Showing papers on "Dihedral angle published in 1968"

Effect of side chains on the conformational energy and rotational strength of the n-pi transition for some alpha-helical poly-alpha-amino acids.

Abstract: Calculations of the dependence of the conformational energy and the rotational strength of the amide n–π* electronic transition (in a series of α-helical polyhel-α- amino acids with different side chains) on conformation have been carried out. The conformational energies were computed by procedures developed in this laboratory; the computation of rotational strengths was carried out by the method of Schellman and Oriel, with a slight modification. Polyamino acids with both nonpolar and polar side chains were considered; in the latter case, it was assumed that the only influence of the polar side chain was on the backbone conformation and on the electrostatic field which perturbs the amide chromophore of the backbone. Only conformations in the range of backbone dihedral angles of the right- and left-handed a-helices were considered, and the assumption of regularity (i.e., uniformity of dihedral angles in every residue) was made. The rotational strength per residue was found to vary markedly with chain length (in oligomers of up to 40 residues long); both the conformational energy per residue and the rotational strength per residue were found to vary significantly with the backbone conformation, which in turn depends on the nature of the side chain. The geometry of the hydrogen bond in the α-helical backbone is the most important factor which influences the dependence of the rotational strength on conformation. The implications of these results, for the interpretation of experimental circular dichroism and optical rotatory dispersion data, are discussed.

Conformations of Acetylcholine

Abstract: Canepa, Pauling and Sorum1 have reported the single crystal structure analysis of acetylcholine bromide and pointed out a correlation between that structure and those of choline chloride and muscarine iodide. In particular the (CH3)3N+–C–C–O– groups of all three are very similar in conformation with the ether oxygen atom gauche to the nitrogen atom. This correlation has been extended by Sundaralingam2, who has shown that in all molecules containing the N+–C–C–O– group the crystal structures of which are known (twelve) the oxygen atom is gauche to the nitrogen atom. Culvenor and Ham3 have concluded, however, on the basis of a nuclear magnetic resonance study of acetylcholine in D2O, that although it is gauche, the conformation of the acetylcholine molecule in solution is that of a normal primary ester4,5 in which the plane of the acetyl group is rotated about the O1–C5 bond (indicated by an arrow in Fig. 1) so as to bisect approximately the H–C5–H angle. This would give a C6–O1–C5–C4 torsion angle of approximately 180° instead of about ± 60° as observed in acetylcholine bromide. An important implication of this suggestion of Culvenor and Ham is that the spatial correlation between their proposed conformation of acetylcholine and the observed conformation of muscarine is much greater than the correlation between the conformation in acetylcholine bromide and muscarine6.

Energies and Grooving Kinetics of [001] Tilt Boundaries in Nickel Oxide

Abstract: Relative energies of symmetrical [001] tilt boundaries in nickel oxide bicrystals were determined from the dihedral angle at the groove root with a surface profilometer. At 1535°C in air, the relative grain boundary energy increases with tilt angle up to about 20 degrees and then remains relatively constant with increasing angle. A strong temperature dependence of boundary energy is explained by segregation of point defects to the boundary. Grooving kinetics are controlled by volume diffusion of nickel, and a theoretical discussion of mass transport in nonstoichio-metric solids is given, predicting this behavior under certain conditions.

Quantum mechanical calculations on barriers to internal rotation

Abstract: SCF LCAO MO calculations are reported for the borazane molecule BH3NH3, for different values of the dihedral angle (0°, 20°, 40°, 60°). The lower energy was found equal to -82.59651 a.u. for the staggered form. The theoretical barrier height, 0.00524 a.u.=3.29 kcal/mole, is very close to the one, 0.00577 a.u., computed by Clementi for the ethane molecule. A study of the electronic density maps seems to indicate that some care should be exercised in the use of the population analysis. According to the criterion of Bader andal., the bond density map seems characteristic of ionic binding.

Energetics of Grain‐Boundary Triple Junctions and Corner‐Twinned Junctions: Transmission Electron Microscope Studies

Abstract: Direct observations of grain‐boundary triple junctions and triple junctions containing a grain‐corner twin have been made in 304 stainless steel foils by transmission and diffraction electron microscopy. The distributions of true dihedral angles at pure triple junctions and the true dihedral angles in twinned grain corners have been computed by creating crystallographically identifiable slip or stacking‐fault traces in grains composing the junction through high‐intensity electron‐beam bombardment of the junction or areas in the immediate vicinity, or through the measurement of the image projection of coherent twin boundaries with the subsequent determination of the foil thickness. The mean dihedral angle for pure triple junctions was observed to be 120°, while that for the corner‐twinned grains was observed to be 105°. The characterization of pure triple junctions and corner‐twinned junctions on the basis of a cyclic crystallographic convention (hkl) for grain‐surface orientations yielded a mean dihedral ...

