Structural arrangements of the ternary metal boride carbide compounds MB2C4 (M=Mg, Ca, La and Ce) from first-principles theory

The structural arrangements of the ternary metal borocarbides MB₂C₄ (M=Mg, Ca; La and Ce) are investigated using density-functional theory (DFT) calculations within the generalized gradient approximation (GGA). Results indicate that these compounds adopt a layered structure consisting of graphite-like B₂C₄ layers alternating with metal sheets. Within the hexagonal layers, the coloring with the -C-C-C-B-C-B- sequence is energetically more stable than that with the -C-C-C-C-B-B- one. The electronic structures of these compounds, mainly determined by the B₂C₄ sheets, can be rationalized with the simple valence electron distribution M²⁺[B₂C₄]²⁻xe⁻, with the metals essentially acting as two-electron donors with respect to the boron-carbon network, the other x electrons remaining in the relatively narrow d and/or f bands of the metals. Accordingly, MB₂C₄ are narrow band-gap semiconductors (ΔE≈0.2-0.4 eV) with M=Mg and Ca. On the other hand, with M=La and Ce, the compounds are conducting with a relatively high density of states at the Fermi level predominantly metal in character with substantial B/Cπ* antibonding state admixture.     

Crystallographic study of interaction between adspecies on metal surfaces

The interaction among adsorbed atoms and molecules (adspecies) on metal surfaces plays a decisive role in catalytic reactions. Such interaction may cause structural changes of the local adsorption geometry which, together with spectroscopic and energetic data, may afford useful physical and chemical insights into the basic mechanisms of surface processes. When the adsorption geometry of a single adspecies is considered as a function of coverage, a deeper understanding of the nature of the adsorbate-substrate bonding can be obtained. Depending on the adsorbate coverage, the magnitude of adsorbate-induced relaxations and reconstructions vary widely. Occasionally, chemisorption systems transform gradually into adsorbate-substrate compounds, such as oxides, nitrides, hydrides, and sulfides. For the case of adsorption of different adspecies, coadsorption, structural data can make a vital contribution to our understanding of reaction intermediates, the promotion effect in heterogeneously catalyzed reactions, and the formation of ultra-thin compound films.     

Diffraction and XPS studies of Cu complexes of intercalated compounds of α-zirconium phosphate. I: Derivation of structural models from powder diffraction data

Reasonable structural models are proposed for the Cu complexes of two intercalated compounds of -Zr(HPO₄)₂·H₂O (-ZrP): one with 1,10-phenanthroline (A) and the other with 2,2-bipyridyl (B). The models are derived from considerations of the unit-cell geometry changes undergone by -ZrP upon insertion of, first, the organic molecules and then copper, in accordance with a set of general criteria of structural analysis called the comparison method.     

Thermodynamic Aspects of the Increased Thermal Stability of Silicon by Doping with Transition or Rare-earth Metals

Summary. The main reason of the degradation of silicon monocrystals at heating is a structural transformation connected with a partial transition of the diamond-like structure into the structure of white tin. The reason for this transformation being observed under high pressures is the appearance of stress zones at the boundaries of variously oriented crystal microvolumes due to heat expansion anisotropy. The high stress concentration in the microvolumes provides sufficient pressure for the indicated phase transformation which results in the observed degradation of the electrophysical properties of silicon. The prevention of the structural transformation is considered to be possible by doping of Si by transition or rare-earth metals which increases the interatomic energy and decreases the thermal expansion coefficient. The choice of the doping additions is based on the bonding energy and the charge density calculated for a system of non-polarised ionic radii. The technology to increase the thermal stability of silicon has been patented^#. Patent of Russia, No 2094904, 13/10/1995     

Crystal and Molecular Structures of (E)-(2-Phenylethenyl)cyclooctatetraene and (E)-(2-(4-Nitrophenyl)ethenyl)cyclooctatetraene. Conformations of Substituted Cyclooctatetraenes

The single crystal x-ray structures of (E)-(2-arylethenyl)cyclooctatetraenes 1 and 2 have been determined. The nitro group in 2 does not influence the geometry of the COT ring relative to that of 1. The influence of various substituents on several structural parameters of the COT ring are analyzed in terms of steric/electrostatic and electronegativity effects of the substituent.     

