Résolution des équations de dispersion–diffusion en présence de forts gradientsResolution of advection–diffusion equation in presence of sharp gradient

In order to resolve the convection terms in the simulation of coastal flows, we propose a numerical method based on the Total Variation Diminishing scheme completed with the Artificial Compression Method to reduce the artificial diffusion and to sharpen the tracer profile in strong gradient regions. This method was applied successfully at discontinuous regions, but out of these regions a limitation of its application is necessary. An application is made to estimate the width of the fresh water band in the Eastern English Channel. To cite this article: C. Sart et al., C. R. Mecanique 330 (2002) 159–165.     

Phase proportioning in CuAlBe shape memory alloys during thermomechanical loadings using electric resistance variation

The microstructure of shape memory alloys changes with the thermomechanical history of the material. During thermomechanical loading, austenite, thermally-induced martensite or stress-induced martensite can be simultaneously present in the material. In applications integrating SMA parts, utilization conditions seriously affect the microstructure and can generate macroscopic strain or stress. Consequently, during thermomechanical loadings, it is important to be able to proportion the different phases and consequently to understand the kinetic transformation. This is very useful in the development of constitutive equations. This study shows, by a series of tests, that the proposed experimental method, based on the measurement of the variation of electric resistance of CuAlBe wires, permits to determine the volume fraction of the different phases present in the material (i.e., austenite, stress-induced martensite and thermally-induced martensite). The proposed method is applied to the most common thermomechanical behavior met in engineering applications of shape memory alloys: pseudoelasticity, pseudoplasticity, recovery-stress and stress-assisted two-way shape memory effect. The proportioning method based on a mixture law integrating the resistivity of pure phases present in the SMA is first performed on different two-phase mixture cases and then applied to a three phase mixture case.     

Deep gravity waves in moist-saturated atmospheric air

The equations of motion of a moist-saturated, dissipative, stratified atmosphere are used in the case of small Schmidt number to describe the propagation of linearized gravity waves in deep cloudy atmosphere: as in dry air, the consequence of the dissipation is the diffusion and the damping of waves through the medium. However, there exist particular cases of stratification, for which the moisture gives rise at locally undamped waves. This property justifies the existence of oscillatory unstable régimes when we study shallow convection in the medium.     

X-ray diffraction study of the structure of carboxymethylcellulose-cationic surfactant complexes.

We have investigated dilute aqueous solutions of an anionic polymer (carboxymethylcellulose) mixed with cationic surfactants of different chain lengths (dodecyl to octadecyl trimethylammonium bromides: DTAB, TTAB, CTAB and OTAB). The structures of the concentrated phases formed above the precipitation threshold were studied by X-ray diffraction. Different body-centred cubic structures with space groups Pm3n were observed in the presence of surfactant with a short aliphatic chain (DTAB), despite the fact that the polymer persistence length is comparable to the repeat distance of the structure (5 nm). For larger surfactant chain lengths (TTAB and CTAB), the structure of the precipitates can be either cubic (Pm3n) or 2D hexagonal depending on the initial surfactant and polymer concentrations. For still larger chain length (OTAB), the structure becomes lamellar. This structural evolution from micellar cubic towards 2D hexagonal and lamellar is attributed to the decrease of the local curvature of the surfactant aggregates, as observed for flexible synthetic polymers and short DNA fragments under similar conditions. Furthermore, the structure of the bulk complexes formed just below the precipitation threshold anticipates the structure seen in the precipitated phases.     

Tetrazo[1,2-b]indazoles: Straightforward Access to Nitrogen-Rich Polyaromatics from s-Tetrazines

The straightforward access to a new class of aza-polyaromatics is reported. Starting from readily available fluorinated s-tetrazine, a cyclization process with azide leads to the formation of an unprecedented tetrazo[1,2-b]indazole or a bis-tetrazo[1,2-b]indazole (cis and trans conformers). Based on the new nitrogen core, further N-directed palladium-catalyzed ortho-C−H bond functionalization allows the introduction of halides or acetates. The physicochemical properties of these compounds were studied by a joint experimental/theoretical approach. The tetrazo[1,2-b]indazoles display solid-state π-stacking, low reduction potential, absorption in the visible range up to the near-infrared, and intense fluorescence, depending on the molecular structure.     

