Quantum Mechanical Investigations on the Formation of Complex Organic Molecules on Interstellar Ice Mantles. Review and Perspectives

The interstellar medium (ISM) is rich in molecules, from simple diatomic to complex organic ones, some of which have a biotic potential. A notable example, in this respect, is represented by the so-called interstellar complex organic molecules (iCOMs). Interestingly, the various phases involved in the formation of Solar-type planetary systems lead to an increasing chemical complexity, in which, at each step, more complex molecules form. In dark molecular clouds, dust grains are covered by ice mantles, mainly made up of H2O but also of other volatiles species such as CO, NH3, CO2, CH4, and CH3OH. Although their mass is one hundred times lower than the gas-phase matter, these ice-covered grains play a fundamental role in the interstellar chemical complexity as some important reactions are exclusively catalyzed by their surfaces. For example, one of the current paradigms on the iCOMs formation assumes that iCOMs are synthesized on the ice mantle surfaces, in which reactants accrete and diffuse to finally react. As the usual approaches employed in astrochemistry (i.e., spectroscopic astronomical observations, astrochemical modeling and laboratory experiments) cannot easily provide details on the iCOMs formation processes occurring on ice mantles at the atomic level, computational chemistry has recently become a complementary tool to fill in this gap. Indeed, it can provide an accurate description (i.e., structures and reactive energy profiles) of these processes. Accordingly, several recent studies simulating the formation of iCOMs on icy surfaces by means of quantum mechanical methods have appeared in the literature. This Review aims to comprehensively analyze most of these works, focusing not only on standard iCOMs but also on simpler organic compounds as well as biomolecules. Perspectives on possible future directions of research using computational chemistry are also proposed.