Trajectory Studies of SN2 Nucleophilic Substitution. 6. Translational Activation of the Cl- + CH3Cl Reaction

Abstract

Classical trajectory simulations are used to study the translational activation of the Cl- + CH3Cl SN2 reaction at energies in the range 20−80 kcal/mol. The trajectories are calculated on the PES3 analytic potential energy surface. The shape of the reactive cross section versus relative translational energy Erel and the translational threshold of 18 kcal/mol are both similar to recent experimental results [J. Phys. Chem. A 1997, 101, 5969]. The reactive trajectories are direct, with negligible trapping in the ion−dipole complexes. The product energy is primarily partitioned to relative translation with small and similar amounts of energy partitioned to vibration and rotation. The velocity scattering angle distribution suggests backward scattering and a rebound mechanism for translational activation at low Erel, with increasing importance of forward scattering and a stripping mechanism as Erel is increased. An analysis of angular momenta terms and their correlations shows that the total angular momentum is well-approximated by the initial orbital angular momentum, which is strongly correlated with the final orbital angular momentum. The decrease in the reactive cross section with CD3Cl isotopic substitution is consistent with the experiments. The principal difference between the trajectories and experiments is the order of magnitude larger cross sections found from the trajectories. No pronounced inadequacies in the PES3 potential energy surface are evident from comparisons with MP2/6-311+G** ab initio calculations.