Trajectory studies of S(N)2 nucleophilic substitution. IX. Microscopic reaction pathways and kinetics for Cl-+CH3Br

摘要

A classical trajectory simulation performed on the PES1(Br) analytic potential energy surface is used to study the atomic-level dynamics of the Cl-+CH3Br-->ClCH3+Br(-)S(N)2 nucleophilic substitution reaction. At low reactant relative translational energies E-rel of less than 5 kcal/mol, the reaction is dominated by an indirect mechanism in which the Cl-...CH3Br complex or both the Cl-...CH3Br and ClCH3...Br- complexes are formed. For E-rel>10 kcal/mol a direct reaction mechanism dominates without the formation of either complex. For E-rel of 5-10 kcal/mol there is a minimum in the S(N)2 rate constant which, for a CH3Br vibrational/rotational temperature T-vr of 300 K, is similar to400 times smaller than the rate at E-rel of 0.1 kcal/mol. The dependence of the trajectory S(N)2 rate constants on E-rel, T-v, and T-r is significantly different than the prediction of a statistical theoretical model. For E(rel)less than or equal to10 kcal/mol there is a much more pronounced decrease in the trajectory S(N)2 rate constant as E-rel is increased as compared to the statistical model, which arises from the inadequacy of the ion-molecule capture component of the statistical model. As E-rel is increased the trajectory Cl-+CH3Br association rate constant becomes much smaller than that predicted by the ion-molecule capture model. Increasing the CH3Br rotational temperature from 300 to 600 K decreases the trajectory S(N)2 rate constant more than the prediction of the statistical model. At low E-rel, where the reaction occurs by an indirect mechanism, the product energy is preferentially partitioned to CH3Cl vibration. For the direct mechanism, which dominates at high E-rel, the majority of the energy is partitioned to product relative translation. (C) 2003 American Institute of Physics. [References: 43]