Unimolecular decay of the Cl−···CH3Cl complex: Influence of symmetry breaking

Abstract

The unimolecular dissociation of quasi-bound states formed in the bimolecular nucleophilic substitution (S_N2) reaction ³⁵Cl⁻ + CH₃³⁷Cl is investigated by means of two-dimensional quantum mechanical calculations. The study, which extends our recent work on the symmetric [³⁵ClCH₃³⁵Cl]⁻ complex, highlights the changes in the dissociation rates when the symmetry with respect to the interchange of the two chlorine atoms is reduced. It is observed that in the intermediate energy range the distribution of rates is similar to the distribution for the gerade states in the case of the symmetric mass combination; rates, which are as small as the smallest rates for the ungerade states, are missing when different chlorine isotopes are used. An explanation of this effect is given in terms of an adiabatic model with emphasis on the non-adiabatic matrix elements, which promote transitions between different adiabatic states.