Diffusion approximation to wave propagation in imperfectly periodic structures

A diffusion approximation to the random evolution modeling of wave propagation in randomly imperfect periodic laminated composites is presented. It is shown that unless the material properties of the various layers satisfy some inequality (Eqn. (2)) all waves have exponentially decaying expected amplitudes.

---

Résumé On utilise une diffusion comme approximation de l'évolution aléatoire modélisant la propagation des ondes dans un matériau composite laminaire avec imperfections aléatoires. On montre qu'à moins que les propriétés des matériaux formant les différentes couches satisfassent l'inéquation (2), toutes les ondes ont une amplitude qui décroit de façon exponentielle.

     

The characterization and thermal stability of a cluster grafted on silica surface

Ru₃(CO)₁₂, supported on silica in the absence of oxygen, reacts with silanol groups of the surface to produce a grafted cluster

, which has been characterized by IR and Raman spectroscopy; the molecular formula of this cluster is in agreement with the stoichiometric balance of CO evolved during its formation from Ru₃(CO)₁₂. The grafted cluster is an intermediate step to produce by thermal decomposition small metallic ruthenium particles of 14 Å together with some Ru(II) carbonyl species encapsulated in the silica surface.     

Lysozymes: a chapter of molecular biology

Hen egg-white lysozyme was the first enzyme whose tertiary structure could be elucidated. The peptide chain of this enzyme is arranged in two sections, of approximately equal size, that are separated by a deep cleft. Substrates (and inhibitors) are bound in this cleft via hydrogen bonds and are hydrolyzed under the action of Glu 35 and Asp 52, which form the active site of the enzyme. Although the lysozymes, which occur in many species of animals and plants, exhibit differences in their chemical behavior, they have the same qualitative biological activity; quantitatively important differences have been noted which also concern the specificity. Infection of E. coli with bacteriophages gives rise to a lysozyme whose formation is controlled by the phage DNA. The fact that mutated lysozymes are produced when the phages are treated with mutagens opens new fields of research in molecular biology.     

Etudes sur la nature des «pinks» par thermogravimétrie et spectrograpnie infrarouge. I

Résumé L'étude thermogravimétrique des 2 réactions à l'état solide SnCl₂+ +CrO₃ et SnCl₂+K₂Cr₂O₇ pour différentes proportions des constituants, montre que le «pink» formé pour un rapport Sn/Cr supérieur à 5 n'est pas une combinaison de chrome et d'étain.Le spectre d'absorption infrarouge de la poudre violette recueillie à partir de 600 ne présente aucune bande d'absorption en dehors de celles de SnO₂ et de Cr₂O₃. Il s'agit essentiellement d'oxyde d'étain dans lequel se trouve finement dispersé de 2 à 8% d'oxyde Cr₂O₃.

---

Summary The thermogravimetric study of two reactions in the solid state, namely SnCl₂+CrO₃ and SnCl₂+ K₂Cr₂O₇ with different proportions of the reactants show that the pink produced by a ratio of Sn/Cr above 5 is not a compound of chromium and tin.The infra red absorption spectrum of the violet powder collected from 600 on shows no absorption band except those of SnO₂ and Cr₂O₃. We are dealing here essentially with a tin oxide containing 2 to 8% of finely dispersed oxide Cr₂O₃.

---

Zusammenfassung Die thermogravimetrische Untersuchung der beiden Festkörperreaktionen zwischen Zinn(II)-chlorid und Chromsäureanhydrid bzw. Zinn(II)-ohlorid und Kaliumbichromat bei verschiedenen Gewichtsverhältnissen ergab, daß das bei einem Sn/Cr-Verhältnis > 5 entstehende Pink keine Verbindung aus Chrom und Zinn ist.Das IR-Absorptionsspektrum des über 600 erhaltenen violetten Pulvers enthält keinerlei Absorptionsbande außer denen des SnO₂ und des Cr₂O₃. Es handelt sich im wesentlichen um Zinnoxyd, in dem sich 2 bis 8% Cr₂O₃ in feiner Verteilung finden.

