Ab initio molecular orbital study on the gas phase SN2 reaction F-+CH3Cl→CH3F+Cl-

Ab-initio molecular orbital (MO) and direct ab initio dynamics calculations have been applied to the gas phase S(N)2 reaction F- + CH3Cl --> CH3F + Cl-. Several basis sets were examined in order to select the most convenient and best fitted basis set to that of high-quality calculations. The Hartree-Fock (HF) 3-21+G(d) calculation reasonably represents a potential energy surface calculated at the MP2/6-311++G(2df,2pd) level. A direct ab initio dynamics calculation at the HF/3 - 21 + G(d) level was carried out for the S(N)2 reaction. A full dimensional ab initio potential energy surface including all degrees of freedom was used in the dynamics calculation. Total energies and gradients were calculated at each time step. Two initial configurations at time zero were examined in the direct dynamics calculations: one is a near collinear collision, and the other is a side-attack collision. It was found that in the near collinear collision almost all total available energy is partitioned into two modes: the relative translational mode between the products (similar to 40%) and the C - F stretching mode (similar to 60%). The other internal modes of CH3F were still in the ground state. The lifetimes of the early- and late-complexes F-... CH3Cl and FCH3...Cl- are significantly short enough to dissociate directly to the products. On the other hand, in the side-attack collision, the relative translation energy was about 20% of total available energy. (Int J Mass Spectrom 181 (1998) 151-157) (C) 1998 Elsevier Science B.V.