排序方式: 共有74条查询结果,搜索用时 15 毫秒
31.
Ben Sdira S Felix CP Giudicelli MB Seigle-Ferrand PF Perrin M Lamartine RJ 《The Journal of organic chemistry》2003,68(17):6632-6638
Chiral p-tert-butylcalix[4]arenes functionalized at the lower rim with amino acid residues have been prepared. The (1)H and (13)C NMR spectra indicate that the macrocycles preferably adopt a cone conformation. Calix[4]arenes bearing amino acid moieties were prepared as a class of receptors selective for anions that are bound through hydrogen bonding with the NH group. The association constants are dependent on the nature of the substituents at the lower rim. Derivative 9 shows the strongest complexation and the largest selectivity for N-tosyl-(L)-alaninate. Finally, a preliminary X-ray crystal study of the difunctionalized receptor 6f shows the "flattened cone" conformation in the solid state. 相似文献
32.
Mohamed Al Amine Benhacine Malika Hamadne Sofiane Bouacida Hocine Merazig 《Acta Crystallographica. Section C, Structural Chemistry》2016,72(3):243-250
The oxalate dianion is one of the most studied ligands and is capable of bridging two or more metal centres and creating inorganic polymers based on the assembly of metal polyhedra with a wide variety of one‐, two‐ or three‐dimensional extended structures. Yellow single crystals of a new mixed‐metal oxalate, namely catena‐poly[[diaquasodium(I)]‐μ‐oxalato‐κ4O1,O2:O1′,O2′‐[diaquairon(III)]‐μ‐oxalato‐κ4O1,O2:O1′,O2′], [NaFe(C2O4)2(H2O)4]n, have been synthesized and the crystal structure elucidated by X‐ray diffraction analysis. The compound crystallizes in the noncentrosymmetric space group I41 (Z = 4). The asymmetric unit contains one NaI and one FeIII atom lying on a fourfold symmetry axis, one μ2‐bridging oxalate ligand and two aqua ligands. Each metal atom is surrounded by two chelating oxalate ligands and two equivalent water molecules. The structure consists of infinite one‐dimensional chains of alternating FeO4(H2OW1)2 and NaO4(H2OW2)2 octahedra, bridged by oxalate ligands, parallel to the [100] and [010] directions, respectively. Because of the cis configuration and the μ2‐coordination mode of the oxalate ligands, the chains run in a zigzag manner. This arrangement facilitates the formation of hydrogen bonds between neighbouring chains involving the H2O and oxalate ligands, leading to a two‐dimensional framework. The structure of this new one‐dimensional coordination polymer is shown to be unique among the AIMIII(C2O4)2(H2O)n series. In addition, the absorption bands in the IR and UV–Visible regions and their assignments are in good agreement with the local symmetry of the oxalate ligand and the irregular environment of iron(III). The final product of the thermal decomposition of this precursor is the well‐known ternary oxide NaFeO2. 相似文献
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34.
Frederic Dehoux Yannick Lecocq Sofiane Benhamadouche Remi Manceau Laurent-Emmanuel Brizzi 《Flow, Turbulence and Combustion》2012,88(1-2):77-100
The present paper focuses on the application of the elliptic blending approach to the modeling of turbulent heat fluxes, in order to account for the influence of solid boundaries. The analytical justification of the extension to the temperature–pressure gradient correlation term of this approach, originally applied to the velocity–pressure gradient, is given. The assumption of weak equilibrium enables the derivation of two new algebraic flux models valid down to the wall. It is shown, with both a priori tests and computations in forced and mixed convection regimes, that the predictions of the streamwise heat-flux and the temperature variance are significantly improved by the use of elliptic blending. A particular attention is devoted to the issue of the modeling of the correlation length scale involved in the elliptic blending for the heat fluxes, which is shown to have a significant influence on the predictions. 相似文献
35.
Marián Slodi?ka Sofiane Dehilis 《Journal of Computational and Applied Mathematics》2009,231(2):715-724
A semilinear reaction-diffusion problem with a nonlocal boundary condition is studied. This paper presents a new and very easy implementable numerical algorithm for computations. This is based on a suitable linearization in time and on the principle of linear superposition. Any method for the space discretization (FEM was taken in this analysis) can be chosen. The derived algorithm is implicit and it does not need any iteration scheme to get a solution with the nonlocal boundary condition. Stability analysis has been performed and the optimal error estimates have been derived. Numerical results have been compared with other known techniques. 相似文献
36.
Most of the models proposed in literature for binary diffusion coefficients of solids in supercritical fluids are restricted to infinite dilution; this can be explained by the fact that most of experimental data are performed in the dilute range. However some industrial processes, such as supercritical fluid separation, operate at finite concentration for complex mixtures. In this case, the concentration dependence of diffusion coefficients must be considered, especially near the upper critical endpoint (UCEP) where a strong decrease of diffusion coefficients was experimentally observed. In order to represent this slowing down, a modified version of the Darken equation was proposed in literature for naphthalene in supercritical carbon dioxide. In this paper, the conditions of application of such a modelling are investigated. In particular, we focus on the order of magnitude of the solubility of the solid and on the vicinity of the critical endpoint. Various equations proposed in literature for the modelling of the infinite dilution diffusion coefficients of the solutes are also compared. Ten binary mixtures of solids with supercritical carbon dioxide were considered for this purpose. 相似文献
37.
