Trajectory Studies of SN2 Nucleophilic Substitution. 8. Central Barrier Dynamics for Gas Phase Cl- + CH3Cl

Abstract

Quasiclassical direct dynamics trajectories, calculated at the MP2/6-31G* level of theory, are used to study the central barrier dynamics for the C1^- + CH₃Cl S_N2 reaction. Extensive recrossings of the central barrier are observed in the trajectories. The dynamics of the Cl^-−CH₃Cl complex is non-RRKM and transition state theory (TST) is predicted to be an inaccurate model for calculating the Cl^- + CH₃Cl S_N2 rate constant. Direct dynamics trajectories also show that Cl^- + CH₃Cl trajectories, which collide backside along the S_N2 reaction path, do not form the Cl^-−CH₃Cl complex. This arises from weak coupling between the Cl^-−CH₃Cl intermolecular and CH₃Cl intramolecular modes. The trajectory results are very similar to those of a previous trajectory study, based on a HF/6-31G* analytic potential energy function, which gives a less accurate representation of the central barrier region of the Cl^- + CH₃Cl reaction than does the MP2/6-31G* level of theory used here. Experiments are suggested for investigating the non-RRKM and non-TST dynamics predicted by the trajectories.