Patrick Chaquin

Polyoxometallates as models for oxide catalysts: Part I. An UV-visible reflectance study of polyoxomolybdates: Influence of polyhedra arrangement on the electronic transitions and comparison with supported molybdenum catalysts

Abstract Diffuse reflectance spectroscopy is a technique commonly used to determine the symmetry and the environment of transition metal ions in supported oxide catalysts by comparison with reference samples either in solid or liquid state. However, the examination of model polyoxomolybdates with well-established structures such as Na2MoO4 · 2H2O, TBA2Mo2O7 (TBA = tetrabutylammonium), α-TBA4Mo8O26, α- and β-TBA4SiMo12O40, TBA2Mo6O19, H8As4Mo12O50, α-TBA6P2Mo18O62, etc., indicates that earlier attributions of the UV-visible bands of supported Mo catalysts may be reconsidered. It is shown that Mo dispersion (Mo cluster size and distance between clusters) or Mo-support interactions have more influence than local Mo symmetry. Indeed, analysis of experimental spectra of polyoxomolybdates indicates that: • —in contrast to an earlier belief, the effect of the local symmetry on the lowest energy transition band of ligand-metal charge transfer is negligible compared to the effect of the more distant environment of the metal. • —a broadening and a red shift of this band are observed when cluster size increases, or when the polarizing effect increases and/or the size of the counter-cation decreases. On the contrary, a narrowing and a blue shift are observed when a polyoxomolybdate is supported, i.e., when its dispersion increases. • —the increasing effects of the different parameters may be classified as follows: local symmetry overall symmetry condensation degree (polyanion size) polarizing effect of the counter-cation size of the counter-cation dispersion on a support • —UV-visible spectra do not distinguish the effect of strong interaction between Mo (via oxobonding MoOMo) exhibited by the influence of polyanion size from weaker effects such as polyanion interaction (for example, via hydrogen bonding), occurring when the counter-cation is small enough, or from electrostatic effects due to polarization by the counter-cation.

Polyoxometallates as models for oxide catalysts: Part II. Theoretical semi-empirical approach to the influence of the inner and outer Mo coordination spheres on the electronic levels of polyoxomolybdates

Abstract In complement to Part I, theoretical MO calculations (EHMO level) show that the main factor which influences the electronic energy levels of polyoxomolybdates is not always the local Mo symmetry (tetrahedral or octahedral). The effect of protonation and/or grafting on a surface oxide is weak but slightly larger for tetrahedral than for octahedral species and could lead to wrong attributions. The condensation of several sites in polyoxomolybdates has a greater effect since it involves a MO delocalization over the whole compound, and therefore a loss of the individuality of each octahedral or tetrahedral unit. It leads to a decrease in the HOMO-LUMO gap yielding to an absorption red shift. The effect of distortion of local symmetry by bond elongation tends to decrease the HOMO-LUMO gap in an isolated species. In a condensed system, this distortion lowers the MoMo interactions and moderates the red shift due to the effect of condensation. This latter effect, however, remains predominant.

Electron localization function comparative study of ground state, triplet state, radical anion, and cation in model carbonyl and imine compounds

The modifications of bonding in carbonyl and imine compounds upon excitation, electron attachment, and ionization were studied within the framework of the electron localization function (ELF). The topological analysis of ELF allows a partition of the molecular space into regions having a clear chemical meaning: the basins. The electronic populations of the basins associated with bonding and nonbonding character, as well as the basin spin densities, were calculated at the MP2 level of the quantum mechanical calculation. Good agreement was found with the classical view in terms of mesomeric structures corresponding to the dominant localization of electrons contained in frontier molecular orbitals (MOs). The variations of the basins population were compared to the predictions of MO theory. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 897–910, 1999

Structure and properties of X2O and XYO (X, Y Cl, Br) molecules: DFT vs. ‘classical’ ab initio calculations

Abstract Equilibrium structures, energies and vibrational frequencies of Cl2O, Br2O and ClOBr isomers have been calculated using five selected DFT techniques. Comparison with available experimental data shows that S-VWN (LSD Slater exchange and LSD Vosko, Wilk and Nusaies correlation functional) and, to a lesser extent, B3-LYP (Beckes three-parameter functional with the correlation functional of Lee, Yang and Parr) yield the most accurate results. These DFT techniques have been compared to recent CCSD(T) calculations by Lee, and, in the case of Cl2O isomers, to a variety of ‘classical’ ab initio methods. Regarding the equilibrium geometries and vibrational frequencies, they give results (especially in the hypervalent XYO series) comparable to CCSD(T). Regarding the energetic data, the DFT results exhibit important discrepancies and should be considered with some caution.

Semi-empirical calculation of electronic absorption wavelengths of polyynes, monocyano- and dicyanopolyynes. Predictions for long chain compounds and carbon allotrope carbyne

Abstract Absorption wavelengths and oscillator strengths have been calculated using the ZINDO method for the allowed low-energy electronic transition 1 Σ + ← 1 Σ + of monocyanopolyynes ( HC y N , y=1–13 ), 1 Σ u + ← 1 Σ g + of polyynes and dicyanopolyynes (HCyH and NC y N , y=1–40 ). For y>18, the geometries were extrapolated from DFT optimization of the shortest members. Extrapolation formulas have been drawn up for longer chains, the asymptotic values of those yield an estimation of the absorption wavelength (ca. 400 nm) of the hypothetical carbon allotrope carbyne.

Structure and selective visible photodissociation of the O3:Br2 and O3:BrCl complexes: An infrared matrix isolation and ab initio study

Abstract Complexes of ozone with chlorine, bromine and bromine chloride were reinvestigated in argon matrix by Fourier Transform Infrared spectroscopy. All three ozone submolecule fundamentals were observed and ν 1 was enhanced in intensity relative to ν 3 . The asymmetric mixed isotopic ozone complexes exhibit a split ν 3 band which indicates unequivalent terminal oxygens. Photolysis of the O 3 /BrCl system which was studied with the 870, 633 and 532 nm laser lines gives only ClBrO as a primary product indicating that the most stable complex between BrCl and O 3 occurs through bonding of ozone with Br. Results show that at 633 nm, ClBrO transforms it into BrClO whereas at 870 nm, it is isomerized to BrOCl. Ab initio calculations undertaken on the structures of O 3 :Cl 2 and O 3 :BrCl support experimental observations.

MATRIX PHOTOCHEMISTRY OF NITROSYL CHLORIDE. INTERCONVERSION OF CLNO AND CLON SPECIES BY IRRADIATION AND TUNNELING EFFECT

Abstract Natural and isotopic nitrosyl chloride (ClNO) isolated in nitrogen or argon matrices were irradiated with light at 266 nm. Weak absorption lines observed after photolysis were assigned to nitrogen hypochlorite (ClON). Spontaneous conversion of ClON into ClNO was observed and kinetic studies were consistent with a tunneling mechanism. Calculations using DFT and CCSD(T) were made for all isotopic species of each isomer. Energies, structures, harmonic vibrational wavelengths were calculated. Photoisomerization of ClNO into ClON as well as that of FNO into FON was also discussed.

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 Titans atmosphere are emphasised.

Experimental and theoretical studies on the gas/solid/gas transformation cycle in extraterrestrial environments

The ubiquity of molecular material in the universe, from hydrogen to complex organic matter, is the result of intermixed physicochemical processes that have occurred throughout history. In particular, the gas/solid/gas phase transformation cycle plays a key role in chemical evolution of organic matter from the interstellar medium to planetary systems. This paper focuses on two examples that are representative of the diversity of environments where such transformations occur in the Solar System: (1) the photolytic evolution from gaseous to solid material in methane containing planetary atmospheres and (2) the degradation of high molecular weight compounds into gas phase molecules in comets. We are currently developing two programs which couple experimental and theoretical studies. The aim of this research is to provide data necessary to build models in order to better understand (1) the photochemical evolution of Titans atmosphere, through a laboratory program to determine quantitative spectroscopic data on long carbon chain molecules (polyynes) obtained in the SCOOP program (French acronym for Spectroscopy of Organic Compounds Oriented for Planetology), and (2) the extended sources in comets, through a laboratory program of quantitative studies of photochemical and thermal degradation processes on relevant polymers (e.g., Polyoxymethylene) by the SEMAPhOrE Cometaire program (French acronym for Experimental Simulation and Modeling Applied to Organic Chemistry in Cometary Environment).

Reactions of O(1D) oxygen atom with 1,1,1-trichloroethane in solid argon

Abstract Matrices formed by the cocondensation of 1,1,1-trichloroethane and ozone in argon were irradiated at 11 K with the laser line at 266 nm. The CH 3 CCl 3 molecule was not photodecomposed at this wavelength. The product identification was made by Fourier transform infrared spectroscopy. Acetylchloride, 2,2,2-trichloroethanol and 1,1-dichloroethylene were observed in the beginning of the reaction. Trichloroethanol is the main product. It is probably formed by insertion of an O atom in a CH bond to give an excited intermediate which may release its excess energy by relaxation in the matrix or by elimination of HOCl, leading to CH 2 CCl 2 . The formation of small amounts of acetylchloride is probably due to direct oxygen attack on the carbon atom. Photolysis of CH 3 COCl and CH 2 CCl 2 was observed upon prolonged irradation between 800 and 250 nm and produced ketene and chloroacetylene, respectively. Fourier transform infrared spectra of CH 3 COCl and CD 3 COCl isolated in argon matrix were also recorded in this work.