Kinetics of the OH+HCl→H2 O+Cl reaction: Rate determining roles of stereodynamics and roaming and of quantum tunneling

The OH + HCl → H₂ O + 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.

Negative collision energy dependence of Br formation in the OH + HBr reaction

The reaction between HBr and OH leading to H(2)O and Br in its ground state is studied by means of a crossed molecular beam experiment for a collision energy varying from 0.05 to 0.26 eV, the initial OH being selected in the state |JOmega> = |3/2 3/2> by an electrostatic hexapole field. The reaction cross-section is found to decrease with increasing collision energy. This negative dependence suggests that there is no barrier on the potential energy surface for the formation pathway considered. The experimental results are compared with the previously reported quantum scattering calculations of Clary et al. (D. C. Clary, G. Nyman and R. Hernandez, J. Phys. Chem., 1994, 101, 3704), and briefly discussed in the light of skewed potential energy surfaces associated with heavy-light-heavy type reactions.