Mechanisms of SN2 reactions: insights from a nearside/farside analysis

Abstract

A nearside/farside analysis of differential cross sections has been performed for the complex-forming S_N2 reaction Cl⁻ + CH₃Br → ClCH₃ + Br⁻. It is shown that for low rotational quantum numbers a direct “nearside” reaction mechanism plays an important role and leads to anisotropic differential cross sections. For high rotational quantum numbers, indirect mechanisms via a long-lived intermediate complex are prevalent (independent of a nearside/farside configuration), leading to isotropic cross sections. Quantum mechanical interference can be significant at specific energies or angles. Averaging over energies and angles reveals that the nearside/farside decomposition in a semiclassical interpretation can reasonably account for the analysis of the reaction mechanism.