Protonated ions as systemic trapping agents for noble gases: From electronic structure to radiative association.

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.

Arsenic in prebiotic species: a theoretical approach.

A recent controversy about the presence of arsenic in biological systems prompted us to investigate the possible replacement of phosphorus by arsenic in prebiotic species small enough to be potentially identified in space. Systematic computational experiments were carried out on simple systems able to form a peptide or analogous bond. Density Functional Theory (DFT) within the B3LYP formalism, MP2 and CCSD(T) methods were used to determine the most stable isomers that can possibly form from the [C,H,O,As] and [C,3H,O,As] sets of atoms. It was found that HAsCO, like HPCO and HNCO was the most stable isomer. With three hydrogen atoms, the peptide-like bond (AsH(2)-CH=O) is not the most stable structure, contrary to NH(2)-CH=O. It is ∼9 kcal mol(-1) higher than the most stable structure, CH(2)[double bond, length as m-dash]As-OH. To assess the plausibility of the As to P substitution, a comparative study of the dimethylphosphate (DMP) and dimethylarsenate (DMA) anions was then carried out. It was found that the gauche-gauche arrangement that mimics the helix structure is the most stable one in both model molecules, showing that there is no structural evidence to discard the hypothesis of the possible inclusion of As in place of P in the DNA architecture. The topological analysis of the ELF function showed a weakening by 50% of two As-O covalent bonds in all the DMA conformers. It means that if As replaces P, the structure of the DNA helix could be weakened. Rotational constants and IR frequencies of the low-lying isomers are given to encourage laboratory experiments on these prototype molecules.

Gas-phase folding of a two-residue model peptide chain: on the importance of an interplay between experiment and theory.

In order to assess the ability of theory to describe properly the dispersive interactions that are ubiquitous in peptide and protein systems, an isolated short peptide chain has been studied using both gas-phase laser spectroscopy and quantum chemistry. The experimentally observed coexistence of an extended form and a folded form in the supersonic expansion was found to result from comparable Gibbs free energies for the two species under the high-temperature conditions (< or = 320 K) resulting from the laser desorption technique used to vaporize the molecules. These data have been compared to results obtained using a series of quantum chemistry methods, including DFT, DFT-D, and post-Hartree-Fock methods, which give rise to a wide range of relative stabilities predicted for these two forms. The experimental observation was best reproduced by an empirically dispersion-corrected functional (B97-D) and a hybrid functional with a significant Hartree-Fock exchange term (M06-2X). In contrast, the popular post-Hartree-Fock method MP2, which is often used for benchmarking these systems, had to be discarded because of a very large basis-set superposition error. The applicability of the atomic counterpoise correction (ACP) is also discussed. This work also introduces the mandatory theoretical examination of experimental abundances. DeltaH(0 K) predictions are clearly not sufficient for discussion of folding, as the conformation inversion temperature is crucial to the conformation determination and requires taking into account thermodynamical corrections (DeltaG) in order to computationally isolate the most stable conformation.

Preliminary study of the influence of environment conditions on the successive hydrogenations of CO.

The successive hydrogenation of CO has been investigated by two methods. The first is hydrogenation of a CO surface. The second is co-injection of CO molecules and H atoms. Both methods have been performed at 3 and 10 K. In the first method, the interaction of H atoms with solid CO at 10 K shows that CO is consumed to form H(2)CO and CH(3)OH. No trace of species such as HCO and CH(3)O is detected. No product was observed when the same experiment was performed at 3 K. In the second method, when H and CO are codeposited at 10 K, HCO and CH(3)O are observed. In fact, the yield of these intermediate species depends on the amount of the H radicals interacting with CO molecules. At 3 K, the presence of H(2) in the solid screens the hydrogenation reaction. This causes a termination for the reaction in the stage of the formation of HCO and H(2)CO. At 10 K, H(2) cannot condense, and the reaction between CO and H is total. In this case, species such as HCO, H(2)CO, CH(3)O, and CH(3)OH are observed.

H3(+) as a trap for noble gases-3: multiple trapping of neon, argon, and krypton in X(n)H3(+) (n = 1-3).

Recent studies on the formation of XH(3)(+) 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 H(3)(+). It is the context in which the possibility for clustering with more than one noble gas (X(n)H(3)(+) 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 H(3)(+) triangle. The binding energy of the nth atom, defined as the X(n)H(3)(+) --> X(n-1)H(3)(+) + 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 H(3)(+) 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 H(3)(+) abundance is small.

Looking for homochirality in the inter-stellar medium.

