Distribution by Scientific Domains
Distribution within Chemistry

Kinds of Planarity

  • molecular planarity

  • Selected Abstracts

    Planarity of acetamides, thioacetamides, and selenoacetamides: Crystal structure of N,N -dimethylselenoacetamide

    Shuqiang Niu
    The planarity of acetamides 1a,3a, thioacetamides 4a,6a, and selenoacetamides 7a,9a, R1R2NC(=E)CH3 where E = O, S, Se, and R1, R2 = H or CH3, was investigated using theoretical calculations at the density functional theory (DFT) level. The calculations showed that the methyl substitution on nitrogen and the change from the amide moiety (NCO) to NCS or NCSe group increased the double bond character of the NC bond. In other words, the planarity of these compounds (1a,9a) increases in the order NH2 < NHCH3 < N(CH3)2 and O < S < Se. The calculations of bending energy suggest that the planar geometry represents the lowest energy conformation for all compounds investigated in this work. N,N-Dimethyl-selenoacetamide (9a), (CH3)2NC(Se) CH3, has the largest bending energy of 10.37 kcal/mol, which suggests that it possesses the greatest planarity among the compounds 1a,9a. However, the solid phase molecular structure of 9a was found to be slightly nonplanar by X-ray crystallography. The slight nonplanarity observed experimentally is very likely the consequence of intermolecular interactions arising within the crystal packing. 2002 Wiley Periodicals, Inc. Heteroatom Chem 13:380,386, 2002; Published online in Wiley Interscience ( DOI 10.1002/hc.10056 [source]

    Development of orientation and mechanical properties of extrusion cast polyamide 11 films in biaxial stretching process

    Sangkeun Rhee
    The development of orientation of extrusion cast polyamide 11 films in the biaxial stretching process was studied with birefringence and wide angle X-ray diffraction (WAXD) pole figures. White-Spruiell biaxial orientation factors of the crystalline phase were calculated with the pole figure data. Both biaxially stretched films were developed. Planarity of hydrogen bonding planes with respect to the film surface was observed from WAXD pole figures. Mechanical properties of the films were studied. Tensile strength and elongation at break were successfully correlated with the out-of-plane birefringences. [source]

    Resonance Structures of the Amide Bond: The Advantages of Planarity

    Jon I. Mujika
    Abstract Delocalization indexes based on magnitudes derived from electron-pair densities are demonstrated to be useful indicators of electron resonance in amides. These indexes, based on the integration of the two-electron density matrix over the atomic basins defined through the zero-flux condition, have been calculated for a series of amides at the B3LYP/6-31+G* level of theory. These quantities, which can be viewed as a measure of the sharing of electrons between atoms, behave in concordance with the traditional resonance model, even though they are integrated in Bader atomic basins. Thus, the use of these quantities overcomes contradictory results from analyses of atomic charges, yet keeps the theoretical appeal of using nonarbitrary atomic partitions and unambiguously defined functions such as densities and pair densities. Moreover, for a large data set consisting of 24 amides plus their corresponding rotational transition states, a linear relation was found between the rotational barrier for the amide and the delocalization index between the nitrogen and oxygen atoms, indicating that this parameter can be used as an ideal physical-chemical indicator of the electron resonance in amides. [source]

    Crystal structures of two acridinedione derivatives

    K. Palani
    Abstract The crystal structures of two acridinedione derivatives, namely 10-(3,4-Dichloro-5-hydroxyphenyl)-3,4,6,7,9,10-hexahydro-1,8(2H, 5H) acridinedione (DHHA, CCDC 206440) and 10-(3,5-Dihydroxy-4-nitrophenyl)-3,4,6,7,9,10-hexahydro-1,8(2H, 5H) acridinedione (DHNA, CCDC206441) are reported here. Both the structures were solved by direct methods and refined by full-matrix least-squares procedures to final R- values of 0.073 and 0.076 respectively. In both the crystal structures, the central pyridine ring in the acridinedione moiety tends to be planar while the outer two rings adopt half-chair (sofa) conformation. The buckling angles 2.2(2) and 11.0(1) for DHHA and DHNA show the degree of planarity of the acridinedione moiety. The C-H,O types of hydrogen bonds help to stabilize the molecules in the unit cell in addition to van der Waals forces. ( 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    A thermodynamics-based estimation model for adsorption of organic compounds by carbonaceous materials in environmental sorbents

    Paul C. M. van Noort
    Abstract A model was developed to estimate Langmuir affinities for adsorption of low-polarity organic compounds from either water or air by carbonaceous sorbents. Sorption enthalpies and entropies provided the basic information for the description of sorption affinities in terms of the entropy of melting and either solubility in water or vapor pressure. For m -xylene, polycyclic aromatic hydrocarbons (PAHs) and chlorobenzenes on 10 different sorbents, 80% of the measured sorption affinities fall within a factor of four of the model estimates. Equations for the limiting distribution coefficients in terms of either octanol,air (KOA) or octanol,water partition (Kow) coefficients were derived from regressions of calculated affinities combined with an estimated relation between experimental Langmuir sorption capacities and Kow. Estimated soot,water distribution coefficients were within a factor of three of measured data for polychlorobiphenyls (PCBs) and lower molecular weight PAHs on automotive soot samples and captured the dependence of PCB distribution coefficients on the extent of ortho substitution. For higher molecular weight PAHs, sorption was underestimated. For soot in sediment,water distribution coefficients of PAHs and PCBs, estimated values captured both the trend of measured data with Kow and the dependence on sorbate planarity. Tentative application to aerosol,air distribution explained the observed independence of distribution coefficient,KOA relations for PCBs on the extent of ortho substitution and suggested nonequilibrium conditions for PAHs in comparison with recent measurements. [source]

    Tuning of Copper(I),Dioxygen Reactivity by Bis(guanidine) Ligands,

    Sonja Herres-Pawlis
    Abstract A series of bis(guanidine) ligands designed for use in biomimetic coordination chemistry, namely bis(tetramethylguanidino)-, bis(dipiperidinoguanidino)-, and bis(dimethylpropyleno)propane (btmgp, DPipG2p and DMPG2p, respectively), has been extended to include bis(dimethylethyleneguanidino)propane (DMEG2p), which has both Namine atoms of each guanidine functionality connected by a short ethylene bridge, as a member. From this series, a family of novel bis(guanidine)copper(I) compounds , [Cu2(btmgp)2][PF6]2 (1), [Cu2(DPipG2p)2][PF6]2 (2), [Cu2(DMPG2p)2][PF6]2 (3), and [Cu2(DMEG2p)2][PF6]22MeCN (4) , has been synthesised. Single-crystal X-ray analysis of 1,4 demonstrated that these compounds contain dinuclear complex cations that contain twelve-membered heterocyclic Cu2N4C6 rings with the Cu atoms being more than 4 apart. Each copper atom is surrounded by a set of two N-donor functions from different ligands, resulting in linear N,Cu,N coordination sites. Depending on their individual substitution patterns, the guanidine moieties deviate from planarity by characteristic propeller-like twists of the amino groups around their N,Cimine bonds. The influence of these groups on the reactivity of the corresponding complexes 1,4 with dioxygen was investigated at low temperatures by means of UV/Vis spectroscopy. The reaction products can be classified into ,-,2:,2 -peroxodicopper(II) or bis(,-oxo)dicopper(III) complex cations that contain the {Cu2O2}2+ core portion as different isomers. The electronic properties of the specific bis(guanidine) ligands are discussed from the viewpoint of their ,-donor and ,-acceptor capabilities, and it is shown that ,-,2:,2 -peroxodicopper(II) complexes are stabilised relative to the bis(,-oxo)dicopper(III) ones if , conjugation within the guanidine moieties is optimised. ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source]

    Towards Cationic Gallium Derivatives: Metallacycles from the Reactions of Organogallium Compounds with Tetraorganodichalcogenoimidodiphosphinates and a New N -(Diphenylthiophosphinyl)thioureato Ligand

    Virginia Montiel-Palma
    Abstract The organometallic complexes of general formulae [Me2Ga{,2 - E,E, -[R2P(E)NP(E,)R,2]}] [R = R, = Ph, E = E, = O (1); R = R, = Ph, E = E, = S (2); R = R, = Ph, E = E, = Se (3); R = R, = Ph, E = O, E, = S (4); R = Me, R, = Ph, E = S, E, = O (5)] and [Me2Ga{,2 - S,S, -[Ph2P(S)NC(S)(C9H10N)]}] (6) were obtained by facile methane elimination reactions from GaMe3 and the acidic ligands L1H [(XPPh2)2NH (X = O, S, Se), (OPPh2)(SPPh2)NH, and (OPMe2)(SPPh2)NH] and L2H [Ph2P(S)NHC(S)(C9H10N)] in toluene. Replacement of one phosphorus atom by a carbon atom in the ligand skeleton of L1H gave the new ligand L2H, which, upon reaction with GaMe3, gave compound 6, which shows no significant structural differences with respect to 1,5. Therefore, L2H does not induce partial planarity in the six-membered ring, indicating the necessity for replacing both phosphorus atoms of the ligand by carbon atoms, as in the ,-diketonate-type derivatives, in order to impose ring planarity. Thus, despite originating from a variety of ligands with differing donor atoms and substituents at the phosphorus atoms, all complexes show little structural differences. ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source]

    On the Structure of Cross-Conjugated 2,3-Diphenylbutadiene

    Cornelis A. van Walree
    Abstract The structure of the cross-conjugated compound 2,3-diphenylbutadiene was investigated by single-crystal X-ray diffraction and computational methods. In the crystal structure the central butadiene fragment adopts an s-gauche geometry [,55.6(2) torsion angle , around the essential single bond], whereas the styrene moieties are close to planarity. MP2/6-311G* calculations show that the s-gauche conformation represents the global minimum along the , coordinate, but also revealed the existence of an s-trans local minimum. While the crystal structure seems to reflect dominance of styrene-like conjugation, the MP2/6-311G* calculations indicate that conjugation in both the styrene and butadiene ,-systems is important. An NBO orbital deletion study shows that the structure is primarily determined by (hyper)conjugation and that steric effects play a minor role. ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source]

    Planarity of acetamides, thioacetamides, and selenoacetamides: Crystal structure of N,N -dimethylselenoacetamide

    Shuqiang Niu
    The planarity of acetamides 1a,3a, thioacetamides 4a,6a, and selenoacetamides 7a,9a, R1R2NC(=E)CH3 where E = O, S, Se, and R1, R2 = H or CH3, was investigated using theoretical calculations at the density functional theory (DFT) level. The calculations showed that the methyl substitution on nitrogen and the change from the amide moiety (NCO) to NCS or NCSe group increased the double bond character of the NC bond. In other words, the planarity of these compounds (1a,9a) increases in the order NH2 < NHCH3 < N(CH3)2 and O < S < Se. The calculations of bending energy suggest that the planar geometry represents the lowest energy conformation for all compounds investigated in this work. N,N-Dimethyl-selenoacetamide (9a), (CH3)2NC(Se) CH3, has the largest bending energy of 10.37 kcal/mol, which suggests that it possesses the greatest planarity among the compounds 1a,9a. However, the solid phase molecular structure of 9a was found to be slightly nonplanar by X-ray crystallography. The slight nonplanarity observed experimentally is very likely the consequence of intermolecular interactions arising within the crystal packing. 2002 Wiley Periodicals, Inc. Heteroatom Chem 13:380,386, 2002; Published online in Wiley Interscience ( DOI 10.1002/hc.10056 [source]

    Ab initio and DFT calculations of benzaldoxime elimination kinetics in the gas phase

    Jose R. Mora
    Abstract The mechanism for the gas-phase molecular elimination kinetics of benzaldoxime was examined at MP2/6-31G, MP2/6-31G(d,p), B3LYP/6-31G, B3LYP/6-31G(d,p), MPW1PW91/6-31G, and MPW1PW91/6-31G(d,p) levels of theory. The products of elimination of this oxime are benzonitrile and water. Calculated thermodynamic and kinetic parameters estimated from B3LYP/6-31G was found to be in better agreement with the experimental values. Transition state structure is best described as a four-membered cyclic structure with good approximation to planarity. NBO charges analysis revealed a little greater polarization of the benzylic C,H,+ rather than N,OH,,. Bond indexes and synchronicity parameters are in agreement with a concerted semi-polar type of mechanism with benzylic CH bond breaking as determining step of the reaction. 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source]

    Coherent superposition of resonance wave function in terms of weighted orthogonalized natural localized configurations

    A. H. Pakiari
    Abstract In this research, the projection technique has been applied in order to decompose the electronic wave function into its weighted orthogonalized resonance components. These components have been constructed by determinants whose orbitals are selected among natural bond orbitals. However, the procedure is general and any other localized orbitals can be used as well. Both , and , delocalize systems have been considered in order to check the reliability of the calculated resonance weights. For ,-systems, the presented procedure could predict significant decrease of weight of certain resonance structures when the molecular planarity was destroyed. Water cyclic clusters were also tested and the results confirmed the existence of strong ,-delocalization in the clusters. 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source]

