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Showing papers on "Dihedral angle published in 1969"


Journal ArticleDOI
TL;DR: The conformation of the 7-membered hydrogen-bonded ring of the dipeptides has been elucidated and an empirically found stereochemical dependence of the constant 3 J NHCH upon the dihedral angle θ of the fragment has served as basis for discussing the possible conformations of the extended form of the Dipeptide molecules in polar solvents.

261 citations


Journal ArticleDOI
TL;DR: In this paper, the statistical weights of equilibrium conformations of macromolecules contain contributions from internal vibrations, and an analysis of such vibrations, in the absence and presence of solvent, is presented from a quantum-statistical-mechanical point of view.
Abstract: The statistical weights of equilibrium conformations of macromolecules contain contributions from internal vibrations. An analysis of such vibrations, in the absence and presence of solvent, is presented from a quantum‐statistical‐mechanical point of view. Several classical approximations to the quantum‐mechanically correct expression, with different degrees of accuracy, are derived. In all of these approximations, all internal degrees of freedom of the polymer are divided into two classes: hard (bond stretching and bond angle bending) and soft (torsional rotations around single bonds, i.e., variation of dihedral angles). Since the hard variables oscillate many times before the soft variables change in value by an appreciable amount, the hard variables can be treated effectively as parameters (i.e., not as independent variables), which are (i) functions of the instantaneous values of the soft variables, or (ii) simply constants, the latter treatment being less accurate. In treatment (i) the molecule is re...

211 citations


Journal ArticleDOI
TL;DR: In this paper, the exact analysis of ABC type absorption due to the CH2OH group of 3β-acetoxy-5β,6β-oxidocholestan-19-ol provides two vicinal HCOH coupling constants for dihedral angles of 80° and 160°.
Abstract: The exact analysis of the ABC type absorption due to the —CH2OH group of 3β-acetoxy-5β,6β-oxidocholestan-19-ol provides two vicinal HCOH coupling constants for dihedral angles of 80° and 160°. A least squares fit of these and previous data in the literature to a "Karplus" relation provides values for the coefficients in such an equation. The theoretical justification for such a relation has been obtained by extended Huckel molecular orbital calculations for methanol with different dihedral angles. The practical limitations in, and applications of the relation are discussed. An effect of solvent on the vicinal coupling constants is also noted and discussed.

119 citations


Journal ArticleDOI
TL;DR: The microwave spectra of four isotopic species of bicyclo[1.1.0] butane have been investigated and the rotational constants of each species have been determined.
Abstract: The microwave spectra of four isotopic species of bicyclo[1.1.0] butane have been investigated and the rotational constants of each species have been determined. Combining these data with those of the previously studied normal isotopic species, a complete set of structural parameters have been found. These include C1–C3 = 1.497 A, C3–C4 = 1.498 A, C3–C6 = 1.071 A, C–H (methylene) = 1.093 A, ∠C1C3H6 = 128°22′, dihedral angle, α, between cyclopropane rings = 121°40′, ∠HCH = 115°34′. These results are discussed with regard to the electronic structure of the molecule.

96 citations


Journal ArticleDOI
TL;DR: The study has revealed the importance of nonplanar distortion of the peptide unit and the need for distortion of covalent bond angles in cyclic peptides and the conformation with negative Δω is expected to be more stable.

60 citations


Journal ArticleDOI
TL;DR: The crystal structure of Al[N(SiMe3)2]3 has been determined by single-crystal X-ray diffraction as mentioned in this paper, which has D3 symmetry with a dihedral angle of 50° between the AlN3 and AlNSi2 planes.
Abstract: The crystal structure of Al[N(SiMe3)2]3 has been determined by single-crystal X-ray diffraction. The crystals are trigonal, space group P1c, with Z= 2 in a unit cell of dimensions a= 15·950 ± 0·009, c= 8·683 ± 0·017 A. The molecule has D3 symmetry with a dihedral angle of 50° between the AlN3 and AlNSi2 planes. The bond lengths and angles were found to be: Al–N 1·78 ± 0·02, N–Si 1·75 ± 0·01, mean Si–C 1·90 ± 0·02 A, Al–N–Si 121 ± 1, mean N–Si–C 112 ± 2°.

58 citations


Journal ArticleDOI
TL;DR: In this article, the microwave spectrum of propionyl fluoride [CH3CH2COF] has been used to prove the existence of two stable rotameric conformations.
Abstract: The microwave spectrum of propionyl fluoride [CH3CH2COF] proves the existence of two stable rotameric conformations. For the cis form (methyl eclipsing oxygen) with a planar heavy‐atom skeleton, rotational constants A = 10 042.53, B = 3762.19, C = 2832.69 MHz and a dipole moment of μtotal = 2.90, μa = 2.22, μb = 1.87 D have been determined. An approximate structure has been fitted to these data. Eight satellite spectra of the cis form have been analyzed yielding a torsional vibration frequency around the central carbon–carbon bond of 85 ± 3 cm−1 and an apparent barrier height V3 = 2400 ± 60 cal/mole hindering the internal rotation of the methyl group. The gauche conformation has rotational constants A = 8701.9, B = 3976.9, C = 3108.7 MHz and a total dipole moment of μtotal = 3.09 D with components μa = 2.93, μb = 0.32, and μc = 0.91 D. The dihedral angle between the CCC and COF plane is found to be 120° ± 2° and the torsional vibration frequency of the gauche form around the central carbon–carbon bond is ...

56 citations


Journal ArticleDOI
TL;DR: In this paper, the molecular structure of S2Cl2 has been determined accurately by electron diffraction and the two bond distances occur within a single peak of the radial distribution curve; the resulting correlation problem was solved by carrying out refinements with constrained amplitudes.
Abstract: The molecular structure of S2Cl2 has been determined accurately by electron diffraction. The ClSSCl chain configuration has the dimensions: S—Cl = 2.057 ± 0.002 A, S—S = 1.931 ± 0.005 A, SSCl = 108.2 ± 0.3°, ϕ(the dihedral angle between the two SSCl planes)= 84.8 ± 1.3°. The two bond distances occur within a single peak of the radial distribution curve; the resulting correlation problem was solved by carrying out refinements with constrained amplitudes. The study confirms the absence of the pyramidal isomer S = SCl2, but the existence of SClClS cannot be ruled out entirely from the electron diffraction results alone. The structure of S2Cl2 is compared with that of S2F2 where the S = SF2 isomer predominates.

51 citations


Journal ArticleDOI
TL;DR: In this article, the structure of the bicyclo [2.2]-octane (BCO) skeleton has been solved with the help of direct methods and refined by full-matrix least-squares analysis of the three-dimensional intensity data.
Abstract: Crystals of bicyclo [2.2.2] octane-1, 4-dicarboxylic acid are monoclinic, a = 6.01 A, b = 16.73 A, c = 10.29 A, β = 115.08°, space group P21/c, with 4 molecules in the unit cell. The structure was solved with the help of direct methods and refined by full-matrix least-squares analysis of the three-dimensional intensity data. Within experimental error the bicyclo [2.2.2]-octane (BCO) skeleton has apparent D3h-symmetry, corresponding to the totally eclipsed conformation. Analysis of the thermal ellipsoids in terms of the translational and rotational motion of the BCO skeleton leads to an r.m.s. amplitude of 5.9 ± 0.2° for rotation about the threefold axis. On the assumption that the bond lengths remain effectively constant during a torsional vibration of BCO, the potential energy surface has been calculated for a range of semi-empirical potential functions. These calculations show that the energy minimum may be slightly displaced from D3h symmetry, but if so the barrier between the two such equivalent minima is only about 0.1 kcal mole−1. The energy eigenvalues and eigenfunctions for a typical variation of potential energy vs torsion angle have been calculated. From the form of the eigenfunction of the ground vibrational state we conclude that BCO has effective D3h symmetry at all temperatures as far as diffraction methods are concerned.

50 citations


Journal ArticleDOI
TL;DR: In this article, the authors investigated the effect of the dihedral angle on the barrier to internal rotation of hydrogen peroxyde and derived a trans-barrier of 0.60 kcal/mole.

42 citations


Journal ArticleDOI
19 Dec 1969-Science
TL;DR: The phosphorus-31 nuclear magnetic resonance spectrum of β-adenosine-3'-β-adeno-5'-phosphoric acid in its aqueous solution was studied and the dihedral angles of the three P-O-C-H systems were estimated.
Abstract: The phosphorus-31 nuclear magnetic resonance spectrum of β-adenosine-39-β-adenosine-59-phosphoric acid in its aqueous solution ( p H = 9.2) was studied. The signal consisted of eight peaks caused by the spin-spin coupling of the phosphorus nucleus with three protons, two on the 59 carbon, and one on the 39 carbon. The coupling constants were 3.4, 6.5, and 8.1 hertz; from these values the dihedral angles of the three P-O-C-H systems were estimated.

Journal ArticleDOI
TL;DR: In this article, the nonbonded interaction energy was computed for xylobiose and xylan as a function of the dihedral angles (ϕ,ψ), indicating that interactions higher than the second neighbor are negligible.
Abstract: The nonbonded interaction energy was computed for xylobiose and xylan as a function of the dihedral angles (ϕ,ψ). The energy maps indicate that interactions higher than the second neighbor are negligible. Of the four possibilities, the left-handed helical conformation with (ϕ,ψ) = (63°,25°) is of the lowest energy. The hydrogen bond search and the energy maps reveal that the xylan helix is stabilized mainly by van der Waals forces. The allowed region map shows that the freedom of rotation of the monomer units in cellulose is more restricted than that of the monomer units in xylan, because of the presence of the CH2OH group in the former. The intramolecular hydrogen bond of the O5 O3′ type is stronger in cellulose.

Journal ArticleDOI
TL;DR: In this article, the 19F spectra of substituted trifluorochlorocyclobutanes which commonly occur as mixtures of two cis-trans isomers have been investigated.
Abstract: The 19F spectra of several substituted trifluorochlorocyclobutanes which commonly occur as mixtures of two cis-trans isomers have been investigated. Substituents of different bulkiness have been introduced to shift the conformational equilibrium of the puckering motion of the cyclobutane ring system. A complete assignment of the resonances of the two isomers was possible, based on chemical shifts and H-F coupling constants. Vicinal F-F coupling constants have been investigated for a possible dependence on the dihedral angle ϕ. It is found that the coupling constants change sign twice for 0 ⩽ gf ⩽ 1>80°, being negative for small and large angles and positive for intermediate angles. The investigations were based on fluorine spectra with completely eliminated H-F couplings by means of noise decoupling.

Journal ArticleDOI
TL;DR: In this paper, the nonbonded energy of isolated helical amylose chains was computed as a function of the dihedral angles (ϕ,ψ), i.e., the relative orientations of the glucose residues in the polysaccharide chain.
Abstract: The nonbonded energy (van der Waals) was computed for isolated helical amylose chains as a function of the dihedral angles (ϕ,ψ), i.e., the relative orientations of the glucose residues in the polysaccharide chain. In conformity with x-ray data, different helical conformations have been proposed for different crystalline modifications of amylose. Right-handed helical conformations were preferred for 71.13, 61.33, and 42.65-type helices from minimum energy considerations. The hydrogen bond search reveals the possibility of formation of strong OH O type hydrogen bonds between contiguous residues in the polymer chain for 61.33 and 71.13 helices. Smooth transition from a 71.13 to a 61.13 helix is possible with small changes in the values of (ϕ,ψ) and without the loss of intramolecular hydrogen bonds, whereas all the intramolecular hydrogen bonds will be broken during the conversion from 61.33 to 42.65 helix.

Journal ArticleDOI
TL;DR: The molecular structure of phenyl silyl ether C6H5OSiH3 has been studied by the sector-microphotometer method of electron diffraction as mentioned in this paper, and the results are discussed and compared with those for similar molecules.
Abstract: The molecular structure of phenyl silyl ether C6H5OSiH3 has been studied by the sector-microphotometer method of electron diffraction. The Si—O bond length is 1.648 ± 0.007 A, the C—O bond length is 1.357 ± 0.009 A, the Si—O—C angle is 121 ± 1° and the dihedral angle between the Si—O—C plane and the aromatic ring is 68 ± 3°. These results are discussed and compared with those for similar molecules.

