Chemical Dynamics Simulations of X- + CH3Y → XCH3 + Y- Gas-Phase SN2 Nucleophilic Substitution Reactions. Nonstatistical Dynamics and Nontraditional Reaction Mechanisms

Extensive classical chemical dynamics simulations of gas-phase X- + CH3Y -> XCH3 + Y- S(N)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(N)2 reactions exhibit important nonstatistical atomic-level dynamics. The X- + CH3Y -> X--CH3Y association rate constant is less than the capture model as a result of inefficient energy transfer from X- + CH3Y relative translation to CH3Y rotation and vibration. There is weak coupling between the low-frequency intermolecular modes of the X--CH3Y complex and higher frequency CH3Y intramolecular modes, resulting in non-RRKM kinetics for X--CH3Y unimolecular decomposition. Recrossings of the [X-CH3-Y](-) central barrier is important. As a result of the above dynamics, the relative translational energy and temperature dependencies of the S(N)2 rate constants are not accurately given by statistical theory. The nonstatistical dynamics results in nonstatistical partitioning of the available energy to XCH3 + Y- reaction products. Besides the indirect, complex forming atomic-level mechanism for the S(N)2 reaction, direct mechanisms promoted by X- + CH3Y relative translational or CH3Y vibrational excitation are possible, e.g., the roundabout mechanism.