bluuebunny/crossref_metadata_2025 huggingface dataset:
Apparatus for the measurement of the electronic excited-state structure of single crystals using X-ray diffraction
Authors: White M. A. , Pressprich M. R. , Coppens P. , Coppens D. D.
Published: July 2002

Instrumentation for measuring the X-ray diffraction pattern of optically excited crystals is described. The experiment uses a high-power (~1 W) laser and a single-crystal diffractometer equipped with a helium cryostat (T < 70 K). The laser beam is modulated by a mechanical chopper and the diffraction signal gated in synchronization with the chopper phase. The modulation method is capable of observing small changes (down to about 0.01%) in the structure factors upon excitation of a fraction of the molecules in the crystal, given adequate counting statistics. The technique can be used for relatively long lived electronic excited states (τ ≃ 0.1–10 ms). The optical system is also suitable for time-resolved measurements using the time structure of synchrotron radiation.
Refinement of 2-Amino-6-(4-methyl-1-piperazinyl)-4-(tricyclo[3.3.1.13,7]dec-1-yl)-1,3,5-triazine at Three Different Temperatures using Image-Plate Data
Authors: Perrakis A. , Schneider T. R. , Antoniadou-Vyzas E. , Dauter Z. , Hamodrakas S. J.
Published: July 2002

The structure of 2-amino-6-(4-methyl-1-piperazinyl)-4-(tricyclo[3.3.1.13,7]dec-1-yl)-1,3,5-triazine was refined using data collected at three different temperatures [room temperature (293 K), 150 K and 85 K] with the use of an image-plate scanner in an attempt to study the effect of temperature on the disorder of the adamantyl group. It is demonstrated that the data collected with the rotation method and an image-plate scanner are of a high enough quality to be used for the refinement of small-molecule structures.
Distribution of Pores in a-Si1−x
C
x
:H Thin Films
Authors: Prado R. J. , Bittencourt D. R. S. , Tabacniks M. H. , Fantini M. C. A. , Carreño M. N. P. , Pereyra I.
Published: July 2002

The aim of this paper is to compare the optical, compositional and morphological properties of a-Si1 − x
C
x
: H films deposited by plasma enhanced chemical vapour deposition (PECVD) using different mixtures of silane (SiH4) and methane (CH4) under minimum attainable deposition pressure. Films deposited at lower silane flow present a higher carbon content and larger optical gap. The morphology of the films was investigated by small-angle X-ray scattering (SAXS) using two different light sources: (i) conventional tube and (ii) synchrotron radiation. The analysis of the data from both experiments was performed in order to determine a size distribution for spherical pores. The results obtained with both light sources are consistent: the increase in the CH4 concentration implies broader size distribution functions, with an increase of the pore size up to 10 nm. Larger pores are found in films deposited at lower silane flow. For all samples, the density of the smaller pores dominates the size distribution. The relative microvoid density is not proportional to the carbon concentration but presents a maximum for the low carbon content films.
Pressure and Temperature Effects in Homopolymer Blends and Diblock Copolymers
Authors: Frielinghaus H. , Schwahn D. , Mortensen K. , Willner L. , Almdal K.
Published: July 2002

Thermal composition fluctuations in a homogeneous binary polymer blend and in a diblock copolymer were measured by small-angle neutron scattering as a function of temperature and pressure. The experimental data were analyzed with theoretical expressions, including the important effect of thermal fluctuations. Phase boundaries, the Flory–Huggins interaction parameter and the Ginzburg number were obtained. The packing of the molecules changes with pressure. Therefore, the degree of thermal fluctuation as a function of packing and temperature was studied. While in polymer blends packing leads, in some respects, to a universal behaviour, such behaviour is not found in diblock copolymers. It is shown that the Ginzburg number decreases with pressure sensitively in blends, while it is constant in diblock copolymers. The Ginzburg number is an estimation of the transition between the universality classes of the `mean-field' approximation and the three-dimensional Ising model. The phase boundaries in blends increase with pressure, while the phase boundary of the studied block copolymer shows an unusual shape: with increasing pressure it first decreases and then increases. Its origin is an increase of the entropic and of the enthalpic parts, respectively, of the Flory–Huggins interaction parameter.
Thermostimulated Sol–Gel Transition in Suspensions of Sulfate-Zirconium Oxychloride
Authors: Chiavacci L. A. , Santilli C. V. , Pulcinelli S. H. , Craievich A. F.
Published: July 2002

The sol-gel transition of an aqueous solution of zirconium oxychloride modified by sulfuric acid (Zr:S 3:1) has been studied by small-angle X-ray scattering (SAXS) performed in situ during one cycle of heating and cooling between 298 and 358 K. The experimental SAXS curves exhibit three regions, at small, medium and high q values, characteristic of the Guinier, 'fractal' and Porod regimes, respectively. The value of 5.5 Å for the radii of the primary particles, obtained from the cross over of the Porod and fractal regimes, is consistent with the size of the inner core of the polynuclear Zr18O4(OH)38.8(SO4)12.6.33H2O molecule. These molecules aggregate as small clusters (31 Å) of fractal structure, with a dimensionality D = 2.16 in the sol. The value of the fractal dimensionality, which is characteristic of ideal branched polymers, decreases during heating, attaining a value of 2.0, characteristic of swollen polymers. Above the critical temperature (329 K), the average size of aggregates increases abruptly to 200 Å and the fractal dimensionality decreases to D = 1.75, this value being characteristic of a cluster-cluster aggregation process.
Formation and Growth of Semiconductor PbTe Nanocrystals in a Borosilicate Glass Matrix
Authors: Craievich A. F. , Alves O. L. , Barbosa L. C.
Published: July 2002