Journal ArticleDOI
TL;DR: In this paper, the infrared and Raman spectra of trimethylene selenide (TMSe) were used to determine the double minimum potential, barrier-to-ring inversion, and the equilibrium separation of the ring diagonal.
Abstract: The infrared and Raman spectra of trimethylene selenide (TMSe) and trimethylene selenide‐2,2,4,4‐d4 (TMSe‐d4) have been recorded from 4000 to 15 cm−1 in gaseous, liquid, and solid states. A vibrational assignment of the fundamentals is consistent with the expected Cs equilibrium configuration. A series of sharp bands associated with the ring puckering vibration have been observed in the mid‐ and far‐infrared regions. The frequencies of the transitions have been employed in determining the following double‐minimum potential, barrier‐to‐ring inversion, and the equilibrium separation of the ring diagonal, 2x in angstroms, for TMSe, respectively: V(cm−1 = 4.258 (± 0.02) × 105x4 − 2.5391 (± 0.007) × 104x2; 378.1 ± 4 cm−1; 0.35 A. For TMSe‐d4 these quantities are: V(cm−1) = 3.242(± 0.1) × 105x4 − 2.208 (± 0.033) × 104x2; 375 ± 10 cm−1; 0.37 A, respectively. The dihedral angle of both molecules is calculated to be 32.5 ± 2°.

Journal ArticleDOI
TL;DR: In this paper, the proton resonance spectra of some aminopolycarboxylate complexes of nickel have been measured and the assignments of the large contact shifts of the CH protons are discussed, and it is suggested that the two protons of the N-CH2 group in a chelate ring are often not equivalent.
Abstract: The proton resonance spectra of some aminopolycarboxylate complexes of nickel have been measured. The assignments of the large contact shifts of the CH protons are discussed, and it is suggested that the two protons of the N—CH2 group in a chelate ring are often non-equivalent. This is ascribed to the puckering of the rings, and a consideration of the structures of the complexes indicates that the magnitude of the shift for an N—CH proton depends on the dihedral angle across the group Ni—N—C—H in a manner similar to that found for the vicinal proton-proton coupling in aliphatic groups in diamagnetic compounds. The ligands N-2-hydroxyethyl iminodiacetate and nitrilo β-propionate diacetate, which are tetradentate in the 1 : 1 complexes, are shown to be tridentate in the 2 : 1 complexes. With the 2 : 1 complexes of iminodiacetate and N-methyl iminodiacetate, spatial isomers appear to be present.

Journal ArticleDOI
TL;DR: In this article, the authors used Gaussian orbital expansions of best-atom Slater-type orbitals through 3p, tested in other calculations on HSSH, to calculate an energy minimum for a dihedral angle in the range 90°-100°.
Abstract: Hydrogen persulfide, HSSH, has been studied by the ab initio self‐consistent‐field molecular‐orbital method for the four dihedral angles of 0°, 90°, 135°, and 180°. The basis set consisted of Gaussian orbital expansions of best‐atom Slater‐type orbitals through 3p, tested in other calculations on H2S. The molecule has a calculated energy minimum for a dihedral angle in the range 90°–100°, in fair agreement with experiment (about 90.5°). The cis barrier is high (7.4 kcal), but the smaller trans barrier (1.9 kcal) seems low for consistency with experimental studies of the torsional vibrational levels. The S–S overlap population shows a pronounced maximum near a dihedral angle of 90°, just where the total energy shows a minimum. This reflects larger S–S bonding character near equilibrium and furnishes semiquantitative support for the discussion based on hyperconjugation used by the experimentalists to explain the structure. A partitioning of the total energy into attractive and repulsive components shows tha...

