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Twelve structures of distinct members of the lipocalin protein family have been solved experimentally. These structures have revolutionised our understanding of the properties of the lipocalins. Many more members of the family hav...
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Twelve structures of distinct members of the lipocalin protein family have been solved experimentally. These structures have revolutionised our understanding of the properties of the lipocalins. Many more members of the family have been crystallised and now await structure solution. The number of solved lipocalin structures is steadily increasing, and with it increases our knowledge of this enigmatic and challenging protein family.
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We report the synthesis and characterisation of tetrakis(2,4,6-triisopropylphenyl)diphosphine. Synthesis is effected by the treatment of PCl3 with an excess of 2,4,6-triisopropylphenyllithium (or the equivalent Grignard reagent) i...
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We report the synthesis and characterisation of tetrakis(2,4,6-triisopropylphenyl)diphosphine. Synthesis is effected by the treatment of PCl3 with an excess of 2,4,6-triisopropylphenyllithium (or the equivalent Grignard reagent) in 70% yield. While under normal circumstances the triarylphosphine would be expected, excessive bulk prevents this, and the resulting diphosphine is, unusually, stable to P-P cleavage by further organolithium moieties. The compound is stable, both thermally (m.p. 185 degrees C) and to air and water in the solid state, although conversion to the equivalent diorganophosphinate ester is effected by boiling ethanolic solutions in air. Crystallisation from hexane/ethanol afforded pale yellow crystals of X-ray quality. The molecule is characterised by m.p., IR, NMR, elemental analysis (C, H, P) and MS. The X-ray structure shows an antiperiplanar conformation with a P-P separation of 2.2461(16) Angstrom. Comparisons are made with other diphosphines, the title compound being only the fourth simple diphosphine to be structurally characterised. (C) 2000 Elsevier Science S.A. All rights reserved. [References: 11]
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We report a corrected crystal structure for the CePt_2In_7 superconductor, refined from single crystal x-ray diffraction data. The corrected crystal structure shows a different Pt-In stacking along the c-direction in this layered ...
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We report a corrected crystal structure for the CePt_2In_7 superconductor, refined from single crystal x-ray diffraction data. The corrected crystal structure shows a different Pt-In stacking along the c-direction in this layered material than was previously reported. In addition, all of the atomic sites are fully occupied with no evidence of atom site mixing, resolving a discrepancy between the observed high resistivity ratio of the material and the atomic disorder present in the previous structural model. The Ce-Pt distance and coordination is typical of that seen in all other reported Ce_nM_mIn_(3n+2 m) compounds. Our band structure calculations based on the correct structure reveal three bands at the Fermi level that are more 3D than those previously proposed, and density functional theory (DFT) calculations show that the new structure has a significantly lower energy.
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In the search for a greater understanding of polymer crystallization,numerous experimental observations with regards to microscopic structures and macroscopic properties have been reported in the past half-century.There are genera...
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In the search for a greater understanding of polymer crystallization,numerous experimental observations with regards to microscopic structures and macroscopic properties have been reported in the past half-century.There are generally two types of experimental results to provide information about the mechanisms of polymer crystal growth,i.e.molecular dynamic/scattering and structural/morphological.Since we cannot follow the trajectory of individual chain molecules when they undergo the transition from liquid to solid state during the crystallization process,structural/morphological analysis of polymer crystals reveal information recorded during this process.Namely,the final structure and morphology of polymer crystals have atomic,stem and global chain conformation information embedded in them during crystallization which provides evidence which can be used to deduce molecular aspects of the polymer crystallization process.It is commonly understood that polymer crystallization,from the thermodynamic perspective,is a first-order transition involving the relaxation of a metastable undercooled melt towards the equilibrium state which is rarely reached in polymer crystals.This process is controlled by a free energy barrier.A molecular model is required to describe the landscape of the free energy barriers and to provide an analytical explanation concerning and predictions about polymer crystallization.The Hoffman-Lauritzen (HL) theory,which was put forward more than 40 years ago,was one of the first analytical theories to illustrate how polymers crystallize.Since then,modifications to the HL theory and suggestions for new approaches have been reported,but the core physical picture of the HL theory has largely remained intact.This article consists of four major parts:(1) we will compare the crystallization of small molecules and long chain molecules,and the relationship between crystallization and crystal habits.The diversity of crystalline structures and morphologies of semi-crystalline polymers must be taken into account when studying the crystallization mechanism of polymers (2) this article also serves as a brief review of the HL theory and its importance in our understanding of polymer crystallization (3) we have tried to answer the question:what is the nucleation barrier?Specifically,we will illustrate that the nucleation barrier in polymer crystallization consists of both enthalpic and entropic components as deduced from experimental results.This barrier,from our perspective,consists of selection processes taking place in different length- and time-scales (4) finally,there is a brief discussion on what issues remain,in particular,in the areas of undercooled liquid structures and the initial stages of crystallization.
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A brief review of works on the crystal chemical features of the structure and methods of describing crystal structure types is presented. A variant of the universal system of a symbolic description of crystal structure types and m...
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A brief review of works on the crystal chemical features of the structure and methods of describing crystal structure types is presented. A variant of the universal system of a symbolic description of crystal structure types and modular structures with the use of structure codes is proposed. The structure code is based on a description of the geometry and topology of the basic structure module. Structure codes are intended for the identification and systematization of the structure types of compounds, the formalization of topological structural transformations, the determination of structural modules, and the derivation of genetic relationships between the structures.
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The crystal structure of the R(2)SnS(5) (R = Pr, Nd, Gd and Tb) compounds has been investigated using X-ray single-crystal diffraction. Crystal architecture and structural relationships among U(3)S(5), Y(2)HfS(5), R(2)SnS(5) compo...
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The crystal structure of the R(2)SnS(5) (R = Pr, Nd, Gd and Tb) compounds has been investigated using X-ray single-crystal diffraction. Crystal architecture and structural relationships among U(3)S(5), Y(2)HfS(5), R(2)SnS(5) compounds are discussed and a structural origin is determined. It is shown that the complex architecture of the crystal structure of Eu(5)Sn(3)S(12) is a result of interweaving of the simple crystal structures. The location of the copper ions in the non-stoichiometric compound Y(2)Cu(0.20)Sn(0.95)S(5) is proposed on the basis of comparative analysis of the R-S interatomic distances in the R(2)SnS(5) series of compounds.
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Intermolecular interactions leading to particular molecular packing have been studied in the crystals of 30 single-substituted nitrobenzenes. The experimental and predicted crystal structures of ortho, meta and para isomers of nit...
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Intermolecular interactions leading to particular molecular packing have been studied in the crystals of 30 single-substituted nitrobenzenes. The experimental and predicted crystal structures of ortho, meta and para isomers of nitrophenol, nitroaniline, nitrobenzoic acid, halogenonitrobenzenes, dinitrobenzene, nitrobenzaldehyde, cyano- and isocyanobenzene have been analyzed. The stacking and herringbone modes of molecular packing and the resulting interactions in the crystals exhibiting, or not exhibiting intermolecular hydrogen bondings, are discussed. The relevance of the intermolecular interactions between nitro groups is stressed.
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Crystal structures of ethylene-vinyl alcohol copolymers (EVOH) with various vinyl alcohol (VA) compositions have been determined on the basis of the X-ray diffraction data measured for the uniaxially and doubly oriented sample spe...
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Crystal structures of ethylene-vinyl alcohol copolymers (EVOH) with various vinyl alcohol (VA) compositions have been determined on the basis of the X-ray diffraction data measured for the uniaxially and doubly oriented sample specimens. The unit cells of EVOH with a VA content higher than 27 mol % are monoclinic of the space group P1121/m, in which the two planar-zigzag chains are statistically packed in a mode similar to that of poly(vinyl alcohol) (PVOH). With a decreasing VA content from 27 to 14 mol %, the crystal system approaches the hexagonal type. The unit cells of EVOH with a VA content below 14 mol % are orthorhombic of the space group Pnam, similar to the packing mode of orthorhombic polyethylene (PE). The crystal structure of EVOH with the VA content in the range of 14-27 mol % is the pseudohexagonal type. In this way the transformation of the unit cell structure between the PVOH type (>27 mol %) and PE type (<14 mol %) was found to occur via the appearance of a hexagonal-type structure. This transformation was supported also by the change in the infrared spectral pattern measured for a series of EVOH copolymers.
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A structural study by means of X-ray and electron diffraction of intermediate oxides between Eu2Mo3O12 and Eu2W3O12 shows the existence of a new Eu-2(Mo1-xWx)(3)O-12 solid solution with the scheelite structural type. The essential...
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A structural study by means of X-ray and electron diffraction of intermediate oxides between Eu2Mo3O12 and Eu2W3O12 shows the existence of a new Eu-2(Mo1-xWx)(3)O-12 solid solution with the scheelite structural type. The essential feature of the x = 1/3 and x = 2/3 compounds is the presence of isolated oxygen tetrahedra, in which, according to X-ray and electron diffraction data, W and Mo are randomly distributed. Eu atoms occupy distorted square antiprisms sites.
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The new compound LiCd2(SeO3)(2)(OH) has been hydrothermally synthesized and characterized by single-crystal X-ray diffraction and IR spectroscopy. It is built up from a network of edge- and vertex-sharing pyramidal [SeO3](2-) grou...
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The new compound LiCd2(SeO3)(2)(OH) has been hydrothermally synthesized and characterized by single-crystal X-ray diffraction and IR spectroscopy. It is built up from a network of edge- and vertex-sharing pyramidal [SeO3](2-) groups, distorted CdO6 octahedra, and CdO7 monocapped trigonal prisms. The cadmium-centred groups form infinite columns, onto which Se atoms (as [SeO3](2-) groups) are grafted. Cross-linking between the columns results in a three-dimensional framework which encapsulates [100] channels occupied by the tetrahedrally-coordinated lithium cations. The H atom of the hydroxyl group appears to participate in a weak, bifurcated, hydrogen bond. Crystal data: LiCd2(SeO3)(2)(OH), M-r=502.67, monoclinic, P2(1)/c (No. 14), a=5.8184 (3) Angstrom, b=10.2790 (5) Angstrom, c=11.5021 (5) Angstrom, beta=90.446(1)degrees, V=687.89 (9) Angstrom(3), Z=4, R(F)=0.021, wR(F-2)=0.046. [References: 19]
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