Assignment of the methanol OH-stretch overtone spectrum using the pattern recognition method

We present the measurement and analysis of the 2OH stretching band of methanol between 7165 cm-1 and 7230 cm-1 cooled down to 26 ± 12 K in a buffer gas cooling experiment. Measurements were performed with a cavity ring-down spectrometer having a detection limit αmin = 2 × 10-10 cm-1. A total of 350 rovibrational transitions were assigned and 62 rovibrational transitions were tentatively assigned. This assignment was performed using the pattern recognition method developed by Rakvoský et al. [Phys. Chem. Chem. Phys., 2021, 23, 20193-20200]. In this work, we extended their method by using information on the relative intensities of the transitions to add one criterion to the validation of the assignments, allowing us to firmly assign 188 additional rovibrational transitions and to tentatively assign 14 more compared to the ir work.

Formation of Methanol via Fischer-Tropsch Catalysis by Cosmic Iron Sulphide

Chemical reactions in the gas phase of the interstellar medium face significant challenges due to its extreme conditions (i. e., low gas densities and temperatures), necessitating the presence of dust grains to facilitate the synthesis of molecules inaccessible in the gas phase. While interstellar grains are known to enhance encounter rates and dissipate energy from exothermic reactions, their potential as chemical catalysts remain less explored. Here, we present mechanistic insights into the Fischer-Tropsch-type methanol (FTT-CH3OH) synthesis by reactivity of CO with H2 and using cosmic FeS surfaces as heterogeneous catalysts. Periodic quantum chemical calculations were employed to characterize the potential energy surface of the reactions on the (011) and (001) FeS surfaces, considering different Fe coordination environments and S vacancies. Kinetic calculations were also conducted to assess catalytic capacity and allocate reaction processes within the astrochemical framework. Our findings demonstrate the feasibility of FeS-based astrocatalysis in the FTT-CH3OH synthesis. The reactions and their energetics were elucidated from a mechanistic standpoint. Kinetic analysis demonstrates the temperature dependency of the simulated processes, underscoring the compulsory need of energy sources considering the astrophysical scenario. Our results provide insights into the presence of CH3OH in diverse regions where current models struggle to explain its observational quantity.

Interaction of methanol with molecular hydrogen: Ab initio potential energy surface and scattering calculations

The potential energy surface (PES) describing the interaction of the methanol molecule with molecular hydrogen has been calculated by the use of the explicitly correlated coupled cluster method, including single, double, and (perturbative) triple excitations [CCSD(T)-F12a] and a correlation-consistent aug-cc-pVTZ basis, with the assumption of fixed molecular geometries. The computed points were fit to a functional form appropriate for time-independent quantum scattering calculations of rotationally inelastic cross sections and rate coefficients. Stationary points on the PES were located, and the global minimum was found to have an energy equal to -254.7 cm-1 relative to the energy of the separated molecules. This PES was used in time-independent close coupling quantum scattering calculations to determine state-to-state cross sections and rate coefficients for rotational transitions within the A- and E-type nuclear spin torsional ground states.

Vacuum UV photodesorption of organics in the interstellar medium: an experimental study of formic acid HCOOH and methyl formate HCOOCH3-containing ices

Being a potential process that could explain gas phase abundances of so-called complex organic molecules (COMs) in the cold interstellar medium (ISM), the UV photon-induced desorption from organics-containing molecular ices has been experimentally studied. In this work, we focused on the observation of the photodesorbed products and the measurement of the associated photodesorption yields from pure and mixed molecular ices, each containing organic molecules whose detection has been achieved in the gas phase of the cold ISM, namely formic acid HCOOH and methyl formate HCOOCH3. Each molecule, in pure ice or in ice mixed with CO or water, was irradiated at 15 K with monochromatic vacuum UV photons in the 7-14 eV range using synchrotron radiation from the SOLEIL synchrotron facility, DESIRS beamline. Photodesorption yields of the intact molecules and of the photoproducts were derived as a function of the incident photon energy. Experiments have revealed that the desorbing species match the photodissociation pattern of each isolated molecule, with little influence of the kind of ice (pure or mixed in CO or H2O-rich environment). For both species, the photodesorption of the intact organics is found to be negligible in our experimental conditions, resulting in yields typically below 10-5 ejected molecules per incident photon. The results obtained on HCOOH and HCOOCH3-containing ices are similar to what has already been found for methanol-containing ices, but contrast with the case of another complex molecule, CH3CN, photodesorption of which has been recently studied. Such experimental results may be linked to the observation of COMs in protoplanetary disks, in which CH3CN is commonly observed whereas HCOOH or methanol are detected only in some sources, HCOOCH3 not being detected at all.

Spiers Memorial Lecture: Astrochemistry at high resolution

Astrochemistry is the science that studies the chemistry in the Universe, namely the combination of two fields: astronomy and chemistry. It started about fifty years ago and it has progressed in leaps and bounds, often triggered by the advent of new telescopes. From the collection of new interstellar molecule detections, astrochemistry has evolved more and more in the quest to understand how they are formed and thrive in the harsh conditions of the interstellar medium. Collaboration between astronomers and chemists has never been more necessary than today, when new powerful astronomical facilities provide us with ever sharper images of the regions where interstellar molecules are present. This review focuses on the special case of interstellar complex organic molecules (iCOMs), one the most debated astrochemical fields and where the astronomers-chemists collaboration and synergy is indispensable. The review will go through the various phases of the formation of planetary system similar to the solar system, providing the most recent observational picture at each step. The current scenarios of the iCOMs formation will be laid down and the critical chemical processes and quantities involved in each of them will be discussed. The major goal of this review is not only to present the progress but, more importantly, to highlight the many areas of uncertainty. A few specific cases will be discussed to give practical examples of why the huge challenge that represents the formation of iCOMs can only be won if chemists and astronomers work together.

Fragmentation of interstellar methanol by collisions with He˙+: an experimental and computational study

Methanol is a key species in astrochemistry as its presence and reactivity provides a primary route to the synthesis of more complex interstellar organic molecules (iCOMs) that may eventually be incorporated in newly formed planetary systems. In the interstellar medium, methanol is formed by hydrogenation of CO ices on grains, and its fate upon collisions with interstellar ions should be accounted for to correctly model iCOM abundances in objects at various stages of stellar evolution. The absolute cross sections (CSs) and branching ratios (BRs) for the collisions of He˙⁺ ions with CH₃OH are measured, as a function of the collision energy, using a Guided Ion Beam Mass Spectrometer (GIB-MS). Insights into the dissociative electron (charge) exchange mechanism have been obtained by computing the entrance and exit multidimensional Potential Energy Surfaces (PESs) and by modelling the non-adiabatic transitions using an improved Landau-Zener-Stückelberg approach. Notably, the dynamical treatment reproducing the experimental findings includes a strong orientation effect of the system formed by the small He˙⁺ ion and the highly polar CH₃OH molecule, in the electric field gradient associated to the strongly anisotropic intermolecular interaction. This is a stereodynamical effect that plays a fundamental role in collision events occurring under a variety of conditions, with kinetic energy confined within intervals ranging from the sub-thermal to the hyper-thermal regime.

Relationship between CH3OD Abundance and Temperature in the Orion KL Nebula

The relative abundances of singly deuterated methanol isotopologues, [CH₂DOH]/[CH₃OD], in star-forming regions deviate from the statistically expected ratio of 3. In Orion KL, the nearest high-mass star-forming region to Earth, the singly deuterated methanol ratio is about 1, and the cause for this observation has been explored through theory for nearly three decades. We present high-angular resolution observations of Orion KL using the Atacama Large Millimeter/submillimeter Array to map small-scale changes in CH₃OD column density across the nebula, which provide a new avenue to examine the deuterium chemistry during star and planet formation. By considering how CH₃OD column densities vary with temperature, we find evidence of chemical processes that can significantly alter the observed gas-phase column densities. The astronomical data are compared with existing theoretical work and support D–H exchange between CH₃OH and heavy water (i.e., HDO and D₂O) at methanol’s hydroxyl site in the icy mantles of dust grains. The enhanced CH₃OD column densities are localized to the Hot Core-SW region, a pattern that may be linked to the coupled evolution of ice mantle chemistry and star formation in giant molecular clouds. This work provides new perspectives on deuterated methanol chemistry in Orion KL and informs considerations that may guide future theoretical, experimental, and observational work.

Exploring the link between star and planet formation with Ariel

The goal of the Ariel space mission is to observe a large and diversified population of transiting planets around a range of host star types to collect information on their atmospheric composition. The planetary bulk and atmospheric compositions bear the marks of the way the planets formed: Ariel's observations will therefore provide an unprecedented wealth of data to advance our understanding of planet formation in our Galaxy. A number of environmental and evolutionary factors, however, can affect the final atmospheric composition. Here we provide a concise overview of which factors and effects of the star and planet formation processes can shape the atmospheric compositions that will be observed by Ariel, and highlight how Ariel's characteristics make this mission optimally suited to address this very complex problem.

Absolute Photoabsorption Cross-Sections of Methanol for Terrestrial and Astrophysical Relevance

We investigate the methanol absorption spectrum in the range 5.5-10.8 eV to provide accurate and absolute cross-sections values. The main goal of this study is to provide a comprehensive analysis of methanol electronic-state spectroscopy by employing high-resolution vacuum ultraviolet (VUV) photoabsorption measurements together with state-of-the-art quantum chemical calculation methods. The VUV spectrum reveals several new features that are not previously reported in literature, for n > 3 in the transitions (nsσ(a') ← (2a″)) (¹A' ← X̃¹A') and (nsσ, npσ, npσ', ndσ ← (7a')) (¹A' ← X̃¹A'), and with particular relevance to vibrational progressions of the CH₃ rocking mode, v₁₁^'(a″), mode in the (3pπ(a″) ← (2a″)) (2¹A' ← X̃¹A') absorption band at 8.318 eV. The measured absolute photoabsorption cross-sections have subsequently been used to calculate the photolysis lifetime of methanol in the Earth's atmosphere from the ground level up to the limit of the stratosphere (50 km altitude). This shows that solar photolysis plays a negligible role in the removal of methanol from the lower atmosphere compared with competing sink mechanisms. Torsional potential energy scans, as a function of the internal rotation angle for the ground and first Rydberg states, have also been calculated as part of this investigation.

A study of the translational temperature dependence of the reaction rate constant between CH3CN and Ne+ at low temperatures

We have measured the translational temperature dependence of the reaction rate constant for CH₃CN + Ne⁺ → products at low temperatures. A cold Ne⁺ ensemble was embedded in Ca⁺ Coulomb crystals by a sympathetic laser cooling technique, while cold acetonitrile (CH₃CN) molecules were produced by two types of Stark velocity filters to widely change the translational temperatures. The measured reaction rate constant gradually increases with the decrease in the translational temperature of the velocity-selected CH₃CN molecules from 60 K down to 2 K, and thereby, a steep increase was observed at temperatures lower than 5 K. A comparison between experimental rate constants and the ion-dipole capture rate constants by the Perturbed Rotational State (PRS) theory was performed. The PRS capture rate constant reproduces well the reaction rate constant at a few kelvin but not for temperatures higher than 5 K. The result indicates that the reaction probability is small compared to typical ion-polar molecule reactions at temperatures above 5 K.

X-ray processing of a realistic ice mantle can explain the gas abundances in protoplanetary disks

The Atacama Large Millimeter Array has allowed a detailed observation of molecules in protoplanetary disks, which can evolve toward solar systems like our own. While CO, [Formula: see text], HCO, and [Formula: see text] are often abundant species in the cold zones of the disk, [Formula: see text] or [Formula: see text] are only found in a few regions, and more-complex organic molecules are not observed. We simulate, experimentally, ice processing in disks under realistic conditions, that is, layered ices irradiated by soft X-rays. X-ray emission from young solar-type stars is thousands of times brighter than that of today's sun. The ice mantle is composed of a [Formula: see text]:[Formula: see text]:[Formula: see text] mixture, covered by a layer made of [Formula: see text] and CO. The photoproducts found desorbing from both ice layers to the gas phase during the irradiation converge with those detected in higher abundances in the gas phase of protoplanetary disks, providing important insights on the nonthermal processes that drive the chemistry in these objects.

Color, composition, and thermal environment of Kuiper Belt object (486958) Arrokoth

The outer Solar System object (486958) Arrokoth (provisional designation 2014 MU₆₉) has been largely undisturbed since its formation. We studied its surface composition using data collected by the New Horizons spacecraft. Methanol ice is present along with organic material, which may have formed through irradiation of simple molecules. Water ice was not detected. This composition indicates hydrogenation of carbon monoxide-rich ice and/or energetic processing of methane condensed on water ice grains in the cold, outer edge of the early Solar System. There are only small regional variations in color and spectra across the surface, which suggests that Arrokoth formed from a homogeneous or well-mixed reservoir of solids. Microwave thermal emission from the winter night side is consistent with a mean brightness temperature of 29 ± 5 kelvin.

First-Principles Study of the Reaction Mechanism of CHO + H on Graphene Surface

Many organic molecules observed in the interstellar medium are considered to be formed on dust grains and populated into the gas phase. We analyzed the reaction of HCO + H on a graphene surface using ab initio molecular dynamics simulations as a case study of the formation and desorption of organic molecules on interstellar dust particles. During the reactions of chemisorbed CHO (chemisorbed at the C atom) with free H, CO was generated and efficiently desorbed from the surface. These results suggest that the reactions, of which the reactant forms a covalent bond with the surface while the product does not, cause efficient desorption of the product upon reaction. In such reactions a repulsive force between the product and the surface would be generated and accelerate translation of the product in a specific direction. In addition, it was also shown that the branching ratio of the reactions between radical species on the surface would be affected by the form of the adsorption on the surface, e.g., when a free H reacted with the CHO chemisorbed at the C atom, CH₂O was not generated.

The chemistry of disks around T Tauri and Herbig Ae/Be stars

CONTEXT

Infrared and (sub-)mm observations of disks around T Tauri and Herbig Ae/Be stars point to a chemical differentiation between both types of disks, with a lower detection rate of molecules in disks around hotter stars.

AIMS

To investigate the underlying causes of the chemical differentiation indicated by observations we perform a comparative study of the chemistry of T Tauri and Herbig Ae/Be disks. This is one of the first studies to compare chemistry in the outer regions of these two types of disks.

METHODS

We developed a model to compute the chemical composition of a generic protoplanetary disk, with particular attention to the photochemistry, and applied it to a T Tauri and a Herbig Ae/Be disk. We compiled cross sections and computed photodissociation and photoionization rates at each location in the disk by solving the FUV radiative transfer in a 1+1D approach using the Meudon PDR code and adopting observed stellar spectra.

RESULTS

The warmer disk temperatures and higher ultraviolet flux of Herbig stars compared to T Tauri stars induce some differences in the disk chemistry. In the hot inner regions, H2O, and simple organic molecules like C2H2, HCN, and CH4 are predicted to be very abundant in T Tauri disks and even more in Herbig Ae/Be disks, in contrast with infrared observations that find a much lower detection rate of water and simple organics toward disks around hotter stars. In the outer regions, the model indicates that the molecules typically observed in disks, like HCN, CN, C2H, H2CO, CS, SO, and HCO+, do not have drastic abundance differences between T Tauri and Herbig Ae disks. Some species produced under the action of photochemistry, like C2H and CN, are predicted to have slightly lower abundances around Herbig Ae stars due to a narrowing of the photochemically active layer. Observations indeed suggest that these radicals are somewhat less abundant in Herbig Ae disks, although in any case the inferred abundance differences are small, of a factor of a few at most. A clear chemical differentiation between both types of disks concerns ices. Owing to the warmer temperatures of Herbig Ae disks, one expects snowlines lying farther away from the star and a lower mass of ices compared to T Tauri disks.

CONCLUSIONS

The global chemical behavior of T Tauri and Herbig Ae/Be disks is quite similar. The main differences are driven by the warmer temperatures of the latter, which result in a larger reservoir or water and simple organics in the inner regions and a lower mass of ices in the outer disk.

Electron-induced origins of prebiotic building blocks of sugars: mechanism of self-reactions of a methanol anion dimer

Reactions of methanol dimers in interstellar medium driven by low energy irradiation may lead to prebiotic precursors.

The composition of cometary ices

The chemical composition of cometary ices provides clues for the conditions of formation and evolution of the early Solar System. A large number of molecules have been identified in cometary atmospheres, from both ground-based observations and space, including in situ investigations. This includes large organic molecules, which are also observed in star-forming regions. This paper presents a review of molecular abundances measured in cometary atmospheres from remote sensing observations with ground-based and space-based telescopes. The diversity of composition observed in comet populations is presented and discussed.This article is part of the themed issue 'Cometary science after Rosetta'.

B-DNA model systems in non-terran bio-solvents: implications for structure, stability and replication

We have computationally analyzed a comprehensive series of Watson–Crick and mismatched B-DNA base pairs, in the gas phase and in several solvents, including toluene, chloroform, ammonia, methanol and water, using dispersion-corrected density functional theory and implicit solvation.