Full dimensional potential energy surface and low temperature dynamics of the H 2 CO + OH → HCO + H 2 O reaction

Jiménez

et al. 2019

J. Phys. Chem. Lett.

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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.

supporting

confidence: 52%

“…10 of Ref. 24). Using this value, the total reaction rate constant obtained with the RPMD method, Eq.…”

mentioning

confidence: 99%

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

Mazo‐Sevillano

Jiménez

et al. 2019

J. Phys. Chem. Lett.

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“…All these methods are not directly applicable to treat the dynamics at low temperatures because for describing long range interactions properly the configuration space increases enormously. In this work we use a method recently proposed37–39 which consists in dividing the electronic Hamiltonian in two terms as H=Hdiab+HMB. H diab is an electronic diabatic matrix, in which each diagonal matrix element describes a rearrangement channel. In the case of H4+ and H5+ these terms were described by a triatomics-in-molecules method (TRIM)37,38, which is an extension of the diatomics-in-molecule (DIM)40,41, while in the case of H 2 CO+OH those terms were described by force fiels 39, as an extension of the reactive force field (RFF) approach42.…”

Section: Ab Initio Calculations and Analytical Pesmentioning

confidence: 99%

Low temperature reaction dynamics for CH₃OH + OH collisions on a new full dimensional potential energy surface

Roncero

Zanchet

Phys. Chem. Chem. Phys.

2018

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Is the rise of the rate constant measured in laval expansion experiments of OH with organic molecules at low temperatures due to the reaction between reactants or to the formation of complexes with the buffer gas?. This question has importance for understanding the evolution of prebiotic molecules observed in different astrophysical objects. Among these molecules methanol is one of the most widely observed, and its reaction with OH has been measured by several groups showing a fast increase of the rate constant under 100K. Transition state theory doesn’t reproduce this behavior and here dynamical calculations are performed on a new full dimensional potential energy surface developed for this purpose. The calculated classical reactive cross sections show an increase at low collision energies due to a complex forming mechanism. However, the calculated rate constant at temperatures below 100 K remains lower than the observed one. Quantum effects are likely responsible for the measured behavior at low temperatures.

“…The reaction dynamics was studied using a QCT method for H 2 CO+OH 33,40 and CH 3 OH +OH. 41 For the H 2 CO + OH reaction, QCT results mimic rather satisfactorily the experimental data, showing an increase below 300 K as the experimental results.…”

Section: Dynamics Of the Gas Phase Reaction: Zero Pressurementioning

confidence: 99%

“…In the two cases, the initial conditions are determined by the adiabatic switching method, [66][67][68][69] using the normal modes calculated at the bottom of the corresponding potential well, as described previously. 33,40 The interaction of He with H 2 CO is so weak and anharmonic that the system fragments when using all the vibrational modes. To avoid fragmentation, in this case the three modes associated with the van der Waals interaction were neglected.…”

Section: Collisions With He: High Pressure Effectsmentioning

confidence: 99%

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

Naumkin¹,

Mazo‐Sevillano

et al. 2019

ACS Earth Space Chem.

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A recent Ring Polymer Molecular Dynamics study of the reactions of OH with methanol and formaldehyde, at zero pressure and below 100 K, has shown the formation of long lived complexes, with long lifetimes, longer than 100 ns for the lower temperatures studied, 20-100 K (del Mazo-Sevillano et al., 2019). These long lifetimes support the existence of multi collision events with the He buffer-gas atoms under experimental conditions, as suggested by several transition state theory studies of these reactions. In this work we study these secondary collisions, as a dynamical approach to study pressure effects on these reactions. For this purpose, the potential energy surfaces of He with H 2 CO, OH, H 2 O and HCO are calculated at highly accurate ab initio level. The stability of some of the complexes is studied using Path Integral Molecular dynamics techniques, determining that OH-H 2 CO complexes can be formed up to 100 K or higher temperatures, while the weaker He-H 2 CO complexes dissociate at approximately 50 K. The predicted IR intensity spectra shows new features which could help the identification of the OH-H 2 CO complex. Finally, the He-H 2 CO + OH and OH-H 2 CO + He collisions are studied using quassi-classical trajectories, finding that the cross section to produce HCO + H 2 O products increases with decreasing collision energy, and that it is ten times higher in the He-H 2 CO + OH case.