Crystal structure and electronic spectrum of SnS2

It is shown that layered metal dichalcogenides are quasi-one-dimensional molecular crystals and form a new class of crystal structures — molecular close packed. Since the minimum structural unit in these crystals is a monomolecular layer, using the symbols employed in atomic close packing to describe them gives a mistaken representation of their structure and symmetry. A new system of notation is proposed which provides complete and exact information about the ordering of the atomic layers in different polytype modifications and about their symmetries. It is found that in molecular close packing and, especially, in tin disulfide, there is not one (as in atomic close packing), but two, simplest structures, 1T and 1H, containing one molecule each in a unit cell and, therefore, two series of superlattices constructed on their basis. An energy model is constructed for the natural superlattice in tin disulfide crystals and the electronic spectra of the 2H, 4H, and 9R polytype modifications are calculated in the Kronig-Penney approximation with rectangular potential barriers as superstructure perturbations of the simplest structures. These results make it possible to explain the observed complicated dependence of the band gaps of these crystals on polytype structure. Fiz. Tverd. Tela (St. Petersburg) 40, 1712–1718 (September 1998)     

Synthesis of oligosaccharides related to the HNK-1 antigen. 5. Synthesis of a sulfo-mimetic of the HNK-1 antigenic trisaccharide

2-Aminoethyl 3,6-di-O-sulfo-β-D-glucopyranosyl-(1→3)-β-D-galactopyranosyl-(1→4)-2-acetamido-2-deoxy-β-D-glucopyranoside, which is the sulfo-mimetic of the antigenic trisaccharide HNK-1, and the corresponding monosulfates, viz., 2-aminoethyl 3-O-sulfo-and 2-aminoethyl 6-O-sulfo-β-D-glucopyranosyl-(1→3)-β-D-galactopyranosyl-(1→ 4)-2-acetamido-2-deoxy-β-D-glucopyranosides, were synthesized. 2-Azidoethyl 2,4-di-O-benzoyl-β-D-glucopyranosyl-(1→3)-2,4,6-tri-O-benzoyl-β-D-galactopyranosyl-(1→ 4)-2-acetamido-3,6-di-O-benzyl-2-deoxy-β-D-glucopyranoside served as the common precursor for the sulfated trisaccharides. This compound was synthesized according to the [2+1] pattern from monosaccharidic precursors: 3,6-di-O-acetyl-2,4-di-O-benzoyl-D-glucopyranosyl trichloroacetimidate, allyl 2-O-benzoyl-4,6-O-benzylidene-β-D-galactopyranoside, and 2-azidoethyl 2-acetamido-3,6-di-O-benzyl-2-deoxy-β-D-glucopyranoside. The structures of the glycosyl donors and glycosylation conditions were optimized for the efficient synthesis of the glucosyl-β-(1→3)-galactose disaccharide block and its subsequent transformation into the target trisaccharide sequence. Dedicated to Academician V. A. Tartakovsky on the occasion of his 75th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1593–1607, August, 2007.     

Features of the oxidative polymerization of aminothiazole in the presence of iron(III) chloride

A new conjugated polymer with semiconducting characteristics-polyaminothiazole-was obtained by oxidative polymerization of 2-aminothiazole in the presence of iron(III) chloride. A reaction scheme is proposed on the basis of the data from IR, ESR, and electronic spectroscopy. It involves the formation of an intermediate aminothiazole-FeCl₃ complex with coordination of the initiator at the nitrogen atom and subsequent oxidation of another molecule of aminothiazole by this complex with the formation of a radical-cation, leading to polymerization. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 43, No. 2, pp. 91–95, March–April, 2007.     

Features of the Pulsed Magnetization Switching in a High-Coercivity Material Based on ε-Fe2O3 Nanoparticles

Physics of the Solid State - The magnetic structure of the ε-Fe2O3 iron oxide polymorphic modification is collinear ferrimagnetic in the range from room temperature to ~150 K. As the...     