Pb- and Te-doped borosilicate glasses are transformed by appropriate heat treatment into a composite material consisting of a vitreous matrix in which semiconductor PbTe nanocrystals are embedded. This composite exhibits interesting non-linear optical properties in the infrared region, in the range 10–20 000 Å. The shape and size distribution of the nanocrystals and the kinetics of their growth were studied by small-angle X-ray scattering (SAXS) during in situ isothermal treatment at 923 K. The experimental results indicate that nanocrystals are nearly spherical and have an average radius increasing from 16 to 33 Å after 2 h at 923 K, the relative size dispersion being time-invariant and approximately equal to 8%. This investigation demonstrates that the kinetics of nanocrystal growth are governed by the classic mechanism of atomic diffusion. The radius of nanocrystals, deduced by applying the simple Efros & Efros [Sov. Phys. Semicond. (1982), 16, 772–775] model using the energy values corresponding to the exciton peaks of optical absorption spectra, does not agree with the average radius determined by SAXS.
Calculation of Partial Specific Volumes and Other Volumetric Properties of Small Molecules and Polymers
Authors: Durchschlag H. , Zipper P.
Published: July 2002

Volumetric properties of macromolecules and low-molecular compounds are necessary auxiliary means for the determination of molar masses from solution-scattering and hydrodynamic techniques. In many cases experimental determinations of partial volumes can or have to be replaced by calculative procedures. A universal approach for the calculation of both low-molecular organic compounds and polymers of different chemical composition and structure in aqueous solution is described. It is based on volume increments for the constituent atoms, ions and/or groups and allows corrections for covolume, ring formation, micellization, ionization etc. Application of this approach is of particular interest in connection with: (i) nonionic and ionic organic solutes; (ii) inorganic electrolytes; (iii) monomeric and micellar detergents and lipids; (iv) carbohydrates and polysaccharides; (v) nucleobases, nucleosides, nucleotides, polynucleotides, nucleic acids; (vi) amino acids, amino-acid residues, peptides, polyaminoacids, nonconjugated and conjugated proteins; (vii) synthetic polymers. The results of the volume predictions show a high degree of reliability, if compared to experimental data. Special approaches dealing with simple and conjugated proteins and protein-ligand complexes in two- and multicomponent solutions allow the prediction of both isomolal and isopotential volumes under a variety of native and denaturing conditions, including the presence of high amounts of additives.
Applications of Polycapillary X-ray Optics in Protein Crystallography
Authors: Li Ping-Wei , Bi Ru-Chang
Published: July 2002

Using polycapillary X-ray optics, the intensity of the Cu Kα radiation from a Rigaku RU200 rotating-anode generator was increased remarkably compared with Supper double focusing mirrors. A crystal of a double mutant of trichosanthin diffracted beyond 1.9 Å with the application of the polycapillary optics and 2.0 Å resolution data with an overall R_{\rm sym} of 8.4% were collected, while the same crystal only diffracted to about 2.62 Å using the radiation from double focusing mirrors. A small crystal of acidic phospholipase A2 from snake venom diffracted to 2.2 Å employing the polycapillary optics and the overall R_{\rm sym} for the data collected was 8.6%. The same crystal only diffracted to about 2.7 Å using double focusing mirrors. The average intensity of reflections and the signal-to-noise ratio for both of the crystals increased dramatically with the application of the polycapillary X-ray optics. This new kind of X-ray optic has been used to collect good-quality diffraction data for a number of protein crystals.
Anisotropy of Intermolecular Interactions from the Study of the Thermal-Expansion Tensor
Authors: Salud J. , Barrio M. , López D. O. , Tamarit J. Ll. , Alcobé X.
Published: July 2002

The anisotropy of the intermolecular interactions in the low-temperature ordered phases of three chemically and structurally related compounds [neopentylglycol, (CH3)2C(CH2OH)2, pivalic acid, (CH3)3C(COOH), and neopentylalcohol, (CH3)3C(CH2OH)], all of which display an orientationally disordered high-temperature phase, has been shown by means of the isobaric thermal-expansion tensor. The variation of the directions of the principal components of the thermal-expansion tensor as a function of temperature, as well as the variation of its principal coefficients, is evidence of the large differences in the intermolecular interactions for each compound; or, more precisely, between the strong intermolecular hydrogen bonds and the weak van der Waals interactions. In addition, the differences in the hydrogen-bonding schemes expecteda priorifrom the molecular structures of the studied compounds have been enhanced. Finally, the volume expansivity as well as the packing coefficient have been analysed in the orientationally disordered high-temperature phase of each of the three compounds.
CIF Applications. XI.A La Mode: a ligand and monomer object data environment. I. Automated construction of mmCIF monomer and ligand models
Authors: Clowney Les , Westbrook John D. , Berman Helen M.
Published: July 2002

The macromolecular Crystallographic Information File (mmCIF) dictionary [Fitzgeraldet al.(1997).The Macromolecular Crystallographic Information File Dictionary, http://ndbserver.rutgers.edu/mmcif] provides a comprehensive description of chemical components used as models in the crystallographic refinement of macromolecular structures. A new ligand and monomer object data environment namedA La Modeis described for building chemical-component models in the mmCIF representation from surveys of high-resolution small-molecule crystal structures. Examples of the application of this system are presented for an intercalating drug component and for a nucleotide unit constructed from independent base, sugar and phosphate components.
Twinning in Natural Diamond. I. Contact Twins
Authors: Yacoot A. , Moore Moreton , Machado W. G.
Published: July 2002

