Models for intrinsic Non-RRKM dynamics. Decomposition of the S_N2 Intermediate Cl^--CH₃Br

Chemical dynamics simulations, based on both an analytic potential energy surface (PES) and direct dynamics, were used to investigate the intrinsic non-RRKM dynamics of the Cl^--CH₃Br ion-dipole complex, an important intermediate in the Cl^-+CH₃Br S_N2 nucleophilic substitution reaction. This intermediate may dissociate to Cl ^-+CH₃Br or isomerize to the ClCH₃-Br ^- ion-dipole complex. The decomposition of microcanonical ensembles of the Cl^--CH₃Br intermediate were simulated, and the ensuing populations vs. Time of the excited intermediate and Cl ^-+CH₃Br and ClCH₃-Br^- products were fit with multi-exponential functions. The intrinsic non-RRKM dynamics is more pronounced for the simulations with the analytic PES than by direct dynamics, with the populations for the former and latter primarily represented by tri-and bi-exponential functions, respectively. For the analytic PES and direct dynamics simulations, the intrinsic non-RRKM dynamics is more important for the isomerization pathway to form ClCH₃-Br^- than for dissociation to Cl^-+CH₃Br. Since the decomposition probability of Cl^--CH₃Br is non-exponential, the Cl ^--CH₃Br unimolecular rate constant depends on pressure, with both high and low pressure limits. The high pressure limit is the RRKM rate constant and for the simulations with the analytic PES the rate constant decreased by a factor of 3.0, 5.6, and 4.3 in going from the high to low pressure limit for total energies of 40, 60, and 80 kcal/mol. For the direct dynamics simulations these respective factors are 2.4, 1.4, and 1.2. A separable phase space model with intermolecular and intramolecular complexes describes some of the simulation results, but overall models advanced for intrinsic non-RRKM dynamics give incomplete representations of the intermediate and product populations vs. Time determined from the simulations.