Structure and Reactivity of Methyl Radical — Cyanide Ion Pairs in Crystal I and Crystal II of Acetonitrile

Abstract

The extent of interaction between methyl radicals and cyanide ions produced in pairs by dissociative electron capture in the two solid phases of acetonitrile has been studied by e.s.r. using CD313CN. Although no interaction is observed when the radical–anion pairs are generated by photobleaching the acetonitrile dimer radical anion in Crystal I, a very weak interaction as evidenced by an isotropic 13C hyperfine splitting of 3.4 G is found for the corresponding species produced from the acetonitrile monomer radical anion in Crystal II. The rate of hydrogen atom abstraction by the methyl radical in Crystal I is at least a factor of 10 greater than in Crystal II at the same temperature over the range 77–113 K. These results show that the weak perturbation of the methyl radical by the cyanide ion does not enhance methyl radical reactivity in hydrogen atom abstraction. Evidence from 13C hyperfine splitting measurements on [Formula: see text] indicates that the configuration of the methyl radical is planar in these radical–anion pairs. It is emphasized that quantum mechanical tunneling provides a satisfactory explanation for the low apparent activation energies, the curved Arrhenius plots, and the abnormally large deuterium isotope effects which characterize hydrogen atom abstraction reactions by methyl radicals in glassy and crystalline solids at low temperatures. Moreover, since the tunneling rate is extremely sensitive to the width of the barrier, methyl radical reactivity is expected to show a very strong dependence on the precise geometry of the reacting partners in the solid state.