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The chemical reactions between certain interstellar molecules are exothermic in nature and barrierless in the entrance channel, allowing these reactions to occur rapidly even at low astronomical temperatures, e.g., C and O2 interaction. Obtaining detailed rovibrational transition parameters for the reaction between C and O2, such as state-selected rate coefficients, is crucial for studying the associated atmospheric and astronomical environments. Hence, this work presents an approach that combines quasi-classical trajectory calculations with machine learning techniques based on Neural Network (NN) and Gaussian Process Regression (GPR) to determine state-selected rate coefficients. Employing this approach, we significantly reduced the computational requirements while simultaneously obtaining the accurate state-selected reaction cross sections and rate coefficients for the collision of C and O2. Both the NN-based and GPR-based models established in this work accurately predict the results calculated from explicit numerical calculations in the explored temperature range of 50–1500 K, achieving a coefficient of determination R2 > 0.96. Most importantly, the current work provides the most comprehensive dataset of rovibrational rate coefficients of v = 0–4, j = 0–70 → v′ = 0–15 for the astrophysical modeling of the C–O2 collision system. [ABSTRACT FROM AUTHOR]

With the advent of JWST ice observations, dedicated studies on the formation reactions of detected molecules are becoming increasingly important. One of the most interesting molecules in interstellar ice is CO2. Despite its simplicity, the main formation reaction considered, CO + OH → CO2 + H through the energetic HOCO* intermediate on ice dust, is subject to uncertainty because it directly competes with the stabilization of HOCO as a final product, which is formed through energy dissipation of HOCO* to the water ice. When energy dissipation to the surface is effective during the reaction, HOCO can be a dominant product. In this study, we experimentally demonstrate that the major product of the reaction is indeed not CO2, but rather the highly reactive radical HOCO. The HOCO radical can later evolve into CO2 through H-abstraction reactions, but these reactions compete with additional reactions, leading to the formation of carboxylic acids (R-COOH). Our results highlight the importance of HOCO chemistry and encourage further exploration of the chemistry of this radical. [ABSTRACT FROM AUTHOR]

Using the EXES instrument on SOFIA, we have obtained velocity-resolved spectra of several pure rotational lines of H2 toward shocked molecular gas within three Galactic sources: the supernova remnant (SNR) IC 443 (clump C), a protostellar outflow in the intermediate-mass star-forming region NGC 2071, and the SNR 3C 391. These observations had the goal of searching for expected velocity shifts between ortho- and para-H2 transitions emitted by C-type shocks. In contrast in our previous similar study of HH7, the result of our search was negative: no velocity shifts were reliably detected. Several possible explanations for the absence of such shifts are discussed: these include a preshock ortho-to-para ratio that is already close to the high-temperature equilibrium value of 3 (in the case of IC 443C), the more complex shock structures evident in all these sources, and the larger projected aperture sizes relative to those in the observations of HH7. [ABSTRACT FROM AUTHOR]

We report the first detection of the metal-bearing molecules sodium sulfide and magnesium sulfide and the tentative detection of calcium monoxide in the interstellar medium toward the Galactic center molecular cloud G+0.693-0.027. The derived column densities are (5.0 ± 1.1) × 1010 cm−2, (6.0 ± 0.6) × 1010 cm−2, and (2.0 ± 0.5) × 1010 cm−2, respectively. This translates into fractional abundances with respect to H2 of (3.7 ± 1.0) × 10−13, (4.4 ± 0.8) × 10−13, and (1.5 ± 0.4) × 10−13, respectively. We have also searched for other Na-, Mg-, and Ca-bearing species toward this source but none of them have been detected and thus we provide upper limits for their abundances. We discuss the possible chemical routes involved in the formation of these molecules containing metals under interstellar conditions. Finally, we compare the ratio between sulfur-bearing and oxygen-bearing molecules with and without metals, finding that metal-bearing sulfur molecules are much more abundant than metal-bearing oxygen ones, in contrast with the general trend found in the ratios between other nonmetal-oxygen- and sulfur-bearing molecules. This further strengthens the idea that sulfur may be a little depleted in G+0.693-0.027 as a result of the low-velocity shocks present in this source sputtering large amounts of material from dust grains. [ABSTRACT FROM AUTHOR]

Planet formation is strongly influenced by the composition and distribution of volatiles within protoplanetary disks. With JWST, it is now possible to obtain direct observational constraints on disk ices, as recently demonstrated by the detection of ice absorption features toward the edge-on HH 48 NE disk as part of the Ice Age Early Release Science program. Here, we introduce a new radiative transfer modeling framework designed to retrieve the composition and mixing status of disk ices using their band profiles, and apply it to interpret the H2O, CO2, and CO ice bands observed toward the HH 48 NE disk. We show that the ices are largely present as mixtures, with strong evidence for CO trapping in both H2O and CO2 ice. The HH 48 NE disk ice composition (pure versus polar versus apolar fractions) is markedly different from earlier protostellar stages, implying thermal and/or chemical reprocessing during the formation or evolution of the disk. We infer low ice-phase C/O ratios around 0.1 throughout the disk, and also demonstrate that the mixing and entrapment of disk ices can dramatically affect the radial dependence of the C/O ratio. It is therefore imperative that realistic disk ice compositions are considered when comparing planetary compositions with potential formation scenarios, which will fortunately be possible for an increasing number of disks with JWST. [ABSTRACT FROM AUTHOR]

Observational data show complex organic molecules in the interstellar medium (ISM). Hydrogenation of small unsaturated carbon double bond could be one way for molecular complexification. It is important to understand how such reactivity occurs in the very cold and low‐pressure ISM. Yet, there is water ice in the ISM, either as grain or as mantle around grains. Therefore, the addition of atomic hydrogen on double‐bonded carbon in a series of seven molecules have been studied and it was found that water catalyzes this reaction. The origin of the catalysis is a weak charge transfer between the π MO of the unsaturated molecule and H atom, allowing a stabilizing interaction with H2O. This mechanism is rationalized using the non‐covalent interaction and the quantum theory of atoms in molecules approaches. [ABSTRACT FROM AUTHOR]

One of the most abundant Al-containing molecules detected in the interstellar medium (ISM) is AlOH. Over the past several years, there have been various pathways proposed for the formation of AlOH in the ISM, including reactions between AlO and H2 or H2O. However, these pathways include an energetic barrier from a transition state that likely prevents the reaction from progressing efficiently in the low temperature/low pressure environment of the ISM. Recently, a barrierless pathway for formation of AlOH from AlO and AlH has been proposed for the formation of AlOH. Even so, only one of these species really needs to contain an aluminum atom. To account for this, alternative but related pathways reacting the known interstellar molecule AlO with XH and AlH with XO (X = Mg, Si, P, or S) to form AlOH are explored with high accuracy quantum chemical calculations via CCSD(T)-F12b/cc-pVTZ-F12. Each third row element has at least one pair of reactants that lead to exothermic formation of AlOH. These reactions can go on to form other aluminum oxides and aluminum oxide clusters that may, in part, lead to the formation of interstellar dust grains. [ABSTRACT FROM AUTHOR]

Methane (CH4) is one of the major components of the icy mantle of cosmic dust prevalent in cold, dense regions of interstellar media, playing an important role in the synthesis of complex organic molecules and prebiotic molecules. Solid CH4 is considered to be formed via the successive hydrogenation of C atoms accreting onto dust: C + 4H → CH4. However, most astrochemical models assume this reaction on the ice mantles of dust to be barrierless and efficient, without considering the states of adsorption. Recently, we found that C atoms exist in either the physisorbed or chemisorbed state on compact amorphous solid water, which is analogous to an interstellar ice mantle. These distinct adsorption states considerably affect the hydrogenation reactivity of the C atom. Herein, we elucidate the reactivities of physisorbed and chemisorbed C atoms with H atoms via sequential deposition and codeposition processes. The results indicate that only physisorbed C atoms can produce CH4 on ice. Combining this finding with a previous estimate for the fraction of physisorbed C atoms on ice, we determined the upper limit for the conversion of C atoms into CH4 to be 30%. [ABSTRACT FROM AUTHOR]

Forsterite is a primary constituent of interstellar dust and planetary systems. It is believed to originate from the outflows of oxygen-rich stars and undergo further processing within the interstellar medium through the action of cosmic rays and shocks. Under these harsh conditions, point defects may form, such as MgO Schottky vacancies. These vacancies can then undergo atom reconstruction as part of a chemical process to maintain the system's crystalline structure. Polycyclic aromatic hydrocarbons (PAHs) are ubiquitously observed interstellar molecules and are thought to form through gas-phase reactions akin to sooting flames. However, their role and impact on dust stability remain unknown. In this study, we employ an atomistic artificial-intelligence-based method, surrogate machine learning trained directly by density functional theory. Specifically, we utilize gofee (global optimization with first-principles energy expressions) to predict possible reconstructions of MgO vacancies on a crystalline forsterite (010) surface as an important component of interstellar dust and planetary systems. We identify nine possible reconstructions involving the formation of unbound Si and O atoms. We investigate their energy stability and find that the reconstruction of Si–O atoms stabilizes the vacancy by about 0.54 eV. Additionally, if PAHs bind with the unbound O and Si atoms of the vacancy, the vacancy is stabilized by approximately 1.76 eV. We demonstrate that PAHs, along with the reconstruction of unbound atoms on the dust surface, affect the stability of the dust, which might open up avenues for diverse chemistry. [ABSTRACT FROM AUTHOR]

Gas-phase abundance ratios between C2H4O2 isomers methyl formate (MF), glycolaldehyde (GA), and acetic acid (AA) are typically on the order of 100:10:1 in star-forming regions. However, an unexplained divergence from this neat relationship was recently observed toward a collection of sources in the massive protocluster NGC 6334I; some sources exhibited extreme MF:GA ratios, producing a bimodal behavior between different sources, while the MF:AA ratio remained stable. Here, we use a three-phase gas-grain hot-core chemical model to study the effects of a large parameter space on the simulated C2H4O2 abundances. A combination of high gas densities and long timescales during ice-mantle desorption (∼125–160 K) appears to be the physical cause of the high MF:GA ratios. The main chemical mechanism for GA destruction occurring under these conditions is the rapid adsorption and reaction of atomic H with GA on the ice surfaces before it has time to desorb. The different binding energies of MF and GA on water ice are crucial to the selectivity of the surface destruction mechanism; individual MF molecules rapidly escape the surface when exposed by water loss, while GA lingers and is destroyed by H. Moderately elevated cosmic-ray ionization rates can increase absolute levels of "complex organic molecule" (COM) production in the ices and increase the MF:GA ratio, but extreme values are destructive for gas-phase COMs. We speculate that the high densities required for extreme MF:GA ratios could be evidence of COM emission dominated by COMs desorbing within a circumstellar disk. [ABSTRACT FROM AUTHOR]

The chemistry of phosphorus (31P) in space is particularly significant due to the key role it plays in biochemistry on Earth. Utilising radio and infrared spectroscopic observations, several key phosphorus-containing molecules have been detected in interstellar clouds, circumstellar shells, and even extragalactic sources. Among these, phosphorus nitride (PN) was the first P-bearing molecule detected in space, and still is the species detected in the largest number of sources. Phosphorus oxide (PO) and phosphine (PH3) were also crucial species due to their role both in chemical networks and in forming biogenic compounds. The still limited high-angular resolution observations performed so far are shading light on the geometrical distribution of these molecules, which represent crucial insights on their formation processes. Observations have also highlighted the challenges and complexities associated with detecting and understanding phosphorus chemistry in space, owing to the low elemental abundance of P relative to other elements. This review article provides a state-of-art picture of the observational results obtained so far on phosphorus compounds in the interstellar medium. Special attention is given to star- forming regions, and to their implications for our understanding of prebiotic chemistry and the potential for life beyond Earth. Our knowledge of the dominant formation and destruction pathways of the most abundant species has improved, but critical questions remain open, among which: what is (are) the main phosphorus carrier(s) in space? Upcoming new facilities are expected to contribute significantly to this field, offering opportunities to both detect new phosphorus-bearing molecules and enlarge the number of sources in which the chemistry of P can be studied. The synergy between observations, theoretical models, laboratory experiments, and computational chemistry is mandatory to significantly progress in our comprehension of the chemistry of this important but poorly studied chemical element. [ABSTRACT FROM AUTHOR]

The detection of various molecular species, including complex organic molecules relevant to biochemical and geochemical processes, in astronomical settings, such as the interstellar medium or the outer solar system, has led to the increased need for a better understanding of the chemistry occurring in these cold regions of space. In this context, the chemistry of ices prepared and processed at cryogenic temperatures has proven to be of particular interest due to the fact that many interstellar molecules are believed to originate within the icy mantles adsorbed on nano- and micro-scale dust particles. The chemistry leading to the formation of such molecules may be initiated by ionizing radiation in the form of galactic cosmic rays or stellar winds, and thus, there has been an increased interest in commissioning experimental setups capable of simulating and better characterizing this solid-phase radiation astrochemistry. In this article, we describe a new facility called AQUILA (Atomki-Queen's University Ice Laboratory for Astrochemistry), which has been purposefully designed to study the chemical evolution of ices analogous to those that may be found in the dense interstellar medium or the outer solar system as a result of their exposure to keV ion beams. The results of some ion irradiation studies of CH3OH ice at 20 K are discussed to exemplify the experimental capabilities of the AQUILA as well as to highlight its complementary nature to another laboratory astrochemistry setup at our institute. [ABSTRACT FROM AUTHOR]

Ice mantles play a crucial role in shaping the astrochemical inventory of molecules during star and planet formation. Small-scale molecular processes have a profound impact on large-scale astronomical evolution. The areas of solid-state laboratory astrophysics and computational chemistry involve the study of these processes. We review laboratory efforts in ice spectroscopy, methodological advances and challenges, and laboratory and computational studies of ice physics and ice chemistry. We place the last of these in context with ice evolution from clouds to disks. Three takeaway messages from this review are: Laboratory and computational studies allow interpretation of astronomical ice spectra in terms of identification, ice morphology, and local environmental conditions as well as the formation of the involved chemical compounds. A detailed understanding of the underlying processes is needed to build reliable astrochemical models to make predictions about abundances in space. The relative importance of the different ice processes studied in the laboratory and computationally changes during the process of star and planet formation. [ABSTRACT FROM AUTHOR]

The present survey represents a continuation of our study of high mass star forming regions in the lines of deuterated molecules, the first results of which were published in [1]. This paper present the results of observations of 50 objects in the line of ortho modification of singly deuterated ammonia NH2D at a frequency of 85.9 GHz, carried out using the 20-m radio telescope of the Onsala Space Observatory (Sweden). This line is detected in 29 sources. The analysis of obtained data, as well as the fact that the gas density in the investigated sources, according to independent estimates, is significantly lower than the critical density for this NH2D transition, indicate non-LTE excitation of NH2D. Based on non-LTE modeling, estimates of the relative content of the NH2D molecule and the degree of deuterium enrichment were obtain, and the dependencies of these parameters on temperature and velocity dispersion were analyzed with and without taking into account detection limits assuming the same gas density in all sources. An anti-correlation between the NH2D relative abundances and the kinetic temperature is revealed in a temperature range of 15–50 K. At the same time, a significant decrease in the ratio of the NH2D/NH3 abundances with increasing temperature, predicted by the available chemical models, is not observed under the adopted assumptions. An anti-correlation was also revealed between the relative content of the main isotopologue of ammonia NH and the velocity dispersion, while no statistically significant correlation with the kinetic temperature of sources in the same temperature range was found. [ABSTRACT FROM AUTHOR]

Thioformyl cyanide, HC(S)CN, and its deuterated isotopologue have been studied by high-level ab initio methods. Based on multidimensional potential energy surfaces including up to four-mode coupling terms and obtained from explicitly correlated coupled-cluster calculations, their geometrical parameters, vibrational transitions, rovibrational spectrum and spectroscopic constants have been studied in detail. The latter two properties, being obtained from vibrational configuration interaction theory (VCI) and vibrational perturbation theory (VPT), have been compared with available experimental data. The rotational and rovibrational spectra have been studied by rovibrational configuration interaction theory (RVCI) and colour coding has been used to obtain detailed insight into the rotational fine structure. [ABSTRACT FROM AUTHOR]

Insufficient theoretical investigation is being conducted on the spectroscopic characteristics within the infrared wavelength range of SiP, an identified interstellar molecule. Using the ic-MRCI method, potential energy functions and dipole moment functions for the ground state (⁠|${X^2}\Pi $|⁠) and low excited states (⁠|${{\rm{A}}^2}{\Sigma ^ + }$|⁠) of the SiP molecule were calculated. Based on experimental spectroscopy data, least squares fitting was used for the potential energy functions of the |${X^2}\Pi $| and |${{\rm{A}}^2}{\Sigma ^ + }$| states. By combining these potential energy and dipole moment functions, the one-dimensional Schrödinger equation was solved to obtain the vibronic energy levels and Einstein A coefficients for the electronic states. Partition functions of the SiP molecule from 0.1 to 3000 K and the radiative properties of |${X^2}\Pi \leftrightarrow {X^2}\Pi $| and |${X^{\rm{2}}}\Pi \leftrightarrow {{\rm{A}}^2}{\Sigma ^ + }$| were derived. Infrared spectroscopy of the |${X^2}\Pi $| state and ultraviolet spectroscopy of the |${X^{\rm{2}}}\Pi \leftrightarrow {{\rm{A}}^2}{\Sigma ^ + }$| transition at 100 K, a temperature crucial for astronomical research, were calculated. Results indicate that the spectral-line intensity of the |${X^{\rm{2}}}\Pi \leftrightarrow {{\rm{A}}^2}{\Sigma ^ + }$| transition is greater, making it more suitable for astronomical observation. The obtained computational results in this paper yield spectroscopic parameters for the characterization of the interstellar molecule SiP, furnishing theoretical underpinnings for subsequent experimental observations. [ABSTRACT FROM AUTHOR]

Infrared spectroscopic observations have established the presence of solid methanol (CH3OH) in the interstellar medium and in solar system ices, but the abundance of frozen CH3OH cannot be deduced without accurate band strengths, optical constants, and reference spectra. In this paper we identify disagreements, omissions, and gaps in the literature on infrared (IR) intensities of methanol ices, including unaddressed concerns that reach back several decades. New spectra are presented with intensity measurements aided by new data on the index of refraction and density of solid CH3OH. The result is that the large discordant results from different laboratory groups can now be reconciled. Multiple ices have been used to determine, apparently for the first time, IR intensities of H2O + CH3OH mixtures of accurately known composition for use with observations of interstellar ices. Also for the first time, measurements on CH3OH ices with different thicknesses have allowed us to report both near-IR band strengths and optical constants for two near-IR features used by planetary scientists. We have used our new IR results to determine vapor pressures of solid CH3OH and have compared them to measurements made with a quartz-crystal microbalance. Thermal annealings of methanol ices have been carried out and phase changes in the solid state examined. Comparisons of our results to earlier work are presented where possible, and electronic versions of our new results are made available. [ABSTRACT FROM AUTHOR]

Understanding the impact of the relativistic motion of a chiral molecule on its optical response is a prime challenge for fundamental science, but it also has a direct practical relevance in our search for extraterrestrial life. To contribute to these significant developments, we describe a multi–scale computational framework that combines quantum chemistry calculations and full–wave optical simulations to predict the chiral optical response from molecules moving at relativistic speeds. Specifically, the effect of a relativistic motion on the transmission circular dichroism (TCD) of three life–essential biomolecules, namely, B–DNA, chlorophyll a, and chlorophyll b, is investigated. Inspired by previous experiments to detect interstellar chiral molecules, we assume that the molecules move between a stationary observer and a light source, and we study the rotationally averaged TCD as a function of the speed of the molecule.We find that the TCD spectrum that contains the signatures of the molecules shifts with increasing speed to shorter wavelengths, with the effects already being visible for moderate velocities. [ABSTRACT FROM AUTHOR]

Deuterium fractionation in the closest vicinity of a protostar is important in understanding its potential heritage to a planetary system. Here, we have detected the spectral line emission of CH3OH and its three deuterated species, CH2DOH, CHD2OH, and CH3OD, toward the low-mass protostellar source B335 at a resolution of 0.″03 (5 au) with Atacama Large Millimeter/submillimeter Array. They have a ring distribution within the radius of 24 au with the intensity depression at the continuum peak. We derive the column densities and abundance ratios of the above species at six positions in the disk/envelope system as well as the continuum peak. The D/H ratio of CH3OH is ∼[0.03–0.13], which is derived by correcting the statistical weight of 3 for CH2DOH. The [CHD2OH]/[CH2DOH] ratio is derived to be higher ([0.14–0.29]). On the other hand, the [CH2DOH]/[CH3OD] ratio ([4.9–15]) is higher than the statistical ratio of 3 and is comparable to those reported for other low-mass sources. We study the physical structure on a few astronomical unit scales in B335 by analyzing the CH3OH (183,15 − 182,16, A) and HCOOH (120,12 − 110,11) line emission. Velocity structures of these lines are reasonably explained as the infalling-rotating motion. The protostellar mass and the upper limit to the centrifugal barrier are thus derived to be 0.03–0.07 M ⊙ and <7 au, respectively, showing that B335 harbors a young protostar with a tiny disk structure. Such youth of the protostar may be related to the relatively high [CH2DOH]/[CH3OH] ratio. [ABSTRACT FROM AUTHOR]

Context. The presence of grains is key to the synthesis of molecules in the interstellar medium that cannot form in the gas phase due to its low density and temperature conditions. In these reactions, the role of the grains is to enhance the encounter rate of the reactive species on their surfaces and to dissipate the energy excess of largely exothermic reactions, but less is known about their role as chemical catalysts; namely, bodies that provide low activation energy pathways with enhanced reaction rates. Different refractory materials with catalytic properties, such as those containing space-abundant d-block transition metals like iron (Fe), are present in astrophysical environments. Aims. Here, we report for first time mechanistic insights into the Fischer-Tropsch-type (FTT) synthesis of ethanol (CH3CH2OH), through ketene (CH2CO) and acetaldehyde (CH3CHO) intermediates, and methane (CH4) via a chain growing mechanism using a single-Fe atom supported on silica (SiO2) surfaces as a heterogeneous astrocatalyst. Methods. Quantum chemical simulations based on extended periodic surfaces were carried out to characterize the potential energy surfaces of the FTT chain growing mechanism. Calculations of the binding energies of reaction intermediates and products and Rice–Ramsperger–Kassel–Marcus kinetic calculations were performed to evaluate catalytic efficiencies and determine the feasibility of the reactions in different astrophysical environments. Results. Mechanistic studies demonstrate that the FTT chain growing mechanism enters into direct competition with FTT methanol formation, since formation of the CH2 chain growth initiator is feasible. The coupling of the CH2 with CO (forming ketene) and subsequent H2 additions yield acetaldehyde and finally ethanol, while direct H2 addition to CH2 produces methane. Thermodynamically, both processes are largely exergonic, but they present energy barriers that require external energy inputs to be overcome. Kinetic calculations demonstrate the strong temperature dependency of the FTT processes as tunneling does not dominate. Conclusions. The results could explain the presence of CH3CH2OH and CH4 in diverse astrophysical regions where current models fail to reproduce their observational quantities. The evidence that the chain growing mechanism is operating opens a new reactivity paradigm toward the formation of complex organic molecules, which is constrained by the temperature-dependent behaviour of the FTT reactions and by making their energy features a crucial aspect. [ABSTRACT FROM AUTHOR]

Context. Among the silicon-bearing species discovered in the interstellar medium, SiS and SiO stand out as key tracers due to their distinct chemistry and variable abundances in interstellar and circumstellar environments. Nevertheless, while the origins of SiO are well documented, the SiS chemistry remains relatively unexplored. Aims. Our objective is to enhance the network of Si- and S-bearing chemical reactions for a gas-grain model in molecular clouds, encompassing both low and high metallicities. To achieve this, we calculated the energies and rate coefficients for six neutral atom-diatom reactions involved in the SiCS triatomic system, with a special focus on the C+SiS and S+SiC collisions. Methods. We employed the coupled-cluster method with single and double substitutions and a perturbative treatment of triple substitutions (CCSD(T)) refined at the explicitly correlated CCSD(T)-F12 level. With these computational results in conjunction with supplementary data from the literature, we construct an extended network of neutral-neutral chemical reactions involving Si- and S-bearing molecules. To assess the impact of these chemical reactions, we performed time-dependent models employing the Nautilus gas-grain code, setting the gas temperature to 10 K and the H2 density to 2 × 104 cm−3. The models considered two initial abundance scenarios, corresponding to low- and high-metallicity levels. Abundances were computed using both the default chemical network and the constrained network, enriched with newly calculated reactions. Results. The temperature dependence for the reactions involving SiS were modelled to the k(T) = α (T/300)β exp (−γ/T) expression, and the coefficients are provided for the first time. The high-metallicity models significantly boost the SiS production, resulting in abundances nearly four orders of magnitude higher compared to low-metallicity models. Higher initial abundances of C, S, and Si, roughly ~2, 190, and 210 times higher, respectively, contribute to this. Around the age of 103 yr, destruction mechanisms become relevant, impacting the abundance of SiS. The proposed production reaction S + SiC → C + SiS, mitigates these effects in later stages. By expanding the gas reaction network using a high-metallicity model, we derived estimates for the abundances of observed interstellar molecules, including SiO, SO, and SO2. Conclusions. We demonstrate the significance of both SiC+S and C+SiS channels in the SiS chemistry. Notably, the inclusion of neutral-neutral mechanisms, particularly via Si+HS and S+SiC channels, played a pivotal role in determining SiS abundance. These mechanisms carry a significance level on a par with that of the well-known and fast ion-neutral reactions. [ABSTRACT FROM AUTHOR]

The formation of nitrogen-containing organic interstellar molecules is of great importance to reveal chemical processes and the origin of life on Earth. Benzonitrile (BN) is one of the simplest nitrogen-containing aromatic molecules in the interstellar medium (ISM) that has been detected in recent years. Methanol (CH3OH) exists widely in interstellar space with high reactivity. Herein, we measured the infrared (IR) spectra of neutral and cationic BN–CH3OH clusters by vacuum ultraviolet (VUV) photoionization combined with time-of-flight mass spectrometry. Combining IR spectra with the density functional theory calculations, we reveal that the BN–CH3OH intends to form a cyclic H-bonded structure in neutral clusters. However, after the ionization of BN–CH3OH clusters, proton-shared N···H···O and N···H···C structures are confirmed to form between BN and CH3OH, with the minor coexistence of H-bond and O-π structures. The formation of the proton-shared structure expands our knowledge of the evolution of the life-related nitrogen-containing molecules in the universe and provides a possible pathway to the further study of biorelevant aromatic organic macromolecules. [ABSTRACT FROM AUTHOR]

Polycyclic aromatic hydrocarbons (PAHs) are widely established as ubiquitous in the interstellar medium (ISM), but considering their prevalence in harsh vacuum environments, the role of ionisation in the formation of PAH clusters is poorly understood, particularly if a chirality-dependent aggregation route is considered. Here we report on photoelectron spectroscopy experiments on [4]helicene clusters performed with a vacuum ultraviolet synchrotron beamline. Aggregates (up to the heptamer) of [4]helicene, the smallest PAH with helical chirality, were produced and investigated with a combined experimental and theoretical approach using several state-of-the-art quantum-chemical methodologies. The ionisation onsets are extracted for each cluster size from the mass-selected photoelectron spectra and compared with calculations of vertical ionisation energies. We explore the complex aggregation topologies emerging from the multitude of isomers formed through clustering of P and M, the two enantiomers of [4]helicene. The very satisfactory benchmarking between experimental ionisation onsets vs. predicted ionisation energies allows the identification of theoretically predicted potential aggregation motifs and corresponding energetic ordering of chiral clusters. Our structural models suggest that a homochiral aggregation route is energetically favoured over heterochiral arrangements with increasing cluster size, hinting at potential symmetry breaking in PAH cluster formation at the scale of small grains. Polycyclic aromatic hydrocarbons such as [4]helicenes are important interstellar molecules. Here, the authors show their evolution of chiral clusters upon aggregation through coincidence spectroscopy and quantum chemical calculations. [ABSTRACT FROM AUTHOR]

To investigate the gas-phase hydrogenation processes of large, astronomically relevant cationic polycyclic aromatic hydrocarbon (PAH) molecules under the interstellar environments, the ion–molecule collision reaction between six PAH cations and H-atoms is studied. The experimental results show that the hydrogenated PAH cations are efficiently formed, and no even–odd hydrogenated mass patterns are observed in the hydrogenation processes. The structure of newly formed hydrogenated PAH cations and the bonding energy for the hydrogenation reaction pathways are investigated with quantum theoretical calculations. The exothermic energy for each reaction pathway is relatively high, and the competition between hydrogenation and dehydrogenation is confirmed. From the theoretical calculation, the bonding ability plays an important role in the gas-phase hydrogenation processes. The factors that affect the hydrogenation chemical reactivity are discussed, including the effect of carbon skeleton structure, the side-edged structure, the molecular size, the five- and six-membered C-ring structure, the bay region structure, and the neighbouring hydrogenation. The infrared spectra of hydrogenated PAH cations are also calculated. These results we obtain once again validate the complexity of hydrogenated PAH molecules, and provide the direction for the simulations and observations under the co-evolution interstellar chemistry network. We infer that if we do not consider other chemical evolution processes (e.g. photoevolution), then the hydrogenation states and forms of PAH compounds are intricate and complex in the interstellar medium. [ABSTRACT FROM AUTHOR]

