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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 34 (1978), S. 969-974 
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: A procedure is given for estimating the variance ratio σ21/σ211, of two models of a crystal structure where σ21 and σ211 are the variances of the weighted (|Fo| - |Fc|) distributions for models I and II, respectively, and |Fo| and |Fc| are the observed and calculated structure factors. The procedure provides a statistical test of the hypothesis that the above ratio is unity for a given pair of models and, thus, whether one of the models offers a significantly better description of the crystal structure. The method is applicable to crystallographic problems in which each of two or more structural models is expressed in terms of the same set of parameters. It has been applied to a number of atom-assignment problems, and the results are compared with those from Hamilton's R-factor ratio test.
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 27 (1971), S. 296-296 
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Comment on the preceding Short Communication.
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  • 3
    Electronic Resource
    Electronic Resource
    Oxford [u.a.] : International Union of Crystallography (IUCr)
    Acta crystallographica 52 (1996), S. 1928-1930 
    ISSN: 1600-5759
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: In 1,1,1-tris(4-hydroxyphenyl)ethane, C20H18O3 (1), monoclinic la, a = 7.9781 (10), b = 18.558 (3), c = 11.1995 (13) Å, \beta–101.668 (9)°, with Z = 4, each of the hydroxyl groups acts as both a donor and an acceptor of hydrogen bonds of the type O—H...O. The molecules are thus connected into square nets, graph set R^4_4(38), pairs of which are then interwoven; the nets are themselves interconnected by further hydrogen bonds to give a continuous three-dimensional network. In 1,1,1-tris(4-hydroxyphenyl)ethane-l,4-diazabicyclo[2.2.2]octane–water (1/1/1), C20H18O3.C6H12N2.H2O (2), triclinic P\overline 1, a = 10.421 (2), b = 10.734 (2), c = 10.9756 (13) Å, \alpha = 76.645 (12), \beta 74.513 (11), \gamma = 89.305 (13)°, with Z = 2, the water molecules are linked to the trisphenol and the diamine units by O—H...O and O—H...N hydrogen bonds, respectively, to form a linear aggregate. These aggregates are linked into chains by the formation of O—H...N hydrogen bonds between the trisphenol and the neighbouring diamine; however, alongside hydrogen-bond formation between the trisphenol and the diamine, there is also partial transfer of a proton, so that the intra-chain links between trisphenol and diamine units are in fact a mixture of O—H...N and N—H...O hydrogen bonds. These chains in the [101] direction are cross-linked by further O—H...O hydrogen bonds, involving only trisphenol O atoms, which form chains in both the [010] and [001] directions, thus generating a continuous three-dimensional network. Adduct (3), 1,1,1-tris(4-hydroxyphenyl)ethane–piperazine (4/3), (C20H18O3)4.(C4H10N2)3, triclinic P\overline 1, a = 12.5049 (11), b = 12.7046 (10), c = 14.6226 (9) Å, \alpha = 113.738 (6), \beta = 100.839 (6), \gamma = 102.438 (7)°, with Z = 1, has two independent trisphenol molecules in general positions and three independent piperazine molecules lying about centres of inversion. The five independent components of the asymmetric unit are linked together by means of three O—H...N and one O—H...O hydrogen bonds to form an open aggregate containing no hydrogen-bonded rings. These aggregates are connected into three sets of interlinked chains in the [010], [001] and [\overline 110] directions: the [010] and [\overline 110] chains employ only O—H...O hydrogen bonds, while the [001] chains employ both O—H...N and N—H...O hydrogen bonds. Additionally, the inversion symmetry at each piperazine unit gives rise to further interlinks.
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  • 5
    Electronic Resource
    Electronic Resource
    Oxford [u.a.] : International Union of Crystallography (IUCr)
    Acta crystallographica 52 (1996), S. 2520-2524 
    ISSN: 1600-5759
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Oxford [u.a.] : International Union of Crystallography (IUCr)
    Acta crystallographica 52 (1996), S. 3009-3013 
    ISSN: 1600-5759
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Type of Medium: Electronic Resource
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