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Context. Almost 20% of the ~200 different species detected in the interstellar and circumstellar media present a carbon atom linked to nitrogen by a triple bond. Of these 37 molecules, 30 are nitrile R-CN compounds, the remaining 7 belonging to the isonitrile R-NC family. How these species behave in their interactions with the grain surfaces is still an open question. Aims. In a previous work, we have investigated whether the difference between nitrile and isonitrile functional groups may induce differences in the adsorption energies of the related isomers at the surfaces of interstellar grains of various nature and morphologies. This study is a follow up of this work, where we focus on the adsorption on carbonaceous aromatic surfaces. Methods. The question is addressed by means of a concerted experimental and theoretical approach of the adsorption energies of CH3CN and CH3NC on the surface of graphite (with and without surface defects). The experimental determination of the molecule and surface interaction energies is carried out using temperature-programmed desorption in an ultra-high vacuum between 70 and 160 K. Theoretically, the question is addressed using first-principle periodic density functional theory to represent the organised solid support. Results. The adsorption energy of each compound is found to be very sensitive to the structural defects of the aromatic carbonaceous surface: these defects, expected to be present in a large numbers and great diversity on a realistic surface, significantly increase the average adsorption energies to more than 50% as compared to adsorption on perfect graphene planes. The most stable isomer (CH3CN) interacts more efficiently with the carbonaceous solid support than the higher energy isomer (CH3NC), however.

Molecular oxygen has been detected in the coma of comet 67P/Churyumov-Gerasimenko with abundances in the 1-10% range by the ROSINA-DFMS instrument on board the Rosetta spacecraft. Here we find that the radiolysis of icy grains in low-density environments such as the presolar cloud may induce the production of large amounts of molecular oxygen. We also show that molecular oxygen can be efficiently trapped in clathrates formed in the protosolar nebula, and that its incorporation as crystalline ice is highly implausible because this would imply much larger abundances of Ar and N2 than those observed in the coma. Assuming that radiolysis has been the only O2 production mechanism at work, we conclude that the formation of comet 67P/Churyumov-Gerasimenko is possible in a dense and early protosolar nebula in the framework of two extreme scenarios: (1) agglomeration from pristine amorphous icy grains/particles formed in ISM and (2) agglomeration from clathrates that formed during the disk's cooling. The former scenario is found consistent with the strong correlation between O2 and H2O observed in 67P/C-G's coma while the latter scenario requires that clathrates formed from ISM icy grains that crystallized when entering the protosolar nebula., The Astrophysical Journal Letters, in press

Complex organic molecules have been observed in the interstellar medium in comets and meteorites. Among these compounds are the building blocks that may have led to prebiotic systems and ultimately to the origin of life. In the panspermia hypothesis, the survival and the transfer of the amino acids from space to planets is a necessary condition for the appearance of life, and, especially, their resistance to the solar UV radiation in ice is a key issue. The case of glycine, which is the smallest molecule in the amino acid family, is presented here. To improve our knowledge on the decomposition mechanisms of glycine, we have undertaken a coupled experimental and computational study. On the one hand, it has been performed a near-edge X-ray absorption spectroscopy (NEXAFS) study at the oxygen K-edge of solid glycine and of glycine diluted into H2O ice, and irradiated at 30 K with soft X-rays. However, extensive quantum chemical simulations using density functional theory have been realized at the B3LYP/cc-pVQZ level. A systematic investigation of the most plausible fragmentations has been performed for neutral glycine itself, ionized glycine, doubly ionized glycine, protonated glycine, and zwitterionic glycine, the well known stable form of glycine trapped in ices. We show that glycine easily decomposes under irradiation. Water does not enhance or protect glycine from photodecomposition, but it increases the production of CO2 in the secondary photoreactions. The concentration of glycine tends to a limit of ∼30% of the initial load in the ice. The role of water ice appears to be that of a containment environment allowing the fragments to remain close, thus leading to reformation of glycine in situ. Under these conditions, it can be concluded that glycine is partially, but not entirely, protected by ice during its journey to the Earth. It is a strong point to be credited to the panspermia hypothesis. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011

We study the efficiency of the noble gases sequestration by the ion H3+ in the form of XH3+ complexes (with X = argon, krypton or xenon) in gas phase conditions similar to those encountered during the cooling of protoplanetary disks, at the epoch of icy planetesimals formation. We show that XH3+ complexes form very stable structures in the gas phase and that their binding energies are much higher than those involved in the structures of X-H2O hydrates or pure X-X condensates. This implies that, in presence of H3+ ions, argon, krypton or xenon are likely to remain sequestrated in the form of XH3+ complexes embedded in the gas phase rather than forming ices during the cooling of protoplanetary disks. The amount of the deficiency depends on how much H3+ is available and efficient in capturing noble gases. In the dense gas of the mid-plane of solar nebula, H3+ is formed by the ionization of H2 from energetic particles, as those in cosmic rays or those ejected by the young Sun. Even using the largest estimate of the cosmic rays ionization rate, we compute that the H3+ abundance is two and three orders of magnitude lower than the xenon and krypton abundance, respectively. Estimating the ionization induced by the young Sun, on the other hand, is very uncertain but leaves the possibility to have enough H3+ to make krypton and xenon trapping efficent. Finally, additional source of H3+ formation may be provided by the presence of a nearby supernova, as discussed in the literature. Recent solar system observations show a deficiency of Ar, and, even more, of Kr and Xe in Titan and in comets. In this article, we consider the possibility that this deficiency is caused by the afore-mentioned process, namely trapping of those noble gases by H3+ ions in the solar nebula.

We report the results of a theoretical study of the effect of ionization on the IR spectrum of naphthalene, using ab initio molecular orbital theory. For that purpose we determined the structures, band frequencies, and intensities of neutral and positively ionized naphthalene. The calculated frequencies and intensities allowed an assignment of the most important bands appearing in the newly reported experimental spectrum of the positive ion. Agreement with the experimental spectrum is satisfactory enough to take into consideration the unexpected and important result that ionization significantly affects the intensities of most vibrations. A possible consequence on the interpretation of the IR interstellar emission, generally supposed to originate from polycyclic aromatic hydrocarbons (PAHs), is briefly presented.

The puzzling difference between the IR spectra of polycyclic aromatic hydrocarbons (PAHs) and those of the corresponding positive ions (PAHs(+)) is a well documented fact, although the basic reason for it is far from clear. In this report, the CH fragment, in its neutral and ionized forms is taken as a case study for investigating the collapse of the CH stretching vibration with ionization. A comprehensive study of the dipole moment function around the equilibrium geometries of the fragments using large scale configuration interaction and quantum Monte Carlo methods shows very different variations with the CH distance: a marked decrease for neutral CH((2)Pi) and a perfect stability for ionized CH(+)((1)Sigma(+)). These results are consistent with strong/weak intensities of the CH vibrations in the neutral/ionized PAHs, the key point being the presence, or not, of a hole in the pi shell. A topological analysis of the electronic densities shows that the collapse of the CH stretching with ionization is directly linked to the compensation between the internal charge transfer contribution and the distortion of the electronic density within the CH bond.

We calculate the minimum mass of heavy elements required in the envelopes of Jupiter and Saturn to match the observed oversolar abundances of volatiles. Because the clathration efficiency remains unknown in the solar nebula, we have considered a set of sequences of ice formation in which the fraction of water available for clathration is varied between 0 and 100 %. In all the cases considered, we assume that the water abundance remains homogeneous whatever the heliocentric distance in the nebula and directly derives from a gas phase of solar composition. Planetesimals then form in the feeding zones of Jupiter and Saturn from the agglomeration of clathrates and pure condensates in proportions fixed by the clathration efficiency. A fraction of Kr and Xe may have been sequestrated by the H3+ ion in the form of stable XeH3+ and KrH3+ complexes in the solar nebula gas phase, thus implying the formation of at least partly Xe- and Kr-impoverished planetesimals in the feeding zones of Jupiter and Saturn. These planetesimals were subsequently accreted and vaporized into the hydrogen envelopes of Jupiter and Saturn, thus engendering volatiles enrichments in their atmospheres, with respect to hydrogen. Taking into account both refractory and volatile components, and assuming plausible molecular mixing ratios in the gas phase of the outer solar nebula, we show that it is possible to match the observed enrichments in Jupiter and Saturn, whatever the clathration efficiency. Our calculations predict that the O/H enrichment decreases from 6.7 to 5.6 times solar (O/H) in the envelope of Jupiter and from 18.1 to 15.4 times solar (O/H) in the envelope of Saturn with the growing clathration efficiency in the solar nebula., Comment: Accepted for publication in The Astrophysical Journal

The repartition of molecular hydrogen in space, and its depletion on solid particles in particular, is an important question of modern astrophysics. In this paper, we report a theoretical study of the physisorption of molecular hydrogen, H2, on a major component of the interstellar dust known as polycyclic aromatic hydrocarbons (PAHs). Two different density functional theory approaches were used: (i) the conventional Kohn−Sham theory and (ii) the subsystem-based approach (Kohn−Sham equations with constrained electron density, KSCED) developed in our group. The approximate exchange-correlation energy functional applied in all calculations and the nonadditive kinetic-energy functional needed in KSCED have a generalized gradient approximation form and were chosen on the basis of our previous studies. The results of both approaches show similar trends: weak dependence of the calculated interaction energies on the size of the PAH and negligible effect of the complexation of two PAH molecules on the adsorption of molecular hydrogen. The KSCED interaction energy calculated for the largest considered PAH (ovalene), amounting to 1.27 kcal/mol, is in excellent agreement with experimental estimates ranging from 1.1 to 1.2 kcal/mol, whereas the one derived from supermolecular Kohn−Sham calculations is underestimated by more than 50%. This result is in line with our previous studies, which showed that the generalized gradient approximation applied within the KSCED framework leads to interaction energies of weakly bound complexes that are superior to the corresponding results of supermolecular Kohn−Sham calculations.

In order to test the hypothesis of ionized polycyclic aromatic hydrocarbons (PAHs) as possible carriers of the UIR bands, we realized a computational exploration on selected PAHs of small dimension in order to identify which changes ionization would induce on their IR spectra. In this study we performed ab initio calculations of the spectra of neutral and positively ionized naphthalene, anthracene, and pyrene. The results are significantly important. The frequencies in the cations are slightly shifted with respect to the neutral species, but no general conclusion can be reached from the three molecules considered. By contrast, the relative intensities of most vibrations are strongly affected by ionization, leading to a much better agreement between the calculated CH/CC vibration intensity ratios and those deduced from observations.

The potential value of quantum chemical methods in the study of interstellar molecules is illustrated by ab initio calculations on sulphur compounds of astrophysical interest. This work includes: (1) assignments of rotation lines from “non-terrestrial” molecules (i.e., those for which laboratory spectra were lacking, such as the HCS+ ion and its metastable isomer HSC+), through structure computations; (2) energy balances for the formation and dissociation reactions of protonated species from simpler sulphur-containing molecules, and (3) a determination of the energy curves for the first Σ and Π states of the (HS)+ system, to be used later in a collision treatment of the charge-exchange reaction between the sulphur and hydrogen atoms. ab]Lea possibilites deg methodes de la chimie quantique dans le domaine des molecules interstellaires sont illustrees par des calculs relatifs a des composes soufres d'interet astrophysique. Les recherches effectuees comprennent: (1) des attributions de raies de rotation provenant de molecules “non-terrestres” (c'est a dire de composes pour lesquels on manquait de donnees de laboratoire, tels l'ion HCS+ et son isomzere metastable (HSC+), a l'aide de calculs de structure; (2) des evaluations de bilans energ etiques pour lea reactions de formation et de destruction des espces protones dans l'espace a partir de molecules soufrees plus simples, et (3) une determination des courbes d'energle des premiers etats Σ et Π du systeme (HS)+ pour une etude collisionnelle ult $ erieure de la reaction d'echange de charge entre les atomes de soufre et d'hydrogene.

Determination par une methode ab initio des constantes de force, frequences de vibration et conformation d'une serie de composes cycliques satures a 3 chainons

It is shown here that the difference of two orders of magnitude between the values of the abundance ratios HCO(+)/CO and HCS(+)/CS in interstellar clouds can be directly related to the difference in the reaction rates of the dissociative recombination reactions of the positive ions. The potential energy curves of the systems (HCO+ + e-) and (HCS+ + e-) in Rydberg and dissociative valence states, as well as those of the positive ions CHO+ and HCS+, are computed by ab initio quantum chemistry methods. The potential surfaces of the two systems show striking differences, suggesting that the dissociative processes, direct and indirect, should be more efficient for HCO. 23 refs.

The hunt for prebiotic species in the interstellar medium is a major concern for understanding the possible role of interstellar chemistry in the synthesis of the molecules at the origin of life. This study was motivated by the fact that some amino acids are currently identified in meteorites but none are yet observed in the interstellar medium. On the one hand, we use DFT and ab-initio methods of quantum chemical calculations to study the relative stability of the isomers detected in the interstellar medium. This study leads to a "Minimum Energy Principle" (MEP) specifying that the most thermodynamically stable isomer corresponds to the larger abundance observed. Applying this principle, we evaluate the possibility of detecting new prebiotic species, particularly amino acids, in the interstellar medium. Then, within the same approach, we verify that the link between abundance and thermodynamic order is valid for amino acids in the meteorites, which allows us to constrain the formation conditions in the parent bodies. On the other hand, we use the plane-wave periodic method in order to study the sticking of molecular species on carbonaceous and icy interstellar grains. We show that there is an efficient selective adsorption for the different isomers. This could lead to an observational bias for the relative abundances of these isomers, which could provide a possible explanation for the few exceptions to the MEP.; La recherche de molécules prébiotiques dans le milieu interstellaire est importante pour comprendre le rôle potentiel de la chimie interstellaire dans la synthèse des molécules à l'origine de la vie. Aujourd'hui nous sommes confrontés à une apparente contradiction observationnelle : des acides aminés ont été identifiés dans les météorites mais aucun n'a été observé de façon certaine dans le milieu interstellaire. Dans un premier temps, nous utilisons les méthodes de calcul moléculaire DFT et ab-initio de la chimie quantique pour étudier la stabilité relative des isomères détectés dans le milieu interstellaire, ce qui nous conduit à établir un "Principe d'Energie Minimale" (PEM) stipulant que l'isomère le plus stable thermodynamiquement est le plus abondant. En s'appuyant sur ce principe, nous évaluons les possibilités de détection de nouvelles molécules prébiotiques dans le milieu interstellaire, en particulier les acides aminés. Puis, en suivant la même démarche, nous vérifions pour les météorites, que le lien entre abondance et ordre thermodynamique est respecté au niveau des acides aminés, ce qui nous permet de contraindre les conditions de formation dans les corps parents. Dans un second temps, nous utilisons les méthodes périodiques en ondes planes pour étudier les collages d'espèces moléculaires sur les grains carbonés et glacés interstellaires. Nous montrons l'existence d'une physisorption sélective pour certains types d'isomères. Elle entraînerait un biais observationnel sur les abondances relatives de ces isomères, ce qui nous donnerait une possibilité d'explication pour les quelques apparentes exceptions au PEM.