A systematic study of the reactions of OH^- with chlorinated methanes. 1. Benchmark studies of the gas-phase reactions

Ab initio electronic structure calculations and variational transition state theory are used to calculate reaction energetics and rate constants for the gas-phase reactions of OH^- with CH_(4-n)Cl_n for n = 1-4. Two reaction pathways are considered, second-order (bimolecular) nucleophilic substitution (S_N2), and proton transfer. Benchmark electronic structure calculations using CCSD(T) and basis sets as large as aug-cc-pVQZ are performed to obtain highly accurate estimates of the enthalpies of reaction. These results are extrapolated to the complete basis set limit for comparison with experiment and to establish the level of theory needed to provide energies that are accurate to better than a few kJ/mol. Energies of critical geometries (reactant complexes, saddle points, and product complexes) are computed for all systems. For the S_N2 reaction, the potential energy and its first and second derivatives along minimum energy paths are computed and used directly in variational transition state theory (VTST) calculations of the rate constants. These calculations indicate that for n = 1-3 the region of the potential in the asymptotic reactant channel controls the reaction rate constants and that the loose-transition-state methods implemented in VARIFLEX provide the best estimates of the reaction rate constants. The reaction with n = 4 has a dynamical bottleneck that lies near the saddle point and is best treated using the VTST methods implemented in POLYRATE.