bluuebunny/crossref_metadata_2025 huggingface dataset:
Lattice dynamics and Debye–Waller factors of some compounds with the sodium chloride structure
Authors: Gao H. X. , Peng L.-M. , Zuo J. M.
Published: July 2002

The lattice dynamics of 19 compounds with the sodium chloride structure have been investigated using shell models. The models are compared with existing experimentally measured phonon-dispersion curves and refined using a multidimensional downhill simplex method. Debye–Waller factors for these compounds are calculated over the temperature range from 1 to 1000 K where appropriate and the results are fitted analytically using polynomial regression. The results are compared with experimentally measured room-temperature Debye–Waller factors and for most of the compounds the agreement is found to be better than 10%. At lower temperatures, it is expected that these results would be more accurate, since the harmonic approximation, which is crucial to the calculation of the Debye–Waller factors, works better. In choosing the models for particular applications, it is recommended that the model with the smallest standard error \sigma is used for fitting the experimentally measured phonon-dispersion curves, or the model that shows best agreement with reliable experimental measurements of more relevant physical quantities, such as Debye–Waller factors in crystallography.
Structures of [Pd(NH3)2
X
2] and its chemical transformation in the solid state
Authors: Kirik S. D. , Solovyov L. A. , Blokhin A. I. , Yakimov I. S.
Published: July 2002

Crystal structures of [Pd(NH3)2
X
2] complexes, where X = Br or I, diamminediiodo-/-dibromopalladium(II), have been studied by X-ray powder diffraction. The series consists of five complexes: cis-[Pd(NH3)2Br2] (I) [a = 13.3202 (7), b = 12.7223 (6), c = 7.05854 (3) Å, Z = 8, space group Pbca], trans-[Pd(NH3)2Br2] (II) [a = 6.7854 (3), b = 7.1057 (3), c = 6.6241 (2) Å, α = 103.221 (3), β = 102.514 (2), γ = 100.386 (3)°, Z = 2, space group P\overline 1], β-trans-[Pd(NH3)2Br2] (III) [a = 8.4315 (3), b = 8.4206 (3), c = 8.0916 (2) Å, Z = 4, space group Pbca], cis-[Pd(NH3)2I2] (IV) [a = 13.9060 (8), b = 13.5035 (8), c = 7.5050 (4) Å, Z = 8, space group Pbca], and β-trans-[Pd(NH3)2I2] (V) [a = 8.8347 (5), b = 8.8410 (5), c = 8.6081 (2) Å, Z = 4, space group Pbca]. Patterson synthesis and Rietveld refinement have been used for structural determination. Molecular structures with column- or parquet-type packing of flat complexes are characteristic of these substances. Corresponding cis- and β-trans compounds are isostructural. The thermal transformations cis→trans→β-trans (cis→β-trans in the case of iodine) are considered. Cl derivatives are also discussed. The transformations proceed irreversibly and are accompanied by decreasing specific volume. Owing to these features, they can be classified as chemical reactions. High-temperature X-ray powder diffraction was used to study the transformations in air. The set of data is consistent with a solid state transformation from cis to trans. According to this model, the columns of molecules remain intact during the process, and the transformation proceeds via the breaking of Pd...X and Pd...N intermolecular bonds. The powder diffraction data have been deposited in ICDD-JCPDS (45-0596, 46-0876, 46-0879, 47-1690, 48-1185).
Structure predictions allowing more than one molecule in the asymmetric unit
Authors: van Eijck Bouke P. , Kroon Jan
Published: July 2002

The UPACK program for crystal structure prediction was extended to allow the possibility of more than one molecule in the asymmetric unit. A search method was developed where the essential parameters (including torsional angles of hydroxyl groups) take random values. Energy minimization and clustering then lead to a list of hypothetical structures. In a test for six hexapyranoses this random search method was found to be approximately equally as efficient as the previously used systematic grid search method for one independent molecule. As a second test, the generation of structures with more than one independent molecule was performed for ethanol. A multitude of possible structures was found, the experimental one (two independent molecules in space group Pc) always being present. However, for \alpha-D-mannose (two independent molecules with five unknown hydroxyl torsional parameters each) the number of hypothetical structures was so large that the experimental structure was never encountered. Finally, a crystal structure generation for hydrates of pyranoses and polyalcohols was carried out. When the total number of unknown parameters was less than 20, the experimental structure was encountered. As usual, the empirical energies were not adequate to select that structure from the list of possible ones. Even then, the procedure proved to be valuable in identifying the most probable hydrogen-bonded network in cases where hydrogen positions in the experimental crystal structure were missing.
Towards a unified description of the AMOB2O5 (A = K, Rb, Cs, Tl; M = Nb, Ta) family of compounds
Authors: Schmid Siegbert , Withers Ray L. , Corker Deborah , Baules Pierre
Published: July 2002

Single-crystal X-ray diffraction data (Mo Kα radiation) are used to re-refine the structure of RbNbOB2O5, rubidium niobium oxo pyroborate. The structure is refined as an incommensurate modulated structure with superspace group symmetry Pmn21(0,0.4,0)s and lattice parameters a = 7.406 (2), b = 3.939 (2) and c = 9.475 (2) Å. Refinement on 3242 unique reflections converged to R = 0.031, while a previous conventional superstructure refinement led to R = 0.090. This lowering of the R factor goes hand-in-hand with a substantial reduction in the number of refined parameters. The refinement strongly suggests that the structure is effectively incommensurately modulated, despite an apparently rational magnitude of the primary modulation wavevector and overlap of satellite reflections.
Synthesis and structure of, and bonding in some derivatives of 2,6,9,10-tetraoxatricyclo[3.3.1.13,8]decane
Authors: Simons J. , Thomas H. G. , Hall S. R. , Raabe G.
Published: July 2002

