摘要:

Abstract1H NMR spectra at 300 and 400 MHz have been recorded for the thyroid hormones thyroxine (T4) and triiodothyronine (T3) over a range of temperatures from 175 to 298 K in MeOH‐d4. Above 200 K the aromatic region of the NMR spectrum of T4 consists of two peaks, one for the H‐2′, H‐6′ protons and one for H‐2,H‐6. As the temperature is lowered each of these resonances first broadens and then splits into two peaks of equal intensity at low temperature. This is consistent with slowed exchange between two equally populated conformers. Similar behaviour is noted for T3, although in this case the spectrum is complicated by the presence of spin‐spin splitting. Consideration of coalescence temperatures and a line‐shape analysis yielded activation barriers in the range 35–38 kJ mol−1. It is concluded that the observed spectral changes are due to dynamic interconversion between proximal and distal forms of the hormones, and that the interconversion takes place by cooperative rotations around the bonds between each phenyl ring and th

ISSN:0749-1581    

DOI:10.1002/mrc.1260260402

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

Abstract13C CP/MAS NMR spectra of nitrogen‐containing compounds often show complex signals when recorded at low field strengths. This effect is due to the13C,14N residual dipolar coupling which is not averaged to zero by magic‐angle spinning. Solid‐state13C NMR spectra at 25 MHz of ribonucleosides are analysed on the basis of the isotropic chemical shifts measured at 50 MHz, together with the asymmetric splittings caused by the presence of14N calculated using a first‐order equation. X‐ray diffraction and NQR data, together with several assumptions regarding the quadrupole tensor at each14N site, are used to obtain a successful spectral s

ISSN:0749-1581    

DOI:10.1002/mrc.1260260403

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractAssignment of the congested proton and carbon NMR spectra of heterocyclic benzo[b]triphenyleno[1,2‐d]thiophene was accomplished through the concerted application of several two‐dimensional NMR experiments. The inherent insensitivity of homonuclear zero quantum coherence to magnetic field inhomogeneity was used to advantage in disentangling the heavily overlapped proton NMR spectrum. Direct heteronuclear pairings were established using heteronuclear correlation with broad band homonuclear vicinal proton decoupling. Limited sample availability and solubility precluded complete establishment of long‐range heteronuclear correlations by conventional means. Proton‐detected long‐range heteronuclear multiple quantum coherence did, however, provide sufficient long‐range heteronuclear correlations to complete the assignment.A comparison of the data obtained from COSY and zero quantum experiments is presented; the ability of each to establish proton connectivities is discussed. Sensitivity limitations inherent in the conventional long‐range optimized heteronuclear chemical shift correlation experiment are contrasted with the advantages of the proton‐detected long‐range heteronuclear multiple quantum correlation experiment. Practical limitations of the latt

ISSN:0749-1581    

DOI:10.1002/mrc.1260260404

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractPerindopril, a potent inhibitor of angiotensin I converting enzyme, was studied by1H NMR and13C NMR in various solvents and at various temperatures. Thecisandtransconformations of the amide bond were assigned. The conformations of the perhydroindole skeleton rings were studied by1H NMR.

ISSN:0749-1581    

DOI:10.1002/mrc.1260260405

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractAromatic13C chemical shifts of 4‐ and 3‐substituted pyridineN‐oxides in aprotic and protic solvents are reported and discussed. Changes in chemical shifts caused by hydrogen bonding and protonation are much smaller than those induced by a substituent, and are not related to the pKavalues ofN‐oxides. Since the effect of hydrogen bonding is not in the same direction as that due to the substituent effect, the estimation of proton transfer from measured chemical shifts is not possible in complexes of pyridineN‐oxides with trifluoroacetic acid. A linear correlation is obtained between theipsocarbon substituent chemical shifts of 4‐ and 3‐substituted pyridineN‐oxides and mono‐substituted benzenes, which supports the suggestion that electronic factors other than Δqare more important

ISSN:0749-1581    

DOI:10.1002/mrc.1260260406

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractA modified1H‐decoupled INEPT pulse sequence has been used for assigning15N resonances, based on differences in small15N,1H coupling constants (1–12 Hz). Both enhancement and distinction of the15N lines are achieved. The usefulness of this method is demonstrated for wyosine triacetate and caffe

ISSN:0749-1581    

DOI:10.1002/mrc.1260260407

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractThe1H NMR spectrum of hexakis(3‐methoxyphenyl)benzene exhibits a methoxy absorption which looks exactly like a triplet. The spacing between the signals is, however, proportional to rather than independent of the field strength. The aryl H‐5 region of this compound and the methyl region of hexakis(3‐methylphenyl)benzene display similar field‐proportional, triplet‐like splittings. This property ofmeta‐substituted hexaphenylbenzenes cannot be explained by either preferential or residual isomerism, even in the presence of many accidental isochronies. A chemical shift model based on the hypothesis that themetasubstituents do not disturb the geometry of the hexaphenylbenzene skeleton or the populations of the rotamers explains the spectra in a straightfor

ISSN:0749-1581    

DOI:10.1002/mrc.1260260408

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractThe radical anions of (E)‐1, 2‐di(1‐naphthyl)ethene, (E)‐1, 2,‐di(1‐napthyl)acetylene, di(1‐naphthyl)diacetylene, 9, 10‐bis(phenylethynyl)anthracene and di(9‐anthryl)acetylene have been characterized by ESR and ENDOR spectroscopy. Their proton hyperfine data andgfactors are

ISSN:0749-1581    

DOI:10.1002/mrc.1260260409

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractThe isotropic and anisotropic ESR spectra of dialkyl oxalate radical anions, including derivatives which were labelled with13C or17O in the carbonyl and2H or13C in the alkyl groups, were measured. The spin density distribution as determined from the experimental nuclear hyperfine coupling constants agrees very well with MNDO‐CI calculations. As a consequence of the strictly planar structure of the radical anions,E/Zisomers are observe

ISSN:0749-1581    

DOI:10.1002/mrc.1260260410

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY

摘要:

AbstractCyclic ketones from cyclobutanone to cyclohexadecanone were labelled with deuterium in the α‐position. The deuterium isotope effect on the13C NMR spectra was measured and in some cases was indicative of perturbation of conformational equilibrium. Similar effects were observed with acyclic ketones. The results were confirmed by temperature‐dependent measurements and molecular mechanics calculat

ISSN:0749-1581    

DOI:10.1002/mrc.1260260411

出版商:John Wiley&Sons, Ltd.    

年代:1988

数据来源: WILEY