摘要:

AbstractVicinal coupling constants of stereochemical significance were obtained in four solvents for zearalenone and specifically deuteriated zearalenone. Comparison with values calculated for several conformations using molecular mechanics calculations indicates that zearalenone exists in a single conformation in solution. The solvent dependence observed for the H‐9′ and H‐10′ signals cannot be attributed to a conformational equi

ISSN:0749-1581    

DOI:10.1002/mrc.1260280802

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY

摘要:

AbstractThe Karplus equation is expanded to contain 11 mutually independent structural terms and 22 adjustable parameters. These terms are selected on an empirical basis and combined linearly (Eqn 4). The new equation reproduces 198 NMR vicinal1H‐1H coupling constants determined in less polar solvents with a standard deviation of 0.33 Hz. The equation can be applied to norbornene derivatives, for which some of the previous modifications did not work well. Among new factors introduced in this modification to the Karplus‐type equation are Mullay's group electro‐negativity used in the evaluation of the α‐ and β‐substituent effects, the CC length and the CCH valence angles of the coupled HCCH system, and the through‐space interactions between a coupled proton and a non‐bonded atom within the molecule. The four most important terms are the two cosine functions, originally used by Karplus, one form of the α‐substituent effect and the CCH valence angles. These four terms

ISSN:0749-1581    

DOI:10.1002/mrc.1260280803

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY

摘要:

AbstractAnalysis of the1H–1H COSY,13C–1H HETCOR and NOESY spectra of diprenorphine, a prototypic 6α, 14α‐endo‐ethanotetrahydrothebaine, yielded1H NMR and13C NMR assignments which should aid in spectral studies of structural congeners from this class of opioid ligands.13C NMR assignments were in accord with those previously reported, with the exception of the C‐17 and C‐18 carbon resonances, which were shown to be reversed. Specific assignments for the non‐equivalent C‐20 and C‐21 methyl groups were possible through NOESY due to intramolecular hydrogen bonding between the 19‐hydroxy and the 6‐methoxy groups. The piperidyl ring presented a chair conformation, similar to that of morphine, with an equatorialN

ISSN:0749-1581    

DOI:10.1002/mrc.1260280804

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY

摘要:

AbstractThe complete assignment of the proton and carbon chemical shifts of 6‐O‐acetyl‐2,3,4‐tri‐O‐(3S‐methylpentanoyl)‐α‐D‐glucopyranosyl‐β‐D‐fructofuranoside, a naturally occurring flavor precursor of tobacco, is presented. These assignments were made by comparison of the spectra obtained from1H‐detected heteronuclear multiple quantum coherence and1H‐detected heteronuclear multiple‐bond connectivit

ISSN:0749-1581    

DOI:10.1002/mrc.1260280805

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY

摘要:

AbstractThe helical molecule 1‐fluorobenzo[c] phenanthrene shows a large (17.4 Hz)13C–19F coupling resulting primarily from direct through‐space overlap between a fluorine lone pair and the sterically opposed CH bond. After application of corrections for estimated through‐bond contributions, comparison of the magnitude of this coupling with the analogous couplings in 4‐fluorophenanthrene, 10‐fluorobenzo[a] pyrene, 11‐fluoro‐5‐methylchrysene and 7‐fluorobenzo[b] fluoranthene reveals an exponential decrease with increasing H …︁ F internuclear distance, consistent with theoretical predictions. No evidence was found for an angular dependence of the coupling. HETCOR experiments showed the signs ofn+1J(HF) in 1‐fluorobenzo[c] phenanthrene, 4‐fluorophenanthrene, 10‐fluorobenzo[a] pyrene and 11‐fluoro‐5‐methylchrysene to be opposite to those of the correspondingnJ(CF), hence negative. Substituents at both positions 4 (parato fluorine) and 9 (parato the sterically opposed CH bond) exert only a minor influence on5J(CF), the couplings in the latter series responding largely to sterically induced distortions of the molecular framework, and in the former series to both steric distortions and an electronic effect. The difference in5J(CF) between isomers is a measure of the electronic effect of the substituent. Electron‐withdrawing substituentsparato fluorine reduce the magnitude of the through‐space coupling whereas electron