Journal ArticleDOI
TL;DR: In this article, the X-ray structure analysis of (Me)2(C5H5NO)2SnCl2 has shown that the compound is a monomeric octahedral transcomplex with a crystallographic center of symmetry.
Abstract: X-Ray structure analysis of (Me)2(C5H5NO)2SnCl2 has shown that the compound is a monomeric octahedral trans-complex with a crystallographic centre of symmetry. The crystals are triclinic, space group P with a= 7·25, b= 7·24, c= 8·75 A, α= 102·6, β= 87·5, γ= 117·2°, and Z= 1. Full-matrix least-squares refinement was carried out to a conventional R of 9·2% for 1007 independent reflexions. Bond lengths are Sn–Cl, 2·58(1); Sn–O, 2·25(2); Sn–CH3, 2·22(2); and N–O, 1·37(2)A. The angle Sn–O–N is 117(1)° and the dihedral angle between the plane of the pyridine ring and the plane of the atoms Sn–O–N is 83°.

Journal ArticleDOI
TL;DR: In this paper, a selective discussion is presented of experimental and theoretical wolk on the dependence of interfacial free energy on crystallographic orientation in both solid-fluid and solid-solid systems.

Journal ArticleDOI
TL;DR: In this article, the Dipole moments and molar Kerr constants for the molecules p-XC6H4·OMe (X = H, Me, Cl, Br, l, NO2, or CN) examined as solutes in carbon tetrachloride and benzene.
Abstract: Dipole moments and molar Kerr constants are reported for the molecules p-XC6H4·OMe (X = H, Me, Cl, Br, l, NO2, or CN) examined as solutes in carbon tetrachloride and benzene. The experimental data are analysed to indicate that (a) the Car–O–C group is virtually coplanar with the aromatic ring if X = NO2 or CN; for X = H, Me, or halogen, the apparent dihedral angle between the planes is ca. 20°(b) stereospecific benzene–solute complex formation occurs in the aromatic solvent.

Journal ArticleDOI
TL;DR: The structure of the complex hydridocarbonyl(fumaronitrile)bis(triphenylphosphine)iridium, IrH(CO)( trans -NCCH CHCN)[P(C 6 H 5 ) 3 ] 2 ] 2, has been determined from a single crystal X-ray analysis as discussed by the authors.

Journal ArticleDOI
TL;DR: In this article, a three-dimensional X-ray diffraction analysis of the structure of bis(diphenylphosphino)acetylene, (C6Hs)2PC=CP(C 6Hs)-2, has been carried out.
Abstract: A three-dimensional X-ray diffraction analysis of the structure of bis(diphenylphosphino)acetylene, (C6Hs)2PC=CP(C6Hs)2, has been carried out. The crystals are monoclinic, space group P21/n with a= 14"709, b= 8"166, c= 17-836 A, p= 95 ° 13', Z= 4. The parameters determined earlier by Brinkel were refined by a block-diagonal least-squares method with 2717 intensities collected with a Hilger-Watts linear diffractometer. The hydrogen atoms were included in the calculations. The final R value, including all reflexions, was 0.061 (R'=0.0064). The molecule has the expected pyramidal configuration about both phosphorus atoms, but in view of the asymmetric orientation of the benzene rings about the P-C bonds, possesses no symmetry, n-Interaction between the phosphorus atoms and the acetylenic group has been advanced to account for the geometry of the molecule. The torsion angle between the directions of the polar axes of the phosphorus lone-pair orbitals is approximately 90 °. Bond lengths and angles do not differ significantly in the two equivalent parts of the molecule. The mean > P-C(Ph) and > P-C = bond lengths are 1.832 and 1.765/~ respectively, and include an angle of 101.1 ° (e.s.d. 0.004/~, and 0-2°). All benzene rings are essentially planar with some significant inand out-of-plane bendings of the exocyclic bonds. The packing arrangement is described.

Journal ArticleDOI
TL;DR: The crystal structure of di-n-butylarsinic acid has been determined from 1257 independent photographically recorded intensities as discussed by the authors, where the acid molecules are present as non-planar, H-bonded dimers across the centers of symmetry in the cell.

Journal ArticleDOI
TL;DR: In this article, the authors showed that the 1 : 1 charge transfer complex between phenothiazine and trinitrobenzene is orthorhombic, space group Pbcn, with a= 7·010 ± 0·01, b= 15·14 ± 0�02, c= 17·15 ± 0 ·02 A, and Z= 4.
Abstract: Crystals of the 1 : 1 charge-transfer complex between phenothiazine and trinitrobenzene are orthorhombic, space group Pbcn, with a= 7·010 ± 0·01, b= 15·14 ± 0·02, c= 17·15 ± 0·02 A, and Z= 4. Packing within the crystal is typical of 1 : 1 donor–acceptor complexes; phenothiazine and trinitrobenzene molecules alternate in a stack parallel to a with a separation of 3·37 A. Each molecule is required by the space group symmetry to lie on a two-fold axis, but the phenothiazine molecules seem to be disordered. Thermal-ellipsoid data can be interpreted in terms of either a dynamic or a static disorder of the phenothiazine molecules requiring in either case a molecule folded along the N–S axis, with an average dihedral angle of about 165–172°. The dynamic interpretation is favoured.

Journal ArticleDOI
TL;DR: In this paper, the molar Kerr constants, dipole moments, refractivities, etc. were reported for phenoxathiin, xanthene, and thioxanthene examined as solutes in carbon tetrachloride and in benzene.
Abstract: The molar Kerr constants, dipole moments, refractivities, etc., are reported for phenoxathiin, xanthene, and thioxanthene examined as solutes in carbon tetrachloride and in benzene. The experimental data are analysed to indicate that the solute molecules have nonplanar, folded conformations defined by dihedral angles, Φ, of 138°(±6°), 160°(±6°), and 135°(±8°), respectively.

Journal ArticleDOI
TL;DR: The nuclear magnetic resonance data of some 9,10-dihydroanthracenes with substituents on the aromatic rings allow the assignment of a pseudoboat conformation for the central ring as mentioned in this paper.
Abstract: The nuclear magnetic resonance data of some 9,10-dihydroanthracenes with substituents on the aromatic rings allow the assignment of a pseudoboat conformation for the central ring. The dihedral angle formed by the two planes in the molecule changes according to the substituents.

Journal ArticleDOI
A.T Ward1
TL;DR: In this article, a normal coordinate vibrational analysis has been performed based on a non-linear four-atom model having SS bond lengths = 2.04 A, S ǫ s bond angles = 105°, and the Sǫ S dihedral angle = 90°.

Journal ArticleDOI
TL;DR: In this article, the dipole moment and molar Kerr constant values at 25° were determined for 2,3-dicyano-2,3dimethylbutane as a solute in carbon tetrachloride (2·13 D, +61) and also determined at 45°(2·30 D) and at 7°(1·98 D).
Abstract: Dipole moment and molar Kerr constant (×1012) values at 25° are recorded for 2,3-dicyano-2,3-dimethylbutane as a solute in carbon tetrachloride (2·13 D, +61) The electric moments have also been determined at 45°(2·30 D) and at 7°(1·98 D) Conformational analysis shows that the results at 25° indicate an equilibrium mixture of 82% of the trans-rotamer and 18% of the gauche-rotamer of the 2,3-dicyano-2,3-dimethylbutane molecule in which the dihedral angle is 85° The temperature-dependence of the dipole moment in carbon tetrachloride solution yields an apparent internal energy difference between the two rotational isomers of 1·31 kcal/mole

Journal ArticleDOI
TL;DR: In this article, a crystal structure analysis of 8,16-oxido-cis(2.2)metacyclophane-1,9-diene has been carried out at -160C.
Abstract: : A crystal structure analysis of 8,16-oxido-cis(2.2)metacyclophane-1,9-diene has been carried out at -160C. The crystal is orthorhombic, Pbca, with a=11.96, b=17.13, c=10.67 A, Z=8, formula C16H10O. The structure was determined by computer-programmed symbolic addition procedures, and was refined by block-diagonal least-squares to a conventional R index of 0.069. Within the limits of experimental error the molecule has mm symmetry. It is folded to a dihedral form so that the angle between the six-membered rings is 123.6. Strain is inferred from the nonplanarity of these rings. The bond angle subtended by the oxygen atom is 99.7, and the length of the double bond in the C-C bridge is 1.359 A. (Author)