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

We compute benchmark structures, frequencies, and relative energies for the stationary points of the potential energy surface of the F + CHCHCl 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 S2 channel leading to Cl + CHCHF (-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 + CH (-19.3), can be formed via anti (-15.0) and syn (-7.3) saddle points, whose best adiabatic energies relative to F + CHCHCl are shown in parentheses in kcal/mol. Besides the S2 and E2 channels, the 0 K reaction enthalpies of the HF + HC-CHCl (29.4), H + HC-CHClF (46.2), H + FHC-CHCl (51.1), and FCl + CHCH (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.