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Twisted Conformation (twisted + conformation)
Selected AbstractsSynthesis and properties of poly(amic acid)s and polyimides based on 2,2,,6,6,-biphenyltetracarboxylic dianhydrideJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 21 2006Katsuya Sakayori Abstract Poly(amic acid)s (PAAs) having the high solution stability and transmittance at 365 nm for photosensitive polyimides have been developed. PAAs with a twisted conformation in the main chains were prepared from 2,2,,6,6,-biphenyltetracarboxylic dianhydride (2,2,,6,6,-BPDA) and aromatic diamines. Imidization of PAAs was achieved by chemical treatment using trifluoroacetic anhydride. Among them, the PAA derived from 2,2,,6,6,-BPDA and 4,4,-(1,3-phenylenedioxy)dianiline was converted to the polyimide by thermal treatment. The heating at 300 °C under nitrogen did not complete thermal imidization of PAAs having glass-transition temperatures (Tg)s higher than 300 °C to the corresponding PIs. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6385,6393, 2006 [source] Dithioether ligands containing a 2,6-disubstituted pyridine linker with two thioether-heterocycle armsACTA CRYSTALLOGRAPHICA SECTION C, Issue 1 2010Archimede Rotondo The structure of 2,6-bis(2-pyridyltsulfanylmethyl)pyridine (pytmp), (I), C17H15N3S2, presents a twisted conformation, with the three planar moieties almost perpendicular to each other. The structures of two related derivatives, namely 2,6-bis(6-methyl-2-pyridylsulfanylmethyl)pyridine (mpytmp), (II), C19H19N3S2, and 2,6-bis(4-methyl-2-pyrimidylsulfanylmethyl)pyridine (mprtmp) n- pentane hemisolvate, (III), C17H17N5S2·0.5C5H12, present extended planar fragments with just one quasi-perpendicular arylsulfanylmethyl side arm, such that the molecules are folded in an L-shaped conformation. All three conformations appear different from those adopted by similar compounds, demonstrating the great flexibility of such species, although such differences in conformational behaviour might drive specific coordination modes. [source] 6,-Hydroxy-5,-methyl-20-oxo-19-norpregn-9(10)-en-3,-yl acetateACTA CRYSTALLOGRAPHICA SECTION C, Issue 5 2009R. M. A. Pinto In the title compound, C23H34O4, which is an intermediate in the synthesis of pregnane derivatives with a modified skeleton that show potent abortion-inducing activity, the conformation of ring B is close to half-chair due to the presence of both the C=C double bond and the axial 5,-methyl group. Rings A and C have conformations close to chair, while ring D has a twisted conformation around the bridgehead C,C bond. Molecules are hydrogen bonded via the hydroxyl and acetoxy groups into infinite chains. Quantum-mechanical ab initio Roothan Hartree,Fock calculations show that crystal packing might be responsible for the low values of the angles between rings A and B, and between ring A and rings C and D, as well as for a different steric position of the methyl ketone side chain compared to the geometry of the free molecule. [source] Two-dimensional hydrogen-bonded networks in 1-(diaminomethylene)thiouron-1-ium nitrate and bis[1-(diaminomethylene)thiouron-1-ium] phosphonate monohydrateACTA CRYSTALLOGRAPHICA SECTION C, Issue 3 2009Jan Janczak Crystals of the title compounds, C2H7N4S+·NO3,, (I), and 2C2H7N4S+·HPO32,·H2O, (II), are built up from 1-(diaminomethylene)thiouron-1-ium cations and nitrate anions in (I), and from phosphonate anions and water molecules in (II). In both crystals, the cations and anions are linked together via N,H...O hydrogen bonds. The 1-(diaminomethylene)thiouron-1-ium cations exhibit a twisted conformation. Both arms of the cations are planar and are turned in opposite directions around the C,N bond involving the central N atom. Hydrogen-bonding interactions join oppositely charged units into layers in the nitrate salt and into double layers in the phosphonate monohydrate salt. In addition, the structures are stabilized by ,,, interactions between the delocalized , bonds of the cations. The significance of this study lies in the illustration of the differences between the supramolecular aggregations in the nitrate and phosphonate salts of a small organic molecule. The different geometries of the counter-ions and their different potential for hydrogen-bond formation results in markedly different hydrogen-bond arrangements. [source] 6-(2-Chloroacetamido)hexanoic acidACTA CRYSTALLOGRAPHICA SECTION C, Issue 8 2006Carolina Gastone Crystals of the title compound, C8H14ClNO3, belong to the space group Cc and are characterized by an asymmetric unit containing two molecules, both with a twisted conformation. The molecular packing is stabilized by N,H,O=C hydrogen bonds between the amide groups of molecules with the same conformation. In addition, hydrogen-bonded cyclic carboxylic acid dimers are established between molecules with a different conformation. The ClCH2,CONH bond has a cis conformation in order to favour an intramolecular Cl,HN electrostatic