Structural Science

 

Acta Cryst. (2000). B56, 545-546  [ doi:10.1107/S0108768100008181 ]

George A. Jeffrey (1915-2000)

B. Craven

Online August 2000

 

Acta Cryst. (2000). B56, 547-557  [ doi:10.1107/S010876810000269X ]

How precise are measurements of unit-cell dimensions from single crystals?

F. H. Herbstein

Synopsis: The best measurements of cell dimensions from (small) single crystals and powders on the most favourable specimens have a precision of some parts in 10⁵, which seems to be the maximum attainable with current, and older, techniques. There has been little real improvement over a period of some 60 years. `Routine' measurements, on less favourable specimens, give precisions of some parts in 10⁴ or even 10³. Some current assessments of the standard uncertainties of cell dimensions are fictional at best.

Online August 2000

 

Acta Cryst. (2000). B56, 558-564  [ doi:10.1107/S0108768100002871 ]

Towards a unified description of the AMOB₂O₅ (A = K, Rb, Cs, Tl; M = Nb, Ta) family of compounds

S. Schmid, R. L. Withers, D. Corker and P. Baules

Synopsis: The structure of RbNbOB₂O₅ is re-refined as an incommensurate modulated structure using the program JANA98.

Online August 2000

 

Acta Cryst. (2000). B56, 565-569  [ doi:10.1107/S0108768100002615 ]

Bond-valence parameters for ammonium-anion interactions

L. García-Rodríguez, Á. Rute-Pérez, J. R. Piñero and C. González-Silgo

Synopsis: A database search for ammonium ion coordination by anions is used to determine bond-valence parameters. The applicability of these parameters is discussed and exemplified.

Online August 2000

 

Acta Cryst. (2000). B56, 570-576  [ doi:10.1107/S0108768100000677 ]

Neutron Rietveld refinement of the incommensurate phase of the ordered perovskite Pb₂CoWO₆

G. Baldinozzi, G. Calvarin, Ph. Sciau, D. Grebille and E. Suard

Synopsis: Rietveld refinement of the modulated structure of lead cobalt tungstate. A displacive model involving significant atomic shifts for Pb atoms and quite a complex mixing of tilt and deformation of the oxygen octahedra is proposed.

Online August 2000

 

Acta Cryst. (2000). B56, 577-583  [ doi:10.1107/S0108768199016559 ]

Structure of uranium(VI) oxide dihydrate, UO₃·2H₂O; synthetic meta-schoepite (UO₂)₄O(OH)₆·5H₂O

M. T. Weller, M. E. Light and T. Gelbrich

Synopsis: The structure of meta-schoepite, UO₃·2H₂O, is constructed from layers of stoichiometry (UO₂)₄O(OH)₆ separated by water molecules. The material is structurally related to schoepite, UO₃·2.25H₂O, by the removal and redistribution of molecules in the water layers.

Online August 2000

 

Acta Cryst. (2000). B56, 584-593  [ doi:10.1107/S0108768100002032 ]

Study of NH₄⁺ in the zeolite phillipsite by combined synchrotron powder diffraction and IR spectroscopy

A. F. Gualtieri

Synopsis: The Rietveld refinement on synchrotron data was combined with IR data to study three natural hydrated NH₄⁺-exchanged phillipsites with different Si/Al ratios. They all display the typical monoclinic P2₁/m cell with NH₄⁺, which occupy sites I and II where it acts as a donor in hydrogen bonding with O atoms of the framework and water molecule.

Online August 2000

 

Acta Cryst. (2000). B56, 594-600  [ doi:10.1107/S0108768100003906 ]

Electron density and atomic displacements in KTaO₃

E. A. Zhurova, Y. Ivanov, V. Zavodnik and V. Tsirelson

Synopsis: The electron density and atomic displacement pattern in the virtual ferroelectric KTaO₃ studied with accurate X-ray diffraction data gives a comprehensive description of the atomic interactions in this crystal.

Online August 2000

 

Acta Cryst. (2000). B56, 601-606  [ doi:10.1107/S0108768100003633 ]

Structural phase transitions of HfV₂ at low temperatures

Y. Zhao, F. Chu, R. B. Von Dreele and Q. Zhu

Synopsis: A high-resolution synchrotron X-ray diffraction study on a HfV₂ Laves C15 phase conducted at low temperatures has illustrated the structural and lattice changes as a function of temperature and definitely revealed the crystal structure of a tetragonal I4₁amd phase for the first time.

Online August 2000

 

Acta Cryst. (2000). B56, 607-617  [ doi:10.1107/S0108768100003219 ]

Analysis of LiKSO₄ crystals in the temperature range from 573 to 943  K

C. B. Pinheiro, M. A. Pimenta, G. Chapuis and N. L. Speziali

Synopsis: Structure solution and refinement details are given for the multi-domain high-temperature (708 T 943  K) phase of LiKSO.

