摘要:

AbstractElectron paramagnetic resonance has been used to identify the radicals produced by photoirradiation of a mixture of polyunsaturated fatty acids in ethanol. One solvent and one solute radical were formed during irradiation under anaerobic conditions at 77 K. The radicals were formed in hydrogen transfer reactions belonging to Type I photosensitization processes, and were produced from the same photosensitizer. The yield of the solvent radical varied linearly with the mixing ratio of the fatty acids in ethanol whereas the solute radical, as expected, varied with the concentration of the solute to the second power. The solute radical decayed by secondary thermal reactions at 77 K into a third radical with no hyperfine interaction structure. When oxygen was present in the sample the superoxide radical or, possibly, a peroxy radical was also produced. At 77 K the effect of the antioxidant spermine was to increase the yield of radical formation by preventing the recombinations of solvated electrons with the ionized sensitizer. Spermine is hence acting as an electron scavenger in this system. These results indicated that photoirradiation will cause radical formation by hydrogen transfer reactions and hence contribute to initiation processes in the oxidation of polyunsaturated fatty acids, and that spermine is not operative as an antioxidant in the initiation phase of the total oxidation.

ISSN:0749-1581    

DOI:10.1002/mrc.1260261102

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractInternal and external reference methods are introduced for the study of kinetic deuterium isotope effects (KDIEs) by natural abundance2H NMR. These methods take account of both inter‐ and intra‐molecular competive reactions of fully protonated and naturally monodeuteriated molecules of the substrate in the reaction. Specifically deuteriated compounds are not necessary using these methods. These techniques have further advantages over other methods; for example, primary and secondary KDIEs of different types can be measured simultaneously. The methodology is summarized and examples of its application are presen

ISSN:0749-1581    

DOI:10.1002/mrc.1260261103

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractThe magnitude of1J(15N13C) inpara‐substituted benzenediazonium ions has been used to assess the importance of the several resonance forms contributing to the ground state of the ion. From a comparison of values observed in 4‐nitro‐ and 4‐dimethylamino‐benzenediazonium ions and their respective 3,5‐dimethyl derivatives, it is concluded that diazo‐type structures are important only with strong electron‐releasing substituents in the aromatic ring. INDO–SOS–MO calculations indicate that the large and negative Fermi contact term dominat

ISSN:0749-1581    

DOI:10.1002/mrc.1260261104

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

Abstract20‐Methylpregna‐4,20‐dien‐3‐one and the corresponding 17α‐hydroxy, 20‐hydroxymethyl and 17α‐hydroxy‐20‐hydroxymethyl derivatives were prepared from the 20‐keto compounds by Wittig reaction or a modified Simmons‐Smith reaction. The13C NMR spectra of these steroids and of 3‐methylene derivatives are analysed. Solvent effects produced by addition of methanol‐d4in the spectra of the 20‐methylene steroids are correlated with intramolecular hydrogen bonding in th

ISSN:0749-1581    

DOI:10.1002/mrc.1260261105

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractSubstituent constants for the isothiocyanato group (Z gemNCS0.92,Z cisNCS0.01 andZ transNCS−0.22) were derived by an additivity procedure, and shown to reproduce the chemical shifts of vinyl protons in a wide variety of α,β‐unsaturated isothiocyanates with an accuracy of ±0.30 ppm in 92% of the cases studied. Deviations are discussed in terms of the steric and field effects of the substitue

ISSN:0749-1581    

DOI:10.1002/mrc.1260261106

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractThe first example ofcis–transisomerism in monoalkylhydroxamic acids, detected by1H,13C and15N NMR spectroscopy, is reported. The15N NMR spectrum of CH3CO15NHOH gave a clear assignment of the OH and NH protons of bothZandEisomers. The assignment was confirmed by1H and13C NMR spectroscopy. A rotational barrier of isomerization (ΔGc≠) of 17.8 kcal mol−1was calculated from the variable‐temperature1H NMR spectra using the method of Shanan‐Atidi a

ISSN:0749-1581    

DOI:10.1002/mrc.1260261107

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractThe configurations of some isoxazolidinium salts have been determined by nuclear Overhauser effect difference spectroscopy (NOEDS). The stereochemical findings give a good explanation of the different reactivities experienced in the five‐membered ring‐opening process as a function of the steric requirement of the sys

ISSN:0749-1581    

DOI:10.1002/mrc.1260261108

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractThe long‐range CH correlation NMR experiment (with decoupling of modulation of intensities due to one‐bond couplings during refocusing) and the double quantum coherence correlation spectroscopy (DQCOSY) experiment were used, together with the standard one‐bond CH correlation and COSY spectra, for complete13C and1H spectral peak assignments of veratrine (a 2:1 mixture of cevadine and veratridine). The long‐range CH correlation experiment determined the long‐range CH connectivity network, thus indirectly providing carbon–carbon connectivity. The DQCOSY experiment proved to be a complementary technique to the standard COSY experiment in unraveling and/or confirming proton–proton connectivities in this highly crowded pr

ISSN:0749-1581    

DOI:10.1002/mrc.1260261109

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractSpin–spin coupling between carbonyl carbons and aromatic protons (or fluorine atoms) in several (η6‐arene)tricarbonylchromium complexes has been observed for the first time. The values of the coupling constants are of the order of 0.6 Hz for protons and 2 Hz for fluroine, and seem to depend on the electron‐donating properties of the aromatic subst

ISSN:0749-1581    

DOI:10.1002/mrc.1260261110

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractA methodology is described which allows the rationalization of the13C chemical shifts of a variety of compounds including alkanes, alcohols and ketones. The geometries of 250 rigid compounds were computed by the MM2 force field. Multiple linear regression analysis was used to model the13C shifts from topological, geometric and energetic parameters. Among these parameters, the most important were those corresponding to interatomic distances, hybridization state of the quaternary carbons and energetic parameters. The final result is a rationalization of the13C chemical shifts with an average RMS error of 1.25 ppm.

ISSN:0749-1581    

DOI:10.1002/mrc.1260261111

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY