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Of the about 300 gas-phase molecular species so far detected in the interstellar medium (ISM), mostly via observations of their rotational lines, around 40% contain nitrogen (N) atoms. Likewise, of the less than a dozen interstellar molecules, firmly or likely detected in the solid-state water-dominated icy matrix by means of infrared observations, two bear N. A crucial parameter that regulates whether a species is in the gas or adsorbed on the icy phase is their binding energy (BE) toward the icy grain. Therefore, an accurate quantification of the BE is of paramount importance to properly model the ISM chemistry through numerical models. However, very few BEs are available in the literature, either determined experimentally or theoretically. In the present study, we calculate the BEs of 21 among the most abundant interstellar N-bearing species. We adopted two structural water ice models, representing a crystalline and an amorphous surface, using a reliable cost-effective procedure based on the density functional theory. While on the crystalline surface model only one BE per species is obtained due to the high symmetry of the unit cell, on the amorphous model from 5 to 10 BEs are obtained, due to its richer surface morphological variety. Most of our computed BEs agree with available experimental and other computational values. Finally, we discuss how the newly computed BEs can help estimate which N-bearing species can be frozen at the water snow line and, therefore, incorporated in water-rich ice planetesimals.

In the denser and colder regions of the interstellar medium (ISM), gas-phase sulfur is depleted by 2 or 3 orders of magnitude with respect to its cosmic abundance. Thus, which species are the main carriers of sulfur is an open question. Recent studies have proposed S _n species as potential sulfur reservoirs. Among the various sulfur allotropes, the most stable one is the S _8 ring, detected in the asteroid Ryugu and Orgueil meteorite. Shorter species, namely S _3 and S _4 , have been found in the comet 67P/C-G, but their presence in the ISM remains elusive. In this study, we compute the binding energies (BEs) of S _n ( n = 1–8) species on an amorphous water-ice surface model and analyze their infrared (IR) and Raman spectral features to provide data for their identification in the ISM. Our computations reveal that these species exhibit lower BEs than previously assumed and that their spectral features experience minimal shifts when adsorbed on water ice, because of the weak and nonspecific S _n –ice interactions. Furthermore, these species display very low IR band intensities and, therefore, very accurate instruments operating in the mid-IR range are required for detecting the presence of these species in dense interstellar environments.

In the coldest (10–20 K) regions of the interstellar medium, the icy surfaces of interstellar grains serve as solid-state supports for chemical reactions. Among their plausible roles, that of third body is advocated, in which the reaction energies of surface reactions dissipate throughout the grain, stabilizing the product. This energy dissipation process is poorly understood at the atomic scale, although it can have a high impact on astrochemistry. Here we study, by means of quantum mechanical simulations, the formation of NH _3 via successive H-additions to atomic N on water ice surfaces, paying special attention to the third-body role. We first characterize the hydrogenation reactions and the possible competitive processes (i.e., H-abstractions), in which the H-additions are more favorable than the H-abstractions. Subsequently, we study the fate of the hydrogenation reaction energies by means of ab initio molecular dynamics simulations. Results show that around 58%–90% of the released energy is quickly absorbed by the ice surface, inducing a temporary increase of the ice temperature. Different energy dissipation mechanisms are distinguished. One mechanism, more general, is based on the coupling of the highly excited vibrational modes of the newly formed species and the libration modes of the icy water molecules. A second mechanism, exclusive during the NH _3 formation, is based on the formation of a transient H _3 O ^+ /NH _2 ^− ion pair, which significantly accelerates the energy transfer to the surface. Finally, the astrophysical implications of our findings relative to the interstellar synthesis of NH _3 and its chemical desorption into the gas are discussed.

The carbon ( ^3 P) atom is a reactive species that, according to laboratory experiments and theoretical calculations, condensates with interstellar ice components. This fact is of uttermost importance for the chemistry in the interstellar medium (ISM) because the condensation reaction is barrierless, and the subsequent species formed are still reactive given their open-shell character. Carbon condensation on CO-rich ices forms the C=C=O ( ^3 Σ ^− ) species, which can be easily hydrogenated twice to form ketene (H _2 CCO). Ketene is very reactive in terrestrial conditions, usually found as an intermediate that is difficult to isolate in chemical synthesis laboratories. These characteristics suggest that ketene can be a good candidate to form interstellar complex organic molecules via a two-step process, i.e., its activation followed by a radical–radical coupling. In this work, reactions between ketene and atomic H and the OH and NH _2 radicals on a CO-rich ice model have been explored by means of quantum chemical calculations complemented by kinetic calculations to evaluate if they are favorable in the ISM. Results indicate that the addition of H to ketene (helped by tunneling) to form the acetyl radical (CH _3 CO) is the most preferred path as the reactions with OH and NH _2 possess activation energies (≥9 kJ mol ^−1 ) hard to surmount in the ISM conditions unless external processes provide energy to the system. Thus, acetaldehyde (CH _3 CHO) and, probably, ethanol (CH _3 CH _2 OH) formation via further hydrogenations, are the possible unique operating synthetic routes. Moreover, from the computed, relatively large binding energies of OH and NH _2 on CO ice, slow diffusion is expected, hampering possible radical–radical couplings with CH _3 CO. The astrophysical implications of these findings are discussed considering the incoming James Webb Space Telescope observations.

The gas-phase reaction networks are the backbone of astrochemical models. However, due to their complexity and nonlinear impact on the astrochemical modeling, they can be the first source of error in the simulations if incorrect reactions are present. Over time, following the increasing number of species detected, astrochemists have added new reactions, based on laboratory experiments and quantum mechanics (QM) computations, as well as reactions inferred by chemical intuition and the similarity principle. However, sometimes no verification of their feasibility in the interstellar conditions, namely their exothermicity, was performed. In this work, we present a new gas-phase reaction network, GRETOBAPE , based on the KIDA2014 network and updated with several reactions, cleaned from endothermic reactions not explicitly recognized as such. To this end, we characterized all the species in the GRETOBAPE network with accurate QM calculations. We found that ∼5% of the reactions in the original network are endothermic, although most of them are reported as barrierless. The reaction network of Si-bearing species is the most impacted by the endothermicity cleaning process. We also produced a cleaned reduced network, GRETOBAPE-red , to be used to simulate astrochemical situations where only C-, O-, N-, and S-bearing species with less than six atoms are needed. Finally, the new GRETOBAPE network, its reduced version, and the database with all the molecular properties are made publicly available. The species property database can be used in the future to test the feasibility of possibly new reactions.

Water is one of the most important and abundant molecules in star-forming regions. In protoplanetary disks, where planets and comets form, H _2 O is in a gas or solid form, depending on the dust temperature, i.e., the distance from the center and its binding energy (BE). Not surprisingly, several experimental and theoretical studies of the H _2 O BE have been published. We report new ab initio calculations carried out on a large model of interstellar ice, where we identified 144 different adsorption sites. The BE associated with those sites ranges between 14.2 kJ mol ^−1 (1705 K) and 61.6 kJ mol ^−1 (7390 K). The distribution of the computed BEs as a function of BE follows a Gaussian peaked at 35.4 kJ mol ^−1 (4230 K) with a standard deviation of 9.7 kJ mol ^−1 (1160 K). The computed pre-exponential factor ( ν ) ranges between 9 × 10 ^12 and 6 × 10 ^14 s ^−1 . We evaluated the impact of the newly calculated BE and ν distributions on the snowline of a generic protoplanetary disk. We found that the region where water is frozen onto the ice is much smaller (a factor of 10 smaller radius) than that computed with the single BE (5600 K) and ν (2 × 10 ^12 s ^−1 ) values commonly adopted by astrochemical models. Besides, ∼10% of water remains frozen in relatively warm (∼150 K) regions, where the single BE and ν model would predict a full release of the ice in the gas phase. This last aspect may have an impact on the quantity trapped in the planetesimals eventually forming rocky planets.

The reactivity of interstellar carbon atoms (C) on water-dominated ices is one of the possible ways to form interstellar complex organic molecules (iCOMs). In this work, we report a quantum chemical study of the coupling reaction of C ( ^3 P) with an icy water molecule, alongside possible subsequent reactions with the most abundant closed-shell frozen species (NH _3 , CO, CO _2 , and H _2 ), atoms (H, N, and O), and molecular radicals (OH, NH _2 , and CH _3 ). We found that C reacts spontaneously with the water molecule, resulting in the formation of ^3 C–OH _2 , a highly reactive species due to its triplet electronic state. While reactions with the closed-shell species do not show any reactivity, reactions with N and O form CN and CO, respectively, the latter ending up in methanol upon subsequent hydrogenation. The reactions with OH, CH _3 , and NH _2 form methanediol, ethanol, and methanimine, respectively, upon subsequent hydrogenation. We also propose an explanation for methane formation observed in experiments through additions of H to C in the presence of ices. The astrochemical implications of this work are: (i) atomic C on water ice is locked into ^3 C–OH _2 , making difficult the reactivity of bare C atoms on icy surfaces, contrary to what is assumed in current astrochemical models; and (ii) the extraordinary reactivity of ^3 C–OH _2 provides new routes toward the formation of iCOMs in a nonenergetic way, in particular ethanol, the mother of other iCOMs once it is in the gas phase.

In cold galactic molecular clouds, dust grains are coated by icy mantles and are prevalently charged negatively, because of the capture of the electrons in the gas. The interaction of the charged grains with gaseous cations is known to neutralize them. In this work, we focus on the chemical consequences of the neutralization process of HCO+, often the most abundant cation in molecular clouds. More specifically, by means of electronic structure calculations, we have characterized the energy and the structure of all possible product species once the HCO+ ion adsorbs on water clusters holding an extra electron. Two processes are possible: (i) electron transfer from the negative water cluster to the HCO+ ion or (ii) a proton transfer from HCO+ to the negative water cluster. Energetic considerations favor electron transfer. Assuming this scenario, two limiting cases have been considered in astrochemical models: (a) all the neutralized HCO+ is retained as neutral HCO adsorbed on the ice and (b) all the neutralized HCO+ gets desorbed to the gas phase as HCO. None of the two limiting cases appreciably contribute to the HCO abundance on the grain surfaces or in the gas.

Formamide has been recognized in the literature as a key species in the formation of the complex molecules of life, such as nucleobases. Furthermore, several studies reported the impact of mineral phases as catalysts for its decomposition/polymerization processes, increasing the conversion and also favoring the formation of specific products. Despite the progresses in the field, in situ studies on these mineral-catalyzed processes are missing. In this work, we present an in situ UV-Raman characterization of the chemical evolution of formamide over amorphous SiO2 samples, selected as a prototype of silicate minerals. The experiments were carried out after reaction of formamide at 160 °C on amorphous SiO2 (Aerosil OX50) either pristine or pre-calcined at 450 °C, to remove a large fraction of surface silanol groups. Our measurements, interpreted on the basis of density functional B3LYP-D3 calculations, allow to assign the spectra bands in terms of specific complex organic molecules, namely, diaminomaleonitrile (DAMN), 5-aminoimidazole (AI), and purine, showing the role of the mineral surface on the formation of relevant prebiotic molecules.

Anion substitution is at present one of the pathways to destabilize metal borohydrides for solid state hydrogen storage. In this work, a solid solution of LiBH4 and LiCl is studied by density functional theory (DFT) calculations, thermodynamic modeling, X-ray diffraction, and infrared spectroscopy. It is shown that Cl substitution has minor effects on thermodynamic stability of either the orthorhombic or the hexagonal phase of LiBH4. The transformation into the orthorhombic phase in LiBH4 shortly after annealing with LiCl is for the first time followed by infrared measurements. Our findings are in a good agreement with an experimental study of the LiBH4-LiCl solid solution structure and dynamics. This demonstrates the validity of the adopted combined theoretical (DFT calculations) and experimental (vibrational spectroscopy) approach, to investigate the solid solution formation of complex hydrides.