N-丙烯酰-2-氯吩噻嗪、N-丙烯酰-2-乙酰基吩噻嗪单体和聚合物的合成以及荧光行为的研究

含芳杂环的丙烯酰类功能性单体及其聚合物的合成一直吸引人们的兴趣,原因之一是由于这类单体可用一般的自由基引发剂引发聚合,这类聚合物所表现出的光化学、电化学性质甚被瞩目。如含吩噻嗪的丙烯酰类衍生物可以和缺电子的化合物形成电荷转移     

Die Kristallstruktur von TlFe3Te3

TlFe₃Te₃ is hexagonal, space groupP6₃/m–C _2h ⁶ ,a=9.350(2) Å,c=4.2230 (7) Å,Z=2. Iron and tellurium atoms occupy the positions 6 (h) withx=0.170,y=0.149 andx=0.046,y=0.357 respectively. Thallium atoms are situated in 2 (d). The structure was determined on the basis of single crystal data obtained form a four circle diffractometer. Refinement yielded andR-value of 4.8% for an asymmetric set of 267 reflections. TlFe₃Te₃ is a new structure type. The structure and its relations to the Mn₅Si_3–, the Nb₃Te₄-and the Tl _x V₆S₈-type are discussed.

     

Structural characterisation and ACE-inhibitory activities of polysaccharide from Gastrodia elata Blume

The structural properties and Angiotensin-I converting enzyme (ACE) inhibition activities of a polysaccharide (PGE) extracted from Gastrodia elata Blume were investigated. PGE was extracted using hot water and purified by Sephadex G-200 followed by ultra-filtration. The structural characterisation of PGE was analysed by FT–IR, NMR spectroscopy, specific rotation determination, periodate oxidation-smith degradation, methylation analysis, GC–MS and Congo red test. The results revealed that PGE was composed by glucose, with an average molecular weight of 1.54 × 10³ kDa. The structure of PGE was 1→3 and 1→4,6-branched-glucopyranose that had a linear backbone of (1 → 4)-linked-d-glucopyranose (Glcp). ACE-inhibitory activity results showed that PGE was efficient to inhibit ACE and the IC₅₀ value was 0.66 mg/mL.     

SYNTHESIS AND CHARACTERIZATION OF AROMATIC LIQUID CRYSTALLINE COPOLYESTERS WITH REGULAR SEQUENCE STRUCTURE

Several novel aromatic liquid crystalline copolyesters with regular sequence structure were prepared by melt Sehotten-Baumann polycondensation via complex monomer. Polarizing microscope with hot stage, thermal analysis and X-ray diffraction were used to investigate the structure and properties of the copolyesters. The effects of structural units, such as flexible spacer, noncolinear meta-linked phenylene unit, crankshaft unit, kink with flexible bridging unit and various substituted benzene rings on melting temperature of aromatic copolyesters were studied and discussed on the basis of crystalline structure of the polymers.     

双帽羧桥三核钨(Ⅳ)原子簇化合物的结构研究

用新的合成途径,以Na_2WO_4·2H_2O为原料,Zn粉为还原剂,合成了三个簇合物(1)[W_3O_2(CH_3CO_2)_6(H_2O)_3]ZnBr_4·8H_2O,(2)H(W_3O_2(CH_3CO_2)_9]·5H_2O,(3)H_2Na_3[W_3O(CCH)_3(CH_3CO_2)_6(H_2)O][H_2W_(12)O_(40)]·13.5H_2O。在测定晶体结构和综合有关文献基础上,说明了反应过程,并进行了EHMO量化计算,探讨了该类型化合物的簇骼成键性质和相关的物理,化学性质。