Growth histories of contact twins of natural diamond have been elucidated by nondestructive techniques of X-ray topography, using both conventional and synchrotron sources. Reflection conditions for the simultaneous imaging of both members of a diamond, twinned on (111), are given. The common `triangular' contact twin, known as a macle in the diamond trade, results from {111}-faceted growth from a central nucleation site, sometimes marked by an inclusion. If this period of growth is followed by one of dissolution, then the twinned rhombic dodecahedron may result. The dissolution shape of a twinned octahedron is the same as the twin of the dissolution shape of the octahedron. A peritropic twin was found to consist of two macles fortuitously joined on their common (111) facets in only approximate twin orientation. A lozenge-shaped diamond was found to contain a twin component in the shape of an arrowhead. In all these variants, the composition `plane' can be far from planar, resulting from intergrowth of one twin component into the other.
Crystallographic CourseWare
Authors: Kastner M. E.
Published: July 2002

Crystallographic CourseWareis a set of computer animations and interactive exercises designed to assist undergraduate and introductory graduate students in their learning of crystallography. Topics discussed include crystal growth, plane- and space-group symmetry elements, unit cells and asymmetric units, reciprocal space, precession photographs, and an introduction to reading theInternational Tables for Crystallography.
LAYER– a computer program for the graphic display of intensity data as simulated precession photographs
Authors: Barbour Leonard J.
Published: July 2002

LAYERis a 32-bit Microsoft Windows-based program that reads intensity data inSHELXtype 4 format and displays any level of the reciprocal-lattice net. The program is primarily intended for instructive purposes, but may also prove useful as a visual complement to statistically based numeric analysis of diffraction data for space-group determination.
Superscrew dislocation contrast on synchrotron white-beam topographs: an accurate description of the direct dislocation image
Authors: Huang X. R. , Dudley M. , Vetter W. M. , Huang W. , Si W. , Carter Jr C. H.
Published: July 2002

A kinematic (geometrical) diffraction simulation model has been developed to provide understanding of direct dislocation images on synchrotron white-beam X-ray topographs, and has been successfully applied to illustrate the contrast formation mechanisms involved in images of micropipe-related superscrew dislocations in silicon carbide crystals. The coincidence of the simulations with the contrast features of the superscrew dislocation images, recorded using a series of synchrotron topography techniques, shows that this model is capable of revealing the detailed diffraction behavior of the highly distorted region around the dislocation core and determining the quantitative characteristics of the dislocations. The simulation technique is thus demonstrated to be a simple but efficient method for interpretation of synchrotron topographs, and may be applied to explain the topographic contrast characters of general crystal defects.
Quantitative texture analysis of small domains with synchrotron radiation X-rays
Authors: Heidelbach F. , Riekel C. , Wenk H.-R.
Published: July 2002

Quantitative analysis of crystallographic preferred orientation (texture) of very small volumes in fine-grained polycrystalline materials has been carried out with a monochromatic X-ray microbeam (≤30 µm) at the microfocus beamline of the European Synchrotron Radiation Facility (ESRF). The experimental procedure is described and illustrated with textures of rolled aluminium, aluminium and steel wires, polymer fibers and natural bone material (apatite).
Automatic orientation determination from Kikuchi patterns
Authors: Morawiec A.
Published: July 2002

Some formal aspects of crystal orientation determination from Kikuchi patterns are discussed, with respect to writing general and fast algorithms for automatic indexing of the patterns and orientation determination. The geometry of the problem is presented in a general form which is also suitable for channeling patterns or X-ray Kossel patterns. Moreover, geometrical ambiguities and other reliability issues are considered. Some strategies designed to handle artifacts of automatic pattern analysis are presented. Finally, the reliability and the accuracy of the described procedures and the sensitivity of the results to the errors of the basic measurement parameters were tested on computer-generated patterns.
Solution structure of detergent micelles at conditions relevant to membrane protein crystallization
Authors: Littrell K. , Urban V. , Tiede D. , Thiyagarajan P.
Published: July 2002

In this study small angle neutron scattering was used to characterize the formation of micelles in aqueous solutions of the detergents DMG and SPC as a function of detergent concentration and ionic strength of the solvent. The effects on the micelle structure of the additives glycerol and PEG, alone as well as in combination typical for membrane protein crystallization, were also explored. This study suggests that the micelles are rod-like in form at the concentrations studied. The size of the micelles was observed to increase with increasing ionic strength but decrease with the addition of glycerol or PEG.
The internal structure of single carbon fibers determined by simultaneous small- and wide-angle scattering
Authors: Paris O. , Loidl D. , Peterlik H. , Müller M. , Lichtenegger H. , Fratzl P.
Published: July 2002

Simultaneous small-angle scattering and wide-angle diffraction using a synchrotron radiation microbeam was applied for the first time to investigate single carbon fibers in a position-resolved way. Taking into account the exact X-ray beam profile and examining the fibers in two scattering geometries allowed a discrimination between different models for the internal arrangement of carbon layers and pores. For a fiber based on polyacrylnitrile the carbon layers were randomly oriented within the fiber cross section, whereas in a mesophase-pitch based fiber the layers were arranged in a radial structure.
Flat points of minimal balance surfaces
Authors: Koch E. , Fischer W.
Published: July 2002

The symmetry conditions for flat points of minimal surfaces have been studied in relation to the order β of points on such surfaces. Using symmetry aspects, a set of rules for the derivation of fiat points have been developed. By means of these rules the flat points for the 45 families of minimal balance surfaces known so far have been determined. As a check for completeness the relation between the genus of a minimal surface and the orders of its flat points has been used.
A new approach for n-beam dynamical calculations
Authors: Watanabe K. , Kikuchi Y. , Hiratsuka K. , Yamaguchi H.
Published: July 2002

