Trajectory studies of S(N)2 nucleophilic substitution. 8. Central barrier dynamics for gas phase Cl-+CH3Cl

Quasiclassical direct dynamics trajectories, calculated at the MP2/6-31G* l evel of theory, are used to study the central barrier dynamics for the Cl- + CH3Cl S(N)2 reaction. Extensive recrossings of the central barrier are ob served in the trajectories. The dynamics of the Cl--CH3Cl complex is non-RR KM and transition state theory (TST) is predicted to be an inaccurate model for calculating the Cl- + CH3Cl S(N)2 rate constant. Direct dynamics traje ctories also show that Cl- + CH3Cl trajectories, which collide backside alo ng the S(N)2 reaction path, do not form the Cl--CH3Cl complex. This arises from weak coupling between the Cl--CH3Cl intermolecular and CH3Cl intramole cular 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 o f the Cl- + CH3Cl reaction than does the MP2/6-31G* level of theory used he re. Experiments are suggested for investigating the non-RRKM and non-TST dy namics predicted by the trajectories.