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 stat e theory are used to calculate reaction energetics and rate constants for t he gas-phase reactions of OH- with CH(4-n)Cln for n = 1-4. Two reaction pat hways are considered, second-order (bimolecular) nucleophilic substitution (S(N)2), and proton transfer. Benchmark electronic structure calculations u sing 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 experimen t 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 (reac tant complexes, saddle points, and product complexes) are computed for all systems. For the SN2 reaction, the potential energy and its first and secon d derivatives along minimum energy paths are computed and used directly in variational transition state theory (VTST) calculations of the rate constan ts. These calculations indicate that for n = 1-3 the region of the potentia l in the asymptotic reactant channel controls the reaction rate constants a nd 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 trea ted using the VTST methods implemented in POLYRATE.