Trajectory studies of SN2 nucleophilic substitution. I. Dynamics of Cl−+CH3Cl reactive collisions

Classical trajectories were used to study the dynamics of the Cl−+CH3Cl→Cl−---CH3Cl association and the Cl−+CH3Cl→ClCH3+Cl− substitution reactions. Substantial deviations are found between the underlying microscopic dynamics of the reactions and the assumptions of statistical rate theories. The energy dependence of the trajectory rate constant for the majority of Cl−---CH3Cl→Cl−+CH3Cl dissociation is in accord with a model in which only the Cl−---C stretch and the two Cl−---CH3Cl bend modes are active degrees of freedom. At 300 K the trajectory rate constant for Cl−+CH3Cl→Cl−---CH3Cl association is approximately forty percent smaller than that of microcanonical variational transition state theory, with the difference increasing with an increase in temperature. For thermal conditions substitution occurs by an indirect mechanism in which the reactive system is initially trapped in the Cl−---CH3Cl potential well. The cross section for this process decreases dramatically as the reactant relative translational energy is increased. The effect of rotational energy is less precipitous. Exciting the C–Cl stretch normal mode of CH3Cl opens up a direct substitution mechanism without trapping in either of the two potential wells. There is a significant decrease in the cross section for this direct substitution when CH3Cl is rotationally excited.