Detailed benchmark ab initio mapping of the potential energy surfaces of the X + C2H6 [X = F, Cl, Br, I] reactions

Abstract

We investigate three reaction pathways of the X + C₂H₆ [X = F, Cl, Br, I] reactions: H-abstraction, methyl-substitution, and H-substitution, with the latter two proceeding via either Walden-inversion or front-side-attack mechanisms. We report classical and adiabatic relative energies of unprecedented accuracy for the corresponding stationary points of the reaction potential energy surfaces (PESs) by augmenting the CCSD(T)-F12b/aug-cc-pVQZ energies by core-correlation, post-CCSD(T) and spin–orbit corrections. Taking these correction terms into account turns out to be essential to reach subchemical, i.e. <0.5 kcal mol⁻¹, accuracy. Our new benchmark 0 K reaction enthalpies show excellent agreement with experimental data. Spin–orbit coupling in these open-shell systems is also monitored throughout the reaction paths and found to be non-negligible even in some transition-state geometries. Barrier heights corresponding to the different channels of the title reactions appear in the same order with increasing energy: H-abstraction, Walden-inversion methyl-substitution, Walden-inversion H-substitution, front-side-attack H-substitution and front-side-attack methyl-substitution, except for X = I where the latter two come in reverse order. Similarly, product channels follow the energy order of the corresponding barrier heights in all four cases. We find strongly reactant-like transition-state structures for the exothermic F + C₂H₆ reaction paths, while more and more product-like transition states are observed along with increasing endothermicity as going from Cl to I. Several entrance and exit channel minima are also identified for the studied reactions with significant spin–orbit effects for the formers.