Benchmark ab initio 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 HOH⋯C₂H₅, HOH⋯CH₃, H₃C⋯CH₃OH, and H⋯C₂H₅OH complexes are characterized with D_e values of 2.4, 1.7, 0.7, and 0.3 kcal mol⁻¹, respectively.