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Current research to understand astrochemistry and astrobiology with regard to the weakly bound astronomical complexes including amino acids in the Inter-Stellar Medium (ISM), comets and meteorites at molecular level are flourishing and many new findings are emerging rapidly. Modern experimental techniques and computational investigations assisted by quantum chemical analysis are playing key role towards exploring the evolution of many molecular complexes in astrochemical environment and planetary atmospheres by affecting their stability as well as their structural and spectral properties. The present work deals with weekly bound complexes of glycine with small molecules like water, ammonia and HF as 1:1:1 Glycine-X-Y and Y-Glycine-X. The affects that influence the complexation, mainly involve through non-covalent interactions with these small molecules have been explored, based on various properties of the red shifted hydrogen bonds of the type: OHO, NHO, FHO, OHN and carbon-cantered blue shifted hydrogen bonds, CHO type, for all the stable complexes of 1:1:1 Glycine-X-Y and Y-Glycine-X with respect to their structural properties and spectral characterization. The computed Ray's asymmetry parameter, κ values for the investigated 1:1:1 Glycine-X-Y and Y-Glycine-X complexes are found to be negative and predicted as prolate-type asymmetric rotor. The computed rotational constants and IR spectra of these different complexes may assist in the interpretation and analysis of observational data and the data obtained in laboratory experiments. The possibilities of formation of such molecular complexes in the interstellar space may provide a direction to explore new possibilities.
A sufficient amount of spectroscopic data for S-substituted heterocyclic molecules are not available in the literature to adequately examine interstellar sulfur depletion. A quantum chemical investigation of quadrupole hyperfine structures of 2CNT, 3CNT, and some of their isotopologues is undertaken here in order to supplement the literature and supply new data that are valuable for the detection of new species in the ISM. The nuclear hyperfine quadrupole structures of 2-cyanothiophene (2CNT), 3-cyanothiophene (3CNT), and their S-33, H-2-isotopologues were analyzed at temperatures prevailing in regions where these compounds are expected to be found (T similar to 5 K to T similar to 150). The geometrical parameters were calculated using density functional theory (M06-2X/6-31G(d,p)) with empirical corrections for systematic errors. The N-14, S-33, and H-2 quadrupole hyperfine constants, and dipole moments for 2CNT, 3CNT, S-33-2C(15)NT, S-33-3C(15)NT, 2C(15)NT-3D, and 2C(15)NT-2D were calculated, and their corresponding hyperfine structures were analyzed. The strongest transition lines and optimal regions of detectability were also discussed.
Collision of NH radical with H2 molecule and their deuterated analogs were studied on an interpolated potential energy surface using ab initio calculation. For this purpose, reactive and non-reactive processes were considered. Reaction probability and cross-section were determined and used for calculating the rate constant. In non-reactive trajectories, the inelastic collision was investigated in order to calculate the scattering angle. The difference between the initial and final energy of each particle was calculated in order to determine the transmitted and converted energy. For all trajectories, the effect of the impact parameter and relative translational energy of particles on the reaction probability, transmitted energy, and scattering angle were determined. Isotope effect was used to specify the effect of the mass of the target and incoming particle on the quantity and quality of products and also on the reaction probability and other observable properties.
Hydrogenated polycyclic aromatic hydrocarbons (PAHs) have been proposed to contribute to the formation of interstellar H2 by acting as a surface catalyst for the adsorption of hydrogen atoms and desorption of H2 molecules. In the present study, imaging photoelectron photoion coincidence (iPEPICO) spectroscopy and tandem mass spectrometry were employed to examine the unimolecular chemistry of four hydrogenated polycyclic aromatic hydrocarbon ions: 9,10-dihydroanthracene (DHA+•) and 1,2,3,4,5,6,7,8-octahydroanthracene (OHA+•), having opposite patterns of hydrogenation, and 1,2,3,4-tetrahydrophenanthrene (THP+•) and 1,2,3,4,9,10-hexahydrophenanthrene (HHP+•). DHA+• exhibits the same reactions previously observed for 1,2-dihydronaphthalene and 9,10-dihydrophenanthrene, namely competing loss of H• and CH3•. However, the energy required for H•-loss, as predicted by RRKM modeling of the iPEPICO results, was lower than the latter ions, presumably due to charge delocalization across the central ring upon dehydrogenation. OHA+• behaves similarly to ionized tetralin, displaying losses of H•, CH3•, C2H4 and C3H5• in its collision induced dissociation (CID) mass spectra, but under iPEPICO conditions CH3•-loss is not observed. THP+• and HHP+• have aspects of both DHA+• and OHA+• chemistries, displaying losses of H•, CH3•, C2H4 and C3H5•. RRKM modeling produced minimum energies for all observed reaction channels, which were also computationally explored at the B3LYP/6–31+G(d,p) level of theory. The results indicate that small PAH ions may not be effective surfaces for the catalytic formation of H2 in the ISM, but rather sources of small hydrocarbons.
The knowledge of the binding energy of molecules on astrophysically relevant ices can help to obtain an estimate of the desorption rate, i.e. the molecules residence time on the surface. This represents an important parameter for astrochemical models, and it is crucial to determine the chemical fate of interstellar complex organic molecules formed on the surface of dust grains and observed in the densest regions of the interstellar medium through rich rotational lines. In this work, we propose a new robust procedure to study the interaction of atoms and molecules with interstellar ices, based on ab initio molecular dynamics and density functional theory, validated by high-level ab initio methods at a CCSD(T)/CBS level. We have applied this procedure to a simple but astronomically relevant molecule, hydrogen fluoride (HF), a promising tracer of the molecular content of galaxies. In total we found 13 unique equilibrium structures of HF binding to small water clusters of up to 4 molecules, with binding energies ranging from 1208 K to 7162 K (2.40 to 14.23 kcal mol−1). We computed a 22-molecules model of amorphous solid water (ASW) surface using ab initio molecular dynamics simulations and carried out a systematic analysis of the binding sites of HF, in terms of binding modes and binding energies. Considering 10 different water clusters configurations, we found a binding energy distribution with an average value of 5313±74 K, and a dispersion of 921±115 K (10.56±0.15 kcal mol−1), and a dispersion of 921±115 K (1.83±0.23 kcal mol−1). Finally, the effect of the electrostatic field of the 22 water molecules on the binding energies was investigated incrementally by symmetry adapted perturbation theory, in order to gauge the effect of the water environment on the binding energies. The results indicate that the extent of the electrostatic interaction of HF with ASW depends strongly on the properties of the binding site on the water cluster. We expect that this work will provide a solid foundation for a systematic development of a binding energy distribution database of small molecules on astrophysically relevant surfaces.
Of the dicyanomine anion (NCNCN), cyanoethynylamine anion (NCNC2H-), and diethynylamine anion (HC2NC2H-), only the mixed, C-s NCNC2H- anion has are large enough dipole moment to support an electronically excited state at 3.0323 eV. This quantum chemical study shows that this value lies 0.0051 eV below the electron binding energy (eBE) and may have correlation to early-onset diffuse interstellar bands. None of these three anions possess further valence excited electronic states beyond the singlet ground states, and triplet excited states are all beyond their respective eBEs.
We report theoretical and experimental line-shape parameters for He-perturbed pure rotational HD lines that are relevant for the studies of gas giants atmospheres. Besides the usual pressure broadening and shift parameters, we also report their speed dependencies and Dicke parameters. The theoretical values, obtained from quantum dynamical calculations, are for the R(j=0-3) lines and S(j=0-2) and temperatures from 10 to 500 K. The measurements, performed using stimulated Raman spectroscopy, were done for the S(j=0-2) rotational Raman lines at 77, 195 and 298 K. We also compare the results of our calculations with pressure broadening and line shift coefficients available in the literature at 77, 195 and 300 K for the studied R lines. We demonstrate that a simple Voigt profile is insufficient to accurately model the shapes of He-perturbed HD lines at conditions relevant to gas giants atmospheres, and one should incorporate also the speed-dependent effects and velocitychanging collisions.
Enantiomeric excess of amino acids observed in the meteoritic samples of carbonaceous chondrites has incited many researchers to search for an extra-terrestrial origin of life on prebiotic Earth. However, in a non-catalytic environment, only racemic amino acids are synthesized. This computational quantum-mechanical study explores non-catalytic mechanistic pathways for stereoinversion in proteinogenic L-glutamic acid, which may be observable under gas-phase conditions of interstellar medium (ISM). The multi-step stereoinversion pathways proposed in this study are traced through a global reaction route mapping (GRRM) strategy utilizing density-functional and coupled-cluster theories. Notably, a few of the pathways are observed to proceed through simultaneous intramolecular hydrogen atom and proton transfer as well as through a proton-coupled electron transfer mechanism. The intermediates explored along the stereoinversion pathways resemble ammonium ylide and imine, the key ingredients in Strecker synthesis of amino acids. The thermodynamic and kinetic analysis of the stereoinversion pathways in different temperature regions of ISM are also carried out, predicting the streoinversion to proceed over any dissociation of intermediates and conformers of glutamic acid along the pathways. However, initial step of the pathways involves an unsurmountable energy barrier though the key step responsible for stereoinversion has a very low energy barrier and is predicted to proceeds with significant rates. The work suggests the possibility of observing stereoinversion of glutamic acid in the warmer regions of ISM.
Most carbon in the Universe is tied up in carbon monoxide or in polycyclic aromatic hydrocarbons. Even so, a vast majority of the molecules detected in various astrophysical media contain at least one carbon atom in them. These could nearly all be classified as hydrocarbons. However, only a fraction of the atoms in the Universe heavier than helium are actually carbon. This review will explore the past astronomical detections of molecules that do not contain carbon and will discuss the present workings and future outlooks of pure, inorganic astrochemistry. Such molecules have bonding structures that are often “atypical,” have notable spectroscopic intensities, and open the door for new chemical insights. Asking novel questions can lead to novel insights, and inorganic astrochemistry provides a strong motivation for asking the most creative chemical questions.
Amidogen (NH2), a b-type asymmetric top molecule with electric dipole moment 1.82 +/- 0.05 Debye, is detected in Sgr B2, in high-mass star-forming regions W31C (G10.6-0.4), W49N (G43.2-0.1), W51 (G49.5-0.4), G34.3+ 0.1, and in several comets. Because of two hydrogen atoms, each with nuclear spin 1/2, its rotational energy levels can be classified into ortho and para groups. We have not considered for fine structure splitting and hyper-fine structure splitting of rotational levels. For 15 rotational levels in the ground vibrational state, having energy up to 400 cm(-1), for each specie, the energies of rotational levels, and Einstein A and B coefficients for radiative transitions between the levels are calculated, using accurate values of spectroscopic data. These radiative transition probabilities along with the collisional rate coefficients (obtained from a scaling law) are employed as input parameters for solving a set of statistical equilibrium equations coupled with the equations of radiative transfer for each group. Several emission lines produced by amidogen are found. For each species of NH2, we have considered some strongest emission lines along with the observed one, which may help for identification of NH2 in the interstellar medium (ISM) and in the cometary material.
PAHs are one of the important components of the carbonaceous matter of the Universe. They are not detected in the darkest regions of the Interstellar Medium and one possible reason could be their chemical transformation through gas phase reactions In particular, their oxidation was considered ineffective because the reaction barriers appear to be too high, based on combustion studies conducted at high temperatures. For the first time, we experimentally studied the oxidation of Coronene, a PAH archetype, at low temperature (50 K), as well as the oxidation of hydrogenated Coronenes. It appears that reactivity is higher than expected and that the fragmentation of coronene is a significant channel of the oxidation. Furthermore, hydrogenated coronenes are very reactive to oxygen. To understand the experimental data, DFT calculations were performed. They confirm a low oxidation barrier (0.11 eV) and show that oxygen is preferentially inserted at the periphery of the coronene and propose a reaction mechanism for fragmentation also involving a hydrogen atom. An estimate of the orders of magnitude shows that PAH oxidation may explain part of the decrease in their abundances in warm environments.
The rotational excitation of HCN by H2O, the main perturber in cometary atmospheres, is investigated using quantum methodologies. We provide approximate rotational de-excitation rate coefficients among the first levels of HCN perturbed by thermalized para-water in the temperature range T = 5K to T =150K. Because of the novelty of the system for quantum rotational excitation, the current study includes a detailed appreciation of the parameters involved in the convergence of the cross-sections and of rate coefficients calculations. A compromise on the convergence of the rate coefficients with respect to the rotational basis set is taken because of the computing time cost. Moreover, because of the cost also involved in calculating the 5D potential energy surfaces necessary for the current dynamical calculations, several potential energy surfaces of increased quality are tested and it is shown that, within the current approximations on the collisional calculations, average quality potential energy surfaces are sufficient for cometary applications.
In this article, I have presented quantum mechanical treatment of methanol formation reaction from methane water-ice cluster after ionization with high energy photons. To mimic water ice structure, six water molecules are arranged in hexagonal form. This is the most stable structure formed by six water molecules. Theoretical photodesorption study has been performed on this structure. I have investigated a unique reaction mechanism of methanol formation from methane water ice cluster. I have shown that methanol forms after ionization via two transition states. Hydrogen molecule forms as one of the side product. Initiation of the reaction occurs by dissociation of O-H bond in the water cluster followed by C-H bond dissociation and formation of C-O bond. The timescale of the initial step, which is dissociation of O-H bond, has been computed as 50 femtosecond in this article.
The gas-phase stereoinversion of amino acid threonine under the condition of interstellar medium (ISM) has been predicted to proceed through isomeric species with diverse chemistry. These species including ammonium ylides, epoxides, contain a variety of functional groups such as geminal-diol, triol besides alkenyl, carboxy, keto, hydroxy, and amino groups. The detection of these species in ISM can help in unravelling the enantiomeric excess observed in meteoritic samples. Towards this, the present work reports rotational and vibrational spectroscopic data computed for the conformers and isomeric intermediates predicted along the stereoinversion pathways of proteinogenic threonine under conditions akin to ISM. The rotational parameters are computed using quantum mechanical methods employing Møller–Plesset perturbation theory whereas for the vibrational analysis, density functional computations are performed using dispersion corrected exchange-correlation functionals. The anharmonic corrections are also computed using vibrational second-order perturbation theory, which, however, fails to account for the hydrogen bonded interactions in the species investigated. The rotational and vibrational transitions predicted for the conformers of threonine are observed to be in good agreement with the available experimental data. The gas-phase spectroscopic data computed for other isomeric species of threonine is quite reliable and can be used to search threonine or other amino acids in ISM by resolving the astrophysical data observed in the microwave and mid-infrared regions.
Successive hydrogenation reactions of isolated CO molecules adsorbed on a bare graphene surface have been studied by density functional theory using a van der Waals functional. Three hydrogenation scenarios, leading to the formation of methanol via the intermediate species: HCO, H2CO, HCOH, H-3 CO and H2COH, have been considered. Hydrogenation and adsorption energies on the surface have been calculated for all the species. The fractions of molecules released in the gas phase after formation on the surface have been calculated with two different chemical desorption models. Our results show that the fraction of methanol molecules released in the gas phase is low (< 6%) whatever the scenario. Conversely, the highest fractions of molecules released in the gas phase have been obtained for formaldehyde, H2CO, and the hydroxymethyl radical, H2COH. The methoxy radical, H3CO, is characterized by a high adsorption energy on the substrate (-0.337 eV).
