André Burneau

Infrared and Raman spectroscopic study of carboxylic acids in heavy water

Infrared and Raman spectra of CH3COOD and CD3COOD were obtained below 1900 cm−1, from dilute solutions in heavy water up to neat liquids. The relation of the intermolecular interactions with the acid concentration is discussed. In the debate concerning the structure of pure liquid acetic acid at room temperature, we favour the assignment of the major species to the centrosymmetric dimer. This is based not only on the literature data for crystal, liquid and vapour acetic acid, but also on the infrared and Raman spectra of liquid acetic and propanoic acids and their 0.1 M solutions in water. On increasing the acid concentration from dilute solutions, the hydrated monomer is progressively replaced by more or less hydrated linear dimer and then by the cyclic dimer. At a molar fraction of 0.5, the three kinds of species coexist. At still higher concentrations, longer oligomers appear while the cyclic dimer dominates the other species. A comprehensive assignment of the infrared and Raman spectra is given on the basis of a previously published ab initio calculation for monomer, cyclic and linear dimers. A narrow well resolved satellite band is observed for the hydrated monomer some 45 cm−1 above the CC bond stretching vibration (νCC), not only for CH3COOD and CD3COOD but also for C2H5COOD and CH2ClCOOD, specifically in dilute heavy water solutions. It is not easily assigned to overtones or combination bands simultaneously for the three molecules. Both a blue-shift and a narrowing of the νCC band are usually observed by ionisation of a carboxylic acid into a carboxylate ion in water. The hypothesis of a contact ion pair {R–COO−, D3O+}, whose protonated equivalent species would not exist in normal water, is thus discussed.

Ab initio study of the vibrational properties of acetic acid monomers and dimers

The vibrational spectra of CH3COOH, CH3COOD, CD3COOH and CD3COOD, either monomeric or in two dimeric structures (centrosymmetric or side-on linear, with two bent hydrogen bonds), were studied by ab initio calculations at the DFT/B3LYP/6-31++G** level. The force field, the potential energy distribution and the derivatives of the electric dipole moment with respect to internal coordinates were analysed in addition to the vibration wavenumbers. A comprehensive discussion of the infrared intensities between 1150 and 1500 cm−1 is made for the twelve structures studied. The calculated infrared intensities of monomers are in close agreement with the experimental spectra. In particular, the puzzling wavenumber increase that had been observed in argon matrices for the intense band assigned to the COH angle bending δCOH, by substituting OD for OH, is explained without invoking the previous hypothesis of the tunnelling of the H atom between the two O atoms. It is related to the decoupling of δCOH from a mode involving the stretching of the C–C and C–O bonds with opposite phases.

Near infrared and ab initio study of the vibrational modes of isolated silanol on silica

Near infrared spectroscopy was used to study the vibrational modes of isolated silanol groups in porous silica monoliths that had been heated in acuo at 750°C. A set of bands is assigned to overtone and combination transitions for normal modes involving mainly the SiOH group: the νOH stretching, the νSi–OH stretching, the δOH bending and the τOH torsion around the SiO bond. The spectra of a deuterated sample were also analysed. The wavenumbers for the fundamental transitions were deduced and some anharmonicity coefficients were calculated. The δOH bending is coupled with a symmetric stretching of SiOSi bridges in an (SiO)3SiOH unit. Pairs of bands involving some SiOH bending character (at 4510/4580 cm−1 for combinations with νOH and at 8100/8165 cm−1 for combinations with 2νOH) were extrapolated to fundamental transitions at 760/835 cm−1. This coupling was not observed for the deuterated sample because the δOD absorption at 605 cm−1 is well separated from SiOSi stretching centred at 815 cm−1. The experimental spectra are discussed with the help of ab initio calculations of the vibrational modes of a molecule modelling an isolated silanol group on silica. The contribution of hot transitions at room temperature, from low lying levels such as τOH and its overtones, is also analysed.

Subwavelength Raman spectroscopy

Near-field Raman spectroscopy has been performed on liquid CCl4. The spectrum, obtained through an aperture significantly smaller (λ/10) than the exciting wavelength (514.5 nm) with a good signal-to-noise ratio, presents polarization effects that are different from the far-field spectrum. The tip is shown to be polarized at 99.9% by the Raman near field. If the probe is modeled as a single dielectric sphere, the estimated Raman near-field amplitude amounts to about 103 V m−1. The frustration of the Raman near field by the probe allows to record the Raman spectrum of only 0.4 attomol, or 240 000 mol.

Sulfate graphite intercalation compounds : new electrochemical data and spontaneous intercalation

During the electrochemical intercalation and deintercalation processes of sulfuric acid into graphite, the electrical resistivity along the graphene layers was followed with a contactless method. At 20 °C, the intercalation process is associated with a large decrease of resistivity; the lowest values are observed for stages 8 to 5 formation, and remain nearly constant until stage 1. However, the intercalation reaction is limited by the oxidation of sulfuric acid and by graphite overoxidation; these effects appear for an apparent charge transfer of C10∗. Between 190 and 300 °C (1 or 1000 bar), pure sulfuric acid intercalates readily into graphite without external chemical or electrochemical oxidation. The formation of sulfate GIC is related to the decomposition of H2SO4 progressively evolving sulfur trioxide, which is a powerful oxidizing agent. Oxidation reaction, increasing with temperature above 250 °C, is expressed by the apparition of corrosion holes. The samples have been characterized by X-ray diffraction, resistivity measurements, and scanning electron microscopy.

