Adsorption of atomic hydrogen as driving vector for solid-gas processes on ices

Abstract

ABSTRACT A consensus has progressively developed that the path towards complex organic molecules could be favoured by the icy mantles of interstellar dust particles. However, the question of whether activation energy is required to promote those reactions is open. This work deals with the simplest process thought to increase complexity i.e. successive additions of a single atom (H). Two situations can be considered, the direct additions of H in the gas phase and the atomic hydrogenation processes on water ice surfaces available in the interstellar medium (ISM). The synthesis of methanol, largely discussed in the literature is revisited as a case study. Computational investigations of the reactions leading from CO to CH3OH show that, with [H...(H2O)n] complexes as hydrogenation vectors, all steps of the process are barrier-less, contrary to the gas phase process and this, whatever the level of theory considered. The key parameter is the position of the CO + [H...(H2O)n] initial system on the global energy scale. At the present level of theory, when H is pre-adsorbed on the ice, the system happens to be high enough above the reaction path, namely, above any intermediates and possible transition states. This is true whatever the dimension of the ice support, even for the simplest model of one H2O molecule. Application of such a simplified synthetic approach, here validated for the synthesis of methanol, could be generalized, providing a simple way to get a fair insight into the important class of atomic hydrogenations on ices in the ISM.