Four-mode calculation of resonance states of intermediate complexes in the S(N)2 reaction Cl-+CH3Cl '-> ClCH3+Cl '(-)

Abstract

A four-dimensional coupled-cluster [CCSD(T)] potential energy surface has been constructed for the collinear symmetric S(N)2 reaction Cl-+CH3Cl'-->ClCH3+Cl'-. Making use of the filter diagonalization method and optical potentials, bound states as well as resonance states up to energies far above the dissociation threshold have been calculated. Most of the bound and resonance states up to the first overtone of the symmetric C-H stretching vibration could be assigned four quantum numbers (symmetric C-H stretching mode, umbrella bending mode, intra- and intermolecular C-Cl stretching modes). The effect of different average lifetimes of resonance states with gerade and ungerade symmetry that was found in previous two-mode calculations [J. Chem. Phys. 114, 5233 (2001)] is preserved in the four-dimensional calculations. The range of the resonance widths has become appreciably more widespread because many resonance states with excitation in the high-frequency C-H stretching mode and the umbrella bending mode are particularly long-living. The linewidths of the resonances and, therefore, the coupling to the continuum become larger with excitation in any mode. However, this increase as a function of a quantum number is not monotonic due to the strong mixing of the many progressions in the intermolecular stretching mode of the intermediate complex. (C) 2003 American Institute of Physics.