Revisiting the OH + H₂ → H₂O + H reaction at the molecular level: the plausible catalytic role of ice in its own reconstruction

¹ Sorbonne Universités, UPMC Univ. Paris 06, UMR, CNRS 7616, Laboratoire de Chimie Théorique, 75252 Paris Cedex 05, France
e-mail: yves.ellinger@sorbonne-universite.fr
² Universidad de Valladolid, Facultad de Ciencias (Dpto. QF), Valladolid, Spain
e-mail: predondo@qf.uva.es

Received: 4 August 2020
Accepted: 26 December 2020

Abstract

Context. In spite of the permanent damage suffered from the radiation field (cosmic rays, X-rays, and intense UV-visible radiations), interstellar grains are still covered by ices mantles whose role in interstellar chemistry is well beyond any doubt. This clearly means that the destruction of the ice cover has to be counterbalanced by efficient reconstruction mechanisms.

Aims. Our goal is to determine whether the ice, which is still present after irradiation, has a catalytic role in the OH + H₂ → H₂O + H reaction for its own reconstruction. We focus on the three plausible reaction paths depending on the way reactants OH or H₂ are adsorbed at the ice surface.

Methods. Calculations were performed in both cluster and solid state approaches, using ab-initio post Hartree-Fock methods for small systems, standard density functional theory (DFT) for larger clusters, and periodic solid state DFT with specific formalisms accounting for weak interactions in systems of infinite dimensions.

Results. Although the end product is the same, that is namely the reconstruction of one H₂O on the subjacent ice, three different reaction paths are found depending on whether H₂ reacts with adsorbed OH(ads), wether OH reacts with adsorbed H₂(ads) or wether both OH(ads) and H₂(ads) are adsorbed on the ice before reacting. In the first case, there is an activation barrier of ~6 kcal mol⁻¹, requiring the tunneling effect for the reaction to proceed, which is in agreement with preceding studies. In the second case, the reaction is a barrierless process leading to the direct reconstruction of the ice. In the third case, the double adsorption increases the activation barrier due to the lowering of the starting energy. This is found regardless of the dimension of the supporting ice aggregates.

Conclusions. Icy grain surfaces play a critical role for their own reconstruction in cold, dense interstellar clouds. The prevalence of tunneling over the direct mechanism should strongly depend on the temperature and local environment.