Trajectory studies of SN2 nucleophilic substitution. II. Nonstatistical central barrier recrossing in the Cl- +CH3 Cl system

Young June Cho, Scott R. Vande Linde, Ling Zhu, William L. Hase

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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- + CH 3Cl 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.

Original languageEnglish
Pages (from-to)8275-8287
Number of pages13
JournalThe Journal of Chemical Physics
Issue number11
StatePublished - 1992


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