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.
Abstract
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.
Glycine (Gly), NH
CH
COOH, is the simplest amino acid. Although it has not been directly detected in the interstellar gas-phase medium, it has been identified in comets and meteorites, and its ...synthesis in these environments has been simulated in terrestrial laboratory experiments. Likewise, condensation of Gly to form peptides in scenarios resembling those present in a primordial Earth has been demonstrated experimentally. Thus, Gly is a paradigmatic system for biomolecular building blocks to investigate how they can be synthesized in astrophysical environments, transported and delivered by fragments of asteroids (meteorites, once they land on Earth) and comets (interplanetary dust particles that land on Earth) to the primitive Earth, and there react to form biopolymers as a step towards the emergence of life. Quantum chemical investigations addressing these Gly-related events have been performed, providing fundamental atomic-scale information and quantitative energetic data. However, they are spread in the literature and difficult to harmonize in a consistent way due to different computational chemistry methodologies and model systems. This review aims to collect the work done so far to characterize, at a quantum mechanical level, the chemical life of Gly, i.e., from its synthesis in the interstellar medium up to its polymerization on Earth.
Abstract
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 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 NH3 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 NH3 formation, is based on the formation of a transient H3O+/NH2− ion pair, which significantly accelerates the energy transfer to the surface. Finally, the astrophysical implications of our findings relative to the interstellar synthesis of NH3 and its chemical desorption into the gas are discussed.
The mechanism of the peptide‐bond formation between two glycine (Gly) molecules has been investigated by means of PBE‐D2* and PBE0‐D2* periodic simulations on the TiO2 (101) anatase surface. This is ...a process of great relevance both in fundamental prebiotic chemistry, as the reaction univocally belongs to one of the different organizational events that ultimately led to the emergence of life on Earth, as well as from an industrial perspective, since formation of amides is a key reaction for pharmaceutical companies. The efficiency of the surface catalytic sites is demonstrated by comparing the reactions in the gas phase and on the surface. At variance with the uncatalyzed gas‐phase reaction, which involves a concerted nucleophilic attack and dehydration step, on the surface these two steps occur along a stepwise mechanism. The presence of surface Lewis and Brönsted sites exerts some catalytic effect by lowering the free energy barrier for the peptide‐bond formation by about 6 kcal mol−1 compared to the gas‐phase reaction. Moreover, the co‐presence of molecules acting as proton‐transfer assistants (i.e., H2O and Gly) provide a more significant kinetic energy barrier decrease. The reaction on the surface is also favorable from a thermodynamic standpoint, involving very large and negative reaction energies. This is due to the fact that the anatase surface also acts as a dehydration agent during the condensation reaction, since the outermost coordinatively unsaturated Ti atoms strongly anchor the released water molecules. Our theoretical results provide a comprehensive atomistic interpretation of the experimental results of Martra et al. (Angew. Chem. Int. Ed. 2014, 53, 4671), in which polyglycine formation was obtained by successive feedings of Gly vapor on TiO2 surfaces in dry conditions and are, therefore, relevant in a prebiotic context envisaging dry and wet cycles occurring, at mineral surfaces, in a small pool.
Chemistry of life: Peptide‐bond formation from unactivated amino acids is catalyzed by a fruitful interplay between TiO2 surface Lewis/Brønsted sites and proton‐transfer assistant molecules (see figure).
The mechanism of the amide bond formation between nonactivated carboxylic acids and amines catalyzed by the surface of amorphous silica under dry conditions is elucidated by combining spectroscopic ...measurements and quantum chemical simulations. The results suggest a plausible explanation of the catalytic role of silica in the reaction. Both experiment and theory identify very weakly interacting SiOH surface group pairs (ca. 5 Å apart) as key specific sites for simultaneously hosting, in the proper orientation, ionic and canonical pairs of the reactants. An atomistic interpretation of the experiments indicates that this coexistence is crucial for the occurrence of the reaction, since the components of the canonical pair are those undergoing the amidation reaction while the ionic pair directly participates in the final dehydration step. Transition state theory based on quantum mechanical free energy potential energy shows the silica-catalyzed amide formation as being relatively fast. The work also points out that the presence of the specific SiOH group pairs is not exclusive of the adopted silica sample, as they can also be present in natural forms of silica, for instance as hydroxylation defects on α-quartz, so that they could exhibit similar catalytic activity toward the amide bond formation.
The formation of amide and peptide bonds on plain amorphous silica surfaces was studied by DFT-D3 methods on cluster silica surface models involving strained SiO rings as sources of reactivity. The ...amide/peptide bond-formation reaction was found to be thermodynamically and kinetically favored compared to the gas-phase processes because of the copresence of surface (SiO)2/(SiO)3 strained ring defects, resulting from the high-temperature treatment of silica, and spatially close SiOH silanol groups. Preliminary extended calculations involving ammonia and formic acid provided insights into the most promising reaction paths for amide bond formation on defective silica surfaces. These paths were also employed to study glycine dipeptide formation. The reactions proceed through two steps: (i) silica ring opening by reaction with carboxylic acids to form a SiOC(O) surface mixed anhydride (SMA) and (ii) reaction of the SMA with amines to form the amide product. The key point of the overall reaction is the synergy between the strained SiO rings and the spatially close silanol groups: SMA formation forces carboxylic acids to be immobilized on the surface, whereas SiOH groups act as effective mild Brønsted catalytic acidic sites through a silanol-assisted proton-relay mechanism in the second step. These results provide some atomistic insights into recent experimental findings on the formation of amides catalyzed by bare silica surfaces.
Abstract 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.
Molecular hydrogen is the most abundant molecular species in the universe. While no doubts exist that it is mainly formed on the interstellar dust grain surfaces, many details of this process remain ...poorly known. In this work, we focus on the fate of the energy released by the H2 formation on the dust icy mantles: how it is partitioned between the substrate and the newly formed H2, a process that has a profound impact on the interstellar medium. We carried out state-of-the-art ab initio molecular dynamics simulations of H2 formation on periodic crystalline and amorphous ice surface models. Our calculations show that up to two-thirds of the energy liberated in the reaction (∼300 kJ mol−1 ∼3.1 eV) is absorbed by the ice in less than 1 ps. The remaining energy (∼140 kJ mol−1 ∼1.5 eV) is kept by the newly born H2. Since it is 10 times larger than the H2 binding energy on the ice, the new H2 molecule will eventually be released into the gas phase. The ice water molecules within ∼4 Å from the reaction site acquire enough energy, between 3 and 14 kJ mol−1 (360–1560 K), to potentially liberate other frozen H2 and, perhaps, frozen CO molecules. If confirmed, the latter process would solve the long standing conundrum of the presence of gaseous CO in molecular clouds. Finally, the vibrational state of the newly formed H2 drops from highly excited states (ν = 6) to low (ν ≤ 2) vibrational levels in a timescale of the order of picoseconds.
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