Resonances in S_N2 reactions: Two-mode quantum calculations for Cl⁻+CH₃Br on a coupled-cluster potential energy surface

An effective two-dimensional potential energy surface has been constructed for the S_N2 reaction Cl^-+CH₃Br-->ClCH₃+Br^- from coupled-cluster calculations with a large basis set. In the quantum dynamics calculations Radau coordinates were employed to describe the Cl-C and C-Br stretching modes. Making use of the filter diagonalization method and an optical potential, bound states as well as resonance states up to energies far above the dissociation threshold have been calculated. The resonance widths fluctuate over several orders of magnitude. In addition to a majority of Feshbach-type resonances there are also exceedingly long-lived shape resonances, which can only decay by tunneling. Owing to a smaller width of the potential barrier and a larger density of states, tunneling through the barrier is more important for Cl^-+CH₃Br than for Cl^-+CH₃Cl despite the larger total mass of this system. Excitation of the C-Br stretching vibration enhances the tunneling probability of the entrance channel complex. (C) 2002 American Institute of Physics