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

Abstract

For the Cl⁻+CH₃Cl S_N2 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 S_N2 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.