摘要:

Abstract13C NMR spectra of 40 substituted acetals, carbonates, ortho ethers, 1,3‐dioxocycloalkanes, 1,3‐oxothiolanes and 1,3‐oxazolidines were studied. The chemical shift of the C‐α carbon atom was found to depend both on the number (K) and type of the hetero substituents at C‐α. The effect of oxygen, nitrogen and sulphur atoms on the C‐α chemical shift was shown to be a non‐additive property relative to the unsubstituted isostructural hydrocarbons. The C‐α chemical shift in the linear and cyclic polyalkoxymethanes can be described by an equation incorporating CO bond increments and contributions from the RO substituents:\documentclass{article}\pagestyle{empty}\begin{document}$$ \delta \left({^{13} {\rm C} - \alpha } \right) = \delta \left({^{13} {\rm CH}_4 } \right) + K\Delta \delta \left({{\rm C} - {\rm O}} \right) + \sum _k \Delta {\rm RO} $$\end{document}The CO increment depends quadratically on the number (K–1) of geminal neighbours of the oxygen atom under study. Good agreement between the experimental and calculated shifts can only be achieved by inclusion of nonbonded interactions betwee

ISSN:0749-1581    

DOI:10.1002/mrc.1260250102

出版商:John Wiley&Sons, Ltd.    

年代:1987

数据来源: WILEY

摘要:

AbstractA13C NMR method has been developed for obtaining simultaneously structural and quantitative information on mixtures of pyrrolizidine alkaloids fromSenecio vulgarisL. (Compositae). The samples contained seneciphylline, senecionine, retrorsine and the correspondingEgeometrical isomers, spartioidine, integerrimine and usaramine. The spin‐lattice relaxation times were measured and, after establishing the instrumental requirements for quantitative13C FT NMR, all the alkaloids of the mixture could be determined. Quantitative1H and13C FT NMR analyses were carried out, and the advantages and limitations of these methods are discusse

ISSN:0749-1581    

DOI:10.1002/mrc.1260250103

出版商:John Wiley&Sons, Ltd.    

年代:1987

数据来源: WILEY

摘要:

AbstractThe1H,2H and13C spectra oftrans‐9‐chloro, ‐bromo‐ and ‐iodo‐decalins and their deuteriated analogues have been recorded in order to study the effect of chloro, bromo and iodo substituents on β antiperiplanar protons. All shifts were completely assigned. Unusually large upfield shifts were detected for β protons antiperiplanar to the halogen substituents. The electronegativity dependence of the β shifts is opposite to that hit

ISSN:0749-1581    

DOI:10.1002/mrc.1260250104

出版商:John Wiley&Sons, Ltd.    

年代:1987

数据来源: WILEY

摘要:

AbstractQuaternary13C dipole–dipole spin–lattice relaxation times are used to analyse prototropic equilibria of 2‐acetylbenzimidazole (1) and 4‐azabenzimidazole in DMSO‐d6solutions. For the first compound, fast exchange between the two chelated structures prevails for13C relaxation times, whereas proton and13C chemical shifts characterize a slow exchange. For 4‐azabenzimidazole,13C and1H spectra do not exhibit absorptions from structures in slow exchange. Analysis of theT1values of carbons 8 and 9 before and after isotopic exchange of the labile amino hydrogen clearly show that the major component of the equilibrium is the

ISSN:0749-1581    

DOI:10.1002/mrc.1260250105

出版商:John Wiley&Sons, Ltd.    

年代:1987

数据来源: WILEY

摘要:

AbstractUsing two‐dimensional NMR spectroscopy, the spatial location of various carboxylate anions relative to the polyene chain of the protonated Schiff base of all‐trans‐retinal was determined. The observed intermolecular NOE cross‐peaks between a proton on the counter ion and a proton near the nitrogen atom indicate the existence of ion‐pair formation between the protonated retinal Schiff base and various counter ions in chloroform. The results suggest that the most likely site of the carboxylate group of the counter ion is in the immediate vicinity of the positively charged nitrogen atom of the retinal Sc

ISSN:0749-1581    

DOI:10.1002/mrc.1260250106

出版商:John Wiley&Sons, Ltd.    