DETECTION AND SORPTION STUDY OF DIOXOURANIUM(VI) IONS ON N-(2-MERCAPTOPROPIONYL)GLYCINE-MODIFIED SILVER COLLOID BY SURFACE-ENHANCED RAMAN SCATTERING

An aqueous silver colloid activated at 514 nm with 10-3MN-(2-mercaptopropionyl)glycine (MPG) involves surface-active sites for the complexation of uranyl ions and displays the νs UO22+ SER spectrum at 840 cm-1, allowing uranyl detection at concentrations as low as 5×10-9M. This level of detection competes successfully with spectrophotometric and fluorimetric determination. A quantitative study of the SER spectra as a function of total uranyl concentrations in the colloid, ranging between 5×10-9 and 1×10-5M, shows that half of the sorption sites are occupied by uranyl for a total uranyl concentration of about 10-7M; from this value, a 100-fold concentration increase induces only a twofold enhancement of the uranyl band at 840 cm-1, although no appreciable change of the colloid appears. As a result of an analysis of the uranyl complexation either in solution or simultaneously with several sorbed MPG molecules, the surface site concentration for uranyl sorption in the colloid used is estimated to be close to 10-8M. This value corresponds to about one complexation site per primary particle of silver, perhaps at contacts between aggregated particles. At such sites, MPG molecules bonded to the metal surfaces through S—Ag bonds still involve amide and carboxylate groups free to coordinate an uranyl ion. The mean enhancement of the Raman cross-section of uranyl at the surface complexation sites reaches 3×105 with respect to uranyl nitrate aqueous solution.

Step towards sum frequency generation spectromicroscopy at a submicronic spatial resolution

A near-field optical device has been developed to collect sum frequency signal, generated in an AsGa single crystal sample by two pulsed lasers, one fixed at a wavelength of 532 nm and the other tunable in midinfrared. The sum frequency signal was collected with an uncoated silica tip. When the tip sample distance was increased, a strong decrease of the collected sum frequency signal was observed: the signal was divided by 2 for an increase of about 100 nm of the distance between the nanoprobe end and the sample surface. Without demonstrating submicronic lateral spatial resolution, this letter is, however, a first step towards a microscopic device of sum frequency generation spectroscopy.

Vibrational Spectroscopic Study of the Complexation of Mercury(II) by Substituted Acetates in Aqueous Solutions

Attenuated total reflectance infrared (ATR-IR) and Raman spectroscopies are used to identify the complexed species of mercury(II) with acetate and some aliphatic derivatives in aqueous solutions, as a function of pH and metal-to-acetate ratio. The spectra of the free carboxylate ions, in their sodium salt solutions, and of the protonated molecules are shown for comparison and assigned below 1800 cm−1. Whatever the stoichiometries assumed [HgCH3COO+ up to Hg(CH3COO)42-], one structure is observed, principally via the stretching modes ν(CO2) and ν(CC), although two strong bands have a νs(CO2) character owing to an accidental coupling of this mode with δs(CH3) in all the acetate complexes. This coupling is demonstrated with the isotopic ligand CD3COO, which displays only one νs(CO2) band on account of the separation of this mode from δs(CD3). The splitting between νa(CO2) and νs(CO2) increases up to about 260 cm−1 for all the carboxylates under study, the value pointing to a hydrated monodentate form.

Effect of heat treatment on boron impurity in Vycor. Part II. Migration, reactivity with vapour water and dissolution in liquid water

With increasing temperature and time of heat pretreatment of Vycor porous glass, near infrared absorption related to the formation of surface boranol groups increased concurrently with the decrease of the absorption spectrum of silanol groups. This trend corresponds to the migration of boron toward the surface, as previously suggested in the literature. The surface boron is much more soluble than the surface silicon species in pure water at room temperature. One pretreatment of Vycor at 900°C for 20 h allows the dissolution of 35% of the total pristine boron, corresponding to 1.6 soluble B atoms nm−2. The thermal migration of boron is driven by a concentration gradient between surface and inner sites, since it is accelerated by dissolving surface boron between thermal stages at a given temperature. On preheated Vycor, a much quicker and more important formation of boranols than silanols by dissociative chemisorption of water is observed under a low vapour pressure and B2O3 aggregates have been detected.

Effect of heat treatment on boron impurity in Vycor. Part I. Near infrared spectra and ab initio calculations of the vibrations of model molecules for surface boranols

Near infrared spectra of surface boranol groups on a preheated Vycor silica glass were observed and interpreted with the help of ab initio calculations of the vibrational properties of BOH groups anchored to silica or to B2O3 clusters. The spectra of deuterated samples were also obtained. A set of narrow lines corresponds to overtone and combination transitions for normal modes involving mainly the hydroxy group, that is the νOH stretching, the δOH bending and the τOH torsion around BO bond. Hot bands at 25°C, whose origin is the τOH level at 530 cm−1, are revealed by cooling the sample at −173°C. A sample evacuated at 760°C displays mainly isolated boranol groups and some very weakly perturbed species, likely vicinal groups, bonded to silica. A thorough analysis of the anharmonic properties of isolated boranol is given. The spectrum assignment is extrapolated to transitions hidden by the very strong absorptions of the bulk material. Fermi resonances appear in diads for species (SiO)2BOH because the 2τOH overtone is close to δOH, near 1030 cm−1.