A TEM and XRD study of the SbNbSe3 misfit layer structures

Two new misfit layer structures have been synthesized within the Sb-Nb-Se system. Powder X-ray diffraction and electron microscopy techniques (electron diffraction, HREM, XEDS) have been used to determine the nature of their structure. According to TEM and XEDS data (for more than 15 crystals studied) both phases are monolayer type, i.e. (SbSe)1+delta (NbSe2). Electron microscopy reveals a composite modulated structure that consists of the periodical intergrowth of a pseudotetragonal SbSe layer, denominated as Q, and a pseudohexagonal layer NbSe2, denominated as H. Both layers fit along b, stack along c and do not fit along a (misfit) giving rise to an incommensurate modulation along this direction. The two phases differ in the symmetry of the Q layers being in one case orthorhombic (for delta = 0.17) and monoclinic in the other (for delta = 0.19). After the characterization of the sample by electron microscopy the unit cells of the basic layers could be refined for both phases by powder X-ray diffraction: aQ = 5.824(2) A, bQ = 5.962(5) A, cQ = 23.927(6) A, alpha = 90 degrees, beta = 90 degrees and gamma = 90 degrees and aH = 3.415(5) A, bH = 5.962(6) A,, cH = 11.962(1) A, alpha = 90 degrees, beta = 90 degrees and gamma = 90 degrees for the orthorhombic phase; aQ = 5.844(2) A, bQ = 5.981(1) A, cQ = 23.919(5) A, alpha = 90 degrees, beta = 90 degrees and gamma = 96.00(3)degrees and aH = 3.439(1) A, bH = 5.994(2) A, cH = 11.956(3) A, alpha = 90 degrees, beta = 90 degrees and gamma = 90 degrees for the monoclinic phase. The phase with the monoclinic Q-sublattice often appears as twinned crystals. The more abundant crystals are disordered intergrowths of both monolayer phases.     

Photochemical two-electron reduction of a dinuclear ruthenium complex containing a bent tetraazatetrapyridopentacene bridging ligand: pushing up the LUMO for storing more energy

The synthesis and characterization of a ditopic bridging ligand, 9,12,21,22-tetraazatetrapyrido[3,2-a:2',3'-c:3″2″-m:2',3'-o]pentaphene (tatppα) and its dinuclear ruthenium complex, [(phen)(2)Ru(tatppα)Ru(phen)(2)][PF(6)](4) (1(4+)), are described. The tatppα ligand is structurally very similar to 9,10,20,33-tetraazatetrapyrido[3,2-a:2',3'-c:3″,2″-l:2',3'-n]pentacene (tatppβ), except that, instead of a linear tetraazapentacene backbone, tatppα has an ortho (or α) substitution pattern about the central benzene ring, leading to a 120° bend. Complex 1(4+) shows tatppα-based reductions at -0.73 and -1.14 V vs Ag/AgCl/saturated KCl and has an absorption spectrum showing the typical Ru(II) dπ → phen-like π* metal-to-ligand charge-transfer transition centered at ～450 nm. In acetonitrile, visible-light irradiation of 1(4+) in the presence of triethylamine leads to two sequential changes in the absorption spectra, which are assigned to the formation of the one- and two-electron-reduced species, with the electrons stored on the tatppα ligand. These assignments were made by comparison of the spectral changes observed in 1(4+) upon stoichiometric chemical reduction with cobaltocene and by spectroelectrochemical analysis. Significantly, DFT calculations are very predictive of the optical and reductive behavior of the tatppα complex relative to the tatppβ complexes and show that modeling is a useful tool for ligand design. The chemical reactivity and differential reflectance spectroelectrochemical data reveal that the reductions are accompanied by radical dimerization of the tatppα ligand to species such as σ-{1}(2)(6+), which is only slowly reversible upon exposure to air and may limit the complexe's 1(4+) utility for driving photochemical H(2) production.     

Hybrid organic-inorganic catalytic mesoporous materials with proton sponges as building blocks

Non-ordered organic-inorganic mesoporous hybrid materials with basic sites have been synthesized following a fluoride-catalysed sol-gel process at neutral pH and low temperatures that avoids the use of structural directing agents (SDAs). Proton sponges have been used as the organic builder of the hybrids, while the inorganic part corresponds to silica tetrahedra. The proton sponges are diamines that exhibit very high basicity and, after functionalization, have been introduced as part of the walls of the mesoporous silica by one-pot synthesis. Several hybrids with different organic loadings have been synthesized and characterized by gas adsorption, thermogravimetric and elemental analysis, solid state MAS-NMR and FTIR spectroscopy. These hybrids show high activity as base catalysts and can be recycled.     

