Détermination du champ de force de valence et des modes normaux de vibration des composés d'addition de l'acétone avec le trifluorure de bore

A valence force field for coordination compounds between acetone and boron trifluoride is computed: it is based upon vibrational spectra of six isotopic species and compared with the force field of free acetone and BF^?₄ ion. The formation of complex modifies the electronic state of the base and consequently the diagonal force constant of the CCO group and of the CH bonds. The force constant of the coordination bond (OB) is estimated as 2.10 mdyne Å^?1. Below 800 cm^?1 all modes involve motions of the acid and base; none corresponds to a stretching of the coordination bond.     

Spectres de vibration et structure d'un cristal d'amidogallate de sodium

Raman spectra (4000-150 cm^?1) of a single crystal of NaGa(NH₂)₄ and infrared spectra (4000-200 cm^?1 ) of a polycrystalline sample have been studied at different temperatures. An assignment of the bands is given. The spectra are discussed assuming S₄ and T_d point group symmetry of the Ga(NH₂)^?₄ ion at low temperature and at room temperature respectively. Metal-ligand and N-H stretching frequencies are compared to those of some other amido metalates.     

Enthalpie de dissolution du fluorure et de l′hydrogenofluorure d'ammonium dans les solutions d'acide fluorhydrique

The standard enthalpies of solution of NH₄F and NH₄HF₂ in aqueous solutions of hydrogen fluoride (in the range of concentration 0–30 mol.1^?1) have been measured and from these results the standard enthalpy of formation of NH₄HF_2(c) has been derived as: δH^o_fNH₄HF_2(c) = ?809.9 ± 0.9 kJ.mol^?1     

Échelle d'acidité dans l'hydrogénodifluorure de potassium (KHF2) fondu à 250°C: Détermination électrochimique

In molten potassium hydrogenodifluoride (KHF₂) fully ionized into K⁺ and HF₂^?, at 250°C, the HF₂^? ion is slightly dissociated according to the equilibrium: HF₂^?HF+F^?. This is a solvent acid base equilibrium, HF being the strongest acid and F^? the strongest base in this system. By means of a voltammetric study we showed that the hydrogen electrode may be used as an acidity indicator electrode in the whole acidity range of the melt. By analysis of the equilibrium potential variation in acidic and in basic media, the HF₂^? dissociation constant (melt autoprotolysis constant): K_D = {HF} {F^?} was determined The experimental value: K_D=10^?2.05 mol² kg^?2 is compared with a calculated one, issued from thermodynamic data. Results obtained with other electrodes (LaF₃ monocrystal electrode and copper electrode) were discussed and compared with those obtained with the hydrogen electrode.     

Selectivite de la reduction d'α-enones polycycliques par les borohydrures: effet de l'addition de tetramethyl-ethylenediamine au borohydrure de tetrabutylammonium

3^1,9octalone, 3^1,9-10-methyl octalone and testosterone were reduced by NBu₄BH₄. alone or in the presence of tetramethylethylenediamine (TMEDA), in THF and in toluene. With TMEDA, the first step of the reduction is the regioselective 1,4 attack by BH^?₄ which leads either to the saturated ketones or to the corresponding saturated alcohols. The results observed under different conditions were interpreted by the intervention of various reductive species: diborane, enoxyborohydrides in the absence of TMEDA, amine-borane in its presence.     

Reaction de metathese des olefines induite photochimiquement en presence d'un complexe de metal de transition. IV.- etude cinetique de l'interconversion catalysee cis-trans du butene-2.

The cis-trans interconversion of olefins in the system W(CO)₆ + CCl₄ + 2-butene is studied, both with initial irradiation of a solution of W(CO)₆ in CCl₄ (photoinduction), and with continuous irradiation of the system, for cis- and trans-butene concentrations between 0.09 and 6.0 M. Analysis of the results of the photoinduction experiments (rate of conversion and kinetic law as a function of the initial concentration of the olefin) allowed us to propose a simple kinetic scheme comprising a cis-trans interconversion of 2-butene and olefin-catalyzed destruction of the catalytic entity (k₂ = (0.62 ± 0.06)x10^?4 M s^?1). In the continuous irradiation experiments the final distribution of the olefin was independent of the initial butene concentration (cis-2-butene/trans-2-butene 3.0) and the reaction kinetics are of first-order (k_obs = (3±1) x10^?4 s^?1. Comparison of the two experiments suggests continuous photochemical regeneration of the catalytically active entity. The first-order reaction kinetics are in agreement with a carbene-metal carbonyl structure of the intermediate     

Spectres de vibration et conformations du propionate et de l'isobutyrate de méthyle

Infrared and Raman spectra (3600–3620cm^?1) of methyl propionate CH₃CH₂-COOCH₃, CH₃CH₂COOCD₃ and methyl isobutyrate (CH₃)₂CHCOOCH₃, (CH₃)₂CHCOOCD₃, in liquid and crystalline states, have been recorded. Rotational isomerism, by rotation around the C-C bond α to the carbonyl group, is detected and the energy difference between the conformers is 1.1 ±0.3 kcal mol^?1 for methyl propionate and 0.5 ±0.1 kcal mol^?1 for methyl isobutyrate. Vibrational assignments in terms of group frequencies are proposed for each conformer, only the more stable being present in the crystal.     

