Effect of irradiation temperatures on hydrogen atom reactions in neopentane and its mixtures irradiated at 4.2 and 770K as studied by electron spin resonance

It has been found that the behavior of hydrogen atoms produced by the homolitic scission of the C-H bond is quite different at 4.2 and 770K. Neopentane--isobutene (2 mol percent) mixtures irradiated at 770K show that tert-butyl radicals are formed by hydrogen atom scavenging in addition to neopentyl radicals. The scavengeable hydrogen atoms are estimated to be about 60 to 70 percent of the hydrogen atoms produced in the system. However, the same mixture irradiated at 4.20K gives only neopentyl radicals which amount to the same as the total radical yield in the mixture irradiated at 770K. This indicates that the hydrogen atoms which are scavengeable at 770K are not scavenged at 4.20K and they reacted with neopentane forming neopentyl radicals. The ESR line width of the neopentyl radicals, their pairwise trapping, and microwave power saturation behavior indicate that the hydrogen atoms reacted with neopentane by a rather short-range reaction at 4.20K while some of them reacted with solutes via long-range migration at 770K. Similar effects of irradiation temperature have been also found in neopentane--cyclohexane (2 mol percent) mixtures as well as in neopentane containing a small amount of impurity. It is suggested that short-range hot abstraction from neopentane takes place at 4.20K while a long-range tunneling reaction of thermal hydrogen atoms with solutes takes place at 770K other than hot abstraction. The results are discussed in relation to guest radical formation in mixed crystals of n-decane-d22 and n-decane-h22 previously reported