Central barrier recrossing dynamics of the Cl^-+CD₃Cl S_N2 reaction

A direct dynamics simulation, at the MP2/6-31G* level of theory, was performed for the Cl^-+CD₃Cl S_N2 reaction. An ensemble of 88 trajectories was initiated at the [Cl- -CD₃- -Cl]^- central barrier, in accord with a 300 K Boltzmann distribution, and integrated in both the forward and reverse directions off the barrier. The dynamics of these trajectories are inconsistent with both RRKM theory and transition state theory (TST). Only 1% of the trajectories dissociate to Cl^-+CD₃Cl products in comparison to the RRKM prediction of greater than 50%. Also, in contrast to the assumption of TST and prediction of RRKM theory, recrossings of the central barrier are observed in the trajectories, giving rise to an upper limit of 0.7 for the correction factor to the 300 K TST rate constant. The results of this study are compared with a previous direct dynamics simulation, also at the MP2/6-31G* level of theory, of the central barrier dynamics for the Cl^-+CH₃Cl S_N2 reaction. Both reactions exhibit extensive non-RRKM dynamics. However, central barrier recrossings are more important for the Cl^-+CH₃Cl reaction and the upper limit to its TST correction factor is 0.2.