Showing papers on "Dihedral angle published in 1972"
TL;DR: Uracil was prepared enriched at C2 with carbon-13 and converted into uridine from which 2,5′-anhydro-1β-D-ribo-furanosyl and 2,2′]-anhydroid-1-β-d-arabino-furanusyl uracil were prepared as mentioned in this paper.
Abstract: Uracil was prepared enriched at C2 with carbon-13 and converted into uridine from which 2,5′-anhydro-1-β-D-ribo-furanosyl and 2,2′-anhydro-1-β-D-arabino-furanosyl uracil were prepared. The 13C2 to ...
157 citations
TL;DR: The crystal structure of adenosine, C10Ht3NsO4, has been determined from the intensities of 1333 reflections, each measured at least four times on an automated diffractomcter as discussed by the authors.
Abstract: The crystal structure of adenosine, C10Ht3NsO4, has been determined from the intensities of 1333 reflections, each measured at least four times on an automated diffractomcter. The crystals are monoclinic, space group P21, with a=4.825 (1), b= 10"282 (2), c= 11.823 (1)/~, fl=99.30 (1) °, and two molecules per cell. Least-squares refinement of coordinates and anisotropic temperature factors for all atoms, including hydrogen, led to an R index of 0-024 and standard deviations of about 0.003 A, for bond distances between pairs of heavy atoms. The anomalous scattering by nitrogen and oxygen was used to confirm the absolute configuration of the sugar ring. The conformation of the ribose ring is C(3') endo; the torsion angle about the glycosidic bond is 9.9 °. Intermolecular interactions include a full complement of hydrogen bonds, a relatively short C(2)-H.. .O(2') contact (3.09 A,), and parallel stacking of adenine rings with an interplanar spacing of 3"57/~.
128 citations
TL;DR: In this article, the molecular structures of disilane and diphosphine have been studied in the gas phase by electron diffraction, and the P-P bond length in P2H4 is 0.021 ± 0.003 A shorter than found in silicon.
128 citations
TL;DR: In this paper, the authors introduced a map, where the mutual distance between C α -atoms of i -th and j -th residue, r i j, is listed against the residue number, in row and column.
Abstract: A protein conformation can be computed by connecting peptide units of usual trans-planar structure successively with a given set of dihedral angles ϕ and ψ. It is, however, not easy to generate the native conformations such as myoglobin and lysozyme by the computation. In order to show the discrepancy between the native conformation and the computed one, we have introduced a map, where the mutual distance between C α -atoms of i -th and j -th residue , r i j , is listed against the residue number, in row and column. This map represents a tertiary structure of the protein (e. g. α-helix, β-structure) as the characteristic patterns. It becomes possible to estimate the difference of the computed conformation from the native one numerically by comparing the corresponding maps. The improvement of the dihedral angles, ϕ and ψ, as made by minimizing the deviation of the computed map from the native one on both myoglobin and lysozyme.
122 citations
TL;DR: In this paper, the vibrational frequencies of biphenyl and its 4,4'-dihalogen derivatives have been computed for various values of the dihedral angle between the two rings.
111 citations
TL;DR: In this article, an interpretation of the higher energy bands is given in terms of a Huckel-type theory based on the interaction between semi-localized π molecular orbitals.
Abstract: The helium I photoelectron spectroscopy of 30 biphenyls is reported. An empirical correlation between certain bands in the spectra and dihedral angle is noted and its use in conformational studies emphasized. An interpretation of the higher energy bands is given in terms of a Huckel-type theory based on the interaction between semi-localized π molecular orbitals. Substituent effects are divided into electronic and steric and means for separating these two factors are outlined.
105 citations
TL;DR: The minimum energy conformation of 2-methoxyethanol (CH3OCH2CH2OH) has been determined from an analysis of its microwave spectrum as discussed by the authors, which is one in which the hydroxyl hydrogen atom is approximately aligned with the nearest sp3 lone pair electrons of the ether oxygen atom.
Abstract: The minimum energy conformation of 2-methoxyethanol (CH3OCH2CH2OH) has been determined from an analysis of its microwave spectrum. The rotational constants of the normal species are: A = 12982.35, B = 2742.48, and C = 2468.10 MHz; the dipole moment components are μa = 2.03, μb = 1.15, and μ = 2.36 ± 0.03 D. For the CH3OCH2CH2OD species: A = 12385.71, B = 2724.74, and C = 2431.42 MHz. The conformation consistent with this data is gauche about each of the C—C, C—O(H) and C—O(ether) bonds, having dihedral angles of 57 ± 3°, 45 ± 5°, and 8 ± 3°, respectively. This distorted conformation is one in which the hydroxyl hydrogen atom is approximately aligned with the nearest sp3 lone pair electrons of the ether oxygen atom. Transitions in three excited torsional states have also been observed but no other rotational isomer was detected.