Enols—tautomers of ketones or aldehydes—are considered key intermediates in the formation of prebiotic sugars and sugar acids. Although laboratory simulation experiments suggest that enols should be ubiquitous in the interstellar medium, the underlying formation mechanisms of enols in interstellar environments are largely elusive. Here, we present the laboratory experiments on the formation of glyoxal (HCOCHO) along with its ynol tautomer acetylenediol (HOCCOH) in interstellar ice analogs composed of carbon monoxide (CO) and water (H2O) upon exposure to energetic electrons as a proxy for secondary electrons generated from Galactic cosmic rays. Utilizing tunable vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry, glyoxal and acetylenediol were detected in the gas phase during temperature-programmed desorption. Our results reveal the formation pathways of glyoxal via radical–radical recombination of two formyl (HĊO) radicals, and that of acetylenediol via keto-enol-ynol tautomerization. Due to the abundance of carbon monoxide and water in interstellar ices, glyoxal and acetylenediol are suitable candidates for future astronomical searches. Furthermore, the detection of acetylenediol in astrophysically relevant ices advances our understanding for the formation pathways of high-energy tautomers such as enols in deep space. [ABSTRACT FROM AUTHOR]

Metal (iso)cyanides dominate the molecular inventory of metal-bearing species in the interstellar medium. Their oxide counterparts, metal (iso)cyanates, have potential as interstellar molecules and have received significant attention. However, cationic complexes HNCOM+ as precursors to metal (iso)cyanates are rarely studied. Herein, we investigated HNCOCa+ by exploiting infrared spectrometry with isotopic substitutions and quantum chemical calculations. For comparison, the light and heavy alkaline earth metal cationic complexes HNCOBe+ and HNCOBa+ were also explored. HNCOCa+ and HNCOBe+ rather than HNCOBa+ can be experimentally generated by the reactions of metal cations with HNCO. The observed antisymmetric and symmetric NCO stretching vibrations in HNCOCa+ (2362.6 and 1330.4 cm−1) are higher than those in free HNCO (2268.5 and 1320.3 cm−1) but lower than those in HNCOBe+ (2426.4 and 1355.2 cm−1). These shifts can be explained by the charge polarization within the NCO fragment in HNCOBe+ and HN-COCa+. Bonding analysis suggests that HNCO–Be+ bond favors covalent character (54%) while HNCO–Ca+ bond has higher electrostatic character (57%). The dominant electrostatic interaction (64%) in HNCO–Ba+ bond results in the low bond energy, which might account for its absence in experiments. [ABSTRACT FROM AUTHOR]

Radiolysis of α-pinene by 61.3 MeV 84Kr15 + ions was analysed with the scope to simulate the effects of heavy ion cosmic ray bombardment on chiral molecules in the interstellar medium. The α-pinene ice samples were irradiated at 10 K and their chemical evolution was monitored by mid-infrared Fourier transform (FTIR) spectroscopy to characterize the reaction products and to determine the extent of racemization. The integrated band strengths have been obtained for all the neutral α-pinene vibrational bands using the experimental band integrated absorbances and the theoretical absolute intensities calculated along the column densities. In the current heavy ion bombardment experiments, small molecules were formed and the precursor, α-pinene, was destroyed instead of being racemized. Twelve hydrocarbons were produced (final fluence of 2.0 × 1012 ions cm−2): methane (CH4), acetylene (C2H2), ethylene (C2H4), propylene (C3H6), propane (C3H8), n-butane (C4H10), butene (C4H8), propyne (C3H4), benzene (C6H6), ethane (C2H6), vinylacetylene (C4H4), and 2-methyl-1,3-butadiene or isoprene (C5H8). The highest formation cross-section (∼ 40 × 10−15 cm2) was observed for the C3H4 and the lowest was for C3H8 (∼ 3 × 10−15 cm2). The radiochemical yields for these molecules follow the same trends as those of their cross-sections. The atom budget calculation confirms that all the expected products have been generated during the radiolysis and supports the conclusion that the proposed A values are accurate. The α-pinene sputtering yield for this ion beam was found to be Y0  = 1.84 × 106 molecules per impact. [ABSTRACT FROM AUTHOR]

We have studied the stability of C59 anions as a function of time, from their formation on femtosecond timescales to their stabilization on second timescales and beyond, using a combination of theory and experiments. The C 59 − fragments were produced in collisions between C60 fullerene anions and neutral helium gas at a velocity of 90 km s−1 (corresponding to a collision energy of 166 eV in the center-of-mass frame). The fragments were then stored in a cryogenic ion beam storage ring at the DESIREE facility, where they were followed for up to 1 minute. Classical molecular dynamics simulations were used to determine the reaction cross section and the excitation energy distributions of the products formed in these collisions. We find that about 15% of the C 59 − ions initially stored in the ring are intact after about 100 ms and that this population then remains intact indefinitely. This means that C60 fullerenes exposed to energetic atoms and ions, such as stellar winds and shock waves, will produce stable, highly reactive products, like C59, that are fed into interstellar chemical reaction networks. [ABSTRACT FROM AUTHOR]

Polycyclic aromatic hydrocarbon (PAH) molecules have long been suggested to be present in the interstellar medium (ISM). Nevertheless, despite their expected ubiquity and sustained searching efforts, identifying specific interstellar PAH molecules from their infrared (IR) spectroscopy has so far been unsuccessful. However, due to its unprecedented sensitivity, the advent of the JWST may change this. Meanwhile, recent years have witnessed breakthroughs in detecting specific PAH molecules (e.g. indene, cyanoindene, and cyanonaphthalene) through their rotational lines in the radio frequencies. As JWST holds great promise for identifying specific PAH molecules in the ISM based on their vibrational spectra in the IR, in this work, we model the vibrational excitation of indene, a molecule composed of a six-membered benzene ring fused with a five-membered cyclopentene ring, and calculate its IR emission spectra for a number of representative astrophysical regions. This will facilitate JWST to search for and identify indene in space through its vibrational bands and to quantitatively determine or place an upper limit on its abundance. [ABSTRACT FROM AUTHOR]

To date, four $ \hbox {C}_2\hbox {H}_4\hbox {O}_2 $ C 2 H 4 O 2 isomers have been identified in the interstellar medium: acetic acid, glycolaldehyde, methyl formate, and (Z)-1,2-ethenediol. The detection of these species raises the question of whether other related carbohydrates could be present in similar astrophysical environments. In this study, we report a systematic investigation of six $ \hbox {C}_2\hbox {H}_4\hbox {O}_2 $ C 2 H 4 O 2 isomers not previously observed spectroscopically in space: anti-(1)-, (2)- and (3)- $ \hbox {CH}_2=\hbox {CHOOH} $ CH 2 = CHOOH , methyldioxirane, and anti- and syn-(2S)-oxiran-2-ol. For each species, we employ coupled-cluster theory at the AE-CCSD(T)/cc-pCVQZ level to provide accurate equilibrium parameters, rotational constants, quartic centrifugal distortion parameters, and permanent electric dipole moment components. Anharmonic force fields were determined within vibrational perturbation theory to second order and evaluated using the MP2 method, thus allowing the evaluation of vibrational corrections. These computations provided, as a by-product, sextic centrifugal distortion constants. Rotational spectra have subsequently been simulated in the millimeter/submillimeter-wave frequency for the first time. It is hoped and anticipated that the accurate theoretical rovibrational spectral data presented herein will aid in experimental investigations of these title molecules in the interstellar medium. [ABSTRACT FROM AUTHOR]

