Home About us Contact
|
Home About us Contact | ||
|
|
||
Unique Reflections (unique + reflection)
Selected AbstractsZero-dose extrapolation as part of macromolecular synchrotron data reductionACTA CRYSTALLOGRAPHICA SECTION D, Issue 5 2003Kay Diederichs Radiation damage to macromolecular crystals at third-generation synchrotron sites constitutes a major source of systematic error in X-ray data collection. Here, a computational method to partially correct the observed intensities during data reduction is described and investigated. The method consists of a redundancy-based zero-dose extrapolation of a decay function that is fitted to the intensities of all observations of a unique reflection as a function of dose. It is shown in a test case with weak anomalous signal that this conceptually simple correction, when applied to each unique reflection, can significantly improve the accuracy of averaged intensities and single-wavelength anomalous dispersion phases and leads to enhanced experimental electron-density maps. Limitations of and possible improvements to the method are discussed. [source] Imidazolium based ionic liquid crystals: structure, photophysical and thermal behaviour of [Cnmim]Br·xH2O (n = 12, 14; x=0, 1)CRYSTAL RESEARCH AND TECHNOLOGY, Issue 11 2008A. Getsis Abstract The long chain imidazolium halides [Cnmim]Br·xH2O (n = 10, 12; x = 0, 1) have been synthesized and their structural and thermal behaviour together with their photophysical properties characterized. X-ray structure analyses of the monohydrates ([C12mim]Br·H2O: triclinic, P1, no. 2, Z = 2, Pearson code aP112, a = 550.0(5) pm, b = 779.4(5) pm, c = 2296.1(5) pm, , = 81.89(5)°, , = 83.76(5)°, , = 78.102(5)°, 3523 unique reflections with Io > 2,(Io), R1 = 0.0263, wR2 = 0.0652, GooF = 1.037, T = 263(2) K; [C14mim]Br,H2O: triclinic, P1, no. 2, Z = 12, Pearson code aP11, a = 549.86(8) pm, 782.09(13) pm, c = 2511.3(4) pm, , = 94.86(2)°, , = 94.39(2)°, , = 101.83(2)°, 2063 unique reflections with Io > 2,(Io), R1 = 0.0429, wR2 = 0.0690, GooF = 0.770, T = 293(2) K) show for both compounds similar bilayered structures. Sheets composed of hydrophilic structure regions constituted by positively charged imidazolium head groups, bromide anions and hydrogen bonded water alternate with hydrophobic areas formed by interdigitated long alkyl chains belonging to imidazolium cations with different orientation. Combined differential scanning calorimetry and polarizing optical microscopy shows that the monohydrates as well as the anhydrous imidazolium salts are thermotropic liquid crystals which adopt smectic mesophases. The mesophase region is larger in case of the monohydrates when compared to the anhydrous compounds indicating that water obviously stabilizes the mesophase. All compounds show an intense whitish photoluminescence with short lived (1,,1,*) and long lived (1,,3,*) transitions. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Intersubsystem chemical bonds in the misfit layer compounds (LaS)1.13TaS2 and (LaS)1.14NbS2ACTA CRYSTALLOGRAPHICA SECTION B, Issue 2 2002Andreas Jobst The modulated structures of incommensurate composite crystals (La0.912S)1.13TaS2 at room temperature and of (La0.949S)1.14NbS2 at T = 115,K are refined against high-resolution X-ray data. The compounds are isostructural with superspace group F,m2m(,,0,0)00s. For (LaS)1.13TaS2, lattice parameters of the first subsystem TaS2 were obtained as a = 3.2922,(1), b = 5.7776,(2) and c = 23.013,(2),Å. For the second subsystem LaS, the same b and c parameters were found, but a = 5.8090,(8),Å. Refinements led to a final structure model with R = 0.036 for 4767 observed unique reflections (R = 0.023 for 2147 main reflections, R = 0.099 for 1554 first-order satellites and R = 0.112 for 1042 second-order satellites). The final model includes modulation parameters up to the second-order harmonics for the displacements of the atoms, for the occupational parameters and for the temperature parameters. A clear correlation is found between the relative positions of the subsystems, the displacement modulation, the occupational modulation and the modulation of the temperature parameters. The analysis shows that the variations in environments are resolved by correlated variations in the temperature factors. For (LaS)1.14NbS2, lattice parameters at T = 115,K of the NbS2 subsystem were obtained as a = 3.3065,(4), b = 5.7960,(5) and c = 22.956,(3),Å. For the LaS subsystem, the same values for b and c were obtained, but a = 5.7983,(7),Å. Refinements led to a final structure model with R = 0.048 for 5909 observed unique reflections (R = 0.034 for 2528 main reflections, R = 0.092 for 2171 first-order satellites and R = 0.113 for 1103 second-order satellites). The final structure