Acta Crystallographica Section B

Structural Science

Volume 52, Part 5 (October 1996)

Cover illustration: Polar diagram and side views of two aza-18-crown-6 macrocyclic compounds. Courtesy of R. Kiralj, B. Kojic-Prodic, M. Zinic, S. Alihodzic and N. Trinajstic

• topical reviews
• research papers
• short communications

topical reviews

Acta Cryst. (1996). B52, 753-769    [doi:10.1107/S0108768196005599]

Electron crystallography

D. L. Dorset

Synopsis: Solution of crystal structures from electron-diffraction intensity data is reviewed. Important applications include small organics, lipids, linear polymers, membrane proteins and even a variety of inorganic materials.

research papers

Acta Cryst. (1996). B52, 770-776    [doi:10.1107/S0108768196001656]

Structure of KLiSi₂O₅ and the hygroscopicity of glassy mixed alkali disilicates

B. H. W. S. de Jong, H. T. J. Supèr, A. L. Spek, N. Veldman, W. van Wezel and V. van der Mee

Synopsis: All lithium-free mixed alkali disilicate glasses are hygroscopic. The crystal structure of KLiSi₂O₅, a sheet silicate topologically equivalent to Li₂Si₂O₅ and Na₂Si₂O₅, has been determined in an attempt to rationalize by classical valence arguments the role of lithium in preventing hygroscopicity.

Acta Cryst. (1996). B52, 777-779    [doi:10.1107/S0108768196006179]

The sigma phase revisited: an alternative pathway to a trial structure

G. Bergman

Synopsis: Trial structure determination is based on the initial assumption of tetrahedral close packing with only unit-cell dimensions, metallic radii and possible space groups as the required information.

Acta Cryst. (1996). B52, 780-789    [doi:10.1107/S0108768196006519]

Incommensurate modulated disorder in Ba_0.85Ca_2.15In₆O₁₂

G. Baldinozzi, F. Goutenoire, M. Hervieu, E. Suard and D. Grebille

Synopsis: The tunnel structure of Ba_0.85Ca_2.15In₆O₁₂ has been refined using the Rietveld method. The incommensurate occupational and displacive modulation has been characterized.

Acta Cryst. (1996). B52, 790-805    [doi:10.1107/S0108768196004594]

New ferroelectric inorganic materials predicted in point group 4mm

S. C. Abrahams

Synopsis: Analysis of the 128 different structures listed under point group 4mm in the Inorganic Crystal Structure Database has resulted in the prediction of 51 ferroelectrics, of which only eight were previously known to exhibit this property.

Acta Cryst. (1996). B52, 806-809    [doi:10.1107/S0108768196004582]

Prediction of ferroelectricity in recent Inorganic Crystal Structure Database entries under space group Pba2

S. C. Abrahams, K. Mirsky and R. M. Nielson

Synopsis: Structural analysis of three recent entries in the Inorganic Crystal Structure Database under space group Pba2 predicts aminoguanidinium(2+) hexafluorozirconate to be ferroelectric, with Ba_0.6K_0.4BiO_2.3 also ferroelectric if the three reported O-atom displacements are confirmed, and (Cl₃PNPCl₃)(MoOCl₄) to be centrosymmetric if the Mo sites are equally occupied; otherwise, pyroelectric but not ferroelectric.

Acta Cryst. (1996). B52, 810-816    [doi:10.1107/S0108768195015801]

Structures of room-temperature and ferroelectric (35 K) phases of deuterated betaine calcium chloride dihydrate

J. M. Ezpeleta, F. J. Zúñiga, W. Paulus, A. Cousson, J. Hlinka and M. Quilichini

Synopsis: The distortion of the ferroelectric phase is analyzed in terms of symmetry modes and then compared with the modulations of the incommensurate and fourfold phases.

Acta Cryst. (1996). B52, 817-822    [doi:10.1107/S0108768196002674]

A revision of the structure of (bipyridyl-N,N')-dicyanoplatinum(II)

W. B. Connick, L. M. Henling and R. E. Marsh

Synopsis: The apparently satisfactory structural determination previously reported is incorrect. New data show a substantially different packing interaction arising from a doubling of the original c axis. Implications of this error are discussed.

Acta Cryst. (1996). B52, 823-837    [doi:10.1107/S0108768196006337]

Bond length-bond order relations and calculated geometries for some benzenoid aromatics, including phenanthridine. Structures of 5,6-dimethylphenanthridinium triflate, [N-(6-phenanthridinylmethyl)-aza-18-crown-6-⁵O,O',O'',O''',O''''](picrate-²-O,O')potassium, and [N,N'-bis(6-phenanthridinyl-N-methyl)-7,16-diaza-18-crown-6-⁴O,O',O'',O''']sodium iodide dichloromethane solvate

R. Kiralj, B. Kojic-Prodic, M. Zinic, S. Alihodzic and N. Trinajstic

Synopsis: The crystal structures of 5,6-dimethylphenanthridinium triflate and two alkali metal complexes of aza-and diaza-18-crown-6 with single- and two-armed fluoroionophores bearing phenanthridine units are presented. The evaluation of C-C and C-N bonds in phenanthridine by semi-empirical and molecular-mechanics calculations is performed and compared with the results of valence-bond and Hückel molecular-orbital calculations.