Structure of tricyclo[3, 3, 0, 02,6]-octane as determined by electron diffraction

Abstract: Tricyclo[3,3,0,02,6]-octane (C8H12) was prepared from cis, cis-1,5-cyclo-octadiene by mercury-sensitized photoisomerization. Sectored differaction photographs were taken with the sample at 0°C. With 70 kV electrons the recorded patterns covered the angular range q= 6–140 A–1. The interatomic distances and bond angles in this highly-strained, rigidly-fused ring system were determined by resolution of the refined radial distribution curve, followed by a least-squares fitting of the calculated molecular intensity pattern to that observed. Convergence was obtained for a structure with D2d symmetry, in which the highly puckered four-member ring has (C—C)= 1.558 ± 0.003 A with a dihedral angle of 127°. The sides of the five-member rings are 1.569 ± 0.005 A while their bases are 1.516 ± 0.010 A; (C—H)av= 1.106 ± 0.003 A. The root-mean-square amplitudes were lower than for less rigidly bound hydrocarbons. Distortions of the four- and five-member rings in this octane are compared with rings in related molecules.

Molecular polarizability. The dipole moments, molar Kerr constants, and conformations of some dialkyl and diaryl disulphides

Abstract: Dipole moments and molar Kerr constants are reported for five organic disulphides R2S2 (where R is methyl, t-butyl, phenyl, p-chlorophenyl, and β-naphthyl)in carbon tetrachloride solution. The experimental data are analysed to show (a) that each molecule has a skew structure with dihedral angles in the range 88-85o between the planes C?-S-S and S-S-C"; (b) that the anisotropic electron polarizabilities of the S-S group in dimethyl disulphide are bL = 4.66 A3, bT = bv = 2.25 A3; and (c) that in the aromatic disulphides the aryl group planes are nearly coplanar with their respective Car-S-S groups.

Molecular polarisability. The conformations as solutes of phenothiazine and of N-methyl- and N-phenyl-phenothiazine

Abstract: Dipole moments and molar Kerr constants are reported for phenothiazine and for N-methyl- and N-phenyl-phenothiazine in benzene solution. The experimental data are interpreted to indicate that (a) the molecules as solutes have apparent conformations defined by dihedral angles, Φ, of ca. 150°, and (b) the substituent on the nitrogen is disposed “equatorially” with respect to the central heterocyclic ring.

Electronic Structures and Reactivities of Aliphatic Polycyclo Compounds. I. Fused Three-membered-ring Compounds

Abstract: The electronic structures of bicyclo(1.1.0)butane, tricyclo(3.1.0.0)hexane, tricyclo(2.1.0.0)pentane, tricyclo(1.1.0.0)butane, prismane, and spiro(2.2)pentane are studied by the extended Huckel method. The calculated total energies of bicyclo(1.1.0) butanes with various dihedral angles, θ, and bridgehead C–C–H angles, α, show that the most stable configuration of bicyclo-butane is at α=135° and θ=115–120°. It is made clear that the s-nature of the carbon atoms of the C–Haxial and C–Hequatorial depends greatly on the dihedral angle of the two cyclopropane rings. The double-bond character between the bridgehead carbon atoms in the bicyclobutane and its homologues is pointed out. The stabilities of the C–C bonds of prismane are shown to be largely dependent on the angle between the C–H bond and the four-membered-ring plane. The electronic structures of some configurations of spiropentane with different dihedral angles are discussed. The physico-chemical properties of these compounds are briefly investigated.

Molecular polarisability. The conformations of some polynuclear aromatic hydrocarbons

Abstract: Molar Kerr constants are reported for 9-phenylanthracene, 9,10-diphenylanthracene, 9,9′-bianthryl, 1-phenylnaphthalene, and 2-phenylnaphthalene as solutes in non-polar media. Analysis of the experimental data indicates that the preferred solute conformations are defined by dihedral angles Φ(between the aromatic ring planes) of ca. 67°, 67°, 70°, 50°, and 20°, respectively.

Moiré patterns formed by inclined gratings

Abstract: Moire patterns formed by two inclined gratings of identical frequency are characterized by a spatial location which depends on the pitch of the gratings and their dihedral angle. A theory on the formation of moire fringes is developed, based on the geometric obstruction of light and on the assumption that the viewing direction is perpendicular to one grating. Two extreme cases are studied where the lines of the gratings are either parallel or normal to the dihedral corner.

The crystal structure of 8,16-oxido-cis-[2,2]metacyclophane

Abstract: : The crystal structure of 8,16-oxido-cis(2.2)metacyclophane, C16H14O, is orthorbombic, space group probably Cmc21, with a=8.78, b=16.04, c=8.42 A, Z=4. The structure was deduced from the three-dimensional Patterson synthesis and refined by block-diagonal least squares to a final agreement residual of 0.057. The molecule has nearly mm symmetry, and is folded to a dihedral form so that the angle between the slightly boat-shaped six-membered rings is 99.6 deg. The bond angle subtended by the oxygen atom is 101.4 deg, while the mean value of those in the methylene bridges is 119.0 deg. The strain implied by these values is consistent with the tendency of the compound to transform into the corresponding pyrene derivative, with ejection of the hetero atom. (Author)

The Synthesis and Nuclear Magnetic Resonance Spectra of Epimeric 16-Deuterio-13α-androstan-17-ols

Abstract: Four epimeric 16-deuterio-5α, 13α-androstane-3β, 17-diols were prepared starting from 3β-hydroxy-5α, 13α-androstan-17-one, and the conformation of ring D in 13α-steroids is discussed based upon the correlation of dihedral angles with the observed coupling constants of H16, 17. The chemistry of the 16β, 17β-epoxy and Δ16 compounds is also described.