With the use of modulated plane waves, a new method for n-beam dynamical calculations has been established on the basis of a paper by Watanabe, Kikuchi, Hiratsuka & Yamaguchi [Phys. Status Solidi A (1988), 109, 119-126]. The computing time is reduced to about one-sixth of what it originally was and a large reduction of memory is achieved, n-beam dynamical calculations of aluminium, copper and gold at several accelerating voltages and orientations were carried out in a completely parallel manner by the present method, the multi-slice method and Bethe's eigen-value method [Fujiwara (1959). J. Phys. Soc. Jpn
14, 1513-1524]. The present method turned out to be competitive with respect to accuracy and speed in comparison with the latter two methods. The new method makes n-beam dynamical calculations of complex systems and defects possible.
On the prediction of crystal morphology. III. Equilibrium and growth behaviour of crystal faces containing multiple connected nets
Authors: Grimbergen R. F. P. , Bennema P. , Meekes H.
Published: July 2002

In this paper, the equilibrium and growth behaviour of faces (hkl) with more than one connected net is studied. It is shown that for these types of orientation different surface phases exist under equilibrium conditions as a function of temperature. Depending on the exact bonding topology at the surface, flat, rough or disordered flat phases are found. Moreover, the growth rate R_{hkl} of such faces can differ significantly from the usually calculated relative growth rates based on the attachment energy. Monte Carlo simulations confirm the results from the Hartman–Perdok analyses and offer a tool for the prediction of the crystal habit as a function of supersaturation.
Direct methods in protein electron crystallography – beef liver catalase in its fully hydrated form at room temperature
Authors: Dorset Douglas L. , Gilmore Christopher J.
Published: July 2002

The crystal structure of beef liver catalase was determined ab
initio in projection to 9 Å resolution using electron diffraction data at room temperature from hydrated specimens maintained in an environmental chamber in the electron microscope. A conservative combination of symbolic addition with maximum entropy and likelihood led to a model with a Patterson correlation coefficient C = 0.89 to the observed data. This independent solution could then be compared favorably to a previous 23 Å analysis of electron micrographs from frozen hydrated preparations. Prediction of the higher-resolution structure by extension of the lower-resolution image-based phase basis set also gave a good match to the direct-methods solution, particularly for the most intense reflections.
X-ray phase determination by the principle of minimum charge
Authors: Elser Veit
Published: July 2002

When the electron density in a crystal or a quasicrystal is reconstructed from its Fourier modes, the global minimum value of the density is sensitively dependent on the relative phases of the modes. The set of phases that maximizes the value of the global minimum corresponds, by positivity of the density, to the density having the minimum total charge that is consistent with the measured Fourier amplitudes. Phases that minimize the total electronic charge (i.e. the average electron density) have the additional property that the lowest minima of the electron density become exactly degenerate and proliferate within the unit cell. The large number of degenerate minima have the effect that density maxima are forced to occupy ever smaller regions of the unit cell. Thus, by minimization of the electronic charge, the atomicity of the electron density is enhanced as well. Charge minimization applied to simulated crystalline and quasicrystalline diffraction data successfully reproduces the correct phases starting from random initial phases.
The joint probability distribution function of structure factors with rational indices. IV. The P1 case
Authors: Giacovazzo Carmelo , Siliqi Dritan , Fernández-Castaño Cristina
Published: July 2002

The method of the joint probability distribution functions of structure factors has been extended to reflections with rational indices. The most general case, space group P1, has been considered. The positional parameters are the primitive random variables of our probabilistic approach, while the reflection indices are kept fixed. Quite general joint probability distributions have been considered from which conditional distributions have been derived: these proved applicable to the accurate estimation of the real and imaginary parts of a structure factor, given prior information on other structure factors. The method is also discussed in relation to the Hilbert-transform techniques.
Modified form factors from multiconfiguration Dirac–Fock wave functions for neutral atoms with Z = 70–100
Authors: Kahane Sylvian
Published: July 2002

Tables of modified X-ray scattering factors for neutral elements with Z = 70–100 have been calculated using multiconfiguration Dirac–Fock wave functions. The physical approximation is a next step beyond the usual form-factor approximation in calculating elastic scattering photon intensities. Differences as large as 3–6% compared to previous calculations of the same kind are obtained; differences from the usual form factors are much larger.
Solving crystal structures without Fourier mapping. I. Centrosymmetric case. Erratum
Authors: Pilz Katrin , Fischer Karl F.
Published: July 2002

Unfortunately, some printing errors escaped our attention in the proof-reading of our paper [Pilz & Fischer (1998). Acta Cryst. A54, 273–282]. As some of them may cause inconvenience or confusion to readers, they are corrected here: p. 275, equation (6a) should read: Q_i = \left[\textstyle \sum \limits_{j = 0}^{i-1}(-1)^{i-j-1}P_{i-j}Q_j\right]\bigg/i.p. 277 (right column), third line after equation (12) should read:  Using D
1 [from (10a)] …. p. 277 (right column), second line after equation (13) should read:  … namely 2^{m+n-1}. p. 279 (right column), lines 10 and 11 in second paragraph should read:  … σ(gh
) = 0.05gh
(or 0.1gh
, respectively) for …. p. 279 (right column), fifth line from bottom should read: … example I (Table 2) …. p. 280 (right column), third line before Section 3.2 should read: E.g. one of them systematically ….
Redetermination of the crystal structures of the `Arppe compound' Bi24O31Cl10 and the isomorphous Bi24O31Br10
Authors: Eggenweiler U. , Keller E. , Krämer V.
Published: July 2002

The crystal structures of Bi24O31
X
10 (X = Cl, Br; 24-bismuth 31-oxide decachloride/-bromide) have been redetermined. The structure models obtained resemble the model described in the literature about 60 years ago, but differ from it in some details of the O substructure and in the fact that the structure for X = Cl is a twofold superstructure of the literature model. A fictional mechanism is given by which the formerly described incorrect O substructure is transferred into the correct one.
Powder and single-crystal X-ray diffraction study of the structure of [Y(H2O)]2(C2O4)(CO3)2
Authors: Bataille Thierry , Louër Daniel
Published: July 2002