80 citations
TL;DR: In this paper, the geminal (HCH) and vicinal (HCCH) proton spin-spin coupling constants are examined in detail, and a semi-empirical approach based on general qualitative relations found by quantum-mechanical calculations is proposed.
Abstract: The geminal (HCH) and vicinal (HCCH) proton spin–spin coupling constants are of fundamental importance in the applications of nuclear magnetic resonance (NMR) spectroscopy in structural and conformational analysis, since they are extremely sensitive to fine details of steric and electronic molecular structure. The nature of the dependence of the constants on individual structural factors (bond length, valence and dihedral angles, electronic structure of the substituents and their orientation, etc.) has been examined in detail. At present a priori theoretical calculations do not allow an exact description of these relations and therefore a semiempirical approach, based on general qualitative relations found by quantum-mechanical calculations, is very promising. The bibliography includes 293 references.
70 citations
TL;DR: In this paper, it was shown that the PE spectrum of a molecule M of Cs or higher symmetry can lead either to the natural order of the corresponding delocalized orbitals or to the inverted order.
Abstract: The competitive through-space and through-bond interaction of two symmetry-equivalent, semi-localized orbitals φa, φb, (= π or lone-pair orbitals) in a molecule M of Cs or higher symmetry can lead either to the natural order of the corresponding delocalized orbitals, i.e., ψ− ≈ (φa - φb)/√2 above ψ+ ≈ (φa - φb)/√2 or to the inverted order, i.e., ψ+ above ψ−. It is shown that one can discriminate between these two situations through an analysis of the PE spectrum of a molecule M′ which differs from M only by an additional semi-localized orbital φc of known symmetry behaviour. The method is exemplified by an investigation of the dependence of the n-orbital sequence in 3,6-bridged 1,4-cyclohexadienes on the dihedral angle between the two CCHCHC moieties.
63 citations
TL;DR: In this article, the Fermi contact contribution to three-bond carbon-proton coupling constants in propane was calculated at the INDO level of approximation using molecular orbital theory.
Abstract: Molecular orbital theory at the INDO level of approximation is used to calculate the Fermi contact contribution to three-bond carbon–proton coupling constants in propane. The calculations predict a dihedral angle dependence of 3J(13C,H) in the 13C—C—C—H fragment similar to that observed for 3J(H,H), 3J(19F,H), and for 3J(31P,H) in the saturated X—C—C—H fragments.
58 citations
TL;DR: In this article, the Fermi contact contribution to two-and three-bond nitrogen-proton coupling constants for a wide variety of compounds was calculated at the INDO level of approximation.
Abstract: Molecular orbital theory at the INDO level of approximation is used to calculate the Fermi contact contribution to two-bond and three-bond nitrogen–proton coupling constants for a wide variety of compounds. The dependence of calculated N,H coupling constants upon the spatial orientation of the nitrogen lone-pair is examined for selected molecules. For both saturated and unsaturated compounds the calculated 2J(15N,H) is large and negative when the lone-pair is oriented cis to the proton, and is small and of either sign when the lone-pair lies trans to the proton. Calculated 3J(N,H) depends both on the orientation of the nitrogen lone-pair and on the HCCN dihedral angle. Computed 2J(N,H) and 3J(N,H) generally follow the experimentally known substituent and structural effects. It is suggested that observed phosphorus–proton coupling constants in tervalent phosphorus compounds are dependent on lone-pair orientation in a manner analogous to the corresponding nitrogen–proton coupling constants. The influence of...
TL;DR: In this paper, a triclinic compound with cell constants a= 5·986(2), b= 3·933(1), c= 12·342(2)A, α= 87·38(3), β= 78·40(3, γ= 89·53(3)°, and Z= 1.
Abstract: The title compound crystallises in the triclinic system with cell constants a= 5·986(2), b= 3·933(1), c= 12·342(2)A, α= 87·38(3), β= 78·40(3), γ= 89·53(3)°, and Z= 1. The crystal and molecular structures were solved from 1458 intensities measured on a diffractometer, and refined anisotropically to R 0·062. The structure is disordered about a crystallographic inversion centre, which requires that the two aromatic rings must be parallel, the dihedral angles about the exocyclic bonds being equal but of opposite sign, in contrast to the structures of previously analysed benzylideneanilines. The metastable nature of the crystal is discussed in terms of the packing arrangement.
TL;DR: In this article, the microwave spectra of the normal and three singly substituted 13C isotopic species of bicyclo[2.0]pentane have been assigned and analyzed using the moments of inertia derived from these spectra.
Abstract: The microwave spectra of the normal and three singly substituted 13C isotopic species of bicyclo[2.1.0]pentane have been assigned and analyzed. Using the moments of inertia derived from these spectra the following structural parameters have been obtained for the carbon skeleton: C1 – C2=C3 – C4=1.528 A, C2 – C3=1.565 A, C1 – C4=1.536 A, C1 – C5=C4 – C5=1.507 A, and the dihedral angle α=67.26°. The disagreement of these results with those of an earlier electron diffraction study has been discussed. It seems likely that a reanalysis of the electron diffraction data will lead to concurrence with the results of the present study. We have compared our structural parameters to those of other cyclic systems and have discussed the essential constancy of the average C – C distances in cyclobutyl systems and also in cyclopropyl systems. Finally, Stark effect measurements have yielded a total dipole moment of 0.27 D.
TL;DR: Extended Huckel theory molecular orbital calculations of the vicinal proton-proton coupling constants in 1,1- and 1,2-difluoroethanes are presented and compared with experimental results as mentioned in this paper.
Abstract: Extended Huckel theory molecular orbital calculations of the vicinal proton–proton coupling constants in 1,1- and 1,2-difluoroethanes are presented and compared with experimental results. Equations, describing the angular dependence of vicinal couplings in substituted CH–CH fragments, are formulated on the basis of the MO calculations. Empirical parameters for these equations are derived from a least-squares treatment of a large number of experimental coupling constants.
TL;DR: In this article, a simple theoretical model is used to suggest that the NCCH coupling constant, Jnh, has a dihedral angle dependence that can serve for estimating the angle Ψ in peptides.
Abstract: A simple theoretical model is used to suggest that the NCCH coupling constant, Jnh, has a dihedral angle dependence that can serve for estimating the angle Ψ in peptides. Some experimental comparisons are made to test the model; they include a new measurement of Jnh in N-acetylglycine.
TL;DR: In this paper, electron diffraction patterns from gaseous N2F4 at 25°C have verified conclusions from earlier work that the material contains both trans and gauche conformers, assuming that these conformers have the same parameter values except for the torsion angle, the following principal results were obtained: 71.2% (8.1)
Abstract: Analysis of electron‐diffraction patterns from gaseous N2F4 at 25°C has verified conclusions from earlier work that the material contains both trans and gauche conformers. Assuming that these conformers have the same parameter values except for the torsion angle, the following principal results were obtained: 71.2% (8.1) trans, rNN=1.492 A (0.007), rNF=1.372 A (0.002), 〈 FNF=103.1° (0.6), 〈 NNF=101.4° (0.4), θ (the dihedral angle N2N1F4, N1N2F1)=64.2° (3.7), lNN=0.048 A (0.005), and lNF=0.044 A (0.002). These distances and root‐mean‐square amplitudes are ra and la values; the errors are 2σ and include estimates of systematic error. The diffraction results indicate that the energy difference Eg—Et is roughly less than 1.0 kcal but that the rotational barrier may be several kilocalories.
TL;DR: The structure of the title compound is monoclinic with a= 6·932(1), b= 26·721(6), c= 11·697(3)A, β= 103·4(1)°, space group P21/c, and Z= 4.5° as discussed by the authors.
Abstract: Crystals of the title compound are monoclinic with a= 6·932(1), b= 26·721(6), c= 11·697(3)A, β= 103·4(1)°, space group P21/c, and Z= 4. The structure was determined from diffractometer data by Patterson and Fourier methods and refined by full-matrix least-squares techniques to R 0·060 for 2589 observed reflections. The structure consists of discrete Pd(bipy)22+ cations and nitrate anions. There is a weak interaction between the metal ion and one of the nitrate ions, the other nitrate ion being hydrogen bonded to a water molecule. The steric strain imposed by the interaction of interligand ortho-hydrogens is reduced by a distortion from a square planar configuration. The ‘bite’ of the 2,2′-bipyridyl ligand results in a rectangular distortion of the PdN4 skeleton such that mean of the angles N–Pd–N, where both nitrogens are from the same ligand, is 80·0(3)°. The corresponding angle involving nitrogens from different ligands is 102·2(3)°. There is also a mutual twist of the two ligands to avoid interligand hydrogen contacts. The mean diagonal angle, N–Pd–N, is 164·5(5)°. The torsion angle is 24·3°
TL;DR: The most probable value of the dihedral angle of the gauche isomer of CH 2 ClCOCl is about 105° as discussed by the authors, and the energy differences between the rotational isomers of these molecules have been evaluated.
