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Chemical processes taking place on ice-grain mantles are pivotal to the complex chemistry of interstellar environments. In this study, we conducted a comprehensive analysis of the catalytic effects of an amorphous solid water (ASW) surface on the reaction between ammonia (NH3) and formaldehyde (H2CO) to form aminomethanol (NH2CH2OH) using density functional theory. We identified potential catalytic sites based on the binding energy distribution of NH3 and H2CO reactants, on a set-of-clusters surface model composed of 22 water molecules and found a total of 14 reaction paths. Our results indicate that the catalytic sites can be categorized into four groups, depending on the interactions of the carbonyl oxygen and the amino group with the ice surface in the reactant complex. A detailed analysis of the reaction mechanism using Intrinsic Reaction Coordinate and reaction force analysis, revealed three distinct chemical events for this reaction: formation of the C–N bond, breaking of the N–H bond, and formation of the O–H hydroxyl bond. Depending on the type of catalytic site, these events can occur within a single, concerted, albeit asynchronous, step, or can be isolated in a step-wise mechanism, with the lowest overall transition state energy observed at 1.3 kcal mol−1. A key requirement for the low-energy mechanism is the presence of a pair of dangling OH bonds on the surface, found at 5% of the potential catalytic sites on an ASW porous surface. [ABSTRACT FROM AUTHOR]

The discovery of C60, C60+, and C70 in the interstellar medium has ignited a profound interest in the astrochemistry of fullerene and related systems. In particular, the presence of diffuse interstellar bands and their association with C60+ has led to the hypothesis that hydrogenated derivatives, known as fulleranes, may also exist in the interstellar medium and contribute to these bands. In this study, we systematically investigated the structural and spectroscopic properties of C60Hn+q (n = 0–4, q = 0,1) using an automated global minimum search and density functional theory calculations. Our results revealed novel global minimum structures for C60H2 and C60H4, distinct from previous reports. Notably, all hydrogenated fullerenes exhibited lower ionization potentials and higher proton affinities compared to C60. From an astrochemical perspective, our results exposed the challenges in establishing definitive spectroscopic criteria for detecting fulleranes using mid-infrared and UV-Vis spectroscopies. However, we successfully identified distinct electronic transitions in the near-infrared range that serve as distinctive signatures of cationic fulleranes. We strongly advocate for further high-resolution experimental studies to fully explore the potential of these transitions for the interstellar detection of fulleranes. [ABSTRACT FROM AUTHOR]

Developing machine learning-based interatomic potentials from ab initio electronic structure methods remains a challenging task for computational chemistry and materials science. This work studies the capability of transfer learning, in particular discriminative fine-tuning, for efficiently generating chemically accurate interatomic neural network potentials on organic molecules from the MD17 and ANI data sets. We show that pre-training the network parameters on data obtained from density functional calculations considerably improves the sample efficiency of models trained on more accurate ab initio data. Additionally, we show that fine-tuning with energy labels alone can suffice to obtain accurate atomic forces and run large-scale atomistic simulations, provided a well-designed fine-tuning data set. We also investigate possible limitations of transfer learning, especially regarding the design and size of the pre-training and fine-tuning data sets. Finally, we provide GM-NN potentials pre-trained and fine-tuned on the ANI-1x and ANI-1ccx data sets, which can easily be fine-tuned on and applied to organic molecules. [ABSTRACT FROM AUTHOR]

Aiming at merocyanine dyes with good linear optical and self-assembly properties, a series of rigid mono-, bi- and tricyclic merocyanines with O- and N-donor units as well as keto or malodinitrile acceptor units was prepared by a convergent approach. With particular focus on tailoring the donor unit, a selection of appropriate derivatives was investigated with respect to their dye properties in solution and in the bulk (UV/Vis, fluorescence, temperature-dependent fluorescence, lifetime). Determination of fluorescence quantum yields revealed the importance of the donor unit and the chromophore size. Larger chromophores and N-donors were beneficial for strong emission in solution, whereas small chromophores and O-donors favored emission in the solid state. To rationalize the different optical properties depending on their donor unit, density functional theory (DFT) calculations were performed. Liquid crystalline derivatives were additionally studied by optical polarization microscopy, differential scanning calorimetry, and X-ray diffraction experiments. For merocyanines with O-donor, fluorinated side chains were mandatory to get stable enantiotropic SmA phases regardless of chromophore size, side chain lengths or acceptor unit. Increased mesophase widths (up to 134 K) were observed upon increasing the chromophore lengths, chain lengths (up to C12) and F/C ratio in the side chain. On the other hand, merocyanines with N-donor and keto acceptor showed enantiotropic SmA phases in the presence of simple alkoxy side chains. The tricyclic merocyanine with N-donor shows an additional SmE phase at lower temperatures. The results revealed the importance of the donor unit to balance optical and mesomorphic properties in merocynanines. [ABSTRACT FROM AUTHOR]

We investigated the behavior of H2, the main constituent of the gas phase in dense clouds, after collision with amorphous solid water (ASW) surfaces, one of the most abundant chemical species of interstellar ices. We developed a general framework to study the adsorption dynamics of light species on interstellar ices. We provide binding energies and their distribution, sticking probabilities for incident energies between 1 meV and 60 meV, and thermal sticking coefficients between 10 and 300 K for surface temperatures from 10 to 110 K. We found that the sticking probability depends strongly on the adsorbate kinetic energy and the surface temperature, but hardly on the angle of incidence. We observed finite sticking probabilities above the thermal desorption temperature. Adsorption and thermal desorption should be considered as separate events with separate time scales. Laboratory results for these species have shown a gap in the trends attributed to the differently utilized experimental techniques. Our results complement observations and extend them, increasing the range of gas temperatures under consideration. We plan to use our method to study a variety of adsorbates, including radicals and charged species. [ABSTRACT FROM AUTHOR]

Atom tunneling in the hydrogen atom transfer reaction of the 2,4,6-tri-tert-butylphenyl radical to 3,5-di-tert-butylneophyl, which has a short but strongly curved reaction path, was investigated using instanton theory. We found the tunneling path to deviate qualitatively from the classical intrinsic reaction coordinate, the steepest-descent path in mass-weighted Cartesian coordinates. To perform that comparison, we implemented a new variant of the predictor-corrector algorithm for the calculation of the intrinsic reaction coordinate. We used the reaction force analysis method as a means to decompose the reaction barrier into structural and electronic components. Due to the narrow energy barrier, atom tunneling is important in the abovementioned reaction, even above room temperature. Our calculated rate constants between 350 K and 100 K agree well with experimental values. We found a H/D kinetic isotope effect of almost 106 at 100 K. Tunneling dominates the protium transfer below 400 K and the deuterium transfer below 300 K. We compared the lengths of the tunneling path and the classical path for the hydrogen atom transfer in the reaction HCl + Cl and quantified the corner cutting in this reaction. At low temperature, the tunneling path is about 40% shorter than the classical path. [ABSTRACT FROM AUTHOR]

Investigating how formamide forms in the interstellar medium is a hot topic in astrochemistry, which can contribute to our understanding of the origin of life on Earth. We have constructed a QM/MM model to simulate the hydrogenation of isocyanic acid on amorphous solid water surfaces to form formamide. The binding energy of HNCO on the ASW surface varies significantly between different binding sites, we found values between ∼0 and 100 kJ mol−1. The barrier for the hydrogenation reaction is almost independent of the binding energy, though. We calculated tunneling rate constants of H + HNCO → NH2CO at temperatures down to 103 K combining QM/MM with instanton theory. Tunneling dominates the reaction at such low temperatures. The tunneling reaction is hardly accelerated by the amorphous solid water surface compared to the gas phase for this system, even though the activation energy of the surface reaction is lower than the one of the gas-phase reaction. Both the height and width of the barrier affect the tunneling rate in practice. Strong kinetic isotope effects were observed by comparing to rate constants of D + HNCO → NHDCO. At 103 K we found a KIE of 231 on the surface and 146 in the gas phase. Furthermore, we investigated the gas-phase reaction NH2 + H2CO → NH2CHO + H and found it unlikely to occur at cryogenic temperatures. The data of our tunneling rate constants are expected to significantly influence astrochemical models. [ABSTRACT FROM AUTHOR]

The discovery of C 60 , C 60 + , and C 70 in the interstellar medium has ignited a profound interest in the astrochemistry of fullerene and related systems. In particular, the presence of diffuse interstellar bands and their association with C 60 + has led to the hypothesis that hydrogenated derivatives, known as fulleranes, may also exist in the interstellar medium and contribute to these bands. In this study, we systematically investigated the structural and spectroscopic properties of C 60 H n + q ( n = 0-4, q = 0,1) using an automated global minimum search and density functional theory calculations. Our results revealed novel global minimum structures for C 60 H 2 and C 60 H 4 , distinct from previous reports. Notably, all hydrogenated fullerenes exhibited lower ionization potentials and higher proton affinities compared to C 60 . From an astrochemical perspective, our results exposed the challenges in establishing definitive spectroscopic criteria for detecting fulleranes using mid-infrared and UV-Vis spectroscopies. However, we successfully identified distinct electronic transitions in the near-infrared range that serve as distinctive signatures of cationic fulleranes. We strongly advocate for further high-resolution experimental studies to fully explore the potential of these transitions for the interstellar detection of fulleranes.

We investigated the behavior of H2, the main constituent of the gas phase in dense clouds, after collision with amorphous solid water (ASW) surfaces, one of the most abundant chemical species of interstellar ices. We developed a general framework to study the adsorption dynamics of light species on interstellar ices. We provide binding energies and their distribution, sticking probabilities for incident energies between 1 meV and 60 meV, and thermal sticking coefficients between 10 and 300 K for surface temperatures from 10 to 110 K. We found that the sticking probability depends strongly on the adsorbate kinetic energy and the surface temperature, but hardly on the angle of incidence. We observed finite sticking probabilities above the thermal desorption temperature. Adsorption and thermal desorption should be considered as separate events with separate time scales. Laboratory results for these species have shown a gap in the trends attributed to the differently utilized experimental techniques. Our results complement observations and extend them, increasing the range of gas temperatures under consideration. We plan to use our method to study a variety of adsorbates, including radicals and charged species.

The final step of the water formation network on interstellar grain surfaces starting from the H + O 2 route is the reaction between H and H 2 O 2 . This reaction is known to have a high activation energy and therefore at low temperatures it can only proceed via tunneling. To date, however, no rate constants are available at temperatures below 200 K. In this work, we use instanton theory to compute rate constants for the title reaction with and without isotopic substitutions down to temperatures of 50 K. The calculations are based on density functional theory, with additional benchmarks for the activation energy using unrestricted single-reference and multireference coupled-cluster single-point energies. Gas-phase bimolecular rate constants are calculated and compared with available experimental data not only for H + H 2 O 2 → H 2 O + OH, but also for H + H 2 O 2 → H 2 + HO 2 . We find a branching ratio where the title reaction is favored by at least two orders of magnitude at 114 K. In the interstellar medium this reaction predominantly occurs on water surfaces, which increases the probability that the two reactants meet. To mimic this, one, two, or three spectator H 2 O molecules are added to the system. Eley-Rideal bimolecular and Langmuir-Hinshelwood unimolecular rate constants are presented here. The kinetic isotope effects for the various cases are compared to experimental data as well as to expressions commonly used in astrochemical models. Both the rectangular barrier and the Eckart approximations lead to errors of about an order of magnitude. Finally, fits of the rate constants are provided as input for astrochemical models.

Investigating how formamide forms in the interstellar medium is a hot topic in astrochemistry, which can contribute to our understanding of the origin of life on Earth. We have constructed a QM/MM model to simulate the hydrogenation of isocyanic acid on amorphous solid water surfaces to form formamide. The binding energy of HNCO on the ASW surface varies significantly between different binding sites, we found values between ∼0 and 100 kJ mol -1 . The barrier for the hydrogenation reaction is almost independent of the binding energy, though. We calculated tunneling rate constants of H + HNCO → NH 2 CO at temperatures down to 103 K combining QM/MM with instanton theory. Tunneling dominates the reaction at such low temperatures. The tunneling reaction is hardly accelerated by the amorphous solid water surface compared to the gas phase for this system, even though the activation energy of the surface reaction is lower than the one of the gas-phase reaction. Both the height and width of the barrier affect the tunneling rate in practice. Strong kinetic isotope effects were observed by comparing to rate constants of D + HNCO → NHDCO. At 103 K we found a KIE of 231 on the surface and 146 in the gas phase. Furthermore, we investigated the gas-phase reaction NH 2 + H 2 CO → NH 2 CHO + H and found it unlikely to occur at cryogenic temperatures. The data of our tunneling rate constants are expected to significantly influence astrochemical models.

The mechanism of carbonylation of alkyl halides using [Fe(CO)3(NO)](-) has been studied using density functional theory (DFT). Our results suggest an SN2 mechanism for the alkylation event followed by a well-defined oxidative addition and alkyl migration. An experimentally elusive common intermediate [Fe(CO)2NO(Ac)] has been identified in two isomers and reacted in silico with a number of ligands (CO, PH3 and PPh3) to give the corresponding iron acyl complexes. Pathways between the two isomers, including direct involvement of a solvent molecule (THF) or iodide, have been elucidated.

The formation of water in the interstellar medium from hydrogen and oxygen atoms on a pristine olivine surface (forsterite (010)) is investigated with an embedded cluster approach. The 55-atom quantum cluster is described at the density functional level while the remaining 1629 atoms of the surface cluster are described with atomistic potentials. Transition states are most easily calculated with our modified implementation of the climbing-image nudged elastic band method in ChemShell. With these computational techniques, we find that gas phase hydrogen atoms can chemisorb (-102 kJ mol(-1)) without an activation barrier on the forsterite (010) surface, concomitantly creating a surface electron at the adjacent magnesium atom site. Subsequently, an oxygen atom chemisorbs strongly to this surface electron site (-432 kJ mol(-1)). The rearrangement of the adjacently chemisorbed O and H to a chemisorbed OH-radical is endothermic by 4 kJ mol(-1) and activated by 27 kJ mol(-1). This chemisorbed OH can then react barrierlessly with a second hydrogen atom to yield adsorbed water (-511 kJ mol(-1)). Alternatively, if O and H do not recombine to form OH, but instead thermally equilibrate, a second hydrogen atom can react with the chemisorbed oxygen atom (-501 kJ mol(-1)) to yield dissociatively adsorbed water (OH(-) and H(+)), which then can rearrange to associatively adsorbed water (-5 kJ mol(-1), DeltaE(double dagger) = 18 kJ mol(-1)) or gas phase water (+91 kJ mol(-1)). The formation of water on a bare dust grain from hydrogen and oxygen atoms is thus catalysed by an olivine surface by stabilising the reaction intermediates and product. Since the reaction proceeds via three chemisorbed intermediates, thermal equilibration is facilitated and back-dissociation of the freshly formed reaction products OH and H(2)O would not occur as frequently as it would in the gas phase or when the reactants are physisorbed on a surface.