Inter-ring Dihedral Angle Research Articles

4-Chloroanilinium hydrogen (2R,3R)-tartrate monohydrate

In the structure of the hydrated 1:1 compound of 4-chloro­aniline with l-tartaric acid, C6H7ClN+·C4H5O6−·H2O, determined at 130 K, the asymmetric unit comprises two 4-chloro­anilinium cations, two hydrogen tartrate anions and two water mol­ecules of solvation, and forms a two-dimensional duplex substructure comprising head-to-tail C11(7) hydrogen-bonded hydrogen tartrate anions and water mol­ecules. The π-associated 4-chloro­anilinium cation pairs [ring centroid separation = 3.576 (4) Å; inter-ring dihedral angle = 0.5 (1)°] are accommodated within the channels of the substructure and are hydrogen-bonded to it peripherally.

Force field and semiempirical MO conformational analysis of dihydropyridine calcium-channel antagonists

Force field and semiempirical MO calculations were used to investigate the conformational features (dihydropyridine ring puckering, inter-ring and side-chain dihedral angles) of a group of 4-aryl substituted dihydropyridine calcium-channel antagonists. The considered compounds were studied both in vacuo and in water (simulated with the Cosmo approach). For derivatives bearing a basic side chain the corresponding protonated structures were also submitted to MO calculations. The investigation highlighted the conformational flexibility of the dihydropyridine derivatives, the Δ H f of the most stable uncharged conformers of each compound lying in a range of 2–7 kcal·mol −1.

Reinvestigation of the structure of 2-hydroxyquinoxaline

$C_8H_6N_20$, M r= 146.15, orthorhombic, $P2_12_12_1$, a = 4.354 (1), b = 7.348 (1), c = 21.320 (1) \AA, V= 682.13 $\AA^ 3$, Z = 4, $D_x = 1.43, D _m = 1.41 Mg m ^{-3}$, \lambda(CuKa) = 1.5418 \AA, \mu = 7.6 $mm ^{-1}$, F(000) = 304, room temperature, R =0.041 for 771 observed reflections [I > 3\rho(I)]. A reinvestigation of the structure of 2-hydroxyquinoxaline based on diffractometer data has yielded more precise structural parameters (bond-length and angle e.s.d.'s of 0.004 \AA, 0.2 °) than the photographic results of Stepien, Grabowski, Cygler & Wajsman [(1976). Acta Cryst. B32, 2048-2050] (R = 0. 110, bond-length and angle e.s.d.'s 0.01 \AA, 0.7°). The compound exists in the keto form in the crystal with C=O = 1.239 (3)A. The quinoxaline rings are essentially planar (inter-ring dihedral angle = 1.6 °) including the exocyclic atoms. The large deviation of H(5) from the ring plane (0.59 \AA) reported earlier is not observed here. The crystal structure is stabilized by strong intermolecular hydrogen bonds: N...O = 2.782 (6) \AA, N--H...O = 162 ° .

Inter-ring dihedral angles in polychlorinated biphenyls from photoelectron spectroscopy

A proposal that the physiological activity of a chlorinated biphenyl is related to its degree of molecular planarity is explored through experimental measurements of inter-ring dihedral angles in the gas phase. A known correlation between the difference between fourth and first ionization energies of substituted biphenyls (via photoelectron spectroscopy), and the thermally averaged inter-ring dihedral angle as established by electron diffraction techniques, is confirmed and exploited for the present purpose. The expectation, based on both qualitative and quantitative theoretical considerations, that increased ortho substitution causes an increase in inter-ring angle, is confirmed. However, the observation that, for 2,2′-substituted halobiphenyls the only observed conformation is the near-syn form, is not easy to explain, and is discussed in terms of a non-bonding attractive force between halogen atoms.

Coenzyme Interactions: III. CHARACTERISTICS OF THE MOLECULAR COMPLEXES OF THIAMINE WITH INDOLE DERIVATIVES

Abstract Proton magnetic resonance and ultraviolet spectroscopic data have shown that the coordination of thiamine and thiamine pyrophosphate with indolyl compounds centers upon the positively charged quaternary nitrogen atom of the thiamine moiety and the benzene ring of the indole moiety. In ultraviolet studies a marked increase in absorbance over the sum of the absorbances of the individual solutions was found when a solution of a thiazolium derivative was mixed with that of an indolyl compound, whereas no interaction was indicated between the indolyl compounds and uncharged thiazole compounds. The proton magnetic resonance signals which show the greatest effect of the interaction are those of the protons which surround the quaternary nitrogen atom of thiamine and those of the protons attached to the benzene ring of the indolyl compounds. The signal of the bridge methylene protons of thiamine in the pure 1:1 complex with indoleacetate appears approximately 2.5 ppm upfield from the signal of the bridge methylene protons of free thiamine. From the magnitude of this change in chemical shift, the distance between the apex of the complexed thiamine molecule and the plane of the indole ring can be estimated as 0.37 nm. This small separation and the relative magnitude of the changes in chemical shift of the various thiamine signals suggest that the average conformation of thiamine in the complex has a large (g90°) inter-ring dihedral angle. Such a conformation places the pyrimidine amino group close to the thiazolium proton.