TL;DR: The microwave spectrum of CH3CH2SCN was investigated in this article, where rotational and centrifugal distortion constants in the ground vibrational state, and rotational constants for two excited vibrational states were obtained.
TL;DR: In this paper, the title compound is orthorhombic, space group Pbca, a= 20·02(1), b= 21·73 (1), c= 15·72(1)A, Z= 4.
Abstract: The title compound is orthorhombic, space group Pbca, a= 20·02(1), b= 21·73(1), c= 15·72(1)A, Z= 4. Least-squares refinement of X-ray diffractometer data using 2115 independent structure amplitudes converged with R 0·05.The co-ordination round each platinum atom is strained square-planar, but the two co-ordination planes are not coplanar, having a dihedral angle of 130° between them. This conformation is partly attributed to severe steric repulsion between the alkyl groups on the phosphorus and on the sulphur atoms. The non-planarity of the (Pt2S2) bridge and the pyramidal configuration at the sulphur atoms are unfavourable to dπ–pπ aromatic bonding in the bridge while allowing dπ–dπ overlap. The Pt–S bonds trans to P have approximately normal lengths for single bonds [2·37(1)A], whilst those cis to P and trans to Cl are much shorter [2·27(1)A]; the Pt–P bonds are also short [2·26(1)A]. The stability of the cis-isomer is attributed to the continuous zig-zag chain of strong cis-bonds P–Pt–S–Pt–P in which one of the bridging sulphur atoms is held by two strong bonds; the trans-isomer is inherently less stable because one Pt–S bond to each bridging sulphur atom will be weakened by the trans-influence of the phosphines. The bonds in the zig-zag chain in the cis-isomer all have appreciable π-character, and a multiple overlap of dπ orbitals resembling conjugation may be present.
TL;DR: The crystal structure of cyclo-di-β-alanyl (monoclinic; a = 11.85, b = 5.20, c = 10.66 A, β = 94.83°; space group C2/c; Z = 4) was determined by X-ray analysis, using direct methods, and refined by full-matrix least squares calculations as discussed by the authors.
Abstract: The crystal structure of cyclo-di-β-alanyl (monoclinic; a = 11.85, b = 5.20, c = 10.66 A, β = 94.83°; space group C2/c; Z = 4) has been determined by X-ray analysis, using direct methods, and refined by full-matrix least-squares calculations. The two cis-peptide groups, related by a twofold axis, are almost exactly planar, and the molecule occurs in a “flexible” chair conformation with N-CH2CH2-C torsion angle of 27°.
TL;DR: In this paper, the crystal and molecular structures of a compound with the formula [(C5H5)CH3NC)Ni]2 have been determined by X-ray crystallography.
Abstract: The crystal and molecular structures of a compound with the formula [(C5H5)CH3NC)Ni]2 have been determined by X-ray crystallography. The dinuclear molecule has virtual C2 symmetry; its important structural features are: (1) bridging CH3NC groups; (2) pentahaprtocyclopentadienylnickel groups; (3) a short Ni-Ni bond, 2.322(1) A. Each Ni2CNC group is planar; the dihedral angle between the two Ni2CN planes, which meet along the Ni-Ni line is 121.1°. The Ni-C-Ni groups are slightly unsymmetrical. The crystals belong to space group P21Jc (No. 14) and the unit cell dimensions are: a = 6.999(2) A, b = 22.07(1) A, c = 9.130(3) A and β = 93.433(5)°.
TL;DR: The crystal structure of 2'-deoxyuridine (CgHI2N2Os) has been determined from data collected on a Hilger and Watts linear diffractometer as discussed by the authors.
Abstract: The crystal structure of 2'-deoxyuridine (CgHI2N2Os) has been determined from data collected on a Hilger and Watts linear diffractometer. The crystals are monoclinic, space group P2x, with cell dimensions a=7.91 +0.01, b=6.710+0.005, c= 18.77+0.01 /~ and fl=96.6+0.1 °. The structure was solved by Patterson interpretation methods. The final R value for 1611 observed reflexions is 0.084. There are two molecules in the asymmetric unit; in molecule I the dihedral angle between the base and the sugar is 53 °, whilst in molecule II it is 47 °. The glycosidic torsion angle ~0cN, is 26 ° in molecule I and 25 ° in molecule II. In molecule I, atom C(2') of the sugar ring is displaced 0.43 A, endo and atom C(Y) 0.19 A, exo relative to the plane through atoms C(I'), O(1') and C(4'). In molecule II, atom C(2') is 0.32/~ endo and atom C(3') is 0-28 A exo. The orientation of the C(5')-O(5') bond is trans to the C(4')-O(1') bond and gauche to C(4')-C(Y) in both molecules.
