Adsorption of HCN on cosmic silicates: a periodic quantum mechanical study

Abstract

Hydrogen cyanide (HCN) represents a small but widely distributed fraction of the interstellar molecules, and it has been observed in all the environments characterizing the formation of a new planetary system. HCN can polymerize to form biomolecules, including adenine (H₅C₅N₅), and it has drawn attention as a possible precursor of several building blocks of life due to the presence of its polymerization products in meteorites, comets and other asteroidal bodies. To elucidate the potential catalytic role that cosmic silicates have played in these processes, we have investigated, at DFT-PBE level inclusive of a posteriori dispersion correction, the energetic and spectroscopic features of the adsorption of HCN molecules on the most relevant crystalline surfaces of the mineral forsterite (Mg₂SiO₄), a common silicate constituent of the interstellar core grains and planetary rocky bodies. The results reveal that HCN adsorbs both in molecular and dissociative ways, within a wide range of adsorption energies (−29.4 to −466.4 kJ mol⁻¹). Thermodynamic and kinetic results show that dissociative adsorption is dominant already at low temperatures, a fact particularly relevant at the protoplanetary conditions (i.e., the latest stages in the star system formation process). The simulated spectroscopic features of the studied adducts show a wide range of different degrees of perturbation of C–H and CN bonds. This finding agrees with previous experimental works, and our results confirm that a complex chemistry is observed when this astrochemically-relevant molecule interacts with Mg₂SiO₄, which may be associated with a considerable potential reactivity towards the formation of relevant prebiotic compounds.