Exploiting micelle-driven coordination to evaluate the lipophilicity of molecules

We present a systematic study based on the calculation of complexation constants between a Zn-complex solubilized in Triton X-100 micellar solutions and a series of linear mono- and dicarboxylic acids, under physiological pH conditions, that allowed the evaluation of the lipophilicity of these molecules. This empirical lipophilicity parameter describes conveniently the partition of organic molecules between hydrophobic microdomains and water. The results can be used to predict the lipophilicity of molecules with similar structure and allows the distinction of intrinsic contributions of the carboxylates and of the methylene groups to the lipophilicity of the molecule.     

Direct observation of nucleation and early stages of growth of GaN nanowires

We report direct observations of the nucleation and early stages of growth of GaN nanowires. The nanowires were formed by exposing Au+Ga droplets to ammonia. The formation process was observed in situ, and controlled in real time using an environmental transmission electron microscope. Observations show that nuclei forms only at thin region of the Au+Ga, following only the VLS mechanism. The droplet–nanowire interface in the initial stages is multi-faceted, and become planar during the growth. Chemical and structural analysis shows that the resulting nuclei are GaN with the wurtzite structure.     

I18 – the microfocus spectroscopy beamline at the Diamond Light Source

The design and performance of the microfocus spectroscopy beamline at the Diamond Light Source are described. The beamline is based on a 27 mm‐period undulator to give an operable energy range between 2 and 20.7 keV, enabling it to cover the K‐edges of the elements from P to Mo and the L₃‐edges from Sr to Pu. Micro‐X‐ray fluorescence, micro‐EXAFS and micro‐X‐ray diffraction have all been achieved on the beamline with a spot size of ～3 µm. The principal optical elements of the beamline consist of a toroid mirror, a liquid‐nitrogen‐cooled double‐crystal monochromator and a pair of bimorph Kirkpatrick–Baez mirrors. The performance of the optics is compared with theoretical values and a few of the early experimental results are summarized.     

New absorbing layers conditions for short water waves

We develop a new PML formulation for the linearized shallow-water equations including the Coriolis force. The construction process is based on the uncoupling of the velocity components with the depth of water. Then the damping effect is only applied to the propagative modes just as was formerly done by Nataf [1] to the linearized Euler equations to enforce the long-time stability. We assess numerically the performance of the new absorbing condition and we illustrate in particular that it is stable for long-time simulations.     

Improving bounds on flavor changing vertices in the two Higgs doublet model from B^0-\bar{B}^0 mixing

We find some constraints on the flavor changing vertices of the two Higgs doublet model, from the measurement. Although bounds from this observable have already been considered, this paper takes into account the role of a new operator not included previously, as well as the vertices , and . Using the Cheng–Sher parametrization, we find that for a relatively light charged Higgs boson (200–300 GeV), we obtain , while the parameter could have values up to about 50. In addition, we use bounds for and obtained from at next to leading order, and study the case where the only vanishing vertex factors are the ones involving quarks from the first family. We obtain that is not sensitive to the change of the parameter , while .     

Effects of Ascorbic Acid on Arenediazonium Salts Reactivity: Kinetics and Mechanism of the Reaction

We have examined the kinetics and mechanism of dediazoniation of o‐, m‐ and p‐methylbenzenediazonium (ArN) tetrafluoroborate in the presence of ascorbic acid (H₂A) at different pHs by combining spectophotometric (VIS‐UV), high performance liquid chromatography (HPLC), and polarographic measurements. Kinetic data show that, at low pH, observed rate constants increase linearly with increasing ascorbic acid concentration, but the saturation kinetics observed at higher pH suggest the formation of a transient diazo‐ether complex preceding the slow step of the reaction. Experimental evidence for the formation of such a complex was obtained from a competitive coupling reaction with the Na salt of `2‐naphthol‐6‐sulfonic acid' and by titration of ascorbic acid (H<sub>2</sub>A) with the arenediazonium ions (electrochemical measurements). HPLC Analysis of dediazoniation products indicates that, in the absence of H<sub>2</sub>A, only the heterolytic phenol derivative, ArOH, is formed quantitatively, in keeping with the predictions of the D<sub>N</sub>+A<sub>N</sub> mechanism. In the pH 2 – 4 range and in the presence of H<sub>2</sub>A, reduction products (ArH) are obtained in addition to heterolytic products (ArOH), corroborating that certain biological reducing agents like ascorbate (HA<sup>−</sup>) are capable of inducing reductive fragmentation of ArN into aryl radicals. All evidence is consistent with two competitive reaction pathways, the thermal decomposition of ArN, and a rate‐limiting decomposition of the transient diazo ether `complex', formed during the reaction of ArN with HA⁻ in a rapid pre‐equilibrium step.