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  • International Union of Crystallography (IUCr)  (7)
  • 1995-1999  (7)
Collection
Publisher
Years
Year
  • 1
    Electronic Resource
    Electronic Resource
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 51 (1995), S. 48-54 
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 32 (1999), S. 11-14 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: A simple algorithm is proposed for quantifying and correcting geometric distortions introduced by fiber-optic tapers in an X-ray charge-coupled-device detector with direct taper coupling.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 54 (1998), S. 31-43 
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The organic salt hexamethylenetetramine sebacate presents a 1D modulation with strong satellite reflections. Single-crystal X-ray intensities of main reflections and satellites up to sixth order have been measured at room temperature and used to refine this incommensurate structure. Two incommensurate structure models of hexamethylenetetramine (hexamine) sebacate are proposed and compared. The first is based on displacive atomic modulation functions and the second one uses displacive modulation and occupational step functions. Both models require high-order harmonic terms to describe the strongly nonsinusoidal modulation components. The structural characteristics of hexamine sebacate are similar in the two models. The structure is a layer structure consisting of alternating sheets of hexamine and sebacic acid connected by H bonds. Each layer of sebacic acid molecules consists of close-packed aliphatic chains with their axis slightly tilted with respect to the layer normal. The chains form alternating areas, each of which is characterized by a common orientation of the chains. These characteristics have also been observed in hexamethylenetetramine suberate.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 55 (1999), S. 543-557 
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The geometry of a modern imaging diffractometer is discussed in detail. A method to find all relevant instrument parameters from the control single-crystal measurement data is proposed and the limitations of such a procedure are indicated. Optimization of the instrument parameters by the least-squares method is presented.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: One of the terms of reference published in 1992 (Acta Cryst. A48, 922–946) in the report of the (at that time) Ad Interim Commission on Aperiodic Crystals was `to co-operate with other Commissions of the IUCr in establishing adequate guidelines and standards for articles to be published in IUCr journals reporting structural investigations of aperiodic crystals and theoretical investigations of aperiodic patterns'. It was soon recognized by the Commission that the establishment of a checklist for the publication of incommensurately modulated structures was perhaps the most urgent task. One of the first attempts to address the problem of publishing incommensurate structures was presented during a discussion organized in 1991 in Leikeitio, Spain, in a workshop dedicated to methods of structural analysis of modulated structures and quasicrystals. Since this meeting, the work of the Commission progressed interactively by numerous exchanges on the occasions of international meetings and mostly by electronic correspondence. The Checklist was accepted in 1994 by the Executive Committee of the IUCr. The opinion of the Commission on Journals was also requested and, following the comments of its members, the Checklist has been completed with an example of an incommensurate crystal structure illustrating each specific item that is required in the Checklist.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 52 (1996), S. 349-364 
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Molecular-geometry calculations in the superspace framework of incommensurate structure analysis are presented in detail. In particular, a method is described for attaching atoms to groups of others without the necessity of introducing further modulation functions. Modified structure-factor expressions are derived to simplify the application of constraints to modulation functions by introducing internal coordinate reference points in a more general way. The consequences of such changes are discussed. The results can be useful to improve the existing algorithms for incommensurate structure refinement and the analysis and presentation of results, especially for recently discovered incommensurate structures of large organic molecules. Some ambiguities still existing in the superspace approach related to molecular geometry are clarified. The results may also be useful for the compilation of a database describing incommensurate structures.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 52 (1996), S. 1036-1047 
    ISSN: 1600-5740
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Hexamethylenetetramine suberate is an organic compound presenting a one-dimensional modulation with strong satellite reflections between 123 and 300 K. Single-crystal X-ray intensities of main reflections and satellites up to fourth order have been measured and used for refining a model based on displacive atomic modulation functions. Harmonic terms up to eighth order were necessary to describe the strongly anharmonic modulation. The structure consists of alternating sheets of hexamethylenetetramine and suberic acid connected by hydrogen bonds. Each layer of suberic acid chains consists of alternating areas, each of which is characterized by a common orientation for all chains. These orientations subtend an angle of roughly 60°. At the interfaces, neighbouring chains in adjacent areas adopt intermediate orientations. The two carboxyl groups participate in hydrogen bonds, the character of which depends on the internal coordinate t. In one group and for all values of t, the same oxygen forms a double bond with the α-carbon and the other a single bond, and in the other the carboxylate form is observed for all values of t. The packings of both moieties are incompatible and are conjectured to be responsible for the incommensurability of the structure.
    Type of Medium: Electronic Resource
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