    Exploring the binding site of the human muscarinic M3 receptor: Homology modeling and docking study

    Liliana Ostopovici
    Abstract The human muscarinic M3 receptor (hM3) and its interactions with selective agonists and antagonists were investigated by means of combined homology and docking approach. Also, two pharmacophoric models for the hM3 agonist and antagonist binding sites were proposed. The three-dimensional (3D) structure of hM3 receptor was modeled based on the high-resolution X-ray structure of bovine rhodopsin from the Protein Data Bank (PDB). To validate the reliability of the model obtained, the main chain torsion angles phi (,) and psi (,) were examined in a Ramachandran plot, and all omega angles were measured for peptidic bond planarity. The characteristics of the active site, the position, and the orientation of ligands in situ, as well as the binding modes of the representative agonists and antagonists, were analyzed by applying a molecular docking technique using the AutoDock 3.0.5 program. Specific interactions responsible for recognition of the hM3 receptor, like ionic bond formed between protonated amine of the ligands and the Asp3.6 side chain were identified. Structure,reactivity relationships have been explained by analyzing the 3D structure of the hM3 model and the ligand conformations resulted from molecular docking simulation. 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source]

    Charge transport in stacking metal and metal-free phthalocyanine iodides.

    Effects of packing, central metals, core modification, dopants, external electric field, substitutions
    Abstract The charge-transport properties of the one-dimensional stacking metal phthalocyanine iodides (M(Pc)I, M = Fe, Co, Ni, Cu) and metal-free phthalocyanine iodide (H2(Pc)I) have been theoretically investigated. On the basis of the tight-binding approximation and two-state theory, both the site-energy corrected energy splitting in dimer and Fock-matrix-based methods are used to calculate the transfer integral. The intermolecular motions, including interplanar translation, rotation, slip, and tilt, exert remarkable impacts on the transfer integral. The order/disorder of the dopant stack and the long-range electrostatic interactions are also demonstrated to be crucial factors for modulation of charge-transport properties. The transfer integral undergoes slight changes under an applied electric field along the stacking direction in the range of 106 , 107 V cm,1. The change of central metals in MPc has little effect on the transfer integrals, but significantly affects the reorganization energies. The extension of the ,-conjugation in macrocyclic ligand brings about considerable influence on the transfer integrals. Peripheral substitutions by animo, hydroxyl, and methyl lead to deviations from planarity of macromolecular rings, and hence affect the valence bands significantly. 2009 Wiley Periodicals, Inc. J Comput Chem, 2009 [source]

    Structure, magnetizability, and nuclear magnetic shielding tensors of bis-heteropentalenes.


    Abstract The geometry of the heteropentalenes formed by two phosphole units has been determined at the DFT level. The magnetic susceptibility and the nuclear magnetic shielding at the nuclei of these systems have also been calculated using gauge-including atomic orbitals and a large Gaussian basis set to achieve near Hartree,Fock estimates. A comparative study of the various isomers, of their flattened analogs, and of the parent phosphole molecule, shows that the [3,4-c] isomer is the most aromatic system in the set considered, assuming diatropicity and degree of planarity as indicators, even if it is the less stable in terms of total molecular energy. Plots of magnetic field-induced current densities confirm diatropicity of P-containing bis-heteropentalenenes, showing, however, significant differences from the analogous systems with distinct heteroatoms. The maps give evidence of spiral flow nearby CC bonds, compatible with prevalent distortive behavior of , electrons exalted by pyramidalization at P, and competing against the , electron compression, which would favor planar structure. 2005 Wiley Periodicals, Inc. J Comput Chem 27: 344,351, 2006 [source]

    Application of standard DFT theory for nonbonded interactions in soft matter: Prototype study of poly- para -phenylene

