DFT study of inner-sphere electron transfer in a gas-phase SN2 reaction at the saturated carbon

The SN2 reaction is investigated from the point of view of an inner-sphere electron transfer reaction. Density functional theory (DFT) calculations are applied to investigate the intrinsic reaction pathway (IRP) of the symmetrical SN2 (inner-sphere electron transfer) reactions, Xa− + CH3Xb → XaCH3 + Xb− (X = F, Cl, Br, I). The geometries of the stationary points in the potential energy surface are compared with available experimental data and with calculations at the MP2 level. The changes in the geometrical parameters, the projected frequencies, charge distribution and dipole moment along the reaction coordinate are discussed. Some relationships between the amount of transferred electron density and the changes in geometrical parameters, vibrational frequencies and energies are given: the greater the amount of transferred electron density, the smaller the decrease in the frequency of the C–Xb stretching mode (from reactant to TS), and the lower the activation energy and the complexation energy. Moreover, the amount of transferred electron density is related to the electronegativity of the halogen atom. Two parameters in the electron transfer theory, the coupling interaction and the reorganization energy, which affect the reactivity are discussed. We conclude that the stronger the CH3–Xb bond, the worse the acceptor.