Electron spin resonance study of the decay of methyl radical-bromide ion pairs in acetonitrile at low temperature

Methyl radicals adjacent to cyanide ions are generated upon photolysis of ..gamma..-irradiated, crystalline acetonitrile, and these radicals decay by hydrogen atom abstraction from neighboring matrix molecules. In this paper the kinetic consequences of a slight alteration in the immediate environment of the methyl radical are examined. It is found that replacement of the cyanide ion with a bromide ion causes the observed rate constant for abstraction to be lower by about a factor of 2 at low temperatures. Once again, the low activation energy and large deuterium kinetic isotope effect are indicative of very important tunnel effects in the abstraction reaction.