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1.
E. V. Alekseev S. S. Nosov E. V. Suleimanov N. G. Chernorukov E. V. Chuprunov 《Journal of Structural Chemistry》2003,44(3):506-510
The crystal structure of HAsUO6 4H2O was solved at 295 K by X-ray analysis. Reasons for structural changes due to temperature variation are analyzed. 相似文献
2.
Berthold Stger Matthias Weil 《Acta Crystallographica. Section C, Structural Chemistry》2014,70(1):7-11
The four isotypic alkaline metal monohydrogen arsenate(V) and phosphate(V) dihydrates M2HXO4·2H2O (M = Rb, Cs; X = P, As) [namely dicaesium monohydrogen arsenate(V) dihydrate, Cs2HAsO4·2H2O, dicaesium monohydrogen phosphate(V) dihydrate, Cs2HPO4·2H2O, dirubidium monohydrogen arsenate(V) dihydrate, Rb2HAsO4·2H2O, and dirubidium monohydrogen phosphate(V) dihydrate, Rb2HPO4·2H2O] were synthesized by reaction of an aqueous H3XO4 solution with one equivalent of aqueous M2CO3. Their crystal structures are made up of undulating chains extending along [001] of tetrahedral [XO3(OH)]− anions connected via strong O—H...O hydrogen bonds. These chains are in turn connected into a three‐dimensional network via medium‐strength hydrogen bonding involving the water molecules. Two crystallographically different M+ cations are located in channels running along [001] or in the free space of the [XO3(OH)]− chains, respectively. They are coordinated by eight and twelve O atoms forming irregular polyhedra. The structures possess pseudosymmetry. Due to the ordering of the protons in the [XO3(OH)]− chains in the actual structures, the symmetry is reduced from C2/c to P21/c. Nevertheless, the deviation from C2/c symmetry is minute. 相似文献
3.
Carolyn Pratt Brock 《Helvetica chimica acta》2023,106(2):e202200170
A survey has been made of the well determined (R≤0.050) organic crystal structures that have only a single glide plane (e. g., are in space group Pc) and that have more than one crystallographically independent formula unit (Z′>1); the goal was to discover what fraction have additional approximate symmetry. Of the 377 unique structures examined 8 % should almost certainly been refined in a higher-symmetry unit cell; 86 % of the remaining 347 have approximate symmetry that is periodic in at least one dimension. While these percentages are similar to those found for P1 structures (C. P. Brock, Acta Crystallogr., Sect. B 2022 , 78, 576–588), the types of approximate symmetry differ because 89 % of the P1, Z′>1 structures were composed of enantiopure material. The 347 reliable Pc structures include 118 that are slightly distorted or mimicked Cc and P21/c structures, 15 of which were reported as having been determined at room temperature. The distortions in another 72 are so large that the approximate symmetry must be seen as periodic in two dimensions only. These results suggest that symmetry lowering may accompany transformation of a crystal nucleus to a macroscopic crystal. 相似文献
4.
《Acta Crystallographica. Section C, Structural Chemistry》2017,73(7):575-581
Kryptoracemates are racemic compounds (pairs of enantiomers) that crystallize in Sohnke space groups (space groups that contain neither inversion centres nor mirror or glide planes nor rotoinversion axes). Thus, the two symmetry‐independent molecules cannot be transformed into one another by any symmetry element present in the crystal structure. Usually, the conformation of the two enantiomers is rather similar if not identical. Sometimes, the two enantiomers are related by a pseudosymmetry element, which is often a pseudocentre of inversion, because inversion symmetry is thought to be favourable for crystal packing. We obtained crystals of two kryptoracemates of two very similar compounds differing in just one residue, namely rac‐N‐[(1S ,2R ,3S )‐2‐methyl‐3‐(5‐methylfuran‐2‐yl)‐1‐phenyl‐3‐(pivalamido)propyl]benzamide, C27H32N2O3, (I), and rac‐N‐[(1S ,2S ,3R )‐2‐methyl‐3‐(5‐methylfuran‐2‐yl)‐1‐phenyl‐3‐(propionamido)propyl]benzamide dichloromethane hemisolvate, C25H28N2O3·0.5CH2Cl2, (II). The crystals of both compounds contain both enantiomers of these chiral molecules. However, since the space groups [P 212121 for (I) and P 1 for (II)] contain neither inversion centres nor mirror or glide planes nor rotoinversion axes, there are both enantiomers in the asymmetric unit, which is a rather uncommon phenomenon. In addition, it is remarkable that (II) contains two pairs of enantiomers in the asymmetric unit. In the crystal, molecules are connected by intermolecular N—H…O hydrogen bonds to form chains or layered structures. 相似文献
5.
The solid-state structure of the triple-decker salt [Cp*Fe(-5:5-C4Me4P)RuCp*] · CF3SO3 shows orientational disorder for the pseudosymmetric cations. A chemically related compound was used to define a restrained structure model. Comparison of different refinement strategies proves that this restrained model is superior to an unrestrained treatment. 相似文献
6.