From powder pattern indexing it has been demonstrated that [Y(H2O)]2(C2O4)(CO3)2, yttrium oxalate carbonate, crystallizes with orthorhombic symmetry, space group C2221, a = 7.8177 (7), b = 14.943 (1), c = 9.4845 (7) Å, V = 1108.0 (1) Å3, Z = 4. This unit cell displays a doubling of the c parameter, arising from weak diffraction lines observed in the powder diffraction pattern, with respect to results reported in the literature. The crystal structure has been solved ab initio using direct methods from powder data and has been confirmed by additional single-crystal data collected with a CCD area detector. The overall crystal structure is similar for both unit cells, except that an alternation of the carbonate groups in the direction parallel to the screw axis is displayed in the larger cell, while with the suggested half unit cell (space group C2mm) the carbonate groups would show only one orientation. The unit-cell determination strategy from single-crystal diffraction, collected with Nonius CAD-4 and Nonius Kappa CCD diffractometers, is discussed with respect to the results extracted from the powder diffraction pattern. The study demonstrates the power and usefulness of the full trace of a powder pattern for the detection of subtle structure details.
Application of graph theory to detect disconnected structures in a crystallographic database: copper oxide perovskites as a case study
Authors: Kotliarov Yuri , Iwata Shuichi
Published: July 2002

Every crystal structure can be described as a graph with atoms as vertices and bonds as edges. Although such a graph loses the space arrangement of atoms and symmetry elements, it can mathematically represent the connectivity between atoms. This topological approach was used to develop a new method for detecting disconnected structures, in which individual atoms or structural fragments are located too far from each other, forming impossibly large gaps. Approximately 2300 perovskite-related crystal structures have been extracted from the Inorganic Crystal Structure Database (in 1999) and the maximum disconnecting distances, and the relations between them and the ionic radii of elements, have been analysed. Several disconnected structures, which are erroneous by our definition, have been revealed. Conventional tests for crystallographic data checking did not detect those entries.
Structures of three cis-β1 and three cis-β2 isomers of [Co(trien)(aminoacidato)]2+ complexes
Authors: Cai Jiwen , Hu Xiaopeng , Feng Xiaolong , Ji Liangnian , Bernal Ivan
Published: July 2002

The crystal structures and absolute configurations of three cis-β1 and three cis-β2 isomers of [Co(trien)(L-aminoacidato)]2+, namely β1-Λ(SSS)(λλδ)/(λδδ)-[Co(trien)(L-isoleucinato)](ClO4)2, L-isoleucinato(triethylenetetramine)cobalt(III) diperchlorate (1), β1-Λ(SRS)(λδλ)-[Co(trien)(L-valinato)](ClO4)2, L-valinato(triethylenetetramine)cobalt(III) diperchlorate (2), β1-Δ(RSS)(δλδ)-[Co(trien)(L-pyroglutamato)]ClO4, L-pyroglutamato(triethylenetetramine)cobalt(III) perchlorate (3), β2-Δ(RRS)(δδλ)-[Co(trien)(L-isoleucinato)]I2·H2O, L-isoleucinato(triethylenetetramine)cobalt(III) diiodide monohydrate (4), β2-Δ(RRS)(δδλ)-[Co(trien)(L-isoleucinato)](ClO4)2·2H2O, L-isoleucinato(triethylenetetramine)cobalt(III) diperchlorate dihydrate (5), and β2-Λ(SSS)(δλλ)[Co(trien)(L-leucinato)](ClO4)2·2H2O, L-leucinato(triethylenetetramine)cobalt(III) diperchlorate dihydrate (6), have been determined. Both β1 and β2 isomers form inter-cation hydrogen-bonding interactions through double or triple hydrogen bonds which link the cations into helices. However, the motifs of the interactions are different in β1 and β2 isomers, resulting in different packing structures. A localized hydrophobic area is observed in the packing structure of (1), a unique feature amongst these six structures. This work describes the first synthesis of the β1 isomer by direct reaction between amino acids and cis-α-[Co(trien)Cl2]Cl in mildly basic solution.
Structure of β-trimyristin and β-tristearin from high-resolution X-ray powder diffraction data
Authors: van Langevelde Arjen , Peschar René , Schenk Henk
Published: July 2002

The crystal structures of β-1,2,3-tritetradecanoylglycerol (β-trimyristin or β-MMM) and β-1,2,3-trioctadecanoylglycerol (β-tristearin or β-SSS) have been determined from high-resolution synchrotron X-ray powder diffraction data. Grid search and Rietveld refinement have been used to determine and refine the structure, respectively. Both substances crystallize in space group P\bar 1 with Z = 2. The unit-cell parameters for β-MMM are a = 12.0626 (6), b = 41.714 (1), c = 5.4588 (3) Å, α = 73.388 (4), β = 100.408 (5) and γ = 118.274 (4)°. For β-SSS the unit-cell parameters are a = 12.0053 (7), b = 51.902 (2), c = 5.4450 (3) Å, α = 73.752 (5), β = 100.256 (6) and γ = 117.691 (5)°. Soft-distance restraints have been applied to the molecules during refinement. For β-MMM the final Rp
value obtained is 0.053 and for β-SSS the final Rp
value is 0.041.
Two fluoradene derivatives: pseudosymmetry, eccentric ellipsoids and a phase transition
Authors: Xia Aibing , Selegue John P. , Carrillo Alberto , Patrick Brian O. , Parkin Sean , Brock Carolyn Pratt
Published: July 2002

Structures of two derivatives of the curved fluoradene ring system (C19H12) have been determined. Both have phases that are highly pseudosymmetric. At room temperature crystals of 7b-triisopropylsilylfluoradene (C28H32Si) have a P\bar 1 cell that contains two independent molecules (Z′ = 2) and that is almost centered. Crystals of 7b-(2,4-dinitrophenyl)fluoradene (C25H14N2O4) have both a P21/c cell with Z′ = 1 and a P21/c cell with Z′ = 2. The molecular volumes in these two P21/c structures differ by 0.7%, but the structures are otherwise virtually the same; the two independent molecules in the larger cell are related by a pseudotranslation. Some of the atomic ellipsoids in the P21/c, Z′ = 1 structure are very large and eccentric, and there are some hints in the diffraction pattern of an incipient phase transition, but the Z′ = 1 and Z′ = 2 phases are clearly different. The P21/c, Z′ = 2 crystal at 295 K probably contains some volume fraction of the Z′ = 1 phase; when the temperature is lowered to 273 K this fraction is decreased markedly. The pronounced pseudosymmetry in the P\bar 1 and P21/c structures that have Z′ = 2 has been investigated by analysing the atomic coordinates, by performing refinements in the smaller pseudocells and by making separate Wilson plots for the classes of reflections which are systematically strong and systematically weak. All three approaches are informative, but they reveal different information. Least-squares fits of coordinates of corresponding atoms measure the similarity of the molecular conformations. The Wilson plots allow a quantitative comparison of the intensities of the strong and weak reflections and thus an assessment of the deviations of the true structure from the smaller pseudocell structure. Comparison of the atomic displacements obtained in the full and pseudocell refinements shows where the structural distortions are largest and provides an indication of their directions."
Use of the Metals Data File (CRYSTMET) in XAFS analysis
Authors: Taguchi Takeyoshi , White Peter S.
Published: July 2002

The X-ray absorption spectrometry (XAS) technique has been widely used to determine the local structure of materials that are poorly suited to study by ordinary diffraction methods, such as fine particles or amorphous matter. XAS is among the major applications at synchrotron radiation facilities and many existing beamlines perform the measurement of XAFS (X-ray absorption fine structure) spectra. XAFS spectra can also be measured with conventional X-ray sources. Measurement of XAFS spectra is relatively straightforward, but real difficulties arise in the analysis and interpretation of the data. Contrary to single-crystal diffraction techniques, the structure is not obtained directly from the measured XAFS data. Model structures must be assumed and the corresponding simulated XAFS spectra must be calculated, with determination of which models best fit the measured data. Model building is a most important part of XAFS analysis, but creation of three-dimensional structures from crystal-chemical considerations can be a very time-consuming task. Utilization of a database of crystal structures and of its built-in structure analysis and display tools can considerably reduce the time and effort required by this task. As XAS is often used to study metals, a database of alloys and intermetallic compounds, such as CRYSTMET, incorporating an array of powerful tools, is then very useful.
Structures and photochemistry of 1,5-disubstituted dibenzobarrelenes
Authors: Jones R. , Rettig S. J. , Scheffer J. R. , Trotter J. , Yang J.
Published: July 2002

The effect of 1,5-disubstitution on the photochemistry of 9,10-ethenoanthracene-11,12-diesters has been studied and correlated with the crystal structures determined for several derivatives; photoproduct structures have been established from a crystal structure analysis of one of the products and from NMR correlations. The crystals studied are: (1)-Cl, dimethyl 1,5-dichloro-9,10-dihydro-9,10-ethenoanthracene-11,12-dicarboxylate, C20H14Cl2O4, P21/n; (1)-CN, 1,5-dicyano derivative, C22H14N2O4, P21/n; (1)-CN.0.5p-xylene, C22H14N2O4.0.5C8H10, P21/c; (1)-CN.xxylene, C22H14N2O4.xC8H10, P21/n; (1)-OMe, 1,5-dimethoxy derivative, C22H20O6, P{\bar 1}; (2b)-Cl, C20H14C12O4, dimethyl 1,5-dichloro-4b,8b,8c,8d-tetrahydrodibenzo[a, f]cyclopropa[c,d]pentalene-8c,8d-dicarboxylate, Pna21. The dibenzobarrelene molecules all have geometries and dimensions similar to those of related materials. In particular, the orientations of the two ester groups in each molecule differ, one being in general almost fully conjugated with the C(11)=C(12) double bond and the other almost non-conjugated. Relationships are sought between the ratios of the two regioisomeric photoproducts and molecular and crystal structures; both electronic and steric effects are considered, but no one effect seems to dominate.
Charge density studies in NH4[Ti(C2O4)2].2H2O crystals at two wavelengths
Authors: Sheu H. S. , Wu J. C. , Wang Y. , English R. B.
Published: July 2002

The crystal structure of ammonium bis-μ-oxalatotitanate(III) dihydrate, NH4[Ti(C2O4)2].2H2O, is reinvestigated by careful single-crystal X-ray diffraction using Mo Kα (room temperature and 130 K) and Ag Kα radiation (room temperature). It crystallizes in noncentrosymmetric hexagonal space group P6422, with cell parameters a = 8.947 (2), c = 10.898 (1) Å, Z = 3, F(000) = 423, Mr
= 278.1, Dx
= 1.833, R = 0.035, wR = 0.030 for 2296 observed reflections with I ≥ 2σ(I), on Ag Kα data. Accurate data measurement was applied using two wavelengths (0.7107 Å for Mo Kα and 0.5609 Å for Ag Kα in order to study the charge density distribution and also to investigate the wavelength effect in such investigations. A total of 22650 and 20298 reflections were measured using Mo Kα radiation at room temperature and 130 K, respectively, and 18361 reflections using Ag Kα radiation. The Ti atom is coordinated by four oxalate dianions with coordination number 8 in an approximate symmetry of D
4 geometry. Each oxalato ligand is bridged between two Ti atoms and forms an infinite polymeric spiral column along the c axis. The deformation density maps, Δρ, obtained from all three data sets are consistent and agree well. Although the formal charge of Ti in this complex is 3+, with only one electron on the 3d orbitals expected, the net atomic charge on Ti from this study is, ~2+. Charge asphericity around the Ti atom caused by the splitting of 3d orbitals is clearly observed in the deformation density. The d-orbital populations are evaluated from multipole refinement. The expected d
1 electron is mainly located in the dZ2

orbital.
A synchrotron X-ray study of the electron density in SmFeO3
Authors: Maslen E. N. , Streltsov V. A. , Ishizawa N.
Published: July 2002

