Dual-level strategy for quantitative kinetics for the reaction between ethylene and hydroxyl radical

The atmospheric reactions are mainly initiated by hydroxyl radical (OH). Here, we choose the C2H4 + OH reaction as a model reaction for other reactions of OH with alkenes. We use the GMM(P).L//CCSD(T)-F12a/cc-pVTZ-F12 theoretical method as the benchmark results close to the approximation of CCSDTQ(P)/CBS accuracy to investigate the C2H4 + OH reaction. The rate constants for the C2H4 + OH reaction at high-pressure limit were calculated by using the dual-level strategy. It integrates the transition state theory rate constant calculated by GMM(P).L//CCSD(T)-F12a/cc-pVTZ-F12 with the canonical variational transition state theory containing small-curvature tunneling (CVT/SCT) calculated by using the M11-L functional method with the MG3S basis set. The rate constants of C2H4 + OH at different pressures were obtained by using both the system-specific quantum Rice-Ramsperger-Kassel (SS-QRRK) theory and master equation method. The calculated results uncover that both the calculated rate constants at different pressures and temperatures are quantitatively consistent with the values obtained by the experimental measurements in the C2H4 + OH reaction. We find that the post-CCSD(T) contributions to the barrier height for the C2H4 + OH reaction are significant with the calculated value of -0.38 kcal/mol. We also find that the rate determining step is only dominated by the tight transition state under atmospheric conditions, whereas previous investigations indicated that the rate constants were controlled by both the loose and tight transition states in the C2H4 + OH reaction. The present findings unravel that it is an important factor for the effect of torsional anharmonicity on quantitative kinetics.