Chemical dynamics simulations of X + CH Y → XCH + Y gas-phase S 2 nucleophilic substitution reactions. Nonstatistical dynamics and nontraditional reaction mechanisms

Extensive classical chemical dynamics simulations of gas-phase X + CH Y→ XCH + Y S 2 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 2 reactions exhibit important nonstatistical atomic-level dynamics. The X + CH Y→ X -CH Y association rate constant is less than the capture model as a result of inefficient energy transfer from X + CH Y relative translation to CH Y rotation and vibration. There is weak coupling between the low-frequency intermolecular modes of the X -CH Y complex and higher frequency CH Y intramolecular modes, resulting in non-RRKM kinetics for X -CH Y unimolecular decomposition. Recrossings of the [X - CH - Y] central barrier is important. As a result of the above dynamics, the relative translational energy and temperature dependencies of the S 2 rate constants are not accurately given by statistical theory. The nonstatistical dynamics results in nonstatistical partitioning of the available energy to XCH +Y reaction products. Besides the indirect, complex forming atomic-level mechanism for the S 2 reaction, direct mechanisms promoted by X + CH Y relative translational or CH Y vibrational excitation are possible, e.g., the roundabout mechanism. © 2012 American Chemical Society.