Structural, textural, and electronic properties of a nanosized mesoporous ZnxTi1-xO2-) solid solution prepared by a supercritical drying route

Mesoporous nanosized TiO2 and Zn(x)Ti(1-x)O(2-x) solid solution having a Zn content below 10 mol % with a particles size between 13 and 17 nm are prepared by a template-free sol-gel method followed by high-temperature supercritical drying in 2-propanol. The structural, textural, and electronic properties of the obtained nanomaterials are methodically investigated by using XRD, SEM, TEM, ED, HREM, EDX, ICP-OES, N(2) adsorption-desorption, Raman spectroscopy, and diffuse reflectance UV-vis spectroscopy. It is shown that the proposed synthesis technique leads to the formation of a Zn(x)Ti(1-x)O(2-x) solid solution based on the anatase crystal structure rather than a two-phase sample. High-resolution electron microscopy and electron diffraction indicate that the distribution of zinc atoms over the anatase structure does not lead to a considerable deformation of the crystal structure.     

Implementation of the finite field perturbation method in the CRYSTAL program for calculating the dielectric constant of periodic systems

The finite field approach has been implemented in the periodic ab initio CRYSTAL program and been used for calculating the dielectric constants of crystalline LiF and MgO (FCC structure) and BeO (wurtzite structure). To maintain the periodicity along the applied field direction, a "sawtooth" potential is used in conjunction with a supercell scheme. Supercells four to five times longer than the primitive cell in the direction of the applied field provide well-converged results. The influence of the computational parameters is discussed. An alternative scheme has also been implemented, for inner check, that consists of applying a static electric field to a slab of increasing thickness in the direction orthogonal to the surface; the dielectric response at the center of the slab is shown to converge rapidly to the bulk value evaluated with the sawtooth field. The method is accurate and permits the determination of nonlinear corrections to the dielectric constant. When used in conjunction with the local density approximation (LDA) scheme, it provides for the dielectric constant of the three above-mentioned compounds values close to those recently obtained with a time-dependent density functional theory approach.     

Synthesis of some isomeric 4,11-dihydro-5H-[1]benzoxocino[4,3-d] or [3,4-c]isoxazol-5-ones and isomeric 4,5,10,11-tetrahydrobenzo[5,6]-cycloocta[2,1-d] or [1,2-c]isoxazol-5-ones

The title compounds were prepared by acid-catalyzed cyclization of aryloxymethyl- or (2-arylethyl)-4-carboxymethylisoxazoles which in turn were synthesized from aryloxymethyl- or (2-arylethyl)isoxazole-4-carboxylic acids by Arndt-Eistert homologation.     

Copper sorption on a straw lignin: experiments and EPR characterization

Spectroscopic and physicochemical data, X-ray photoelectron spectroscopy (XPS), solid-state (13)C cross-polarization magic-angle-spinning NMR, GC/MS, specific surface area, site density, and surface acidity constants have been recorded or determined for a ligno-cellulosic substrate (LS) extracted from straw. Its copper(II)-bound ability has also been studied. The LS solid that exhibits two types of binding sites, carboxylic and phenolic groups, has a great affinity for copper(II), with a maximum of adsorption at around 4 mg g(-1) as early as the pH reaches 6. The structural characterization of the Cu(II) surface complexes has been investigated using EPR spectroscopy. The CuO(4) chromophore of the inner-sphere surface complexes is a slightly distorted square. A surface oxidation occurred simultaneously, which leads to semiquinonic radicals. Furthermore, our empirical approach, which used a correlation between thermodynamics data and EPR parameters, has led to a surface complexation constant log beta equal to 12.6. This indicates that copper(II) surface complexes are relatively stable.     

Electroorganic synthesis. Reductive alkylation of functionalized 1,2-dithiole-3-thiones

Two-electron reduction of some substituted 1,2-dithiole-3-thiones 1 followed by alkylation of the dianionic intermediates leads through electrosynthesis to mixture of Z and E isomers of the corresponding substituted alkyl-(3-thioalkyl)-2-propenedithioates 2, 3 in satisfactory yield. The structure of those products was established by ¹³C and ¹H nmr and mass spectroscopy. The isomers ratios were determined by nmr spectroscopy.     

