摘要:

AbstractThe tautomeric equilibria in phenol–triethylamine complexes have been quantitatively investigated. A method of calculation of thermodynamic parameters based on reproducing the temperature dependences of13C chemical shifts is described. The results are in agreement with UV and IR predictions for similar complexes. The chemical shifts characterizing molecular forms of the complexes depend on the type of phenol derivativ

ISSN:0749-1581    

DOI:10.1002/mrc.1260260602

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractA pulse sequence has been implemented for two‐dimensional POMMIEJ(CH)‐resolved13C NMR spectroscopy and has been used to explore three methods for the automated acquisition of data for two‐dimensional spectrum editing. In the first two methods, sets of three two‐dimensional data matrices are acquired in either sequential or interleaved modes, by use of three values, π = ϕ/6, π/2 and 5π/6, for the phase shift of the multiple quantum read pulse. Computation of linear combinations of these three data matrices by use of a Pascal program that had been written earlier for two‐dimensional DEPT spectrum editing yielded two‐dimensional POMMIEJ(CH)‐resolvedCH,CH2andCH3subspectra. In the third method, these subspectra are constructed directly during acquisition, by rotation of the phase shifts of the multiple quantum read pulse and the receiver. Selected small peptides and carbohydrate derivatives were used as model compounds for this study, and their1H and13C NMR assignments were confirmed by two‐dimensional COSY and heteronuclear CH chemical shift correlation techniques. The results show that if appropriate equipment is available, the two‐dimensional POMMIE method is easier to implement forJ(CH)‐resolved spectral editing than the two‐dimensional DEPT technique. However, these methods are comparable in their ability to

ISSN:0749-1581    

DOI:10.1002/mrc.1260260603

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractX‐ray diffraction of a single crystal of 4β‐phenyl(or mesityl)‐3β‐methyl‐2α(or 2β)‐bromobutyrolactone confirmed the envelope conformation of the butyrolactone unit. In both cases the aryl group is equatorial and the methyl group is axial; the 2α‐bromine is axial and the 2β‐bromine is equatorial. The coupling constants observed by1H NMR for a series of α‐bromobutyrolactones show two types: with a mesityl group at C‐4 there is always a pure conformation with an equatorial mesityl group; with a phenyl group at C‐4 the two reverse envelope forms are possible, the favoured effects being equatorial Ar>axial Br>equatorial Me. In the generalized Karplus equation a strong influence of the relative orientation of the bromine and the carbonyl on the corresponding electronegativity parameters was observed. These parameters were evaluated by means of the electronegativity group concept and weighted by a stereochemistry‐depe

ISSN:0749-1581    

DOI:10.1002/mrc.1260260604

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractThe previously accepted13C spectral assignment of hellebrigenin was re‐examined and revised signal assignments are proposed for C‐1, C‐7 and C‐11. Results obtained in a (1H,1H) COSY experiment and NOE experiments enabled us to assign the1H NMR spectrum of hellebrigenin. The13C NMR spectrum was subsequently assigned unambiguously by the correlations observed in a 2D (13C,1H) chemical shift correlation experiment and a COLOC exp

ISSN:0749-1581    

DOI:10.1002/mrc.1260260605

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractBarriers to the pyramidal inversion of sulphur bonds were investigated for five pentacarbonylchromium complexes of cyclic and acyclic thioethers. The expected effects from the conjugation of p lone electron pairs of sulphur with neighbouring π‐electron systems on the free energy of activation for the inversion process were not observ

ISSN:0749-1581    

DOI:10.1002/mrc.1260260606

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractThe synthesis of diethyl 2‐(1,1‐dimethylhydrazono)propyl phosphonate (1) and its 1‐benzoyl derivative (2) is described. The1H and13C NMR spectra of 1 and 2 have been completely assigned with the aid of two‐dimensional1H13C correlation NMR spectroscopy. Spectral data for ethyl α‐benzoyl‐β‐(1,1‐dimethylhydrazino)crotonate (5) were also obtained.The results show that 1 exists predominantly in theE‐ andZ‐isomeric forms in a ratio ofca2:1, and 2 occurs almost exclusively in theE‐form. Data for 5 indicate that this compound exists in theZ‐ an

