摘要:

AbstractStereochemical assignments of the C21steroids 12‐β‐O‐acetyltenacigenin A and tenacigenin A were performed by use of two‐dimensional phase‐sensitive COSY, ROESY,1H and13C shift correlation spectroscopy (HETCOR and FLOCK), and ambiguous assignments were resolved through molecular modeling comparisons. This combination of techniques is applicable for the assignment of the highly overlapped1H signals of thes

ISSN:0749-1581    

DOI:10.1002/mrc.1260310302

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractA series of phenylethylamine and amphetamine derivatives were synthesized in order to determine the nature of the internal and overall molecular dynamics in these systems.13C and2H NMR relaxation times were measured. For the latter measurements, compounds specifically deuteriated in the side‐chains were synthesized. The relaxation data were analysed via a computer model incorporating the spectral density function for axially symmetric anisotropic motion. The motion of these molecules was found to be substantially anisotropic, with rotation about the symmetry axis of the molecule being at least twice as fast as rotation normal to the symmetry axis. The results of more complex models of axially symmetric anisotropic molecular motion, involving internal rotation or conic diffusion of the side‐chains, were found to be sensitive to variation of the input data, but the correlation times for internal rotation of the ring—C‐β bond were longer than might have been expected. Similarly, owing to its flexibility, the effective correlation times of atoms in the side‐chain were found to be faster than those of the ring carbons, but did not always decrease along the side chain. Intermolecular interactions or aggregation states in CDCl3may reduce segmental motion of side‐chains. A particular feature of the current study is that it was demonstrated that deuteriation of the side‐chain gives a unique opportunity to determine independently side‐chain correlation times via the quadrupolar relaxation times of the2H nucleus, and to compare these results with those of the carbon atoms. These correlation times were found to be in very good agreement with those obtained

ISSN:0749-1581    

DOI:10.1002/mrc.1260310303

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractAssignment of the13C resonances of the highly delta opioid receptor selective cyclic peptideenkephalin (DPDPE) was accomplished by using single‐quantum inverse proton detected 2D heteronuclear correlation spectroscopy. The pulse sequences described provided better sensitivity and resolution than the corresponding, commonly used, multiple‐quantum evolution experiments, since during the SQ‐evolution period there is no line broadening due to scalar and dipolar1H‐1H interactions. The proton detected C,H correlation spectrum, in combination with the H‐TOCSY relay C,H correlation, gave the assignment of all protonated carbons. The C,H correlation optimized for long‐range coupling gave the assignments of all non‐protonated carbons, such as the carbonyl carbons of the peptide bonds. These spectra also allowed the sequential assignment of

ISSN:0749-1581    

DOI:10.1002/mrc.1260310304

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractThe15N chemical shifts and1J(15N,1H) coupling constants were measured for a series of 5‐substituted indoles and 4‐substituted acetanilides. The cross‐conjugation effect in both series was demonstrated by linear correlations between the15N SCS values versus an empirical balance of the deuterium‐induced isotope shifts of the carbons directly bonded to the NH(D) residue. In the case of indoles this empirical balance involves the average of2Δ(C‐2) and2Δ(C‐7a), whereas for acetanilides values obtained by subtraction of2Δ(CO) from2Δ(

ISSN:0749-1581    

DOI:10.1002/mrc.1260310305

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

Abstract13C NMR spectra of 36endo–endo,exo–endoandexo–exotetracyclic fused norbornyl compounds are interpreted. Stereochemical influences on1H and13C chemical shifts are compared and discussed. Only upfield δ and ε effects are o

ISSN:0749-1581    

DOI:10.1002/mrc.1260310306

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractPrincipal component analysis is applied to the13C NMR chemical shifts of 29 tetracyclic dodecane derivatives, the spectra of most of which have been recently assigned. These derivatives are subdivided into three classes:exo‐endo,exo–exoandendo–endo. Only the seven13C shifts of one of the two norbornyl moieties of these derivatives are used in the construction of the data matrix, the other moiety being considered as a ring substituent. This data treatment allows these 29 systems to become a test system for a previously worked (through principal component analysis) standard or training set of13C shifts of 55 substituted norbornanes. Only one two‐dimensional principal component projection is necessary for identifying the main structural effects in the chosen norbornyl moiety of the test system, highlighting trends in the13C data of these test derivatives and confirming their recent13C spectral assi

ISSN:0749-1581    

DOI:10.1002/mrc.1260310307

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractMultifrequency ESR analysis allows a new interpretation of the correlation times involved in nuclear relaxation. In this study the metal‐ATP interaction, so important in ecosystem energy mechanisms, was interpreted in the light of new multifrequency ESR and NMR experimental data. ESR spectra of the Mn(II)‐ATP complex were recorded at 2–4 GHz (S‐band), 9 GHz (X‐band) and 35 GHz (Q‐band). A loop‐gap resonator was used to enhance the ηQ0product at low frequencies. The ESR line shapes at different temperatures and frequencies led to the interpretation of the predominant relaxation mechanism involved. The NMR relaxation rates, also recorded at different temperatures and frequencies, are discussed in terms of a motional model taking into account a distribution of correlation times. Structural and dynamic features were analysed in order to determine the effective correlation time contributing to nuclear paramagnetic relaxation. A comprehensive analysis of the multifrequency ESR spectra is crucial for the correct interpretation of the NMR spectra and of the metal

ISSN:0749-1581    

DOI:10.1002/mrc.1260310308

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractImproved two‐dimensional1H NMR sequences including shaped pulses are proposed for increasing the separation of cross peaks in the case of severe spectral overcrowding. Ways around limitations in soft 2D experiments are proposed for increasing the resolution and the apparent peak separation in both dimensions. Modulated shaped pulses leading to multi‐selective excitation are proposed for reducing the size of data matrices at a givenF2resolution and for simplifying coherence transfer pathways. Separation of the signals inF1is achieved by either introduction of refocusing soft pulses or constant‐time experiments. These methods were applied to inclusion complexes of cyclodextrin derivatives where, although of considerable value in deriving the 3D structure of the complex, a total assignment of NMR spectra is not, at first glance, a simple task. It is demonstrated that the proposed solutions overcome the apparently discouraging complexity of the 1D NMR spectra and yield a wealth of important data for further structural determin

ISSN:0749-1581    

DOI:10.1002/mrc.1260310309

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractA method is described for obtaining quantitative intensity data from solution‐state spectra by a combination of coaxial tubes and sample replacement to take account of spectrometer electronic effects. The protocol was applied to two sodium silicate solutions of high silica content, and it is shown that there is no ‘invisible’ silica at the concentrations st

ISSN:0749-1581    

DOI:10.1002/mrc.1260310310

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY

摘要:

AbstractThe1H and13C NMR spectra of the parent tetramethyl 9aH‐quinolizine‐1,2,3,4‐tetracarboxylate and 20 of its alkyl‐substituted derivatives were obtained and analysed. The study included measurements of one‐bond carbon–carbon and also proton–proton coupling constants across three, four, five and six bonds. An analysis of the data allowed an estimation of electron distribution and steric requirements in this group of compounds. No correlation was observed between1J(C,C) values and the corresponding

ISSN:0749-1581    

DOI:10.1002/mrc.1260310311

出版商:John Wiley&Sons, Ltd.    

年代:1993

数据来源: WILEY