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A consensus has progressively developed that the path towards complex organic molecules could be favoured by the icy mantles of interstellar dust particles. However, the question of whether activation energy is required to promote those reactions is open. This work deals with the simplest process thought to increase complexity i.e. successive additions of a single atom (H). Two situations can be considered, the direct additions of H in the gas phase and the atomic hydrogenation processes on water ice surfaces available in the interstellar medium (ISM). The synthesis of methanol, largely discussed in the literature is revisited as a case study. Computational investigations of the reactions leading from CO to CH3OH show that, with [H...(H2O)n] complexes as hydrogenation vectors, all steps of the process are barrier-less, contrary to the gas phase process and this, whatever the level of theory considered. The key parameter is the position of the CO + [H...(H2O)n] initial system on the global energy scale. At the present level of theory, when H is pre-adsorbed on the ice, the system happens to be high enough above the reaction path, namely, above any intermediates and possible transition states. This is true whatever the dimension of the ice support, even for the simplest model of one H2O molecule. Application of such a simplified synthetic approach, here validated for the synthesis of methanol, could be generalized, providing a simple way to get a fair insight into the important class of atomic hydrogenations on ices in the ISM. [ABSTRACT FROM AUTHOR]

The deficiencies of argon, krypton, and xenon observed in the atmosphere of Titan as well as anticipated in some comets might be related to a scenario of sequestration by H3+ in the gas phase at the early evolution of the solar nebula. The chemical process implied is a radiative association, evaluated as rather efficient in the case of H3+ , especially for krypton and xenon. This mechanism of chemical trapping might not be limited to H3+ only, considering that the protonated ions produced in the destruction of H3+ by its main competitors present in the primitive nebula, i.e., H2O, CO, and N2, might also give stable complexes with the noble gases. However the effective efficiency of such processes is still to be proven. Here, the reactivity of the noble gases Ar, Kr, and Xe, with all protonated ions issued from H2O, CO, and N2, expected to be present in the nebula with reasonably high abundances, has been studied with quantum simulation method dynamics included. All of them give stable complexes and the rate coefficients of their radiative associations range from 10-16 to 10-19 cm³ s-1, which is reasonable for such reactions and has to be compared to the rates of 10-16 to 10-18 cm³ s-1, obtained with H3+. We can consider this process as universal for all protonated ions which, if present in the primitive nebula as astrophysical models predict, should act as sequestration agents for all three noble gases with increasing efficiency from Ar to Xe. [ABSTRACT FROM AUTHOR]

The detection of ArH+ has revived the interest in the search for noble gas containing species. Despite helium being the second most abundant element in the universe (He/H ~ 1/10), it has never been observed in any other form than that of a neutral/ionized atom in the interstellar medium. Because He is the "most noble" gas, its non-observation as part of neutral molecular systems is understandable. It is more surprising for charged species, especially HeH+ whose spectral signatures are well documented in the laboratory. The purpose of this work was to find a simple positive ion containing He, and likely to be observed as an alternative to undetected HeH+. Among the HeX2+ diatomics formed with first row atoms, we focused on X = C because of both its relative abundance and the magnitude of its ionization potentials with respect to He. The formation of CHe2+ by radiative association is the center of this study. The question was addressed by means of numerical simulations using high level ab initio calculations of the CHe2+ potential surface, followed by a quantum chemical determination of the rate coefficients for the corresponding radiative association in the range of 10 to 1000K. The radiative association path shows a potential well deep enough to accommodate 20 vibrational levels, and no barrier to oppose the reaction. The rate coefficient varies from ~ 4.5 × 10-20 cm3s-1 to ~ 2.5 × 10-22 cm3s-1 for the temperatures considered. The present study suggests that the existence of this species has to be searched for mainly in highly irradiated regions. [ABSTRACT FROM AUTHOR]

The recent publication that some bacteria could use arsenic instead of phosphorus for building their DNA triggered a large controversy in the astro/exobiology community. Most comments claim that such a substitution is not possible. Here, we address the same question of the presence of As in DNA from a pure theoretical point of view, beyond any biological consideration. By means of “First principle“ quantum calculations we found that there is no energetical or structural argument to reject the As to P substitution in the DNA helix. However, a topological analysis of the electron density shows that As-DNA is much more fragile and most probably will not survive because it lacks the covalent bonds that insure the stability of biological P-DNA.

Life on Earth, as we know it to day, is inseparable from homochirality. The presence of an enantiomeric excess in carbonaceous chondrites is well documented; among the different hypotheses considered to explain this undisputed fact, we chose to investigate the possibilities open by a selective adsorption on a chiral surface that would engage a process of enantiomeric enrichment. The chiral surface is that of the α-quartz { 10 10¯ } $\left\{ {10\,\overline {10} } \right\}$ and the chiral molécule is lactic acid, HOCH(CH3)COOH. In this theoretical work we rely on numerical simulations based on Density Functional Theory (DFT) that proved efficient in the super-molecule and periodic approaches to this category of phenomena. In view of the different types of adsorption sites, a statistical treatment was necessary; it shows that (S)-lactic acid is preferentially adsorbed with a selectivity of ~0.7 kcal/mol with respect to the (R) isomer.

Recent studies on the formation of XH3+ noble gas complexes have shown strategic implications for the composition of the atmospheres of the giant planets as well as for the composition of comets. One crucial factor in the astrophysical process is the relative abundances of the noble gases versus H3+. It is the context in which the possibility for clustering with more than one noble gas (XnH3+ up to n=3) has been investigated for noble gases X ranging from neon to krypton. In order to assert our results, a variety of methods have been used including ab initio coupled cluster CCSD and CCSD(T), MP2, and density functional BH&HLYP levels of theory. All complexes with one, two, and three noble gases are found to be stable in the Ne, Ar, and Kr families. These stable structures are planar with the noble gases attached to the apices of the H3+ triangle. The binding energy of the nth atom, defined as the XnH3+→Xn-1H3++X reaction energy, increases slightly with n varying from 1 to 3 in the neon series, while it decreases in the argon series and shows a minimum for n=2 in the krypton series. The origin of this phenomenon is to be found in the variations in the respective vibrational energies. A topological analysis of the electron localization function shows the importance of the charge transfer from the noble gases toward H3+ as a driving force in the bonding along the series. It is also consistent with the increase in the atomic polarizabilities from neon to krypton. Rotational constants and harmonic frequencies are reported in order to provide a body of data to be used for the detection in laboratory prior to space observations. This study strongly suggests that the noble gases could be sequestered even in an environment where the H3+ abundance is small. [ABSTRACT FROM AUTHOR]

The affinity of H3+ to combine with noble gases X has been investigated from neon to xenon using ab initio coupled cluster [CCSD and CCSD(T)] and density functional BH&HLYP levels of theory. For all noble gases, the stable structures belong to a C2v symmetry with an apex of the H3+ triangle pointing to the noble gas. The structure of the complexes changes gradually from a practically pure Ne–H3+ arrangement to a situation close to XeH+–H2. A topological analysis of the electron localization function is used to illustrate the changes in the bonding along the series. The lowest dissociation energies of NeH3+ and ArH3+ (∼1 and ∼7 kcal/mol) correspond to the breaking of the complexes according to X+H3+, while the lowest dissociation energies of KrH3+ and XeH3+ (∼8 and ∼3 kcal/mol) correspond to the breaking according to XH++H2. Rotational constants and harmonic frequencies are reported. Apart from XeH3+ whose dipole moment (μ=2.6 D) may not be large enough, all the other complexes with dipole moments in the range of 6–8 D should be reasonable targets for detection by microwave spectroscopy. The present calculations are intended to stimulate both laboratory experiments and spatial observations since the possible sequestration of noble gases by H3+ may have strong implications on the composition of astrophysical objects. [ABSTRACT FROM AUTHOR]

Examines the use of density functional theory with an approximate kinetic energy functional to determine the structure and stability of weak van der Waals complexes. Relation of the functionals with the approximate exchange energy functional; Definition of Kohn-Sham equations with constrained electron density (KSCED) formalism; Implementation of KSCED.

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 eV. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The structure of the ground and lowest two excited states of H2NO have been determined in large scale configuration interaction calculations using a multiconfiguration self-consistent description of the molecular orbitals. These treatments are based on a systematic building of the correlation contribution which has been designed to account for the characteristics of the nitroxide group. This approach shows that the aminoxyl functional group is more than a three electron group shared by two atoms, but, in fact, a nine electron entity. Our best estimate of the geometry of the ground electronic state, obtained after second-order configuration interaction using a large basis of atomic natural orbitals, is pyramidal. However, since the potential depth between 0° and 40° is lower or of the same order of magnitude as the estimated inversion frequency, the conclusion that this molecule behaves like a planar system is totally justified. The structure of the excited (n-π*) and (π-π*) states have been determined and the transitions energies are in accordance with the experimental results on the highly substituted stable nitroxide radicals. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]