Theoretical and experimental study of the electronic structure of tin dioxide

The electronic structure of tin dioxide has been theoretically studied within the linearized augmented plane wave method using the Wien2k program package. The total and local partial electron densities of states have been calculated. The X-ray emission K-spectrum of oxygen has been calculated. The X-ray absorption spectra of the tin M _4,5-edge and oxygen K-edge have been calculated by simulating the supercell and core hole. The calculated results have been compared with the experimental data obtained using synchrotron radiation.     

Study of sorption of platinum and palladium cyanometallate complexes as the key to understanding the mechanism of binding the [Au(CN)<Subscript>2</Subscript>]<Superscript>−</Superscript> anion with carbon adsorbents

Goldcarb WSC-207C GR activated carbons with platinum and palladium complexes adsorbed from aqueous solutions of K₂[Pt(CN)₄], K₂[Pt(CN)₆], and K₂[Pd(CN)₄] were studied by the methods of X-ray photoelectron spectroscopy and IR spectroscopy with Fourier transformation, and also by MALDI massspectrometry. Platinum and palladium cyanide complexes are not reduced onto surface of active charcoal while adsorption. A certain part of the [Pd(CN)⁴]²⁻ and [Pt(CN)₄]²⁻ anions directly bound to active centers on the activated carbon is oxidized more deeply and functions as particles-anchors, forming oligomers resembling Krogmann salt. A correspondence between the structure of the complex cyanometallate ion and a mode of its binding with active carbon is found. The adsorption of complex species with a linear structure or a square planar structure is defined by a possibility of the formation of donor-acceptor and metallophilic bonds. The mode of the anion [Au(CN)₂]⁻ binding with the surface of active carbon was considered.     

Tunneling resonances in structures with a two-step barrier

Electron transport through an asymmetric heterostructure with a two-step barrier N⁺GaAs/N⁻GaAs/Al_0.4Ga_0.6As/Al_0.03Ga_0.97As/N⁻GaAs/N⁺GaAs was investigated. Features due to resonance tunneling both through a size-quantization level in a triangular quantum well, induced by an external electric field in the region of the bottom step of the barrier (Al_0.03Ga_0.97As layer), and through virtual levels in two quantum pseudowells of different width are observed in the tunneling current. The virtual levels form above the bottom step or above one of the spacers (N⁻GaAs layer) as a result of interference of electrons, in the first case on account of reflection from the Al_0.4Ga_0.6As barrier and a potential jump at the Al_0.03Ga_0.97As/N⁻GaAs interface and in the second case — from the Al_0.4Ga_0.6As barrier and the potential gradient at the N⁻GaAs/N⁺GaAs junction, reflection from which is likewise coherent. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 10, 814–819 (25 May 1998)     

Effect of borax additives on the rheological properties of sodium hyaluronate aqueous solutions

The rheological properties of sodium hyaluronate aqueous solutions are studied, and the effect of borax additives on them is investigated. It is shown that, at low concentrations, sodium hyaluronate behaves as a typical linear polyelectrolyte in the limit of a high concentration of the salt in both a 0.1 M NaCl aqueous solution and a salt-free solvent. The addition of 1 mole of borax per base-mole of the polymer to the solution of sodium hyaluronate significantly decreases the specific viscosity of the solution if no salt is added and has practically no effect on the viscosity of the solution in 0.1 M NaCl. The viscosity of a semidilute solution of sodium hyaluronate without the added salt decreases as the shear rate is increased in the range 1.5–656 s^?1. With an increase in temperature, viscosity decreases and its dependence on shear rate becomes less pronounced. The same effect is exerted by small amounts of borax. The properties of salt-free solutions are explained by the presence of admixtures of low-molecular-mass ions in them that screen the Coulomb repulsion of charges linked to sodium hyaluronate chains, and the effect of borax may be rationalized by the screening effect of ions resulting from the hydrolysis of borax.