Showing papers on "Conformational isomerism published in 1969"
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 NHCH 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
118 citations
TL;DR: A vibrational analysis of the spectra of anisole and of the ortho, meta and para substituted fluoro, chloro and bromo derivatives has been carried out in this article.
111 citations
TL;DR: The peracetylated diethyl dithioacetals of d -ribose, d -arabinose and d -xylose were studied in chloroform-d solution by nmr spectroscopy at 100 MHz.
75 citations
TL;DR: In this article, the 1C (d)-Ribopyranose tetraacetate was shown to exist, in acetone-d 6 at room temperature, as a 9:11 mixture of 1C(d) and C1(d ) conformers in rapid equilibrium.
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TL;DR: The coupling constants of the cis and trans isomers of 2-ureidocyclohexane carboxylic (UCHA) and of the erytho and threo isomer of 2,3-diphenyl-3-uridopropionic acid (DPhUPA) with gauche hydrogens are preferred as discussed by the authors.
41 citations
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TL;DR: In this paper, an examination of the conformations adopted by acyclic sugar derivatives has been made and it is shown that 1,3-eclipsed rather than 1,2-gauche interactions are important in determining relative conformer population and that the observed conformer distribution, as determined by analysis of PMR spectra, is close to that calculated from consideration of non-bonded interactions.
TL;DR: In this paper, the authors used the newly reported moment of the cis-isomer to extend their earlier conclusions and specify the distribution of possible isomers using the experimental p and m(rnK8) in carbon tetrachloride and benzene.
Abstract: Cahill, Gold, and Owen1 have recently examined the microwave spectrum of methyl vinyl ether and identified the more stable of the two rotational isomers as the planar cis conformation with a dipole moment in the ground vibrational state of 0.96&0.02 D, determined through the Stark effect. This moment is relevant to the interpretation of the Kerr constant of methyl vinyl ether in terms of the preferred molecular conformations. In our previous w o r k , h o l a r Kerr constants were measured for methyl vinyl ether as a vapour and as a solute in carbon tetrachloride and benzene; the results were analysed assuming that both the cis-isomer and the other \"unidentified\" rotamer had a dipole moment of 1.10 D (the observed value in CC14). The inadequacy of this assumption was pointed out. If, however, we use the newly reported moment of the cis-isomer, we are able greatly to extend our earlier conclusions and to specify the distribution of possible isomers using the experimental p and m(rnK8) in carbon tetrachloride solution.
TL;DR: The application of Hoffmann's Extended Huckel MO method to diazoacetone and α-diazo-acetophenone has been found to provide a quantum mechanical basis for previous postulations that α-Diazo ketones can exist in two conformations as discussed by the authors.
TL;DR: In this paper, the infrared and Raman spectra of chlorocyclopentane, bromocyclophane, and α-bromophane-d 1 were recorded and the fundamentals assigned.
TL;DR: In this paper, the N.M.R. vicinal coupling constants in trichloroethane, 1,1,2,2-tetrabromo and tetrachlorethane have been measured and the energy differences between rotamers have been estimated from the temperature dependence of the coupling constants.
Abstract: The variation with temperature and solvent of the N.M.R. vicinal coupling constants in 1,1,2-tribromo and trichloroethane, 1,1,2,2-tetrabromo and tetrachlorethane, and in 1,1-dichloro-2-methyl propane have been measured. The energy differences between rotamers have been estimated from the temperature dependence of the coupling constants. For 1,1-dichloro-2-methylpropane ΔE = 480 ± 150 cal/mole and is virtually independent of the medium. The results for the remaining four compounds are strongly dependent on the nature of the solvent and can only be interpreted satisfactorily by means of an electrostatic theory of the medium effect. Values of ΔE vap are estimated for these compounds. The results are compared with those obtained from infra-red and Raman spectroscopy. Agreement is satisfactory only when effects of the medium are taken into account.
TL;DR: In this paper, infrared spectra have been obtained for mchlorobenzaldehyde, mbromo-chloro and m−bromobenzalbenaldehyde, as well as other rotational isomers in the neat liquid, and the stability of these compounds is attributed to the inductive effect of the halogen.
