Trajectory studies of S N2 nucleophilic substitution. II. Nonstatistical central barrier recrossing in the Cl−+CH3Cl system

Abstract

For the Cl−+CH3Cl SN2 nucleophilic substitution reaction transition-state theory predicts that crossing the central barrier region of the potential-energy surface is the rate-controlling step. In this work classical trajectories are initialized at the central barrier. Four different models are considered for the potential-energy surface. A significant amount of central barrier recrossing is observed in the trajectories, which suggests that transition-state theory is an incomplete model for calculating the Cl−+CH3Cl SN2 rate constant. Two types of recrossings are observed in the trajectories: intermediate recrossings in which trajectories linger near the central barrier and complex recrossings in which trajectories trapped in the Cl−⋅⋅⋅CH3Cl complex return to the central barrier region. Intermediate recrossings are important if, in the trajectory initial conditions, zero-point energy is added to the vibrational modes orthogonal to the reaction coordinate. Rice–Ramsperger–Kassel–Marcus (RRKM) theory predicts extensive dissociation of the Cl−⋅⋅⋅CH3Cl complex to Cl−+CH3Cl and negligible complex recrossings in the trajectory calculations. In contrast to this prediction, negligible Cl−+CH3Cl formation is observed and continual complex recrossings occur, on a time scale longer than the complex’s RRKM lifetime. These results indicate the ergodic assumption is invalid for the Cl−⋅⋅⋅CH3Cl complex. Phase-space bottlenecks which give rise to the intermediate and complex recrossings are considered.