Five derivatives of 2,6,9,10-tetraoxatricyclo[3.3.1.13,8]decane were synthesized and their molecular structures in the solid state were determined by means of X-ray diffraction analysis. In addition, the structures of all the molecules were optimized at different levels of computational quantum chemistry (HF/6-31G*, B3LYP/6-31G*). Experimentally determined bond lengths were compared with their calculated counterparts, and striking differences between the Hartree–Fock (HF) results and the experimental data could be traced back to the lack of correlation energy in the geometry optimizations. Two of the compounds under consideration are diastereomers and their relative stability was not only calculated with a conventional ab initio method (ZPE+MP2/6-31G*//HF/6-31G*), but also within the framework of density functional theory (B3LYP/6-31G*). In some of the calculations the influence of the solvent was included in single point calculations by means of an electrostatic model. Moreover, combining experimental as well as computational results, the exclusive axial orientation of a Cl atom in one of the compounds could be explained by an interplay of kinetic and energetic effects.
Structures of tetrathiabenzoquinone derivatives and the order–disorder phase transition
Authors: Matsumoto Shinya , Mizuguchi Jin
Published: July 2002

The tetrathiabenzoquinone derivatives 2,3,7,8-tetrahydro-benzo[1,2-b:4,5-b′]bis[1,4]dithiin-5,10-dione (1), C10H8O2S4, and 3,4,9,10-tetrahydro-2H,8H-benzo[1,2-b:4,5-b′]bis[1,4]dithiepin-6,12-dione (2), C12H12O2S4, have attracted attention as new organic pigments. Compound (1) has also been known to exhibit a large third-order non-linear susceptibility. In order to study the electronic structure in the solid state, the crystal structure has been investigated. Compound (1) was found to possess two crystal phases. Phase I is a room-temperature phase in which the molecular conformation is disordered. The other phase II is an ordered phase found at 223 K. Compound (2) is found to be isomorphous with (1) (phase II). Both (1) and (2) form characteristic one-dimensional molecular columns along the stacking c axis and two-dimensional or quasi two-dimensional molecular sheets parallel to the ab plane. Several close van der Waals contacts were observed between the neighboring molecules. Compound (1) was also found to undergo a reversible order–disorder phase transition at ca 265–269 K by X-ray diffraction as well as differential scanning calorimetry.
Competition of hydrogen-bond acceptors for the strong carboxyl donor
Authors: Steiner Thomas
Published: July 2002

A database study on the competition of hydrogen-bond acceptors for the strong carboxyl donor in crystals is reported. The relative success in attracting this donor is determined for 34 types of acceptor (O, N, S, halogen and π acceptors), and a correlation between the success in competition and the average hydrogen-bond distance is established.
Synthons and design in metal phosphates and oxalates with open architectures
Authors: Rao C. N. R. , Natarajan Srinivasan , Choudhury Amitava , Neeraj S. , Vaidhyanathan R.
Published: July 2002

We briefly describe the structures of open-framework metal phosphates with different dimensionalities, such as the one-dimensional linear-chain and ladder structures, two-dimensional layer structures and three-dimensional structures with channels. We demonstrate the role of the zero-dimensional four-membered ring monomer and of the one-dimensional ladder structure as the starting building units or synthons involved in the formation of the complex architectures. Thus, we show how the one-dimensional ladder structure transforms to two- and three-dimensional structures under mild conditions. The two-dimensional layer structures also transform to three-dimensional structures, while the zero-dimensional monomer transforms to layered and three-dimensional structures under ordinary reaction conditions. These transformations provide an insight into the possible pathways involved in the building up of the complex structures of metal phosphates. The isolation of amine phosphates during the hydrothermal synthesis of metal phosphates and also the facile reactions between amine phosphates and metal ions to yield a variety of open-framework materials have thrown light on the mechanism of formation and design of these structures. The existence of a hierarchy of open-framework metal oxalates and their ready formation by employing amine oxalates as intermediates provides additional support to the observations made earlier with regard to the phosphates.
Ab initio structure determination of monoclinic 2,2-dihydroxymethylbutanoic acid from synchrotron radiation powder diffraction data: combined use of direct methods and the Monte Carlo method
Authors: Tanahashi Yusaku , Nakamura Hisayoshi , Yamazaki Satoru , Kojima Yuko , Saito Hideshi , Ida Takashi , Toraya Hideo
Published: July 2002

The crystal structure of 2,2-dihydroxymethylbutanoic acid (C6H12O4) in monoclinic form has been determined ab initio from synchrotron radiation powder diffraction data. Two O and five C atoms were first derived by direct methods. Two missing O atoms and one C atom were found by the Monte Carlo method without applying constraint to their relative positions. Positional and isotropic displacement parameters of these non-H atoms were refined by the Rietveld method. Molecules are linked by hydrogen bonds and they make sheet-like networks running parallel to the (010) plane. The Monte Carlo method is demonstrated to be a powerful tool for finding missing atoms in partially solved structure.
The self-hosting structure of β-Ta
Authors: Arakcheeva Alla , Chapuis Gervais , Grinevitch Vladimir
Published: July 2002

Using electrodeposition from a bath of molten fluorides, single crystals of tetragonal β-tantalum have been obtained for the first time at normal pressure. The unit-cell parameters are a = 10.211 (3), c = 5.3064 (10) Å, space group P\bar 421
m. The β-Ta structure belongs to the σ-type Frank–Kasper structures which are typical for binary intermetallic compounds and β-U. In comparison to the σ-type, additional intercalated Ta atoms (population factor ∼0.01) have been detected between the atoms located in the channels of the structure. The shorter interatomic distances observed between the channel atoms in comparison with the atoms of the framework justify the `self-hosting' characteristic. β-Ta exhibits common features with the complex tetragonal structures of the high-pressure phases for the elements Rb, Ba, Sr, Bi and Sb.
Structure solution of the basic decagonal Al–Co–Ni phase by the atomic surfaces modelling method
Authors: Cervellino Antonio , Haibach Torsten , Steurer Walter
Published: July 2002