Abstract: Infrared spectra have been obtained for m‐chlorobenzaldehyde, m‐bromobenzaldehyde, and m‐iodobenzaldehyde. Each of these compounds exists as a mixture of two rotational isomers in the neat liquid. The cis‐O form of m‐chloro‐ and m‐bromobenzaldehyde is the low‐energy form, and is the only form present in the solid state. The trans‐O form of m‐iodobenzaldehyde is the only form present in the solid state of that compound, but the difference in energy between rotamers in the neat liquid is small. The stability of the cis‐O form of m‐bromo‐ and m‐chlorobenzaldehyde relative to the trans‐O form is attributed to the inductive effect of the halogen.
TL;DR: In this article, the infrared spectra of gaseous, liquid, and polycrystalline symmetrical dimethylhydrazine and its deuterium analogs have been recorded from 70-4000 cm−1.
Abstract: The infrared spectra of gaseous, liquid, and polycrystalline symmetrical dimethylhydrazine and its deuterium analogs have been recorded from 70–4000 cm−1. The Raman spectra of the liquid and solid samples have also been recorded and depolarization values have been measured. A comparison of the infrared and Raman spectra of the respective compounds indicates that the liquid phase consists of approximately a 69:31% mixture of gauche conformers. An assignment of the observed frequencies based on band positions, relative intensities, and isotopic shifts is presented for both molecular conformations. Although the torsional fundamentals were observed for the molecules in both the gaseous and crystalline states, the bands observed for the solids were more clearly defined and were thus employed for the barrier height calculations. The threefold methyl rotational barrier is calculated to be approximately 3.3 kcal/mole, whereas the magnitude of the barrier hindering internal rotation about the N–N bond is estimated to be 5.0 kcal/mole.
TL;DR: The crystal and molecular structure of µ-sulphido-carbonyltris(triphenylphosphine)diplatinum(II) has been determined from three-dimensional X-ray diffractometer data as mentioned in this paper.
Abstract: The crystal and molecular structure of µ-sulphido-carbonyltris(triphenylphosphine)diplatinum(II) has been determined from three-dimensional X-ray diffractometer data. The crystals are triclinic with Z= 2 in a unit cell of dimensions a= 10·687, b= 25·240, c= 9·307 A, α= 95·17, β= 104·22, and γ= 97·67°. The structure was refined by least-squares methods using 2897 independent reflections to R 0·069. The molecules conform essentially to the P space group, although there is disorder in the carbonyl and one of the triphenylphosphine groups to give two conformational isomers. The complex contains a single covalent Pt–Pt bond which forms with sulphur a three-membered ring. The Pt–Pt bond distance is 2·647 (σ 0·002)A, and the Pt–S distances are 2·218 (σ 0·009) and 2·227 (σ 0·009)A. The Pt–S–Pt angle is 73·1° and the mean Pt–Pt–S angle is 53·5°. The platinum atoms and their immediate neighbours are essentially planar, although the co-ordination about the metal atoms is far from square-planar.
TL;DR: In this article, the thermal dependence of the NMR spectra of 14 p-substituted nitrosobenzenes is described and the barriers to restricted rotation have been calculated, and an attempt to correlate these with Hammett σ functions has been made.
TL;DR: The temperature dependence of the n.m.r. spectra of tetra-alkylhydrazines has been interpreted in terms of a substantial barrier to rotation about the N-N bond.
Abstract: The temperature dependence of the n.m.r. spectra of tetra-alkylhydrazines may be interpreted in terms of a substantial barrier to rotation about the N–N bond.
TL;DR: In this paper, the conformational analysis of a stereoisomeric pair of 4-hydroxymenthones, (1R:4R)-(−)-4-hydroxmenthone (1) and (1 R:4S)-(+)-4hydroxyisomenthone (2), was made by means of temperature-dependent circular dichroism.
Abstract: The conformational analysis of a stereoisomeric pair of 4-hydroxymenthones, (1R:4R)-(−)-4-hydroxymenthone (1) and (1R:4S)-(+)-4-hydroxyisomenthone (2), was made by means of temperature-dependent circular dichroism It was demonstrated that α-hydroxyketone (1) exists in the conformational equilibrium: 1a\leftrightarrows1b; the conformer 1a exists predominantly in a nonpolar medium, whereas the conformer 1b is more stable than 1a in a polar medium On the other hand, the other isomer, (2), exists preferentially in the state of the conformer 2b in either a polar or a nonpolar solvent The results were interpreted in terms of the competition of the steric factor and intramolecular hydrogen-bonding
TL;DR: Hindered rotation in the urethane link of N-benzyloxy carbonyl- and N-t-butoxycarbonyl -L-proline esters was revealed by n.m. spectroscopy as mentioned in this paper.
