Use of an improved ion-solvent potential-energy function to calculate the reaction rate and α-deuterium and microsolvation kinetic isotope effects for the gas-phase S_N2 reaction of Cl^-(H₂O) with CH₃Cl

We present calculations of the rate constants and secondary kinetic isotope effects for the gas-phase S_N2 reaction Cl^-(H ₂O)+CH₃Cl based on a new chloride-water potential-energy function that has been specifically converged for heavy-water isotope effects. The results are compared to new calculations employing five chloride-water potential-energy functions that have been developed for simulations of aqueous solutions. In all calculations the ClCH₃Cl^- solute intramolecular potential is taken from a previous semiglobal fit to ab initio calculations including electron correlation. We also examine two different intramolecular water potentials, and we examine the effect of treating the CH₃ internal rotation at the ClCH₃Cl^-(H ₂O) transition state as a hindered rotation. Both the CH ₃/CD₃ (α-deuterium) and H₂O/D ₂O (microsolvation) kinetic isotope effects are studied.