Unimolecular decay of the Cl-center dot center dot center dot CH3Cl complex: Influence of symmetry breaking

The unimolecular dissociation of quasi-bound states formed in the bimolecul ar nucleophilic substitution (S(N)2) reaction Cl-35(-) + (CH3Cl)-Cl-37 is i nvestigated by means of two-dimensional quantum mechanical calculations. Th e study, which extends our recent work on the symmetric [(ClCH3Cl)-Cl-35-Cl -35](-) complex, highlights the changes in the dissociation rates when the symmetry with respect to the interchange of the two chlorine atoms is reduc ed. It is observed that in the intermediate energy range the distribution o f 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 tra nsitions between different adiabatic states.