Electron effects of the substituents on the structure of 4,4′-dipyridylium dications

The crystal structure of N,N′-di(2-hydroxyethyl)-4,4′-dipyridylium diperchlorate C₁₄H₁₈Cl₂N₂O₁₀ is determined by X-ray diffraction at 293 K. The crystals are monoclinic, a = 6.446(1) Å, b = 14.479(3) Å, c = 9.912(1) Å, β = 99.24(2)°, space group P2₁/c, and Z = 2; 1562 reflections measured; and wR2 = 0.086 and R1 = 0.033 for 1209 reflections with F > 4σ(F). It is found that, in the absence of charge transfer from the counterion to the dication, the planarity of the dipyridylium skeleton of the molecule is primarily due to the conjugation between the lone electron pairs of the hydroxyl groups and the π system of the dication. This inference is supported by the intramolecular O?N contact (2.847 Å).     

Light harvesting analysis of a nano‐cylinder structure on crystalline silicon using the Mie scattering model

In this study, nano spheres of different diameters (d = 200 nm, 300 nm, and 400 nm) were applied in nano-sphere lithography with dry etching to construct nano-cylinder structures on the surface of crystalline silicon wafers. The structure was then bombarded by a Kaufman-type directive ion source to shape and modify the surface topography as pillar-like or cone-like structures. Finally, an AR-coating layer was deposited on the surface of the nano-cylinder structure. When the size of the nano-cylinder structure approached the wavelength of light, Mie scattering analysis was utilized to describe the optical properties formed by the structures. The results showed that the haze parameters could reach 87.8% in the visible region and 91.1% in the infra-red region.     

Numerical investigation of the effect of heat shield shape on the oxygen impurity distribution at the crystal–melt interface during the process of Czochralski silicon crystal growth

In this study, a numerical simulation is performed to investigate the effect of the shape of the heat shield on the oxygen concentration in the melt. The results show that the oxygen concentration in the melt can be significantly decreased by increasing the speed of the argon gas near the crucible wall. This can be achieved by enlarging the horizontal length of the heat shield. The oxygen concentration at the melt–crystal interface varies with the length of the crystal growth. In the initial stage, there is a significant decrease in the oxygen concentration as the growth length increases. There is also a significant reduction in the emission of oxygen from the crucible wall due to the lower melt depth and crucible temperature. The transportation of oxygen impurity towards the melt–crystal interface is suppressed by the vortex motion in the melt. When the crystal exceeds a certain length, the oxygen concentration in the melt–crystal interface starts to increase with increasing crystal length, due to the drop in vortex motion in the melt.     

X-ray diffraction analysis on the structure of the glasses in the system PbOSiO2

The structures of PbO·SiO₂ and 2PbO·SiO₂ glasses have been analyzed by use of X-ray diffraction data and pair function method. For PbO·SiO₂ glass, a model consisting of chains of PbO₃ pyramids and silicate chains showed good agreement with the observed RDF. For 2PbO·SiO₂ glass, the present authors reported previously a model in which chains of PbO₃ pyramids are connected with SiO₄ tetrahedra, while the chromatographic analyses of silicate anions by Götz et al. and Smart et al. showed that silicate anions are distributed from monomer to polymer in the glass. We reexamined the structure of this glass referring to these results. Three representative models containing isolated SiO₄, Si₄O₁₂ rings and (SiO₃)_n chains respectively as well as PbO₃ chains were constructed and the RDFs were calculated with changing structure parameters. These three models showed satisfactory agreement with the observed data, showing that silicate anions are distributed from monomer to polymer in 2PbO·SiO₂ glass and an increase of SiO₂ content leads to polymerization of silicate anions to longer chains up to PbO·SiO₂ composition, while the chains of PbO₃ pyramids remain unchanged.     

Three-dimensional structure of thymidine phosphorylase from E. coli in complex with 3′-azido-2′-fluoro-2′,3′-dideoxyuridine

The three-dimensional structures of thymidine phosphorylase from E. coli containing the bound sulfate ion in the phosphate-binding site and of the complex of thymidine phosphorylase with sulfate in the phosphate-binding site and the inhibitor 3′-azido-2′-fluoro-2′,3′-dideoxyuridine (N₃F-ddU) in the nucleoside-binding site were determined at 1.55 and 1.50 Å resolution, respectively. The amino-acid residues involved in the ligand binding and the hydrogen-bond network in the active site occupied by a large number of bound water molecules are described. A comparison of the structure of thymidine phosphorylase in complex with N₃F-ddU with the structure of pyrimidine nucleoside phosphorylase from St. Aureus in complex with the natural substrate thymidine (PDB_ID: 3H5Q) shows that the substrate and the inhibitor in the nucleoside-binding pocket have different orientations. It is suggested that the position of N₃F-ddU can be influenced by the presence of the azido group, which prefers a hydrophobic environment. In both structures, the active sites of the subunits are in the open conformation.     

Influence of Changes in Central Core on the Mesomorphic Properties of 4,4′-Dialkoxy-2,2′-Dihydroxybenzalazine and Derivatives

Two new series of compounds that have mesomorphic properties were synthesized: the 4,4′-dialkoxy-2,2′-dihydroxy-a-methylbenzalazines and the 4,4′-dialkoxy-2,2′-dihy-droxybenzalazines. The first series shows crystalline and smectic polymorphism.

A comparative study of the mesomorphic properties of these series with those of the 4,4′-dialkoxy-2,2′-dihydroxy-α,α'-dimethylbenzalazines was carried out to establish the influence of the central groups on the mesomorphic properties.     

