摘要:

AbstractA new version of the 2D combination of TOCSY and H,C‐COSY is proposed which uses the DEPT technique for the heteronuclear transfer. Hence the resulting 2D spectrum can be easily corrected to pure absorption mode for optimum resolution in both dimensions. In contrast to an earlier published version with a partially refocused INEPT sequence and delayed heteronuclear decoupling inF2, the new variant does not generate any base‐line distortions interfering with interesting signals. In addition, experimental spectra obtained from cyclosporin A and a cyclic hexapeptide also show improved sensitivity compared with the INEPT version. In all these experiments, the information about direct HC connectivities is lost owing to the TOCSY transfer preceding the heteronuclear transfer. This information can, however, be provided by a version with only partial heteronuclear decoupling inF1, which can be easily obtained by shifting the refocusing 180°13C pulse out of the centre oft1. Thus all couplings can be reduced to a fraction of their original value, as long as the large1Jcoupling still causes a visible doublet splitting in the 2D spectrum. The reduced heteronuclear couplings lead to a spectrum which is easy to interpret, since the resulting small splittings of the direct cross‐peaks are unlikely to interfere with other signals. In addition, all long‐range couplings inF1are also scaled down, thus causing less line broadening and therefore higher s

ISSN:0749-1581    

DOI:10.1002/mrc.1260280112

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY

摘要:

AbstractThe magnitude of the NMR spin‐spin coupling constant,3J(CH), between a vicinal13C–1H pair depends,inter alia, on the value of the torsion angle ΦCH(13CCCH) and is influenced by the presence of an electronegative substituent located on the coupling13C nucleus. The form and magnitude of the effect of the orientation ΨXCof such an α‐substituent were examined. The coupling constant between C‐1 and a hydrogen atom located on C‐3 in a series of α‐substituted propanes were studied by means of the semi‐empirical INDO method. In the calculations both Φ and Ψ(X13CCC) were systematically varied in steps of 30°. These calculations reveal that the variation of Ψ at a constant Φ has a pronounced effect on the calculated coupling constantJcalc. The magnitude of this effect is shown to be strongly dependent on the electronegativity χ of the α‐substituent. Thus, it is shown thatJcalcdepends on Φ and Ψ, in addition to χ. The resulting set of two‐dimensional Karplus‐type surfaces can be described by an equation that contains only nine adjustable parameters. Measurement of3J(CH) incis‐ andtrans‐2,2,6,6‐tetradeuterio‐4‐tert‐butylcyclohexanol confirmed some of the theoretical predictions. In theciscompound (ΦCH= 180°, ΨOC= 60°)3J(C‐1,H‐3eq) is 7.1 Hz, whereas in thetranscompound (ΦCH= 180°, ΨOC= 180°)3J(CH

ISSN:0749-1581    

DOI:10.1002/mrc.1260280113

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY

摘要:

AbstractThe DANTE‐Z pulse train [(θ)x‐τ‐(θ)x‐τ]n(π/2) (Acq)±, applied to solid‐state samples in an asynchronous manner with respect to the rotor period of the magic‐angle spinning, is able to extract the spectrum of a single CSA (chemical shift anisotropy) side‐band family without any intensity alteration. The effects of such a selectivity sequence is described by means of a phenomenological approach. A related method, based on selective destruction of magnetization viaT2processes, is proposed for retaining only those resonances for which CSA effects are totally averaged out by magic‐

ISSN:0749-1581    

DOI:10.1002/mrc.1260280114

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY

摘要:

AbstractTin‐119 NMR data are presented for thirteen 1,2,4,5‐tetrastannacyclohexanes. The one‐bond tin‐tin coupling constant shows a monotonic decrease with increasing methyl substitution at the ring carbon atoms. In a series of compounds the two‐path couplings2+4J(SnSn) and3+3J(SnSn) can be assigned and interpreted on the basis of a sign change for the former coupling. Values of4J(SnSn) for substituted 2,3,5,6‐tetrastannaheptanes suggest that the magnitude of the four‐bond component of2+4J(SnSn) in the stannacycles can be

ISSN:0749-1581    

DOI:10.1002/mrc.1260280115

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY

摘要:

AbstractCarbon‐13 spectral assignments of three friedelanes and nineteen secofriedelanes have been made from mutual correlations, 2D X‐H correlation analyses and comparison with the carbon shifts of friedelin, which were firmly established by an INADEQUATE experiment that required reassignment of some of its resonances. Specific methyl and methine proton resonance assignments for some friedelanes, established from X‐H correlation studies, are also rep

ISSN:0749-1581    

DOI:10.1002/mrc.1260280116

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY

摘要:

AbstractA compilation of13C NMR chemical shifts for 13 pairs of 3‐ and 5‐cyano‐substituted pyrazole regioisomers is reported. All of the ring carbon and cyano carbon13C chemical shifts show a regular, predictable correlation with the particular isomer, whether 3‐cyano or 5‐cyano. These shifts occurred in very narrow ranges, precluding any confusion of assignment within the group of compounds studied. x‐ray crystallographic analysis was performed on one of

ISSN:0749-1581    

DOI:10.1002/mrc.1260280117

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY

ISSN:0749-1581    

DOI:10.1002/mrc.1260280118

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY

ISSN:0749-1581    

DOI:10.1002/mrc.1260280119

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY

ISSN:0749-1581    

DOI:10.1002/mrc.1260280101

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY