Development of computational strategies to simulate structures, reactivity and spectroscopic features of molecules at the interstellar ice interface

Molecules in space are detected through their spectroscopic signatures, and there are several telescopes scanning the Universe to capture signals from new molecules. Laboratory experiments and simulations complement observations.Computational astrochemistry provide atomistic perspectives that are difficult to achieve experimentally. The physical conditions of the ISM impose severe constraints on chemical reactivity. We re-investigated the formation process of ethanimine, a prebiotic molecule, in gas-phase. Rigorous quantum chemical (QC) and kinetics calculations allow for a reliable description of the chemical process. However, the predicted ratio of the two isomers of the ethanimine still needs to be improved with respect to observations. A possible explanation comes from the consideration of reactivity occurring on the surface of interstellar dust grains. About 1 % of the ISM material is made of grains and dust covered by icy mantles and solid-state reactions cannot be overlooked. Ice surfaces can play a pivotal role in the formation of interstellar complex organic molecules (iCOMs). We developed a multiscale approach to account for the solid icy matrix while using accurate methods for the chemical relevant zone. We tested the accuracy of such strategy for the isomerization of HCN. We extended this strategy to the simulation of infrared fingerprints of iCOMs adsorbed on ice. Astronomical observations of ice spectroscopic fingerprints suggest amorphous structures to be dominant. Laboratory experiments point to low density amorphous (LDA) ice as the most probable structure to be formed in such extreme environment. By using both QC and molecular dynamics (MD) simulations we were able to get reliable models of both crystalline ice (CI) and LDA. We studied the adsorption of acetic acid, methyl formate and glycolaldehyde on top of both CI and LDA and used the multiscale strategy here developed for the proper simulation of the carbonyl stretching frequency. To include the effect of the mineral core we performed MD analysis of the ice morphology while growing on top of silica models of the interstellar grains.Summarizing the research carried out during this PhD thesis aimed at developing reliable and robust yet feasible computational protocols to realistically describe interstellar ices and the chemical processing occurring on their surfaces.