issue contents

ISSN: 2052-5206

April 2011 issue

Highlighted illustration

Cover illustration: The cover picture highlights an artefact in the electrostatic potential of the macrolide antibiotic roxithromycin as derived via multipole refinement of X-ray diffraction data.  Ignoring the hydrogen disorder (left) in the oxime chain leads to a falsely positive electrostatic potential there as a result of the multipole model's being flexible enough to fit the averaged electron density.  If the invariom formalism, which uses theoretical electron density, is applied to the disordered region (right) the artefact disappears.  A procedure for obtaining reliable electron density distributions in the presence of disorder has been proposed [Holstein et al. (2010). Acta Cryst. B66, 568-577].

research papers

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Rubidium sulfide, Rb2S, with an antifluorite-type structure at 1 bar, has been studied up to 8 GPa using angle-dispersive X-ray diffraction in a diamond–anvil cell. Two different high-pressure phases have been characterized: (i) an anticotunnite-type structure (P > 0.7 GPa) and (ii) an Ni2In-type structure (P > 2.6 GPa).

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The disordered crystal structure of (NH4)3H(SO4)2 in the high-temperature phase I at 413 K was studied using single-crystal neutron diffraction.

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Single crystals of tetra(isopropylammonium) decachlorotricadmate (II) with their unique two-dimensional [Cd3Cl10] network exhibit rich polymorphism in a narrow temperature range (phase transitions at 353, 294 and 259 K). The mechanisms of these transformations are discussed.

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The phase transition between 190 and 200 K in [Ni(H2O)6](NO3)2·(15-crown-5)·H2O has been followed by determining the structure at 22 temperatures in the range 90−273 K. An attempt has been made to describe the transition using mean-field theory.

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Experimental charge-density studies were performed for two of the three polymorphic modifications of barbituric acid–urea co-crystals. The topological properties of intermolecular interactions confirmed the tendency of barbituric acid to adopt different mesomeric forms in the polymorphs. The mesomeric forms influence the various systems of the observed hydrogen bonds.

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Possible crystal structures of ethyl-tert-butyl ether were predicted by global lattice-energy minimizations. The densest calculated structure corresponds to the experimental one which was determined from low-temperature X-ray powder data.

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A single-crystal X-ray diffraction study in the temperature range 100–300 K has revealed two reversible non-destructive phase transitions with a continuous decrease in volume per molecule in the β-form of chlorpropamide: β (Pbcn, Z′ = 1)  βII (P2/c, Z′ = 2)  βIII (P2/n, a′  =  2a, Z′ = 4); the sequence corresponds to cooling.

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The types of packing of hexamethylenetetramine (HMTA) complexes, either co-crystals or salts, are explained with regard to the molecular shape of the HMTA molecule and the number and type of functional groups of the partner molecule, with which the HMTA molecule interacts. One-dimensional assemblies featuring zigzag-shaped chains are derived from the tetrahedral arrangement of the N atoms on the HMTA.
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