ISSN:0749-1581    

DOI:10.1002/mrc.1260280806

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY

摘要:

AbstractThe reaction B(NCS)4−+ BX4−(X = F, Cl, Br) leads to mixed BXn(NCS) 4−n−(n= 1, 2, 3) species easily detected in solution by11B NMR. Further, with X = Cl,1J(11B,14N) values could be measured from the11B NMR spectra (25.6, 24.5 and 21.4 Hz forn= 3, 2 and 1, respectively). For X = F, the relevant1J(11B,14N) values (24.3, 25.2 and 28 Hz forn= 3, 2 and 1, respectively) increase in the same order as the related1J(11B,19F) (5.5, 12.7 and 20.0 Hz, respectively). No such1J(11B,14N) could be resolved f

ISSN:0749-1581    

DOI:10.1002/mrc.1260280807

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY

摘要:

AbstractThe potential of Gaussian soft pulses in the selective excitation of individual13C multiplets is demonstrated for heteronuclear13C–1H NMR spectroscopy. The efficiency of these techniques is demonstrated for the rapid and accurate recording of both proton chemical shifts and heteronuclear coupling constants in multi‐labelled oligosacchari

ISSN:0749-1581    

DOI:10.1002/mrc.1260280808

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY

摘要:

AbstractThe preferred conformations of the methylsulphinyl group in a number of phenyl methyl sulphoxides substituted in the phenyl ring with one, two or three fluorine atoms have been studied by employing a multinuclear NMR approach. From the1H,13C and17O chemical shifts of the methylsulphinyl group, and the torsional angles obtained either from MO calculations or experimental determinations for a number of the compounds, empirical correlations have been derived which enabled us to extract the preferred orientation of the SO bond or of the methyl group relative to the phenyl ring in all the molecules examined. For a limited number of the compounds, the structure of the preferred conformer is satisfactorily close to the solid‐state structure found in an independent study carried out with x‐ray diffraction. The twist of the SO bond relative to the phenyl ring changes as a function of the number oforthosubstituents, being highest when two substituents are present. The lowest twist, torsional angle nearly zero, was found inortho‐monosubstituted compounds and the SO bond is oriented towards the opposite side with respect to the substituent. Analyses were also carried out of the conformational dependence of long‐rangenJ(HF) andnJ(CF) coupling constants, where the first nucleus in the parentheses refers to the methylsulphinyl group. The behaviour of these coupling constants as a function of internal rotation around the exocyclic CS bond was derived from semi‐empirical INDO MO FPT calculations, and scaled interpolating equations were derived in order to reproduce the experimental results. It thus emerges that5J(HF),4J(CF),5J(CF) and6J(CF) are conformationally dependent and show, experimentally, significant changes. These parameters can be usefully employed for the conformational analysis of methyl p

ISSN:0749-1581    

DOI:10.1002/mrc.1260280809

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY

摘要:

AbstractThe13C NMR chemical shifts of the α‐carbon atoms in cations [NR4]+, [NR3CH2Ph]+and [PR4]+[R = Me, Et, Prn, Bunor Pen(n‐pentyl)] are found at anomalously high field in comparison with the corresponding shifts in RCl or RNH2. This is ascribed in part to the effect of the positive charge. The effect is detectable but less pronounced with β‐carbon atoms, but is not detectable in subsequent carbon atoms in longer alkyl

ISSN:0749-1581    

DOI:10.1002/mrc.1260280810

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY

摘要:

Abstract31P nuclear magnetic resonance (NMR) studies of the four phases (I, II, II′ and III) in the barium ethyl octyl phosphate‐water system were carried out. The concentration dependence of the31P chemical shift, half‐height width and line shape provided information on the phase structures. For phase I the critical micelle concentration (CMC) is found at 1.2 wt.%, and above this concentration there is a monomer ⇌ micelle equilibrium. Phase II has a monomer ⇌ micelle equilibrium system, similar to phase I. Phase II′ is an aggregate system and is structurally organized. The structure of phase III is more organized than that of phase II′ and depends strongly on concentration. The3J(POCC) values were also used to establish the restricted state of the POCC single bon

ISSN:0749-1581    

DOI:10.1002/mrc.1260280811

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY