Model calculations of primary hydrogen isotope effects

Abstract

Model calculations of primary hydrogen isotope effects for four and five-centre transition states, taking account of bending vibrations and proton tunnelling, are reported. The results reinforce those based on consideration of stretching vibrations alone, and suggest that both zero-point energy and tunnelling contributions are at a maximum for a symmetrical transition state. For proton transfer to halide ions, large isotope effects persist for product-like transition states owing to the low frequency of hydrogen bending vibrations. Swain's relation between deuterium and tritium isotope effects is well obeyed. It seems better to consider the reaction co-ordinate mode for an atom transfer reaction as deriving from a translation rather than from a vibration in the reactants.