Ricardo Baggio 《Acta Crystallographica. Section C, Structural Chemistry》2019,75(7):837-850
An intuitive method is presented for detecting pseudosymmetries in Z′ > 1 cases as a complement to well‐proven strategies already available in the literature. It is based in the simple idea that the mid‐points between equivalent atoms in symmetrically related molecules are disposed according to simple well‐known patterns, which are easily recognizable by optical inspection. A number of Z′ = 4 cases in the literature are analyzed, which allows some of the potentialities of the method to be revealed. 相似文献
7.
Zi‐Qun Yu Jing‐Quan Wang Ya‐Xi Huang Sanda M. Botis Yuanming Pan Jin‐Xiao Mi 《Acta Crystallographica. Section C, Structural Chemistry》2015,71(6):440-447
The ADDSYM routine in the program PLATON [Spek (2015). Acta Cryst. C 71 , 9–18] has helped researchers to avoid structures of (metal–)organic compounds being reported in an unnecessarily low symmetry space group. However, determination of the correct space group may get more complicated in cases of pseudosymmetric inorganic compounds. One example is NaVO2F2, which was reported [Crosnier‐Lopez et al. (1994). Eur. J. Solid State Inorg. Chem. 31 , 957–965] in the acentric space group P21 based on properties but flagged by ADDSYM as (pseudo)centrosymmetric P21/m within default distance tolerances. Herein a systematic investigation reveals that NaVO2F2 exists in at least four polymorphs: P21, (I), P21/m, (II), P21/c, (III), and one or more low‐temperature ones. The new centrosymmetric modification, (III), with the space group P21/c has a similar atomic packing geometry to phase (I), except for having a doubled c axis. The double‐cell of phase (III) arises from atomic shifts from the glide plane c at (x, , z). With increasing temperature, the number of observed reflections decreases. The odd l reflections gradually become weaker and, correspondingly, all atoms shift towards the glide plane, resulting in a gradual second‐order transformation of (III) into high‐temperature phase (II) (P21/m) at below 493 K. At least one first‐order enantiotropic phase transition was observed below 139 K from both the single‐crystal X‐ray diffraction and the differential scanning calorimetry analyses. Periodic first‐principles calculations within density functional theory show that both P21/c superstructure (III) and P21 substructure (I) are more stable than P21/m structure (II), and that P21/c superstructure (III) is more stable that P21 substructure (I). 相似文献
8.
Cihan 《数学物理学报(B辑英文版)》2002,22(3)
In this paper the authors investigate hypersurfaces M of a semi-Euclidean space Esn+1, n ≥ 4, satisfying (aC +βR). H = LkQ(g, Hk), k = 1,2,3. Using obtained results they show additional curvature properties of investigated hypersurfaces. 相似文献
9.
D. Abou‐Ras J. Gibmeier G. Nolze A. Gholinia P. Konijnenberg 《Crystal Research and Technology》2008,43(3):234-239
The tetragonal crystal‐structure type of chalcopyrites (chemical formula AIBIIICVI2) is a superstructure of sphalerite type. The c /a ratio differs generally from the ideal value 2, i.e., the crystal structure is pseudocubically distorted. For CuInSe2 and CuGaSe2 thin films, simulations demonstrate that it is theoretically possible to reveal the tetragonality in electron backscatter‐diffraction (EBSD) patterns for CuGaSe2, whereas it may not be possible for CuInSe2. EBSD experiments on CuGaSe2 thin films using the ”Advanced Fit” band‐detection method show that it is possible to extract accurate misorientation‐angle distributions from the CuGaSe2 thin film. Pole figures revealing the texture of the CuGaSe2 thin film are shown, which agree well with X‐ray texture measurements from the same layer. (© 2008 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
10.
María Soledad Garraza María Emilia Gimenez Daniel Roberto Vega Ricardo Baggio 《Acta Crystallographica. Section C, Structural Chemistry》2019,75(7):851-858
The prodrug cloxazolam [systematic name: 13‐chloro‐2‐(2‐chlorophenyl)‐3‐oxa‐6,9‐diazatricyclo[8.4.0.02,6]tetradeca‐1(10),11,13‐trien‐8‐one], C17H14Cl2N2O2, crystallizes in the triclinic space group P, with four chemically identical independent molecules in the asymmetric unit. However, in order to facilitate the analysis of the striking pseudosymmetry relating the four independent molecules, the structure has been analysed and reported in the nonconventional centred B space‐group setting. Pseudosymmetry is an eminently local property, valid only in the realm of the unit‐cell boundary and not propagating to the whole crystal structure. It has been analyzed using the MP procedure described in the preceding article [Baggio (2019). Acta Cryst. C 75 , 837–850]. The molecules consist of a rigid core made up of three rings (five‐, six‐ and seven‐membered) and an extra six‐membered ring joined to the latter group by a single C—C bond, together with a clamping intramolecular C—H…O interaction preventing free rotation and providing additional rigidity. The four molecules in the asymmetric unit pair into dimers with almost exact twofold pseudosymmetry, further linked into (001) slabs as the building bricks of the structure. Interpenetration of slabs finally leads to a three‐dimensional structure of unusual compactness for an organic structure, with a Kitaigorodskii packing index of ca 0.71. 相似文献