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Symmetry Elements (symmetry + element)
Selected AbstractsV-Shaped Thiophene-Based Oligomers with Improved Electroluminescence Properties,ADVANCED FUNCTIONAL MATERIALS, Issue 4 2005G. Barbarella Abstract The synthesis via the Stille coupling of a new family of oligomers derived from benzo[b]thiophene is reported. Owing to their branched molecular structure lacking any symmetry element, these compounds display a low tendency to crystallization and better film-forming properties than their linear counterparts. Spin-coated films show photoluminescence efficiencies up to 50,%. Light-emitting diodes with spin-coated films as the active layers display markedly improved performance with respect to similar devices based on linear oligothiophenes, with luminance values up to more than 10,000,cd,m,2. Semiempirical PM3 and ZINDO/S calculations provide insight into the molecular geometries and electron distribution of the frontier orbitals of the new compounds. Cyclic voltammetry data indicates that the transformation of the thienyl sulfur of benzo[b]thiophene to the corresponding thienyl- S,S -dioxide leads an increase in electron affinity by 0.5,0.7,V, analogous to that of the corresponding linear oligomers. [source] The application of eigensymmetries of face forms to anomalous scattering and twinning by merohedry in X-ray diffractionACTA CRYSTALLOGRAPHICA SECTION A, Issue 3 2010H. Klapper The face form (crystal form) {hkl} which corresponds to an X-ray reflection hkl is considered. The eigensymmetry (inherent symmetry) of such a face form can be used to derive general results on the intensities of the corresponding X-ray reflections. Two cases are treated. (i) Non-centrosymmetric crystals exhibiting anomalous scattering: determination of reflections hkl for which Friedel's rule is strictly valid, i.e.I(hkl) = I() (Friedel pair, centric reflection), or violated, i.e.I(hkl) , I() (Bijvoet pair, acentric reflection). It is shown that those reflections hkl strictly obey Friedel's rule, for which the corresponding face form {hkl} is centrosymmetric. If the face form {hkl} is non-centrosymmetric, Friedel's rule is violated due to anomalous scattering. (ii) Crystals twinned by merohedry: determination of reflections hkl, the intensities of which are affected (or not affected) by the twinning. It is shown that the intensity is affected if the twin element is not a symmetry element of the eigensymmetry of the corresponding face form {hkl}. The intensity is not affected if the twin element belongs to the eigensymmetry of {hkl} (`affected' means that the intensities of the twin-related reflections are different for different twin domain states owing to differences either in geometric structure factors or in anomalous scattering or in both). A simple procedure is presented for the determination of these types of reflections from Tables 10.1.2.2 and 10.1.2.3 of International Tables for Crystallography, Vol. A [Hahn & Klapper (2002). International Tables for Crystallography, Vol. A, Part 10, edited by Th. Hahn, 5th ed. Dordrecht: Kluwer]. The application to crystal-structure determination of crystals twinned by merohedry (reciprocal space) and to X-ray diffraction topographic mapping of twin domains (direct space) is discussed. Relevant data and twinning relations for the 63 possible twin laws by merohedry in the 26 merohedral point groups are presented in Appendices A to D. [source] 2,3,4,5,6-Pentanitroaniline 1,2-dichloroethane disolvate: `push,pull' deformation of aromatic rings by intramolecular charge transferACTA CRYSTALLOGRAPHICA SECTION C, Issue 8 2006Sergiy V. Rosokha The title compound, C6H2N6O10·2C2H4Cl2, forms layered stacks of pentanitroaniline molecules, which possess twofold symmetry. The voids between these stacks are occupied by dichloroethane molecules, which reside near a 2/m symmetry element and display pseudo-inversion symmetry. The C atoms in one of the two solvent molecules are threefold disordered. In the pentanitroaniline molecule, considerable distortion of the benzenoid ring, coupled with the short C,N(H2) bond and out-of-plane NO2 twistings, point to significant intramolecular `push,pull' charge transfer at the amino- and nitro-substituted (ortho and para) positions, as theoretically quantified by natural bond orbital analysis of the ,-electron density. [source] A Hough-like approach to the measurement and detection of symmetry in two- and three-dimensional structuresJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 6 2003Luís Beltrán-del-Río In this work an algorithm for the detection and measurement of symmetry in material aggregates is presented. The algorithm is based on the Hough transform in the sense that points (and pairs and triplets of points) `vote' for all the symmetry elements that are compatible with them. [source] Comments on tables of magnetic space groupsACTA CRYSTALLOGRAPHICA SECTION A, Issue 2 2009Hans Grimmer Litvin [Acta Cryst. (2008), A64, 419,424 and supplementary material] extends much of the information contained in Volume A of International Tables for Crystallography for the 230 space-group types to the 1651 types of Shubnikov space groups, using Opechowski,Guccione (OG) notation for the space groups with a black,white lattice. It is pointed out that OG notation has crucial disadvantages compared to Belov,Neronova,Smirnova (BNS) notation. It is shown how Litvin's diagrams of symmetry elements for the orthorhombic Shubnikov space groups can be interpreted in terms of BNS symbols and how those containing e -glides can be simplified. A number of mistakes in the diagrams of Litvin are corrected. [source] The piezoelectric effect of second order in stress or strain: its form for crystals and quasicrystals of any symmetryACTA