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

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. III. The spectral signatures of the H2SiN+ molecular ion

The recent detection of SiN in the outer envelope of the IRC+10216 carbon star has renewed the interest for the gas phase interstellar silicon chemistry. In this contribution, we present a theoretical study of the H2SiN+ molecular ion, the silicon hydrogenated counterpart of the previously studied SiNH + 2. On many points, the differences relative to the SiNH + 2 isomer have been found to be dramatic. As an example, the dipole moment is computed to be 3.8 D while being only 0.5 D in SiNH + 2. The radio, infrared and electronic signatures have been evaluated at a quantitative level. The rotational constants and vibrational frequencies have been determined using Möller–Plesset MPn (n=2,3,4), coupled cluster (CCSDT) and complete active space self-consistent field (CASSCF) methods for H2SiN+ and some of its isotopomers. These quantities have been corrected using a scaling procedure derived from previous studies on the HNSi, HSiN, HSiNH2, H2SiNH, and SiNH + 2 species in order to provide quantitative results. The failure of single-reference perturbation theories to predict a relevant infrared spectrum is discussed. Intense bands around 550, 950, and 2300 cm–1 are predicted. The electronic spectrum has been obtained using a coupled multiconfiguration SCF–perturbation treatment (MC/P): It is characterized by a large number of excited states, none of them having a strong transition moment. The lowest excited state is predicted to lie 0.54 eV above the ground state, but the first allowed transition having a nonnegligible oscillator strength has to be searched at 6.44 e