Direct and steady state rate constants of C2H6 + X (X = H, Cl, OH): influence of the van der Waals well

The reaction of ethane with an atom or radical is the benchmark system to study the kinetics and dynamics of the polyatomic chemical reaction. The rate constants and kinetic isotope effects for C2H6+ X (X = H, Cl, OH) reactions are calculated using the quantum instanton method with the path integral Monte Carlo technique. In order to reveal the influence of the van der Waals well at the reactant complex on rate constants, both the direct and steady state rate constants are computed. Owing to the absence of the recrossing effect, the quantum instanton method overestimates the rate constants of C2H6 + H by several tens of percent as compared to the RPMD and experimental results. The direct rate constants are always larger than the steady state ones for C2H6 + Cl and C2H6 + OH, which is mainly due to the difference of free energy barrier height and quantum tunneling. The results reveal that if the reaction barrier height remains unchanged, the experimental rate constants will switch from the direct ones to the steady state ones upon increasing the depth of the van der Waals well. In addition, the direct kinetic isotope effects are consistent with experiment, while they become smaller than the steady state kinetic isotope effects as the van der Waals well is deepened.