Different Crystal Structures (different + crystal_structure)

Distribution by Scientific Domains


Selected Abstracts


Diagrammatic Separation of Different Crystal Structures of A2BX4 Compounds Without Energy Minimization: A Pseudopotential Orbital Radii Approach

ADVANCED FUNCTIONAL MATERIALS, Issue 12 2010
Xiuwen Zhang
Abstract The A2BX4 family of compounds manifest a wide range of physical properties, including transparent conductivity, ferromagnetism, and superconductivity. A 98% successful diagrammatic separation of the 44 different crystal structures of 688 oxide A2BX4 compounds (96% for 266 oxide-only) is described by plotting the total radius of the A atom RA versus the radius of the B atom RB for many A2BX4 compounds of known structure types and seeking heuristically simple, straight boundaries in the RA versus RB plane that best separate the domains of different structure types. The radii are sums RA,=,Rs(A),+,Rp(A) of the quantum-mechanically calculated "orbital radii" Rs(Rp), rather than empirical radii or phenomenological electronegativity scales. These success rates using first-principles orbital radii uniformly exceed the success rates using classic radii. Such maps afford a quick guess of the crystal structure of a yet unmade A2BX4 compound by placing its atomic orbital radii on such maps and reading off its structure type. [source]


Heterogeneity and dynamics in villin headpiece crystal structures

ACTA CRYSTALLOGRAPHICA SECTION D, Issue 5 2009
Jianmin Meng
The villin headpiece domain (HP67) is the C-terminal F-actin-binding motif that confers F-actin-bundling activity to villin, a component of the actin bundles that support the brush-border microvilli. It has been investigated extensively by both experimental and theoretical measurements. Our laboratory, for example, has determined both its NMR and its crystal structures. This study presents the structures of HP67 and its pH-stabilized mutant (H41Y) in a different crystal form and space group. For both constructs, two molecules are found in each asymmetric unit in the new space group P61. While one of the two structures (Mol A) is structurally similar to our previously determined structure (Mol X), the other (Mol B) has significant deviations, especially in the N-terminal subdomain, where lattice contacts do not appear to contribute to the difference. In addition, the structurally most different crystal structure, Mol B, is actually closer to the averaged NMR structure. Harmonic motions, as suggested by the B -factor profiles, differ between these crystal structures; crystal structures from the same space group share a similar pattern. Thus, heterogeneity and dynamics are observed in different crystal structures of the same protein even for a protein as small as villin headpiece. [source]


Diagrammatic Separation of Different Crystal Structures of A2BX4 Compounds Without Energy Minimization: A Pseudopotential Orbital Radii Approach

ADVANCED FUNCTIONAL MATERIALS, Issue 12 2010
Xiuwen Zhang
Abstract The A2BX4 family of compounds manifest a wide range of physical properties, including transparent conductivity, ferromagnetism, and superconductivity. A 98% successful diagrammatic separation of the 44 different crystal structures of 688 oxide A2BX4 compounds (96% for 266 oxide-only) is described by plotting the total radius of the A atom RA versus the radius of the B atom RB for many A2BX4 compounds of known structure types and seeking heuristically simple, straight boundaries in the RA versus RB plane that best separate the domains of different structure types. The radii are sums RA,=,Rs(A),+,Rp(A) of the quantum-mechanically calculated "orbital radii" Rs(Rp), rather than empirical radii or phenomenological electronegativity scales. These success rates using first-principles orbital radii uniformly exceed the success rates using classic radii. Such maps afford a quick guess of the crystal structure of a yet unmade A2BX4 compound by placing its atomic orbital radii on such maps and reading off its structure type. [source]


The magnetic moments and their long-range ordering for Fe atoms in a wide variety of metallic environments

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 15 2010
A. Ayuela
Abstract The magnitudes of Fe magnetic moments, together with their long-range ordering when appropriate, are compared and contrasted in a variety of metallic environments. Thus, Fe, in its stable body-centered-cubic (bcc) phase under ambient conditions, is considered under pressures p, which can yield different crystal structures at high p, including fcc. The modification of the ferromagnetism in bcc Fe is surveyed as one passes through a bc-tetragonal lattice to the fcc form. In the latter, evidence is presented, both from theory and experiment, that the ordering is antiferromagnetic in character. Then, binary metallic alloys with Fe atoms as the majority component are considered, Fe,Co and Fe,Ga being focal points in both ordered and disordered materials. Finally, some discussion is given, involving again both experiment and theory, of the possible spin polarization of neighboring Cs atoms when Fe impurity atoms are inserted into the low conduction electron density characterizing this heavy alkali metal under ambient conditions. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source]


Experimental and predicted crystal structures of Pigment Red 168 and other dihalogenated anthanthrones

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 5 2010
Martin U. Schmidt
The crystal structures of 4,10-dibromo-anthanthrone (Pigment Red 168; 4,10-dibromo-dibenzo[def,mno]chrysene-6,12-dione), 4,10-dichloro- and 4,10-diiodo-anthanthrone have been determined by single-crystal X-ray analyses. The dibromo and diiodo derivatives crystallize in P21/c, Z = 2, the dichloro derivative in , Z = 1. The molecular structures are almost identical and the unit-cell parameters show some similarities for all three compounds, but the crystal structures are neither isotypic to another nor to the unsubstituted anthanthrone, which crystallizes in P21/c, Z = 8. In order to explain why the four anthanthrone derivatives have four different crystal structures, lattice-energy minimizations were performed using anisotropic atom,atom model potentials as well as using the semi-classical density sums (SCDS-Pixel) approach. The calculations showed the crystal structures of the dichloro and the diiodo derivatives to be the most stable ones for the corresponding compound; whereas for dibromo-anthanthrone the calculations suggest that the dichloro and diiodo structure types should be more stable than the experimentally observed structure. An experimental search for new polymorphs of dibromo-anthanthrone was carried out, but the experiments were hampered by the remarkable insolubility of the compound. A metastable nanocrystalline second polymorph of the dibromo derivative does exist, but it is not isostructural to the dichloro or diiodo compound. In order to determine the crystal structure of this phase, crystal structure predictions were performed in various space groups, using anisotropic atom,atom potentials. For all low-energy structures, X-ray powder patterns were calculated and compared with the experimental diagram, which consisted of a few broad lines only. It turned out that the crystallinity of this phase was not sufficient to determine which of the calculated structures corresponds to the actual structure of this nanocrystalline polymorph. [source]


The structure of orange HgI2.

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 6 2002

The metastable orange crystals of HgI comprise three different crystal structures, all of which are built from corner-linked HgI supertetrahedra. Two of them are end members with the maximum degree of order (MDO) of a polytypic layer structure; the third shows a three-dimensional linkage. This paper presents the determination from X-ray diffraction data of the tetragonal polytypic structures and their stacking disorder. Diffraction patterns show sharp Bragg reflections and rods of diffuse intensity with pronounced maxima. In a first step, the diffuse intensity was neglected and all maxima were treated as Bragg reflections. The crystal was supposed to be a conglomerate of the two MDO structures diffracting independently, and their parameters and volume ratio were refined against the single data set. The geometries and anisotropic displacement parameters of the layers in the two structures are shown to be nearly identical. Layer contacts in the two stacking modes are identical. The structures are fractal complications of the stable red form of HgI. In a second step, the stacking disorder has been quantitatively analyzed with a Markov chain model. Two probabilities describing next-nearest-layer interactions were visually adjusted to observed intensity profiles extracted from image-plate detector data. Results consistently show that the crystal comprises nearly equal volumes of MDO structures with an average domain thickness of about 5 layers or 30,Ĺ. [source]


Four 7-aryl-substituted pyrido[2,3- d]pyrimidine-2,4(1H,3H)-diones: similar molecular structures but different crystal structures

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 4 2009
Jorge Trilleras
Molecules of 1,3-dimethyl-7-(4-methylphenyl)pyrido[2,3- d]pyrimidine-2,4(1H,3H)-dione, C16H15N3O2, (I), are linked by paired C,H...O hydrogen bonds to form centrosymmetric R22(10) dimers, which are linked into chains by a single ,,, stacking interaction. A single C,H...O hydrogen bond links the molecules of 7-(biphenyl-4-yl)-1,3-dimethylpyrido[2,3- d]pyrimidine-2,4(1H,3H)-dione, C21H17N3O2, (II), into C(10) chains, which are weakly linked into sheets by a ,,, stacking interaction. In 7-(4-fluorophenyl)-3-methylpyrido[2,3- d]pyrimidine-2,4(1H,3H)-dione, C14H10FN3O2, (III), an N,H...O hydrogen bond links the molecules into C(6) chains, which are linked into sheets by a ,,, stacking interaction. The molecules of 7-(4-methoxyphenyl)-3-methylpyrido[2,3- d]pyrimidine-2,4(1H,3H)-dione, C15H13N3O3, (IV), are also linked into C(6) chains by an N,H...O hydrogen bond, but here the chains are linked into sheets by a combination of two independent C,H...,(arene) hydrogen bonds. [source]


Four 2-amino-6-aryl-4-methoxy-11H -pyrimido[4,5- b][1,4]benzodiazepines: similar molecular structures but different crystal structures

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 12 2008
Ricaurte Rodríguez
2-Amino-4-methoxy-6-phenyl-11H -pyrimido[4,5- b][1,4]benzodiazepine, C18H15N5O, (I), and its 6-(2-fluorophenyl)-, 6-(3-nitrophenyl)- and 6-(4-methoxyphenyl)- analogues, viz. C18H14FN5O, (II), C18H14N6O3, (III), and C19H17N5O2, (IV), respectively, all adopt molecular conformations which are almost identical, containing boat-shaped seven-membered rings. In each structure, paired N,H...N hydrogen bonds link the molecules into centrosymmetric dimers. In each of (I),(III), the dimers are further linked, forming a different three-dimensional framework in each case, while in compound (IV) the dimers are linked into sheets. The significance of this study lies in the observation of different crystal structures in four compounds whose molecular structures are very similar. [source]


Heterogeneity and dynamics in villin headpiece crystal structures

ACTA CRYSTALLOGRAPHICA SECTION D, Issue 5 2009
Jianmin Meng
The villin headpiece domain (HP67) is the C-terminal F-actin-binding motif that confers F-actin-bundling activity to villin, a component of the actin bundles that support the brush-border microvilli. It has been investigated extensively by both experimental and theoretical measurements. Our laboratory, for example, has determined both its NMR and its crystal structures. This study presents the structures of HP67 and its pH-stabilized mutant (H41Y) in a different crystal form and space group. For both constructs, two molecules are found in each asymmetric unit in the new space group P61. While one of the two structures (Mol A) is structurally similar to our previously determined structure (Mol X), the other (Mol B) has significant deviations, especially in the N-terminal subdomain, where lattice contacts do not appear to contribute to the difference. In addition, the structurally most different crystal structure, Mol B, is actually closer to the averaged NMR structure. Harmonic motions, as suggested by the B -factor profiles, differ between these crystal structures; crystal structures from the same space group share a similar pattern. Thus, heterogeneity and dynamics are observed in different crystal structures of the same protein even for a protein as small as villin headpiece. [source]


Macrocyclic Hexaureas: Synthesis, Conformation, and Anion Binding

CHEMISTRY - A EUROPEAN JOURNAL, Issue 19 2009
Denys Meshcheryakov Dr.
Abstract Varied flexibility: Cyclic oligoureas are formed by using anions as templates. Linking of six xanthene and/or diphenyl ether fragments by urea groups leads to the formation of five macrocyclic compounds with a 48-membered ring with variable flexibility (see picture). Their interaction with anions shows a strong influence of acetate and chloride ions on the cyclization from four precursor molecules. Five macrocylic compounds XXXXXX, XXDXXD, XDXDXD, XDDXDD, and DDDDDD with 48-membered rings, in which six xanthene and/or diphenyl ether fragments are linked through six urea (-NH-C(O)-NH-) groups, have been synthesized. In the cyclization step, a linear diamine was allowed to react with the appropriate diisocyanate by using a [5+1] (i.e., "XDXDX+D" for XDXDXD), [4+2] (DDDDDD), or [3+3] (XDDXDD) procedure. Compounds XXXXXX and XXDXXD were prepared from two molecules of the dimeric amine XX and two molecules of the respective monomeric diisocyanate (X or D) in a [2+1+2+1] (or 2×[2+1]) reaction. The (nonoptimized) yields in the cyclization step ranged from 45 to 80,%. The linear precursor diamines or diisocyanates were obtained by analogous condensation reactions by using partial protection with a tert -butoxycarbonyl group. All the macrocyclic compounds and synthetic intermediates were characterized by 1H,NMR and mass spectra. Three different crystal structures were obtained for XDDXDD, which show the molecule in a more or less strongly folded conformation determined by intramolecular hydrogen bonding. The interaction of the hexaureas with selected anions was studied by 1H,NMR spectroscopy and UV absorption spectrophotometry. [source]