Methyl Radicals in Solid Acetonitrile: Decay or No Decay

A comparison of both the e.s.r. spectra and the thermal stability of the radicals produced in γ radiolysis and 185 nm photolysis of acetonitrile shows that the former are not free methyl radicals. The initial species formed by γ radiolysis can be converted by photobleaching into a methyl radical weakly associated with a cyanide ion. The methyl radical of this CH3---CN−pair decays via hydrogen abstraction, whereas the free methyl radicals produced by 185 nm photolysis of acetonitrile are quite stable at 77 K. We ascribe the low energy of activation for the hydrogen abstraction reaction in the γ irradiated samples not primarily to quantum mechanical tunnelling but to perturbation by the cyanide ion.