Phenomena at Interstellar Core and Mantle Grains: a Quantum Mechanical Approach

The evolution of an interstellar cloud into a planetary system is inextricably linked to the growth of chemical complexity. In the interstellar medium (ISM), chemical reactions occur either in the gas phase or on the surface of dust grains, which are refractory nanoparticles made of siliceous or carbonaceous material. In the coldest regions of space, these dust grains are coated with water-dominated ice mantles, and they play a crucial role in the growth of molecular complexity. This thesis aims to advance our comprehension of the physico-chemical processes that are regulated by dust grains through the use of computational chemistry techniques. Particular emphasis is placed on the adsorption processes that influence the chemical composition of the gas phase, especially with respect to S-bearing species, whose main reservoirs during the early stages of planetary formation remain unidentified. Furthermore, a combined experimental and theoretical determination of the binding energies, which govern the adsorption and desorption processes, reveals the significance of employing both approaches to enhance the interpretation of experimental outcomes and to overcome their respective limitations. Furthermore, the thesis investigates the role of chemical reactions on the surface of dust grains in the growth of chemical complexity in the ISM. The formation mechanisms of complex organic molecules, defined as species containing six or more atoms, including at least one carbon atom, are characterised. In particular, this research investigates the formation mechanisms of acetaldehyde, ethanol and urea, with a focus on reactions involving both closed-shell and radical species. The findings reveal that dust grains play a variety of roles in these processes, including supplying and concentrating reactants on their surface, acting as catalysts and, eventually, as third bodies that dissipate the energy released during exothermic reactions. This research provides new insights into the critical role of dust grains in the chemical evolution that precedes planetary system formation.