Comparaison des effets α-methyle sur des sites 13C et 15N a l'aide du modele topologique (α-Me) par difference lineaire (DARC-PULFO)

The extension to ¹⁵N NMR of the model by linear difference previously proposed for (α-Me.) ¹³C shifts gathers in an unifying treatment the different behaviours observed for (α-Me.) shifts in ¹⁵M and ¹³C of, aliphatic amines, hydrochlorides and alkanes.     

Electrochemical study of intermetallic metal hydride as an anode material for Ni–MH batteries

The electrochemical behavior of Cobalt-free LaNi_3.55Mn_0.4Al_0.3Co_0.4Fe_0.35 alloy electrode in alkaline solution was investigated using electrochemical impedance spectroscopy (EIS) at different number of charge/discharge cycles. A physicochemical model is developed in order to simulate impedance data. Kinetic parameters are obtained by fitting the electrochemical impedance spectrum performed at different number of cycles. The charge-transfer resistance decreases with increasing number of charge/discharge of cycles, whereas exchange current density and hydrogen diffusion coefficient parameters increase with increasing number of cycles. In addition, the specific surface area of LaNi_3.55Mn_0.4Al_0.3Co_0.4Fe_0.35 alloy electrode increases due to pulverization and the formation of new active sites during charge/discharge cycling. The results of EIS measurements indicate that the performance of the LaNi_3.55Mn_0.4Al_0.3Co_0.4Fe_0.35 metal hydride electrode was markedly improved with increasing number of cycles which is mainly attributed to the increase in the reaction surface area and the improvement in the electrode surface activation.     

A Mirror Effect in the Intensities of the Secondary Transitions of Benzyl Acetate and Phenethyl Propionate

The intensity of the secondary transition (towards 260 nm)of the benzene chromophore in toluene derivatives, has been linked to distorsions of the D_6h symmetry which are induced by the electronic effects of the substituent. These distorsions arise through the C_α H₂ σ_π hyperconjugative coupling, involving φ and the substituent on C_α     

Thickness dependent sensing mechanism in sorted semi-conducting single walled nanotube based sensors

Single walled carbon nanotube (SWCNT) networks present outstanding potential for the development of SWCNT-based gas sensors. Due to the complexity of the transport properties of this material, the physical mechanisms at stake during exposure to gas are still under debate. Previously suggested mechanisms are charge transfer between gas molecules and SWCNT and Schottky barrier modulation. By comparing electrical measurements with an analytical model based on Schottky barrier modulation, we demonstrate that one mechanism or the other is predominant depending on the percolation of metallic carbon nanotubes. Below the metallic SWCNT percolation threshold, sensing is dominated by the modulation of the Schottky barrier, while above this threshold, it is only attributed to a charge transfer between SWCNT and gas molecules. Both mechanisms are discussed in terms of sensitivity and resolution leading to routes for the optimization of a gas sensor architecture based on highly enriched semiconducting carbon nanotube films.     

In situ study of the deposition of (ultra)thin organic phosphonic acid layers on the oxide of aluminum

The interest in self-assembling monolayer deposition on various oxide substrate surfaces is steeply increasing in the last decades. Although many studies are being performed, literature does not come with a general insight in the adsorption of these layers on oxide surfaces. Also for the deposition of phosphonic acids on aluminum oxides, there is no global consensus. In this paper, we present an original in situ analysis in order to eludicate the real layer formation mechanism. First of all, the state of the phosphonic acid molecules was determined using DOSY NMR, making sure that no structures other than free molecules were present at the concentration used. With in situ atomic force microscopy and in situ visual ellipsometry, multilayers of phosphonic acids, showing 3D island growth, were determined. It was shown that using the variation of the in situ obtained roughness and bearing ratio, together with the equivalent thickness modeled by ellipsometry, the growth of the layers occurs in situ in three different stages. They consist of increasing number of islands growth, followed by filling up the gaps between islands. At last, within the adsorption time frame measured, the islands grow further in dimensions but not in numbers. This closely corresponds with the behavior of the octylphosphonic acid films analyzed by ex situ techniques.     

Local relaxation in lanthanum silicate oxyapatites by Raman scattering and MAS‐NMR

To characterize the local relaxation in the structure of lanthanum silicate oxyapatite materials, six compositions with different cation and oxygen stoichiometries (La₈Ba₂Si₆O₂₆, La₉BaSi₆O_26.5, La₁₀Si_5.5Mg_0.5O_26.5, La_9.33SiO₂₆, La_9.67SiO_26.5 and La_9.83Si_5.5Al_0.5O_26.5) were investigated by combining Raman scattering and ²⁹Si and ²⁷Al magic‐angle spinning nuclear magnetic resonance (MAS‐NMR) spectroscopies. Only [SiO₄]⁴⁻ species were evidenced and the hypotheses of [Si₂O₇]⁶⁻ and [Si₂O₉]⁸⁻ entities were ruled out. Both oxygen excess and cation vacancies induce local distortions in the structure, which leads to nonequivalent [SiO₄]⁴⁻ species, characterized by different ²⁹Si MAS‐NMR signals and by splitting of Raman signals. Copyright © 2011 John Wiley & Sons, Ltd.     

La3TaO7 derivatives with Weberite structure type: Possible electrolytes for solid oxide fuel cells and high temperature electrolysers

In this study, with the aim to enhance the ionic conduction of known structures by defect chemistry, the La₂O₃-Ta₂O₅ system was considered with a focus on the La₃TaO₇ phase whose structure is of Weberite type. In order to predict possible preferential substitution sites and substitution elements, atomistic simulation was used as a first approach. A solid solution La_3−xSr_xTaO_7−x/2 was confirmed by X-ray diffraction and Raman spectroscopy; it extends for a substitution ratio up to x = 0.15. Whereas La₃TaO₇ is a poor oxide ion conductor (σ_700 °C = 2 × 10⁻⁵S.cm⁻¹), at 700 °C, its ionic conductivity is increased by more than one order of magnitude when 3.3% molar strontium is introduced in the structure (σ_700 °C = 2 × 10⁻⁴S.cm⁻¹).     

Nucleophilic attack on phosphate diesters: a density functional study of in-line reactivity in dianionic, monoanionic, and neutral systems

A density functional study of the hydrolysis reaction of phosphodiesters with a series of attacking nucleophiles in the gas phase and in solution is presented. The nucleophiles HOH, HO-, CH3OH, and CH3O- were studied in reactions with ethylene phosphate, 2'3'-ribose cyclic phosphate and in their neutral (protonated) and monoanionic forms. Stationary-point geometries for the reactions were determined at the density functional B3LYP/6-31++G(d,p) level followed by energy refinement at the B3LYP/6-311++G(3df,2p) level. Solvation effects were estimated by using a dielectric approximation with the polarizable continuum model (PCM) at the gas-phase optimized geometries. This series of reactions characterizes factors that influence the intrinsic reactivity of the model phosphate compounds, including the effect of nucleophile, protonation state, cyclic structure, and solvent. The present study of the in-line mechanism for phosphodiester hydrolysis, a reaction of considerable biological importance, has implications for enzymatic mechanisms. The analysis generally supports the associative mechanism for phosphate ester hydrolysis. The results highlight the importance for the reaction barrier of charge neutralization resulting from the protonation of the nonbridging phosphoryl oxygens and the role of internal hydrogen transfer in the gas-phase mechanism. It also shows that solvent stabilization has a profound influence on the relative barrier heights for the dianionic, monoanionic, and neutral reactions. The calculations provide a comprehensive data set for the in-line hydrolysis mechanisms that can be used for the development of improved semiempirical quantum models for phosphate hydrolysis reactions.     

Is wood pre‐treatment essential for tree‐ring nitrogen concentration and isotope analysis?

Tree-ring nitrogen concentrations and isotope ratios (δ(15)N) are gaining in popularity for environmental research although their use is still debated because of nitrogen mobility in tree stems. Modern studies generally present results on wood that is pre-treated to remove soluble nitrogen compounds and to minimize the impact of radial translocation on tree-ring nitrogen environmental records. However, the necessity to use such pre-treatment has never been fully assessed. Here we compare the nitrogen concentrations and δ(15)N values of two wood preparation protocols applied to beech and red spruce tree rings for the removal of soluble compounds from ring pairs with non pre-treated tree rings. For both tree species, pre-treatment did not minimize the radial patterns of tree-ring nitrogen concentrations and the increasing concentration trends that are coincident with the heartwood-sapwood boundary. Therefore, even if the tree-ring nitrogen concentrations are slightly modified by pre-treatment, these concentrations are considered to reflect internal stem processes rather than environmental conditions in both species. The δ(15)N values were similar for untreated and pre-treated ring pairs, suggesting that wood pre-treatment did not substantially change the δ(15)N values and temporal trends in ring series. In addition, tree-ring δ(15)N series of untreated and pre-treated wood did not show any sign of influence of the heartwood-sapwood boundary in either tree species, indicating that nitrogen translocation did not generate significant isotopic fractionation. We therefore suggest that untreated ring δ(15)N values of beech and red spruce trees can be used for environmental research.