Quantum mechanical investigation on the formation of silicate building blocks on interstellar ice mantles

Silicon monoxide (SiO), a diatomic molecule present in the interstellar medium, probably contributes to the formation of silicate dust grains since orthosilicic acid (Si(OH)4), a SiO derivative, is a precursor of silica and silicates. The formation of Si(OH)4 is investigated here by means of quantum chemical computations, characterizing the interaction and reactivity of SiO with water ice mantles (modelled through different cluster systems) that coat interstellar dust grains in dense clouds. Several reaction pathways involving SiO and H2O were simulated with the aim of forming Si(OH)2 and subsequently Si(OH)4. Results highlight that the ice mantle not only supplies a reactant, H2O, but also acts as a chemical catalyst, reducing the energy barrier of the process. The formation of the Si(OH)2 precursor is a crucial step, occurring easily by reaction of SiO with H2O. Subsequently, both the Si(OH)2 −−→ +O OSi(OH)2 −−−→ +H2O Si(OH)4 and Si(OH)2 −−→ +2O O2Si(OH)2 −−→ +2H Si(OH)4 paths have been found to be energetically favourable towards Si(OH)4 formation due to being barrierless, although formation of H2Si(OH)2 by H addition to Si(OH)2 is a potential competitive channel. This study provides evidence that orthosilicic acid can form due to the addition of H2O, O, and H to SiO on water ice mantles, indicating that silicate grains can potentially form and grow in cold interstellar regions, in addition to originating from material ejected by asymptotic giant branch stars and supernovae remnants. Our results show that grain surface reactions in dense clouds are important not only in the synthesis of volatile molecular species but also in the formation of silicate refractory material precursors.