A direct ab-initio dynamics study on a gas phase S N 2 reaction F − +CH 3 Cl → CH 3 F+Cl − : dynamics of near-collinear collision

Direct ab-initio dynamics calculations have been applied to a gas phase S N 2 reaction F − +CH 3 Cl→CH 3 F+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 - +CH 3 Cl) 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 CH 3 Cl 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 CH 3 F remain in the ground state. The lifetimes of early- and late-complexes F − ⋯CH 3 Cl and FCH 3 ⋯Cl − were short enough to dissociate directly to products, while the energy was not completely distributed within the lifetime. It is concluded that the S N 2 reaction F − +CH 3 Cl proceeds non-statistically via a direct mechanism in the case of near-collinear collision.