Chemical dynamics simulations of X ^- + CH ₃Y → XCH ₃ + Y ^- gas-phase S _N2 nucleophilic substitution reactions. Nonstatistical dynamics and nontraditional reaction mechanisms

Extensive classical chemical dynamics simulations of gas-phase X ^- + CH ₃Y→ XCH ₃ + Y ^- S _N2 nucleophilic substitution reactions are reviewed and discussed and compared with experimental measurements and predictions of theoretical models. The primary emphasis is on reactions for which X and Y are halogen atoms. Both reactions with the traditional potential energy surface (PES), which include pre- and postreaction potential energy minima and a central barrier, and reactions with nontraditional PESs are considered. These S _N2 reactions exhibit important nonstatistical atomic-level dynamics. The X ^- + CH ₃Y→ X ^--CH ₃Y association rate constant is less than the capture model as a result of inefficient energy transfer from X ^-+ CH ₃Y relative translation to CH ₃Y rotation and vibration. There is weak coupling between the low-frequency intermolecular modes of the X ^--CH ₃Y complex and higher frequency CH ₃Y intramolecular modes, resulting in non-RRKM kinetics for X ^--CH ₃Y unimolecular decomposition. Recrossings of the [X - CH ₃ - Y] ^- central barrier is important. As a result of the above dynamics, the relative translational energy and temperature dependencies of the S _N2 rate constants are not accurately given by statistical theory. The nonstatistical dynamics results in nonstatistical partitioning of the available energy to XCH ₃ +Y ^- reaction products. Besides the indirect, complex forming atomic-level mechanism for the S _N2 reaction, direct mechanisms promoted by X ^- + CH ₃Y relative translational or CH ₃Y vibrational excitation are possible, e.g., the roundabout mechanism.