Astrochemistry Research Articles

Computational studies on the possible formation of glycine via open shell gas-phase chemistry in the interstellar medium.

Glycine is the simplest proteinogenic amino acid. It has significant astrobiological implications owing to the ongoing investigation for its detection in the interstellar medium (ISM). Hence, a suitable mechanistic elucidation for its formation in the ISM is of current research interest. In the present work, by employing electronic structure calculations [UCCSD(T) and density functional theory (DFT)], various plausible chemical pathways in the gas phase have been examined for the formation of glycine (whose existence has been indirectly proposed in the ISM) and other simple amino acids (yet to be detected in the ISM) from some simpler molecules present in the ISM. This work suggests that step 1: HO-CO (radical) + CH2NH → NHCH2COOH (radical) and step 2a: NHCH2COOH (radical) + H2 → glycine + H (radical) have very small barriers of 0.14 kcal mol-1 and ∼3 kcal mol-1, respectively (easily surmountable at a temperature of ∼50 K under putative interstellar conditions). Hence this should likely be feasible in interstellar gas-phase chemistry. Therefore, HO-CO (radical), CH2NH, and H2 could be the possible precursors for the formation of glycine (subject to the presence of the HO-CO radical). The energetic information related to the interstellar reactions, and how this work takes the putative interstellar conditions into account are presented. This paper also highlights how a reaction found to be unsuitable for interstellar molecular evolution via surface chemistry could nonetheless occur via gas-phase chemistry. Based on our results, this work also recommends detecting three new open-shell molecules, HO-CO radical, NHCH2COOH radical, and NH2CHCOOH radical, in the ISM.

GGCHEMPY: A Pure Python-based Gas-grain Chemical Code for Efficient Simulation of Interstellar Chemistry* *Supported by CASSACA.

In this paper, we present a new gas-grain chemical code for interstellar clouds written in pure Python (GGCHEMPY (GGCHEMPY is available on https://github.com/JixingGE/GGCHEMPY)). By combining with the high-performance Python compiler Numba, GGCHEMPY is as efficient as the Fortran-based version. With the Python features, flexible computational workflows and extensions become possible. As a showcase, GGCHEMPY is applied to study the general effects of three-dimensional projection on molecular distributions using a two-core system which can be easily extended for more complex cases. By comparing the molecular distribution differences between two overlapping cores and two merging cores, we summarized the typical chemical differences such as N2H+, HC3N, C2S, H2CO, and C2H, which can be used to interpret 3D structures in molecular clouds.

Kinetic and dynamic studies of the NH2+ + H2 reaction on a high-level ab initio potential energy surface.

Gas-phase ion-molecule reactions have attracted considerable attention due to their importance in the fields of interstellar chemistry, plasma chemistry, and combustion chemistry. The reaction of an amino radical cation with a hydrogen molecule is one of the crucial steps in the gas-phase formation of ammonia in the interstellar medium (ISM). The dynamics and kinetics of the NH2+ + H2 reaction are studied using the quasi-classical trajectory approach on a newly constructed ab initio potential energy surface (PES) for the ground electronic state. The PES is fitted by the fundamental invariant-neural network method, resulting in a total root mean square error (RMSE) of 0.061 kcal mol-1. Dynamics calculations show that, on one hand, the vibrational excitation of H2 largely promotes the reaction. On the other hand, the fundamental excitation of each vibrational mode of NH2+ inhibits the reaction at low collision energies which has a negligible effect at high collision energies except for the symmetric stretching mode. The relatively higher efficacy of the symmetric stretching mode than that of the asymmetric stretching mode can be rationalized by the underlying reaction mechanisms. In addition, the calculated rate coefficients of the reaction agree reasonably well with the available experimental results.

On the synthesis of N–O bearing species in astrophysical ices – an infrared spectroscopic study using heavy-ion irradiation of solid N2:CO samples

ABSTRACT The interstellar chemistry of nitrogen is considerably less understood than the chemistry of other common elements, such as carbon and oxygen. Even though a relatively large number of species containing nitrogen atoms have already been detected in the interstellar medium, only six of them bear a nitrogen–oxygen (N–O) bond. Some astrophysical and primeval Earth models suggest that N–O species, such as hydroxylamine (NH2OH), are potential precursors of prebiotic amino acids, and even peptides. In this work, we have analyzed an apolar ice mixture of N2:CO of astrophysical interest to investigate possible formation mechanisms of N–O bearing molecules due to processing of the sample by 64Ni24+ 538 MeV ions (8.4 MeV/u) at 14 K. The results show the formation of simple nitrogen oxides ($\rm {N_{1 - 2}}{O_y})$, but no CN–O species of any kind. We have also determined the formation cross-sections of some of the products, as well as the destruction cross-sections of precursors and products. The results presented here are discussed in light of our previous work on the processing of a NH3:CO ice mixture, which have found no N–O bearing molecules at all.

Experiment and theory elucidate the pathways for H3+formation in the ultrafast double ionization in methanol

Experiment and theory elucidate the pathways for H₃⁺formation in the ultrafast double ionization in methanol

Vibrationally excited molecular hydrogen production from the water photochemistry

Vibrationally excited molecular hydrogen has been commonly observed in the dense photo-dominated regions (PDRs). It plays an important role in understanding the chemical evolution in the interstellar medium. Until recently, it was widely accepted that vibrational excitation of interstellar H2 was achieved by shock wave or far-ultraviolet fluorescence pumping. Here we show a further pathway to produce vibrationally excited H2 via the water photochemistry. The results indicate that the H2 fragments identified in the O(1S) + H2(X1Σg+) channel following vacuum ultraviolet (VUV) photodissociation of H2O in the wavelength range of λ = ~100-112 nm are vibrationally excited. In particular, more than 90% of H2(X) fragments populate in a vibrational state v = 3 at λ~112.81 nm. The abundance of water and VUV photons in the interstellar space suggests that the contributions of these vibrationally excited H2 from the water photochemistry could be significant and should be recognized in appropriate interstellar chemistry models.

Theoretical computations on the efficiency of acetaldehyde formation on interstellar icy grains

Context. Interstellar grains are known to be important actors in the formation of interstellar molecules such as H2, water, ammonia, and methanol. It has been suggested that the so-called interstellar complex organic molecules (iCOMs) are also formed on the interstellar grain icy surfaces by the combination of radicals via reactions assumed to have an efficiency equal to unity. Aims. In this work, we aim to investigate the robustness or weakness of this assumption. In particular, we consider the case of acetaldehyde (CH3CHO), one of the most abundant and commonly identified iCOMs, as a starting study case. In the literature, it has been postulated that acetaldehyde is formed on the icy surfaces via the combination of HCO and CH3. Here we report new theoretical computations on the efficiency of its formation. Methods. To this end, we coupled quantum chemical calculations of the energetics and kinetics of the reaction CH3 + HCO, which can lead to the formation of CH3CHO or CO + CH4. Specifically, we combined reaction kinetics computed with the Rice-Ramsperger–Kassel–Marcus theory (tunneling included) method with diffusion and desorption competitive channels. We provide the results of our computations in the format used by astrochemical models to facilitate their exploitation. Results. Our new computations indicate that the efficiency of acetaldehyde formation on the icy surfaces is a complex function of the temperature and, more importantly, of the assumed diffusion over binding energy ratio f of the CH3 radical. If the ratio f is ≥0.4, the efficiency is equal to unity in the range where the reaction can occur, namely between 12 and 30 K. However, if f is smaller, the efficiency dramatically crashes: with f = 0.3, it is at most 0.01. In addition, the formation of acetaldehyde is always in competition with that of CO + CH4. Conclusions. Given the poor understanding of the diffusion over binding energy ratio f and the dramatic effect it has on the formation, or not, of acetaldehyde via the combination of HCO and CH3 on icy surfaces, model predictions based on the formation efficiency equal to one should to be taken with precaution. The latest measurements of f suggest f = 0.3 and, if confirmed for CH3, this would rule out the formation of acetaldehyde on the interstellar icy surfaces. We recall the alternative possibility, which was recently reviewed, that acetaldehyde could be synthesized in the gas phase starting from ethanol. Finally, our computations show the paramount importance played by the micro-physics involved in the interstellar surface chemistry and call for extensive similar studies on different systems believed to form iCOMs on the interstellar icy surfaces.

Time-resolved relaxation and fragmentation of polycyclic aromatic hydrocarbons investigated in the ultrafast XUV-IR regime

Polycyclic aromatic hydrocarbons (PAHs) play an important role in interstellar chemistry and are subject to high energy photons that can induce excitation, ionization, and fragmentation. Previous studies have demonstrated electronic relaxation of parent PAH monocations over 10–100 femtoseconds as a result of beyond-Born-Oppenheimer coupling between the electronic and nuclear dynamics. Here, we investigate three PAH molecules: fluorene, phenanthrene, and pyrene, using ultrafast XUV and IR laser pulses. Simultaneous measurements of the ion yields, ion momenta, and electron momenta as a function of laser pulse delay allow a detailed insight into the various molecular processes. We report relaxation times for the electronically excited PAH*, PAH+* and PAH2+* states, and show the time-dependent conversion between fragmentation pathways. Additionally, using recoil-frame covariance analysis between ion images, we demonstrate that the dissociation of the PAH2+ ions favors reaction pathways involving two-body breakup and/or loss of neutral fragments totaling an even number of carbon atoms.

Rotational excitation of H3O+ cations by para-H2: improved collisional data at low temperatures

ABSTRACT The hydronium cation plays a crucial role in interstellar oxygen and water chemistry. While its spectroscopy was extensively investigated earlier, the collisional excitation of H3O+ is not well studied yet. In this work, we present state-to-state collisional data for the rotational de-excitation of both ortho- and para-H3O+ due to para-H2 impact. The cross sections are calculated within the close-coupling formalism using our recent, highly accurate, rigid-rotor potential energy surface for this collision system. The corresponding thermal rate coefficients are computed up to 100 K. For para-H3O+, the lowest 20 rotation-inversion states were considered in the calculations, while for ortho-H3O+, the lowest 11 states are involved (up to j ≤ 5), so all levels with rotational energy below 420 K (292 cm−1) are studied. In order to analyse the impact of the new collisional rate coefficients on the excitation of H3O+ in astrophysical environments, radiative transfer calculations are also provided. The most relevant emission lines from an astrophysical point of view are studied, taking into account the transitions at 307, 365, 389, and 396 GHz. We show that our new collisional data have a non-negligible impact (from a few per cents up to about a factor of 3) on the brightness and excitation temperatures of H3O+, justifying the revision of the physical conditions in the appropriate astrophysical observations. The calculated rate coefficients allow one to recalculate the column density of hydronium in interstellar clouds, which can lead to a better understanding of interstellar water and oxygen chemistry.

Ultrafast Vibrational Relaxation Dynamics in XUV-Excited Polycyclic Aromatic Hydrocarbon Molecules

An investigation of how polycyclic aromatic hydrocarbons respond to extreme ultraviolet radiation provides insight into internal molecular dynamics that can help improve models of interstellar chemistry.

On the formation of CN bonds in Titan's atmosphere-a unified reaction valley approach study.

In this work, we investigated the formation of protonated hydrogen cyanide HCNH+ and methylene amine cation CH[Formula: see text] (both identified in Titan's upper atmosphere) from three different pathways which stem from the interaction between CH4 and N+(3P). As a mechanistic tool, we used the Unified Reaction Valley Approach (URVA) complemented with the Local Mode Analysis (LMA) assessing the strength of the CN bonds formed in these reactions. Our URVA studies could provide a comprehensive overview on bond formation/cleavage processes relevant to the specific mechanism of eight reactions R1- R8 that occur across the three pathways. In addition, we could explain the formation of CH[Formula: see text] and the appearance of HCNH+ and CHNH[Formula: see text] along these paths. Although only smaller molecules are involved in these reactions including isomerization, hydrogen atom abstraction, and hydrogen molecule capture, we found a number of interesting features, such as roaming in reaction R3 or the primary interaction of H2 with the carbon atom in HCNH+ in reaction R8 followed by migration of one of the H2 hydrogen atoms to the nitrogen which is more cost effective than breaking the HH bond first; a feature often found in catalysis. In all cases, charge transfer between carbon and nitrogen could be identified as a driving force for the CN bond formation. As revealed by LMA, the CN bonds formed in reactions R1-R8 cover a broad bond strength range from very weak to very strong, with the CN bond in protonated hydrogen cyanide HCNH+ identified as the strongest of all molecules investigated in this work. Our study demonstrates the large potential of both URVA and LMA to shed new light into these extraterrestrial reactions to help better understand prebiotic processes as well as develop guidelines for future investigations involving areas of complex interstellar chemistry. In particular, the formation of CN bonds as a precursor to the extraterrestrial formation of amino acids will be the focus of future investigations. Formation of CN bonds in Titan's atmosphere visualized via the reaction path curvature.

An Innovative Approach for the Generation of Species of the Interstellar Medium.

The large amount of unstable species in the realm of interstellar chemistry drives an urgent need to develop efficient methods for the in situ generations of molecules that enable their spectroscopic characterizations. Such laboratory experiments are fundamental to decode the molecular universe by matching the interstellar and terrestrial spectra. We propose an approach based on laser ablation of nonvolatile solid organic precursors. The generated chemical species are cooled in a supersonic expansion and probed by high‐resolution microwave spectroscopy. We present a proof of concept through a simultaneous formation of interstellar compounds and the first generation of aminocyanoacetylene using diaminomaleonitrile as a prototypical precursor. With this micro‐laboratory, we open the door to generation of unsuspected species using precursors not typically accessible to traditional techniques such as electric discharge and pyrolysis.

An Innovative Approach for the Generation of Species of the Interstellar Medium

AbstractThe large amount of unstable species in the realm of interstellar chemistry drives an urgent need to develop efficient methods for the in situ generations of molecules that enable their spectroscopic characterizations. Such laboratory experiments are fundamental to decode the molecular universe by matching the interstellar and terrestrial spectra. We propose an approach based on laser ablation of nonvolatile solid organic precursors. The generated chemical species are cooled in a supersonic expansion and probed by high‐resolution microwave spectroscopy. We present a proof of concept through a simultaneous formation of interstellar compounds and the first generation of aminocyanoacetylene using diaminomaleonitrile as a prototypical precursor. With this micro‐laboratory, we open the door to generation of unsuspected species using precursors not typically accessible to traditional techniques such as electric discharge and pyrolysis.

Structures and Properties of Known and Postulated Interstellar Cations

Abstract Positive ions play a fundamental role in interstellar chemistry, especially in cold environments where chemistry is believed to be mainly ion driven. However, in contrast with neutral species, most of the cations present in the astrochemical reaction networks are not fully characterized in the astrochemical literature. To fill this gap, we have carried out new accurate quantum chemical calculations to identify the structures and energies of 262 cations with up to 14 atoms that are postulated to have a role in interstellar chemistry. Optimized structures and rotational constants were obtained at the M06-2X/cc-pVTZ level, while electric dipoles and total electronic energies were computed with CCSD(T)/aug-cc-pVTZ//M06-2X/cc-pVTZ single-point energy calculations. The present work complements the study by Woon & Herbst, who characterized the structure and energies of 200 neutral species also involved in interstellar chemistry. Taken together, the two data sets can be used to estimate whether a reaction, postulated in present astrochemical reaction networks, is feasible from a thermochemistry point of view and, consequently, to improve the reliability of the present networks used to simulate the interstellar chemistry. We provide an actual example of the potential use of the cations plus neutral data sets. It shows that two reactions, involving Si-bearing ions and present in the widely used reaction networks KIDA and UMIST, cannot occur in the cold interstellar medium because they are endothermic.

Rovibrationally Resolved Photodissociation of AlH via Excited Electronic States

Abstract Photodissociation processes are of great importance for modeling interstellar chemistry since it is a key destruction pathway for small molecules. Here, we present a detailed ab initio study of AlH photodissociation. Potential energy curves and transition dipole moments for AlH are obtained by using the internally contracted multireference configuration interaction method and the Davidson correction (icMRCI+Q), as well as the aug-cc-pV6Z basis set. Except for the X1Σ+, A1Π, and C1Σ+ states, five higher excited 31Σ+, 21Π, 31Π, 41Σ+, and 41Π states are considered in order to investigate the photodissociation pathways in the vacuum ultraviolet region. State-resolved cross sections of transitions from all the rovibrational levels of the X1Σ+ state to seven singlet excited states are computed for photon wavelengths ranging from 500 Å to the threshold. Photodissociation cross sections in local thermal equilibrium (LTE) are obtained at temperatures from 500 to 10,000 K. Applications of the LTE cross sections to compute photodissociation rates in the standard interstellar radiation field and blackbody radiation field are briefly discussed.

Genesis of Polyatomic Molecules in Dark Clouds: CO2 Formation on Cold Amorphous Solid Water.

Understanding the formation of molecules under conditions relevant to interstellar chemistry is fundamental to characterize the chemical evolution of the universe. Using reactive molecular dynamics simulations with model-based or high-quality potential energy surfaces provides a means to specifically and quantitatively probe individual reaction channels at a molecular level. The formation of CO2 from collision of CO(1Σ) and O(1D) is characterized on amorphous solid water (ASW) under conditions typical in cold molecular clouds. Recombination takes place on the subnanosecond time scale and internal energy redistribution leads to stabilization of the product with CO2 remaining adsorbed on the ASW on extended time scales. Using a high-level, reproducing kernel-based potential energy surface for CO2, formation into and stabilization of CO2 and COO are observed.

The 13CO-rich atmosphere of a young accreting super-Jupiter.

Isotope abundance ratios have an important role in astronomy and planetary sciences, providing insights into the origin and evolution of the Solar System, interstellar chemistry and stellar nucleosynthesis1,2. In contrast to deuterium/hydrogen ratios, carbon isotope ratios are found to be roughly constant (around 89) in the Solar System1,3, but do vary on galactic scales with a 12C/13C isotopologue ratio of around 68 in the current local interstellar medium4-6. In molecular clouds and protoplanetary disks, 12CO/13CO ratios can be altered by ice and gas partitioning7, low-temperature isotopic ion-exchange reactions8 and isotope-selective photodissociation9. Here we report observations of 13CO in the atmosphere of the young, accreting super-Jupiter TYC8998-760-1b, at a statistical significance of more than six sigma. Marginalizing over the planet's atmospheric temperature structure, chemical composition and spectral calibration uncertainties suggests a 12CO/13CO ratio of [Formula: see text](90% confidence), a substantial enrichment in 13C with respect to the terrestrial standard and the local interstellar value. As the current location of TYC8998-760-1b at greater than or equal to160 astronomical units is far beyond the CO snowline, we postulate that it accreted a substantial fraction of its carbon from ices enriched in 13C through fractionation.

Dynamical investigations of the O(3P) + H2O reaction at high collision energies on an accurate full-dimensional potential energy surface

The reaction between O(3P) and H2O is of significance in combustion, atmospheric and interstellar chemistry. Using an accurate full-dimensional potential energy surface (PES) for the lowest triplet state of the H2O2 system, the quasi-classical trajectory method is employed to run dynamical simulations between O(3P) and H2O at hyperthermal collision energies of 50–100 kcal mol−1, which may access the hydrogen abstraction channel OH + OH, the hydrogen elimination channel H + HO2 and the oxygen exchange channel O + H2O. The integral cross sections (ICSs), differential cross sections, product energy/state distributions and reaction mechanisms are studied for the channels leading to products OH + OH and H + HO2.

Anomalous formation of trihydrogen cations from water on nanoparticles

Regarded as the most important ion in interstellar chemistry, the trihydrogen cation, {{rm{H}}}_{{{3}}}^{+}, plays a vital role in the formation of water and many complex organic molecules believed to be responsible for life in our universe. Apart from traditional plasma discharges, recent laboratory studies have focused on forming the trihydrogen cation from large organic molecules during their interactions with intense radiation and charged particles. In contrast, we present results on forming {{rm{H}}}_{{{3}}}^{+} from bimolecular reactions that involve only an inorganic molecule, namely water, without the presence of any organic molecules to facilitate its formation. This generation of {{rm{H}}}_{{{3}}}^{+} is enabled by “engineering” a suitable reaction environment comprising water-covered silica nanoparticles exposed to intense, femtosecond laser pulses. Similar, naturally-occurring, environments might exist in astrophysical settings where hydrated nanometer-sized dust particles are impacted by cosmic rays of charged particles or solar wind ions. Our results are a clear manifestation of how aerosolized nanoparticles in intense femtosecond laser fields can serve as a catalysts that enable exotic molecular entities to be produced via non-traditional routes.

A computational study on the formations of formamide analogues: Interesting chemistry by silicon analogues

Current research attentions are all focussed on formamide, which is considered as a possible precursor for prebiotic chemistries and is related to origin of life research. No reports on silicon and sulphur analogues of formamide exist in literature. In this work, quantum mechanical computations (B3LYP, MP2, wB97xD and CCSD(T) methods) were carried out, to get some insights on the formations of these analogues. Comparison of the reaction path these analogues with formamide, interesting behaviours for silicon analogues, like, relative stabilities and barrier heights were observed. The reason for larger stabilities of Si-analogues and reduced barrier heights is attributed to the involvement of d-orbitals of silicon in the pseudo-hypercoordination environments along the reaction paths. Thus, the present study calls for more extensive future research works, on the less explored silicon-based chemistries. We hope this study will be useful for understanding about the astrochemistry of formamide analogues, and specifically interstellar silicon chemistry.