    Marcelo Alves-Santos
    Abstract We present a detailed analysis of the application of density functional theory (DFT) methods to the study of structural properties of molecular and supramolecular systems, using as a paradigmatic example three para -phenylene-based systems: isolated biphenyl, single chain poly- para -phenylene, and crystalline biphenyl. We use different functionals for the exchange correlation potential, the local density (LDA), and generalized gradient approximations (GGA), and also different basis sets expansions, localized, plane waves (PW), and mixed (localized plus PW), within the reciprocal space formulation for the hamiltonian. We find that regardless of the choice of basis functions, the GGA calculations yield larger interring distances and torsion angles than LDA. For the same XC approximation, the agreement between calculations with different basis functions lies within 1% (LDA) or 0.5% (GGA) for distances, and while PW and mixed basis calculations agree within 1 for torsion angles, the localized basis results show larger angles by , 8 and a nonmonotonic dependence on basis size, with differences within 6. The most prominent features, namely the torsion between rings for isolated molecule and infinite chain, and planarity for the molecule in crystalline environment, are well reproduced by all DFT calculations. 2005 Wiley Periodicals, Inc. J Comput Chem 27: 217,227, 2006 [source]

    Acetylcholinesterase inhibitors from Stephania venosa tuber

    Kornkanok Ingkaninan
    Acetylcholinesterase (AChE) inhibitors have lately gained interest as potential drugs in the treatment of Alzheimer's disease. Three AChE inhibitors were isolated from tubers of a Thai medicinal plant, Stephania venosa (Bl) Spreng. They were identified as quaternary protoberberine alkaloids, stepharanine, cyclanoline and N -methyl stepholidine. They expressed inhibitory activity on AChE with IC50 values (concentration that caused 50% inhibition of activity) of 14.1K 0.81, 9.23 3.47 and 31.30 3.67 ,M, respectively. The AChE inhibitory activity of these compounds was compared with those of the related compounds, palmatine, jatrorrhizine and berberine, as well as tertiary protoberberine alkaloids isolated from the same plant, stepholidine and corydalmine. The results suggest that the positive charge at the nitrogen of the tetrahydroisoquinoline portion, steric substitution at the nitrogen, planarity of the molecule or substitutions at C-2, ,3, ,9, and ,10 affect the AChE inhibitory activity of protoberberine alkaloids. [source]

    1,1,1-Trichloro-3-(1-phenethylamino-ethylidene)- pentane-2,4-dione,synthesis, spectroscopic, theoretical and structural elucidation

    Tsonko M. Kolev
    Abstract 1,1,1-Trichloro-3-(1-phenethylamino-ethylidene)-pentane-2,4-dione is spectroscopically and structurally elucidated by means of linear-polarized IR spectroscopy (IR-LD) of oriented solids as a colloidal suspension in nematic liquid crystal. Structural information and IR-spectroscopic assignment are supported by quantum chemical calculations at MP2 and B3LYP level of theory and 6-311++G** basis set. The geometry is characterized with an inramolecular hydrogen bond of NH,OC with length of 2.526, and a NHO angle of 140.5(1). The NHC(CH3)CCCO(CH3) fragment is nearly flat with a maximal deviation of total planarity of 10.4. Copyright 2007 John Wiley & Sons, Ltd. [source]

    Ortho effect and steric inhibition of resonance: basicities of methyl-substituted acetophenones,

    Eva Otyepkov
    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]

    Flexible nanocrystalline-titania/polyimide hybrids with high refractive index and excellent thermal dimensional stability

    Guey-Sheng Liou
    Abstract In this study, a novel synthetic route was developed to prepare polyimide,nanocrystalline,titania hybrid optical films with a relatively high titania content (up to 50 wt %) and thickness (20,30 ,m) from soluble polyimides containing hydroxyl groups. Two series of newly soluble polyimides were synthesized from the hydroxy-substituted diamines with various commercial tetracarboxylic dianhydrides. The hydroxyl groups on the backbone of the polyimides could provide the organic,inorganic bonding and resulted in homogeneous hybrid solutions by controlling the mole ratio of titanium butoxide/hydroxyl group. AFM, SEM, TEM, and XRD results indicated the formation of well-dispersed nanocrystalline-titania. The flexible hybrid films could be successfully obtained and revealed relatively good surface planarity, thermal dimensional stability, tunable refractive index, and high optical transparency. A three-layer antireflection coating based on the hybrid films was prepared and showed a reflectance of less than 0.5% in the visible range indicated its potential optical applications. 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1433,1440, 2010 [source]

    Synthesis and electroluminescent properties of fluorene-based copolymers containing electron-withdrawing thiazole derivatives