Structure factors for small, hydrothermally grown samarium orthoferrite, SmFeO3, were measured with focused λ = 0.7 Å synchrotron radiation. Approximate high symmetry in the Δρ images indicates that cations deform the electron density far more strongly than the O atoms. The most obvious effect is on distribution of the Fe atoms. The influence of the nearest low-symmetry (Cs
) O coordination on the electron density of the Sm cation is weak by comparison with that of the Sm–Fe interactions, as is illustrated by the high symmetry of the Δρ map near the Sm atom. The Sm–Fe interactions appear to affect the magnetic ordering and spin configuration of the Fe atoms. Space group Pnma, orthorhombic, Mr
= 254.20, a = 5.6001 (3), b = 7.7060 (7), c = 5.3995 (6) Å, V = 233.01 (4) Å3, Z = 4, Dx
= 7.246 Mg m−3, μ
0.7 = 28.5 mm−1, F(000) = 448, T = 293 K, R = 0.017, wR = 0.021, S = 3.83 (8) for 1329 unique reflections.
Prediction of ferroelectricity in recent Inorganic Crystal Structure Database entries under space group Pba2
Authors: Abrahams S. C. , Mirsky K. , Nielson R. M.
Published: July 2002

Three new entries in space group Pba2 in the 1995 edition of the Inorganic Crystal Structure Database have been examined in the light of previously developed criteria for the prediction of ferroelectricity. The structural prediction [Abrahams (1989). Acta Cryst. B45, 228–232] that seven of the 21 entries presented under this space group in the 1988 edition most likely corresponded to previously unrecognized ferroelectric materials is thereby updated. The atomic coordinates of [(NH2)2C(NHNH3)]ZrF6, among the new entries, are shown to satisfy the structural criteria; the maximum displacement along the polar axis required of any atom to undergo spontaneous polarization reversal is no larger than ~ 0.9 Å for a C or N atom and ~ 1.4 Å for a H atom, within the aminoguanidinium (2+) cation. By contrast, all atoms in the two independent ZrF6 anions are within 0.1 Å of an arrangement with zero spontaneous polarization. The characteristic force constant of the organic group is presently unknown, hence the Curie temperature for the crystal cannot be estimated. In the second new entry, the only atoms in the structure of the superconductor Ba0.6K0.4BiO23 at 403 K which depart significantly from a centrosymmetric arrangement are three of the four independent O atoms, one of which is at a site only 6% occupied. If these displacements are not artefacts, then this material is ferroelectric with an estimated Curie temperature in the range 410–740 K; a subsequent study, however, reported the structure in space group Pbam. The atomic coordinates of the final 1995 entry, (Cl3PNPCl3)(MoOCl4), do not differ significantly from centrosymmetry, in which case the material is not ferroelectic, apart from unequal occupancy of the Mo-atom sites. If the disorder model is correct and the two Mo sites are unequally occupied, then these sites cannot become equivalent and (Cl3PNPCl3)(MoOCl4) must remain polar without the possibility of becoming ferroelectric.
Tautomerism in 3{5}-(dimethoxyphenyl)pyrazoles
Authors: Halcrow M. A. , Powell H. R. , Duer M. J.
Published: July 2002

The single-crystal X-ray structures of 3{5}-(2′,5′-dimethoxyphenyl)pyrazole (HI) and the hemihydrate of 3 {5}-(3/,4′-dimethoxyphenyl)pyrazole (IV) have been determined. Compound (HI) exists purely as the 5-substituted prototropomer in the crystal; the pyrazole pyrollic N—H proton is involved in a three-way hydrogen bond, involving an intramolecular contact with a methoxy oxygen donor and an intermolecular interaction to the pyridinic N atom of a neighbouring molecule, forming discrete hydrogen-bonded dimers. There is no evidence of degenerate proton transfer within the dimeric units from CPMAS 13C NMR spectroscopy, in contrast to other known pyrazoles that associate in this manner. In (IV).1/2H2O, however, the pyrrolic proton is disordered over both N(1) and N(2) via hydrogen bonding to the solvate water molecule. CPMAS 13C NMR spectroscopy shows that the prototropic disorder in (IV).1/2H2O is static at temperatures up to 370 K. Solution 1H and 13C NMR data in DMSO-d
6 show that for both (HI) and (IV) the 3- and 5-substituted tautomeric forms are similarly populated in this solvent, suggesting both that the intramolecular hydrogen bond in (HI) has been disrupted and that the two tautomers of (HI) and (IV) are close in energy.
Structures and Photochemistry of Inclusion Compounds of 9,10-Dihydro-9,10-ethenoanthracene-11,12-bis(diphenylphosphine Oxide)
Authors: Fu T. Y. , Liu Z. , Olovsson G. , Scheffer J. R. , Trotter J.
Published: July 2002

Inclusion complexes of 9,10-dihydro-9,10-etheno-anthracene-11,12-bis(diphenylphosphine oxide) (1) as host are synthesized using a variety of guest solvent molecules and the photochemistry of the host molecule is studied in solution and in the crystalline complexes. The crystal structures of four complexes are determined and correlated with their photochemical reactivity. In each case only one dibenzosemibullvalene photoproduct is obtained in the photolysis. Since three of the complexes studied crystallize in the chiral space group P212121, irradiation of single crystals produces a chiral photoproduct in >90% enantiomeric excess. Determination of the absolute configurations of reactants and products allows elucidation of the key structural features that control the enantiospecific solid-state photorearrangements.
Intermolecular N-H...O Hydrogen Bonding Assisted by Resonance. II. Self Assembly of Hydrogen-Bonded Secondary Enaminones in Supramolecular Catemers
Authors: Bertolasi V. , Gilli P. , Ferretti V. , Gilli G.
Published: July 2002

The crystal structures of 15 compounds containing the 2-en-3-amino-1-one heterodienic system and forming intermolecular N—H...O hydrogen bonds assisted by resonance (RAHB) are reported: (1) 3-phenylamino-2-cyclohexen-1-one; (2) 3-(4-methoxyphenylamino)-2-cyclohexen-1-one; (3) 3-(4-chlorophenylamino)-2-cyclohexen-1-one; (4) 3-(4-methoxyphenylamino)-2-methyl-2-cyclohexen-1-one; (5) 3-(4-methoxyphenylamino)-5-methyl-2-cyclohexen-1-one; (6) 3-isopropylamino-5,5-dimethyl-2-cyclohexen-1-one; (7) 3-phenylamino-5,5-dimethyl-2-cyclohexen-1-one; (8) 3-(3-methoxyphenylamino)-5,5-dimethyl-2-cyclohexen-1-one; (9) N,N-3-aza-pentane-1,5-bis[1-(3-oxo-5,5-dimethyl-1-cyclohexenyl)]; (10) 3-phenylamino-6,6-dimethyl-2-cyclohexen-1-one; (11) 3-(2-methoxyphenylamino)-6,6-dimethyl-2-cyclohexen-1-one; (12) 3-(3-chlorophenylamino)-6,6-dimethyl-2-cyclohexen-1-one; (13) 3-(4-chlorophenylamino)-6,6-dimethyl-2-cyclohexen-1-one; (14) 1-(4-chlorophenyl)-4-(4-chlorophenylamino)-6-methyl-2-pyridone; (15) 3-(4-chlorophenylamino)-5-phenyl-2-cyclopenten-1,4-dione. All compounds form intermolecular N—H...O=C hydrogen bonds assisted by resonance connecting the heteroconjugated enaminonic groups in infinite chains. Chain morphologies are analyzed to find out crystal engineering rules able to predict and interpret the crystal packing. Simple secondary enaminones [i.e. (1)–(13) together with a number of structures retrieved from the Cambridge Structural Database] are found to form hydrogen bonds having π-delocalizations, as characterized by a C=O bond-length average of 1.239 ± 0.004 Å, and hydrogen-bond strengths, represented by the N...O average distance of 2.86 ± 0.05 Å, very similar to those previously found for amides. Enaminones, however, can be easily substituted by chemical groups able to influence both π-conjugations and N...O hydrogen-bond distances. Some substituted enaminones, retrieved from the literature, display, in fact, N...O hydrogen-bond distances as short as 2.627 Å and large π-delocalizations with C=O double-bond distances as long as 1.285 Å. These effects appear to be associated with (a) the presence of further π-conjugated systems involving the C=O and NH groups of the enaminone moiety or (b) the transformation of the enaminone carbonyl group in an amidic function.
Lattice-Dynamical Estimation of Atomic Displacement Parameters in Carbonates: Calcite and Aragonite CaCO3, Dolomite CaMg(CO3)2 and Magnesite MgCO3
Authors: Pilati T. , Demartin F. , Gramaccioli C. M.
Published: July 2002

Using crystallographic information and empirical potentials derived from fitting the vibrational frequencies of all the substances under study, together with those of a group of silicates and oxides, a Born–von Karman rigid-ion lattice-dynamical model has been applied to the whole Brillouin zone in calcite, aragonite (α- and β-CaCO3, respectively), magnesite (MgCO3) and dolomite [CaMg(CO3)2]. The Raman and IR spectra are satisfactorily reproduced and interpreted by these calculations; there is also very good agreement with atomic anisotropic displacement parameters (a.d.p.'s) derived from accurate crystal structure refinement by various authors and with the experimental values of thermodynamic functions over a wide range of temperatures. On these vibrational grounds, the stability of calcite with respect to aragonite at high temperature can be accounted for.
A Reversible Non-disruptive Phase Transition Shown by the Zinc Iodide Dimethylformamide Complex ZnI2(dmf)2
Authors: Edwards R. A. , Easteal A. J. , Gladkikh O. P. , Robinson W. T. , Turnbull M. M. , Wilkins C. J.
Published: July 2002

At 228 K crystals of ZnI2(dmf)2 show a reversible phase transition which does not disrupt the lattice. Above the transition temperature the space group is C2/c and the cell contains eight symmetrically equivalent molecules. Cooling to below the transition temperature has little effect on the cell parameters or on the Zn- and I-atom positions, but the space group is now P21/n and the asymmetric unit comprises two conformationally different molecules. These arise from cooperative rotations of either ca +25 or −43° about the Zn—O bond of one of the dmf ligands in the high-temperature form. This displacive transition involves large movements of some atoms. The corresponding chloride and bromide are isomorphous with the higher temperature C2/c form, but it is only with the iodide that the weaker intermolecular forces permit the unusual phase change. The transition has been followed by differential scanning calorimetry, which gives an enthalpy change of 1.44 (5) kJ mol−1.