Structure cristalline d'un dihydrogeno-sulphato-iodate de potassium

The iodato-sulfate K₄H₂(S₂I₂O₁₄) is monoclinic, $\text{P2}{}_1\text{n}$ with a unit cell: a = 13.84(1) Å, b = 7.173(5) Å, c = 7.443(5) Å, β = 93.16(1)°, and Z = 2. The crystal structure of this salt has been solved using 1956 independent reflections, with a final R value of 0.03. The main feature of this salt is the existence of a finite new heteropolyanion: (I₂S₂O₁₄)^?6.     

Étude de l'oxydation et de la réduction électrochimiques de l'or par ellipsométrie avec excitation de plasmons de surfaces (SEW)

We studied the variation of optical parameters of voltage-sweep oxidized films on gold electrodes (evaporated gold on glass; 0.5 M H₂SO₄; 0<V<1.8 V/ENH; dV/dt=0.03 V s^?1). By ellipsometry with surface plasmons, we can measure large variations of Δ and ψ with oxygen coverage (20° for Δ; 10° for ψ). By associating optical and coulometric data, and assuming different values for the thickness d_f⁰ of diatomic Au₂O₃ layer corresponding to V=1.6 V, θ=1.2 (2 Å<d_f⁰<10 Å) we calculate at each coverage, the complex dielectric constant ?~, during oxidation and reduction steps. Anodic sweep: if 1.3 V<V<1.45 V (0<θ<0.38), ε₂?0. Only chemisorbed oxygen is present on the surface. If 1.45<V<1.60 V (0.38<θ<1.2) ε₁ and ε₂ increase according to the generalized Lorenz-Lorentz equation applied to a model of lateral growth of Au₂O₃ nuclei. If V>1.6 V, ε₁ and ε₂ have very slight variations with θ. Cathodic sweep: the slight variation of ε₂ with coverage for θ≥0.6 is in agreement with a model of uniform reduction of oxidized film until the Au₂O₃ layer becomes monoatomic. Optical, crystallographic, and coulometric data make us consider d_f⁰=3.7 Å to be the most likely value.     

Etude de la structure des oligomeres de l'isoprene desactives par les halogenures d'alcoyle

The deactivation of oligoisoprenyllithium RM_n^?, Li⁺ by an alkyl halide R′X yields two kinds of products: RM_nR′ and RM_nR; the microstructure of the isoprenic units in the oligomer is mainly 4,1 and 4,3 and it is determined by the addition to the active chain of an isoprene molecule or of the alkyl halide molecule. The microstructures of RM_nR′ and of RM_nR are different. The results indicate two kinds of halogen-lithium reactions leading to the products RM_nR: between the excess of initiator RLi and the alkyl halide or between oligoisoprenyllithium and the alkyl halide.     

Effets de micelles cationiques sur l'hydrolyse alcaline d'acetanilides a groupe acyle active par des substituants electro-attracteurs

Micellar effects of CTAB upon the alkaline hydrolysis of CF₃-CO-N(CH₃)C₆H₅, CHCl₂-CO-N(CH₃)C₆H₄X and CH₂Cl-CO-N(CH₃)C₆H₄X, (X=p-OCH₃H,p-Cl) are reported. Variations of k_obs, and of kinetic order of the reaction with respect to HO^? ion, are interpreted as an acceleration of HO^?-catalyzed steps, and a decrease of catalysis by water for decomposition of tetrahedral intermediates; these two effects oppose each other in HO^? and H₂O catalyzed steps. Differences between micellar and DMSO effects suggest a very small local concentration of HO^? ions in micelles.     

Sur de nouveaux ferrites oxyfluorés d'yttrium ou de gadolinium à structure grenat

The oxyfluoride garnets of formula Y₃Fe_5?xM_xO_12?xF_x and Gd₃Fe_5?xM_xO_12?xF_x (M = 3d transition element) result from partial substitution of O^2? by F^? in Y₃Fe₅O₁₂ and Gd₃Fe₅O₁₂ oxides. The cationic charge compensation is obtained by replacing the Fe³⁺ ions by divalent ions as Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺ or Zn²⁺ ions. The site occupied by some of these ions (Mn²⁺, Ni²⁺, Zn²⁺) is determined by magnetic or Mössbauer measurements.     