    In Hwan Jung
    Abstract We synthesized two fluorene-based copolymers poly[(2,5-bis(4-hexylthiophen-2-yl)thiazolo[5,4-day]thiazole-5,5,-diyl)-alt-(9,9,-dioctylfluorene-2,7-diyl)] (PF-TTZT), and poly[(5,5,-bis(4-hexylthiophen-2-yl)-2,2,-bithiazole-5,5,-diyl)-alt-(9,9,-dioctylfluorene-2,7-diyl)] (PF-TBTT), which contain the electron-withdrawing moieties, thiazolothiazole, and bithiazole, respectively. Through electrochemical studies, we found that these two polymers exhibit stable reversible oxidation and reduction behaviors. Moreover, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of PF-TBTT are lower than those of PF-TTZT, and the bandgap of PF-TBTT is smaller than that of PF-TTZT. Thus the bithiazole moiety in PF-TBTT is more electron-withdrawing than the thiazolothiazole moiety in PF-TTZT. Light-emitting devices with indium tin oxide (ITO)/poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate)(PEDOT)/polymer/bis(2-methyl-8-quinolinato)-4-phenylphenolate aluminum (BAlq)/LiF/Al configurations were fabricated. The performance of the PF-TBTT device was found to be almost three times better than that of the PF-TTZT device, which is because electron injection from the cathode to PF-TBTT is much easier than for PF-TTZT. We also investigated the planarity and frontier orbitals of the electron donor-acceptor (D-A) moieties with computational calculations using ab initio Hartree,Fock with the split-valence 6-31G* basis set. These calculations show that TBTT has a more nonplanar structure than TTZT and that the bithiazole moiety is more electron-withdrawing than thiazolothiazole. These calculations are in good agreement with the experimental results. Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7148,7161, 2008 [source]

    Hydrogen-Bonded Shape-Persistent Aryl Hydrazide Polymers: Side-Chain-Tuned Formation of Vesicles and Organogels

    Cen Zhou
    Abstract A new class of aromatic, hydrazide-based, zigzag polymer has been synthesized using the Yamazaki polymerization conditions. Hydrophobic, hydrophilic or amphiphilic side chains are introduced to the backbones to tune their solubility in organic solvents of different polarities. The side chains form successive, intramolecular, six-membered ROHN hydrogen bonds, which increase the planarity of the backbones. The new shape-persistent polymers are revealed to self-assemble into vesicles or fibers to gelate organic solvents of different polarities. The polymeric backbones may be regarded as a conceptual extension of the emerging foldamers, which are usually constructed from oligomeric backbones. [source]

    Probability density of the multipole vectors for a Gaussian cosmic microwave background

    Mark R. Dennis
    ABSTRACT We review Maxwell's multipole vectors, and elucidate some of their mathematical properties, with emphasis on the application of this tool to the cosmic microwave background (CMB). In particular, for a completely random function on the sphere (corresponding to the statistically isotropic Gaussian model of the CMB), we derive the full probability density function of the multipole vectors. This function is used to analyse the internal configurations of the third-year Wilkinson Microwave Anisotropy Probe quadrupole and octopole, and we show that the observations are consistent with the Gaussian prediction. A particular aspect is the planarity of the octopole, which we find not to be anomalous. [source]

    In-plane polarization of GaN-based heterostructures with arbitrary crystal orientation

    Q. Y. Wei
    Abstract The total polarization fields of pseudomorphic InxGa1,xN/GaN and AlxGa1,xN/GaN heterostructures with 0,,,x,,,0.4 have been calculated as a function of the crystal orientation. Especial attention is placed on the direction and magnitude of in-plane piezoelectric polarization, which is not negligible for the non-polar and semi-polar growth. For an arbitrary crystal orientation, the piezoelectric polarization prevails in the InGaN/GaN system while the spontaneous polarization prevails in the AlGaN/GaN system. The in-plane potential due to polarization fields in non-polar epilayers is found to depend on the degree of planarity of the heterojunctions, and on the respective lateral dimensions. [source]

    In-situ high-pressure study of the ordered phase of ethyl propionate

    Roman Gajda
    Ethyl propionate, C5H10O2 (m.p. 199,K), has been in-situ pressure-frozen and its structure determined at 1.34, 1.98 and 2.45,GPa. The crystal structure of the new high-pressure phase (denoted ,) is different from phase , obtained by lowering the temperature. The freezing pressure of ethyl propionate at 296,K is 1.03,GPa. The molecule assumes an extended chain s-trans,trans,trans conformation, only slightly distorted from planarity. The closest intermolecular contacts in both phases are formed between carbonyl O and methyl H atoms; however, the ethyl-group H atoms in phase , form no contacts shorter than 2.58,. A considerable molecular volume difference of 24.2,3 between phases , and , can be rationalized in terms of degrees of freedom of molecules arranged into closely packed structures: the three degrees of freedom allowed for rearrangements of molecules confined to planar sheets in phase ,, but are not sufficient for obtaining a densely packed pattern. [source]

