Identification of atomic-level mechanisms for gas-phase X- + CH3Y SN2 reactions by combined experiments and simulations.

Abstract

For the traditional model of gas-phase X(-) + CH3Y SN2 reactions, C3v ion-dipole pre- and postreaction complexes X(-)---CH3Y and XCH3---Y(-), separated by a central barrier, are formed. Statistical intramolecular dynamics are assumed for these complexes, so that their unimolecular rate constants are given by RRKM theory. Both previous simulations and experiments have shown that the dynamics of these complexes are not statistical and of interest is how these nonstatistical dynamics affect the SN2 rate constant. This work also found there was a transition from an indirect, nonstatistical, complex forming mechanism, to a direct mechanism, as either the vibrational and/or relative translational energy of the reactants was increased. The current Account reviews recent collaborative studies involving molecular beam ion-imaging experiments and direct (on-the-fly) dynamics simulations of the SN2 reactions for which Cl(-), F(-), and OH(-) react with CH3I. Also considered are reactions of the microsolvated anions OH(-)(H2O) and OH(-)(H2O)2 with CH3I. These studies have provided a detailed understanding of the atomistic mechanisms for these SN2 reactions. Overall, the atomistic dynamics for the Cl(-) + CH3I SN2 reaction follows those found in previous studies. The reaction is indirect, complex forming at low reactant collision energies, and then there is a transition to direct reaction between 0.2 and 0.4 eV. The direct reaction may occur by rebound mechanism, in which the ClCH3 product rebounds backward from the I(-) product or a stripping mechanism in which Cl(-) strips CH3 from the I atom and scatters in the forward direction. A similar indirect to direct mechanistic transition was observed in previous work for the Cl(-) + CH3Cl and Cl(-) + CH3Br SN2 reactions. At the high collision energy of 1.9 eV, a new indirect mechanism, called the roundabout, was discovered. For the F(-) + CH3I reaction, there is not a transition from indirect to direct reaction as Erel is increased. The indirect mechanism, with prereaction complex formation, is important at all the Erel investigated, contributing up ∼60% of the reaction. The remaining direct reaction occurs by the rebound and stripping mechanisms. Though the potential energy curve for the OH(-) + CH3I reaction is similar to that for F(-) + CH3I, the two reactions have different dynamics. They are akin, in that for both there is not a transition from an indirect to direct reaction. However, for F(-) + CH3I indirect reaction dominates at all Erel, but it is less important for OH(-) + CH3I and becomes negligible as Erel is increased. Stripping is a minor channel for F(-) + CH3I, but accounts for more than 60% of the OH(-) + CH3I reaction at high Erel. Adding one or two H2O molecules to OH(-) alters the reaction dynamics from that for unsolvated OH(-). Adding one H2O molecule enhances indirect reaction at low Erel, and changes the reaction mechanism from primarily stripping to rebound at high Erel. With two H2O molecules the dynamics is indirect and isotropic at all collision energies.