CRYSTALLOGRAPHICA SECTION A, Issue 6 2007Hans Grimmer The restrictions on the coefficients describing physical effects depend on the orientation of the symmetry elements of the crystal or quasicrystal with respect to the Cartesian coordinate system employed. They are given for the piezoelectric effect of second order in stress or strain for all the orientations that can be expressed by the sequence of elements in the Hermann,Mauguin symbol of the point group. In the literature, the restrictions are usually given only for a particular orientation, which sometimes is not specified. [source] Symmetry of platelet defects in diamond: new insights with synchrotron lightACTA CRYSTALLOGRAPHICA SECTION B, Issue 5 2010Alexei Bosak Mapping of reciprocal space for Ia -type diamond single crystals with synchrotron radiation has uncovered a variety of diffuse scattering features, some of them have not been observed before. The main component of diffuse scattering in the form of diffuse rods corresponds to a set of platelets which join together blocks of diamond structure. The platelets are ordered structural entities with lattice periodicity , where is a unit-cell dimension of diamond. Intensity distribution along the rods has been measured and used for recognition of symmetry elements of the platelet structure. These findings, together with previously reported transmission electron microscopy (TEM) observations, provide strong constraints for atomistic modelling of the platelet structure. [source] Hydrogen bonding in enantiomeric versus racemic mono-carboxylic acids; a case study of 2-phenoxypropionic acidACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2003Henning Osholm Sørensen The structural and thermodynamic backgrounds for the crystallization behaviour of racemates have been investigated using 2-phenoxypropionic acid (PPA) as an example. The racemate of PPA behaves normally and forms a racemic compound that has a higher melting point and is denser than the enantiomer. Low-temperature crystal structures of the pure enantiomer, the enantiomer cocrystallized with n -alkanes and the racemic acid showed that hydrogen-bonded dimers that form over crystallographic symmetry elements exist in all but the structure of the pure enantiomer. A database search for optically pure chiral mono-carboxylic acids revealed that the hydrogen-bonded cyclic dimer is the most prevalent hydrogen-bond motif in chiral mono-carboxylic acids. The conformation of PPA depends on the hydrogen-bond motif; the antiplanar conformation relative to the ether group is associated with a catemer hydrogen-bonding motif, whereas the more abundant synplanar conformation is found in crystals that contain cyclic dimers. Other intermolecular interactions that involve the substituent of the carboxylic group were identified in the crystals that contain the cyclic dimer. This result shows how important the nature of the substituent is for the crystal packing. The differences in crystal packing have been related to differences in melting enthalpy and entropy between the racemic and enantiomeric acids. In a comparison with the equivalent 2-(4-chlorophenoxy)-propionic acids, the differences between the crystal structures of the chloro and the unsubstituted acid have been identified and related to thermodynamic data. [source] Application of graph theory to detect disconnected structures in a crystallographic database: copper oxide perovskites as a case studyACTA CRYSTALLOGRAPHICA SECTION B, Issue 6 2000Yuri Kotliarov Every crystal structure can be described as a graph with atoms as vertices and bonds as edges. Although such a graph loses the space arrangement of atoms and symmetry elements, it can mathematically represent the connectivity between atoms. This topological approach was used to develop a new method for detecting disconnected structures, in which individual atoms or structural fragments are located too far from each other, forming impossibly large gaps. Approximately 2300 perovskite-related crystal structures have been extracted from the Inorganic Crystal Structure Database (in 1999) and the maximum disconnecting distances, and the relations between them and the ionic radii of elements, have been analysed. Several disconnected structures, which are erroneous by our definition, have been revealed. Conventional tests for crystallographic data checking did not detect those entries. [source] Experimental phasing with SHELXC/D/E: combining chain tracing with density modificationACTA CRYSTALLOGRAPHICA SECTION D, Issue 4 2010George M. Sheldrick The programs SHELXC, SHELXD and SHELXE are designed to provide simple, robust and efficient experimental phasing of macromolecules by the SAD, MAD, SIR, SIRAS and RIP methods and are particularly suitable for use in automated structure-solution pipelines. This paper gives a general account of experimental phasing using these programs and describes the extension of iterative density modification in SHELXE by the inclusion of automated protein main-chain tracing. This gives a good indication as to whether the structure has been solved and enables interpretable maps to be obtained from poorer starting phases. The autotracing algorithm starts with the location of possible seven-residue ,-helices and common tripeptides. After extension of these fragments in both directions, various criteria are used to decide whether to accept or reject the resulting poly-Ala traces. Noncrystallographic symmetry (NCS) is applied to the traced fragments, not to the density. Further features are the use of a `no-go' map to prevent the traces from passing through heavy atoms or symmetry elements and a splicing technique to combine the best parts of traces (including those generated by NCS) that partly overlap. [source] | ||||||||||