     

Versatile access to C-4-substituted 2-amino-1,3-azoles from hydropyridines in oxidative conditions

[Chemical reaction: see text] Various substituted 2-aminotetrahydroazolopyridines and 2-aminohexahydroazolopyridines have been prepared by bromine-mediated addition of protected guanidine or urea to hydropyridine derivatives. The pH-dependent regioselective cleavage of the resulting aminal function led to the 2-aminoazole products III. The yields of the bicycles of type II, and their conversion into azoles III depends on the electronic properties of the substituents on the nitrogen of the tetrahydopyridine.     

Photochemical reactivity of trifluoromethyl aromatic amines: the example of 3,5-diamino-trifluoromethyl-benzene (3,5-DABTF)

This work presents the application of an on-line photoreactor to the detection of 3,5-diamino-trifluoromethyl-benzene (3,5-DABTF) in aqueous solutions. When irradiated at 310 nm, this compound is defluorinated to 3,5-diaminobenzoic acid by a nucleophilic substitution of the fluoride by water. Concomitantly, defluorination intermediates polymerize through amide bonds to give dark-colored compounds. We take advantage of the photocatalyzed defluorination and the subsequent decrease in pH to develop an original and specific photoreactor. Continuous recording of pH and temperature in the outlet of the reactor by a dual electrode gives us an opportunity to optimize the system. In the photoreactor, 3,5-DABTF is immediately and totally transformed as attested by the rapid drop of the flowing solution pH from 6.2 to 3.2 and the chromatographic analysis of the outgoing solutions. The detection remains effective from 1 to 1000 parts per million.     

The chemistry of trifluoromethyl imines and related acetals derived from fluoral

Trifluoromethylated nitrogen-containing molecules have been shown to have important biological effects and their synthesis is in the focus of the pharmaceutical industry. In the last few years, simple nitrogen derivatives of fluoral, i.e. imines, acetals and oxazolidines, have emerged as powerful building blocks to synthesize these target molecules and relevant precursors. This review summarizes the chemistry of these "N-derivatives of fluoral", with special highlight on the syntheses of peptidomimetic units (amino alcohols, amino acids...) and heterocycles (piperidines, beta-lactams...).     

Trichlorosilane isocyanate as coupling agent for mild conditions functionalization of silica-coated surfaces

Despite the importance of the isocyanate group in chemistry, very few examples of isocyanate-modified silicas have been reported, and all of the strategies described so far led to partial or total hydrolysis or condensation of the isocyanate group. By synthesizing trichlorosilane isocyanate as the coupling reagent, we show that oxidized silicon wafers are successfully modified with the isocyanate group. Our method is achieved in mild conditions, at low temperature, without side-reactions and allows the formation of a self-assembled monolayer (SAM) of isocyanates. The isocyanate group then offers a flexible way to further functionalize silica substrates with different nucleophiles, due to its high and specific reactivity.     

Critical contribution of nonlinear chromatography to the understanding of retention mechanism in reversed-phase liquid chromatography

The retention of most compounds in RPLC proceeds through a combination of several independent mechanisms. We review a series of recent studies made on the behavior of several commercial C18-bonded stationary phases and of the complex, mixed retention mechanisms that were observed in RPLC. These studies are essentially based on the acquisition of adsorption isotherm data, on the modeling, and on the interpretation of these data. Because linear chromatography deals only with the initial slope of the global, overall, or apparent isotherm, it is unable fully to describe the complete adsorption mechanism. It cannot even afford clues as to the existence of several overlaid retention mechanisms. More specifically, it cannot account for the consequences of the surface heterogeneity of the packing material. The acquisition of equilibrium data in a wide concentration range is required for this purpose. Frontal analysis (FA) of selected probes gives data that can be modeled into equilibrium isotherms of these probes and that can also be used to calculate their adsorption or affinity energy distribution (AED). The combination of these data, the detailed study of the best constants of the isotherm model, the determination of the influence of experimental parameters (e.g., buffer pH and pI, temperature) on the isotherm constants provide important clues regarding the heterogeneity of the adsorbent surface and the main properties of the adsorption mechanisms. The comparison of similar data obtained for the adsorption of neutral and ionizable compounds, treated with the same approach, and the investigation of the influence on the thermodynamics of phase equilibrium of the experimental conditions (temperature, average pressure, mobile phase composition, nature of the organic modifier, and, for ionizable compounds, of the ionic strength, the nature, the concentration of the buffer, and its pH) brings further information. This review provides original conclusions regarding retention mechanisms in RPLC.