There is a consensus that the interaction of organic molecules with the surfaces of naturally-occurring minerals might have played a crucial role in chemical evolution and complexification in a prebiotic era. The hurdle of an overly diluted primordial soup occurring in the free ocean may have been overcome by the adsorption and concentration of relevant molecules on the surface of abundant minerals at the sea shore. Specific organic–mineral interactions could, at the same time, organize adsorbed molecules in well-defined orientations and activate them toward chemical reactions, bringing to an increase in chemical complexity. As experimental approaches cannot easily provide details at atomic resolution, the role of in silico computer simulations may fill that gap by providing structures and reactive energy profiles at the organic–mineral interface regions. Accordingly, numerous computational studies devoted to prebiotic chemical evolution induced by organic–mineral interactions have been proposed. The present article aims at reviewing recent in silico works, mainly focusing on prebiotic processes occurring on the mineral surfaces of clays, iron sulfides, titanium dioxide, and silica and silicates simulated through quantum mechanical methods based on the density functional theory (DFT). The DFT is the most accurate way in which chemists may address the behavior of the molecular world through large models mimicking chemical complexity. A perspective on possible future scenarios of research using in silico techniques is finally proposed.

Mineral surfaces have been demonstrated to play a central role in prebiotic reactions, which are understood to be at the basis of the origin of life. Among the various molecules proposed as precursors for these reactions, one of the most interesting is formamide. Formamide has been shown to be a pluripotent molecule, generating a wide distribution of relevant prebiotic products. In particular, the outcomes of its reactivity are strongly related to the presence of mineral phases acting as catalysts toward specific reaction pathways. While the mineral–products relationship has been deeply studied for a large pool of materials, the fundamental description of formamide reactivity over mineral surfaces at a microscopic level is missing in the literature. In particular, a key step of formamide chemistry at surfaces is adsorption on available interaction sites. This report aims to investigate the adsorption of formamide over a well-defined amorphous silica, chosen as a model mineral surface. An experimental IR investigation of formamide adsorption was carried out and its outcomes were interpreted on the basis of first principles simulation of the process, adopting a realistic model of amorphous silica.

The present work reports ab initiomolecular dynamics simulations, based on density functional theory using the PBE functional, of Li+- Na+- and K+-montmorillonites, considering three types of isomorphic substitutions in the montmorillonite layer: tetrahedral (Tsub), octahedral (Osub) and both (OTsub). These simulations allow us to evaluate the effect of each type of substitution on the inner- outer-sphere complex formation of the alkali cations. It is observed that, for the three kinds of substituted montmorillonites, K+remains bound to the surface confirming its role as swelling inhibitor. In contrast, Li+tends to hydrate and coordinate to 4 water molecules in all cases, except for OTsub, for which one of the two Li+cations remains bound to the oxygens close to the substituted tetrahedral site. Finally, Na+presents an intermediate behaviour, binding to the surface in Tsubmontmorillonite but being hydrated in Osub. These simulations show that the hydration/adsorption behaviour of alkali cations in the swelling process of montmorillonite depends on the affinity of the cation for water and the surface, as well as on the type of substitution that controls the negative charge on surface oxygen atoms. [ABSTRACT FROM AUTHOR]

The role of dispersive interactions on the structure, energetic and vibrational features of brucite [Mg(OH)2], portlandite [Ca(OH)2] and kaolinite [Al2Si2O5(OH)4] layered materials has been addressed for the first time. Dispersion contribution is included with a −C6/R6empirical correction to the B3LYP functional (B3LYP-D* recipe) which has recently been employed to study molecular crystals. To decrease the spurious effect of the basis set superposition error, Gaussian basis sets of triple-zeta plus polarization functions were adopted. Comparing B3LYP and B3LYP-D* results shows the latter to provide significant improvement as far as structure and energetic data are concerned. For the treated systems the cell parameter controlling the inter-layer distance, usually overestimated by the pure B3LYP, is in good agreement with experiment. The inter-layer interaction energy is dramatically increased by the dispersion contribution which, for kaolinite, fully justifies the request of strong Lewis basic molecules needed to swell the material. In general, B3LYP harmonic frequencies are scarcely changed by the dispersive correction albeit some modes sensitive to the inter-layer separation may be significantly perturbed at B3LYP-D*. The present results are encouraging although fine tuning of the proposed empirical correction for inorganic systems might be needed. [ABSTRACT FROM AUTHOR]

The reaction of glycine Gly with a strained SiO2four-membered ring defect D2 at the surface of an interstellar silica grain dust has been studied at ONIOM2B3LYP6-31Gd,p:MNDO level within a cluster approach in the context of hypothetical reactions occurring in the interstellar medium. The D2 opens up exothermically for reaction with Gly ΔrU0−26.3 kcal mol−1 to give a surface mixed anhydride Ssurf–O–CO–CH2NH2as a product. The reaction barriers, ΔU≠0, are 0.1 and 10.4 kcal mol−1for reactive channels involving COOH and NH2as attacking groups, respectively. Calculations show the surface mixed anhydride to be rather stable under the action of interstellar processes, such as reactions with isolated H2O and NH3molecules or the exposure to cosmic rays and UV radiation. The hydrolysis of the surface mixed anhydride to release again Gly was modelled by microsolvation from 1 to 4 H2O molecules mimicking what could have happened to the interstellar dust after seeding the primordial ocean in the early Earth. Results for these calculations show that the reaction is exergonic and activated, the ΔrG298becoming more negative and the ΔG≠298being dramatically reduced as a function of increasing number of H2O molecules. The present results are relevant because they show that defects present at interstellar dust surfaces could have played a significant role in capturing, protecting and delivering essential prebiotic compounds on the early Earth. [ABSTRACT FROM AUTHOR]

Abstract  The structure and vibrational spectrum of boehmite have been investigated at the quantum-mechanical level with the CRYSTAL code, using a Gaussian-type basis set and the B3LYP Hamiltonian. Three space groups are considered in this study: Cmcm, Cmc21, P21/c. Cmcm turns out to correspond to a transition state, whereas Cmc21 and P21/c are minimum energy structures. The difference among them is the position of H atoms only, the Al-O frame being essentially the same. Harmonic frequencies at the Γ point have been computed. The comparison between calculated and experimental frequencies shows a good agreement for the Al-O part of the spectrum (under 790 cm−1). For the Al-OH bending modes (800–1,300 cm−1) an absolute differences of 50–100 cm−1 is observed; for the OH stretching modes (3,200–3,500 cm−1) it increases to 120–200 cm−1: anharmonicity is large because OH groups are involved in strong hydrogen bonds. [ABSTRACT FROM AUTHOR]

A classical interatomic force field for hydroxyapatite has been parameterized from periodic ab initio calculations carried out on the hexagonal structure (space group P63). The GULP program has been used for fitting geometry and phonon frequencies computed with the CRYSTAL06 program using the B3LYP hybrid functional and Gaussian-type basis set of polarized double zeta quality. Polarization effects and covalent bonding have been included through the shell-ion model potential. Excellent agreement has been found in reproducing structural features, lattice dynamics, the OH stretching vibrations and relative phase stabilities between the monoclinic structure (space group P21/b) and the hexagonal one. Transferability from hydroxyapatite to other calcium phosphates has also been demonstrated. [ABSTRACT FROM AUTHOR]

Both glycine and water exhibit a high affinity towards the hydroxyapatite HA surfaces. What happens when they are co-adsorbed at the HA (001) surface? B3LYP periodic calculations reveal that glycine displaces the pre-adsorbed water interacting directly with the HA surface. [ABSTRACT FROM AUTHOR]