Benchmark ab Initio Characterization of the Complex Potential Energy Surface of the F- + CH3CH2Cl Reaction.

We compute benchmark structures, frequencies, and relative energies for the stationary points of the potential energy surface of the F- + CH3CH2Cl reaction using explicitly correlated ab initio levels of theory. CCSD(T)-F12b geometries and harmonic vibrational frequencies are obtained with the aug-cc-pVTZ and aug-cc-pVDZ basis sets, respectively. The benchmark relative energies are determined using a high-level composite method based on CCSD(T)-F12b/aug-cc-pVQZ frozen-core energies, CCSD(T)-F12b/cc-pCVTZ-F12 core electron correlation effects, and CCSD(T)-F12b/aug-cc-pVDZ zero-point energy corrections. The SN2 channel leading to Cl- + CH3CH2F (-33.2) can proceed via back-side (-11.5), front-side (29.1), and double-inversion (18.0) transition states, whereas the bimolecular elimination (E2) products, Cl- + HF + C2H4 (-19.3), can be formed via anti (-15.0) and syn (-7.3) saddle points, whose best adiabatic energies relative to F- + CH3CH2Cl are shown in parentheses in kcal/mol. Besides the SN2 and E2 channels, the 0 K reaction enthalpies of the HF + H3C-CHCl- (29.4), H- + H3C-CHClF (46.2), H- + FH2C-CH2Cl (51.1), and FCl- + CH3CH2 (49.7) product channels are determined. Utilizing the new benchmark data, the performance of the DF-MP2, MP2, MP2-F12, CCSD(T), and CCSD(T)-F12b methods with aug-cc-pVDZ and aug-cc-pVTZ basis sets is tested.