Metastructures: homeomorphisms between complex inorganic structures and three-dimensional nets

Simple three-dimensional nets are the basis for families of structures homeomorphic to these nets. At their vertices may be single atoms, coordination polyhedra or clusters of coordination polyhedra.

Charge density in NiCl₂.4H₂O at 295 and 30 K

The charge density in NiCl₂.4H₂O has been determined at 295 and 30 K, with H-atom parameters from X-ray and, independently, from neutron data; at 30 K one water molecule is disordered.

Determination of the modulated structure of Sr_14/11CoO₃ through a (3 + 1)-dimensional space description and using non-harmonic ADPs

The structure of a new hexagonal perovskite phase, Sr_14/11CoO₃, was determined from single-crystal X-ray diffraction data, using (3 + 1)-dimensional formalism. Crenel functions and a Gram–Charlier anharmonic development of the atomic displacement factor were used to accurately describe the Co atoms. A displacement of the prismatic Co atoms was clearly evidenced.

The crystal structure of Tm₅Re₂O₁₂

In pentathulium dirhenium dodecaoxide, ReO₆ octahedra, monocapped trigonal TmO₇ prisms and TmO₆ octahedra form chains of ReO₆ octahedra with metallic rhenium–rhenium bonds.

Symmetry classification of the layered perovskite-derived A_nB_nX_3n+2 structures

By considering the effects of combinations of octahedral tilts on the symmetries of layered A_nB_nX_3n+2 structures, the space groups associated with different tilt systems were determined for structures with different layer thicknesses (n values) and stacking arrangements. Comparison of these results with literature data suggests that the symmetries of most observed A_nB_nX_3n+2 compounds are fully determined by the tilt systems adopted by rigid BX₆ octahedra.

The use of CCD area detectors in charge-density research. Application to a mineral compound: the α-spodumene LiAl(SiO₃)₂

Data collected on both Nonius (Kappa CCD) and Siemens–Bruker (Smart) CCD detector diffractometers were used for the electron density determination of the mineral α-spodumene. Careful comparisons were made with point-detector CAD-4 system derived results.

Crystal morphology prediction and morphology variation in NaIO₄ and NaIO₄.3H₂O

Crystal morphologies for the title compounds were successfully predicted using attachment energies computed with only the Coulomb potential. There is a systematic variation between the early-growth morphologies and the morphologies of the mature crystals.

Modulated structures of Cs₂HgCl₄: the 5a superstructure at 185 K and the 3c superstructure at 176 K

The superstructures of the -type compound CsHgCl are found to involve rotations, distortions and shifts of the HgCl groups. The superspace approach gives a unified description of the low-temperature modulated structures and the normal phase at room temperature.

Renormalization of the phase transition in lead phosphate, Pb₃(PO₄)₂, by high pressure: lattice parameters and spontaneous strain

Single-crystal X-ray diffraction has been used to show that lead phosphate, Pb₃(PO₄)₂, undergoes an improper ferroelastic phase transition from C2/c to R3¯m (or R3m) symmetry at 1.81 ± 0.01 GPa at 298 ± 1 K. The spontaneous strain components evolve linearly with pressure, indicating that the effective critical exponent for the phase transition is β = ½.

Charge distribution as a tool to investigate structural details: meaning and application to pyroxenes

It is shown that the charge distribution method is well suited to analyse the correctness of a structure and to investigate the structural changes with composition, temperature and pressure. Examples of applications to pyroxenes are given. A computer program is available.

Electron density study of KNiF₃ by the vacuum-camera-imaging plate method

The first investigation of electron density in inorganic compounds made by the vacuum-camera-imaging plate method is reported. The results are compared with those from very accurate conventional four-circle measurements.

Electron density and electrostatic potential of KNiF₃: multipole, orbital and topological analyses of vacuum-camera-imaging plate and four-circle diffractometer data

Multipole, orbital and topological analyses of the electron density, and calculation of the electrostatic potential, of KNiF₃ has been performed using precision X-ray diffraction data measured by vacuum-camera-imaging plate and by four-circle diffractometer methods. The results are compared and some reasons for their discrepancy are discussed.

P1 or P1¯? Or something else?

Approximately 20% of the entries under space group P1 (No. 1) in the October 1998 release of the Cambridge Structural Database belong in other space groups, usually (No. 2). Over half of these corrections have been made in the latest release.

Structure and thermal behavior of the new superprotonic conductor Cs₂(HSO₄)(H₂PO₄)

The structure of Cs₂(HSO₄)(H₂PO₄), a new proton conductor, has been solved by single crystal X-ray methods. The structure contains sheets of hydrogen-bonded (S,P)O₄ groups, interleaved with zigzag rows of Cs ions.

Preparation and characterization of tetra­chlorocobaltates(II) of α,ω-alkylenedi­ammonium. Magnetic and thermal properties. Crystal structure of [NH₃(CH₂)₅NH₃]CoCl₄

The compound [NH₃(CH₂)₅NH₃)]CoCl₄ contains slightly distorted tetrachlorocobaltate anions hydrogen-bonded to diprotonated cadaverine cations. The magnetic behaviour is in agreement with the ⁴A₂ ground state of the CoCl anion.

Crystal structure of layered bis­(ethylenedi­amine)nickel hexavanadate as a new representative of the V₆O₁₄ series

The crystal structure of hydrothermally prepared [Ni(NH₂CH₂CH₂NH₂)₂](V₆O₁₄) was determined and appears to have disordered stacking of the vanadium oxide layers. Combinatorial analysis of the layered V₆O₁₄ structures was conducted, giving ten possible symmetry groups of the V₆O₁₄ layers.