IR and theoretical studies of monocarbonyl Ni complexes formed by adsorption of CO at low pressure on silica-supported Ni(II) ions

This work reports on the reactivity of coordination vacancies of Ni(II) ions grafted onto the tridentate silica support (Ni(II)(3c) ions) with respect to CO used as a probe molecule. The adsorption of CO at 77 K in the 0.3 to 3.5 Pa CO pressure range is studied by FTIR on two samples differing in the dispersion of nickel. Quantum chemical calculations by the DFT method are performed to investigate, using a cluster approach, the binding of Ni to silica and, after CO adsorption, the geometry of the resulting carbonyl Ni complexes. Silica is modeled by using clusters composed of three types of monodentate ligands, SiO(-), SiOSi and/or SiOH, found on the surface of silica. This work is devoted to the monocarbonyl complexes. Whatever the sample, only one type of monocarbonyl is formed from Ni(II)(3c) ions. It is shown that the charge of the silica cluster is the major parameter influencing the CO IR frequency whereas the nature and the size of the silica cluster do not affect the CO bond length, confirming that local electrostatic interactions predominate. Only the 1- charged silica cluster Si(5)O(3)(-), composed of SiO(-), 2SiOSi fragments, respectively, reproduces the Ni[bond]O distances derived from EXAFS for the Ni(II)(3c) grafted site and gives CO frequencies in good agreement with the experimental values. It is shown that CO is stabilized by a magnetic transition from the (3)Ni(2+) triplet to the (1)Ni(2+) singlet state occurring upon adsorption.     

Compositional and 2H NMR studies of bis(benzene)chromium composites of mesoporous vanadium-niobium mixed oxides

New mesoporous niobium oxides with 5, 10, and 15 mol% vanadium(V) doped into the walls of the structure were synthesized by the ligand-assisted templating method with an octadecylamine template. These materials were characterized by XRD, XPS, EPR, elemental analysis, and nitrogen adsorption before being treated with excess bis(benzene)chromium to give new composites with an organometallic phase in the walls. All materials were also characterized by EPR, XRD, nitrogen adsorption, XPS, SQUID magnetometry, and elemental analysis. The materials with higher percentages of vanadium absorbed more bis(benzene)chromium, because this process depends largely on the electron transfer between the organometallic and the walls of the mesostructure and vanadium(V) is a stronger oxidant than niobium(V). Conductivity studies on these materials revealed that the ratio of Cr(0) to Cr(l) in the pores was more important than the absolute Cr loading level in governing electron transport properties but that increasing the V content led to more insulating behavior regardless of the Cr concentration. Solid-state 2H NMR studies on perdeuteriobenzene analogues of these composites showed the presence of the neutral and cationic Cr species in different ratios depending on the loading. Tumbling of these species was also slow on the NMR time scale, indicating that the charge-carrying Cr species are not rapidly moving through the pore channels of the mesostructure. This suggests that the walls of the structure may play a key role in charge transfer in these composites, contrary to what was previously believed.     

Study of support effects on the reduction of Ni2+ ions in aqueous hydrazine

We have studied the effect of silica of quartz-type on the reducibility of nickel acetate in aqueous hydrazine (80 degrees C, pH = 10-12) and metal particle formation. The obtained materials were characterized by X-ray diffraction, transmission electron microscopy, and thermodesorption experiments. With nickel acetate alone, the reduction was partial (45%) and a metal film at the liquid-gas interface or a powdered metal precipitate with an average particle size of 120 nm was obtained. In the presence of silica as the surfactant, the reduction of nickel acetate was total and the nickel phase deposited as a film on the support with an average particle size of 25 nm. Supported nickel acetate was also totally reduced. Crystallites of a mean particle size of about 3 nm were obtained. Decreasing the nickel content or increasing the hydrazine/nickel ratio decreased the metal particle size. Whiskers were formed for low nickel loadings. Hydrogen thermal treatment of the reduced phase showed that the organic acetate fragment, belonging to the precursor salt, still remained strongly attached to the nickel phase. The amount of the retained organic matrix depended on the metal particle size. Surface defects are suggested as active sites, which enhanced nickel ion reduction in the presence of silica as the surfactant or support. Metal-support interactions and the nucleation/ growth rate were the main factors determining the size and morphology of the supported metal particles formed. The organic matrix covered the reduced nickel phase.     

Geometric molecular similarity from volume-based distance minimization: Application to saxitoxin and tetrodotoxin

The geometric molecular dissimilarity between two molecules is defined as the difference between the volume of their union minus the volume of their intersection. This dissimilarity has the mathematical properties of a distance. This distance is minimized under all rotations and translations using a discrete Broyden, Fletcher, Goldfarb & Shanno (B.F.G.S.) algorithm. The optimal geometric superimposition of saxitoxin and tetrodotoxin is discussed. © 1995 by John Wiley & Sons, Inc.