EFFECTS OF REACTION-PATH CURVATURE ON REACTION DYNAMICS AND RATES - REACTION-PATH HAMILTONIAN CALCULATIONS FOR GAS-PHASE S(N)2 REACTION CL-+CH3CL

To obtain essential information on the reaction dynamics for the proto type gas-phase S(N)2 reaction Cl-+ CH3Cl --> ClCH3 + Cl-, the characte ristic features of the potential energy surface in the local region ar ound the reaction path were examined by the reaction path Hamiltonian constructed with high-level ab initio molecular-orbital calculations. After the structures of relevant stationary states and the intrinsic r eaction coordinate were determined, the transverse vibrational modes, the corresponding frequencies, and the coupling elements between the p airs of normal modes induced by the reaction coordinate motion were ca lculated at each point along the intrinsic reaction coordinate. It was found that a quite large reaction path curvature appears in the intri nsic barrier slope near the bottom of each of the pre-and postreaction stable-state complexes. This large curvature was clarified to cause t he internal vibrational excitation of the products and the requirement of the vibrational excitation of the reactants for reaction occurrenc e. The complex recrossings across the transition-state theory dividing surface, previously characterized by Hase et al. [J. Chem. Phys. 96, 8275 (1992)] in which trajectories trapped in the Cl-... CH3Cl complex return to the central barrier region, were demonstrated to be attribu ted to this large curvature. Furthermore, not only the variational eff ects but also the reaction path curvature effects on the intermediate recrossings that were also characterized by Hase et al., in which traj ectories linger near the central barrier, were found to be negligible. (C) 1998 John Wiley & Sons, Inc.