    Interplay between dipolar, stacking and hydrogen-bond interactions in the crystal structures of unsymmetrically substituted esters, amides and nitriles of (R,R)- O,O,-dibenzoyltartaric acid

    Urszula Rychlewska
    The compounds analysed are the O,O,-dibenzoyl derivatives of (R,R)-tartaric acid, asymmetrically substituted by ester, amide and nitrile groups. Benzoylation does not introduce drastic changes to the molecular conformation. All investigated molecules adopt the planar T conformation of the four-carbon chain with noticeably smaller departures from the ideal conformation than observed in the nonbenzoylated analogs. Primary and secondary amides always orient the C=O bond antiperiplanar (a) with respect to the nearest C*,O bond, while methylester groups adjust their conformation to that adopted by the amide substituent situated at the other end of the molecule. Tertiary amides and carboxyl groups place their carbonyls at the same side as the nearest C*,O bond (the s form), but often deviations from coplanarity of the two bonds are significant and higher than those observed in the nonbenzoylated series. The results presented demonstrate the importance of dipole/dipole interactions between CO and ,C*H groups in stabilizing the molecular conformation, and between carbonyl groups in stabilizing crystal packing of the molecules that lack classical hydrogen-bond donor groups. An illustration is provided as to how a small change in mutual orientation of molecules arranged in a close-packed fashion causes a change in the character of intermolecular interactions from van der Waals to sandwich stacking between the benzoyloxy phenyls, and to dipolar between the benzoyloxy carbonyls. Hydrogen-bonded molecules tend to orient in a head-to-tail mode; the head-to-head arrangement being limited to cases in which terminal carbonyl groups are situated at one side of the molecule. The orientation of the benzoyloxy substituents with respect to the carbon main chain is such that the (O=)C,O,C,H bond system often deviates significantly from planarity. [source]

    Green chemistry synthesis: 2-amino-3-[(E)-(2-pyridyl)methylideneamino]but-2-enedinitrile monohydrate and 5-cyano-2-(2-pyridyl)-1-(2-pyridylmethyl)-1H -imidazole-4-carboxamide

    Muhammad Altaf
    The title compounds, C10H9N5OH2O (L1H2O) and C16H12N6O (L2), were synthesized by solvent-free aldol condensation at room temperature. L1, prepared by grinding picolinaldehyde with 2,3-diamino-3-isocyanoacrylonitrile in a 1:1 molar ratio, crystallized as a monohydrate. L2 was prepared by grinding picolinaldehyde with 2,3-diamino-3-isocyanoacrylonitrile in a 2:1 molar ratio. By varying the conditions of crystallization it was possible to obtain two polymorphs, viz. L2-I and L2-II; both crystallized in the monoclinic space group P21/c. They differ in the orientation of one pyridine ring with respect to the plane of the imidazole ring. In L2-I, this ring is oriented towards and above the imidazole ring, while in L2-II it is rotated away from and below the imidazole ring. In all three molecules, there is a short intramolecular N,H...N contact inherent to the planarity of the systems. In L1H2O, this involves an amino H atom and the C=N N atom, while in L2 it involves an amino H atom and an imidazole N atom. In the crystal structure of L1H2O, there are N,H...O and O,H...O intermolecular hydrogen bonds which link the molecules to form two-dimensional networks which stack along [001]. These networks are further linked via intermolecular N,H...N(cyano) hydrogen bonds to form an extended three-dimensional network. In the crystal structure of L2-I, symmetry-related molecules are linked via N,H...N hydrogen bonds, leading to the formation of dimers centred about inversion centres. These dimers are further linked via N,H...O hydrogen bonds involving the amide group, also centred about inversion centres, to form a one-dimensional arrangement propagating in [100]. In the crystal structure of L2-II, the presence of intermolecular N,H...O hydrogen bonds involving the amide group results in the formation of dimers centred about inversion centres. These are linked via N,H...N hydrogen bonds involving the second amide H atom and the cyano N atom, to form two-dimensional networks in the bc plane. In L2-I and L2-II, C,H..., and ,,, interactions are also present. [source]

