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

Abstract

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.

Temperature Dependence of Rate Processes Beyond Arrhenius and Eyring: Activation and Transitivity

“…However, calculation of reaction rate constants has been limited by the arduous procedures both to accurately characterize reactive activated complexes of many body systems and to overcome the inherent difficulties of producing a number of trajectories with statistical consistency and reasonable completeness in the filling of the dynamically relevant parts of the phase-space. Recently, several works have been yielding values with reliable accuracy: overestimates due to limited sampling of phase-space, when experimental values are available for comparison, may exploit empirical calibration (Coutinho et al, 2016, 2017, 2018a).…”

“Transitivity”: A Code for Computing Kinetic and Related Parameters in Chemical Transformations and Transport Phenomena

“…The electronic structure properties of the reactants, of the products, and of the transition state were calculated employing the MP2/aug-cc-pVDZ calculation level using Gaussian 09 [89]. A complete study of this reaction using our methodology can be found in Reference [44].…”

“…A characterizing feature of the code is the consistent use of the d-formulation, which recently culminated in a series of successful applications from phenomenological to first-principles descriptions of pure and applied chemical kinetics and material science. Examples are available:From the phenomenology of elementary processes (such as the H 2 + F [94], OH + HBr [80], F + HD [65] and C + CH + [95] reactions) to complex processes (such as food systems [96], plant respiration [25], plasma chemistry [97], and solid-state diffusive reaction [98]);Calculation of the kinetic rate constants for chemical reactions from the potential energy surface features profile, such as the CH 4 + OH [60], CH 3 OH + H [99], OH + HCl [44], OH + HI [43], to proton rearrangement of enol forms of curcumin [100], OH + H 2 [101], and chiral nucleophilic substitution reaction [102]. …”

“…Calculation of the kinetic rate constants for chemical reactions from the potential energy surface features profile, such as the CH 4 + OH [60], CH 3 OH + H [99], OH + HCl [44], OH + HI [43], to proton rearrangement of enol forms of curcumin [100], OH + H 2 [101], and chiral nucleophilic substitution reaction [102].…”

“…Recently, there is a vast activity evaluating whether advances in molecular dynamics simulations can provide quantitatively rate constants [39,40,41]. However, the extraction of quantitative information on rate constants from molecular dynamics simulations is an important issue but a very difficult one to tackle [42,43,44].…”