TL;DR: In this article, a method for adjusting the dihedral angles (φi, ψi, χj) of backbone and side chains by a least-squares approximation to the x-ray coordinates is described.
TL;DR: The structure of the title compound (I) has been determined from three-dimensional photographic X-ray data as discussed by the authors, which contains a six-carbon atom ligand bridging two [CO(CO)2] moieties.
Abstract: The structure of the title compound (I) has been determined from three-dimensional photographic X-ray data. The complex crystallizes as a racemate in the orthorhombic space group Pna21(No 33) with a= 9·92, b= 11·97, c= 13·70 A, Z= 4. The structure was refined by least-squares methods to R 0·097 for 757 independent observed reflections. The complex contains a six-carbon atom ligand bridging two [CO(CO)2] moieties. Trifluoromethyl groups are attached to carbon atoms 1,3 and 6 within the six-carbon atom ligand. The bridging ligand is composed of two allylic functions joined by a C–C single bond; there is a dihedral angle of 60° between the two allylic planes. The allyl groups are each bonded to both cobalt atoms, which are separated by a normal bonding distance of 2·459(6)A. The cobalt atoms are seven-co-ordinate if the allyl groups are regarded as tridentate ligands. The two cobalt atoms and the two terminal carbon atoms of the C6-chain are coplanar: There is a symmetrical arrangement of these four atoms.
TL;DR: In this paper, several sets of ϕ and ψ which reproduce similar conformations to the native structure of lysozyme or myoglobin were obtained by using the procedure described in a preceding paper.
Abstract: By using the procedure described in a preceding paper, several sets of ϕ and ψ are obtained which reproduced similar conformations to the native structure of lysozyme or myoglobin. Contrary to the expectation, the value of f has no maximum along a line connected any two of the minima in the 2 n -dimensional phase space. This result shows the existence of many solutions for ϕ and ψ which generate the native conformation under restricted condition of fixed geometry in the main chain. Energy calculation is performed on the conformations obtained above to examine atomic collisions along the main chain. After refinement to remove the collisions, a conformation is finally obtained for lysozyme which has a total energy of -270 kcal/mol and still has an low value of f . Similarly a conformation can be found for myoglobin having an energy of -580 kcal/mol in total.
TL;DR: In this paper, the complex dicarbonylbis (diphenylethylphosphine)platinum, Pt(CO)2[P(C6H5)2(C2H5)]2, crystallizes in either of the enantiomorphous space groups P3121 (No. 152) and P3221(No. 154) with cell dimensions a = 10.64(1), c = 22.06(1) A, U = 2163 A3; pc = 1.564 g/cm3
TL;DR: In this article, the electron diffraction pattern of gaseous methyl allenyl ether at room temperature was obtained and the following bond lengths and bond angles for the molecular structure were obtained (standard deviations in parentheses): CC = 1.318 (0.004) A, C-O-C' = 115.0 (1.2)
TL;DR: In this article, an analysis of the Raman spectra of 1, 2-diphenylethane, 1,2-di-(p-chlorophenyl)ethane, meso-2,3-di-phenylbutane, and its pp′-dibromo-derivative reveals the general absence of corresponding frequencies from the i.r. spectra.
Abstract: An examination of the Raman spectra of 1,2-diphenylethane, 1,2-di-(p-chlorophenyl)ethane, meso-2,3-di-phenylbutane, and its pp′-dibromo-derivative reveals the general absence of corresponding frequencies from the i.r. spectra of the solids. This fact, taken together with the general appearance of certain extra bands in the i.r. and Raman spectra of various solutions, strongly suggests the existence of only the trans-rotamer in the solid phase but of a dynamic equilibrium mixture of both trans- and gauche-rotamers in solution. This conclusion is also considered to be essentially true in the case of 1,2-diphenylethane, notwithstanding the lack of extra bands in the i.r. spectra of its solutions. The relatively weak intensities of the bands attributed to the gauche-rotamers suggest that in these compounds the proportion of the gauche-rotamer is appreciably less than that of the trans in solution. The greater number of absorption bands observed in the i.r. spectra of the racemic (±) than in that of the corresponding solid meso-compound, is in accord with predictions based on simple selection rules. The dihedral angles of some of the gauche-rotamers are determined by molecular polarisability measurements.