摘要:

AbstractThe proton‐coupled13C NMR spectra of 1‐trans‐methoxy‐ (2) and 1‐cis‐methoxy‐1,3‐trans‐butadiene (3) are consistent with a methoxy heavy atom planar conformation withs‐synands‐antiorientations, respectively.Ab initio6–31G* calculations confirmed such conformations. They were taken as model compounds to determine the influence of electrostatic interactions on the methoxy13C NMR parameters. A shielding increase of 4 ppm is observed in 2, with respect to 3, for the OMe13C chemical shift in ans‐synconformation and is ascribed to the attraction between the methyl and vinyl moieties as proposed by Li and Chesnut. The methyl1J(C,H) coupling is not affected by this interaction, showing that the carbon 2p electrons are more polarizable tha

ISSN:0749-1581    

DOI:10.1002/mrc.1260320402

出版商:John Wiley&Sons, Ltd.    

年代:1994

数据来源: WILEY

摘要:

AbstractThe carbon‐13 chemical shifts were assigned for all ring positions in cyclohexanone and 4‐tert‐butylcyclohexanone with H, F, Cl, Br, I, MeO, MeS, MeSe, Me2N, Me ortert‐butyl in the 2‐position. The substituent‐induced shifts were calculated by difference from unsubstituted cyclohexanone or 4‐tert‐butylcyclohexanone. Both stereoisomers (cisandtrans) were available for the 4‐tert‐butylcyclohexanones in all but one case. Comparison of the substituent‐induced shifts for thecis(equatorial 2‐substituent) andtrans(axial 2‐substituent) isomers provides stereochemical insight into the interactions between the 2‐substituent and the carbonyl group that bring about non‐additivity of the substituent effects. In the 2‐equatorial isomer, the dipole–dipole interaction between the functional groups causes non‐additivities for the C‐2 carbon that depend largely on the electronegativity of the 2‐substituent. In the 2‐axial isomer, hyperconjugation or other orbital interactions between the groups cause non‐additivities for the C‐2 carbon that depend largely on the polarizability in addition

ISSN:0749-1581    

DOI:10.1002/mrc.1260320403

出版商:John Wiley&Sons, Ltd.    

年代:1994

数据来源: WILEY

摘要:

AbstractA high preference for the boat conformation of tricyclo[4.3.1.12,5]undec‐3‐en‐10‐one was established on the basis of1H NMR spectroscopy with the aid of molecular mechanics calculations (MM3 p

ISSN:0749-1581    

DOI:10.1002/mrc.1260320404

出版商:John Wiley&Sons, Ltd.    

年代:1994

数据来源: WILEY

摘要:

AbstractA glycoside extracted from the rhizomes ofAgave americanawas identified as 3‐O‐[{{β‐D‐xylosyl(1 → 3)}{β‐D‐xylosyl(1 → 3) ‐ β ‐D‐ glucopyranosyl(1 → 2)} ‐ β ‐D‐ glucopyranosyl(1 → 4)} ‐ β ‐D‐ galactopyranosyl] ‐ (25R) ‐ 5α ‐ spirostan‐3β‐ol by concerted use of proton and carbon‐13 2D NMR methods. Complete assignments for the sugar resonances were obtained with standard methods, but the presence of chemical shift degeneracies prevented an unequivocal determination of two of the glycosidic linkage points by the HMBC experiment. Further two‐dimensional experiments failed to resolve the ambiguity, but a clear identification of the linkages was obtained using a selective

ISSN:0749-1581    

DOI:10.1002/mrc.1260320405

出版商:John Wiley&Sons, Ltd.    

年代:1994

数据来源: WILEY

摘要:

AbstractAntipain [(1‐carboxy‐2‐phenylethyl)carbamoyl‐L‐arginyl‐L‐valyl‐DL‐arginal] is a potent inhibitor of papain and related proteases. The assignment of1H NMR signals of the interconverting forms in equilibrium in aqueous solution at pH 5.8 were investigated using two‐dimensional DQF‐COSY, DOUBLE‐RELAYED‐COSY, HOHAHA and ROESY nuclear magnetic resonance techniques. The assignments of protons of several forms in equilibrium of theD‐ andL‐arginal antipain isomers were determined in aqueous solution at 300 K, and their corresponding stereochemistry is tentatively proposed. Addition of equimolar amounts of papain resulted in spectral changes and chemical shifts which are compatible with enzyme

ISSN:0749-1581    

DOI:10.1002/mrc.1260320406

出版商:John Wiley&Sons, Ltd.    