Karray A Zarai Z Gargouri Y Verger R Bezzine S 《Journal of colloid and interface science》2011,363(2):620-625
The interfacial kinetic and binding data for the pancreatic and intestinal sPLA2 from bird and mammals show that these enzymes have dramatically different ability to bind and hydrolyse phospholipids. The main conclusions from our experimental data indicate that phosphatidylcholine monolayers (PC), in contrast to phosphatidylethanolamine (PE) and phosphatidylglycerol (PG), were resistant to the hydrolysis by human intestinal sPLA2. Conversely, chicken intestinal sPLA2 was found to be able to hydrolyse all the phospholipids tested, including PC. The experiments show also that the interfacial penetrating ability of chicken sPLA2 (from intestine and pancreas) was higher than their mammalian's orthologs. This observation is confirmed by the activity of pancreatic chicken PLA2 measured on PC film showing that the interfacial pressure window that permits sPLA2 activity was very large, between 5 and 20 dynes cm(-1), compared with the porcine pancreatic sPLA2-IB which was inactive at pressure above 15 dynes cm(-1). In trying to establish a structure-function relationship, we examined the surface electrostatic potentials of the various sPLA2 from chicken and mammals. We reported in this study that the binding, orientation and persistence of sPLA2 at the lipid-water interface is probably governed by the electrostatic and hydrophobic forces operative at this surface. These variations argue strongly that these enzymes are not isoforms and that they are expected to have functions other than the release of lipid mediators for the biosynthesis of the eicosanoids. 相似文献
38.
The electrical and rheological properties of an ethylene vinyl acetate (EVA) copolymer filled with bamboo charcoal were investigated. The composites were prepared by melt process in an internal batch mixer. Size distribution analysis showed that d(50) and d(90) values of the bamboo charcoal particles are 12.7 and 40 μm, respectively, with a mean diameter of 22 μm. Scanning electron microscopy proved that the particles of bamboo charcoal present a rectangular shape. The electrical percolation threshold was observed at 0.18 volume fraction (35 wt%) of bamboo. Beyond the percolation threshold, a considerable increase in electrical properties was observed up to a limit value of 10-2 S/m. The rheological percolation was studied from different rheological models. As a result, the rheological percolation threshold was observed at 0.3 volume fraction (50 wt%) of bamboo charcoal contents. So, the electrical percolation occurs before the rheological percolation. This is principally due to the filler’s characteristics such as the specific surface area, the aspect ratio, and the surface properties. Finally, the bamboo charcoal confers high electrical properties to the EVA composite without inducing strong changes in its viscoelastic properties. 相似文献
39.
Rafika Bouchene Sofiane Bouacida 《Acta Crystallographica. Section C, Structural Chemistry》2019,75(2):120-127
Two new coordination polymers, namely poly[[(3‐aminopyrazin‐4‐ium‐2‐carboxylate‐κ2N1,O)di‐μ‐chlorido‐cadmium(II)] monohydrate], {[CdCl2(C5H5N3O2)]·H2O}n, (1), and poly[2‐amino‐3‐carboxypyrazin‐1‐ium [(3‐aminopyrazine‐2‐carboxylato‐κ2N1,O)di‐μ‐chlorido‐cadmium(II)] monohydrate], {(C5H6N3O2)[Cd(C5H4N3O2)Cl2]·H2O}n, (2), have been synthesized from the reaction of cadmium(II) chloride and 3‐aminopyrazine‐2‐carboxylic acid (Hapca) under mild conditions in acidic media. The two coordination polymers have been characterized by single‐crystal X‐ray diffraction and show chloride‐bridged zigzag chains with octahedrally coordinated metal ions, where Hapca acts as a bidentate ligand via the π‐conjugated N atom and a carboxylate O atom. The chains are further interconnected via noncovalent interactions into three‐dimensional supramolecular networks. The dominant H…O and H…Cl interactions for both compounds were quantified using Hirshfeld surface analysis. The thermal stability and topological analysis of the two‐dimensional networks of (1) and (2) are also discussed. 相似文献
40.
In this article we study the problem where \(\Delta ^{2}:=\Delta (\Delta )\) is the biharmonic operator, \(a,b>0\) are constants, \(N\le 7,\) \(p\in (4,2_{*})\) for \(2_{*}\) defined below, and \(V(x)\in C(\mathbb {R}^{N},\mathbb {R})\). Under appropriate assumptions on V(x), the existence of least energy sign-changing solution is obtained by combining the variational methods and the Nehari method.
相似文献
$$\begin{aligned} \Delta ^{2}u-\left( a+b\int _{\mathbb {R}^{N}}\left| \nabla u\right| ^{2}dx\right) \Delta u+V(x)u=\left| u\right| ^{p-2}u\ \text { in }\mathbb {R}^{N}, \end{aligned}$$