The reaction of stratospheric methane with chlorine atoms is a termination step in the catalytic Cl/ClO ozone destruction cycle. It is also a potential mechanism for the elimination of stratospheric methane, a potent greenhouse gas. Accurate kinetic parameters such as activation energy and pre-exponential factor are valuable for modeling these processes. Computational modeling employing high-level methods such as CCSD(T) with large basis sets have determined kinetic parameters with sub-kJ mol–1 accuracy but at considerable computational cost. We have calculated gas-phase transition state enthalpy for the reaction CH4 + Cl. → CH3. + HCl employing recently developed DFT functionals that show broad agreement when tested against thermochemistry and thermochemical kinetics data sets. The functionals, which represent rungs three and four of Jacob’s Ladder include long-range corrected and double-hybrid functionals with and without dispersion correction (MPW1K, MPWB1K, PW6BK, PW6B95, CAM-B3LYP, wB97X, wB97X-D, M06-2X, M11, MN15, B2PLYP and B2PLYPD) with the large basis set aug-cc-pVQZ to minimize basis set superposition error. Transition state enthalpy was also calculated as a single-point energy calculation at the CCSD(T)/CBS level based on M06-2X transition state molecular geometry. All DFT functionals gave very similar molecular geometries. Comparison of calculated activation enthalpies with experiment gave absolute deviations |δΔH‡| ranging from 2.1 kJ mol–1 to 8.7 kJ mol–1. In comparison, activation enthalpy at the CCSD(T) level deviates from experimental by 0.4 kJ mol–1. Three DFT functionals (MPWB1K, M06-2X, and MN15) gave deviations from experiment of 4 kJ mol–1 or less and represent a reasonable compromise between accuracy and computational effort in theoretical investigations of this reaction.

Predicting the enthalpy of activation for abstraction of a hydrogen atom from methane by a chlorine atom: Benchmark comparison of recent DFT functionals

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.