Benchmark <i>ab initio</i> characterization of the abstraction and substitution pathways of the OH + CH₄/C₂H₆ reactions

Abstract

We report a comprehensive ab initio investigation of the OH + CH4/C2H6 reactions using a high-level composite approach based on CCSD(T)-F12b/aug-cc-pVTZ geometries and CCSD(T)-F12b/aug-cc-pVnZ n = 5/Q energies augmented with additive corrections of post-CCSD(T) correlation, core correlation, scalar relativity, spin-orbit coupling, and zero-point energy. Besides the hydrogen-abstraction (HA) channel leading to H2O + CH3/C2H5 (ΔH0 = -14.37/-18.19 kcal mol-1), we reveal, for the first time, hydrogen-substitution (HS) and methyl-substitution (MS) pathways resulting in H + CH3OH/C2H5OH (ΔH0 = 13.19/7.12 kcal mol-1) and CH3 + CH3OH (ΔH0 = -2.20 kcal mol-1) products, respectively. The adiabatic barrier heights for HA, MS, and HS in OH + CH4/C2H6 are 4.78/2.18, 39.60, 43.53/41.73(52.48) kcal mol-1, respectively, where substitution proceeds with Walden-inversion or (front-side-attack retention). In the entrance channels van der Waals wells with depths of 0.5-0.8 kcal mol-1 are found and in the exit channels the HOHC2H5, HOHCH3, H3CCH3OH, and HC2H5OH complexes are characterized with De values of 2.4, 1.7, 0.7, and 0.3 kcal mol-1, respectively.