A direct ab-initio dynamics study on a gas phase SN2 reaction F−+CH3Cl → CH3F+Cl−: dynamics of near-collinear collision

Direct ab-initio dynamics calculations have been applied to a gas phase SN2 reaction F−+CH3Cl→CH3F+Cl−. An ab-initio potential energy surface including all degrees of freedom was used. Total energies and gradients were calculated at each time step. Results for near-collinear collision are reported. The initial geometrical configurations of (F-+CH3Cl) at time zero were randomly generated in the range 180±3° for the collision angle ∠F–C–Cl and of r(F−–C)=6.0–6.5 Å. The vibrational phase of CH3Cl was generated to take a temperature of 10 K. It was found that almost all available energy is partitioned into the relative translational mode between the products (∼43%) and the C–F stretching mode (∼57%) at zero collision energy. The other internal modes of CH3F remain in the ground state. The lifetimes of early- and late-complexes F−⋯CH3Cl and FCH3⋯Cl− were short enough to dissociate directly to products, while the energy was not completely distributed within the lifetime. It is concluded that the SN2 reaction F−+CH3Cl proceeds non-statistically via a direct mechanism in the case of near-collinear collision.