年代:1994

数据来源: WILEY

摘要:

AbstractChemical shifts and coupling constants of hydroxyl protons for a series of simple carbohydrates are reported and several trends extracted: (1) chemical shifts are relatively independent of the experimental conditions if referenced to water; (2) chemical shifts are sensitive to the orientation of the HO group and its neighbors; and (3) coupling constants fall within a relatively narrow range, which is indicative of rotational averaging. These observations suggest that NMR parameters of hydroxyl protons can provide important diagnostic information about the stereochemistry of simple carbohydrates and may prove useful for analysis of more complex compounds.

ISSN:0749-1581    

DOI:10.1002/mrc.1260320407

出版商:John Wiley&Sons, Ltd.    

年代:1994

数据来源: WILEY

摘要:

AbstractThe measurement of the1H NMR spectrum of a sealed sample of benzene dissolved in a nematic liquid crystal provides a convenient and sensitive test of both NMR probe temperature stability and of the temperature gradient across the sample.

ISSN:0749-1581    

DOI:10.1002/mrc.1260320408

出版商:John Wiley&Sons, Ltd.    

年代:1994

数据来源: WILEY

摘要:

Abstract183W NMR spectra for a series of octahedral W(VI) imidophenyl, ‘WNPh’ and W(VI) imidoethyl, ‘WNEt,’ complexes were obtained using inverse detection based on non‐specific long‐range interactions. The metal couples to both theorthoandparaprotons of the NPh moiety (four and six bonds, respectively). The183W shift is shown to be sensitive to the nature of the ligand and the coordination number. The effect of geometric isomerism on δ183W is shown to be several hundred ppm. Some δ14N and δ15N and1J(183W,15N) data are reported. A number of new geometric isomers, involving the position of the NR ligand with respect to the remaining ligands, ha

ISSN:0749-1581    

DOI:10.1002/mrc.1260320409

出版商:John Wiley&Sons, Ltd.    

年代:1994

数据来源: WILEY

摘要:

AbstractThe effects of the various xenon and krypton isotopes on the nuclear shielding of19F in xenon and krypton difluoride in solution were studied by19F NMR spectroscopy. For both molecules, linear relationships were observed between the one‐bond secondary isotope shift of the19F nucleus,1Δ;19F(m′/128Xe) and1Δ19F(m′/78Kr), and the relative mass factor, (m′ −m)/m′, and also between the shifts and the quadratic mean bond displacement, 〈(Δr)2〉. On the basis of the latter correlation and the linear relationship between 〈Δr〉 and 〈(Δr)2〉 for the linear combination of the first and second derivatives of the19F shielding, the values −1742 and −5804 ppm Å−2, respectively, were determined. Further, the temperature dependence of the129Xe and19F chemical shifts and the129Xe,19F indirect spin‐spin coupling of XeF2were determined. The129Xe chemical shift is extremely sensitive to temperature with Δδ/ΔT= −0.4718 ppm K−1, whereas for19F Δδ/ΔT= +0.0042 ppm K−1. In contrast, the129Xe,19F indirect spin–spin coupling is independent of temperature over the ra

ISSN:0749-1581    

DOI:10.1002/mrc.1260320410

出版商:John Wiley&Sons, Ltd.    

年代:1994

数据来源: WILEY

摘要:

AbstractThe1H and19F NMR spectra of the α‐ and β‐pyranose anomers of 4‐deoxy‐4‐fluoro‐D‐glucose (4FG) and 6‐deoxy‐6‐fluoro‐D‐galactose (6FGA) in methanol‐d4, DMSO‐d6, acetone‐d6and D2O solution are reported. Computer analysis of the ABMX spectra of the CHCH2F fragments gives accurate vicinal HH and HF coupling constants. An iterative computational analysis of the observed vicinal couplings in this fragment for 6FG, 6FGA and other molecules allows the determination of both the individual rotamer couplings and the rotamer populations. Consideration of the derived rotamer couplings strongly suggests that the correct assignment for the prochiral C‐6 methylene protons in 6FG is that with the 6Sproton having the larger coupling to H‐5. This is the reverse of the assignment of these protons inD‐glucose. In contrast, the assignment of these protons in 6FGA follows that given previously forD‐galactose. The relative energies for the conformations about the C‐5C‐6 bond for 4FG, 6FG and 6FGA are given from the derived rotamer populations. For 6FGA the rotamer in which the fluorine is antiperiplanar to C‐4 is particularly favoured. For 4FG the rotamer with OH anti‐periplanar to the ring O is highly unfavoured, but the other two rotamers are of almost equal energy. Consideration of the effect of replacing hydroxyl by fluorine in these molecules indicates that any hydrogen bonding involving the C‐4 or C‐6 hydroxyls plays little part in determining the confor

ISSN:0749-1581    

DOI:10.1002/mrc.1260320411

出版商:John Wiley&Sons, Ltd.    

年代:1994

数据来源: WILEY