In this report we address the question of whether some chiral molecules have a probability of being detected in the interstellar medium (ISM). To this end we rely on the Minimum Energy Principle which states that the most abundant isomer of a given generic formula should be that of lowest energy. The relative stability of the chiral molecules with respect to the other possible species of the same chemical formula are calculated by means of quantum simulations based on density functional theory (DFT). The result is that no chiral isomer in the C(3)H(6)O (acetone), C(2)H(5)ON, C(3)H(7)ON (amide), C(2)H(5)O(2)N, C(3)H(7)O(2)N (amino acid) families is the most stable species. This is also true of the C(2)(H(2)O)(2) and C(3)(H(2)O)(3) species when restricted to the sugar families, but another chiral molecule of the same chemical formula, i.e. lactic acid HOCH(CH(3))COOH is the most stable of all structures. Two other molecules with an NH(2) group, namely, NH(2)CH(CH(3))CN, the precursor of alpha-alanine and NH(2)CH(CH(3))OH, the simplest chiral molecule, are also the most stable species in their respective families. These three molecules satisfy the conditions for being detected according to the Minimum Energy Principle. With dipoles moments of 2.3, 2.7 and 1.6 Debye respectively, they make appealing targets. The present study should encourage laboratory experiments to determine rotational constants of higher precision prior to submission of observation proposals.

Understanding selectivity of hard and soft metal cations within biological systems using the subvalence concept. I. Application to blood coagulation: direct cation-protein electronic effects vs. indirect interactions through water networks.

Following a previous study by de Courcy et al. ((2009) Interdiscip. Sci. Comput. Life Sci. 1, 55-60), we demonstrate in this contribution, using quantum chemistry, that metal cations exhibit a specific topological signature in the electron localization of their density interacting with ligands according to its "soft" or "hard" character. Introducing the concept of metal cation subvalence, we show that a metal cation can split its outer-shell density (the so-called subvalent domains or basins) according to it capability to form a partly covalent bond involving charge transfer. Such behaviour is investigated by means of several quantum chemical interpretative methods encompasing the topological analysis of the Electron Localization Function (ELF) and Bader's Quantum Theory of Atoms in Molecules (QTAIM) and two energy decomposition analyses (EDA), namely the Restricted Variational Space (RVS) and Constrained Space Orbital Variations (CSOV) approaches. Further rationalization is performed by computing ELF and QTAIM local properties such as electrostatic distributed moments and local chemical descriptors such as condensed Fukui Functions and dual descriptors. These reactivity indexes are computed within the ELF topological analysis in addition to QTAIM offering access to non atomic reactivity local index, for example on lone pairs. We apply this "subvalence" concept to study the cation selectivity in enzymes involved in blood coagulation (GLA domains of three coagulation factors). We show that the calcium ions are clearly able to form partially covalent charge transfer networks between the subdomain of the metal ion and the carboxylate oxygen lone pairs whereas magnesium does not have such ability. Our analysis also explains the different role of two groups (high affinity and low affinity cation binding sites) present in GLA domains. If the presence of Ca(II) is mandatory in the central "high affinity" region to conserve a proper folding and a charge transfer network, external sites are better stabilised by Mg(II), rather than Ca(II), in agreement with experiment. The central role of discrete water molecules is also discussed in order to understand the stabilities of the observed X-rays structures of the Gla domain. Indeed, the presence of explicit water molecules generating indirect cation-protein interactions through water networks is shown to be able to reverse the observed electronic selectivity occuring when cations directly interact with the Gla domain without the need of water.

Electronic structure of simple phosphorus containing molecules [C,xH,O,P] candidate for astrobiology (x=1, 3, 5).

The present report is a prospective study aimed at finding phosphorus containing compounds for astrobiology. Since PN, PC and HCP are the only species detected so far, it was deemed reasonable to enlarge the quest for phosphorus compounds to mixed carbon oxygen containing compounds [C,xH,O,P] analogue to the CHON family. Ab initio Møller-Plesset (MP2), Coupled Cluster (CCSD(T)) and Density Functional Theory (DFT) were used. State of the art level of theory, CCSD(T)/cc-pVQZ, was necessary to show that CH3-PH2=O is the most stable isomer, with CH3-PH-OH close by in the [C,5H,O,P] sub-family. This structure has the same C-P-O connectivity as the most stable compound of the [C,3H,O,P] sub-family, CH3-P=O but differs from the simplest [C,H,O,P] system HP=C=O. Rotational constants B=7.1377 and C=6.0636 GHz associated with a dipole moment of 4.2 Debye together with an IR spectrum with very strong bands at 1214, 2282, 2264 and 1039 cm(-1) have been calculated for CH3-PH2=O. For CH3-P=O, one has B=7.9881 and C=6.4659 GHz, a dipole moment of 2.9 Debye and four IR bands at 1198, 623, 835, 1256 cm(-1) of medium intensity. The simplest HPCO system with B=5.5206 and 5.3952 GHz and a dipole moment of 0.8 Debye has only one very strong IR frequency at 2037 cm(-1). The above values should be precise enough to encourage laboratory experiments on these prototype molecules.