Thermal hydrogen atoms

The comparatively high ionisation potential of sulphur hexafluoride and its inertness toward attack by thermal hydrogen atoms have lead to its use as a specific scavenger for electrons in several irradiated systems. This has already been illustrated in section 1.7.2. The ionisation processes in SF6 have been studied by beam techniques171, but to date there has been no investigation of its radiolysis per se. Such a study would be well worthwhile. 

Apart from this behavior, the concentration of frarw-vinylene increases with dose, at first linearly but then tends to a limiting value which depends on the type of radiation (7). This limit is possibly caused by the capture by frarw-vinylene of the thermalized hydrogen atoms released elsewhere by radiation. From the limiting concentration of unsaturation the number of collisions made by each thermalized hydrogen atom before it abstracts can be deduced this lies in the range 103 to 104. 

Hagopian, A. K. E., and R. H. Johnsen The Reaction of Thermal Hydrogen Atoms with Frozen Organic Substrates. J. Phys. Chem. 72, 1949 (1968). 

A more recent addition to the above reaction scheme is the distinction between thermal and hot hydrogen atoms. A yield of thermal hydrogen atoms of —1.3 is obtained by McCrumb and Schuler (11). All these reactions and yields may be summarized as follows ... 

If Reaction 6 or 7 occurs, the hydrogen yield is reduced, and if Reaction 8 occurs, the cyclohexene yield is increased relative to that of bicyclohexyl. Since hot hydrogen atoms are considered to abstract from cyclohexane much more readily than thermal hydrogen atoms, only thermal hydrogen atoms will be involved in Reactions 6 to 8. 

The yields do not vary between 5 X 1026 and 8 X 1027 e.v./gram sec., suggesting that at these dose rates practically all the thermal hydrogen atoms are reacting with themselves or cyclohexyl radicals. 

The results at the lower Febetron dose rates of 1024-1025 e.v./gram sec. show a decrease in the ratio of cyclohexene to bicyclohexyl. This may be caused by a reduction in the reaction of thermal hydrogen atoms with radicals and indicate a return to the low dose rate (i.e., 1016 e.v./gram sec.) mechanism. However, dosimetry at these dose rates at the present time is not sufficiently accurate to warrant further discussion. 

The cyclohexene and bicyclohexyl yields in the Febetron irradiations at N20 concentrations above 0.1M are. — 3.5 and — 1.9, respectively. Since the unimolecular yield is decreased by N20, the extra yield of cyclohexene that must presumably come from Reaction 8 (H + CeHn -> C6Hio + H2), is. — 0.8. Since this is the same as the extra yield in pure cyclohexane it appears that the yield of thermal hydrogen atoms is not affected by adding N20. 

As with N20 the shapes of yield vs. concentration curves show little or no dependence on dose rate. As stated this is to be expected if these reactions occur in or near spurs. Again the difference between the cyclohexene yield and the bicyclohexyl yields is greater than that caused by unimolecular dissociation, implying that SF6 like N20 does not affect the yield of thermal hydrogen atoms. 

The effect of benzene in the Febetron irradiations is markedly different from that of the electron scavengers. The difference between the cyclohexene and bicyclohexyl yields is much smaller and implies that benzene decreases the yield of thermal hydrogen atoms. Thus, benzene is not acting as an electron scavenger as has been suggested (17). 

P), and by Wooley and Cvetanovic [13] in which photolysis of hydrogen sulphide in the presence of large amounts of carbon dioxide produced fully thermalized hydrogen atoms. 

Alkanes.—Hydrogen Substitution. Whereas thermal hydrogen atoms react with alkanes exclusively by hydrogen abstraction, tritium atoms goierated by nuclear recoil also undergo the energetic substitution reaction (52) in high yield. ... 

The measured terminal to central ratio of 1.4 contrasts sharply with the ratio of about 15 found for the thermal hydrogen atom addition to propylene (38). The value of 1.4 indicates only a moderate preference for reaction at the CH2 end of propylene, indicating that atomic fluorine is a rather indiscriminate, highly reactive species. Both the F recoil and N2F4 Aotolysis e 5)eriments show substantial H atom abstraction from propylene... 

---