The atomic surfaces modelling technique has been used to solve the structure of the basic Ni-rich Al–Co–Ni decagonal phase. Formula Al70.6Co6.7Ni22.7, space group P\overline{10}, five-dimensional unit-cell parameters: d
1 = d
4 = 4.752 (3) Å, d
2 = d
3 = 3.360 (2) Å, d
5 = 8.1710 (2) Å; α12 = α34 = 69.295°, α13 = α24 = 45°, α14 = 41.410°, α23 = α
i5 = 90° (i = 1–4), V = 291.2 (7) Å5; Dx
= 3.887 Mg m−3. Refinement based on |F|; 2767 unique reflections (|F| > 0), 749 parameters, R = 0.17, wR = 0.06. Describing the structure of quasicrystals embedded in n-dimensional superspace in principle takes advantage of n-dimensional periodicity to select the minimal set of degrees of freedom for the structure. The method of modelling of the atomic surfaces yielded the first fully detailed structure solution of this phase. Comparison with numerous former, less accurate models confirms several features already derived, but adds a new essential insight of the structure and its complexity. The atoms fill the space forming recurrent structure motifs, which we will (generically) refer to as clusters. However, no unique cluster exists, although differences are small. Each cluster shows a high degree of structural disorder. This gives rise to a large configurational entropy, as much as expected in a phase which is stable at high temperature. On the other side, the cluster spatial arrangement is perfectly quasiperiodic. These considerations, corroborated by analysis of the structural relationship with neighbouring periodic phases, strongly suggest the existence of a non-local, long-range interaction term in the total energy which may be essential to the stability.
Symmetry-general ab initio computation of physical properties using quantum software integrated with crystal structure databases: results and perspectives
Authors: Le Page Yvon , Saxe Paul W. , Rodgers John R.
Published: July 2002

The timely integration of crystal structure databases, such as CRYSTMET, ICSD etc., with quantum software, like VASP, OresteS, ElectrA
etc., allows ab initio cell and structure optimization on existing pure-phase compounds to be performed seamlessly with just a few mouse clicks. Application to the optimization of rough structure models, and possibly new atomic arrangements, is detailed. The ability to reproduce observed cell data can lead to an assessment of the intrinsic plausibility of a structure model, even without a competing model. The accuracy of optimized atom positions is analogous to that from routine powder studies. Recently, the ab initio symmetry-general least-squares extraction of the coefficients of the elastic tensor for pure-phase materials using data from corresponding entries in crystal structure databases was automated. A selection of highly encouraging results is presented, stressing the complementarity of simulation and experiment. Additional physical properties also appear to be computable using existing quantum software under the guidance of an automation scheme designed following the above automation for the elastic tensor. This possibility creates the exciting perspective of mining crystal structure databases for new materials with combinations of physical properties that were never measured before. Crystal structure databases can accordingly be expected to become the cornerstone of materials science research within a very few years, adding immense practical value to the archived structure data.
On the structure of aragonite – Lawrence Bragg revisited
Authors: Bevan D. J. M. , Rossmanith Elisabeth , Mylrea Darren K. , Ness Sharon E. , Taylor Max R. , Cuff Chris
Published: July 2002

The structure of aragonite was first determined by Lawrence Bragg in 1924 in what is the now standard space-group setting Pnma (No. 62). Subsequent studies have all taken his structure as their starting points, despite Bragg's own stated doubts and some earlier etching studies which indicated that the underlying symmetry may really be polar. We have reinvestigated the structure and found that there are many reflections with significant intensity among those that should be systematically extinct in Pnma. Some of these reflections have been subjected to further experimental analysis and have been shown not to be due to Renninger effects. A possible model that satisfies these observations is one where the true structure is in space group P\overline 1 and the structure is twinned about the three axial twofold rotation axes of Pnma. The space group P1 cannot be ruled out. Evidence for these conclusions is presented. The crystal chemistry of aragonite is revisited and described in terms of the stuffed alloy CaC. The carbonate group is confirmed to be non-planar in the crystal.
New software for searching the Cambridge Structural Database and visualizing crystal structures
Authors: Bruno Ian J. , Cole Jason C. , Edgington Paul R. , Kessler Magnus , Macrae Clare F. , McCabe Patrick , Pearson Jonathan , Taylor Robin
Published: July 2002

Two new programs have been developed for searching the Cambridge Structural Database (CSD) and visualizing database entries: ConQuest and Mercury. The former is a new search interface to the CSD, the latter is a high-performance crystal-structure visualizer with extensive facilities for exploring networks of intermolecular contacts. Particular emphasis has been placed on making the programs as intuitive as possible. Both ConQuest and Mercury run under Windows and various types of Unix, including Linux.
Group-Theoretical Analysis of Octahedral Tilting in Perovskites. Erratum
Authors: Howard Christopher J. , Stokes Harold T.
Published: July 2002

An error has been noted within Fig. 1 of the paper by Howard & Stokes (1998). There is a group–subgroup relationship between I4/mcm (a
0
a
0
c
−) and C2/c (a
−
b
−
b
−), and this should be indicated on the figure by a continuous line joining the corresponding boxes. The corrected version of the figure is shown here.
Predicting the crystal structure of organic molecular materials
Authors: Chaka A. M. , Zaniewski R. , Youngs W. , Tessier C. , Klopman G.
Published: July 2002