Temperature dependence of the generation and decay of E′ centers induced in silica by 4.7 eV laser radiation

We report a study of the generation of silicon dangling bonds (E′ centers) induced in fused silica by 4.7 eV laser irradiation in the 10 < T < 475 K temperature range, carried out by in situ optical absorption spectroscopy. The generation of the defects, occurring by transformation of pre-existing precursors, results to be a thermally activated process, quenched below 150 K and with a 0.044 eV activation energy. At T > 200 K the induced defects undergo a post-irradiation decay due to their reaction with mobile H₂. The interplay between generation and annealing gives rise to a bell-shaped temperature dependence of the concentration of induced E′ centers, peaking at 250 K.     

Structural Influence on the Intensitiy Ratio of the T′ and T87Rb NMR Lines and the Superconductivity in Rb3C60

Abstract

The intensity ratio of the T′/T⁸⁷Rb NMR lines in Rb₃C₆₀ could be changed by different cooling rates after annealing at 380 °C. The results are discussed with respect to magnetically different surroundings of the Rb ions through the two standard orientations of the C₆₀ ^3-ions. Additionally, the superconducting volume fraction is influenced by the different cooling procedures.     

Crystal structure of diheterolevulosan II: α-d-Fructofuranose-β-d-fructopyranose 1,2′:2,1′ dianhydride and molecular mechanics calculations on diheterolevulosan II and IV with chair and boat conformations of the central 1,4-dioxane ring

C₁₂H₂₀O₁₀,M _r=324.28, orthorhombic,P2₁2₁2₁,a=9.4476(8),b=10.2247(8),c=13.5827(11) Å,V=1312.07(18) ų,Z=4,D _x=1.642 g cm^?3, MoKα radiation (λ=0.71073 Å)μ=1.4 cm^?1,F(000)=688,T=295 K, finalR=0.041 for 1089 reflections withI≥2.5σa(I). The molecule consists of aβ-d-fructopyranose and a α-d-fructofuranose ring which are linked by a 1,4-dioxane ring. The pyranose ring and the dioxane ring both have a chair conformation, and the furanose ring has an envelope conformation. All six hydroxyl groups act as donors in intermolecular hydrogen bonding. Each molecule is hydrogen-bonded to eight neighbor molecules by fourteen hydrogen bonds, forming a three-dimensional framework. The hydrogen bonds of varying geometry give rise to five sharp hydroxyl stretch vibrations in the infrared spectrum. In diheterolevulosan IV the central dioxane ring is a boat. Molecular mechanics calculations on diheterolevulosan II and IV having different boat and chair conformations of the dioxane ring show that the lowest energy conformations correspond to the conformations observed in the crystal structure. For comparison the calculations also have been applied to 1,4-dioxane having boat and chair conformations.     

On the influence of various pre-ageing treatments on the structure changes taking place at elevated temperatures in alloys of the type Al?Zn5?Mg1

Investigated were two alloys of Type Al-5 wt% Zn-1 wt% Mg, one of them contained additionally 0.2 wt% of Mn, Cr, Fe as well as Si (technical alloy), be means of hardness and TEM investigations.     

X-ray diffraction structure of the 1,1′-bis(diphenylphosphino)ferrocene (dppf)-bridged complex [(dppf)AgCl]2: An unexpected product from the reaction betweencis-(bpy)2RuCl2 and dppf in the presence of AgBF4

The reaction betweencis-Ru(bpy)₂Cl₂ (where bpy=bipyridine) and the diphosphine ligand 1,1-bis(diphenylphosphino)ferrocene (dppf) in the presence of AgBF₄ has led to the isolation of the title compound [Ag(dppf)Cl]₂. [Ag(dppf)Cl]₂ has been structurally characterized by X-ray diffraction analysis, which confirms the bridging mode adopted by the ancillary dppf ligand and the centrosymmetric nature of this molecule. Dimeric [Ag(dppf)Cl]₂ crystallizes in the triclinic space group ,a=11.426(1) Å,b=11.509(1) Å,c=12.786(1) Å, =68.96(2)°, =70.66(2)°, =71.24(2)°,V=1441(1) ų,Z=1,d _calc=1.608 g·cm^–3;R=0.0445,R _w=0.0566 for 4486 observed reflections withl>3(l)     

X-ray diffraction study of the first monomeric dioxygermylene Ge(OCH2CH2NMe 2)2 stabilized by two Ge ← N intramolecular coordination bonds: The influence of Ge ← N intramolecular coordination bonds on the structure and packing of molecules in crystals

The molecular and crystal structures of the first monomeric dioxygermylene Ge(OCH₂CH₂NMe ₂)₂ (I) stabilized by two Ge ← N intramolecular coordination bonds are determined by X-ray diffraction analysis. The Ge ← N bond lengths in two independent molecules are equal to 2.329(2), 2.337(2) and 2.324(2), 2.346(2) Å, respectively. The central germanium atom plays the role of a spiro atom between two five-membered heterocycles in the envelope conformation and adopts a trigonal-bipyramidal configuration with a lone electron pair in the equatorial position. Compound I has two planar-chiral isomers which crystallize jointly. Molecules in the crystal have a layered packing (molecular layers are perpendicular to the [001] direction) composed of four translationally independent equivalent layers. These layers are spaced at 1/4c intervals and displaced with respect to each other by approximately +0.35a, ?1/4b, ?0.35a, and +1/4b, respectively. The characteristic displacement of the layers and their proper symmetry (pba2) that differs from the symmetry of the crystal (space group P2₁2₁2₁) determine a large variety of pseudosymmetry operations.