    7-Methoxy-2,3-dioxo-1,4-dihydroquinoxalin-6-aminium chloride monohydrate

    Jrgen Brning
    Single crystals of the title compound, C9H10N3O3+Cl,H2O, were obtained by recrystallization from hydrochloric acid. The cations stack along the crystallographic a direction. The 2,3-dioxo-1,4-dihydroquinoxaline group shows a significant deviation from planarity [r.m.s. deviation from the best plane = 0.063,(2),]. Hydrogen bonding links the cations, chloride anions and water molecules to form an extended three-dimensional architecture. [source]

    A stair-like two-dimensional silver(I) coordination polymer of N,-(3-cyanobenzylidene)nicotinohydrazide

    Cao-Yuan Niu
    The structure of the title compound, poly[[[,3 - N,-(3-cyanobenzylidene)nicotinohydrazide]silver(I)] hexafluoroarsenate], {[Ag(C14H10N4O)](AsF6)}n, at 173,K exhibits a novel stair-like two-dimensional layer and a three-dimensional supramolecular framework through C,H...Ag hydrogen bonds. The AgI cation is coordinated by three N atoms and one O atom from N,-(3-cyanobenzylidene)nicotinohydrazide (L) ligands, resulting in a distorted tetrahedral coordination geometry. The organic ligand acts as a ,3 -bridging ligand through the pyridyl and carbonitrile N atoms and deviates from planarity in order to adapt to the coordination geometry. Two ligands bridge two AgI cations to construct a small 2+2 Ag2L2 ring. Four ligands bridge one AgI cation from each of four of these small rings to form a large grid. An interesting stair-like two-dimensional (3,6)-net is formed through AgI metal centres acting as three-connection nodes and through L molecules as tri-linkage spacers. [source]

    Three quinolone compounds featuring O...I halogen bonding

    Jurica Bauer
    Ethyl 1-ethyl-6-iodo-4-oxo-1,4-dihydroquinoline-3-carboxylate, C14H14INO3, (I), and ethyl 1-cyclopropyl-6-iodo-4-oxo-1,4-dihydroquinoline-3-carboxylate, C15H14INO3, (II), have isomorphous crystal structures, while ethyl 1-dimethylamino-6-iodo-4-oxo-1,4-dihydroquinoline-3-carboxylate, C14H15IN2O3, (III), possesses a different solid-state supramolecular architecture. In all three structures, O...I halogen-bonding interactions connect the quinolone molecules into infinite chains parallel to the unique crystallographic b axis. In (I) and (II), these molecular chains are arranged in (101) layers, via,,, stacking and C,H..., interactions, and these layers are then interlinked by C,H...O interactions. The structural fragments involved in the C,H...O interactions differ between (I) and (II), accounting for the observed difference in planarity of the quinolone moieties in the two isomorphous structures. In (III), C,H...O and C,H..., interactions form (100),molecular layers, which are crosslinked by O...I and C,H...I interactions. [source]

    Unsolvated 5,10,15,20-tetra-4-pyridylporphyrin, its sesquihydrate and its 2-chlorophenol disolvate: conformational versatility of the ligand

    Sophia Lipstman
    Unsolvated 5,10,15,20-tetra-4-pyridylporphyrin, C40H26N8, (I), its sesquihydrate, C40H26N81.514H2O, (II), and its 2-chlorophenol disolvate, C40H26N82C6H5ClO, (III), reveal different conformational features of the porphyrin core. In (I), the latter is severely deformed from planarity, apparently in order to optimize the intermolecular interactions and efficient crystal packing of the molecular entities. The molecular framework has a C1 symmetry. In (II), the porphyrin molecules are located on symmetry axes, preserving the marked deformation from planarity of the porphyrin core. The molecular units are interlinked into a single-framework supramolecular architecture by hydrogen bonding to one another via molecules of water, which lie on twofold rotation axes. In (III), the porphyrin molecules are located across centres of inversion and are characterized by a planar conformation of the 24-membered macrocyclic porphyrin ring. Two trans -related pyridyl substituents are hydrogen bonded to the 2-chlorophenol solvent molecules. The interporphyrin organization in (III) is similar to that observed for many other tetraarylporphyrin compounds. However, the organization observed in (I) and (II) is different and of a type rarely observed before. This study reports for the first time the crystal structure of the unsolvated tetrapyridylporphyrin. [source]