ISSN:0749-1581    

DOI:10.1002/mrc.1260260607

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractA general strategy, using well established techniques, is outlined for the assignment of signals in the13C NMR spectra of the trichothecene mycotoxins and some related rearrangement products. Heteronuclear NOE difference spectra were employed to assign resonances from quaternary carbons. An example of the use of spin–echoJmodulation is give

ISSN:0749-1581    

DOI:10.1002/mrc.1260260608

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractUsing1H and13C NMR,13C‐labelled modifications of 2‐amino‐4‐(5‐nitro‐2‐furyl)thiazole (ANFT) and itsN‐formyl analogue N‐[4‐(5‐nitro‐2‐furyl)‐2‐thiazolyl]formamide (FANFT), as well as the 5‐(S)‐substituted glutathione andN‐acetylcysteine conjugates of prostaglandin H synthase‐catalyzed peroxidative metabolism of ANFT were studied.1H and13C NMR chemical shifts, heteronuclear coupling constants, spin–lattice relaxation times (T1) and nuclear Overhauser enhancements (NOE) were obtained. Three‐bond coupling constants of the13C‐labelled nucleus with the C‐5 protons of ANFT and FANFT were in close agreement but were absent in the conjugates owing to the 5‐substitution. Two‐bond coupling of the13C‐labelled nucleus was observed with the amide proton of FANFT, but not with ANFT because of rapid exchange of the amine protons. Values ofT1were smaller in the conjugates than in FANFT or ANFT owing to their larger molecular weights.1H spectra and NOEs provide further support for the prevalence of the 2‐aminothiazole tautomer over the 2‐iminothiazoline tautomer. Evidence is presented for a proton transfer of the C‐5 proton of ANFT, which proceeds over a time of the order of weeks in dimethyl sulphoxide and is autocatalytic. FANFT showed a different proton‐transfer reaction involving the amide proton, which occurs on a time scale of the order of days in dimethyl sulphoxide. The FANFT molecule revealed no evidence for transfer of its C‐5 proton, in contradistinction to ANFT. From a calculation of the percentage of the13C relaxation due to the dipole–dipole mechanism, it can be concluded that for this class of sp2‐hybridized carbon (the C‐2 position of the 2‐aminothiazole system) a considera

ISSN:0749-1581    

DOI:10.1002/mrc.1260260609

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractSubstituent constants for the thiocyanato group (Z gemSCN= 0.94,Z cisSCN= 0.45 andZ transSCN= 0.41) have been derived by an additivity procedure, and shown to reproduce the chemical shifts of vinyl protons in a wide variety of α,β‐unsaturated thiocyanates with an accuracy of ±0.30 ppm in 94% of the cases studied. Deviations due totBu and COR groups are discussed in terms of steric and fie

ISSN:0749-1581    

DOI:10.1002/mrc.1260260610

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractThe repeatability and reproducibility of quantitative deuterium NMR at the natural abundance have been determined according to the ISO norms in a collaborative studyThis study was organized by the Community Bureau of Reference, EEC (Bruxelles). The list of the participants is given at the end.involving 16 spectrometers.A repeatability of better than 0.8% and a confidence interval of 0.25% have been determined from the results obtained in the 15 different European laboratories that participated in the collaborative study. The reproducibility of the measurements carried out on very different NMR systems is of the order of 2–3%, but this can be improved to 1% with more homogeneous spectrometer systems.The influence of the spectrometer performances on the precision of the quantitative data has been analysed in terms of statistical inference, variance analysis and multidimensional factorial representation. The results indicate the advantages of using highly automated procedures to decrease the random error in the measuremen

ISSN:0749-1581    

DOI:10.1002/mrc.1260260611

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY