. II-6) of the thermal conductivity chromatograph system. This chromatograph was equipped with a Gow Mac thermal conductivity detector and a 3.5 m x 0.25 in O.D. column of molecular sieve (5A) . Hydrogen and methane were easily separated by using nitrogen carrier gas at a flow rate of 20 ml/min at 110°C (see Fig. IV-2) . Analysis of condensible organic products : After the non-condensibles were analyzed, the liquid nitrogen Dewar was removed from the trap, and the trap was heated by electrical resistance wire* which was wrapped around it. Valve 3 was then opened allowing the nitrogen carrier gas to sweep the condensible products into the silica gel column of the flame-ionization chromatograph unit. This chromatograph was equipped with an Aerograph flame ionization detector, a 30 cm pre-column of 100-110 mesh Analabs silica gel, a 5 m x 0.25 in O.D. column of 60-200 mesh silica gel (W.H. Curtin), and an F & M Scientific Corporation Model 40 Linear Temperature Programmer. The pre-column, which was outside the oven compartment, was wrapped with heating wire and could readily be heated to 200°C within about two minutes . The irradiated products of hexaf luoroethane , ethane, and their mixtures were analyzed using the above arrangement. The only difference in the analyses of these various samples was in the initiation time of the

PAGE 33

-23temperature programming. For hexaf luoroethane samples, programming was started 15 minutes after injection of the sample onto the column. For ethane the time was 20 minutes after injection, and for the mixtures it was 30 minutes. The pre-column was heated up at the same time the temperature programmer was started. The oven compartment was programmed from 25°C to 200°C at a rate of 3° per minute. The pre-column reached 200°C in a matter of minutes. Some typical chromatograms are shown in the next chapter. The sensitivity of the flame ionization detector to the radiolysis products was calibrated relative to that for hexaf luoroethane. The radiolysis products were identified by their retention times and also by trapping the individual product via a splitting valve into a liquid nitrogen-cooled U-tube. The contents of the tube were then analyzed in the Bendix Time of Flight mass spectrometer (see next section) . Mass spectrometric product identification : The gas chromatograph described above was also used to trap individual products for mass spectrometric analysis. This was accomplished by placing a stream splitting valve in front of the flame ionization detector (see Fig. II-6) As the product mixture was fractionated by the gas chromatograph, the major portion of each effluent peak was diverted to a liquid nitrogen cooled trap attached to the outlet of the splitter valve; the remaining portion of each peak went to the chromatographic detector as usual. The trap consisted of a U-tube to which two Fischer-Porter 4 mm, 0-ring sealed Teflon-glass valves were attached (see Fig. II-8) . The U-trap was packed with Pyrex helices. The trapping procedure was as follows: the irradiated sample was analyzed in the usual manner. When the flame

PAGE 34

24 Fig. II-8. Product trapping vessel.

PAGE 35

-25ionization detector started to register the peak of the radiolysis prodnct to be trapped, the U-tube was attached to the outlet of the splitting valve and then immediately submerged in a Dewar containing liquid nitrogen. After passage of the desired component, both Teflon-glass valves were closed and the sample was stored in liquid nitrogen. The sample was then attached to the vacuum line and the nitrogen carrier gas and condensed oxygen in the trap were pumped out. The trap was connected to the "semi-direct" inlet of the Bendix Model 14-107 Time of Flight mass spectrometer for analysis. A General Automation SPC-12 minicomputer had been interfaced with the mass spectrometer and programmed to acquire, analyze, and print out reduced data giving mass numbers and normalized intensities (26). To prepare the large amounts of reaction products needed for mass spectrometric analysis, the "spark discharge technique" was employed (5). A 500 ml, round-bottomed flask provided with two stainless steel electrodes about an inch apart, equipped with a Fischer-Porter Teflon plug needle valve, served as the spark discharge vessel (see Fig. II-9) . The vessel was filled with from 5 to 10 torr of gas in the usual manner and removed from the vacuum line. One electrode was grounded to a cold water pipe while the other was in contact with a Tesla coil, which was powered through a Variac. The lowest setting on the variac that would maintain a discharge was used (about 30 volts) and the sample was sparked from 5 to 10 minutes. Hydrogen fluoride analysis : After the sample had been irradiated, the radiolysis vessel was cooled in liquid nitrogen, and 2 ml of 95 percent ethanol-5 percent water solution was added to the solid sample through a breakseal fitting. The vessel was removed from the liquid

PAGE 36

-26Fig. II-9. Spark discharge vessel.

PAGE 37

-27nitrogen and was temporarily sealed with a rubber cap. While the sample was warming up to room temperature, it was shaken for 15 minutes to help recover the hydrogen fluoride absorbed on the vessel walls. The ethanolic solution was transferred into a 5 ml polyethylene beaker. The radiolysis vessel was then rinsed out with another 2 ml of 95% ethanol, which was added to the beaker. This solution was then titrated using a procedure _3 described by Heckel and Marsh (27). A solution of 1 x 10 M lanthanum nitrate (Fisher Scientific Co.) in a 10 microliter gas tight Hamilton syringe was used as a titrant. The electrode potential between the Orion Model (#94—09) fluoride ion activity electrode and a Beckman Model 39270 fiber junction calomel electrode was monitored by a high input impedance (10 mega ohms) Hickok digital volt-ohm meter. During the titration, the solution was stirred with a Teflon coated magnetic stirrer. A titration curve for standardizing the electrode and a typical titration curve for an irradiated sample is shown in Fig. 11-10. i

PAGE 38

-28Fig. 11-10. Titration curves for quantitative analysis of F ion. known solution, 2. A x 10"^ moles NaF ; O, radiolysis sample .

PAGE 39

III. THE GAMMA RADIOLYSIS OF HEXAFLUOROETHANE A. Experimental Results Hexaf luoroethane was irradiated at room temperature over the absorbed 20 20 dose range of 0.738 x 10 to 9.84 x 10 eV/gram. Most radiolyses were carried out at 50 torr, however experiments at pressures of 20 and 100 torr were also performed. The product G values were independent of pressure over this range. Figure III-l shows a typical product chromatogram. The three major products are tetrafluoromethane , octafluoropropane, and decaf luorobutane , and the two minor products are perf luoropentane and perf luorohexane . No unsaturated fluorocarbon compounds were found. The major products and the minor product C^F ^ were identified by their retention times and their mass spectral cracking patterns. C F.^ was only identified by mass spectrometry. Figures III-2 and III-3 show the yields of the various products are linear when plotted as a function of absorbed dose. From these plots are obtained the G values, the number of molecules formed per 100 eV of energy absorbed by the hexafluoroethane. These yields are listed in Table III-l. Table III-l also shows the effect of adding oxygen to the hexafluoroethane prior to radiolysis. The G value for tetrafluoromethane was reduced by approximately 75%, while the other radiolysis products were eliminated completely. 29 -

PAGE 40

30 asuodsa-g ao^aa^aQ Elution Time (minutes)

PAGE 41

CF yield, micromoles -31Fig. III-2. Production of CF 4 (pure, O ; 10% O 2 , ) as a function of dose.

PAGE 42

-32o CM I X CO > QJ Q) CO o p saxouiojoxiu ‘ppaiA ^onpojj IH~3. Production of C F (pure, O ) and C.F (pure,D ) as a function of dose.

PAGE 43

TABLE III-l Radiolysis Yields for Hexafl uoroethane . 33 -

PAGE 44

-34B. Discussion There have been radiolysis studies of hexaf luoroethane in both the gas pahse (28-30) and liquid phase (31). In addition Kevan and coworkers did the rare gas-sensitized radiolysis of both phases (32, 33). Table III-l compares the results of this work with that of Cooper and Haysom (30) and also Kevan (32). In the pure system there seems to be reasonable agreement in the yields of the major products. Kevan reported the formation of C F.^ an ^ C F did not give any yield data on them. Cooper and Haysom reported seeing higher molecular weight products, but were not able to identify these products with certainty. In all three studies, the material balance is not particularly good with a fluorine to carbon ratio of 3.35 to 1 for Kevan's results, 3.52 to 1 for Cooper and Haysom's results, and 3.56 to 1 for this work. It is seen in all three studies that when a radical scavenger is added to C F , the higher molecular weight products disappear. There is some disagreement with respect to the effect of scavengers on the formation of tetrafluoromethane. When Cooper added 5% chlorine to C F , 2 6 the G value of CF^ was found to be 0.40. When 5% hydrogen chloride was added, the G value was reduced to 0.26, and when 5% bromine was added, the yield of CF was undetectable. Kevan added 1% oxygen to C„F, and found CF^ had a G value of 1.3. In the present investigation, 10% 0 ^ was added and the G value of CF^ was found to be 0.61. It is quite possible that 1% oxygen is not sufficient to completely scavenge the CF^* radical. Cooper and Haysom reported that in all their scavenging experiments it took at least 3% scavenger to completely eliminate the radical processes. The unscavengeable yield of CF will be discussed later in 4 this section.

PAGE 45

-35In light of the radiolysis results and scavenger studies, it seems clear that the major products in the radiolysis of hexaf luoroethane arise chiefly through radical reactions. Based on the above results, we can propose the following mechanism: C 2 F 6 + 6 — C 2 F 6 * 4e (HI-1) C F * 2 6 > 2CF 3 (HI-2) * C F • + F(III-3) — C 2 F 4 + F 2 (III-4) F+ CF • * CF, 4 (in-5) F • + r f V4 y C 2 F 5 (II 1—6 ) F+ C F • — C 2 F 6 (HI-7) cf 3 . + cf 3 . r C 2 F 6 (III-8) CF 3 * + C 2 F 4 > C 3 F 7* (HI-9) F+ C F ? * — C 3 F 8 (III-10) CF 3 " + C 2 F 5‘ — C /8 (III-ll) C 2 F 5 + C 2 F 5 * C 4 F 10 (III-12) C 2 V + C 3 F 7 " -* C 5 F 12 (III-13) C 3 V + C 3 V * — C 6 F 14 (III-14) Primary excitation (III-l) is probably followed most frequently by carbon-carbon bond rupture (III-2) as the dissociation of the C-C bond requires approximately 25 kcal/mole less energy than C-F bond rupture (14, 15). Reaction (III-4) is proposed because C^F a PP ears to be an essential precursor to account for formation of the higher molecular weight products. Net formation of C.F, is not observed, and the 2 4 strongly endothermic reaction (III— 4) is probably a minor process. Evidence from photochemical studies indicate that unlike hydrocarbons,

PAGE 46

-36fluorocarbon radicals do not disproportionate (34-36) and fluorine atoms do not abstract fluorine from fluorocarbons (37-39). Therefore Reactions (III-5) through (III-14) would be the most probable radical reactions to explain the experimental results as they all have zero or small activation energies (40, 41). Instead of postulating Reaction (III— 4) in order to account for the higher molecular weight products, another possibility may be that the C F • radicals combine to form excited C.F,., which then can collision2 0 4 10 ally deactivate (Reaction (III-12b)) or decompose to two C 2 F • radicals (Reaction (III-12c)) or to CF^* and C^F^* radicals (Reaction (III-12d)). C 2 F 5* + C 2 F 5* * +M — C 4 F 10 + M (III-12b) ( Vio>* -> c 2V + c 2 f 5 . ( III-12c) (C W* -> cf 3 . + c 3 f 7 . (III-12d) Kevan and Hamlet (28) postulated that the unscavengeable CF, was 4 due to either molecular dissociation of excited C„F r (III-15) 2 6 C 2 F 6* * CF 4 + CF 2 (HI-15) or to one of the following ion-molecule reactions: CF 3 +C 2 F 6 — • CF 4 + c 2 F 5 + (III-16) C 2 F 5 +C 2 F 6cf 4 + c 3 f 7 + (II I— 1 7 ) As mentioned in the 'introduction, CF^ + and C 2 F 5 + are the main ions in the mass spectrum of C^F^, so one might expect that either or both of their reactions could be important. Reaction (III-17) can be eliminated

PAGE 47

-37as the c ^ F 7 ion ^ as never been observed in the mass spectrometric investigations of C.F (42-45). In fact, it has been shown that 0 F„ + 2 o 2 5 produced either as primary fragments or as reaction products are essentially unreactive with C„F, (43-46) . 2. b Although Reaction (III-16) is thought to be endothermic, Marcotte and Tiernan (43) observed it using a tandem mass spectrometer and suggested that this reaction occurred because in their experiments CF^"*~ had as much as 2.9 eV excess internal energy. More recently Ausloos and co-workers (45) investigated this reaction using a photoionization mass spectrometer, in which it was shown that CF^ ions having no internal energy undergo Reaction (III-16) , with a rate constant of 4 x 10 _11 3 cm /molecule-second at pressures below 0.01 torr. Although this reaction is slow, it does appear that it is the major source of the unscavenged CF^ when oxygen is used as a scavenger. Cooper and Haysom's results which show only small amounts of CF^ in the presence of chlorine and hydrogen chloride, and no CF^ in the presence of bromine, seem to imply that there is some type of ionic process involved when these scavengers are present which would interfere with Reaction (111-16) . They suggest this possibility particularly when HC1 is the scavenger. Amphlett and Whittle (47) report that photochemically generated CF^radicals react with HC1 to give CF^H exclusively, whereas Cooper and Haysom found that when C 2 F & was irradiated in the presence of 5% HC1, not only CF^H (G = 1.6), but also CF^Cl (G = 1.0) was formed. Since Reaction (III-16) is fairly slow, it probably could be suppressed in the presence of reactive additives which remove CF^ + . Reaction (III-15) is still a possibility, but Cooper and Haysom's results with added bromine seem to cast doubt on it, as there is no

PAGE 48

-38reason to believe that small amounts of bromine would interfere with the primary decomposition of excited C„F . . Z o C . Summary The gamma-radiolysis of gaseous C 2 F £> was i nvest ig atec * at 50 torr, both pure and with 10% oxygen added. For the pure system, the radiolytic products and their respective G values were CF , 2.27; C F , 0.23; 4 3 o C.F , 0.09; C F 0.015; and C F ., 0.009. All radiolysis products 4 1U D LA b except for CF^ (G = 0.61) were eliminated when 10% 0 ^ was added as scavenger. The results were discussed mainly in terms of excited ^ decomposing into free radicals, which can then combine. The unscavenged CF^ was accounted for by the following ion-molecule reaction: CF, + + C 0 F, — > CF. + C„F. 3 2 6 4 2 5 +

PAGE 49

IV. THE GAMMA RADIOLYSIS OF ETHANE A. Experimental Results Ethane was irradiated at room temperature over the absorbed dose 20 21 range of 1.26 x 10 to 1.34 x 10 eV/gram. Radiolyses were carried out at 20 and 50 torr. There seem to be little, if any difference in product G values at these two pressures. Figure IV-1 shows a typical chromatogram of the non-condensible products and Fig. IV-2 shows a chromatogram of the condensible organic products. In all 17 organic products were identified. The major products are hydrogen, methane, ethylene, propane, acetylene, and n-butane. Figures IV-3 through IV-8 show the yields of the major products are essentially linear when plotted as a function of absorbed dose. The results are summarized in Table IV-1. The minor products are propylene, i-pentane, n-pentane, 1-butene, cis-2-butene, trans-2-butene, hexane, and pentene; their G values are listed in Table IV-1. Both major and minor products were identified by their retention times and their mass spectral cracking patterns. In the case of pentene and hexane, however, only the empirical formulas could be established. Due to similarities in both G.C. retention times and mass spectral cracking patterns of the several isomers of each compound, it could not be determined with certainty which isomers were produced. Table IV-1 also summarizes the effect of 0^ on the system. When 0^ was added to the ethane, it is noticed that among the major products the hydrogen yield was 'reduced by approximately 50%, the n-butane yield by 39 -

PAGE 50

40 V . ) ( 1 ) 3 C Q) B C o d i1 w 9SU0dS9)J aoa0933Q Fig. IV 1. Gas chromatogram of the non-condensible products in the radiolysis of ethane.

PAGE 51

Fig. IV-2 . Gas chromatogram of irradiated ethane (nitrogen gas flow rate of 30 ml /min on a 5 m x 0.25" O.D. glass column packed with 60-200 mesh silica gel). c o •H 4 J 03 O 00 PP 00 u PP i CN CN CJ i — 1 00 1 1 PP pp CO CM i

PAGE 52

42 Cfl 0) u p c •H e QJ e c o •H U p w Cp CN I > M fcO •rH 4-t asuodsa^ ao^oa^aa

PAGE 53

asuodsa^ ao^oaiaQ Elution time (minutes)

PAGE 54

-44TABLE IV-1 G Values for the Radiolysis Products from Ethane Product Pure System 10% Oxy H 2 7.24 3.12 CH, 4 0.53 0.24 C 3 H 8 0.49 0.14 n C 4 H 10 2.13 0.27 C 2 H 4 0.12 1.52 C 2 H 2 0.05 0.29 i C 4 H 10 0.016 0 C 3 H 6 0.008 0.032 iC 5 H 12 0.013 0 nc 5 H 12 0.004 0 0.007 0.011 trans-2-C H 4 o 0.004 0.007 cis-2-C H 0 4 o 0.013 0.019 C 6 H 14 peaks ^ 0.020 0 (2 peaks) 0.009 0 i

PAGE 55

yield, micromoles -45Fig. IV-3. Production of hydrogen (pure, O ; 10% 0^, of dose. D ) as a function

PAGE 56

CH yield, micromoles -46Fig. IV-4. Production of CH 4 (pure, O ; 10% 0 9 , ) as a function of dose.

PAGE 57

-47Fig. IV-5. Production of CgHg (pure, O ; 10% 0 ? , D ) as a function of dose.

PAGE 58

yield, micromoles 48 Production of n-C^H 10 (pure, O ; 10% 0 2 , ) as a function of dose. Fig. IV6.

PAGE 59

C H yield, micromoles -49Fig. IV7 . Production of C 2 H^ (pure, O ; 10% 0 , ) as a function of dose.

PAGE 60

yield, micromoles -50Fig. IV-8. Production of C H (pure, O ; 10% 0 , dose. 22 2 D ) as a function of

PAGE 61

-5190%, the propane yield by 70%, and the methane yield by 50%. On the other hand, the major unsaturated products were greatly enhanced. For example, the ethylene yield increased by a factor of ten, and the acetylene yield increased by a factor of 6. Among the minor products, added oxygen essentially eliminated the saturated products such as ibutane, i-pentane, n-pentane, and hexane. All the unsaturated product yields, except for the pentenes, were increased. B. Discussion Table IV-2 compares this work with the results of several other ethane radiolysis investigations. One notices quite a variation in results. Peterson and co-workers (48), Heckel and Niessner (49a, b) , and Holland and Stone (50) did their experiments at very low doses, while Yang and Manno (51), von Biinau (52) and this laboratory performed their investigations at higher doses. One notices at higher doses the yields of hydrogen and especially the unsaturated compounds are lower than for the low dose work, while the yields of higher molecular weight compounds are higher. This probably illustrates the influence which the build-up of radiolysis products can have on the observed product yields. In all of the above studies, the material balance is not very good with a hydrogen to carbon ratio of 3.9:1 for Yang and Manno, 3.6:1 for von Biinau, 3.52:1 for Peterson and co-workers, 3.44:1 for Heckel and Niessner, 3.65:1 for Holland and Stone and 3.72:1 for this work. Table IV3 shows the effect of scavenger on the radiolysis of ethane reported by avrious investigators. While there seems to be discrepancies among the several investigations, probably due to differences in experimental conditions, such as dose, pressure, etc., certain general trends

PAGE 62

A Comparison of the G Values for the Radiolysis Products from Ethane -52X C 03 X o LO m e rr"» m CN i — 1 CO 0) • • • • • . i — j i — i o PC e o U 00 o o CN « — 1 c 3 03 Oh < U r— 1
PAGE 63

G Values for the Radiolysis Products from Ethane with Scavenger Added -53 u o CN CN o CN -< LO o 3 X) o H Cm CN PC O t-H « oo
PAGE 64

-54emerge. By comparing Table IV2 with IV-3, it is noticed that when scavenger is added, the hydrogen yield is reduced by more than half, the lower molecular weight saturated compounds such as methane, propane, and butane are drastically reduced, the higher molecular weight saturates are eliminated, while the unsaturated compounds are increased. (In the case of the higher dose studies, they are increased quite substantially. ) As is evident from Tables IV-2 and IV-3, there have been numerous publications concerning the various aspects of the radiolysis of ethane (48-61). Also the vacuum ultraviolet photochemistry (62-71) and the ion-molecule reactions (72-80) of ethane have been extensively researched. From the information obtained from these investigations, it is possible to propose a set of reactions to help explain the products formed in the radiolysis of ethane. Table IV-4 summarizes the overall radiolysis mechanism. When ethane is irradiated, both excited and ionized molecules are formed (Reaction IV-1). The excited ethane can decompose to give excited ethylene and hydrogen, an excited ethyl radical and hydrogen atom, methane and methylene, or two methyl radicals. Reactions (IV-2) to (IV-5) have been well established through photochemical studies (62-71), indicating that while Reactions (IV-2) and (IV-3) are the dominate processes, Reactions (IV-4) and (IV-5) play an increasingly important role at higher energies. Through some very simple but elegant experiments using mixtures of C" 2^6 a nd ^2^6Â’ as we ^ as CH^CD^, Okabe and McNesby (62) showed that in all the vacuum ultraviolet photochemical irradiations, only a minor amount of HD was formed, thereby indicating that the excited ethane

PAGE 65

-55TABLE IV-4 Proposed Radiolysis Mechanism for Ethane I. Primary Physical Processes: Energy Absorption C 9 H 6 (C 2 H 6 )* or (C 2 H 6 + + e") II. Primary Chemical Processes: Neutral Fragmentation < C 2 V* y (CH 3 -CH)* + H y (C 2 H 5 -)* + Hy ch 4 + ch 2 y CH 3 * + CH 3 * (CH 3 -CH)* y (ch 2 -ch 2 )* (ch 2 -ch 2 )* + M *\ C 2 H 4 + M (ch 2 -ch 2 )* y C 2 H 2 + H 2 (ch 2 -ch 2 )* y C 2 H 3+ H ‘ (c 2 h 5 -)* + m C 2 H 5 + M (c 2 h 5 -)* y C 2 h 4 + Hy C n H • + EL 2 3 2 III. Secondary Chemical Processes: Carbene Insertion CH 2 + C 2 H 6 * < C 3 H 8 ) * (C^g)* + M * C 3 H 8 +M < C 3 H 8>* -> ch 3 + c 2 h 5 . (IV-1) (IV2) (IV3) (IV-4) (IV-5) ( IV-6) (IV7) ( IV-8) ( IV-9) (IV-10) ( IV-11) ( IV-12) ( IV-13) ( IV-14) ( IV-15)

PAGE 66

-56TABLE IV4 (Corftinued) IV. Primary Chemical Processes: Ionic Fragmentation C„H , b C„H. + H 2 6 2 5 C_H_ y C „ H n + H 2 5 2 3 + C.H . y C n H, + H 2 6 2 4 4C H y C_H„ + H 2 4 2 2 ( IV-16) ( IV-17) ( IV-18) (IV-19) V. Secondary Chemical Processes: Ion-Molecule Reactions C 2 H 6 + + C 2 H 6 C 2 H 5 + C 2 H 6 C 2 H 4 + + C 2 H 6 C 2 H 3 + C 2 H 6 c 2 h 2 +c 2 h 6 (C 2 H 6 ) 2 C 4 H 11 + C 4 H 10 -> > C 4 H 9 + C 4 H 8 C 4 H 11 + + H ‘ ( IV-20) c 4 h 9 + + h 2 + h( IV-21) C 3 H 9 + + CH 3* ( IV-22) C 3 H 8 + + CH 4 (IV-23) c 4 h 9 + + h 2 ( IV-24) C q H_ + + CH. j / 4 (IV-25) C 2 H 5 + + C 2 H 6 ( IV-26) C 4 H 9 + + H * ( IV2 7) C 4 H 8 + + H 2 ( IV-28) c 3 h 7 + + ch 3 . ( IV-29) C H + + CH. J o 4 1 ( IV-30) C 4 h 7 + + H 2 ( IV-31) C„H c + + CH, -3 J 4 (IV32) C 2 H 5 + + C 2 H 4 ( IV-33) C.H_ + + H4 7 (IV34) C 3 H 5 + + CH 3* (IV-35) C 2 H 4 + + C 2 H 4 ( IV-36)

PAGE 67

-57V. VI. VII. TABLE IV-4 (Continued) Vn +A — * Vio + AH+ ( IV-37) C 3 H 9 + + A > C 3 H g + AH + ( IV-38) Secondary Chemical Processes: Neutralization + ^2^6 + 6 ^ or ^ ^ — * C 2«6* (IV-39) M + (saturated) + e (or X ) M* (IV-40) N (unsaturated) + e (or X ) > N* ( IV-41) R (carbonium ion) + e (or X ) ^ (R*)* (IV-42) Secondary Chemical Processes: Radical Reactions H * + C 2 H 6 y C 2 H 5* + H 2 (IV-43) CH 3 " + C 2 H 6 y C 2 h 5 + CH 4 ( IV-44) H+ C„H. 2 4 y C 2 H 5( IV-45) ch 3 . + c 2 h 4 C 3 H 7( IV-46) CH 3 ' + C 2 V y C 3 H 8 (IV-4 7) C 2 H 5 " + C 2 H 4 ' y W ( IV-48) C 2 H 5 + C 2 H 5 C 4 H 10 ( IV-49) y C 2 H 6 + C 2 H 4 (IV-50) C 2 H 5+C 3 H 7* ' y C 5 H 12 ( IV-51) C 2 H 6 + C 3 H 6 ( IV-52) y C 3 H 8 + C 2 H 4 ( IV-53) S’ 1 ?' + SV ' C 6 H 14 ( IV-54) y C 3 H 8 + C 3 H 6 ( IV-55)

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-58TABLE IV-4 (Continued) VII. c 4 h 9 + c 2 h 5 ~ * C 6 H 14 (IV-56) ~ * C 4 H 10 + C 2 H 4 (IV-57) ~ * C 2 H 6 + C 4 H 8 ( IV-58) R . + c 2 h 4 + R-CH -CH • 2 4 polymer ( IV-59)

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-59eliminates molecular hydrogen from a terminal crabon (Reaction (IV-2)) . This leaves an excited ethylene species which rearranged (Reaction (IV6 )), and then can collisionally stabilize to ethylene (Reaction (IV-7)), decompose to acetylene and hydrogen (Reaction (IV8 )), or decompose to + H* (Reaction (IV-9)). The excited ethyl radical is usually assumed to immediately decompose to ethylene and a hydrogen atom (Reaction (IV-11)) (61), but it is possible for some to either collisionally stabilize or decompose to C^H^* and H^. The methylene radical can insert into ethane to form excited propane (Reaction (IV-13)), which can either collisionally stabilize or undergo decomposition by C-C bond cleavage to form methyl and ethyl radicals. From the various scavenging studies in the radiolysis of ethane, it is clear that ionic reactions also play a very definite role. The mass spectrum of ethane (see Table 1-1) shows that the most abundant ion is ^ 2^4 f°ll° we d by > ^ 2^3 ’ ^ 2^2 ’ anc * ^ 2 ^ 5 + " T ^ e react l° n scheme which accounts for the major fragments of the ethane ion is shown by Reactions (IV-16) through (IV-19). The parent ethane ion can fragment to either C^H,. or ^ 2^4 > an d both of these may possess enough energy to undergo further decomposition. Therefore one might expect to find radiolysis products formed through the reactions of , C 2 H 5 + ’ C 2 H 4 + ’ ^ 2^3 ’ an< ^ ^ 2^2 ’ react -l° ns which these ions undergo with ethane have been extensively investigated by high pressure mass spectrometric (72-78) and ion cyclotron resonance studies (79, 80). Reactions (IV-20) through (IV-38) list the major and many of the minor ion-molecule reactions thought to play a part in the radiolysis of ethane. The ethane ion can react with ethane and form ethane dimer ion (Reaction (IV-20)), which dissociates leading to the formation of C^H^ + ,

PAGE 70

-60^ 4“ -f-f* C^Hg , ^3^9 » anc * ^3^8 ' Field and co-workers (77) showed the yield of + ^ C 2 H 6^2 W3S falrl y low in the high pressure (up to 5 torr) mass spectrometric studies, so only very minor amounts of the unscavenged radiolysis products are due to these reactions. The addition of ethyl ion to ethane is much more significant and would result in the formation of 4" -fC /. H 1] ’ which can dissociate mainly to C^H g but also forms some 4‘ and C^H,. (61, 75). The ethylene ion can react with ethane to form CyH , which has been shown to chiefly dissociate to C H^ + and C„H, + , ^ 3 7 3 6 4~ -f-fbut also some C^Hg , C^Hg , and have been observed (77). The vinyl ion can add to ethane to form the C^Hg ion, which decomposes to 4“ -f" "I CgH^ , with and C 2 H,being minor reactions (75, 79). Finally the acetylene ion can react with ethane to form C.H 0 , which can dissociate 4 O to form C.H + + + J 4’ 1 7 ’ ^3^5 » ar *d ^2^4 ‘ This also appears to be a minor process (61, 77). It should be mentioned that the addition of the major • + + + ions to etnane resulting in the formation of (C^),, , , C^H 1Q , 4 4* ^4^9 ’ anc ^ ^4^8 res P ectiv ely have been observed in high pressure mass spectrometric studies (61, 74, 75, 79). These ions can dissociate, but in the radiolysis experiments some of these ions should collisionally stabilize considering the higher pressures of the radiolysis experiments. Ausloos and co-workers (61) found that when they irradiated 1:1 mixtures of C 2 Hg and C 2 Dg in the presence of nitric oxide scavenger over 70% of the partially deuterated butane formed was C.DH,. indicating the 4 5 5 main reactions were C 2 H 5 + + C 2°6 C.H-D* + A 4 3 6 C 2 D 5 + + C 2 H 6 C.D H 4 5 b A C 4 H 5°6 + C . D C H C + AD 4 5 5 C.D r H, + 4 5 d C , D r H _ + AH H 4 _> 0 + ( IV-25a) (IV-37a) (IV25b) (IV3 7b)

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-61where A Is a proton acceptor. This certainly indicates that Reaction (IV-25) followed by (IV-37) is an important ionic process in the radiolysis of ethane. By using similar deuterium labeling techniques, they were also able to give strong evidence that the from Reaction (IV-22) undergoes a similar proton transfer reaction (Reaction (IV-38)). Following these various ion-molecule reactions, charge neutralization of the positive ions by either free electrons or negative ions will take place. Charge neutralization usually yields neutral excited species since an amount of energy equivalent to the ionization energy of the ionic species may be available if a free electron is involved in the neutralization. Unfortunately, at the present time little is known about the chemical changes which may result from the neutralization process, as it is very difficult to know with certainty exactly what ions are undergoing neutralization and then how they fragment (81). Reactions (IV-39) through (IV-43) represent the various types of charge neutralization processes which are probably occurring. The parent ethane ion can be neutralized by either an electron or a negative species X (for example, a hydride ion) forming an excited ethane molecule, which is either collisionally stabilized or can undergo reactions mentioned earlier (Reactions (IV-2) through (IV-5)). M + , a saturated species, or N , an unsaturated species, can be neutralized to form an excited species, which will undergo similar reactions to those of excited ethane or ethylene. R , a carbonium ion, can react with an electron or a negative ion to form an excited radical, which then can fragment to smaller species. Subsequent to neutralization, reactions involving the neutral free radicals will occur (Reactions (IV-43) through (IV-59)). Gevantman and

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-62Williams (53) used iodine to identify and estimate the radicals formed in the radiolysis of ethane. Principal radical products detected by the appearance of the corresponding iodides were H* , CH^*, and in smaller amounts, n-C„H •, n-C.H *, and CH„. 3 / 4 y 2, It is well known from pyrolyses and photochemical studies that hydrocarbon radicals can undergo hydrogen abstraction reactions, combination reactions, and disproportionation reactions (82-84). Reactions (IV-43) through (IV-59) would be the most probable radical reactions to explain the experimental results. Radical combination reactions are more likely to occur for simple hydrocarbon radicals than disproportionation reactions or hydrogen abstraction reactions, as they have zero or very low activiation energies and high A factors, with associated rate constants close to collision rates, based on simple collision theory (83). That hydrogen and hydrocarbon radicals are internally scavenged by the ethylene formed in the radiolysis seems clear as the yield of ethylene dramatically increases in the presence of a radical scavenger. The above discussion does demonstrate that the radiolysis of ethane is fairly complex. It is noticed that there is a variety of pathways to the different radiolysis products. The unscavenged yield of hydrogen is formed chiefly from the molecular elimination of hydrogen from excited ethane (Reaction (IV-2)), with other sources being decomposition of either excited ethylene (Reaction (IV-8)) or excited ethane ion (Reaction (IV-18)). Small amounts might also arise from the hydrogen abstraction Reaction (IV-39), in which the hydrogen has excess energy (i.e. , hot) or from some of the numerous ion-molecule reactions mentioned. The scavengeable hydrogen originates from a combination of hydrogen atoms or from hydrogen abstraction from substrate (Reaction (IV-43)). There

PAGE 73

-63are several processes which lead to the formation of hydrogen atoms, including the decomposition of excited molecules (Reactions (IV-3), (IV-9) , and (IV-11)) or ions (Reaction (IV-16)), and also some from the various ion-molecule reactions. The nonradical methane (i.e., unscavenged) is known to originate from the decomposition of excited ethane to methane and methylene (58) , and various ion-molecule reactions such as Reactions (IV-25), (IV-32) , and (IV-30) (61). The radical methane yield can be accounted for by hydrogen abstraction (Reaction (IV-43)) by the methyl radical. The methyl radicals are produced by several ways such as decomposition of excited C„H, (Reaction (1V-5)) and excited propane (Reaction (IV-15)), Z o and by various ion-molecule reactions such as (IV-22) and to a lesser extent (IV-29) and (IV-35). Most of the propane is formed by methylene insertion into ethane (Reaction (IV-13)) and by the combination of methyl and ethyl radicals (Reaction (IV-47)), with minor amounts probably coming from various disproportionation reactions such as (IV-53) and (IV-55) . The unscavenged propane can be accounted for by the proton transfer reaction (IV-38) of the C H + ion formed in Reaction (IV-22). J y A combination of ethyl radicals (Reaction (IV-49)) is the major source of normal butane. The ethyl radicals are produced by decomposition of excited ethane (Reaction (IV-3)), hydrogen abstraction reactions (IV-43) and (IV-44) , with minor amounts also coming from Reaction (IV-15) and neutralization of the formed in various ionic reactions mentioned. The nonradical butane is thought to be due to the proton + transfer reaction (IV-37) of the C.H, , ion formed in Reaction (IV-24) . 4 11 When Ausloos and co-workers (58) irradiated mixtures of C„D. and Z o

PAGE 74

-64C„H, in the presence of nitric oxide, the ethylene consisted of mostly C 2 U 4 and C^H^, with only minor amounts of C^H^D an< ^ ^2^*3^’ that the decomposition of excited ethane to ethylene and hydrogen is the major source of ethylene. Other sources may be the various disproportionation reactions such as Reaction (IV-50) , (IV-51) , and (IV-57) or charge neturalization in some of the ionic processes. That ethylene is being consumed in radical reactions such as Reactions (IV-45) , (IV-48), and (IV-59) seems fairly evident by the fact that the yield of ethylene is substantially enhanced in the presence of oxygen. Ethylene itself is known to be an excellent scavenger of thermal hydrogen atoms (85) . The main source of acetylene appears to be the decomposition of excited ethylene (Reaction IV8) . A minor amount may be produced in the charge neutralization of the various ionic processes. The fact that the yield of acetylene is enhanced in the presence of oxygen indicated that the acetylene is probably also scavenging hydrogen atoms. Using (C 2 D^) 2 CDCD as an ion interceptor, Ausloos and co-workers + (61) showed that the C^H ion, which is produced in Reactions (IV-25) and (IV-32) , can form propylene by a hydride transfer reaction (Reaction (IV-60)). Also some of the propylene yield may be produced by disproportionation reactions such as (IV-52.) and (IV-55) . C.H c + + (C 0 D c ) 0 CDCD. > CJ D + C,D_, , + (IV-60) The C , H-, ion formed in Reactions (IV-31) and (IV-34) should be able to undergo a similar hydride transfer reaction to form butene. The fact that the pentanes, hexanes, and pentenes are eliminated when oxygen is pi'esent implies that they were formed by radical combination reactions

PAGE 75

-65C. Summary The gamma-radiolysis of gaseous ethane was studied at 50 torr, both pure and with 10% oxygen added. For the pure system, the major radiolytic products and their respective G values were H^, 7.24; CH^ , 0.53; C 3 H g , 0.49; n-C 4 H 1Q) 2.13; C^, 0.12; and C^, 0.05. The minor products included i-C^H^, CgHg, C^Hg, C^H^, C^H^, and C^H^. It was observed that the addition of 10% oxygen reduced the hydrogen and methane t yields by approximately 50%, the n-butane yield by 90%, and the propane yield by 70%, while increasing the ethylene yield by a factor of ten and the acetylene yield by a factor of six. Among the minor products, added oxygen essentially eliminated the saturated products such as i-C^H^, C..H, „ , and C c H 1 . All the unsaturated minor products yields, except for ^5^10’ were slightly enhanced. The results were discussed in terms of both excited and ionized ethane. The excited ethane decomposed chiefly to excited ethylene and hydrogen, but some and H*, CH^ and CH^, or two methyl radicals were also produced. The ionized ethane can fragment into ions (C^*, C 2 Hg + , C 2 H 4 + , C ? H 2 + ) and hydrogen. The fragment ions then undergo ion molecule reactions with ethane. The non-scavengeable products were accounted for primarily by the decomposition of ethane to excited ethylene and hydrogen and by various ion-molecule reactions, while the scavengeable products were attributed to hydrogen abstraction reactions and' radical combination reactions.

PAGE 76

V. THE GAMMA RADIOLYSIS OF MIXTURES OF HEXAFLUOROETHANE AND ETHANE A. Experimental Results A series of mixtures whose compositions ranged from pure hexafluoroethane to pure ethane were irradiated at a pressure of 50 torr. The exact compositions of the mixtures are shown in Table V-l in units of mole fraction of ethane, partial pressures, and fraction of the total energy absorbed in ethane. This last quantity is just the fraction of the total electron stopping power of the mixture that is due to ethane. For example, in an equimolar mixture of hexaf luoroethane and ethane, only 23% of the energy is absorbed by the ethane. Figure V-l shows a typical chromatogram from the radiolysis of an equimolar mixture of ethane and hexaf luoroethane . Twenty-two products were identified. The major products were hydrogen , methane, ethylene, 1 . 1dif luoroethylene , trif luoromethane, propane, acetylene, n-butane, and propylene. Additionally, small amounts of pentaf luoroethane, 1.1. 1trif luoroethane , 1 , 1, 1, 2 , 2 -pentaf luoropropane, 1, 1, 1-trif luoropropane, pentanes, butenes, hexanes, and pentenes were formed. It is interesting to note that no pure fluorocarbon compounds were detected in the irradiated mixtures. All major products were identified by their retention times and by their mass spectral cracking patterns. Standards were available for all the reaction products except for the mixed propanes. The yields of all the major radiolysis products of the mixtures of 66 -

PAGE 77

-67TABLE V-l Composition of Irradiated Mixtures 2 Fraction C 2 H 6 p(c 2 h 6 ) torr p(c 2 f 6 ) torr Fraction Absorbed 0 0 50 0 0.125 6.25 43. 75 0.041 0.25 12.5 37.5 0.09 0.50 25 25 0.23 0.75 37.5 12.5 0.47 0.90 45 5 0.73 0.95 47.5 2.5 0.85 1.00 50 0 1.00 '2 6

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1 o u o co 2 co i — l 03 M-4 rH 03 60 X Qj • C Q X o c 03 r m 03 CM c • 03 O rC h X 03 M-l g O m 03 H 03 2 4-J c X o CM CM m m •H i — 1 i — 1 Ph CO PC e rc PC CM PS CM c m m u vO m m 4-1 a CJ cj CJ CJ 00 o rH CM CO rH i — 1 CM CM CM CM o o 1 — 1 rH co PS PE PE Ph 00 CM m CJ v£> PE PE a CJ Ph co PE CO CM i i cm PE CO CJ CJ •H (2 CJ CJ CJ vO oo CTn O rH CM i — 1 rH iH I > 00 •H Ph

PAGE 79

69 ssuodss^ JO^Da^aQ

PAGE 80

70 asuodsa^

PAGE 81

-71the pure and oxygen scavenged systems are plotted as a function of absorbed dose in Figs. V-4 through V-19. The main purpose of these yield graphs is threefold: (1) to record and preserve the original data; (2) to show the effect of composition of mixtures on the linearity and slopes of the yields of various products with respect to energy absorbed; and (3) to show the relative amount of scatter in the data. These graphs show that for any given mixture composition, the individual product yield increases with increasing absorbed dose. One might expect that as the amount of ethane in the mixture is decreased, the yields of the hydrocarbon radiolysis products should decrease also. Looking at Figs . V-4 through V-19, one notices this is not the case. It should be pointed out that as the amount of ethane is decreased in the mixture, the amount of hexaf luoroethane is increased, causing the absorbed dose in eV/gram to decrease, as hexaf luoroethane is over four times as heavy as ethane. Another factor strongly influencing these yield graphs is the effect of energy transfer from the hexaf luoroethane to the ethane. This effect is clearly demonstrated in Figs. V-2 and V-3, which are summary graphs of the yields of the major radiolysis products as a function of mixture composition. In an ideal mixture, the yields of hydrogen and the hydrocarbon radiolysis products would decrease linearly with the fraction of energy absorbed by the ethane. Figures V-2 and V-3 show however that the yields of hydrogen, propane, n-butane, ethylene and acetylene are substantially enhanced over their ideal mixture lines. These figures also show that the yields of CF^H, C 2 F<.H, CF^CH^ , and HF increase with increasing amounts of hexaf luoroethane . The G values for the major products at all mixture compositions in the unscavenged system are listed in Table V-2. Table V-3 lists all the minor radiolysis products.

PAGE 82

-72The yield graphs for the oxygen scavenged system (see Figs. V-5 through V-20) demonstrate the same general trends as for the oxygen free system. The yields of all the major radiolysis products as a function of mixture composition are compiled in Fig. V-3. As in the oxygen free system, the yields of hydrogen and all the hydrocarbon radiolysis products, except for methane, are considerably higher than would be expected for an ideal mixture. In comparing the oxygen scavenged mixture system (see Table V-4) with the unscavenged mixture system (Table V-2) , one notices that oxygen has the same effect on the hydrocarbon radiolysis products in the mixtures as it does in pure ethane (see Table IV-1) — the saturated products are significantly reduced, while the unsaturated compounds are greatly increased. In the oxygen scavenged mixture system, the hydrogen yield was decreased by approximately 60%, the n-butane yield by 90%, the propane yield by 70%, and the methane yield by 50%. On the other hand, the ethylene yield increased by a factor of ten, and the acetylene yield by a factor of six. The yields of CF^H, C^F^H, and HF are essentially unaffected by the addition of oxygen. The minor products in the oxygen scavenged system are listed in Table V-5. It is noticed that the higher molecular weight products and mixed products were eliminated, while the unsaturated products were enhanced in the presence of oxygen. B. Discussion As mentioned in the previous section, no pure fluorocarbon compounds were detected in the irradiated mixtures, but virtually all the hydrocarbon products were. These results are consistent with the generalization made initially by Fallgatter and Hanrahan (7) concerning the

PAGE 83

-73TABLE V-2 G Values for the Major Radiolysis Products from Ethane-Hexaf luoroethane Mixtures Fraction of Energy Absorbed in C„H, 2 6 Product 0.09 0.23 0.47 0.73 0.85 1.00 H 2 0.91 2.43 5.12 6.51 7.03 7.24 CH, 4 0.05 0.21 0.30 0.38 0.43 0.53 C 3 H 8 0.18 0.82 0.61 0.54 0.50 0.44 n_C 4 H 10 0.68 2.44 2.19 2.21 2.20 2.13 C 2 H 4 0.06 0.12 0.17 0.19 0.15 0.12 C 2 H 2 0.01 0.10 0.08 0.09 0.06 0.05 cf 3 h 0.44 0.28 0.20 0.12 0.06 0 C 2 F 5 H 0.24 0.16 0.12 0.07 0.03 0 cf 2 ch 2 0.27 0.17 0.16 0.09 0.04 0 HF 0.70 0.59 0.41 0.27 0.13 0

PAGE 84

-74TABLE V-3 G Values for the Minor Radiolysis Products from Ethane-Hexaf luoroethane Mixtures Fraction of Energy Absorbed in C H, 2 b Product 0.09 0.23 0.47 0.73 0.85 1.00 ^VlO 0.007 0.022 0.020 0.020 0.019 0.016 iC 5 H 12 0.003 0.010 0.011 0.013 0.014 0.013 nc 5 H 12 <.001 0.004 0.004 0.005 0.005 0.004 C 6 H 14 0.005 0.014 0.015 0.018 0.020 0.020 C 3 H 6 0.005 0.024 0.020 0.015 0.012 0.008 0.002 0.011 0.009 0.008 0.009 0.007 cis-2-C H q 4 o 0.004 0.016 0.016 0.015 0.013 0.013 trans-2-C . H 0 4 o <.001 0.003 0.003 0.004 0.004 0.004 C 5 H 10 0.003 0.011 0.010 0.011 0.010 0.009 ch 3 cf 3 0.092 0.040 0.021 0.009 0.005 0 CH 3 C 2 F 5 0.049 0.040 0.019 0.010 0.003 0 CF 3 C 2 H 5 0.062 0.053 0.032 0.013 0.004 0

PAGE 85

-75TABLE V-4 G Values for the Major Radiolysis Products from Ethane-Hexaf luoroethane Mixtures with 10% added Oxygen Scavenger Fraction of Energy Absorbed in C H, 2 . 6 Product 0.09 0.23 0.47 0.73 0.85 1.00 H 2 0.67 1.33 1.98 2.50 2.83 3.12 CH. 4 0.03 0.05 0.09 0.11 0.15 0.24 C 3 H 8 0.08 0.20 0.19 0.19 0.17 0.14 n_C 4 H 10 0.24 0.32 0.30 0.30 0.29 0.27 C 2 H 4 0.85 1.10 1.20 1.24 1. 31 1.52 C 2 H 2 0.16 0.26 0.26 0.28 0.28 0.29 CF 3 H 0.45 0.35 0.22 0.15 0.07 0 C 2 F 5 H 0.20 0.14 0.09 0.05 0.02 0 cf 2 ch 2 0 0 0 0 0 0 HF 0.61 0.48 0.31 0.21 0.08 0 I

PAGE 86

-76. TABLE V-5 G Values for the Minor Radiolysis Products from Ethane-Hexaf luoroethane Mixtures with 10% added Oxygen Scavenger Fraction of Energy Absorbedin Product 0.09 0.23 0.47 0.73 0.85 1.00 i-C.lL . 0 0 0 0 0 0 4 10 i-C.H. „ 0 0 0 0 0 0 5 11 n C 5 H 12 0 0 0 0 0 0 C 6 H 14 0 0 0 0 0 0 S H 6 0.015 0.036 0.033 0.032 0.032 0.031 1 C 4 H 8 0.006 0.012 0.011 0.012 0.010 0.011 cis-2-C 4 H 8 0.008 0.018 0.021 0.018 0.020 0.019 trans-2-C H 0.003 0.005 0.006 0.007 0.007 0.007 C 5 H 10 ch 3 cf 3 CH 3 C 2 F 5 cf 3 c 2 h 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

PAGE 87

77 w G Value, H ? an T B A 0 Fig. V 2 a. Yields of H2, CH^, and C2H4 in the radiolysis of C2H5 C2Fg mixtures a a function of mixture composition. Read right-hand scale for H 2 yield.

PAGE 88

G Value -78Fraction of Energy Absorbed in C H, 2 6 Fig. V-2b. Yields of n-C^H^Q, CgHg, and CH 2 CF 2 in the radiolysis of ^2^6 ~ *"2^6 maxtures as a function of mixture composition.

PAGE 89

G Value 79 Fraction of Energy Absorbed in Fig. V2 c. Yields of HF, CF^H, C2F5H, and C2H2 in the radiolysis of C„H, C„F, mixtures as a function of mixture composition. 2 6 2 6

PAGE 90

-80G Value, 3n T B A D Fig. V-3a. Yields of H 2 , C 2 H 2 , and in the radiolysis of C 2 Hg C 2 F^ mixtures with 10% 0 2 as a function of mixture composition. Read right-hand scale

PAGE 91

G Value 81 Fraction of Energy Agsorbed in C„H, / o Fig. V3 b. Yields of CH^, C3Hg, and n-C^H^Q in the radiolysis of ^2^6 “ < -'2^6 m i xtures with 10% 63 as a function of mixture composition.

PAGE 92

G Value 82 Fraction of Energy of Absorbed in C„H,, i b Fig. V3 c. Yields of HF, CF^H, and C2F5H in the radiolysis of ^2^6 ~ ^2^6 m i xt ures with 10% O2 as a function of mixture composition .

PAGE 93

yield, micromoles -83Fig. V-4a. Yields of H 2 for various mixtures of C 2 Hg and C 2 F 5 as a function of dose. Percent of energy absorbed by C 2 Hg : 9%, ± ; 73%, ; 100%, O .

PAGE 94

yield, micromoles -84Fig. V-4b. Yields for H 2 for various mixtures of C 2 H^ and C 2 Fg as a function of dose. Percent of energy absorbed in £2 ^ 6 : 23%, A ; 47%, ; 85%, • .

PAGE 95

yield, micromoles -85Fig. V-5a. Yields for H 2 for function of dose. 47%, ; 73%, ; C 2 Hg C 2 Fg mixtures with 10% O 2 as Percent of energy absorbed in C^Hg 100%, O a

PAGE 96

yield, micromoles 86 Fig. V-5b. Yields for H 2 for 02% C 2 F^ mixtures with 10% O 2 as a function of dose. Percent of energy absorbed in C^H^: 91 , A ; 23%, A ; 85%, * .

PAGE 97

CH. yield, micromoles -87Fig. V6 a. Yields of CH^ for various mixtures of C2Hg and C2Fg as a function of dose. Percent of energy absorbed in C2Hg: 9%, ; 47%, ; 73%, Q .

PAGE 98

CH yield, micromoles 88 0 2 4 6 8 10 12 -20 Dose, eV/g x 10 Fig. V-6b . Yields of CH^ for various mixtures of C 2 Hg and C 2 Fg as a function of dose. Percent of energy absorbed by 02^ : 23%, A ; 85%, • ; 100%, Q •

PAGE 99

CH yield, micromoles -89Fig. V-7a. Yields of CH^ for £ 2^6 ~ *"2^6 m i xtures with 10% O 2 as a function of dose. Percent of energy absorbed in C^Hg: 23%, A ; 85%, • ; 100%, O .

PAGE 100

CH yield, micromoles -90Fig. V-7b . Yields of CH^ for 02^ C 2 F, mixtures with 10% O 2 as a function of dose. Percent of energy absorbed in 9%, ; 47%, ; 73%, .

PAGE 101

micromoles 91 Fig. V8 a. Yields of CgHg for various mixtures of C2Hg and C2Fg as a function of dose. Percent of energy absorbed by : 9 %, ; 47 %, ; 100 %, Q .

PAGE 102

micromoles -920 2 4 6 8 10 12 Dose, eV/g x 10 -20 Fig. V-8b. Yields of CgHg for various mixtures of C 2 Hg and C 2 Fg as a function of dose. Percent of energy absorbed by C2H6 : 23%, A ; 73%, ; 85%, • .

PAGE 103

-93Fig. V-9a. Yields of C 3 Hg for C 2 Hg C 2 Fg mixtures with 10% O 2 as a function of dose. Percent of energy absorbed in C 2 Hg: 9%, ; 23%, A ; 85%, * .

PAGE 104

C H. yield, micromoles Yields of CgHg for C 2 Hg C 2 F^ mixtures with 10% O 2 as a function of dose. Percent of energy absorbed in C 2 Hg : 47%, ; 73%, ; 100%, Q . Fig. V-9b.

PAGE 105

yield, micromoles -95Fig. V-lOa. Yields of n-C^H^Q for various mixtures of C2Hg and C2 Fg as a function of dose. Percent of energy absorbed by C^Hg : 9%, ; 23%, A ; 73%, .

PAGE 106

yield, micromoles 96 Fig. V-lOb. Yields of n-C^H^Q for various mixtures of C2Hg and C2Fg as a function of dose. Percent of energy absorbed in C2H5: 47 %, ; 85 %, • ; 100 %, Q .

PAGE 107

yield, micromoles -970 2 4 6 8 10 12 -20 Dose, eV/g x 10 Yields of n-C^H^Q for 02^ 02^ mixtures with 10% O 2 as a function of dose. Percent of energy absorbed in C 2 Hg : 9%, A ; 73%, ; 100%, O • Fig. V-lla.

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yield, micromoles -98Fig. V-llb. Yields of n-C^H^Q for C 2 mixtures with 10% O 2 as a function of dose. Percent of energy absorbed in C 2 H 5 : 23%, A ; 47%, ; 85%, • .

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yield, micromoles -990 2 4 6 8 10 12 -20 Dose, eV/g x 10 Fig. V-12a. Yields of C 2 H 4 for various mixtures of C 2 H 5 and C 2 Fg as a function of dose. Percent of energy absorbed by C 2 H 6 : 9%, ; 47%, ; 100%, Q •

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yield, micromoles 100 Fig. V-12b . Yields of C 2 H 4 for various mixtures of C 2 Hg and £2^6 as a function of dose. Percent of energy absorbed by C^Hg : 23%, A ; 73%, ; 85%, * .

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micromoles 101 Fig. V-13a. Yields of C 2 H 4 for C 2 H 5 C 2 F 5 mixtures with 10% O 2 as a function of dose. Percent of energy absorbed in C 2 Hg : 23%, A ; 73%, ; 100 %, Q •

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102 Fig. V-13b. Yields of C 2 H 4 for C 2 H 5 C 2 Fg mixtures with 10% O 2 as a function of dose. Percent of energy absorbed in C 2 H 5 : 9%, ; 47%, ; 85%, * .

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-1031246 8 10 12 -20 Dose, eV/g x 10 Fig. V-14. Yields of CF 3 H for various mixtures of C 2 Hg and C 2 Fg as a function of dose. Percent of energy absorbed by C^Hg: 9%, A ; 23%, A ; 47%, ; 73%, .

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-1040 2 4 6 8 10 12 -20 Dose, eV/g x 10 Fig. V-15. Yields of CF 3 H for C 2 H 5 C 2 Fg mixtures with 10% 03 as a function of dose. Percent of energy absorbed in C 2 Hg: 9%, ; 23%, A ; 47%, ; 73%, .

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-105Fig. V16 . Yields of C2F5H for various mixtures of C2Hg and C2Fg as a function of dose. Percent of energy absorbed by C2H5: 9%, ; 23%, A ; 47%, ; 73%, .

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-106w 0J rH O B o CJ •H B "0 r— I CJ •H pc LO CN CJ Fig. V-17. Yields of C 2 F 5 H for C 2 Fg C 2 Fg mixtures with 10% O 2 as a function of dose. Percent of energy absorbed in C 2 Hg: 9%, ; 23%, A ; 47%, ; 73%,

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CH CF yield, micromoles -107Fig. V-18. Yields of CH 2 CF 2 for various mixtures of C 2 Hg and C 2 F 5 function of dose. Percent of energy absorbed by C 2 H 5 : 9%, ; 23%, A ; 47%, ; 73%, . : as a

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HF yield, micromoles -108Fig. V-19. Yields of HF for various mixtures of £2^6 an d ^2^6 as a function of dose. Percent of energy absorbed by C 2 H 6 : 9%, ; 23%, A ; 47%, H ; 73%, ; 85%, • .

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HF yield, micromoles -1090 2 4 6 8 10 12 Dose, eV/g x 10 Fig. V-20. Yields of HF for C 2 Hg C 2 Fg mixtures with 10% O 2 as a function of dose. Percent of energy absorbed in C 2 Hg: 9%, ; 23%, A ; 47%, M ; 73%, ; 85%, • .

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110 cyclohexane— per f luoro cyclohexane system (and later observed in the cyclobutane-perf luorocyclobutane system (9)) that the products from the radiolysis of fluorocarbon-hydrocarbon mixtures resemble the products from the pure hydrocarbon component rather than from the pure fluorocarbon component. Also noted in the previous section was the fact that the yields of hydrogen and most of the hydrocarbon products are enhanced over their ideal mixture lines for both the scavenged and unscavenged mixtures. This is a strong indication that the hexaf luoroethane is transferring excitation (and/or charge) energy to the ethane (Reaction (V-l) ) . Figures (V-2) and (V-3) reveal that this is a major process in C 2 F 6 +C 2 H 6 C 2 F 6 + C 2 H 6 (V-l) this particular mixture system. That ethane is suspectible to energy transfer processes was reported by von Bunau (52). When he irradiated rare gas-ethane mixtures in which approximately 90% of the energy was absorbed by the rare gas, he observed that the hydrocarbon yields were essentially of the same magnitude as if all of the energy were abosrbed in ethane. It also should be noted that the ionization potential of C F is 14.6 eV (86), while that of C H is 11.6 eV (87). This implies Zb Zb that C F may transfer charge or excitation energy to C H , as charge or Zb Zb energy transfer occurs almost exclusively from a molecule of higher ionization potential to a molecule of lower ionization potential. With these factors in mind, and the experimental results in Tables V-2 through V-5, the mechanism in Table V-6 is proposed. When the mixture is irradiated, both excited C_F. and C„H, are 2 6 2 6 formed. The excited C^F^ can then transfer its energy to ethane, decompose to two CF^* radicals, or C^F,.+ F>, or fragment to form CF^ +

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111 TABLE V -6 Radiolysis Mechanism for the Mixtures of Ethane and Hexaf luoroethane C 2 F 6 r \A; r lAr+ C 2 F 6* (or C 2 F 6 + + e "> (III-l) C 2 H 6 — 'WW* c 2 H 6 *(° r C 2 H 6 + 4e‘) (iv-i) C 2 F 6* <+) + C 2 H 6 y C 2 H 6* <+> + C 2 F 6 (v-i) a r p * L 2*6 > CF 3 + + CF 3 * + e~ (or CF 3 ") (V-2) r f * V6 > C 2 F 5 + + F ' (V-3) CF 3 + + C 2 H 6 y cf 3 h + c 2 h 5 + (V-4) C 2 F 5 + + C 2 H 6 y c 2 f 5 h + c 2 h 5 + (V-5) ' C 4 H 11 + + F " C 4 H 10 + HF ( IV-37) c 3 h 9 + + F“ y C 3 H g + HF (IV38) F+ CMH, — * HF + C_H • (V6 ) 2 6 2 5 CF 3 * + C 2 H 6 y cf 3 h + c 2 h 5 (V-7) C 2 F 5+ C 2 H 6 y c 2 f 5 h + c 2 h 5 . (V8 ) CF 3 « + CH 3 * y (CF 3 CH 3 )*' (V-9) (CF 3 CH 3 )* + M y CF 3 CH 3 + M (V-10) (cf 3 ch 3 )* y CF 2 CH 2 + HF (V-11) CF 3+ C 2 H 5* y CF 3 C 2 H 5 (V-12) C 2 F 5* + CH 3y CH 3 C 2 F 5 (V-13) The excited or ionized C 2 H 5 can then undergo the various reactions listed in Table IV-4. Since most of the hydrocarbon products were significantly enhanced over their ideal mixture values, it follows that the reactions in Table IV-4 (particularly Reactions (IV-2), ( IV-4) , (IV8 ) , ( IV-13) , ( IV18) , (IV-4 7) , and (IV-49)) play a prominent role in the mixtures.

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112 or C„F r + . The excited C„H, (formed from either direct absorption of 2 5 2 b radiation or from energy or charge transfer from excited or ionized C„F,) can undergo the numerous reactions discussed in Chapter IV and Z b summarized in Table IV-4. Since there are no pure fluorocarbon products, many of the reactions mentioned for the pure ' C„F, system have been drastically altered by the 2 6 presence of ethane. First of all, the C^F^ is protected from radiolytic decomposition by the presence of ethane, since the excited or ionized C_F, is able to transfer radiolytic effect to C„H, . Also, instead of 2 6 zb having simply perf luorocarbon radical recombination reactions as was the case for the pure C F , other processes become important when ethane is present. For example, although F* does not abstract fluorine from C F , it has been demonstrated that it readily abstracts hydrogen from hydrocarbons, the process having an activation energy of less than 0.5 kcal/ mole (84). Also Whytock and co-workers have reported that CF^* and C F,.* radicals can abstract hydrogen from ethane with activation energies around 9 kcal/mole (88) . Oxygen scavenging results in the mixtures indicate that ionic species also play a more prominent role than in the pure C^F^ system. As mentioned in Chapter III, C^F^^ is essentially unreactive toward + ' C_F,, and the reaction of CF with C F, is slow. However when ethane 2 6 3 Z 6 is present, both CF^ + and C^F^^ can undergo exothermic (approximately 30 kcal/mole) hydride transfer reactions (V-4) and (V-5) . Both of these ion-molecule reactions have been observed by this laboratory and others ( 45 ). CF 3 +C 2 H 6 * CF 3 H + C 2 H 5 C 2 F 5 +C 2 H 6 -* C 2 F 5 H + C 2 H (V-4) + (V-5)

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-113Thus we can say that C 0 F^ affects C_H, chiefly by transferring Zb Zb charge and/or excitation energy to it. ( -'2^6 ^ as e ff ect on the different kinds of hydrocarbon products, but strongly affects their yields (in most cases substantially increases them). On the other hand, C„H, does not transfer energy to C_F^ , but the C„H, has a drastic effect Zb Zb Zb on the types of products that are generated from excited or ionic C_F , Z b since, unlike C„F r , C H reacts readily with C„F fragments. Since Z b Z b Zb C„F r seems to be protected from radiolytic decomposition and since the Z b C„F fragments that are formed can react with C„H, , there would be few Zb Zb C„F, fragments to "interfere" with the numerous C„H, fragments. Thus one would not expect pure fluorocarbon products, however pure hydrocarbon products would be expected, along with some mixed fluorocarbon-hydrocarbon products. In light of the above discussion, the reactions proposed for the formation of hydrogen and the hydrocarbon products are essentially the same as were proposed for that of the pure ethane system (see Chapter IV-B) . For example, the main processes that contribute to hydrogen formation would still be molecule elimination of hydrogen (Reaction (IV-2)), decomposition of excited ethylene (Reaction (IV-8)) or excited ethane ion (Reaction (IV-18)), and hydrogen abstraction from ethane (Reaction (IV-43)). As mentioned earlier mass spectrometric studies indicate that the formation of CF^H and C^F^H chiefly originate via hydride ion transfer from to CF^ (Reaction (V-4)) and C^F^ + (Reaction (V-5)). Scavenging experiments, which show the yields of CF^H anc ^ ^2^5^ are essent; *a Hy unaffected by the presence of oxygen, seem to imply that abstraction of hydrogen by CF^* (Reaction (V-7)) and C^F^* (Reaction (V-8)) plays a minor role in this system.

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-114The formation of HF is primarily due to hydrogen atom abstraction from ethane (Reaction (V-6)). The fact that the yield only slightly diminishes when oxygen is added is not surprising as the activation energy for hydrogen abstraction from ethane by fluorine atoms is generally assumed to be about 0.28 kcal/mole and the reaction is believed to occur on nearly every collision (89) . Other sources of HF would be elimination of HF from excited CF^CHg (Reaction (V-ll)) and hydrogen transfer to F ion from various proton donors in the system (Reactions (IV3 7c) and (IV-38a)) (90). C.H + + F _ *C.H ir . + HF ( IV-37c) 4 11 4 1U C_H q + + F~ * C_H q + HF (IV-38a) J y Jo As in the pure system, C^H^ + and CgH^" 1 " ions are thought to account for most of the unscavenged yields of C^H^q and C^Hg respectively (see Chapter IV-B) . The yields of CH^CF^ and CH 2 CF 2 are essentially eliminated by the presence of oxygen suggesting that these products arise exclusively by radical reactions. The CHg* and CF^* radicals can combine to form excited CH^CF^ (Reaction (V-9)), which can either collisionally stabilize to form CHgCFg (Reaction (V-10)) or decompose to form CH^CF^ and HF (Reaction (V-ll)). Decomposition of CH^CF^ to CH 2 CF 2 and HF was first observed by Alcock and Whittle (91) in 1965 and since then has been observed by several investigators (92) . CFgC 2 H^ and CHgC 2 F,also seem to be formed by radical combination reactions (Reactions (V-12) and (V-13)), as they are eliminated when oxygen is added. The results in the C„H, C„F. system take on added significance by zo Zb comparing these results with those of the cyclohexane-perf luorocyclohexane

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-115system (1-4, 6-8, 10) and the cyclobutane-perf luorocyclobutane system (5, 9). As mentioned in the beginning of this section, all three systems obey the generalization that the products of the radiolysis of mixtures resemble the products of the hydrocarbon component rather than the fluorocarbon component. When c-C^F £ was added to c-C^H^ the hydrogen yield was sharply decreased from the value in pure c-C^H^* the bicyclohexyl dimer yield was increased, the cyclohexane yield was decreased slightly, and c-C^F H was formed. As discussed in the Introduction, this indicated that the hydrocarbon was protected from decomposition by the presence of fluorocarbon. c-C^F ’*" S an exce H ent electron scavenger (being able to form c-C F , with a lifetime of b 1 Z approximately 450 microseconds) and it interferes with the charge neutralization process between c-C H + ions and electrons, thereby 6 lz preventing decomposition of C H . In the cyclobutane-perf luorocyclo6 lz butane system, the yields of many of the cyclobutane radiolysis products were higher than would be expected for an ideal mixture while the perf luoroethylene yield was decreased and the higher fluorocarbon products were eliminated. These results lead the authors to suggest that there was energy transfer from c-C^Fg to c-C^Hg. There still seemed to be some tendency toward decomposition of the perf luorocyclobutane as was i evidenced by the rise in the HF yield at low concentration of c-C^Fg. The fact that the hydrogen yield in this system was linear with respect to the energy absorbed by c-C H indicated there was a trade-off between a tendency for protection by c-C.F 0 (via the electron capture process) 4 o and a rather noticeable tendency toward energy transfer from c-C^Fg to c-C.H„, leading to formation of products other than hydrogen. In the 4 o ethane-perf luoroethane system, the hydrogen yield and most of the other

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-116hydrocarbon yields are substantially higher than for an ideal mixture. This suggests that charge and/or energy transfer from C„F, to C„H, is a 2 b Zb major process. Unlike the previous systems, there seems to be little evidence of electron capture by the C„F to protect the decomposition Z b of the positive hydrocarbon ions. As mentioned in the Introduction, C F ion has been postulated but has not been seen. If it exists, it Z o has a lifetime less than a microsecond. This contrasts greatly with c-C F and c-C F , which have relatively long-lived negative ions. This seems to imply that would have less tendency to protect the decomposition of the hydrocarbon by electron capture than would either c-CgF^ or c-C^Fg. These results suggest that as the chain length is lowered, energy (and/or charge) transfer to the hydrocarbon becomes much more important than protection of the hydrocarbon by electron capture . C. Summary The gas phase radiolysis of mixtures of ethane and hexaf luoroethane were studied at 50 torr pressure for systems containing 0 to 100% of the fluorocarbon, both pure and with 10% added oxygen. For the pure system, twentytwo products were identified, and the yields over the entire mixture range have been recorded. The major radiolytic products were H 2 , CH 4 , CgHg , n-C 4 H 1Q , C^, , CF^H, C^H, CF 2 CH 2 , and HF. The minor products included i-C c H 10 , n-C c H 10 , C,H, . , C 0 H,, 1-C.H 0 , cis-2-C,H 0 , 5 12 5 12 6 14 36 48 48 trans-2-C^Hg , CH^CF^, CHgC 2 Fj., and CF^C^H;.. It was observed that the addition of 10% oxygen substantially reduced the saturated hydrocarbon products, while the unsaturated hydrocarbon products were enhanced.

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117 Among the mixed products, HF, CF^H, and C^F^H were essentially unaffected by added oxygen, while CH^CF^, CF 2 CH 2Â’ CF^C^H,., and CH^C^F,. were eliminated. The fact that pure fluorocarbon compounds were not formed in the mixtures, and that the hydrocarbon products were significantly enhanced over their ideal mixture lines indicated that decomposition of ethane is sensitized by the presence of perf luoroethane and that a major process in the radiolysis of the mixtures is the ability of the perf luoroethane to transfer charge and/or excitation energy to ethane, which subsequently decomposes, forming the hydrocarbon products. Among the mixed products CH^CF^, CH 2 CF 2 , CF^C^H,., and CH^C^F,. were attributed to radical combination reactions, while CF^H and C 2 F^H were attributed to hydride ion transfer reactions from ethane, with HF formed via a simple hydrogen atom abstraction reaction. The results of this system were compared with the cyclohexane-perf luorocyclohexane system and the cyclobutaneperf luorocyclobutane system. This comparison revealed that in the radiolysis of f luorocarb'on-hydrocarbon mixtures, the products resemble the pure hydrocarbon component rather than the pure fluorocarbon component and that as the chain length is lowered, energy (and/or charge) transfer to the hydrocarbon becomes much more important than protection of hydrocarbon by electron capture.

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REFERENCES 1. L,A. Rajbenbach, "Nondissociation Electron Attachment Reactions in the y-Radiolysis of Solutions of Cyclic Perf luorocarbons in Alkanes," J. Am. Chem.’ Soc., £i8, 4275 (1966). 2. L.A. Rajbenbach and U. Kaldor, "Yield of Scavengeable Hydrogen Atoms, Electrons, and Positive Charges in the Radiolysis of Liquid Hexane," J. Chem. Phys., 47, 242 (1967). 3. N.H. Sagert , "y-Radiolysis of Cyclohexane with Electron Scavengers. III. Perf luorocarbons as Electron Scavengers," Can. J. Chem. , 46 , 95 (1968). 4. N.H. Sagert and A.S. Blair, "The y-Radiolysis of Cyclohexane with Electron Scavengers in the Vapor Phase," J. Chem. Phys., 46^, 3284 (1968) . 5. E. Heckel and R.J. Hanrahan, "The X-Radiolysis of Perf luorocyclobutane and Mixtures of Perf luorocyclobutane and Methane in the Gas Phase," Advan. Chem. Ser. , £!2, 120 (1968). 6. L.A. Rajbenbach, "Radiolysis of Solutions of Perf luorocarbons in n-Hexane," J. Phys. Chem., J3_, 356 (1969). 7. M.B. Fallgatter and R.J. Hanrahan, "The y-Radiolysis of Cyclohexane-Perf luorocyclohexane Solutions," J. Phys. Chem., _74, 2806 (1970) . 8. N.H. Sagert, "Radiolysis of Cyclohexane with Perf luorocyclohexane at very high Dose Rates," Can. J. Chem., 48, 501 (1970). 9. E. Heckel and R.J. Hanrahan, "Radiolytic Processes in Mixtures of Cyclobutane and Perf luorocyclobutane in the Gas Phase," Int. J. Radiat. Phys. Chem., _5, 281 (1973). 10. G.A Kennedy and R.J. Hanrahan, "The Effect of Iodine and other Scavengers on the Gamma Radiolysis of Liquid-Phase CyclohexanePerf luorocyclohexane Solutions," J. Phys. Chem., _78., 366 (1974). 11. George A. Kennedy, Ph.D. Dissertation, "Effects of Free Radical Scavengers in the Radiolysis of Systems Containing Perf luorocyclohexane," University of Florida, 1969. 12. M.M. Bibby and G. Carter, "Ionization and Dissociation in some Fluorocarbon Gases," Trans. Far. Soc., &2, 2637 (1966). 118 -

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-11913. L.G. Christophorou, "Atomic and Molecular Radiation Physics," Wiley-Interscience , New York, 1971, p. 501. 14. K.A.G. MacNeil and J.C.J. Thynne, "Ionization and Dissociation of Hexaf luoroethane, and of 1 , 1 , 1-Trif luoroethane and Fluoroform, by Electron Impact," Int. J. Mass Spectry. Ion Phys., 2 , 1 (1969). 15. P.W. Harland and J.L. Franklin, "Partitioning of Excess Energy in Dissociative Resonance Capture Processes," J. Chem. Phys., _61, 1621 (1974) . 16. "Catalog of Mass Spectral Data," American Petroleum Institute Project 44, Carnegie Institute of Technology, Pittsburgh, Pennsylvania, Serial No. 2. 17. Ibid . , Serial No. 444. 18. R.J. Hanrahan, "A Co-60 Gamma Irradiator for Chemcial Research," Intern. J. Appl. Radiation Isotopes, ]_3, 254 (1962). 19. M.C. Sauer, Jr. and L.M. Dorfman, "The Radiolysis of Ethylene: Details for the Formation of Decomposition Products," J. Phys. Chem. , 66, 322 (1962) . 20. G.G. Meisels, "Gas-Phase Dosimetry by Use of Ionization Measurements," J. Chem. Phys., 4^, 51 (1964). 21. W.H. Bragg, "Studies in Radioactivity," Macmillan and Co., London, 1912. p. 94. 22. L.H. Gray, "An Ionization Method for the Absolute Measurement of X-Ray Energy," Proc. Roy. Soc., A156 , 578 (1936). 23. G.J. Hine and G.L. Brownell, "Radiation Dosimetry," Academic Press Inc., New York, 1956, p. 25. 24. C. Chang, Ph.D. Dissertation, "The Radiation Chemistry of Hexaf luoroacetone in the Gas Phase," University of Florida, 1970, Appendix 1. i 25. E. Heckel and R.J. Hanrahan, "The Complete Analysis of Gas Phase Photolysis and Radiolysis Product Mixtures by Gas Chromatography," J. Chromatog. Sci. , 1 _ , 418 (1969). 26. M.B. Fallgatter and R.J. Hanrahan, "SPC-12 Mass Spectrometer Routines: Real-Time Data Acquisition and Data Reduction for Bendix TOF Instruments," University of Florida, 1970. Available from COSMIC Program Library, Univ. of Georgia under library number COS-02260. 27. E. Heckel and P.F. Marsh, "Titration of Subnanomole Quantities of Fluoride Ions in Polar Nonaqueous Solvents," Anal. Chem., 44, 2347 (1972) .

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120 28. L. Levan and P. Hamlet, "Radiolysis of Hexaf luoroethane , " J. Chem. Phys. , 42 , 2255 (1965) . 29. W.C. Askew, T.M. Reed, III, and J.C. Mailen, "Perf luoroalkanes in Ionizing Radiation," Radiat. Res., 33 , 283 (1968). 30. R. Cooper and H.R. Haysom, "Free Radical Yields in the Radiolysis of Gaseous Hexaf luoroethane , " J. Chem. Soc., Trans. II, _69, 904 (1973) . 31. A. Sololowska and L. Kevan, "Radiolysis of Liquid Hexaf luoroethane , " J. Phys. Chem., 71, 2220 (1967). 32. L. Kevan, "Energy Transfer in Radiolysis of Rare-Gas-C 9 F . Mixtures," J. Chem. Phys., 44 , 683 (1966). 1 6 33. A. Sokolowska and L. Kevan, "Energy Transfer in Radiolysis of RareGas-C 2 F 6 Liquid Mixtures," J. Phys. Chem., 12 , 253 (1968). 34. G.O. Pritchard, G.H. Miller, and J.R. Dacey, "Radical Recombination Reactions. Part II. Trif luoromethyl and Heptaf luoro-n-propyl radicals," Can. J. Chem., _39, 1968 (1961). 35. B.G. Tucker and E. Whittle, "Photolysis of Hexaf luoroacetone with Halogens and with Nitric Oxide," Trans. Faraday Soc., 6 ^, 484 (1965) . 36. A.S. Gordon, "Photolysis of Mixes of Perf luoroacetone and Perf luorodiethyl Ketone. Reactions of CF • and C F • Radicals," Can. J. Chem. , 44, 529 (1966) . 325 37. P.B. Ayscough and E.W.R. Steacie, "The Photolysis of Hexafluoroacetone," Proc. Roy. Soc. (London) Ser. A., 234, 476 (1956). 38. H. Sutcliffe and I. McAlpine, "The Radiation Chemistry of Polyfluorinated Organic Compounds," Fluorine Chem. Rev., 6 , 1 (1972). 39. N.J. Parks, K.A. Krohn, and J.W. Root, "Chemistry of Nuclear Recoil F Atoms. IV. Hot F-to-HF and F-to-F 2 Abstraction in CH 3 CF 3 ," J. Chem. Phys., 55 , 2690 (1971). , 40. R.D. Gilles and E. Whittle, "Photolysis of Mixtures of Acetone and Hexaf luoroacetone. Combination Reactions," Trans. Faraday Soc., 61, 1425 (1965). 41. R.R. Pettijohn, G.E. Mutch, and J.W. Root, "Chemically Activated 1 ^CH 3 CF 3 from Cross Combination of with CF 3 . An Introductory Experimental Study," J. Phys. Chem., _79, 2077 (1975). 42. J. King, Jr. and D.D. Elleman, "Ion Cyclotron Resonance Study of the Secondary Ion-Molecule Reactions in Hexaf luoroethane , " J. Chem. Phys. , 48, 412 (1968) .

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121 43. R.E. Marcotte and T.O. Tiernan, "Mass Spectrometric Studies of Ion-Neutral Reactions in Perfluoroethane, " J. Chem. Phys. , 54 , 3385 (1971). 44. T. Su and L. Kevan, "Ion Cyclotron Resonance Studies of Ionic Reactions in Perf luorocarbons . Excited Ions and Their Deexcitation," J. Phys. Chem., 77 _, 148 (1973). 45. L.W. Sieck, R. Gordon, Jr., and P. Ausloos, "Reactions of Fluorocarbon Ions in C„F,," J. Res. Nat. Bur. Stand., 78 A, 151 (1974). 2 b — 46. L. Kevan and J.H. Futrell, "Determination of Collision Rate Constants of Fluorocarbon Ions by Ion Cyclotron Resonance," J. Chem. Soc., Trans II, 68, 1742 (1972). 47. J.C. Amphlett and E. Whittle, "Reactions of Trif luoromethyl Radicals with Bromine, Chlorine, and Hydrogen Chloride," Trans. Faraday Soc., 61, 484 (1965). 48. C.M. Wodetzki, P.A. McCusker, and D.B. Peterson, "Effect of Density on the Radiolysis of Ethane," J. Phys. Chem., t>9, 1045 (1965). 49. a. E. Heckel and C.R. Niessner, private communication. b. C.R. Niessner, M.S. Thesis ,"Radioly tic Processes in Gaseous Mixtures of Ethane and Perf luoromethane , " East Carolina University, 1972. 50. P.T. Holland and J.A. Stone, "The y-Radiolysis of Ethane III. The Effect of Density on Ion Scavenging Reactions above the Critical Temperature," Can. J. Chem., .52, 221 (1974). 51. K. Yang and P.J. Manno, "The Role of Free Radical Processes in the y-Radiolysis of Methane, Ethane, and Propane," J. Am. Chem. Soc., 81, 3507 (1959). 52. G. von Biinau, "Uber den Einfluss bon Edelgasen auf die Strahlenchemie des Athans , " Ber. Bunsenges. Physik Chem., 69 _, 16 (1965). 53. L.H. Gevantman and R.R. Williams, Jr., "Detection and Identification of Free Radicals in the Radiolysis of Alkanes and Alkyl Iodides," J. Phys.. Chem., 56, 569 (1952). 54. L.M. Dorfman, "Radiolysis of Ethane: Isotopic and Scavenger Studies," J. Phys. Chem., 62, 29 (1958). 55. R.A. Back, "Scavenger Studies in the y-Radiolysis of Hydrocarbon Gases at very low Conversions," J. Phys. Chem., 64_, 124 (1960). 56. K. Yang and P.L. Grant, "Ethane Radiolysis at very low Conversions," J. Phys. Chem., 65 _, 1861 (1961). 57. L.J. Stief and P. Ausloos, "Radiolysis of Ethane-1, 1,1-d," J. Chem. Phys., 36, 2904 (1962). J

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122 58. H.H. Carmichael, R. Gorden, Jr., and P. Ausloos, "Effect of Electrical Fields and Density in the Radiolysis of Ethane," J. Chem. Phys., 42, 343 (1965). 59. C.M. Wodetzki, P.A. McCusker, D.B. Peterson, "Effect of Additives in Radiolysis of Ethane at High Densities," J. Phys. Chem., 69 , 1057 (1965). 60. J. Gawlowski and J.A. Herman, "Radiolysis of Ethane + Sulfur Hexafluoride Gaseous Systems. Formation of SF • Radical and its Reactivity," Can. J. Chem., 52 _> 3631 (1974). 61. P. Ausloos, R.E. Rebbert, and L. Wayne Sieck, "Ion-Molecule Reactions in the Radiolysis of Ethane," J. Chem. Phys., 54 , 2612 (1971). 62. H. Okabe and J.R. McNesby, "Vacuum Ultraviolet Photolysis of Ethane: Molecular Detachment of Hydrogen," J. Chem. Phys., _34, 668 (1961). 63. R.F. Hampson, Jr., J.R. McNesby, H. Akimoto and J. Tanaka, "Mechanism of the Photolysis of Ethane at 1470 X," J. Chem. Phys., 40, 1099 (1964). 64. H. Akimoto, K. Obi, and J. Tanaka, "Primary Process in the Photolysis of Ethane at 1236 A," J. Chem. Phys., 4 _2, 3864 (1965). 65. R.F. Hampson, Jr. , and J.R. McNesby, "Vacuum Ultraviolet Photolysis of Ethane at High Temperature," J. Chem. Phys., 42^, 2200 (1965). 66. J.A. Pirog and J.R. McNesby, "Vacuum-Ultraviolet Photolysis of Ethane in Liquid-Nitrogen Solution," J. Chem. Phys., 42, 2490 (1965). 67. A.H. Lauger and J.R. McNesby, "Photolysis of Ethane at the Argon Resonance Lines 1067 and 1048 X," J. Chem. Phys., 4_2, 3329 (1965). 68. W.M. Jackson, J. Faris, and N.J. Buccos, "Vacuum-Ultraviolet Photolysis of Ethane Films," J. Chem. Phys., 4_5, 4145 (1966). 69. B.C. Roquitte, "Flash Photolysis of Hydrocarbons in the FarUltraviolet. 1. Ethane," J. Phys. Chem., 74, 1204 (1970). 70. S.G. Lias, G.J. Collins, R.E. Rebbert, and P. Ausloos, "Photolysis of Ethane at 11.6-11.8 eV," J. Chem. Phys., 52., 1841 (1970). 71. R.R. Hutchins and R.R. Kuntz, "PhotoelectronInduced Decomposition of Ethane," J. Phys. Chem., 75 _, 2903 (1971). 72. M.S.B. Munson, J.L. Franklin, and F.H. Filed, "High Pressure Mass Spectrometric Study of Alkanes," J. Phys. Chem., 68, 3098 (1964). G.A.W. Derwish, A. Galli, A. Giardini-Guidoni , and G.G. Volip, "Ion-Molecule Reactions in Methane and Ethane," J. Chem. Phys., 40 , 5 (1964). 73 .

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-12374. S.K. Searles, L.W. Sieck, and P. Ausloos, "Reaction of C2^6 +: Formation of the (C 9 H 6 ) 2 + Ion," J. Chem. Phys., 53, 849 (1970). 75. A.S. Blair, E.J. Heslin, and A.G. Harrison, "Bimolecular Reactions of Trapped Ions. IV. Reactions in Gaseous Ethane and Mixtures with C 2 H 2 and CD 4 ," J. Am. Chem. Soc. , 94, 2935 (1972). 76. R.C. Dunbar, J. Shen, and G.A. Olah, "Ion-Molecule Reactions of Ethane at Low Electron Energy," J. Chem. Phys., 5J3, 3794 (1972). 77. S.L. Bennett, S.G. Lias, and F.H. Field, "Ion-Molecule Reactions in Ethane," J. Phys. Chem., ]b_, 3919 (1972). 78. E. Heckel and R.J. Hanrahan, "Ion-Molecule Reactions in the Systems CF 4 -CH 4 and CF^C^," J. Chem. Phys., 62, 1072 (1975). 79. S. Wexler and L.G. Pobo, "Ion Cyclobron Resonance Studies of Ionic Reactions in Ethane and of Hydrogen Exchange in D -C H and H -C D Mixtures," J. Am. Chem. Soc., 93, 1327 (1971). 1 1 b 80. T. McAllister, "Ion Cyclotron Double Resonance of Ion-Molecule Reactions in Ethane," J. Chem. Phys., _56, 5192 (1972). 81. G.G. Meisels, "Organic Gases," in Fundamental Processes in Radiation Chemistry , P. Ausloos, ed., John Wiley and Sons, New York, 1968, p. 347. 82. J.H. Purnell and C.P. Quinn, "The Pyrolyses of Paraffins," in Photochemistry and Reaction Kinetics , P.G. Ashmore, F.S. Dainton, T.M. Sugden, eds. , Cambridge University Press, Cambridge, 1967, p. 330. 83. W.V. Sherman, "Free-Radical Intermediates in the Radiation Chemistry of Organic Compounds," in Advances in Free Radical Chemistry , Vol. 3, G.H. Williams, ed. , Academic Press, New York, 1969, p. 1. 84. J.A. Kerr, "Rate Processes in the Gas Phase," in Free Radicals , Vol. 1, J.K. Kochi, ed. , John Wiley and Sons, Inc., New York, 1973, p. 1. I 85. G.G. Meisels, "Radiolysis of Ethylene. II. Primary Free Radicals and Their Reactions," J. Am. Chem. Soc., 87^, 950 (1965). 86. P. Sauvageau, J. Doucet, R. Gilbert, and C. Sandorfy, "Vacuum Ultraviolet and Photoelectron Spectra of Fluoroethanes , " J. Chem. Phys. , 61, 391 (1974) . 87. H. Henry and S. Fliszar, "Charge Distribution and Chemical Effects. VIII. Ionization Potentials of Normal and Branched Alkanes," Can. J. Chem. , 52, 3799 (1974) . D.A. Whytock, J.D. Clarke, and P. Gray, "Reactions of Perf luoroethyl Radicals with Propane and Neopentane," J. Chem. Soc., Trans. 1, 68 , 689 (1972). 88 .

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-12489. R.L. Williams and F'.A. Rowland, "Hydrogen Atom Abstraction by Fluorine Atoms," J. Phys. Chem. , 77 , 301 (1973). 90. S.A. Sullivan and J.L. Beauchamp, "Competition between Proton Transfer and Elimination in the Reactions of Strong Bases with Fluoroethanes in the Gas Phase. Influence of Base Strength on Reactivity," J. Am. Chem. Soc. , 9^8, 1160 (1976). 91. W.G. Alcock and E. Whittle, "Reactions of Trif luoromethyl Radicals with Organic Halides," Trans. Fara. Soc., _61, 244 (1965). 92. B.D. Neely and H. Carmichael, "Kinetic Isotope Effects in the Dehydrof luorination of Chemically Activated 1 , 1, 1-Trif luoroethane , " J. Phys. Chem., J73, 307 (1973).

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BIOGRAPHICAL SKETCH Michael D. Scanlon was born October 17, 1944 in Buffalo, New York. He graduated from Bishop Timon High School in Buffalo in 1962. He entered Canisius College in Buffalo on a New York State Regents Scholarship, and received a Bachelor of Science in Chemistry in 1966. He received a Master of Arts in Chemistry from the State University of New York at Buffalo in February, 1971. During the time he pursued his degree of Doctor of Philosophy from the University of Florida, he held research assistantships in the Department of Chemistry. I 125 -

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I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Professor of Chemistry I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Gardiner H. Myers Associate Professor of Chemistry I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Jbhn A. Wethington, 5 Professor of Nuclear Engineering Sciences

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This dissertation was submitted to the Graduate Faculty of the Department of Chemistry in the College of Arts and Sciences and to the Graduate Council, and was accepted as partial fulfillment of the requirements for the degree of Doctor of Philosophy. August, 1976 Dean, Graduate School I