issue contents

Journal logoSTRUCTURAL

ISSN: 2053-2296

Absolute structure (December 2012)

Guest Editor(s): H. D. Flack

For the validation of absolute-structure determination, over the years there has been a heavy reliance on the statistics (values and standard uncertainties) of derived parameters or something equivalent, and little study of the fit of the model to the observed quantities. There has been little questioning of the quality of crystals, data collection and correction methodologies. Clearly, absolute-structure determination is still a rich field, wide open for development, especially for light-atom compounds of great importance to the pharmaceutical and chemical industries.

Highlighted illustration

Cover illustration: Perindoprilat monohydrate: the active metabolite of the antihypertensive and cardiovascular drug perindopril [Bojarska, Maniukiewicz, Sieron, Kopczacki, Walczynski & Remko (2012). Acta Cryst. C68, o443-o446].

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The use of Friedel averages and differences is described for the following operations: determining whether a crystal is centrosymmetric or not, determining the point group of a crystal and validating intensity data after structure solution.

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Leverage analysis can be used to investigate the importance of weak data and the effectiveness of restraints. It can also improve the precision of estimates of selected parameters: the standard uncertainty of the Flack parameter can be reduced to 0.1 or below even for light-atom crystal structures.

Acta Cryst. (2011). C67, 21-34
doi: 10.1107/S010876731004287X
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The practical use of Friedel-difference intensities for space-group determination, least-squares refinement and structure validation is described.

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The structure of the unusual `hemimorphic' wulfenite crystals from the Mežica mine was refined in the space group I\overline{4} as an inversion twin.

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The single crystals of the tetra­gonal modification of dysprosium orthomolybdate obtained in the present work are stable at room temperature. The crystal structure is formed by corner-sharing MoO4 tetra­hedra and Dy polyhedra coordinated by seven O atoms.