This paper describes a novel method for predicting the crystal structure of organic molecular materials which employs a series of successive approximations to focus on structures of high probability, without resorting to a brute force search and energy minimization of all possible structures. The problem of multiple local minima is overcome by assuming that the crystal structure is closely packed, thereby eliminating 217 of the 230 possible space groups. Configurations within the 13 remaining space groups are searched by rotating the reference molecule about Cartesian axes in rotational increments of 15°. Initial energy minimization is performed using (6–12) Lennard–Jones pair potentials to produce a set of closely packed structures. The structures are then refined with the introduction of a Coulombic potential calculated using molecular multipole moments. This method has successfully located local minima which correspond to the observed crystal structures of several saturated and unsaturated hydro-C atoms with no a priori information provided. For large polycyclic aromatic hydrocarbons, additional refinements of the energy calculations are required to distinguish the experimental structure from a small number of closely packed structures. Our methodology for a priori crystal structure prediction represents the most efficient algorithm presented to date, in a field where the first successes have only been described within the past year and have been few and far between. Since our algorithm is capable of locating a large number of reasonable structures with similar energy in a short period of time, and is more likely to locate a minimum corresponding to the experimental structure, our program provides a superior framework to determine the level of theory required to calculate the intermolecular potential. For all but highly asymmetric hydrocarbons, however, distinguishing the observed structure from a large number of highly probable structures requires more rigorously calculated intermolecular interactions than pair potentials, plus an ad hoc electrostatic potential, and is thus beyond the scope of this paper. All calculations were performed on the Ohio Supercomputer Center's Cray Y-MP.
Solid-state structure of a layered hydrogen-bonded salt: guanidinium 5-benzoyl-4-hydroxy-2-methoxybenzenesulfonate methanol solvate
Authors: Russell V. A. , Ward M. D.
Published: July 2002

Guanidinium 5-benzoyl-4-hydroxy-2-methoxybenzene-sulfonate methanol solvate [C(NH2)3
+.(C14H11O3)SO3
−.CH3OH] crystallizes into a layered structure containing a two-dimensional hydrogen-bonded network typical of guanidinium alkane- and arenesulfonates. All six guanidinium protons and six sulfonate oxygen lone-pair acceptors participate in hydrogen bonding to form nearly planar pseudohexagonal hydrogen-bonded sheets, which can be viewed as parallel connected hydrogen-bonded ribbons. The 5-benzoyl-4-hydroxy-2-methoxybenzene groups are oriented to the same side of each ribbon, but the orientation of these groups on adjacent ribbons alternates with respect to the hydrogen-bonded sheet. The planar sheets stack with interdigitation of the arene groups, resulting in a structure in which layers of 5-benzoyl-4-hydroxy-2-methoxybenzene groups are separated by ionic hydrogen-bonded sheets. Each methanol molecule forms a hydrogen bond to one of the sulfonate O atoms, resulting in this oxygen forming a total of three hydrogen bonds, and fills void volume between the interdigitated 5-benzoyl-4-hydroxy-2-methoxybenzene groups of neighboring sheets. The benzophenone hydroxyl proton forms an intramolecular hydrogen bond to the carbonyl oxygen.
Structure of vanadocene in the temperature interval 108–357 K and the nature of the ring disorder
Authors: Antipin M. Yu. , Boese R.
Published: July 2002

The crystal structure of vanadocene, bis(π5–2,4-cyclopentadien-1-yl)vanadium, Cp2V, has been studied at the four temperatures 108, 170, 297 and 357 K by X-ray diffraction in order to elucidate the nature of the Cp-ring disorder and compare the results obtained with similar data for formally isostructural ferrocene, cobaltocene and nickelocene. A careful analysis of the anisotropic displacement parameters (ADP's) for Cp2V showed that at 108 K the crystal structure is ordered (staggered Cp2V molecules occupy centres of symmetry in the space group P21/n, Z = 2), which distinguishes vanadocene from its formal analogues and some Cp-ring disorder starts at ca 170 K. A model of this disorder has been proposed with two distinct orientations of the ring, differing in temperature-dependent occupancy factors and by a rotation in the ring plane of almost 36°. The fractional contributions of the second (minor) ring orientation are 0.05, 0.17 and 0.35, respectively, at 170, 297 and 357 K. The energetic characteristics of the ring rotation motion in the Cp2V crystal were estimated from ADP analysis and compared with similar data for other 3d-metallocenes.
Benzene-1,2-disulfonate dianion: another compound with meshed cogwheel substituents?
Authors: Nagel N. , Eller P. , Bock H.
Published: July 2002

Crystal structures of the dipotassium and potassium tetramethylammonium salts of benzene-1,2-disulfonic acid have been determined at 200 K and a PM3 enthalpy hypersurface for the substituent rotations of the free benzene-1,2-disulfonate dianion was calculated. The cogging of the two sulfonate groups as well as the possible pathways of conformational isomerization are discussed and compared with already investigated molecules containing adjacent threefold substituents.
Experimental and theoretical morphologies of diuron, N'-(3,4-dichlorophenyl)-N,N-dimethylurea
Authors: Pfefer G. , Boistelle R.
Published: July 2002

Crystals of diuron, N′-(3,4-dichlorophenyl)-N,N-dimethylurea, C9H10Cl2N2O, were grown from ethanol at low supersaturation. The crystal faces were indexed using a two-circle optical goniometer and X-ray diffraction was used to orientate the crystal morphology with respect to the unit cell. The experimental morphologies were compared with the morphologies predicted by the BFDH (Bravais, Friedel, Donnay, Harker) and attachment energy (AE) methods and calculated from two crystal structures. Good agreement was obtained between experimental and theoretical habits, despite the fact that the crystals exhibit 27 faces belonging to 13 crystallographic forms.
Synchrotron X-ray electron density in the layered LaOCl structure
Authors: Maslen E. N. , Streltsov V. A. , Streltsova N. R. , Ishizawa N.
Published: July 2002

The deformation density Δρ in lanthanum oxychloride, LaOCl, has been determined for a small, naturally faced single-crystal using 0.7 Å synchrotron X-radiation. Accumulation of electron density in the interlayer region stabilizes the structure that contains layers of positively charged Cl atoms. Space group P4/nmm, tetragonal, Mr
= 190.36, a = 4.1198 (7), c = 6.883 (2) Å, V = 116.8 (5) Å3, Z = 2, Dx
= 5.411 Mg m−3, μ
0.7 = 17.28 mm−1, F(000) = 164, T = 293 K, R = 0.011, wR = 0.011, S = 1.98 (7) for 416 unique reflections.
Structure of tris(cyclohexylammonium) phosphoenolpyruvate monohydrate and crystal chemistry of phosphoenolpyruvates
Authors: Souhassou M. , Schaber P. M. , Blessing R. H.
Published: July 2002

The crystal structure of (C6H11NH3
+)3.Pep3−.H2O, where Pep3− = (O−)2P(O)—O—C(CH2)—CO−
2, is reported and the systematic structural variations among 19 crystallographic occurrences of H3Pep, H2Pep−, HPep2− and Pep3− species, which are important phosphate donors in the ATP cycle of bioenergetics, are reviewed. Tris(cyclohexylammonium) phosphoenolpyruvate monohydrate, (C6H11NH3
+)3.[O3POC(CH2)CO2]3−.H2O, Mr
= 483.6, m.p. 418–420 K; T = 296 (1) K; orthorhombic, P212121; a = 16.7042 (5), b = 24.4881 (6), c = 6.38910 (10) Å; V = 2613.49 (11) Å3, Z = 4, Dx
= 1.23, Dm
= 1.22 mg mm−3, μ = 0.14 mm−1 for λ(MoKα) = 0.7107 Å; F = 1056 e; R(|F|) = 0.0608 for 6056 hkl and hk\bar l data with (sin θ)/λ ≤ 0.65 Å−1.
Zeolite Rho Loaded with Methylamines. II. Dimethylamine Loadings
Authors: Weidenthaler C. , Fischer R. X. , Abrams L.
Published: July 2002

Samples of two differently prepared zeolite rho loaded with different amounts of dimethylamine (DMA) were studied in their hydrated forms by X-ray diffraction. Both zeolites are partially dealuminated, as indicated by nonframework A1 which is assumed to be Al2O3 or AlOOH. Series I was prepared from dry-calcined NHn-rho at 873 K, series II from steam-calcined NHn-rho at 773 K. The samples were loaded with different amounts of DMA. Rietveld refinements yielded the following results for series I: (1) H3.8(H-DMA)5Cs0.2Al9Si39O96.Al2O3.21H2O, X-ray data collection at room temperature, Im\overline 3m, a = 15.0590 (2) Å, R

wp
= 0.089; (2) (H-DMA)8.8Cs0.2Al9Si39O96.Al2O3.18H2O, X-ray data collection at room temperature, Im\overline 3m, a = 15.0680 (2) Å, R

wp
= 0.091. Series II: (3) H0.3(H-DMA)5Cs0.7Al6Si42O96.2.5Al2O3.24H2O, X-ray data collection at room temperature, Im\overline 3m, a = 15.0596 (2) Å, R

wp
= 0.120. DMA resides on the center axis through the \alpha-cage with the N atoms pointing to the single eight-ring and the two methyl groups oriented towards the center of the \alpha-cage.
Structures of the Optically Active Monofluoro-Substituted Mandelic Acids: Relation to Their Racemic Counterparts and Thermochemical Properties
Authors: Larsen S. , Marthi K.
Published: July 2002

The crystal structures of the three optically active monofluoro-substituted mandelic acids (C8H7FO3, M

r
= 170.14) have been determined from low-temperature X- ray diffraction data. (R)-(−)-ortho-Fluoromandelic acid, monoclinic, P21, a = 8.356 (2), b = 10.842 (2), c = 8.544 (2) Å, β = 94.13(2)°, Z = 4, m.p. 361.8 (5) K. (R)-(−)-meta-Fluoromandelic acid, monoclinic, P21, a = 8.493 (3), b = 5.8426 (7), c = 15.628 (3) Å, β = 104.10(2)°, Z = 4, m.p. 394.2 (5) K. (R)-(−)-para-Fluoromandelic acid, monoclinic, C2, a = 8.464 (2), b = 5.8518 (13), c = 15.868 (2) Å, β = 107.56 (2)°, Z = 4, m.p. 425.8 (5) K. The hydrogen-bonding schemes in these structures have been analysed and compared with the hydrogen-bond patterns in the equivalent racemic acids. The structural data from six other structures were included in the analysis, which led to a general description of the great variety of hydrogen-bond motifs observed in α-hydroxycarboxylic acids. Differences between crystal structures of the racemic and enantiomerically pure fluoromandelic acids have been related to their melting enthalpies. The observed differences in the binary phase diagrams of the three acids can be rationalized in terms of their structural differences.
Disorder in Cyclodextrin Clathrates: X-ray Diffuse Scattering Analysis of α-Cyclodextrin Cyclopentanone Hydrate
Authors: Le Bas G. , Doucet J.
Published: July 2002

Most of the cyclodextrin clathrates display disorder, consequently the structure determination is often difficult to carry out. Previous studies by X-ray and neutron diffraction have shown that the \alpha-cyclodextrin cyclopentanone hydrate crystallizes in a channel-like structure with hexagonal P6 symmetry. Two inclusion complexes were found in the asymmetric unit; there is an orientational disorder of the guest with six positions found in one host cavity, three in the other. In addition, certain host hydroxyl groups and water molecules were found to be disordered and their disorder was correlated with the guest disorder. In this paper we investigate X-ray diffuse scattering phenomena in order to go further in the interpretation of disorder. Complicated diffuse scattering patterns revealed a variety of disorder phenomena. Some were not shown by the average structure determination. The interpretation of the results (structure in domains, supercell and vacancies) helps to explain the rotational disorder of the guest and some observed strains of the averaged structure. The description of different microstructures may be of interest for crystal growth and inclusion chemistry.
X-ray Study of M
II(ClO4)2.6H2O (M
II = Zn, Ni): Twinning, Disorder and Phase Transitions
Authors: Ghosh S. , Mukherjee M. , Seal A. , Ray S.
Published: July 2002

X-ray studies of zinc and nickel perchlorate hexahydrate, Zn(ClO4)2.6H2O and Ni(ClO4)2.6H2O, respectively, at different temperatures have been carried out to correlate the structural changes with phase transitions in the compounds. The crystals are pseudohexagonal (P63
mc), exhibiting a three-component orthorhombic twinning (Pmn21). At high temperatures a slight deviation of the b/a ratio of the three twinned orthorhombic cells from 31/2 results in a three-component splitting of each spot in the X-ray photograph, which on cooling to room temperature coalesce into single ones, thus restoring the original b/a ratio. The diffuse streaks disappear in the high-temperature photographs due to a decrease in the probability of error in repetition along the b axes of the three orthorhombic cells with temperature. A successful refinement of the heat-treated ordered Zn(ClO4)2.6H2O crystal verifies the continuous perchlorate–water arrangement and three-component twinning of the orthorhombic cell. Low-temperature X-ray photographs indicate no structural change.
Molecular Motion and Conformational Interconversion of Azobenzenes in Crystals as Studied by X-ray Diffraction
Authors: Harada J. , Ogawa K. , Tomoda S.
Published: July 2002

Crystal structures of (E)-azobenzene (1), (E)-2,2′- dimethylazobenzene (2), (E)-3,3′-dimethylazobenzene (3) and (E)-4,4′-dimethylazobenzene (4) were determined by X-ray diffraction at various temperatures. An apparent shrinkage of the N=N bond and its temperature dependence were observed and are interpreted in terms of an artifact caused by the torsional vibration of the N—Ph bonds in crystals. In the crystal structures of (1), (3) and (4) the dynamic disorder was observed. The disorder is accounted for by the torsional vibration whose amplitude is large enough to give rise to the conformational interconversion. No disorder was observed for a crystal of (2). This is ascribed to the large difference in energy of the two conformers as free molecules. The true length of the N=N bond in azobenzenes was estimated to be 1.26–1.27 Å.
Pentachlorotoluene: Rotational and Compositional Disorder
Authors: Brock C. P. , Fu Y.
Published: July 2002

The structure of pentachlorotoluene, which is isostructural with most other C6Cl
n
(CH3)_{6-n} derivatives, has been redetermined at 294 (1) K. The methyl group is distributed almost randomly among six sites; there is little discrimination between the Cl and CH3 substituents. The TL thermal-motion description suggests that the in-plane libration is moderate ( <\psi2> = 13 deg2); the barrier to in-plane rotation is estimated as 35 (3) kJ mol−1. Compositional disorder is a possibility.
Structures and Magnetic Properties of Dimeric Copper(II) Benzoylformates
Authors: Harada A. , Tsuchimoto M. , Ohba S. , Iwasawa K. , Tokii T.
Published: July 2002

Structures of six dimeric copper(II) benzoylformates (I)–(VI) have been determined by single-crystal X-ray diffraction and the magnitudes of the spin-exchange interaction (H = −2JS1S2) have been determined based on the magnetic susceptibility measurement. (I): Tetrakis(\mu-benzoylformato-O,O′)bis(pyridine)dicopper(II), [Cu(C8H5O3)2C5H5N]2, −2J = 648 cm^{-1}. (II): Tetrakis(\mu-benzoylformato-O,O′)bis(2-methylpyridine)dicopper(II), [Cu(C8H5O3)2C6H7N]2, −2J = 656 cm^{-1}. (III): Tetrakis(\mu-benzoylformato-O,O′)bis(3-methylpyridine)dicopper(II) monobenzene solvate, [Cu(C8H5O3)2C6H7N]2.C6H6, −2J = 649 cm^{-1}. (IV): Tetrakis(\mu-benzoylformato-O,O′)bis(4-methylpyridine)dicopper(II), [Cu(C8H5O3)2C6H7N]2, −2J = 625 cm^{-1}. (V): Bis(2,3-dimethylpyridinium) tetrakis(\mu-benzoylformato-O,O′)bis(chloride)dicopper(II) dibenzene solvate, 2C7H10N.[Cu(C8H5O3)2Cl]2.2C6H6, −2J = 618 cm^{-1}. (VI): Tetrakis(benzoylformato-O,O′)bis(caffeine)dicopper(II) dibenzene solvate, [Cu(C8H5O3)2C8H10N4O2]2.2C6H6, −2J = 651 cm^{-1} (caffeine = 3,7-dihydro-l,3,7-trimethyl-1 H-purine-2,6-dione). In the binuclear cage structure the coordination geometry around the CuII atoms is typical square pyramidal with Cu...Cu distances 2.725 (1)–2.843  (1) Å. An ab initio molecular orbital calculation for the benzoylformate ion indicates that the unusually strong antiferromagnetic interaction in dimeric copper(II) benzoylformates is attributed to the electronic effect of the \alpha-keto group in the bridging carboxylate moiety. This is in accordance with a positive linear relationship between −2J of the copper(II) carboxylates and the diagonal part of the 2p

x
orbital population of the carboxylate C atom in the symmetrical HOMO, highest occupied molecular orbital (x is parallel to the C—R bond axis in the RCOO^{-} ion).
Structure Refinement of Monoclinic Astrophyllite
Authors: Shi N. , Ma Z. , Li G. , Yamnova N. A. , Pushcharovsky D. Yu.
Published: July 2002

The crystal structure of monoclinic magnesium astrophyllite has been redetermined. The samples were from Khibina alkaline massifs, Russia. K2NaNa(Fe,Mn)4Mg2Ti2[Si4O12]2(OH)4(OH,F)2, Mr
= 1202.31, monoclinic, A2, a = 10.370 (3), b = 23.129 (5), c = 5.322 (1) Å, β = 99.55 (2)°, V = 1258.8 (5) Å3, Z = 2, Dx
= 3.173 Mg m−3, Mo Kα, λ = 0.71069 Å, μ = 3.74 mm−1, F(000) = 1179, T = room temperature, R = 0.057 for 1720 reflections. The determination indicated that the coordination number of Ti is five. In tetrahedra of astrophyllite the open-branched Si—O chain connected with Ti—O semi-octahedra by corner-sharing formed a continuous sheet. Na atoms have two types of coordination. Their coordination numbers are eight and six. The Na atoms with coordination number eight are situated between layers. Other types of Na atoms are situated within the layers and form Na—O octahedra with partial occupancies.
Structure Refinements of the Layered Intergrowth Phases SbIIISbV x

A
1−x
TiO6 (x≃0, A = Ta, Nb) Using Synchrotron X-ray Powder Diffraction Data
Authors: Ling C. D. , Thompson J. G. , Schmid S. , Cookson D. J. , Withers R. L.
Published: July 2002

The structures of the layered intergrowth phases SbIIISb^{\rm V}_xAl-xTiO6 (x \simeq 0, A = Ta, Nb) have been refined by the Rietveld method, using X-ray diffraction data obtained using a synchrotron source. The starting models for these structures were derived from those of Sb^{\rm III}_3Sb^{\rm V}_xA
3−xTiO14 (x = 1.26, A = Ta and x = 0.89, A = Nb), previously solved by single-crystal X-ray diffraction. There were no significant differences between the derived models and the final structures, validating the approach used to obtain the models and confirming that the n = 1 and n = 3 members of the family, Sb^{\rm III}_nSb^{\rm V}_xA
n−xTiO4n+2 are part of a structurally homologous series.
Hypothetical Crystal Structures of Benzene at 0 and 30 kbar
Authors: van Eijck B. P. , Spek A. L. , Mooij W. T. M. , Kroon J.
Published: July 2002

Possible crystal structures of benzene were generated without any prior crystallographic information, using a systematic grid search method. Only structures with one molecule in the asymmetric unit were considered. 31 space groups were investigated, and the resulting structures were clustered and checked for stability upon removal of the space-group symmetry. In the enthalpy range ∼10 kJ mol−1 30 structures were found at zero pressure and 20 structures at a pressure of 30 kbar. Their pressure-dependent rankings and interconversions are discussed. The results are compared to previous, less complete, investigations. A possible structure for the high-pressure phase benzene (II), on which only limited powder diffraction data have been published, is suggested.
Uric Acid Dihydrate Revisited
Authors: Parkin S. , Hope H.
Published: July 2002

At 120 K, crystals of pure laboratory-grown uric acid dihydrate [7,9-dihydro-1H-purine-2,6,8(3H)-trione dihydrate], C5H4N4O3.2H2O, are monoclinic, P21/c, with a = 7.237 (3), b = 6.363 (4), c = 17.449 (11) Å, β = 90.51 (1)°, Z = 4, wR
2 (all data) = 0.1094, R
1 = 0.0406 for data with I > 2σ(I). The crystals exhibit pseudo-orthorhombic twinning with refined twin fractions of 0.89 and 0.11. Within each twin, disorder about a noncrystallographic twofold exists with refined occupancies of 0.83 and 0.17. Packing consists of layers of hydrogen-bonded uric acid separated by layers of hydrogen-bonded water. The epitaxy of uric acid dihydrate with its anhydrous counterpart is readily explicable from the refined model. Treatment of the structure as twinned and disordered in space group P21/c fully accounts for well documented violations of systematic absences in orthorhombic space groups.
Opening and Narrowing of the Water H—O—H Angle by Hydrogen-Bonding Effects: Re-inspection of Neutron Diffraction Data
Authors: Steiner T.
Published: July 2002

For 71 water molecules donating two Ow—H...O hydrogen bonds, the correlation of the covalent H—O—H angle and the O...Ow...O angle is inspected from 49 well refined organic and organometallic neutron diffraction crystal structures. Compared to sample average, the water angle is opened for large and narrowed for small O...Ow...O angles. Notably, the H—O—H angle is widened compared with the gas phase value even for small O...Ow...O. Related behavior is observed for chloride anion acceptors. The correlation exhibits a considerable scatter which should not be interpreted as experimental inaccuracies, but as secondary effects. Possible secondary effects are multi-center hydrogen bonding and effects of coordination to the water O atom. In a comparative test, low-temperature X-ray diffraction data were shown to be completely unsuitable for this type of analysis. The dependence of the C—O—H angle on the C—O...O angle in hydrogen bonds donated by hydroxyl groups in carbohydrates is also shown.
Li+/Na+ β-alumina: a combined single-crystal neutron and X-ray diffraction study. Erratum
Authors: Edström K. , Gustafsson T. , Thomas J. O. , Farrington G. C.
Published: July 2002

Lists of atomic coordinates, anisotropic displacement parameters and structure factors have been deposited with the IUCr (Reference: AB0365) for this article [Edström, Gustafsson, Thomas & Farrington, Acta Cryst. B53, 631–638]. Copies may be obtained through The Managing Editor, International Union of Crystallography, 5 Abbey Square, Chester CH1 2HU, England.
Room-Temperature Superstructure of ZrV2O7
Authors: Evans J. S. O. , Hanson J. C. , Sleight A. W.
Published: July 2002

The structure of ZrV2O7, zirconium pyrovanadate, has been refined from single-crystal synchrotron X-ray data. As with other phases in the AM
2O7 family, ZrV2O7 shows a set of strong reflections, which can be explained on the basis of a cubic unit cell with a = 8.765 Å, and a family of much weaker reflections due to a 3 × 3 × 3 superstructure. The superstructure has been refined to RF
= 0.036 (a = 26.296 Å, Pa3¯, 6972 reflections) and contains highly regular ZrO6 and VO4 polyhedra. Of the six unique V2O7 groups, two are constrained by symmetry to contain linear V—O—V linkages, while the remaining four are free to bend away from 180°. The structural distortions from the ideal high-symmetry structure to the observed room-temperature structure are described.
Equation of State of α-Al2O3 (Corundum) from Quasi-Harmonic Atomistic Simulations
Authors: Catti M. , Pavese A.
Published: July 2002

A two-body interatomic potential function, including fractional atomic charges and a shell model for oxygen, and supplemented by an O—Al—O bond-angle energy term, was fitted to the structural, elastic and vibrational properties of \alpha-Al2O3, corundum, at ambient conditions. Full quasi-harmonic calculations were then carried out on a p,T grid of 54 points in the domain 0–40 GPa and 300–1700 K. The crystal structure was equilibrated at each point, taking into account the anisotropy of vibrational pressure and the thermal dependence of elastic constants, so as to obtain unit-cell edges, atomic coordinates, bulk modulus, thermal expansion coefficient and other thermodynamic properties. Polynomial approximations were developed to represent the p,T dependence of these quantities. Comparison with experimental results for the separate p (T = 300 K) and T (p = 0) behaviours shows very good agreement, with average deviations of 0.1% for the unit-cell volume and 6% for the thermal expansion coefficient. The coupled p,T dependence of the properties of corundum is predicted to be very small for the bulk modulus (\partial^2K_T/\partial p\partial T=8.4\times10^{-5} K−1), but not at all negligible for the volume [(1/V)\partial^2V/\partial p\partial T in the range −1.2 to −7.5 × 10−7 GPa−1 K−1 over the p,T domain explored].
Arrays with Local Centers of Symmetry in Space Groups Pca21 and Pna21
Authors: Marsh R. E. , Schomaker V. , Herbstein F. H.
Published: July 2002

Of the several hundred structures in the Cambridge Structural Database [version 4.6 (1992), Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, England] having space groups Pca21 or Pna21 and more than one molecule in the asymmetric unit (Z > 4), approximately three-quarters contain local centers of symmetry. These local centers, which are not crystallographic centers, occur predominantly near x = 1\over8, y = 1\over4 in Pca21 or near x = 1\over8, y = 0 in Pna21; this also holds for the limited number of examples we have examined of pseudo-centrosymmetric molecules with Z = 4. Local centers at these points create unusual correlations between corresponding atoms in the two molecules.
Three conformational polymorphs and order–disorder phase transition in a calix[4]arene bis(crown)
Authors: Thuéry Pierre , Nierlich Martine , Calemczuk Roberto , Saadioui Mohamed , Asfari Zouhair , Vicens Jacques
Published: July 2002

Three polymorphs of a calix[4]arene fixed in the 1,3-alternate conformation by two bridges, a crown-6 on one side and a crown-6 including a photoisomerizable azobenzene unit on the other, 8,11,14,28,31,34,37,40,43,57,60,63-dodecaoxa-2,3-diazanonacyclo[62.2.2.24,7.116,45.126,55.015,20.022,27.044,49.051,56]doheptaconta-1(66),2,4,6,15,17,19,22,24,26,44,46,48,51,53,55,64,67,69-nonadecene, have been characterized by single-crystal X-ray diffraction. In all the polymorphs the azobenzene group is in the more stable trans conformation. The polymorphism arises from the high conformational flexibility of the bridges, which are arranged differently and are highly disordered in the two monoclinic forms (1) and (2a). Form (2a) presents a phase transition near 268 (2) K, leading to a triclinic form (2b), differing from the high-temperature form by some ordering of the chains and a subsequent reduction in crystallographic site multiplicity. The evolution of the cell parameters and peak width with temperature has been investigated down to 173 (2) K. Adiabatic calorimetry measurements indicate a smooth transition centered at 265.0 (1) K, with a heat capacity jump of 60 mJ g−1 K−1, suggesting a second-order nature for the transition.
TEMPO radicals showing magnetic interactions. I. 4-(4-Halobenzylideneamino)TEMPO and related compounds
Authors: Iwasaki Fujiko , Yoshikawa Joseph H. , Yamamoto Hajime , Kan-nari Eiji , Takada Kiwamu , Yasui Masanori , Ishida Takayuki , Nogami Takashi
Published: July 2002

X-ray crystal structure analyses of the 4-(4-halobenzylideneamino)TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidyl-1-oxyl) radicals 4-(4-fluorobenzylideneamino)-2,2,6,6-tetramethylpiperidyl-1-oxyl, 4-(4-chlorobenzylideneamino)-2,2,6,6-tetramethylpiperidyl-1-oxyl, 4-(4-bromobenzylideneamino)-2,2,6,6-tetramethylpiperidyl-1-oxyl and 4-(4-iodobenzylideneamino)-2,2,6,6-tetramethylpiperidyl-1-oxyl, and other 4-Ar—CH=N—TEMPO [Ar = 4-Ph-Ph, 2,2,6,6-tetramethyl-4-(4-phenylbenzylideneamino)piperidyl-1-oxyl, 4-Py, 2,2,6,6-tetramethyl-4-(4-pyridylmethylideneamino)piperidyl-1-oxyl, and 3,5-diCl-Ph, 4-(3,5-dichlorobenzylideneamino)-2,2,6,6-tetramethylpiperidyl-1-oxyl] radicals have been performed at room temperature. Some of these radicals show intermolecular ferromagnetic interactions at extremely low temperatures. X-ray analysis revealed that crystals of the 4-I-Ph derivative showed two modifications [(a) and (b)]; measurements of the magnetic properties of these crystals showed a ferromagnetic transition at an extremely low temperature for modification (a) and an antiferromagnetic interaction for modification (b). The structural features of these TEMPO radical crystals can be classified into three groups: (i) the crystal structures of the 4-Cl-Ph, 4-I-Ph(a) and 4-Ph-Ph derivatives, which show a ferromagnetic transition; (ii) the structures of the 4-Br-Ph and 4-Py derivatives, which show a ferromagnetic interaction (θ > 0); (iii) miscellaneous: the antiferromagnetic 4-F-Ph and 4-I-Ph(b) derivatives (θ < 0) and the ferromagnetic 3,5-diCl-Ph (θ > 0) derivative. Sheet-like arrangements of O atoms and intra-sheet interactions through the CH2 or CH3 groups of the TEMPO rings are related to the mechanisms of the ferromagnetic interactions.