Synthese d'acides perfluores de hauts poids moleculaires

Perfluorinated olefins of general formula CF₃(CF₂)_pCFCF(CF₂)_mCF₃ (3 ? m + p ? 25) are oxidized by potassium permanganate or ruthenium tetroxide. The linear high molecular weight perfluorinated carboxylic acids so obtained are characterized by ¹⁹F N.M.R. spectroscopy.     

Structure cristalline et expansion thermique de l'iodure de nickel hexahydrate

The structure of NiI₂, 6 H₂O has been determined by X-ray diffraction techniques. The dimensions of the hexagonal unit cell are: a = 7.638 ± 0.005 Å and c = 4.876 ± 0.005 Å, with Z = 1. The space group is $\text{P}\text{3}\text{m1}$. The structure was deduced from Patterson and Fourier syntheses and refined by least-squares method to a final R value of 0.09. It is almost a layer structure, each layer is composed by n complex ions [Ni(H₂O)₆]²⁺ surrounded by 2 n ions I^?; this is in agreement with the good cleavage of crystals and with the marked anisotropy in the thermal expansion. But the structure may also be regarded as a derivation of NiAs structure, with only $\text{1}\text{8}$ of octahedral holes occupied. A structural classification of compounds MX₂, 6 H₂O is proposed.     

Etude par rmn de31p de la reaction de l'hexamethylphosphotriamide avec le trichlorure de phosphoryle

The reaction of HMPT with POCl₃ was studied by ³¹P NMR at various temperatures and stoechiometries. Progressive substitution of chlorine atoms of phosphoryl chloride by HMPT molecules was observed. Six new species were involved in the system. The main produce was the $\text{1}\text{1}$ adduct, (Me₂N)₃P⁺Cl, O₂PCl₂^?, analogous to Vilsmeier complex.     

Conformation d'hétéropeptides à l'aide de la spectrométrie Raman

The conformational analysis of some heteropeptides Ala_x Pro Ala_y (x = 2 or 3; y = 1, 2 or 3) has been investigated using Raman spectrometry. Vibrational assignments of characteristic amide I and III bands are made for C⁵, C⁷, C⁷C⁵, β turn conformations. Some characteristic end-group vibrations are listed also. The proline group has been shown to induce a β turn in a peptide chain. Raman speclroscopy can give the molecular conformation of solid oligopeptides with only 0.3–0.4 mg of sample.     

Photo-oxygenations de composes aromatiques sensibilisees par des accepteurs d'electron

Photo-oxygenation of aromatic compounds sensitized by electron acceptors, like 9-10 dicyanoanthracene (DCA) is shown to proceed by two distinct mechanisms each one beginning by an electron-transfer step: in the first way superoxide ion [O₂^?] is involved and in the second one singlet oxygen [¹O₂^*] is produced according to an unusual process.     

Synthèse et propriétés magnétiques de solutions solides entre l'oxyde de chrome CrO2 et l'oxyhydroxyde de cobalt CoOOH

Solid solutions of general formula (1 ? x)CrO₂, xCoOOH have been prepared by hydrothermal synthesis under very high pressure conditions (80 kb). Cr⁶⁺O₃ and CoCr⁶⁺O₄ were used as starting materials. Homogeneous ferromagnetic phases were obtained when 0 ? x ? 0.5. X ray powder patterns clearly demonstrate the isotype with InOOH, an orthorhombic distorted rutile type structure. The results of the magnetic measures performed on samples with different compositions indicate that part of the Cr⁴⁺ cation have been reduced to Cr³⁺ and that the general formula of the solid solutions should be written Cr⁴⁺_1?(x+y)Cr³⁺_yCo²⁺_xO₂H_x+y.     

Generalite de l'effet structural d'un motif bitertiaire en rmn 13C: effet gem-6 de 6ème substitution methyle

In di-tertiary groups like (Me₃C)₂¹³C, the sixth methyl substituent $\text{inverts}$ or $\text{attenuates}$ the induced ¹³C chemical shift of sp² or sp³ sites (B_σ and y_π effects).     

Dosage de l'americium par coulometrie a potentiel impose

A coulometric determination of americium by reduction of Am(VI) to Am(V) at a potential of + 1.3 V is proposed. The accuracy of the method was evaluated by analyzing ²⁴¹Am solutions prepared by weighing and dissolving AmO₂ of known purity. The results must be corrected by a factor of 1.017, in order to eliminate the systematic error due to autoreduction. After this correction, the relative standard deviation of the method ranged from 0.44% for américium concentrations of 3.125·l0^-3M to 0.96% for those of 1.25·10^-4M.