Abstract: Hindered rotation in the urethane link of N-benzyloxycarbonyl- and N-t-butoxycarbonyl-L-proline esters is revealed by n.m.r. spectroscopy.
TL;DR: In this article, the conformational equilibria in trans-2,3-dihalogenotetralins are studied with the aid of dipole moments and NMR spectroscopy.
Abstract: The conformational equilibria in trans-2,3-dihalogenotetralins are studied with the aid of dipole moments and NMR spectroscopy. It is concluded that the diaxial conformer strongly predominates in solution. From the combination of dipole moment and NMR data the thermodynamic quantities ΔH0 and ΔS0 are calculated for the dichloro compound. A comparison is made with the results obtained earlier for other vicinal dihalogeno compounds.
TL;DR: The meso and dl configurations assigned to the diastereomers of 2,5-dimethyl-3,4-diphenyl-3-4-hexanediol have been based on the IR and PMR spectra as mentioned in this paper.
TL;DR: In this paper, the magnetic resonance spectra of ON-dialkylnitramines (RH:NO2R′) in geometrically isomeric forms, which do not appear to be readily interconvertible.
Abstract: 1 H Nuclear magnetic resonance spectra provide evidence for the existence of ON-dialkylnitramines (RH:NO2R′) in geometrically isomeric forms, which do not appear to be readily interconvertible. The spectra of some isomeric dialkyl nitrosohydroxylamines [RN(NO)OR′ and RN(O):NOR′] suggest for the structure RN(NO)OR′ an easier interconversion of conformers than is observed in N-nitrosodialkylamines. No signs of rotational isomerism were observed at 35° in the alkoxyurethanes which were prepared as intermediates.
TL;DR: In this paper, a method of interpretation of the MCD spectra with respect to the CD spectra is proposed, which suggests there is considerable trigonal splitting (∼1500cm−1) in solution, and refutes the possibility that the two bands in the Tlg(Oh) region originate from different conformational isomers in solution.
TL;DR: In this paper, it was shown that cis-1,3-diisocyanatocyclohexane exists in the diequatorial conformation; this is taken as evidence that a ground-state interaction between isocyanato groups in this monomer, which readily cyclopolymerizes, is not a significant factor in the cyclopolymersization mechanism.
Abstract: In 1,3-disubstitued cyclohexanes, in general, the diaxial conformation of the cis isomer is, energetically, the least favored conformation. An interspacial electronic interaction in the ground state of a cis-1,3-disubstituted cyclohexane would be expected to increase the proportion of this conformer in the equilibrium mixture. Such an interaction would provide an energetically favorable pathway for cyclopolymerization. From nuclear magnetic resonance studies on cis-and trans-1,3-diisocyanatocyclohexane the conformational equilibrium in the cis isomer was determined. It is shown that cis-1,3-diisocyanatocyclohexane exists in the diequatorial conformation; this is taken as evidence that a ground-state interaction between isocyanato groups in this monomer, which readily cyclopolymerizes, is not a significant factor in the cyclopolymerization mechanism. The value of the free energy barrier, ΔG‡, for trans-1,3-diisocyanatocyclohexane was calculated as 11.1 kcal/mole.
TL;DR: In this article, a comprehensive solvent, concentration, and temperature study of the 850 to 400 cm−1 region of the infrared spectrum of cyclohexyl iodide was made.
Abstract: A comprehensive solvent, concentration, and temperature study has been made of the 850 to 400 cm−1 region of the infrared spectrum of cyclohexyl iodide. This causes us to reject the assignment of Larnaudie and of Chiurdoglu and Reisse of v(C-I)eq at 654 cm−1 and v(C-I)ax at 638 cm−1, and to suggest that the two modes are almost coincident in n-hexane solution at 656.2 cm−1 (eq) and 655.6 cm−1 (ax).
TL;DR: In this article, the energy difference Δϵ between CH 2 conformers, resulting from rotation about single bonds of the chain backbone in trans -1,4-polybutadiene, cis-1, 4-poly butadiene and 1,5- hexadiene, was evaluated from i.r. measurements on the bands characteristic of bending vibration of CH 2 groups.