Planar Molecules (planar + molecule)

Distribution by Scientific Domains

Selected Abstracts

Features of homotetrameric molecular association in protein crystals

Uma V. Katre
The crystal structures of proteins showing homotetrameric association, a common feature observed in many lectins, have been analyzed in order to understand the characteristics of tetrameric association in terms of the arrangement of subunits and their biological significance. The analysis could group the tetramer units into the following four categories. (i) Tetrahedral molecules, in which the four monomers form a nearly perfect tetrahedral arrangement. The angle between the axes of any two monomers is ,109°. (ii) Molecules that form a sandwiched dimer of dimers in which the two dimers are arranged perpendicular to each other, one upon the other. (iii) Planar molecules, in which the four monomers lie in one plane and the corresponding sides of adjacent monomers face in opposite directions. This can be considered as a flattened tetrahedral shape. (iv) Planar closed molecules, in which all four monomers lie in one plane arranged in a head-to-tail fashion in a square. The first group and its variant, the third group, are the most commonly found arrangements in crystal structures. Each arrangement has its own importance for biological function. Some tetrameric assemblies that deviate from the majority described above also have relevance to their biological function. [source]

Orientational disorder in ,-cobalt(III) sepulchrate trinitrate

Andreas Schönleber
The crystal structure of ,-(1,3,6,8,10,13,16,19-octaazabicyclo[6.6.6]eicosane)cobalt(III) trinitrate, [Co(C12H30N8)](NO3)3, consists of a sepulchrate moiety that serves as a macrobicyclic nitrogen cage for the Co3+ cation, which is six-coordinated by N atoms, and three nitrate anions. The Co,sepulchrate group lies on a threefold axis (site symmetry 32), as do two symmetry-related and ordered nitrate groups (site symmetry 3), with which it is connected via N,H...O hydrogen bonds [Co,N = 5.1452,(12),Å]. The third nitrate group is disordered as a result of symmetry requirements around the origin (site symmetry 32), and is further away from the Co,sepulchrate cage [Co,N = 6.3160,(8),Å]. The structure is described by applying orientational disorder over six equivalent orientations for the disordered nitrate group, which is considered as an ideal planar molecule of regular trigonal geometry with its molecular plane rotated out of the ab plane and the molecular centre of gravity slightly shifted away from the origin. This new model for disorder clearly improves a previous crystal structure determination. [source]

Reaction of Phenanthrene-9,10-dione with Phenanthrene-9,10-diol: Synthesis and Characterization of the First ortho -Quinhydrone Derivative

Fausto Calderazzo
Treatment of phenanthrene-9,10-dione (PQ) with phenanthrene-9,10-diol (PQH2), as prepared by catalytic hydrogenation of PQ, in toluene solution or in the solid state afforded crystalline ,9,10-phenanthrenequinhydrone' (PQH), the first example of an ortho -quinhydrone. PQH was characterized by analytical and spectroscopic methods, including X-ray and CP/MAS 13C-NMR analyses. The crystal structure of PQH showed pairs of planar molecules linked by H-bonds and organized in columns parallel to the crystallographic axis a. The solid-state structure of PQH was compared with those of the parent compounds, PQ and PQH2, the latter being reported for the first time. PQH was found to be stable in the solid state only, the components PQ and PQH2 being formed upon dissolution in media of even low polarity such as toluene. [source]

Molecular axes and planes as an Eigenvalue problem

A statistical approach is used to define the long molecular axis and the main molecular plane. While the concept of molecular axis and plane is immediately useful for simple rod-like or planar molecules, it can be easily generalized to arbitrary molecules as well as molecular subgroups. From the molecular axes and planes, the orientation of the molecule with respect to the lattice as well as intermolecular orientations can be characterized. The method is applied for a comparison of pentacene and para -quinquephenyl crystal structures. [source]

13C and 1H nuclear magnetic resonance of methyl-substituted acetophenones and methyl benzoates: steric hindrance and inhibited conjugation

Abstract The 1H and 13C NMR spectra of 14 methyl-substituted acetophenones and 14 methyl-substituted methyl benzoates were assigned and interpreted with respect to the conformation of the Car,C(O) bond. The substituent effects are proportional in the two series and can be divided into polar and steric: each has different effects on the 13C SCS of the individual atoms. In the case of C atoms C(O), C(1) and CH3(CO), the steric effects were quantitatively separated by comparing SCS in the ortho and para positions. The steric effects are proportional for the individual C atoms and also to steric effects estimated from other physical quantities. However, they do not depend simply on the angle of torsion , of the functional group as anticipated hitherto. A better description distinguishes two classes of compounds: sterically not hindered or slightly hindered planar molecules and strongly sterically hindered, markedly non-planar. In order to confirm this reasoning without empirical correlations, the J(C,C) coupling constants were measured for three acetophenone derivatives labeled with 13C in the acetyl methyl group. The constants confirm unambiguously the conformation of 2-methylacetophenone; their zero values are in accord with the conformation of 2,6-dimethylacetophenone. The zero values in the unsubstituted acetophenone are at variance with previous erroneous report but all J(C,C) values are in accord with calculations at the B3LYP/6-311++G(2d,2p)//B3LYP/6,311+G(d,p) level. Copyright © 2004 John Wiley & Sons, Ltd. [source]

1,3-Bis(ethylamino)-2-nitrobenzene, 1,3-bis(n -octylamino)-2-nitrobenzene and 4-ethylamino-2-methyl-1H -benzimidazole

Christopher P. Walczak
1,3-Bis(ethylamino)-2-nitrobenzene, C10H15N3O2, (I), and 1,3-bis(n -octylamino)-2-nitrobenzene, C22H39N3O2, (II), are the first structurally characterized 1,3-bis(n -alkylamino)-2-nitrobenzenes. Both molecules are bisected though the nitro N atom and the 2-C and 5-C atoms of the ring by twofold rotation axes. Both display intramolecular N,H...O hydrogen bonds between the amine and nitro groups, but no intermolecular hydrogen bonding. The nearly planar molecules pack into flat layers ca 3.4,Å apart that interact by hydrophobic interactions involving the n -alkyl groups rather than by ,,, interactions between the rings. The intra- and intermolecular interactions in these molecules are of interest in understanding the physical properties of polymers made from them. Upon heating in the presence of anhydrous potassium carbonate in dimethylacetamide, (I) and (II) cyclize with formal loss of hydrogen peroxide to form substituted benzimidazoles. Thus, 4-ethylamino-2-methyl-1H -benzimidazole, C10H13N3, (III), was obtained from (I) under these reaction conditions. Compound (III) contains two independent molecules with no imposed internal symmetry. The molecules are linked into chains via N,H...N hydrogen bonds involving the imidazole rings, while the ethylamino groups do not participate in any hydrogen bonding. This is the first reported structure of a benzimidazole derivative with 4-amino and 2-alkyl substituents. [source]