年代:1987

数据来源: WILEY

摘要:

AbstractNMR spectral data are given for the A‐ring in ten 4‐en‐3‐one steroids, and the strategy of the spectral analysis is described. The data are interpreted in terms of an equilibrium between normal and inverted half‐chair conformations, with the normal conformation predominant in all cases except when 2β‐acetoxy substitution forces inversion. Substitution at the 17α position has no measurable effect upon the A‐ring conformation while 6α and 19‐nor substitution have only a small effect. Unsaturation at the C‐5 position results in a flattened A‐ring. The results obtained from NMR are compared with those obtained by x‐ray crystallography. The A‐ring conformation shows far less variation in solution

ISSN:0749-1581    

DOI:10.1002/mrc.1260250107

出版商:John Wiley&Sons, Ltd.    

年代:1987

数据来源: WILEY

摘要:

AbstractHigh‐resolution1H NMR (500 MHz) spectra were recorded for morphine, 3‐O‐acetylmorphine, 6‐O‐acetylmorphine and diacetylmorphine (heroin). Complete assignment of all proton resonances was made for these substances following determination of accurate chemical shift and coupling data by spectral simulation of all tightly coupled proton resonance patterns. Chemical shifts and proton couplings are discussed in terms of the conformation of rings C and D. From a varied collection of morphine analogs, torsion angles for ring D protons were obtained from crystallographic studies. The corresponding coupling constants were calculated, they demonstrate similar conformation of the ring D for the compared morphine

ISSN:0749-1581    

DOI:10.1002/mrc.1260250108

出版商:John Wiley&Sons, Ltd.    

年代:1987

数据来源: WILEY

摘要:

AbstractA33S NMR spectrum has been obtained from an asphaltene sample for the first time. Asphaltene from the Athabasca tar sands bitumen, Alberta, Canada, was oxidized with RuVIII, which selectively converts sulphide sulphur into the corresponding sulphone. The33S NMR spectrum of this sample at 30.7 MHz featured a broad signal centred at 320 ppm downfield from CS2. The width of this signal was too broad, owing to the quadrupole relaxation of the nuclei in the solution and/or the many types of sulphone molecules, to permit the assignment to any particular sulphone types. This situation was not improved by the use of proton broad band decoupling at elevated temperatures. Nevertheless, the mere presence of the aliphatic sulphones provides the first unambiguous evidence for the presence of aliphatic sulphides in a petroleum asphaltene. Similar results were obtained from sulphones prepared by peroxide oxidation of high molecular weight sulphides isolated from bitumen from the Wabasca, Alberta deposit.

ISSN:0749-1581    

DOI:10.1002/mrc.1260250109

出版商:John Wiley&Sons, Ltd.    

年代:1987

数据来源: WILEY

摘要:

AbstractThe13C chemical shifts in the spectra of friedelan‐7‐one (1), methyl 3‐oxofriedelan‐25‐oate (2), methyl friedelan‐25‐oate (3), 7‐oxofriedelan‐3‐α‐yl acetate (4), friedelan‐3‐one (5), 2‐hydroxy‐3, 4‐seco‐friedelan‐3‐oic acid (6), methyl 2‐oxo‐3,4‐seco‐friedelan‐3‐oate (7), methyl 2‐hydroxy‐3,4‐seco‐friedelan‐3‐oate (8) and 3,4‐seco‐friedelan‐2,3‐diol (9) have been fully assigned using two‐dimensional heteronuclear correlation NMR spectroscopy, BB proton decoupled, DEPT and selective heteronuclear spin decoupling methods. These assignments and the observation of substantial NOEs between 26‐methyl and 28‐methyl and between 28‐methyl and 30‐methyl support the boat to boat conformations for the D and E rings in 1. Corrections a

ISSN:0749-1581    

DOI:10.1002/mrc.1260250110

出版商:John Wiley&Sons, Ltd.    

年代:1987

数据来源: WILEY

摘要:

AbstractAdditive shielding parameters for the phenylseleno group (ZPhSe‐gem= 1.36;ZPhSe‐cis= 0.31;ZPhSe‐trans= 0.29) have been estimated. Examples of the calculation of chemical shifts for olefinic protons in vinylic selenides by means of these increments are given. A comparison with 2‐pyridylseleno and methylseleno substituents has been made. The standard deviations of theZvalues are 0.06 forZPhSe‐gem, 0.10 forZPhSe‐cisand 0.05 for

ISSN:0749-1581    

DOI:10.1002/mrc.1260250111

出版商:John Wiley&Sons, Ltd.    

年代:1987

数据来源: WILEY