Clathration of Volatiles in the Solar Nebula and Implications for the Origin of Titan's atmosphere

We describe a scenario of Titan's formation matching the constraints imposed by its current atmospheric composition. Assuming that the abundances of all elements, including oxygen, are solar in the outer nebula, we show that the icy planetesimals were agglomerated in the feeding zone of Saturn from a mixture of clathrates with multiple guest species, so-called stochiometric hydrates such as ammonia hydrate, and pure condensates. We also use a statistical thermodynamic approach to constrain the composition of multiple guest clathrates formed in the solar nebula. We then infer that krypton and xenon, that are expected to condense in the 20-30 K temperature range in the solar nebula, are trapped in clathrates at higher temperatures than 50 K. Once formed, these ices either were accreted by Saturn or remained embedded in its surrounding subnebula until they found their way into the regular satellites growing around Saturn. In order to explain the carbon monoxide and primordial argon deficiencies of Titan's atmosphere, we suggest that the satellite was formed from icy planetesimals initially produced in the solar nebula and that were partially devolatilized at a temperature not exceeding 50 K during their migration within Saturn's subnebula. The observed deficiencies of Titan's atmosphere in krypton and xenon could result from other processes that may have occurred both prior or after the completion of Titan. Thus, krypton and xenon may have been sequestrated in the form of XH3+ complexes in the solar nebula gas phase, causing the formation of noble gas-poor planetesimals ultimately accreted by Titan. Alternatively, krypton and xenon may have also been trapped efficiently in clathrates located on the satellite's surface or in its atmospheric haze.Comment: Accepted for publication in The Astrophysical Journa

An ab initio and DFT study of (N2)2 dimers

The structure of van der Waals dimers (N2)2 is studied using ab initio and density functional calculations. The potential energy surfaces corrected a priori for basis set superposition errors are necessary for determining both the geometries and the vibrational frequencies. With both ab initio MP2, MP4 and DFT PW91–PW91 levels of theory, the T-shaped and canted conformations appear to be the most stable, within 1–5 cm−1 of each other. The DFT PW91–PW91 dissociation energy is 67 cm−1 and an upper limit to the barrier to internal motion is 30 cm−1, both in excellent agreement with the values deduced from IR measurements

Second-order perturbation theory using correlated orbitals. II. A coupled MCSCF perturbation strategy for electronic spectra and its applications to ethylene, formaldehyde and vinylidene

In this second paper, the philosophy of coupling multiconfigurational variational wave functions to perturbation treatments (MC/P methodology) is extended to the calculation of electronic spectra. The corresponding methodology is presented with emphasis on its flexibility and an overview of other available approaches is given. The contracted MC/P scheme is then applied to ethylene H2C=CH2, formaldehyde H2C=O vinylidene H2C=C. It is shown that combining well-designed averaged zeroth-order MCSCF wave functions to a barycentric Møller-Plesset (BMP) partition of the electronic Hamiltonian provides accurate spectra, contrary to Epstein-Nesbet partitions. The MC/BMP transition energies compare with experimental data within a few hundreds of cm−1. These results have been obtained using a polarized double-zeta quality basis set augmented by a set of semi-diffuse functions (6–31 + G*) and by an extra set of diffuse orbitals to account for Rydberg states. Since non-dynamic correlations effects that are important for a proper description of the manifold of the excited states of interest are included in the MCSCF zeroth-order space will all remaining correlation effects (non-dynamic and dynamic) are treated at the perturbation level, the present study lets anticipate applications of the MC/P methodology to medium size systems without much computational trouble

Isomerization versus hydrogen exchange reaction in the HNC HCN conversion

For the HNC/HCN interconversion we show that the push-pull hydrogen exchange reaction H+CNHright harpoon over leftHCNHright harpoon over leftHCN+H is favoured over internal isomerization; the formation of H2CN or CNH2 followed by rearrangement to HCNH and subsequent elimination are more energy demanding processes. Both push-pull forward and reverse reactions present activation barriers. However, the activation energy on the H+CNH entrance channel (4.2±1.0 kcal/mol) is four times smaller than on the HCN+H path. As a consequence, it can be anticipated that there will be a range of temperatures where the H+CNH reaction will be efficient while the reverse HCN+H process is still inhibited. This process, much less endothermic than internal isomerization, should become an important path for HNC/HCN conversion with increasing temperature in star forming regions

De l’Astrochimie à l’Exobiologie : un fil directeur vers la vie

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On the HCN/HNC abundance ratio : a theoretical study of the H + CNH HCN + H exchange reaction

The hydrogen exchange reaction H+CNH->HCN+H may be a key step in gas-phase interstellar nitrogen chemistry. It is one of the reactions supposed to cause increasing HNC depletion with increasing temperature in dense interstellar clouds. In this paper we report the results of extensive ab-initio calculations on the H+CNHH+HCN system that partially confirm this hypothesis. It is shown that both forward and reverse reactions possess activation barriers. However, the activation energy of the H+CNH channel (4.2+/-1.0kcal/mol.) is four times smaller than for the endothermic HCN+H path. Calculations on the rate of the forward reaction show that tunneling under the entrance channel barrier allows a small rate coefficient at the temperatures under 100K and that the rate coefficient increases steadily with increasing temperature for T>100K

The PAH hypothesis : a computational experiment on the combined effects of ionization and dehydrogenation on the IR signatures

IR spectra of anthracene and pyrene derivatives, serving as models for isolated, linear and isolated, compact PAHs, respectively, have been calculated using ab-initio quantum mechanical methods. The separate and combined effects of ionization and multiple dehydrogenation have been studied. This study confirms and refines the trends of our preliminary paper on the smallest possible PAH, naphthalene. If small PAHs are responsible for any UIR bands, they should be ionized and partially dehydrogenated, with a few triple bonds at the periphery of the carbon skeleton. In the appendix are given the complete IR spectra of all the isomers of the derivatives of anthracene and pyrene calculated for the purpose of this study. Tables I are for anthracene and Tables II for pyrene. Positions of the the missing hydrogens in the dehydrogenated species are referred as in Figures 1 and 2 of the original publication

Structure électronique et couplages à longue distance dans un radical libre bicyclique

Le bicyclo-1,1,1-pentane et le radical bicyclo-1,1,1-pentyle qui en dérive par arrachement d’un hydrogène méthylénique, ont été étudiés par une méthode d’orbitales moléculaires self-consistante simplifiée. Les couplages à longue distance sont interprétés en fonction de la géométrie du centre radi- calaire. Il est montré que, pour un radical plan ou en inversion rapide, les forts couplages aHy sont positifs et dus aux hydrogènes anti; pour un radical pyramidal bloqué, les interactions à longue distance dépendent du sens et de l’angle de torsion de la liaison exocyclique

Interstellar silicon-nitrogen chemistry. 1, The microwave and the infrared signatures of the HSiN, HNSi, HSiNH?2, HNSiH2 and HSiNH+ species

The experimental and the theoretical interests for the silicon chemistry have been renewed by the recent detection of SiN in space. In this contribution a theoretical study of the HSiN, HNSi, HSiNH2 and HNSiH2 molecular systems is presented that aims to help in the interpretation of available experimental results as well as in the attribution of new interstellar lines. The main goal of this report remains, however, the calibration of ab initio calculations on still-unknown silicon-nitrogen systems: the infrared and the microwave signatures of the HSiNH+ cation are reported as a direct application. The signatures of the five molecules under investagation have been computed at increasing levels of post-Hartree-Fock theories, using up to a 6–311 + + G** atomic orbital expansion. Accurate geometries and Be rotational constants have been determined at the Möller-Plesset MPn(n = 2, 3, 4), CASSCF and CCSD(T) theoretical plateaus for HNSi. The comparison with experimental data allows then to derive the scaling factors needed to obtain accurate rotational constants for related species: they are applied as such on the crude constants determined for HSiN, HSiNH2, HNSiH2, and finally HSiNH2 in its floppy linear singlet ground state and in its lowest cis-bent a3A' state as well. Dipole moments are reported in order to assess the feasability for these species to be detected owing to their rotational signatures either in the laboratory or in space using millimetric radioastronomy techniques. Infrared (IR) signatures are computed at the same levels of theory and compared to the recent matrix isolation experiments devoted to HSiN, HNSi, HSiNH2 and HNSiH2. The calculations unambiguosly confirm that all these species have been effectively produced and observed. They also lead to the determination of accurate IR scaling factors that are significantly larger than the usual ones. Such an approach allows then to quantitatively predict the IR spectra of the still-unknown HSiNH+ entity. The study of the IR spectra furthermore points out the failure of single-reference correlation methods to obtain predictive IR signatures in some cases, as is unambigously illustrated in the case of the HSiN species

Interstellar silicon-nitrogen chemistry. 4. Which reaction paths to HSiN and HNSi ? : an extensive ab initio investigation with crucial consequences for molecular astrophysics

In order to provide a possible explanation for the lack of detection of both HSiN and HNSi in the interstellar medium, an ab initio study of the Si+ + NH3 reaction is presented: it includes accurate energetic considerations and sketches dynamics discussions as well. It is unambiguously concluded that the X1A1 ground state of the SiNH2+ cation is the only exit channel of this reaction assuming interstellar conditions. The rotational and vibrational constants of this species are reported to stimulate its experimental and astrophysical searches. Upon dissociative recombination, it is likely that SiNH2+ can evolve toward HNSi: unfortunately, the dramatic weakness of the dipole moment of the latter species (0.05 D) makes it an unlikely candidate for today's radiotelescopes. At variance with HNSi, the high dipole moment value of HSiN (4.5 D) would make it a much more attractive candidate for astrophysical searches, but under interstellar conditions, we show that it can derive neither from the unimolecular HNSi ↔ HSiN equilibration nor from the Si+ + NH3, N + SiH3+ or N+ + SiH3 reactions as sometimes incorrectly stated in the astrophysical models that deduce interstellar silicon chemistry from that of carbon. Throughout this study, the very hazardous character of conclusions deduced from isoelectronic considerations should be considered as the leading feature: the finishing stroke to such isoelectronic analogies is given by our study of the H+ + HNSi ↔ HSiN + H+ reactions which leads to the conclusion that HSiN might be unlikely to survive interstellar hydrogenation processes