The significant role of water in reactions occurring on the surface of interstellar ice grains: Hydrogenation of pure ketene H2CCO ice versus hydrogenation of mixed H2CCO/H2O ice at 10 K

Water ice plays an important role in reactions taking place on the surface of interstellar ice grains, ranging from catalytic effects that reduce reaction barrier heights to effects that stabilize the reaction products and intermediates formed, or that favor one reaction pathway over another, passing through water-involvement in the reaction to produce more complex molecules that cannot be formed without water or water-derived fragments H, O and OH. In this context, we have combined experimental and theoretical studies to investigate ketene (CH2CO) + H solid-state reaction at 10 K in the presence and absence of water molecules under interstellar conditions, through H-bombardment of CH2CO and CH2CO/H2O ices. We show in the present study that with or without water, the ketene molecule reacts with H atoms to form four reaction products, namely CO, H2CO, CH4 and CH3CHO. Based on the amounts of CH2CO consumed during the hydrogenation processes, the CH2CO + 2H reaction appears to be more efficient in the presence of water. This underlines the catalytic role of water ice in reactions occurring on the surface of interstellar ice grains. However, if we refer to the yields of reaction products formed during the hydrogenation of CH2CO and CH2CO/H2O ices, we find that water molecules favor the reaction pathway to form CH3CHO and deactivate that leading to CH4 and H2CO. These experimental results are in good agreements with the theoretical predictions that highlight the catalytic effect of H2O on the CH2CO + H reaction, whose potential energy barrier drops from 4.6 kcal mol-1 (without water) to 3.8 and 3.6 kcal mol-1 with one and two water molecules respectively.

Conformational preference analysis in C2H6 using orbital forces and non-covalent interactions; comparison with related systems

Dynamic Orbital Forces (DOF) and Non-Covalent Interactions (NCIs) are used to analyze the attractive/repulsive interactions responsible of the conformational preference of ethane and some related compounds. In ethane, it is found that the stabilization of the staggered conformation with respect to the adiabatic eclipsed one arises from both attractive and repulsive interactions in CH₃⋯CH₃. Attractive ones are predominant in a ratio 2 : 1, with an important role of a σ MO. On the contrary, the stabilization of the staggered conformation with respect to the vertical eclipsed one arises almost only from repulsive π interactions. Weak long-range H⋯H repulsions also favour the staggered conformation. From the sum of DOFs, yielding intrinsic bond energies, the rotation barrier can be decomposed into a weakening of the C-C bond (ca. 7 kcal mol^-1), moderated by a strengthening of C-H ones (ca. 4 kcal mol^-1). This evidences the decrease of hyperconjugation in the eclipsed conformation with respect to the staggered one. In the compounds CH₃-SiH₃, SiH₃-SiH₃, CH₃-CF₃ and CF₃-CF₃, the conformational preference is predominantly or exclusively due to repulsive interactions, with respect as well to adiabatic as to vertical eclipsed structures.

Formation of CO, CH4, H2CO and CH3CHO through the H2CCO + H surface reaction under interstellar conditions

The reaction of ketene (H₂CCO) with hydrogen atoms has been studied under interstellar conditions through two different experimental methods, occurring on the surface and in the bulk of H₂CCO ice. We show that ketene interaction with H-atoms at 10 K leads mainly to four reaction products, carbon monoxide (CO), methane (CH₄), formaldehyde (H₂CO) and acetaldehyde (CH₃CHO). A part of these results shows a chemical link between a simple organic molecule such as H₂CCO and a complex one such as CH₃CHO, through H-addition reactions taking place in dense molecular clouds. The H-addition processes are very often proposed by astrophysical models as mechanisms for the formation of complex organic molecules based on the abundance of species already detected in the interstellar medium. However, the present study shows that the hydrogenation of ketene under non-energetic conditions may also lead efficiently to fragmentation processes and the formation of small species such as CO, CH₄ and H₂CO, without supplying external energy such as UV photons or high energy particles. Such fragmentation pathways should be included in the astrophysical modeling of H₂CCO + H in the molecular clouds of the interstellar medium. To support these results, theoretical calculations have explicitly showed that, under our experimental conditions, H-atom interactions with the CC bond of ketene lead mainly to CH₃CHO, CH₄ and CO. By investigating the formation and reactivity of the reaction intermediate H₃C-CO radical, our calculations demonstrate that the H₃C-CO + H reaction evolves through two barrierless pathways to form either CH₃CHO or CH₄ and CO fragments.

