A Six-Body Potential Energy Surface for the S_N2 Reaction Cl^N-(g) + CH₃Cl(g) and a Variational Transition-State-Theory Calculation of the Rate Constant

Correlated calculations (MP2/6-31G^***) of the energies and frequencies at the saddle point, ion-dipole complex, and reactants plus additional energy calculations at selected geometries in the strong interaction region are used to parameterize a multidimensional potential energy function for the title reaction. Semiclassical variational transition-state theory is used to calculate the gas-phase rate coefficient and to determine a semiempirical value of the barrier height to reproduce the experimental value of this coefficient at 300 K. The semiempirical gas-phase barrier height is 3.1 kcal/mol. A new potential energy function with this barrier height is created and used to calculate the temperature-dependent rate coefficients and phenomenological activation energies for both CH₃C1 and CD₃C1 over the 200-1000 K temperature range. The activation energy is predicted to show a large temperature dependence. The kinetic isotope effect is predicted to be 1.04 at room temperature.