Donor-π-Acceptors Containing the 10-(1,3-Dithiol-2-ylidene)anthracene Unit for Dye-Sensitized Solar Cells

Two donor-acceptor molecular tweezers incorporating the 10-(1,3-dithiol-2-ylidene)anthracene unit as donor group and two cyanoacrylic units as accepting/anchoring groups are reported as metal-free sensitizers for dye-sensitized solar cells. By changing the phenyl spacer with 3,4-ethylenedioxythiophene (EDOT) units, the absorption spectrum of the sensitizer is red-shifted with a corresponding increase in the molar absorptivity. Density functional calculations confirmed the intramolecular charge-transfer nature of the lowest-energy absorption bands. The new dyes are highly distorted from planarity and are bound to the TiO(2) surface through the two anchoring groups in a unidentate binding form. A power-conversion efficiency of 3.7?% was obtained with a volatile CH(3) CN-based electrolyte, under air mass 1.5 global sunlight. Photovoltage decay transients and ATR-FTIR measurements allowed us to understand the photovoltaic performance, as well as the surface binding, of these new sensitizers.     

Nitration in the imidazo[1,2-a]pyrazine series. Experimental and computational results

Nitration was carried out on a series of imidazo[1,2-a]pyrazine derivatives. The reactivities of diversely substituted derivatives and of all positions of substitution were analysed and experimental results compared with ¹³-nmr data and semi empirical calculations (AMI). Although the unsubstituted heterocycle is highly resistant to nitration, electron-donating groups such as alkoxy or alkylamino on position 8 enhance the reactivity of the imidazo[1,2-a]pyrazine derivatives towards electrophilic substitution and, more specifically, nitration. The ¹³-nmr experiments, electronic distributions and Molecular Electrostatic Potential isodensity surfaces calculated on the neutral forms are in good agreement with experimental results indicating position 3 is the most reactive position towards nitration.     

Taking Advantage of Ortho- and Peri-Substitution to Design Nine-Membered P,O,Si-heterocycles**

A family of cyclic phosphine-disiloxane featuring peri-substituted naphthyl(Nap)/acenaphthyl(Ace) scaffolds has been prepared and fully characterized including X-ray structure, which enables a detailed structural analysis. This straightforward synthesis takes advantage of both ortho- and peri-substitution of Nap/Ace-substituted phosphine oxides. The synthetic method allows diversifying the polycyclic aromatic platform (Nap and Ace) as well as the Si substituents (Me and Ph). Despite a strong steric congestion, the P-atom remains reactive toward oxidation or coordination. In particular, Au(I) complex could be prepared. All the compounds display absorption/luminescence in the UV-Vis range. Surprisingly, the P-trivalent derivatives display unexpected luminescence in the green in solid-state.     

Naphthyl-Fused Phosphepines: Luminescent Contorted Polycyclic P-Heterocycles

This article presents the synthesis of a new family of naphthyl-fused phosphepines through Ni-mediated C−C coupling. Interestingly, the chlorophosphine oxide intermediate shows strong resistance toward oxidation/hydrolysis owing to a combination of steric hindrance and pnictogen interactions. However, it can undergo substitution reactions under specific conditions. The optical/redox properties and the electronic structure of these new π-systems were studied experimentally (UV/Vis absorption, emission, cyclic voltammetry) and computationally (TD-DFT calculations, NICS investigation). Taking advantage of the luminescence of these derivatives, a blue-emitting OLED has been prepared, highlighting that these novel π-conjugated P-heterocycles appear to be promising building blocks for solid-state lighting applications.     

Fast,easy, and efficient method for the purification of phenolic isomers using a selective solid-phase scavenging process

The need for fast and efficient purification methods that can be easily handled and moreover automated is considerably increasing with the new techniques of high-throughput chemical synthesis. Following our previous work on the use of simple polymeric scavengers in fast reactions and purifications of organic substances, this article presents the results found during the development of a new method for the purification of phenolic substances. The purification method was found to be regulated by the interaction of acidic phenol groups with a basic polystyrene resin. Furthermore, the scavenging of phenolic isomers proved to be very selective for a given isomer. But the most interesting aspect of this method is that it is based on a simple contact in situ with the resin and that the adsorption/desorption process of the phenol was found to be solvent-dependent. The phenols can thus be freed from impurities, or other isomers, by a simple and fast contact with the resin in the first solvent, filtration, and washings, followed by liberation of the purified phenol by a last soaking in another solvent for desorption. The method was successfully applied to the purification of a crude reaction mixture issued from the Fries rearrangement of phenyl acetate, as well as to small libraries of phenolic derivatives.