摘要:

AbstractA spin–echo sequence is proposed for the suppression of the generally unnecessary strong direct cross‐peaks in two‐dimensional heteronuclear NOE and relayed incoherent transfer spectroscopy. The procedure also increases the intensity of “remote peaks,” thereby enhancing the sensitivity of these t

ISSN:0749-1581    

DOI:10.1002/mrc.1260260302

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractThe stereochemical and structural analysis of matrine was carried out using nuclear magnetic resonance spectroscopy. Several methods, including13C1H two‐dimensional shift correlation spectroscopy,1H1H two‐dimensional correlated spectroscopy and measurements of vicinal1H1H coupling constants were used for chemical shift assignments and configurational analysis. Nuclear Overhauser effect difference spectroscopy was used to establish the sterochemistry. This investigation demonstrates the power of two‐dimensional NMR techniques and the nuclear Overhauser effect (NOE) in the structural determination of natur

ISSN:0749-1581    

DOI:10.1002/mrc.1260260303

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractThe13C NMR spectra of some 1‐ and 2‐substituted perimidines and perimidinium salts are discussed and assigned. Although the spectra of most of the 2‐substituted perimidines are comparatively simple, several examples display13C and1H spectral characteristics indicative of inhibition of prototropic tautom

ISSN:0749-1581    

DOI:10.1002/mrc.1260260304

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

Abstract13C NMR studies of alkali metal cation complexation by six related crown ethers are described. The crown ethers (and ions studied) were 2,3‐naphtho‐20‐crown‐6 (Na+, K+, Rb+, Cs+), 2,3‐naphtho‐17‐crown‐5 (Na+, K+, Rb+, Cs+), 2,3‐naphtho‐14‐crown‐4 (Na+, K+, Rb+, Cs+), 1,8‐naphtho‐21‐crown‐6 (Na+, K+, Rb+, Cs+), 1,8‐naphtho‐18‐crown‐5 (Na+, K+, Rb+, Cs+) and 1,5‐naphtho‐22‐crown‐6 (K+, Rb+, Cs+). For each crown ethercation combination,13C NMR spectra were obtained for a number of different ion concentrations in CD3OD. Data were analysed by a non‐linear least‐squares treatment to give complexation constants and chemical shifts that would result at the limit of complete 1:1 complexation. Analyses of the limiting chemical shifts for a number of carbons indicate regular, cation‐dependent trends (for Cs+through Na+). Deviations from these trends are interpreted as reflecting differences in conformations of crown ether–cation complexes. Conformational differences between cation complexes that occur near the point of naphthalene attachment cause deviations from the trend in the aromatic carbon chemical shifts, while conformational differences further from the naphthalene cause trend de

ISSN:0749-1581    

DOI:10.1002/mrc.1260260305

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractThe13C NMR spectra of 21 catechin derivatives substituted at C‐6/C‐8 by bromine and/or hydrogen and at oxygen by methyl, acetyl and/or hydrogen have been analysed in deuteriated acetone and chloroform. When a methoxy group is flanked by two bulkyorthosubstituents (6‐Br and C‐4), the MeO andortho/pararing carbons (C‐4a, C‐6, C‐8) are deshielded owing to steric inhibition of resonance, which permits a distinction between 6‐ and 8‐substituted catechin methyl ethers. Hindered acetoxy groups do not give rise t

ISSN:0749-1581    

DOI:10.1002/mrc.1260260306

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractFluorine‐19 substituent chemical shifts (SCS) are reported for a series of 4‐substituted bicyclo[2.2.2]octane‐1‐carbonyl fluorides containing a limited but a representative number of dipolar substituents. The19F SCS, which cover a very narrow range (<0.5 ppm), are found to correlate poorly against polar field parameters (σF). Polar field susceptibility parameters (ρF) for the fluorine probe were determined indirectly and their origin is briefly discussed in the light of the carbonyl13C SCS which reflect π polarization of the

ISSN:0749-1581    

DOI:10.1002/mrc.1260260307

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractRelayed coherence transfer (RELAY) two‐dimensional NMR spectroscopy was demonstrated to be an effective technique for overcoming the problem of overlapping multiplets in the analysis of the1H NMR spectra of steroids. Cross‐sections through appropriate proton resonances resulted in resolved spectra of the spin systems for each of the saturated rings of the estrogenic hormones 17β‐ and 17α‐estradiol. During RELAY experiments with a fixed delay time of 0.042 s it was possible to observe RELAY cross‐peaks between H‐11α and 18‐CH3which resulted from magnetization transfer via long‐range coupling between 18‐CH3and H‐12α. The difference in stereochemistry at C‐17 of these estradiol epimers allowed an examination of the effect of substituent configuration on the RELAY cross‐peak intensity in order to determine the ability of the RELAY method to provide qualita

ISSN:0749-1581    

DOI:10.1002/mrc.1260260308

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractThe valley‐to‐peak ratio of a 1:1 doublet undergoing chemical exchange can be calculated from the line shape equation. Numerical analysis provides a table that permits the evaluation of the rate constant from that ratio. This method is exact, without the approximations of previous equations that suffer from systematic errors. Statistical analysis shows how the experimental error depends on the rate constant and how to weight rate constants of unequal reliability for least‐squares analysis. From the tabulated errors it is possible to recognize conditions to optimize the evaluation of rate constants, so that it is realistic to obtain them with a precision of

ISSN:0749-1581    

DOI:10.1002/mrc.1260260309

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractTwo‐dimensional1H and13C shift‐correlated NMR experiments have been used for the unambiguous assignments of all proton and carbon chemical shifts of two cadinanes and bicadin

ISSN:0749-1581    

DOI:10.1002/mrc.1260260310

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractHydride1HT1values are reported for a selected series of ruthenium, iridium and platinum complexes. TheseT1values range from 6.9 to 0.05 s with the shortest value, 0.05 s, assigned to a complex containing both hydride and coordinated molecular hydrogen, i.e. “M(H2)”. There are nuclear Overhauser enhancements arising both from protons on coordinated ligands and other hydride ligands. It is suggested that the molecular weight of the complex and the measurement conditions can be important factors fo

ISSN:0749-1581    

DOI:10.1002/mrc.1260260311

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY