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Symmetric Molecules (symmetric + molecule)
Selected AbstractsSynthesis of An Enantioenriched C2 -Symmetric Molecule by a Chiral-Base-Mediated Kinetic Resolution of an (Arene)tricarbonylchromium(0) ComplexEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 8 2006M. Paola Castaldi Abstract Trimethylsilyl substituents have been used to control the conformational preferences of a 1,2-disubstituted (arene)tricarbonylchromium(0) complex. The kinetic resolution of the mono-methyl derivative (±)- 11 using a chiral base/iodomethane quench sequence led to the synthesis of the enantioenriched C2 -symmetric bis-ether (+)- 13. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] hypercloso -Hexa(amino)hexaboranes: Structurally Related to Known hypercloso -Dodecaboranes, Metastable with Regard to Their Classical CycloisomersEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 36 2009Wahid Mesbah Abstract B6(NMe2)6 (2a) is the first neutral hypercloso -hexaborane to be characterized by X-ray structural analysis. The geometry of 2a is in nice agreement with that of the Ci symmetric molecule computed at the B3LYP/6-311+G** level of theory. Two B3 triangles with long B···B distances in 2a are reminiscent of those in Hawthorne's benzyloxy-substituted hypercloso -dodecaboranes 6a,b. Upon heating to 200 °C, 2a transforms into Nöth's classical cyclohexaborane 1a. Computations at the B3LYP/6-311+G** + ZPE level of theory show 1a to be 21.6 kcal,mol,1lower in energy than 2a, that is, the latter is metastable. hypercloso -Hexaborane B6(NEt2)6 (2b), which was reported to be thermodynamically more stable than 1b, is computed to be 22.4 kcal,mol,1less stable than 1b. Pure 1b is shown here not to transform into 2b upon standing in solution, which is in contrast to reports in the literature for a mixture containing 1b. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Cross-correlated and conventional dipolar carbon-13 relaxation in methylene groups in small, symmetric moleculesCONCEPTS IN MAGNETIC RESONANCE, Issue 2 2007Leila Ghalebani Abstract A theory for dipolar cross-correlated relaxation processes in AMX or AX2 spin system, with special reference to 13C-methylene groups, is reviewed briefly. Simple experiments and protocols for measuring the transfer rates between the carbon-13 Zeeman order and the three-spin order, and for their analogues in the transverse plane, are discussed using a concentrated solution of the disaccharide trehalose as a model system. Experimental data sets consisting of conventional carbon-13 relaxation parameters (T1, T2, and NOE), along with the cross-correlated relaxation rates, are also presented for some small, rigid, polycyclic molecules. These data are interpreted using spectral density functions appropriate to spherical or symmetric tops reorienting according to small-step rotational diffusion model. The analysis results in a consistent picture of the auto- and cross-correlated spin relaxation processes. © 2007 Wiley Periodicals, Inc. Concepts Magn Reson Part A 30A: 100,115, 2007. [source] Synthesis of thiophene/phenylene co-oligomers.JOURNAL OF HETEROCYCLIC CHEMISTRY, Issue 5 2003We report the synthesis of various thiophene/phenylene co-oligomers with a total number of thiophene and benzene (phenylene) rings of 6 to 8. These compounds include a phenyl-capped sexithiophene, a thienyl-capped quaterphenylene, as well as block and alternating co-oligomers. The synthesis is based on either the Suzuki coupling reaction or the direct dimerization coupling. The latter method produces symmetric molecules with an even total ring number. These reaction schemes enabled us to obtain the target compounds in high quality. Although the resulting materials are difficult to dissolve in organic solvents and therefore difficult to identify by usual 1H nmr spectroscopy, they have successfully been identified through Fourier-transform ir spectroscopy. The specific group frequencies of ring-stretching and out-of-plane deformation modes are characteristic of the substitution pattern of the individual thiophene and benzene rings. [source] Automated assignment of graph-set descriptors for crystallographically symmetric moleculesACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2000W. D. Samuel Motherwell Algorithms for the automatic assignment of graph-set notation for intermolecular networks have been extended to molecules having internal crystallographic symmetry, for patterns up to the second level. This provides a means of achieving systematic and consistent assignments for networks containing symmetric molecules. These methodologies have been implemented in the program RPLUTO. Examples are given of the application of the method to a number of molecules with hydrogen-bonded and other intermolecular networks, illustrating the diversity of the patterns that occur. [source] Asymmetries in the nucleosome core particle at 2.5,Å resolutionACTA CRYSTALLOGRAPHICA SECTION D, Issue 12 2000Joel M. Harp The 2.5,Å X-ray crystal structure of the nucleosome core particle presented here provides significant additions to the understanding of the nucleosome, the fundamental unit of chromatin structure. Extensions are made to the structure of the N-terminal histone tails and details are provided on hydration and ion binding. The structure is composed of twofold symmetric molecules, native chicken histone octamer cores and the DNA palindrome, which were expected to form a perfectly twofold symmetric nucleosome core particle. In fact, the result is asymmetric owing to the binding of the DNA to the protein surface and to the packing of the particles in the crystal lattice. An analysis is made of the asymmetries by comparisons both within the nucleosome core particle and to the structure of the histone octamer core of the nucleosome. [source] Optical, Redox, and NLO Properties of Tricyanovinyl Oligothiophenes: Comparisons between Symmetric and Asymmetric Substitution PatternsCHEMISTRY - A EUROPEAN JOURNAL, Issue 21 2006Juan Casado Dr. Abstract A series of tricyanovinyl (TCV)-substituted oligothiophenes was synthesized and investigated with a number of physical methods including UV/Vis, IR, and Raman spectroscopy, nonlinear optical (NLO) measurements, X-ray diffraction, and cyclic voltammetry. Mono- or disubstituted oligomers were prepared by the reaction of tetracyanoethylene with mono- or dilithiated oligomers. The comparative effects of the symmetric and asymmetric substitutions in the electronic and molecular properties have been addressed. These oligomers display dramatic reductions in both their optical and electrochemical band gaps in comparison with unsubstituted molecules. The analysis of the electronic properties of the molecules was assisted by density functional theory calculations, which are in excellent agreement with the experimental data. TCV substitution influences the energies of the frontier orbitals, especially with respect to the stabilization of LUMO orbitals. X-ray structural characterization of a monosubstituted oligomer exhibits ,-stacking with favorable intermolecular interactions. NLO results agree with the role of the intramolecular charge-transfer feature in the asymmetric samples. These results furthermore exalt the role of conformational flexibility in the disubstituted compounds and reveal an unexpected nonlinear optical activity for symmetric molecules. Regarding the electronic structure, the interpretation of the vibrational data reflects the balanced interplay between aromatic and quinoid forms, finely tuned by the chain length and substitution pattern. The electronic and structural properties are consistent with the semiconducting properties exhibited by these materials in thin film transistors (TFTs). [source] | ||||||||||||||||||||||