Symmetry specificity in the unimolecular decay of the Cl-center dot centerdot center dot 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 substitut ion reaction Cl- + CH3Cl is investigated by means of two-dimensional quantu m mechanical calculations on a coupled-cluster [CCSD(T)] potential energy s urface. The dynamics calculations employ Radau coordinates to describe the two C-Cl stretching degrees of freedom, filter diagonalization, and an abso rbing (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 i s found to be strongly symmetry specific: For energies above the reaction b arrier, the smallest rates of the ungerade states are about two orders-of-m agnitude smaller than the smallest rates of the gerade states. An explanati on is given in terms of an adiabatic model formulated in hyperspherical coo rdinates. The nonadiabatic coupling elements, which control the energy tran sfer between the two modes and therefore determine the decay rate, are subs tantially larger for the gerade states. Ultimately, the differences are cau sed by the different structures of the gerade and the ungerade wave functio ns at the barrier. (C) 2001 American Institute of Physics.