Intramolecular and intermolecular geometry of thiophenes with oxygen-containing substituents

The structures of six thiophenes possessing oxygen-containing substituents have been determined and their intramolecular and intermolecular geometry analysed.

Structure and molecular modelling of protected dipeptide fragment (Boc–Phe–Leu–OBzl) of enkephalin

The crystal structure with seven conformers in an asymmetric unit is determined. Conformational characteristics of a flexible dipeptide are described using solid-state data and molecular modelling; predominance of folded over extended shapes is found. The presence of protecting groups does not reduce the flexibility of the molecule.

Self-assembly of NH-pyrazoles via intermolecular N—H⋯N hydrogen bonds

In a survey of NH-pyrazoles, a limited number of compounds are found to form homonuclear N⁺—H⋯N positive-charge-assisted hydrogen bonds with N⋯N distances as short as 2.71 Å, while most NH-pyrazoles pack by means of neutral N—H⋯N bonds which are formally assisted by resonance. The resonance does not appear to shorten this bond except in 3,5 symmetrically substituted pyrazoles which display N⋯N distances as short as 2.82 Å associated with an increased internal resonance of the ring and N—H⋯N proton disorder.

Interplay of hydrogen bonding and other molecular interactions in determining the crystal packing of a series of anti-β-keto­arylhydrazones

The crystal structures of six anti-β-ketoarylhydrazones containing the conjugated H—N—N=C—C=O heterodienic group are reported. The anti-β-ketoarylhydrazone group is able to form two main types of interaction: the intermolecular N—H—O hydrogen bonding assisted by resonance (RAHB-inter) linking the molecules in infinite chains, and a dipole–dipole (DD) interaction between two carbonyl groups linking the molecules in antiparallel dimers. Different packing arrangements are determined by the presence of various substituents at the phenyl ring, which are able to switch from RAHB-inter to DD interactions or to non-resonant hydrogen bondings.

A comparative study of the crystal structures of tetrahalogenated hydro­quinones and γ-hydro­quinone

The crystal structures of the title hydroquinones display topologically similar O—H⋯O networks. It is observed that the C—F group resembles neither C—H group nor C—Cl and C—Br groups in its packing characteristics and that it disfavours the structural alternatives provided by the three latter groups.

Order–disorder phenomena determined by high-resolution powder diffraction: the structures of tetra­kis­(tri­methylsilyl)methane C[Si(CH₃)₃]₄ and tetra­kis­(tri­methylsilyl)silane Si[Si(CH₃)₃]₄

A new phase transition has been discovered for tetrakis(trimethylsilyl)methane C[Si(CH₃)₃]₄. The crystal and molecular structures of C[Si(CH₃)₃]₄ and tetrakis(trimethylsilyl)silane Si[Si(CH₃)₃]₄ have been determined for all phases by Rietveld refinement. The phase transitions have been characterized by DSC, NMR and molecular modeling.

Graph-set analysis of hydrogen-bond patterns: some mathematical concepts

The mathematical basis of graph-set analysis of hydrogen-bond patterns in crystals is presented by defining terms and developing tools for its treatment and subsequent implementation in suitable software algorithms.

Visualization and characterization of non-covalent networks in molecular crystals: automated assignment of graph-set descriptors for asymmetric molecules

A method of visualizing intermolecular networks in the crystalline state, based on expansion of the structure through link atoms in contact with the unique molecule(s) in the crystal chemical unit, is described. Algorithms have been developed for the automatic assignment of graph-set notation, up to second level, to networks that involve asymmetric molecules.

TEMPO radicals showing magnetic interactions. II. 4-(Benzyl­idene­amino)-TEMPO and related compounds

Crystal structures of 4-(Ar-methyleneamino)-TEMPO radicals (Ar = Ph, 4-MeS-Ph, 4-Me-Ph, 4-PhO-Ph, 4-MeSO-Ph, 3-pyridyl and 2-naphthyl; TEMPO = 2,2,6,6-tetramethylpiperidyl-1-oxyl) have been determined and intermolecular magnetic interactions based on the structures are discussed.

2,4,6-Triiso­propylbenzenesulfonamide: Monte Carlo structure solution from X-ray powder diffraction data for a molecular system containing four independent asymmetric rotors

The crystal structure of 2,4,6-triisopropylbenzenesulfonamide, C₁₅H₂₅NO₂S, has been solved by the Monte Carlo method from X-ray powder diffraction data collected at 120 (1) K using synchrotron radiation. The stucture contains molecules linked by N—H⋯O hydrogen bonds into two-dimensional sheets built from and rings.

Local similarity in organic crystals and the non-uniqueness of X-ray powder patterns

Symmetry operations applied only to finite regions of a three-dimensional crystal may, under certain conditions, lead to new crystals and solids. Examples for this new phenomenon are shown to exist in organic crystals.

What is the best crystal size for collection of X-ray data? Refinement of the structure of glycyl-L-serine based on data from a very large crystal

Highly redundant data collected from a very large crystal with a CCD detector give an excellent structure after correction for systematic errors.

Volumetric measure of isostructurality

A new isostructurality index based on the overlap of molecular volumes is proposed and tested on various isostructural crystal pairs including associates.

Structure refinement of Cu₈GeS₆ using X-ray diffraction data from a multiple-twinned crystal. Erratum

An error in the figures is reported in the paper by Onoda et al. [Acta Cryst. (1999), B55, 721–725.

Crystallography across the sciences

Patterson peaks