Online August 2000

 

Acta Cryst. (2000). B56, 618-625  [ doi:10.1107/S0108768100003645 ]

Ab initio structure determination and Rietveld refinement of a high-temperature phase of zirconium hydrogen phosphate and a new polymorph of zirconium pyrophosphate from in situ temperature-resolved powder diffraction data

A. M. Krogh Andersen and P. Norby

Synopsis: The crystal structures of a new high-temperature phase of zirconium hydrogen phosphate and a new polymorph of zirconium pyrophosphate have been solved and refined from in situ temperature-resolved synchrotron powder diffraction data.

Online August 2000

 

Acta Cryst. (2000). B56, 626-638  [ doi:10.1107/S0108768100005401 ]

A new relation between bond valence and bond distance

F. Mohri

Synopsis: A new empirical relationship between bond distances and bond valences is derived from the hypothesis that bond valence is proportional to the average electron density between bonding atoms.

Online August 2000

 

Acta Cryst. (2000). B56, 639-647  [ doi:10.1107/S0108768100002044 ]

Identification of two allotwins of mica polytypes in reciprocal space through the minimal rhombus unit

M. Nespolo, G. Ferraris and H. Takeda

Synopsis: Two rare examples of allotwins are identified in an oxybiotite mica, using the X-ray precession method and geometrical analysis of the minimal unit in reciprocal space. 1M and 2M₁ polytypes are attached by rotations of either 120 or 60°, respectively.

Online August 2000

 

Acta Cryst. (2000). B56, 648-653  [ doi:10.1107/S0108768100003621 ]

Aplanarity of CO₃ groups: a theoretical investigation

B. Winkler, J. Zemann and V. Milman

Synopsis: Density functional theory-based calculations have been used to demonstrate that the aplanarity of CO groups in some carbonates is a ground-state property.

Online August 2000

 

Acta Cryst. (2000). B56, 654-658  [ doi:10.1107/S0108768100004900 ]

A new stacking motif: complex alloy structures interpreted as modulated structures

M. Uchida and Y. Matsui

Synopsis: The structures of -Al₄Mn, -Al₁₇₇Cr₄₉Ni and -Al₄Cr, which have been described in terms of the aggregation of clusters until now, are interpreted as modulated structures. The observed stacking motif of atoms can be found in many phases related to icosahedral and decagonal quasicrystals.

Online August 2000

 

Acta Cryst. (2000). B56, 659-665  [ doi:10.1107/S0108768100005462 ]

Structures and phase transitions of B-Ta₂O₅ and Z-Ta₂O₅: two high-pressure forms of Ta₂O₅

I. P. Zibrov, V. P. Filonenko, M. Sundberg and P.-E. Werner

Synopsis: The crystal structures of B-Ta₂O₅ and Z-Ta₂O₅, two high-pressure forms of Ta₂O₅, have been determined and refined from X-ray powder data. The structures are closely related and an intergrowth between them was indicated by the HRTEM study.

Online August 2000

 

Acta Cryst. (2000). B56, 666-672  [ doi:10.1107/S0108768100002056 ]

Synthesis, structure and magnetic properties of tetrakis--carboxylato-bis(dodecylnicotinato)dicopper(II) complexes; crystal and molecular structure of the decyl carboxylate derivative

M. Rusjan, Z. Chaia, O. E. Piro, D. Guillon and F. D. Cukiernik

Synopsis: Dodecylnicotinate bis-adducts of binuclear copper carboxylates, of the general formula Cu₂(O₂CC_{n - 1}H_{2n - 1})₄(C₅H₄NCOOC₁₂H₂₅)₂, were synthesized for n = 10, 12, 14, 16, 18 and 20 and their crystal structure, thermal behavior and magnetic properties studied. The molecular structure of the decyl derivative has been determined from single-crystal X-ray diffraction data.

Online August 2000

 

Acta Cryst. (2000). B56, 673-676  [ doi:10.1107/S0108768100002652 ]

Frequency of Z' values in organic and organometallic crystal structures

T. Steiner

Synopsis: The frequencies of Z' values (formula units per asymmetric crystal unit) are documented for organic and organometallic crystal structures, using the Cambridge Structural Database.

Online August 2000

 

Acta Cryst. (2000). B56, 677-681  [ doi:10.1107/S0108768199016547 ]

Determining the crystal structure of twinned 2-methylpyrazine

R. Boese, A. Gehrke, M. Kapon and F. H. Herbstein

Synopsis: The structure of merohedrally twinned 2-methylpyrazine at 123 and 168  K has been solved using SHELXL97.

Online August 2000

 

Acta Cryst. (2000). B56, 682-689  [ doi:10.1107/S0108768100002329 ]

Crystal-to-crystal photodimerization of trans-cinnamamides

H. Hosomi, Y. Ito and S. Ohba

Synopsis: Single-crystal photoirradiations of trans-cinnamamide and the 4-methyl and 4-chloro derivatives have been carried out to observe the transformation from a pair of monomers related by a center of symmetry into an -type dimer. The lower photoreactivity of trans-cinnamamide than those of the other two is not only due to a longer CC distance between the nearest neighboring C=C double bonds, but also due to partial breakdown of the hydrogen-bond network in the process of photodimerization.

Online August 2000

 

Acta Cryst. (2000). B56, 690-696  [ doi:10.1107/S0108768100002202 ]

X-ray studies on crystalline complexes involving amino acids and peptides. XXXV. Invariance and variability in amino acid aggregation in the complexes of maleic acid with L-histidine and L-lysine

J. V. Pratap, R. Ravishankar and M. Vijayan

Synopsis: The histidine complex provides an example of the retention of the basic features of amino acid aggregation in different environments and in the presence of different molecules, while the pattern observed in the lysine complex serves as a note of caution and demonstrates how difficult it is to predict crystal structures even though broad generalizations are useful.

Online August 2000

 

Acta Cryst. (2000). B56, 697-714  [ doi:10.1107/S0108768100004584 ]

A test of crystal structure prediction of small organic molecules

J. P. M. Lommerse, W. D. S. Motherwell, H. L. Ammon, J. D. Dunitz, A. Gavezzotti, D. W. M. Hofmann, F. J. J. Leusen, W. T. M. Mooij, S. L. Price, B. Schweizer, M. U. Schmidt, B. P. van Eijck, P. Verwer and D. E. Williams

Synopsis: The results are reported of a blind test of crystal structure prediction of four organic molecules using the principal computer programs currently available. Successful predictions for all four molecules were obtained using calculated lattice energy, but no program was successful in all cases.

Online August 2000

 

Acta Cryst. (2000). B56, 715-719  [ doi:10.1107/S0108768100002172 ]

Non-centrosymmetric racemates: space-group frequencies and conformational similarities between crystallographically independent molecules

B. Dalhus and C. H. Görbitz

Synopsis: The distribution of crystalline racemates with more than one molecule in the asymmetric unit among the common space groups has been determined. The conformational similarities between crystallographically independent enantiomers in 114 non-centrosymmetric racemates were quantified using the r.m.s. deviation for a molecular superposition. The r.m.s. distribution curve shows that the conformations of the crystallographically independent molecules are in general very similar.

Online August 2000

 

Acta Cryst. (2000). B56, 720-727  [ doi:10.1107/S0108768100002810 ]

Structural relationships in crystals accommodating different stereoisomers of 2-amino-3-methylpentanoic acid

B. Dalhus and C. H. Görbitz

Synopsis: The spontaneous resolution of the racemate DL-allo-isoleucine upon crystallization is rationalized on the basis of an analysis of interatomic distances in the 1:1 DL-isoleucine:DL-allo-isoleucine complex. An unfavorable CC contact prevents the two enantiomers from forming a stable crystal structure with the same hydrogen-bond arrangement as in 1:1 DL-isoleucine: DL-allo-isoleucine and related amino acid racemates.

Online August 2000

 

Acta Cryst. (2000). B56, 728-737  [ doi:10.1107/S0108768100004390 ]

Electron density and electrostatic properties of two peptide molecules: tyrosyl-glycyl-glycine monohydrate and glycyl-aspartic acid dihydrate

V. Pichon-Pesme, H. Lachekar, M. Souhassou and C. Lecomte

Synopsis: The electron density and the electrostatic properties of the title peptide molecules have been determined from high-resolution diffraction data collected at 123  K. The topological characteristics of the (3,-1) critical points of C=O, C-N and C-C bonds are compared in 17 peptide links.

Online August 2000

 

Acta Cryst. (2000). B56, 738-743  [ doi:10.1107/S0108768100005309 ]

Single crystal structure and molecular dynamics analysis of a myo-inositol derivative

J. Dillen, M. W. Bredenkamp and M.-L. Prinsloo

Synopsis: The dynamical disorder found in the crystal of a myo-inositol derivative is studied using molecular dynamics calculations.

Online August 2000

 

Acta Cryst. (2000). B56, 744  [ doi:10.1107/S0108768100003785 ]

P1 or P? Corrigendum

R. E. Marsh

Synopsis: The space group of bis((phenyl-O,N,N-azoxy)oxy)methane, previously revised from P1 to C2, is further revised to Fdd2.

Online August 2000

 

Acta Cryst. (2000). B56, 745  [ doi:10.1107/S0108768100009721 ]

Structure predictions allowing more than one molecule in the asymmetric unit. Erratum

B. P. van Eijck and J. Kroon

Synopsis: Erratum to Acta Cryst. (2000). B56, 535-542.

Online August 2000