In the denser and colder (≤20 K) regions of the interstellar medium (ISM), near-infrared observations have revealed the presence of submicron-sized dust grains covered by several layers of H2O-dominated ices and “dirtied” by the presence of other volatile species. Whether a molecule is in the gas or solid-phase depends on its binding energy (BE) on ice surfaces. Thus, BEs are crucial parameters for the astrochemical models that aim to reproduce the observed evolution of the ISM chemistry. In general, BEs can be inferred either from experimental techniques or by theoretical computations. In this work, we present a reliable computational methodology to evaluate the BEs of a large set (21) of astrochemical relevant species. We considered different periodic surface models of both crystalline and amorphous nature to mimic the interstellar water ice mantles. Both models ensure that hydrogen bond cooperativity is fully taken into account at variance with the small ice cluster models. Density functional theory adopting both B3LYP-D3 and M06-2X functionals was used to predict the species/ice structure and their BEs. As expected from the complexity of the ice surfaces, we found that each molecule can experience multiple BE values, which depend on its structure and position at the ice surface. A comparison of our computed data with literature data shows agreement in some cases and (large) differences in others. We discuss some astrophysical implications that show the importance of calculating BEs using more realistic interstellar ice surfaces to have reliable values for inclusion in the astrochemical models.

Carbon monoxide (CO) is the second most abundant gas-phase molecule after molecular hydrogen (H2) of the interstellar medium (ISM). In molecular clouds, an important component of the ISM, it adsorbs at the surface of core grains, usually made of Mg/Fe silicates, and originates complex organic molecules through the catalytic power of active sites at the grain surfaces. To understand the atomistic, energetic, and spectroscopic details of the CO adsorption on core grains, we resorted to density functional theory based on the hybrid B3LYP-D* functional inclusive of dispersion contribution. We modeled the complexity of interstellar silicate grains by studying adsorption events on a large set of infinite extended surfaces cut out from the bulk Mg2SiO4 forsterite, the Mg end-member of olivines (Mg2x Fe2–2x SiO4), also a very common mineral on the Earth’s crust. Energetic and structural features indicate that CO is exclusively physisorbed with binding energy values in the 23–68 kJ mol–1 range. Detailed analysis of data revealed that dispersive interactions are relevant together with the important electrostatic contribution due to the quadrupolar nature of the CO molecule. We performed a full thermodynamic treatment of the CO adsorption at the very low temperature typical of the ISM as well as a full spectroscopic characterization of the CO stretching frequency, which we prove to be extremely sensitive to the local nature of the surface-active site of adsorption. We also performed a detailed kinetic analysis of CO desorption from the surface models at different temperatures characterizing the colder regions of the ISM. Our computed data could be incorporated in the various astrochemical models of interstellar grains developed so far and thus contribute to improve the description of the complex chemical network occurring at their surfaces.

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.

Carbon monoxide (CO) is the second most abundant molecule in the gas-phase of the interstellar medium. In dense molecular clouds, it is also present in the solid-phase as a constituent of the mixed water-dominated ices covering dust grains. Its presence in the solid-phase is inferred from its infrared (IR) signals. In experimental observations of solid CO/water mixed samples, its IR frequency splits into two components, giving rise to a blue-and a redshifted band. However, in astronomical observations, the former has never been observed. Several attempts have been carried out to explain this peculiar behaviour, but the question still remains open. In this work, we resorted to pure quantum mechanical simulations in order to shed some light on this problem. We adopted different periodic models simulating the CO/H 2 O ice system, such as single and multiple CO adsorption on water ice surfaces, CO entrapped into water cages and proper CO:H 2 O mixed ices. We also simulated pure solid CO. The detailed analysis of our data revealed how the quadrupolar character of CO and the dispersive forces with water ice determine the energetic of the CO/H 2 O ice interaction, as well as the CO spectroscopic behaviour. Our data suggest that the blueshifted peak can be assigned to CO interacting via the C atom with dangling H atoms of the water ice, while the redshifted one can actually be the result of CO involved in different reciprocal interactions with the water matrix. We also provide a possible explanation for the lack of the blueshifted peak in astronomical spectra. Our aim is not to provide a full account of the various interstellar ices, but rather to elucidate the sensitivity of the CO spectral features to different water ice environments.

Infrared (IR) spectroscopy is the main technique used to identify and characterize silicate dust grains in astronomical environments. From IR spectra, the fraction of crystalline dust particles can be estimated and used to help understand the processing of dust occurring in different astronomical environments such as the interstellar medium (ISM) and circumstellar shells. Narrow spectral lines are assigned to crystalline grains, while broad signals are usually assumed to originate from amorphous material. Herein, we accurately calculate the IR spectra and energetic stabilities of several amorphous and crystalline silicate nanograins with an astronomically common Mg-rich olivinic (Mg 2 SiO 4 ) stoichiometry and with sizes ranging from hundreds to thousands of atoms. First, unlike at larger length scales, crystalline forsterite-like grains at the nanoscale are found to be energetically metastable with respect to amorphous grains. However, from our careful analysis, we further show that the IR spectra of such nanosilicate grains cannot be unambiguously used to identify their structural nature. In particular, our work indicates that amorphous and crystalline silicate nanograins both exhibit broad IR spectra typical of noncrystalline grains, raising potential issues for estimates of the fraction of crystalline silicate dust in the ISM.