Quantum Roaming in the Complex-Forming Mechanism of the Reactions of OH with Formaldehyde and Methanol at Low Temperature and Zero Pressure: A Ring Polymer Molecular Dynamics Approach

Abstract

The quantum dynamics of the title reactions are studied using the Ring Polymer Molecular Dynamics (RPMD) method from 20 to 1200 K using recently proposed full dimensional potential energy surfaces which include long range dipole-dipole interactions. A V-shaped dependence of the reaction rate constants is found with a minimum at 200-300 K, in rather good agreement with the current experimental data. For temperatures above 300 K the reaction proceeds following a direct H-abstraction mechanism. However, below 100 K the reaction proceeds via organic-molecule⋯OH collision complexes, with very long lifetimes, longer than 10 −7 s, associated to quantum roaming arising from the inclusion of quantum effects by the use of RPMD. The long lifetimes of these complexes are comparable to the time scale of the tunnelling to form reaction products. These complexes are formed at zero pressure due to quantum effects and not only at high pressure as suggested by Transition State Theory (TST) calculations for OH + methanol and other OH reactions. The zero-pressure rate constants reproduce quite well measured ones below 200 K, and this agreement opens the question of how important the pressure effects on the reaction rate constants are, as implied in TST-like formalisms. The zero pressure mechanism is the only applicable to very low gas density environments, such as the interstellar medium, unrepeatable by the experiments.

Experimental Studies of Gas-Phase Reactivity in Relation to Complex Organic Molecules in Star-Forming Regions

“…[214][215][216][217] by several different groups. The reaction was first studied at low temperature by the Leeds group, who measured the rate coefficient by OH LIF at 63 K and 80 K. They found the rate coefficient for the reaction between OH and methanol was almost two orders of magnitude larger at…”

Zero- and High-Pressure Mechanisms in the Complex Forming Reactions of OH with Methanol and Formaldehyde at Low Temperatures

“…However, quantum effects are important at low temperatures, such as tunneling and zero-point energy (ZPE). In order to include these effects, very recently Ring Polymer Molecular Dynamics (RPMD) calculations were performed on the reactions of OH with formaldehyde and methanol 42 showing the formation of extremely long lived collision complexes, with lifetimes longer than 100 ns, and a very large capture rate. This finding opens the possibility that the zeropressure reaction cross section of these reactions also increases below 100 K, and it is important to determine as accurately as possible such effects.…”

“…RPMD method, which has already shown itself as a reliable alternative [56][57][58][59][60] was used recently to study the reactions of OH with formaldehyde and methanol. 42 RPMD is a semiclassical formalism based on Path Integral Molecular Dynamics (PIMD) which includes quantum effects as ZPE 61 and tunneling, 62 and it has been demonstrated to be a very powerful technique to describe low-temperature reaction dynamics. 60? In Ref., 42 the RPMD description of the two reactions under study followed two very different mechanisms, one direct at T> 200 K and a second indirect at T<200 K, as it is shown in Fig.…”

“…In the case of CH 3 OH + OH, the k tunnel (T)/[k tunnel (T) + k rediss (T)] ratio was obtained from the low pressure TST studies performed by Ocaña et al, 30 following the method explained before. 42 The total reaction rate constant is the sum of the direct and complex forming contribution. In this study, we have added the results obtained for 20 K for the CH 3 OH + OH reaction, which were not included before, and that are shown in Fig.…”

Gas phase reaction kinetics of complex organic molecules at temperatures of the interstellar medium: The OH + CH3OH case

“…All these results have been obtained using CRESU reactors (French acronym for Cinétique de Réaction en Ecoulement Supersonique Uniforme or Reaction Kinetics in Uniform Supersonic Flows), a well-established technique for the study of gas-phase reaction kinetics at very low temperatures (Potapov et al 2017). From a theoretical point of view the interest for reactions involving COMs has considerably grown recently and several reactions mentioned in Table 1 have been investigated at interstellar temperatures with a special attention paid on OH + CH 3 OH (Shannon et al 2013;Gao et al 2018;Roncero et al 2018;Ocaña et al 2019;Nguyen et al 2019;del Mazo-Sevillano et al 2019) because of its potential importance in the formation of CH 3 O (see next section). In the present paper we will review the recent experimental and theoretical works carried out concerning this reaction and will comment about its significance for the chemistry of molecular clouds.…”