Kinetics of the OH+HCl→H2O+Cl reaction: Rate determining roles of stereodynamics and roaming and of quantum tunneling

The OH + HCl → H2O + Cl reaction is one of the most studied four‐body systems, extensively investigated by both experimental and theoretical approaches. Here, as a continuation of our previous work on the OH + HBr and OH + HI reactions, which manifest an anti‐Arrhenius behavior that was explained by stereodynamic and roaming effects, we extend the strategy to understand the transition to the sub‐Arrhenius behavior occurring for the HCl case. As previously, we perform first‐principles on‐the‐fly Born–Oppenheimer molecular dynamics calculations, thermalized at four temperatures (50, 200, 350, and 500 K), but this time we also apply a high‐level transition‐state‐theory, modified to account for tunneling conditions. We find that the theoretical rate constants calculated with Bell tunneling corrections are in good agreement with extensive experimental data available for this reaction in the ample temperature range: (i) simulations show that the roles of molecular orientation in promoting this reaction and of roaming in finding the favorable path are minor than in the HBr and HI cases, and (ii) dominating is the effect of quantum mechanical penetration through the energy barrier along the reaction path on the potential energy surface. The discussion of these results provides clarification of the origin on different non‐Arrhenius mechanisms observed along this series of reactions. © 2018 Wiley Periodicals, Inc. This article is part of a series aiming at elucidating the mechanisms involved in the non‐Arrhenius behavior of the four‐body OH + HX (X = F,Cl, Br, and I) reactions, assessing the influence of stereodynamics and roaming, and of quantum tunneling in kinetics of the OH + HCl reaction. If on the one the hand, OH + HBr and OH + HI reactions manifest anti‐Arrhenius behavior, explained by stereodirectional and roaming effects. On the other hand, the case of HCl involves a drastic change in the experimental and theoretical Arrhenius plot, which exhibit a sub‐Arrhenius character.