Benchmark ab Initio Characterization of the Complex Potential Energy Surface of the Cl^- + CH₃I Reaction

Benchmark stationary-point structures, vibrational frequencies, and classical/adiabatic relative energies (kcal/mol) are reported for the Cl^- + CH₃I reaction along the back-side attack (ΔE_TS = -5.48/-5.54) inversion, front-side attack (ΔE_TS = 36.73/35.89) and double-inversion (ΔE_TS = 46.97/42.55) retention S_N2 pathways, the proton-transfer channel, and the hydride-substitution reaction path. The structures and frequencies are obtained by the frozen-core CCSD(T), CCSD(T)-F12a, and CCSD(T)-F12b methods with the aug-cc-pVnZ [n = D, T, and Q for structures and n = D and T for frequencies] basis sets and all-electron CCSD(T) with aug-cc-pwCVnZ [n = D and T for structures and n = D for frequencies]. The benchmark relative energies are determined using the focal-point analysis approach based on electron correlation methods up to CCSDT(Q), extrapolations to the complete basis set limits using aug-cc-pVnZ [n = 2(D), 3(T), 4(Q), and 5] bases, core correlation contributions obtained at CCSD(T)/aug-cc-pwCVQZ, and, for the adiabatic energies, zero-point energy corrections at the CCSD(T)-F12b/aug-cc-pVTZ level of theory. We usually find significant method and modest basis dependence for the energies. The post-CCSD(T) and core correlation effects are often about 0.4 kcal/mol, but almost cancel each other. The explicitly correlated CCSD(T)-F12 methods are recommended for geometry and frequency computations as well as for energy computations if the basis set dependence is significant.