interaction. Weak intra- and intermolecular CH2,O=C interactions are also present. [source] Polymorphism Versus Thermochromism: Interrelation of Color and Conformation in Overcrowded Bistricyclic Aromatic EnesCHEMISTRY - A EUROPEAN JOURNAL, Issue 12 2006P. Ulrich Biedermann Dr. Abstract The nature of the thermochromic form of overcrowded bistricyclic aromatic enes (BAEs) has been controversial for a century. We report the single-crystal X-ray structure analysis of the deep-purple and yellow polymorphs of 9-(2,7-dimethyl-9H -fluoren-9-ylidene)-9H -xanthene (11), which revealed the molecules in a twisted and a folded conformation, respectively. Therefore, the deeply colored thermochromic form B of BAEs is identified as having a twisted conformation and the ambient-temperature form A as having a folded conformation. This relationship between the color and the conformation is further supported by the X-ray structures of the deep-purple crystals of the twisted 9-(9H -fluoren-9-ylidene)-9H -xanthene (10), and of the yellow crystals of the folded 9-(11H -benzo[b]fluoren-11-ylidene)-9H -xanthene (12). Based on this conclusive crystallographic evidence, eleven previously proposed rationales of thermochromism in BAEs are refuted. In the twisted structures, the tricyclic moieties are nearly planar and the central double bond is elongated to 1.40 Ĺ and twisted by 42°. In the folded structures, the xanthylidene moieties are folded by 45° and the fluorenylidene moieties by 18,20°. Factors stabilizing the twisted and folded conformations are discussed, including twisting of formal single or double bonds, intramolecular overcrowding, and the significance of a dipolar aromatic "xanthenylium-fluorenide" push,pull structure. [source] The Effects of Fluorine and Chlorine Substituents across the Fjords of Bifluorenylidenes: Overcrowding and StereochemistryEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 22 2006Sergey Pogodin Abstract The bistricyclic aromatic enes (BAEs) (E)- and (Z)-1,1,-difluorobifluorenylidene, 1,8,1,,8,-tetrafluorobifluorenylidene, (E)- and (Z)-3,3,-difluorobifluorenylidene, 3,6,3,,6,-tetrafluorobifluorenylidene, and their chlorinated analogues were subjected to a DFT study of overcrowding in their fjord regions. The B3LYP hybrid functional was employed to calculate energies and geometries of the twisted conformations of these BAEs. The diastereomers E11,F2 and Z11,F2 have identical twist angles (, = 37.1°) and similar degrees of overcrowding, but differ in the degree and mode of pyramidalization, ,. In E11,F2, ,(C9) = +,(C9,) = 7.0° (syn -pyramidalization), while in Z11,F2, ,(C9) = ,,(C9,) = 1.0° (anti -pyramidalization). By contrast, in E11,Cl2 and Z11,Cl2, , = 40.6° and 42.7°, respectively. Introducing four halogen substituents results in higher twist angles: , = 40.3° in 181,8,F4 and 52.6° in 181,8,Cl4. Surprisingly, Z11,F2 is more stable than E11,F2 (,H298 = ,1.9 kJ/mol), whereas Z11,Cl2 is less stable than E11,Cl2 (,H298 = 2.2 kJ/mol). Both results are consistent with the experimental relative stabilities of these diastereomers. The unexpected stability of Z11,F2 is explained by a combination of steric and electronic effects. Calculations of Coulomb energies for point charge systems of atoms C, F, and H in the fjord regions shows stabilization of the (Z) diastereomer by ,45.5 kJ/mol. The dipole,dipole interactions in the fjord region destabilize Z11,F2 by 6.4 kJ/mol relative to E11,F2. Careful examination of the NMR spectra of E11,F2 and Z11,F2 shows, in the latter, evidence of long-range fluorine,fluorine coupling over seven bonds (11.4 Hz) and carbon,fluorine coupling over six bonds (4.8 Hz).(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Ortho effect and steric inhibition of resonance: basicities of methyl-substituted acetophenones,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 10 2003Eva Otyepková Abstract The basicity of 12 methyl-substituted acetophenones was measured spectrophotometrically in 46,90 vol.% sulfuric acid. The acidity function was constructed and the pKa values were calculated by a new algorithm proposed by Pytela. The substituent effects were divided into polar and steric, assuming that the former are approximately equal in the ortho and para positions. Polar effects of the methyl group bring about stronger basicity as expected; the effect is more intense than the acid weakening in equally substituted benzoic acids. Steric effects of ortho methyl groups are base strengthening. This is not due to steric inhibition of resonance since the conformation remains planar in most derivatives. Two ortho methyl groups are necessary to distort the planarity; their steric effect is more than doubled compared with one methyl group. These results do not agree with the common idea of twisted conformations with gradually increasing twisting angle but are better rationalized by the existence of two groups of derivatives, planar and non-planar. Copyright © 2003 John Wiley & Sons, Ltd. [source] | ||||||||||