Symmetry specificity in the unimolecular decay of the Cl-•••CH3Cl complex:: Two-mode quantum calculations on a coupled-cluster [CCSD(T)] potential energy surface

The decay of resonance states in the complex-forming nucleophilic substitution reaction Cl- + CH3Cl is investigated by means of two-dimensional quantum mechanical calculations on a coupled-cluster [CCSD(T)] potential energy surface. The dynamics calculations employ Radau coordinates to describe the two C-Cl stretching degrees of freedom, filter diagonalization, and an absorbing (optical) potential. The resonance widths and the corresponding decay rates vary by several orders-of-magnitude, reflecting the large degree of separability of the intramolecular and the intermolecular mode. The decay is found to be strongly symmetry specific: For energies above the reaction barrier, the smallest rates of the ungerade states are about two orders-of-magnitude smaller than the smallest rates of the gerade states. An explanation is given in terms of an adiabatic model formulated in hyperspherical coordinates. The nonadiabatic coupling elements, which control the energy transfer between the two modes and therefore determine the decay rate, are substantially larger for the gerade states. Ultimately, the differences are caused by the different structures of the gerade and the ungerade wave functions at the barrier. (C) 2001 American Institute of Physics.