Orbital energies and nuclear forces in DFT: Interpretation and validation

The bonding and antibonding character of individual molecular orbitals has been previously shown to be related to their orbital energy derivatives with respect to nuclear coordinates, known as dynamical orbital forces. Albeit usually derived from Koopmans' theorem, in this work we show a more general derivation from conceptual DFT, which justifies application in a broader context. The consistency of the approach is validated numerically for valence orbitals in Kohn-Sham DFT. Then, we illustrate its usefulness by showcasing applications in aromatic and antiaromatic systems and in excited state chemistry. Overall, dynamical orbital forces can be used to interpret the results of routine ab initio calculations, be it wavefunction or density based, in terms of forces and occupations.

The "Inverted Bonds" Revisited: Analysis of "In Silico" Models and of [1.1.1]Propellane by Using Orbital Forces

This article dwells on the nature of "inverted bonds", which refer to the σ interaction between two sp hybrids by their smaller lobes, and their presence in [1.1.1]propellane. Firstly, we study H₃ C-C models of C-C bonds with frozen H-C-C angles reproducing the constraints of various degrees of "inversion". Secondly, the molecular orbital (MO) properties of [1.1.1]propellane and [1.1.1]bicyclopentane are analyzed with the help of orbital forces as a criterion of bonding/antibonding character and as a basis to evaluate bond energies. Triplet and cationic states of [1.1.1]propellane species are also considered to confirm the bonding/antibonding character of MOs in the parent molecule. These approaches show an essentially non-bonding character of the σ central C-C interaction in propellane. Within the MO theory, this bonding is thus only due to π-type MOs (also called "banana" MOs or "bridge" MOs) and its total energy is evaluated to approximately 50 kcal mol^-1 . In bicyclopentane, despite a strong σ-type repulsion, a weak bonding (15-20 kcal mol^-1 ) exists between both central C-C bonds, also due to π-type interactions, though no bond is present in the Lewis structure. Overall, the so-called "inverted" bond, as resulting from a σ overlap of the two sp hybrids by their smaller lobes, appears highly questionable.

Analysis of Reaction Processes On the Basis of the Evolution of Dynamic Orbital Forces: Examples of Cycloadditions, SN 2 Substitution, Nucleophilic Addition, and Hydrogen Transposition

The derivative of the energy of a canonical molecular orbital (MO) [or dynamical orbital forces (DOFs)] with respect to a bond length provides a reliable index of the bonding/antibonding character of this MO on this bond. The DOFs of selected MOs as a function of the reaction coordinate were computed for a panel of model reaction mechanisms: [2+4] (Diels-Alder) cycloaddition, [2+2] cycloaddition, second-order nucleophilic substitution (S_N 2), nucleophilic addition to a carbonyl group, and [1,2] hydrogen transposition. The results highlight the nature of the reorganization of the main MOs and the stage of the reaction coordinate (RC) at which it occurs. For instance, in the Diels-Alder reaction, one can identify a part of the reaction that is dominated by repulsive four-electron interactions and another part dominated by attractive two-electron interactions. Also, the shape of the DOF as a function of the reaction coordinate reveals the existence of avoided MO crossings and their location on the RC. Even for spontaneous reactions with monotonic variation in the potential energy, extrema of the MO energy and sudden electron rearrangements can be put into evidence. This study provides quantitative support to classical MO analyses of reactivity such as correlation diagrams and frontier approximation.

Communication: The formation of CHe(2+) by radiative association

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 HeX(2+) 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 CHe(2+) 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 CHe(2+) 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) cm(3)s(-1) to ∼2.5 × 10(-22) cm(3)s(-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.

Organohelium compounds: structures, stabilities and chemical bonding analyses

This paper deals with the possibility of forming short and relatively strong carbon-helium bonds in small typical organic molecules through substitution of one or several H atoms by He(+). A structural and energetics study (based on high-level calculations) of this unusual bonding, as well as a topological characterization of the resulting cations, is undertaken. Stable species generally requires substitution of about half of the hydrogen atoms for formation. Under these conditions, the number of such species appears to be potentially unlimited. "True" C-He bonds exhibit equilibrium distances ranging from 1.327 (C2H2He2(2+)) to 1.129 Å (He2CO(2+)). The energies of neutral He releasing range from approximately 5 kcal mol(-1) [He2CO(2+), (Z)-C2H2He2(2+)] to 25 kcal mol(-1) (C2HHe3(3+)), but remain most frequently around 10 kcal mol(-1). However, most of He(+)-substituted hydrocarbons are metastable with respect to C-C cleavage, except derivatives of ethene. Atoms in molecules (AIM) and electron localization function (ELF) topological descriptors classify the C-He bond as a weak charge-shift interaction [S. Shaik, D. Danovich, B. Silvi, D. L. Lauvergnat, P. C. Hiberty, Chem. Eur. J. 2005, 11, 6358-6371] in agreement with a recent publication by Rzepa [S. H. Rzepa, Nat. Chem. 2010, 2, 390-393]. He2CO(2+) is the only investigated compound that presents a C-He bonding ELF basin, which indicates a non-negligible covalent contribution to the bond. Other modifications in the electronic structure, such as the breaking of the triple bond in ethyne derivatives or the loss of aromaticity in C6H3He3(3+), are also nicely revealed by the ELF topology.

FTIR and ab Initio Study of the 1/1 Complex between Water and Carbon Dioxide in Solid Nitrogen

Infrared spectra of the 1/1 complex between water and carbon dioxide isolated in a nitrogen matrix are reported. Isotopic substitution of water (HOD, D2O) suggests the existence of two very weak complexes with a nearly T-shaped structure where the C atom is bonded to the water oxygen: one of C2v symmetry and another one of Cs symmetry, in which a slight distortion results in a very weak interaction between one hydrogen atom of water and one oxygen atom of carbon dioxide. This result is supported by force constant calculations, consistent with a double, symmetrical, and unsymmetrical force field, and ab initio calculations. At the MP2/aug-cc-PVTZ level corrected from basis set superposition error the C2v structure is a saddle point with an imaginary frequency for H2O rocking, in contrast to a Cs structure, which would have nonequivalent hydrogen atoms. Nevertheless, the energies of both structures, with a stabilization energy of 1.97 kcal mol-1, differ by less than 0.01 kcal mol-1 from each other. A comparison of calculated and observed frequency shifts allows us to suggest the presence in the matrix of both C2v and Cs complexes.

Electron pair localization function: A practical tool to visualize electron localization in molecules from quantum Monte Carlo data

In this work we introduce an electron localization function describing the pairing of electrons in a molecular system. This function, called "electron pair localization function," is constructed to be particularly simple to evaluate within a quantum Monte Carlo framework. Two major advantages of this function are the following: (i) the simplicity and generality of its definition; and (ii) the possibility of calculating it with quantum Monte Carlo at various levels of accuracy (Hartree-Fock, multiconfigurational wave functions, valence bond, density functional theory, variational Monte Carlo with explicitly correlated trial wave functions, fixed-node diffusion Monte Carlo, etc). A number of applications of the electron pair localization function to simple atomic and molecular systems are presented and systematic comparisons with the more standard electron localization function of Becke and Edgecombe are done. Results illustrate that the electron pair localization function is a simple and practical tool for visualizing electronic localization in molecular systems.

IR and UV spectroscopic data for polyynes: predictions for long carbon chain compounds in Titan's atmosphere

A better understanding of the complex organic chemistry occurring in the methane rich atmosphere of Titan can be achieved via the comparison of observations with results obtained by theoretical models. Available observations are still few but their analysis requires the knowledge of a large set of data, namely frequencies and absolute band intensities. Cross sections are also needed to develop the chemical schemes of photochemical models, in particular the schemes leading to the formation of haze particles visible on Titan. Unfortunately, some of these parameters are not well known, especially if one takes into account the extreme physical conditions of the studied object. This lack of data is particularly enhanced for polyynes because these compounds are highly unstable at the usual pressure and temperature conditions of a laboratory and therefore are very difficult to study. We have developed UV and IR studies, coupling experimental and theoretical approaches, in order to extrapolate the parameters available for short polyynes to longer carbon chains. In the mid-UV range, when the length of the chain increases, the absorption system of polyynes is shifted to longer wavelength and its oscillator strength increases linearly. In the IR range, with the increase of the number of carbon bonds, the positions of the CCC and CCH bending modes shift to lower energy, the latest converging rapidly to a fixed value of 620.5 cm-1 for an infinite length polyyne. Implications for detection and evolution of polyynes in Titan's atmosphere are emphasised.

Bromation régiosélective en série aromatique. II. Approche théorique du mécanisme de la substitution électrophile par l'ion tribromure Br3−

A theoretical study (SCF PS-HONDO + CI) of the stability of the tribromide anion Br3− has been carried out. On the basis of qualitative MO arguments and MNDO calculations, we propose a first approach to the mechanism of electrophilic aromatic substitution by Br3−. It involves an axial attack of linear Br3−, the most electrophilic sites of which are terminal bromine atoms, although they bear a negative charge. This model suggests some arguments to explain the para versus ortho selectivity experimentally observed. Calculations performed on a series of MBr3 systems emphasize the importance of the countercation for the rate of the reaction as well as for its selectivity. Keywords: bromination, tribromide, mechanism, parabromophenol.

Bromation régiosélective en série aromatique. I: Monobromation en position para de phénols et d'aminés aromatiques par le tribromure de tétrabutylammonium

The reaction of tetrabutylammonium tribromide (TBABr3) with phenols and aromatic amines in aprotic and non-basic solvents at 20 °C gives exclusively the corresponding para-brominated compounds in high yields. A mechanism involving electrophilic substitution by the tribromide anion Br3− itself is suggested to account for the results, especially the regioselective para bromination. Keywords: bromination, tetrabutylammonium tribromide, phenols, aromatic amines.