An investigation of the rotational spectrum of the interstellar molecule thioformaldehyde between 110 and 377 GHz through a pyrolysis reaction revealed a multitude of absorption lines assignable to H $ _2 $ 2 CS and H $ _2 $ 2 C $ ^{34} $ 34 S in their lowest four excited vibrational states besides lines of numerous thioformaldehyde isotopologues in their ground vibrational states reported earlier as well as lines pertaining to several by-products. Additional transitions of H $ _2 $ 2 CS in its lowest four excited vibrational states were recorded in selected regions between 571 and 1386 GHz. Slight to strong Coriolis interactions occur between all four vibrational states with the exception of the two highest lying states because both are totally symmetric vibrations. We present combined analyses of the ground and the four interacting states for our rotational data of H $ _2 $ 2 CS and H $ _2 $ 2 C $ ^{34} $ 34 S. The H $ _2 $ 2 CS data were supplemented with two sets of high-resolution IR data in two separate analyses. The $ v_2 = 1 $ v 2 = 1 state has been included in analyses of Coriolis interactions of low-lying fundamental states of H $ _2 $ 2 CS for the first time and this improved the quality of the fits substantially. We extended furthermore assignments in J of transition frequencies of thioketene in its ground vibrational state. [ABSTRACT FROM AUTHOR]

The recent identification of interstellar molecules HCCCHCCC and HCCCCS in TMC-1 has prompted an ab initio investigation of these isomers. The isomers of HCCCHCCC have been studied previously but not so much HCCCCS. In this paper we carry out high-level CCSD(T) calculations to characterise the lowest eleven isomers of HCCCCS. We also apply complete basis set extrapolation including core correlation and relativistic effects to further investigate the key isomers and provide high-precision rotational constants which will hopefully aid astronomers in their assignments. Source: This image has been adapted from licensed under CC BY-SA 3.0. [ABSTRACT FROM AUTHOR]

The presence of carbon-chain molecules in the interstellar medium (ISM) has been known since the early 1970s and > 130 such species have been identified to date, making up ∼ 43 % of the total of detected ISM molecules. They are prevalent not only in star-forming regions in our Galaxy but also in other galaxies. These molecules provide important information on physical conditions, gas dynamics, and evolutionary stages of star-forming regions. Larger species of polycyclic aromatic hydrocarbons (PAHs) and fullerenes (C60 and C70), which may be related to the formation of the carbon-chain molecules, have been detected in circumstellar envelopes around carbon-rich Asymptotic Giant Branch (AGB) stars and planetary nebulae, while PAHs are also known to be a widespread component of the ISM in most galaxies. Recently, two line survey projects toward Taurus Molecular Cloud-1 with large single-dish telescopes have detected many new carbon-chain species, including molecules containing benzene rings. These new findings raise fresh questions about carbon-bearing species in the Universe. This article reviews various aspects of carbon-chain molecules, including observational studies, chemical simulations, quantum calculations, and laboratory experiments, and discusses open questions and how future facilities may answer them. [ABSTRACT FROM AUTHOR]

The presence of Polycyclic Aromatic Hydrocarbon (PAH) molecules in the interstellar medium, recently confirmed by the detection of cyano-naphthalenes, has renewed the interest of extensive spectroscopic and physical-chemistry studies on such large species. The present study reports the jet-cooled rovibrational infrared study of three centrosymmetric two-ring PAH molecules, viz., naphthalene (C10H8), [1,5] naphthyridine (C8H6N2), and biphenyl (C12H10), in the in-plane ring C–H bending (975–1035 cm−1) and C–C ring stretching (1580–1620 cm−1) regions. For the two most rigid PAHs, the accuracy of spectroscopic parameters derived in ground and several excited states (six for naphthalene and six for [1,5] naphthyridine) has significantly improved the literature values. In addition, comparison between experiments and quantum chemical calculations confirms the predictive power of the corrected calculated rotational parameters. The more flexible structure of biphenyl makes the analysis of high resolution jet-cooled spectra of ν19 and ν23 modes recorded at about 1601 and 1013 cm−1, respectively, particularly challenging. The presence of three torsional vibrations below 120 cm−1 together with small values of the rotational constants prevented us from determining the ground and v19 = 1 excited rotational constants independently. In the ν23 band region, the presence of two bands rotationally resolved and separated by only 0.8 cm−1 raises the question of possible splittings due to a large amplitude motion, most probably the torsion of the aliphatic bond between the two phenyl rings. [ABSTRACT FROM AUTHOR]

Understanding the fundamental mechanisms that underlie the synthesis of fullerene molecules in the interstellar medium (ISM) and in the environments of astrophysical objects is an open question. In this regard, using classical molecular dynamics, we demonstrate the possibility of in situ formation of fullerene molecules, such as C60 from graphite, which is known to occur in the ISM, in particular, circumstellar environments. Specifically, when graphite is subjected to thermal and mechanical stimuli that are typical of circumstellar shells, we find that the graphite sheet edges undergo significant restructuring and curling, leading to edge-induced interlayer-interactions and formation of mechanically strained five-membered-ring structural units. These units serve as precursors for the formation of fullerene structures, such as pristine and metastable C60 molecules. The pathways leading to molecular C60 formation consist of a series of steps that involve bond-breakage and subsequent local rearrangement of atoms, with the activation energy barriers of the rate-limiting step(s) being comparable to the energetics of Stone–Wales rearrangement reactions. The identified chemical pathways provide fundamental insights into the mechanisms that underlie C60 formation. Moreover, they clearly demonstrate that top-down synthesis of C60 from graphitic sources is a viable synthesis route at conditions pertaining to circumstellar matter. [ABSTRACT FROM AUTHOR]

We measure the CO-to-H2 conversion factor (α CO) in 37 galaxies at 2 kpc resolution, using the dust surface density inferred from far-infrared emission as a tracer of the gas surface density and assuming a constant dust-to-metal ratio. In total, we have ∼790 and ∼610 independent measurements of α CO for CO (2–1) and (1–0), respectively. The mean values for α CO (2–1) and α CO (1–0) are 9.3 − 5.4 + 4.6 and 4.2 − 2.0 + 1.9 M ⊙ pc − 2 (K km s − 1) − 1 , respectively. The CO-intensity-weighted mean is 5.69 for α CO (2–1) and 3.33 for α CO (1–0). We examine how α CO scales with several physical quantities, e.g., the star formation rate (SFR), stellar mass, and dust-mass-weighted average interstellar radiation field strength ( U ¯ ). Among them, U ¯ , ΣSFR, and the integrated CO intensity (W CO) have the strongest anticorrelation with spatially resolved α CO. We provide linear regression results to α CO for all quantities tested. At galaxy-integrated scales, we observe significant correlations between α CO and W CO, metallicity, U ¯ , and ΣSFR. We also find that α CO in each galaxy decreases with the stellar mass surface density (Σ⋆) in high-surface-density regions (Σ⋆ ≥ 100 M ⊙ pc−2), following the power-law relations α CO (2 – 1) ∝ Σ ⋆ − 0.5 and α CO (1 – 0) ∝ Σ ⋆ − 0.2 . The power-law index is insensitive to the assumed dust-to-metal ratio. We interpret the decrease in α CO with increasing Σ⋆ as a result of higher velocity dispersion compared to isolated, self-gravitating clouds due to the additional gravitational force from stellar sources, which leads to the reduction in α CO. The decrease in α CO at high Σ⋆ is important for accurately assessing molecular gas content and star formation efficiency in the centers of galaxies, which bridge "Milky Way–like" to "starburst-like" conversion factors. [ABSTRACT FROM AUTHOR]

Carbon monoxide (CO) plays a vital role in interstellar chemistry, existing abundantly in both gaseous and frozen environments. Understanding the radiation-driven chemistry of CO-rich ices is crucial for comprehending the formation and desorption of C-bearing molecules in the interstellar medium (ISM), particularly considering the potential impact of temperature on these processes. We report experimental data on irradiation processing of pure CO ice by cosmic ray analogues (95.2 MeV 136Xe23+ ions) at temperatures of 10, 15, and 20 K, in the IGLIAS set-up coupled to the IRRSUD beamline at GANIL (Caen, France). The evolution of the irradiated frozen samples was monitored by infrared spectroscopy. The computational PROCODA code allows us to quantify the chemical evolution of the samples, determining effective reaction rates coefficients (ERCs), molecular abundances at the chemical equilibrium (CE) phase, and desorption processes. The model integrated 18 chemical species – 8 observed (CO, CO2, C3, O3, C2O, C3O, C3O2, and C5O3) and 10 non-observed but predicted (C, O, C2, O2, CO3, C4O, C5O, C2O2, C2O3, C4O2) – linked via 156 reactions. Our findings reveal temperature-driven influences on molecular abundances at chemical equilibrium, desorption yields and rates, and ERC values. Certain reaction routes exhibit distinct thermochemical behaviours of gas- and ice-phase reactions which may be attributed to the presence of neighbouring molecules within the ice matrix. This study provides pivotal insights into the chemical evolution of CO-enriched ice under irradiation, impacting solid-state astrochemistry, clarifying molecular abundances, and advancing our understanding of ISM chemistry and temperature effects on ionized radiation-processed frozen ices. [ABSTRACT FROM AUTHOR]

To understand the origin of interstellar molecules we rely on astrochemical models, the gas-phase networks of which contain ≥7000 reactions. However, just a tiny fraction of them have parameters derived in laboratory experiments. Theoretical quantum mechanical (QM) calculations can also provide this information. Unfortunately, sometimes theoretical predictions and experimental values disagree, as is the case for the paradigmatic reaction CH3OH + OH → CH3O + H2O. Both laboratory experiments and QM calculations found an unexpected increase in the rate coefficients with decreasing temperature. However, experimental and theoretical estimates of the rate coefficients diverge by up to two orders of magnitude at the low temperatures of interest in interstellar chemistry. This work aims to test whether astronomical observations can help untangle this confusing situation. To this end, we first carried out new QM calculations to derive the rate coefficients of the major destruction reaction of the methoxy radical, CH3O + H, and then we compared astronomical observations from the IRAM/NOEMA Large Programme SOLIS with astrochemical model predictions. Our new rate coefficient for the CH3O + H reaction is 5–10 times larger than that in the astrochemical data base KIDA in the 10–100 K range. When including the new methoxy destruction rate coefficients, the comparison between observations and model predictions favours the rate coefficients of the CH3OH + OH reaction from QM calculations. We conclude that QM calculations are an important alternative to laboratory experiments when it comes to the harsh conditions of interstellar objects and that astronomical observations can be used to constraint the rate coefficients of relevant reactions. [ABSTRACT FROM AUTHOR]

The materials that form the diverse chemicals and structures on Earth—from mountains to oceans and biological organisms—all originated in a universe dominated by hydrogen and helium. Over billions of years, the composition and structure of the galaxies and stars evolved, and the elements of life, CHONPS, were formed through nucleosynthesis in stellar cores. Climactic events such as supernovae and stellar collisions produced heavier elements and spread them throughout the cosmos, often to be incorporated into new, more metal-rich stars. Stars typically form in molecular clouds containing small amounts of dust through the collapse of a high-density core. The surrounding nebular material is then pulled into a protoplanetary disk, from which planets, moons, asteroids, and comets eventually accrete. During the accretion of planetary systems, turbulent mixing can expose matter to a variety of different thermal and radiative environments. Chemical and physical changes in planetary system materials occur before and throughout the process of accretion, though many factors such as distance from the star, impact history, and level of heating experienced combine to ultimately determine the final geophysical characteristics. In Earth's planetary system, called the Solar System, after the orbits of the planets had settled into their current configuration, large impacts became rare, and the composition of and relative positions of objects became largely fixed. Further evolution of the respective chemical and physical environments of the planets—geosphere, hydrosphere, and atmosphere—then became dependent on their local geochemistry, their atmospheric interactions with solar radiation, and smaller asteroid impacts. On Earth, the presence of land, air, and water, along with an abundance of important geophysical and geochemical phenomena, led to a habitable planet where conditions were right for life to thrive. [ABSTRACT FROM AUTHOR]

The potential interstellar medium molecule, thiirane, has received considerable attention of experimental and theoretical studies in recent years. The anharmonic force field and spectroscopic constants of thiirane are calculated using CCSD(T) and B3LYP methods employing correlation consistent basis sets. Molecular structure, ground state rotational constants, quartic and sextic centrifugal distortion constants, and fundamental frequencies of thiirane are compared with the available experimental data. The B3LYP and CCSD(T) results can reproduce them well. The rotational parameters and vibrational wave numbers of isotopologues for thiirane are also determined using B3LYP method. The new data obtained in this study might be used to assist astronomical search for thiirane. [ABSTRACT FROM AUTHOR]

Ice chemistry in the dense, cold interstellar medium (ISM) is probably responsible for the formation of interstellar complex organic molecules (COMs). Recent laboratory experiments performed at T ∼ 4 K have shown that irradiation of CO:N2 ice samples analog to the CO-rich interstellar ice layer can contribute to the formation of COMs when H2 molecules are present. We have tested this organic chemistry under a broader range of conditions relevant to the interior of dense clouds by irradiating CO:15N2:H2 ice samples with 2 keV electrons in the 4–15 K temperature range. The H2 ice abundance depended on both, the ice formation temperature and the thermal evolution of the samples. Formation of H-bearing organics such as formaldehyde (H2CO), ketene (C2H2O), and isocyanic acid (H15NCO) was observed upon irradiation of ice samples formed at temperatures up to 10 K, and also in ices formed at 6 K and subsequently warmed up and irradiated at temperatures up to 15 K. These results suggest that a fraction of the H2 molecules in dense cloud interiors might be entrapped in the CO-rich layer of interstellar ice mantles, and that energetic processing of this layer could entail an additional contribution to the formation of COMs in the coldest regions of the ISM. [ABSTRACT FROM AUTHOR]

Detailed chemical kinetic mechanisms for the synthesis of complex organic molecules in the interstellar medium are at an early stage of developement. That such synthesis must take place is well‐known from chemical analysis of sampled asteroids. As molecular complexity increases the number of possible structural isomers also increases with the consequence that the nascent species may adopt a different spatial arrangement, to the lowest energy one. As part of a program of investigations of the hydrogen atom transfer reaction or tautomerization of imidic acid–amide species H‐O=C‐N‐ ⇌$\rightleftharpoons$ O=C‐N‐H we have studied the kinetics for a number of nucleobases, namely cytosine, thymine and uracil where a cyclic form of tautomerism (lactim–lactam) is encountered. Together with a fourth, 5‐aza‐uracil (1,3,5‐triazine‐2,4(1H,3H)‐dione), we report on the rates of reaction at low temperatures 50–200 K for both the direct unimolecular process and the similar transformation mediated by an additional water molecule. We show that these tautomerization reactions can be categorized into three classes, and highlight the importance of quantum mechanical tunneling on the rate constants at these low temperatures. We further present some thermochemistry data, such as formation enthalpies, entropies, isobaric heat capacities and enthalpy functions. [ABSTRACT FROM AUTHOR]

Molecular ions are key intermediates in the build-up of chemical complexity in interstellar clouds. Among the more than 300 interstellar molecules, only eight negative ions, i.e. C2n+1N− (n  = 0–3) and HC2n− (n  = 2–5), have been astronomically observed. Understanding the formation mechanism of these ions under the interstellar conditions is essential for astrochemical modelling and establishing the astrochemical networks. Cyanopolyynes including the parent molecule HC3N are carbon-chain molecules that have been observed in a variety of astronomical objects such as the Titan's atmosphere. Herein, two cyanoacetylene anions HC3N‒ and CH3C3N‒ were generated in solid Ne matrix at 3 K and characterized with matrix-isolation infrared spectroscopy, as aided by isotopic substitutions and the ab initio calculations at the UCCSD(T)-F12a/cc-pVTZ-F12 level using second-order vibrational perturbation theory. Upon red-light irradiation at 625 nm, both ions undergo electron detachment by reformation of the neutral species. Importantly, the concomitant dehydrogenation of HC3N‒ has also been observed in the matrix, providing new insight into the intriguing mechanism for the formation of C3N‒ in the upper atmosphere of Titan. [ABSTRACT FROM AUTHOR]

Context. Detailed astrochemical models are a key component to interpret the observations of interstellar and circumstellar molecules since they allow important physical properties of the gas and its evolutionary history to be deduced. Aims. We update one of the most widely used astrochemical databases to reflect advances in experimental and theoretical estimates of rate coefficients and to respond to the large increase in the number of molecules detected in space since our last release in 2013. Methods. We present the sixth release of the UMIST Database for Astrochemistry (UDfA), a major expansion of the gas-phase chemistry that describes the synthesis of interstellar and circumstellar molecules. Since our last release, we have undertaken a major review of the literature which has increased the number of reactions by over 40% to a total of 8767 and increased the number of species by over 55% to 737. We have made a particular attempt to include many of the new species detected in space over the past decade, including those from the QUIJOTE and GOTHAM surveys, as well as providing references to the original data sources. Results. We use the database to investigate the gas-phase chemistries appropriate to both O-rich and C-rich conditions in TMC-1 and to the circumstellar envelope of the C-rich AGB star IRC+10216 and identify successes and failures of gas-phase only models. Conclusions. This update is a significant improvement to the UDfA database. For both the dark cloud and C-rich circumstellar envelope models, calculations match around 60% of the abundances of observed species to within an order of magnitude. There are a number of detected species, however, that are not included in the model either because their gas-phase chemistry is unknown or because they are likely formed via surface reactions on icy grains. Future laboratory and theoretical work is needed to include such species in reaction networks. [ABSTRACT FROM AUTHOR]

Interstellar aromatic molecules such as polycyclic aromatic hydrocarbons and polycyclic nitrogen and oxygen bearing molecules are thought to be abundant in the interstellar medium (ISM). In this class of molecules, benzonitrile (c-C6H5CN) plays an important role as a proxy for benzene. It has been detected through rotational emission in several astrophysical sources and is one of the simplest N-bearing polar aromatic molecules. Even in the cold ISM, the population of the rotational levels of benzonitrile might not be at equilibrium. Consequently, modelling its detected emission lines requires a prior computation of its quenching rate coefficients by the most abundant species in the ISM (He or H2). In this paper, we focus on the excitation of c-C6H5CN by collision with He. We compute the first potential energy surface (PES) using the explicitly correlated coupled cluster method in conjunction with large basis sets. The PES obtained is characterized by a potential well depth of −97.2 cm−1 and an important anisotropy. Scattering computations of the rotational (de-)excitation of c-C6H5CN by He atoms are performed by means of the coupled states approximation that allow to obtain collisional rates for rotational states up to j  = 9 and temperatures up to 40 K. These rate coefficients are then used to examine the effect of C6H5CN excitation induced by collisions with para-H2 in molecular clouds by carrying out simple radiative transfer calculations of the excitation temperatures and show that non-equilibrium effects can be expected for H2 densities up to 105–106 cm−3. [ABSTRACT FROM AUTHOR]

The diffusion of molecules on interstellar grain surfaces is one of the most important driving forces for the molecular complexity in the interstellar medium. Due to the lack of laboratory measurements, astrochemical modeling of grain surface processes usually assumes a constant ratio between the diffusion energy barrier and the desorption energy. This over-simplification inevitably causes large uncertainty in model predictions. We present a new measurement of the diffusion of CO $ _2 $ 2 molecules on the surface of non-porous amorphous solid water (np-ASW), an analog of the ice mantle that covers cosmic dust grains. A small coverage of CO $ _2 $ 2 was deposited onto an np-ASW surface at 40 K, the subsequent warming of the ice activated the diffusion of CO $ _2 $ 2 molecules, and a transition from isolated CO $ _2 $ 2 to CO $ _2 $ 2 clusters was seen in the infrared spectra. To obtain the diffusion energy barrier and pre-exponential factor simultaneously, a set of isothermal experiments were carried out. The values for the diffusion energy barrier and pre-exponential factor were found to be $ 1300\pm 110 $ 1300 ± 110 K and $ 10^{7.6\pm 0.8} $ 10 7.6 ± 0.8 s $ ^{-1} $ − 1 . A comparison with prior laboratory measurements on diffusion is discussed. [ABSTRACT FROM AUTHOR]

This study reports the infrared and electronic absorption spectra of amine-containing molecules. We investigate how these spectra differ for molecules in their neutral and ionic forms in gas and astrophysical H 2 O ice. Additionally, we analyze the pure rotational spectra of these molecules in the gas state. These are prebiotic molecules such as propargylamine (C 3 H 5 N), four isomers of C 3 H 7 N (1-aminopropene, 2-propenamine, allylamine, and cyclopropylamine), and two isomers of C 3 H 9 N (n-propyl amine and 2-aminpropane) considered for the study. We analyzed various properties of molecules in different states: gas phase, water solvent, and ionic states. We used the coupled-cluster method (CCSD) with a 6–311++G** basis set for our studies. The rotational constants obtained at this level of theory are well matching with the available experimental determinations. These constants are further used to calculate the rotational spectra and their transitions. The calculated vibrational frequencies of these molecules matched well with experimental results at this level of theory. To study the influence of solvent, we employed the IEFPCM model at the same level of theory. We used the equation of motion coupled cluster theory (EOM-CCSD) at the CCSD/6–311++G** level of theory to examine the electronic absorption spectra. We reported the HOMO-LUMO gap, electronic transitions, and oscillator strength of these transitions. We identified the best lines from microwave and infrared regions to observe these molecules in interstellar environments. All calculations were performed using the Gaussian 16 and PGOPHER program packages. [ABSTRACT FROM AUTHOR]

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 (3P) with an icy water molecule, alongside possible subsequent reactions with the most abundant closed-shell frozen species (NH3, CO, CO2, and H2), atoms (H, N, and O), and molecular radicals (OH, NH2, and CH3). We found that C reacts spontaneously with the water molecule, resulting in the formation of 3C–OH2, 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, CH3, and NH2 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 3C–OH2, 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 3C–OH2 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. [ABSTRACT FROM AUTHOR]