Acta Cryst. (1996). B52, 838-841    [doi:10.1107/S0108768196000493]

Structure and conformation of the hydrate of 8,11-dihydroxypentacyclo[5.4.0.0^2,6.0^3,10.0^5,9]undecane-8,11-carbolactam

H. G. Kruger, F. J. C. Martins, A. M. Viljoen, J. C. A. Boeyens, L. M. Cook and D. C. Levendis

Synopsis: Crystallization of the title cage compound at high pH proceeds by way of a hydrogen-bond network around a central H₂O of hydration. This locks the functional region into a specific orientation, irrespective of chirality. Rather than occupy inversion-related positions, enantiomers therefore share sites to produce disorder around the handle of the cage.

Acta Cryst. (1996). B52, 842-853    [doi:10.1107/S0108768196004624]

Trimorphism of 2,2'-dipyridylamine: structures, phase transitions and thermodynamic stabilities

H. Schödel, C. Näther, H. Bock and F. Butenschön

Synopsis: The crystal structure of an unknown third modification of 2,2^'-dipyridylamine was determined at 150 K and room temperature and the phase transitions and thermodynamic stabilities of all three forms were investigated by differential thermal analysis, powder diffraction and temperature-dependent crystallization experiments.

Acta Cryst. (1996). B52, 854-864    [doi:10.1107/S0108768196004661]

Neutron diffraction study of [Pt(en)₂][Pt(en)₂I₂](ClO₄)₄ at 20 K: structure and evidence of a new phase transition

J.-F. Bardeau, A. Bulou, W. T. Klooster, T. F. Koetzle, S. Johnson, B. Scott, B. I. Swanson and J. Eckert

Synopsis: [Pt(en)₂][Pt(en)₂I₂](ClO₄)₄ containing mixed-valence PtI chains has been studied for the first time by neutron diffraction. A phase transition characterized by a doubling of the c parameter has been discovered in the vicinity of 160 K and the refinement of the structure at 20 K revealed that the space group is C2/c.

Acta Cryst. (1996). B52, 865-875    [doi:10.1107/S0108768196005241]

Structure of tris(cyclohexylammonium) phosphoenolpyruvate monohydrate and crystal chemistry of phosphoenolpyruvates

M. Souhassou, P. M. Schaber and R. H. Blessing

Synopsis: The crystal structure of (C₆H₁₁NH₃₊)₃.Pep^3-.H₂O is reported and the structures of 19 other phosphoenolpyruvates are reviewed.

Acta Cryst. (1996). B52, 876-881    [doi:10.1107/S0108768196005368]

X-ray studies on crystalline complexes involving amino acids and peptides. XXX. Structural invariance and optical resolution through interactions with an achiral molecule in the histidine complexes of glycolic acid

S. Suresh and M. Vijayan

Synopsis: Aqueous solutions of DL-histidine and glycolic acid yield crystals of L-histidine glycolate and D-histidine glycolate, in addition to those of DL-histidine glycolate; the crystal structures of DL-histidine glycolate and L-histidine glycolate provide a structural rationale for the separation of optical isomers.

Acta Cryst. (1996). B52, 882-891    [doi:10.1107/S0108768196007409]

Symmetry-modified conformational mapping and classification of the medium rings from crystallographic data. IV. Cyclooctane and related eight-membered rings

F. H. Allen, J. A. K. Howard and N. A. Pitchford

Synopsis: Symmetry-modified Cremer-Pople principal component and cluster analyses are used to map and classify the crystallographically observed conformations of parent cyclooctanes and a variety of unsaturated and heterocyclic analogues.

short communications

Acta Cryst. (1996). B52, 892-895    [doi:10.1107/S010876819600599X]

The ab initio crystal structure determination of vapour-deposited methyl fluoride by high-resolution neutron powder diffraction

R. M. Ibberson and M. Prager

Synopsis: The crystal structure of methyl fluoride (m.p. = 195 K) at 5 K has been routinely solved ab initio from high-resolution neutron powder diffraction data using a vapour deposition technique for sample preparation. The monoclinic structure may be described in terms of dipole-dipole intermolecular interactions and is distinct from the structures of the other methyl halides.

Acta Cryst. (1996). B52, 896-898    [doi:10.1107/S0108768196006702]

Neutron powder diffraction study of -Ti(HPO₄)₂.H₂O and -Hf(HPO₄)₂.H₂O; H-atom positions

M. A. Salvadó, P. Pertierra, S. García-Granda, J. R. García, J. Rodríguez and M. T. Fernández-Díaz

Synopsis: The complete crystal structure, including H-atom positions, of -Ti(HPO₄)₂.H₂O (-TiP) and -Hf(HPO₄)₂.H₂O (-HfP) were determined by Rietveld refinement and Fourier synthesis, using constant-wavelength neutron diffraction data.

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