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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 547-552 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Neutron powder diffraction experiments show that, in the presence of ice and at helium-gas pressures exceeding 0.28 GPa, a gas hydrate with the composition He(6+x)H2O is formed. Its host lattice closely resembles that of ice II. However, the stability range of the helium hydrate is considerably more extended as compared to that of ice II: helium hydrate melts and recrystallizes from the liquid and is more stable than ice III/IX or ice V, at least up to 0.5 GPa. The helium content increases with increasing-pressure/decreasing-temperature, but does not appear to follow an ideal solution behavior, as do the guests in some heavier noble-gas clathrates. The lattice parameters of helium hydrate are significantly different from those of ice II, while the atomic arrangement of the host lattice is very similar, showing full orientational order of the water molecules.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 0009-2940
    Keywords: Spirosilicates, zwitterionic ; Silicon, pentacoordinate ; Bond cleavage, Si - C ; Solid-state NMR, 29Si and 15N ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: New Zwitterionic λ51-Spirosilicates: Syntheses, Single-Crystal X-Ray Structure Analyses, and Solid-State NMR StudiesThe zwitterionic spirocyclic λ51-silicates bis[3,4,5,6-tetrabromo-1,2-benzenediolato(2-)][2-(pyrrolidinio)ethyl]silicate (5; and its monohydrate 5 · H2O) and bis[1,2-benzenediolato(2-)]1-[(dimethylammonio)methyl]silicate (6) were synthesized by various methods including Si - C bond cleavage reactions. The crystal structures of 5, 5·H2O, and 6 were investigated by X-ray diffarction. Furthermore, 5, 5·H2O, 6, and the related zwitterionic λ51-spirosilicates 1·1/4 CH3CN, 2·CH3CN, 3·CH3CN, and 4 were characterized by solid-state NMR spectroscopy (29Si and 15N CP/MAS). The pentacoordinate silicon atoms of 5, 5·H2O (two cystallographically independent zwitterions and two crystallographically independent water molecules), and 6 (two crystallographically independent zwitterions) are surrounded by four oxygen atoms and one carbon atom. The coordination polyhedrons around the silicon atoms of 5 and 6 can be described as distorted (5) or nearly ideal (6) trigonal bipyramids, the carbon atoms being in equatorial positions. 5 forms intramolecular and 6 intermolecular (→ for-mation of dimeric units) N - H…O hydrogen bonds. The co-ordination polyhedrons around the two crystallographically independent silicon atoms of 5·H2O can be described as a nearly ideal and slightly distorted square pyramid, respectively, the carbon atoms being in the apical positions. In the crystal lattice of 5·H2O, intermolecular N - H…O O - H…O hydrogen bonds between the zwitterions and water molecules are observed. The results obtained by X-ray diffraction and solid-state NMR spectroscopy are consistent for each compound studied.
    Additional Material: 7 Ill.
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  • 3
    ISSN: 0009-2940
    Keywords: Fluorosilicates, zwitterionic ; Pentacoordinate silicon ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: On the Existence of Zwitterionic λ5-(Ammonioorganyl)tetrafluorosilicates: Synthesis and Crystal and Molecular Structure of Tetrafluoro(pyrrolidiniomethyl)silicateThe zwitterionic tetrafluoro(pyrrolidiniomethyl)silicate (6) was synthesized by the reaction of trimethoxy(pyrrolidinomethyl)silane (7) with hydrogen fluoride in ethanol/hydro-fluoric acid (yield 83%). 6 crystallizes in the space group P1/c with two crystallographically distinct molecules in the asymmetric unit. In both molecules the pentacoordinate silicon atom is surrounded by four fluorine atoms and one carbon atom, the latter being in an equatorial position. The coordination polyhedron of the silicon atoms can be described as a slightly distorted trigonal bipyramid. The zwitterionic structure was also proved for dissolved 6 (solution in CD3CN, NMR-spectroscopic studies).
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
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