Effet sterique sur les vitesses de transfert protonique: cyclohexanols substitues dans le dimethylsulfoxyde

Steric effects on proton transfer from, and to, hydroxylic oxygen have been studied in a series of seventeen α-methyl and a-benzyl cyclohexanols in anhydrous DMSO, under both acid and base catalysed conditions, using dynamic MNR techniques. The protonation rate constants (k₁ ? 10⁶ M^-1 s^-1 at 25°C) obey a Taft-Ingold relationship, containing only a steric contribution E_s = E_s^OH + E_s^α, where: E_s^OH = 0 or 0.15 for an axial or equatorial hydroxyl respectively and E_s^α = ?0.070 (or ?0.115) for substituting an α-hydrogen by a methyl (or benzyl) group. An equatorial hydroxylic function is therefore 40% more reactive than its axial homologue. These kinetic data are fairly consistent with structural information resulting from IR spectroscopy (v_co and v_OH vibrations) and from NMR (hydroxylic chemical shifts and coupling constants).     

Fe-doped SnO2 transparent semi-conducting thin films deposited by spray pyrolysis technique: Thermoelectric and p-type conductivity properties

In this paper, we report structural, electrical, optical, and especially thermoelectrical characterization of iron (Fe) doped tin oxide films, which have been deposited by spray pyrolysis technique. The doping level has changed from 0 to 10 wt% in solution ([Fe]/[Sn] = 0–40 at% in solution). The thermoelectric response versus temperature difference has exhibited a nonlinear behavior, and the Seebeck coefficient has been calculated from its slope in temperature range of 300–500 K. The Hall effect and thermoelectric measurements have shown p-type conductivity in SnO₂:Fe films with [Fe]/[Sn] ≥ 7.8 at%. In doping levels lower than 7.8 at%, SnO₂:Fe films have been n-type with a negative thermoelectric coefficient. The Seebeck coefficient for SnO₂:Fe films with 7.8 at% doping level has been obtained to be as high as +1850 μV/K. The analysis of as-deposited samples with thicknesses ～350 nm by X-ray diffraction (XRD) and scanning electron microscopy (SEM) has shown polycrystalline structure with clear characteristic peak of SnO₂ cassiterite phase in all films. The optical transparency (T%) of SnO₂:Fe films in visible spectra decreases from 90% to 75% and electrical resistivity (ρ) increases from 1.2 × 10^?2 to 3 × 10³ Ω cm for Fe-doping in the range 0–40 at%.     

Structure and magnetism of [M3](6/7+) metal chain complexes from density functional theory: analysis for copper and predictions for silver

The ground-state electronic structure of the trinuclear complex Cu3(dpa)4Cl2 (1), where dpa is the anion of di(2-pyridyl)amine, has been investigated within the framework of density functional theory (DFT) and compared with that obtained for other known M3(dpa)4Cl2 complexes (M = Cr, Co, Ni) and for the still hypothetical Ag3(dpa)4Cl2 compound. Both coinage metal compounds display three singly occupied x2-y2-like (delta) orbitals oriented toward the nitrogen environment of each metal atom, generating antibonding M-(N4) interactions. All other metal orbital combinations are doubly occupied, resulting in no delocalized metal-metal bonding. This is at variance with the other known symmetric M3(dpa)4Cl2 complexes of the first transition series, which all display some delocalized bonding through the metal backbone, with formal bond multiplicity decreasing in the order Cr > Co > Ni. An antiferromagnetic coupling develops between the singly occupied MOs via a superexchange mechanism involving the bridging dpa ligands. This magnetic interaction can be considered as an extension to the three aligned Cu(II) atoms of the well-documented exchange coupling observed in carboxylato-bridged dinuclear copper compounds. Broken-symmetry calculations with approximate spin projection adequately reproduce the coupling constant observed for 1. Oxidation of 1 removes an electron from the magnetic orbital located on the central Cu atom and its ligand environment; 1+ displays a much weaker antiferromagnetic interaction coupling the terminal Cu-N4 moieties via four ligand pathways converging through the x2-y2 orbital of the central metal. The silver homologues of 1 and 1+ display similar electronic ground states, but the calculated magnetic couplings are stronger by factors of about 3 and 4, respectively, resulting from a better overlap between the metal centers and their equatorial ligand environment within the magnetic orbitals.     

Influence of the precipitation agent in the deposition-precipitation on the formation and properties of Au nanoparticles supported on Al2O3

Au nanoparticles supported on Al2O3 were prepared by deposition-precipitation of HAuCl4 with different precipitation agents NaOH and urea. The samples were investigated by means of different characterization techniques such as X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and transmission electron microscopy (TEM). The results show that depending on the precipitation agent, the Au particles have a different Au-Au coordination number and size after calcination at 523 K. Whereas the use of NaOH leads to the formation of Au nanoparticles with a Au-Au coordination number of 6.7 and a mean diameter below 2 nm, those prepared with urea have a mean size of 3.1 nm. The Au-Au coordination number could be determined as 8.6. At the smaller particles obtained with NaOH, hints for Au-O interactions were found. For these particles TEM results advise a rather flat lenticular morphology. Different deposition mechanisms depending on the precipitation agent are discussed as the reason for the formation of nanoparticles with different shapes, sizes, and valence states.     

Nature of the interference of nitric acid in the determination of nickel and vanadium by atomic absorption spectrometry with electrothermal atomization

Effects of the concentration of nitric acid in the determination of nickel and vanadium in the presence of other metals by flarneless atomic absorption spectrometry have been studied. Specific complexation of the metals in the aqueous phase suppresses the interferences. A method has been developed which allows the use of calibration curves from dilute acidic solutions in the determination of samples with high nitrate concentrations. The method is suitable for solutions reproducing the mineralization of airborne particulates.     

Development of highly active and selective novel Pd based acetoxylation catalysts and prevention of catalyst deactivation by Bi modification

A first approach to successful prevention of catalyst deactivation while simultaneously achieving extremely high selectivity of benzyl acetate (> or = 95%) at significantly high toluene conversion (> 70%) by gas phase acetoxylation over novel Pd-Sb-Bi/TiO2 catalysts.     

Transition from anatase to rutile phase in titanium dioxide (TiO2) nanoparticles synthesized by complexing sol–gel process: effect of kind of complexing agent and calcinating temperature

In this work, the structural and optical properties of titanium dioxide (TiO₂) nanopowders are studied. The TiO₂ nanoparticles were synthesized by complexing sol–gel process and effect of complexing agents on transition of the anatase phase to rutile phase during the heat treatment have been investigated. In addition, we have studied the grain size of TiO₂ powders and their dependence on the type of complexing agent. The analysis of the XRD patterns, FT-IR and UV–Vis spectroscopy, BET surface area and TEM images show that the synthesis of nanoparticles with acetyl acetone (AcAc) as complexing agent yielded the smallest size of nanoparticles about 22–35 nm. Our results indicate that with increasing the calcinating temperature, the size of the nanoparticles is increased and the energy gap reduced, too. Also, the optical band gap was obtained in the range of 3.4–4.1 and 3.06–3.74 eV for anatase and rutile phases, respectively.     

Structure comparison of PMN–PT and PMN–PZT nanocrystals prepared by gel-combustion method at optimized temperatures

We have synthesized and were performed a comparison of structures and optical properties between relaxor ferroelectric PMN–PT and PMN–PZT nanopowders. A gel-combustion method has been used to synthesize PMN–PT and PMN–PZT nanocrystalline with the perovskite structure. The precursors employed in the gel-combustion process were lead nitrate, magnesium acetate, niobium ammonium oxalate and zirconium nitrate. The nanopowders were characterized using the X-ray diffraction (XRD) and transmission electron microscopy (TEM) observation. Fourier transform infrared (FTIR) spectroscopy was employed to monitor the transformation of precursor solutions during the thermal reactions leading to the formation of perovskite phase.     

Effect of the synthesis route on the structural properties and shape of the indium oxide (In2O3) nano-particles

Nano-crystalline indium oxide (In₂O₃) particles have been synthesized by sol–gel and hydro-thermal techniques. A simple hydro-alcoholic solution consisting indium nitrate hydrate and citric acid (in sol–gel method) and 1, 4-butandiol (in hydro-thermal method) have been utilized. The structural properties of indium oxide nano-powders annealed at 450 °C (for both methods) have been characterized by the X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and specific surface area (SSA) analysis. Structural analysis of the samples shows cubic phase in sol–gel and cubic-hexagonal phase mixture in hydro-thermally prepared particles. The nano-particles prepared by sol–gel method have nearly spherical shape, whereas hydro-thermally-made ones display wire- and needle-like shape in addition to the spherical shape. The obtained In₂O₃ nano-particles surface areas were 23.2 and 55.3 in sol–gel and hydro-thermal methods, respectively. The optical direct band gap of In₂O₃ nano-particles were determined to be 4.32 and 4.24 eV for sol–gel and hydro-thermal methods, respectively. These values exhibit 0.5 eV blue shift from that the bulk In₂O₃ (3.75 eV), which is related to the particle size reduction and approaching the quantum confinement limit of nano-particles.     

Cooperative ordering of collagen triple helices in the dense state

Extracellular matrixes such as bone, skin, cornea, and tendon have ordered structures comprised for the most part of collagen, an elongated protein of well-defined dimensions and composition. Here we show how the cooperative ordering of collagen triple helices in the dense fluid state is exploited to produce dense ordered collagen matrixes. The spontaneous formation of a birefringent phase occurs at critical concentrations that increase from 50-60 to 80-85 mg/mL as the acetic acid concentration of the solvent increases from 5 to 500 mM. We studied by small-angle X-ray scattering (SAXS) the local liquidlike positional order across the isotropic/anisotropic phase transition by unwinding the cholesteric phase with moderate shearing stress. Interparticle scattering gives rise to a broad interference peak. The average distance between triple helices, dav, is thus estimated and decreases linearly as a function of phi-1/2 from 12.7 +/- 0.9 nm (22.5 mg/mL) to 5.0 +/- 0.6 nm (166.4 mg/mL). Equilibrium concentrations and the order parameter of the nematic phase agree reasonably well with theoretical predictions for semiflexible macromolecules. Striated fibrils with a high degree of alignment were obtained by fine-tuning the delicately balanced electrostatic interactions, which yielded strong elastic gels with a hierarchical organization very similar to that of major biological tissues. Typical Bragg reflections corresponding to the 67 nm period characteristic of collagen fibrils in biological tissues were recorded by SAXS with ordered collagen matrixes reconstituted in vitro.