TRAJECTORY STUDIES OF S(N)2 NUCLEOPHILIC-SUBSTITUTION - 6 - TRANSLATIONAL ACTIVATION OF THE CL-+CH3CL REACTION

Classical trajectory simulations are used to study the translational a ctivation of the Cl- + CH3Cl S(N)2 reaction at energies in the range 2 0-80 kcal/mol. The trajectories are calculated on the PES3 analytic po tential energy surface. The shape of the reactive cross section versus relative translational energy E-rcl and the translational threshold o f 18 kcal/mol are both similar to recent experimental results [J. Phys . Chem. A 1997, 101, 5969]. The reactive trajectories are direct, with negligible trapping in the ion-dipole complexes. The product energy i s primarily partitioned to relative translation with small and similar amounts of energy partitioned to vibration and rotation. The velocity scattering angle distribution suggests backward scattering and a rebo und mechanism for translational activation at low E-rel, with increasi ng importance of forward scattering and a stripping mechanism as E-rel is increased. An analysis of angular momenta terms and their correlat ions shows that the total angular momentum is well-approximated by the initial orbital angular momentum, which is strongly correlated with t he final orbital angular momentum. The decrease in the reactive cross section with CD3Cl isotopic substitution is consistent with the experi ments. The principal difference between the trajectories and experimen ts is the order of magnitude larger cross sections found from the traj ectories. No pronounced inadequacies in the PES3 potential energy surf ace are evident from comparisons with MP2/6-311+G* ab initio calculat ions.