Trajectory studies of S_N2 nucleophilic substitution. III. Dynamical stereochemistry and energy transfer pathways for the Cl ^-+CH₃CI association and direct substitution reactions

Classical trajectory calculations are performed to determine differences in the microscopic dynamics for two fundamental processes for the Cl _a ^-+CH₃Cl_b→Cl_aCH ₃Cl_b ^- reactive system: Cl_a ^--CH₃Cl_b complex formation and directly attaining the [Cl_a-CH₃-Cl_b]^- central barrier without first forming the complex. This latter process becomes important when the C-Cl_b stretch mode is excited in the CH ₃Cl_b reactant. The total cross section for complex formation and directly attaining the central barrier increases as n _C-Clb is increased. The value for the Cl_a ^--C-Cl_b angle θ as the reactants interact, the dynamical stereochemistry, is found to be a very important property for distinguishing between the mechanisms for the two fundamental processes. Directly attaining the central barrier requires oriented reactants with θ≈π, while orientation suppresses complex formation. Substantial reactant orientation only occurs for CH₃Cl_b rotational temperatures less than 300 K. The complex is formed by a T→R energy transfer process which involves coupling between the reactant orbital angular momentum and CH₃Cl_b rotational angular momentum. Complex formation does not involve energy transfer to the CH₃Cl_b vibrational modes, which is consistent with an earlier finding that the CH ₃Cl_b intramolecular modes are inactive during decomposition of the Cl_a ^--CH₃Cl_b complex. Orientation of CH₃Cl_b enhances coupling between the Cl_a ^-+CH₃Cl_b radial motion and the C-Cl_b stretch mode. This coupling leads to the above direct substitution process and extensive deactivation of the excited C-Cl_b stretch during direct unreactive collisions. Considerably less deactivation results from Cl_a ^--CH₃Cl_b complex formation followed by dissociation to the reactants. Rotationally exciting CH₃Cl_b eliminates orientation and, thus, suppresses deactivation of the C-Cl_b stretch.