model is similar to that of (LaS)1.13TaS2. In particular, it is found that the values of the modulation parameters are almost equal and it is concluded that the modulations are independent of the temperature and the replacement of Ta with Nb, and thus represent a general mechanism of resolving the strain between the mutually incommensurate layers. [source] Structure solution of the basic decagonal Al,Co,Ni phase by the atomic surfaces modelling methodACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2002Antonio Cervellino 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 , five-dimensional unit-cell parameters: d1 = d4 = 4.752,(3),Å, d2 = d3 = 3.360,(2),Å, d5 = 8.1710,(2),Å; ,12 = ,34 = 69.295°, ,13 = ,24 = 45°, ,14 = 41.410°, ,23 = ,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. [source] Structural basis for the phase transitions of Cs2HgCl4ACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2001Bagautdin Bagautdinov The a0× b0× 2c0 twofold superstructure of dicaesium mercury tetrachloride, Cs2HgCl4, at T = 120,K has been determined by single-crystal X-ray diffraction using synchrotron radiation. Lattice parameters were found as a = 9.7105,(2), b = 7.4691 (1), c = 26.8992 (4) Å, and , = 90.368,(1)° with the supercell space group P21/c. Refinements on 1828 observed unique reflections converged to R = 0.053 (wR = 0.057) using anisotropic temperature factors for all atoms. This phase is the stable phase of Cs2HgCl4 below 163,K. A quantitative comparison is made of the distortions of the 2c0 superstructure with the undistorted phase that is stable at room temperature, and with the 3c0 and 5a0 superstructures that are stable at temperatures between 163,K and room temperature. The principal difference between the 2c0 superstructure and all other phases of Cs2HgCl4 is that the Cs cations are displaced away from the centers of their coordination polyhedra in the 2c0 superstructure. The structural basis for the driving force of the series of phase transitions in this compound is found in the variations of the environments of Cs atoms and in the variations of the distortions of the HgCl4 tetrahedra. [source] An Unprecedented 2D 4f-3d-5d Multimetal-Isonicotinic Acid Complex: Synthesis, Structural Characterization and Magnetic PropertiesCHINESE JOURNAL OF CHEMISTRY, Issue 9 2008Wen-Tong CHEN Abstract A novel heterometallic metal-isonicotinic acid inorganic-organic hybrid complex [Zn0.5(H2O)]{(Hg2Cl5)- [Er(C6NO2H4)3(H2O)2]}(HgCl2)·0.5CH3OH·0.5H2O (1) has been successfully synthesized via a hydrothermal reaction and structurally characterized by single-crystal X-ray diffraction. Complex 1 crystallizes in the space group C2/c of the monoclinic system with eight formula units in a cell: a=34.165(4) Å, b=9.4692(8) Å, c=24.575(3) Å, , =115.090(5)°, V=7200(1) Å3, C18.50H21Cl7ErHg3N3O10Zn0.50, Mr=1495.25, Dc=2.759 g/cm3, T=293(2) K, µ(Mo K,) =15.954 mm,1, F(000) =5400 and R1/wR2=0.0561/0.0909 for 3157 observed reflections [I>2,(I)] and 6468 unique reflections. Complex 1 is characteristic of a novel 2D {(Hg2Cl5)[Er(C6NO2H4)3(H2O)2]} layered structure constructed from the [Er(C6NO2H4)3(H2O)2] chains interconnected by the Hg2Cl5, linkers. The 2D {(Hg2Cl5)[Er(C6NO2H4)3(H2O)2]} layers, mercury chloride and the lattice water molecules are held together via hydrogen bonds to form a three-dimensional framework with the methanol molecules and the hydrated zinc ions located in the cavities. The magnetic properties show that complex 1 exhibits antiferromagnetic-like interactions. [source] Synthesis and Structural Characterization of 1,4-Di(2-methoxyphenyl)-2,5-piperazinedioneCHINESE JOURNAL OF CHEMISTRY, Issue 5 2007Shu-Sheng Zhang Abstract A new derivative of 2,5-piperazinedione, 1,4-di(2-methoxyphenyl)-2,5-piperazinedione (I), was synthesized by the cyclocondensation reaction of N -2-methoxyphenyl chloroacetamide, and its structure was characterized by elemental analysis, IR, 1H NMR and single crystal X-ray diffraction method. The crystal belongs to monoclinic system, space group P21/c with unit cell dimensions a=0.56934(10) nm, b=1.3880(2) nm, c=1.00329(17) nm, ,=90.376(3)°, V=0.7928(2) nm3, Z=2, Dc=1.367 g·cm,3, ,=0.98 cm,1, R and wR being 0.0606 and 0.1564 respectively for 1549 unique reflections with 1247 observed reflections [I>2,(I)]. The molecule has a crystallographically imposed symmetry center. The three rings in the molecule are each coplanar with their attached groups, excluding methyl H atoms and the H atoms attached to the piperazinedione ring, while the whole molecule is not planar, with dihedral angles of 74.7(1)° between the piperazinedione and each of the two aromatic rings. The crystal structure is stabilized by van der Waals and dipole-dipole forces. [source] | ||||||||||