摘要:

AbstractThe use of the nominal value of the chemical shift of the solvent to calibrate the spectrometer in13C NMR spectroscopy was found to introduce errors due to the effect which the solute has on the solvent. In addition, hexamethyldisiloxane (HM) is proposed as an internal standard; owing to its high boiling point it is easier to manipulate than tetramethylsilane (TMS) and it is therefore possible to prepare solutions of known concentrations. In order to convert the data obtained with this standard to the TMS scale, the chemical shift of HM was determined in 16 solvents using cyclohexane as external standard in a spherical cell (5% w/w concentration of HM) as a function of the solvent factorg2. Comparing these results with a similar plot obtained previously for TMS by other workers, it is possible to convert one type of data to the other by a simple linear expression.

ISSN:0749-1581    

DOI:10.1002/mrc.1260280102

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY

摘要:

AbstractVariable‐angle sample‐spinning (VASS) NMR was investigated for the case of limited rotation speed of the spinner. Side‐band intensities and line‐shapes for the central transition of half‐integer quadrupolar spin nuclei were calculated as a function of the parameter\documentclass{article}\pagestyle{empty}\begin{document}$ X = \frac{1}{4}[I(I + 1) - \frac{3}{4}]v_{\rm Q} ^2 (1 + \eta ^2 /3)/v_{\rm L} v_{\rm R}$\end{document}. It is shown that complex sideband patterns can be analysed using a very fast computationa

ISSN:0749-1581    

DOI:10.1002/mrc.1260280103

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY

摘要:

AbstractAlthough dibromomethane is a simple molecule, little work has been directed toward the analysis of its reorien‐tational motion. This is surprising, since two components of its inertia tensor are nearly degenerate and may, in principle, be treated as a quasi‐symmetric top. The13C spin–lattice relaxation times and nuclear Overhauser enhancements were measured at various temperatures in the liquid phase. Field‐dependent measurements indicated the presence of the scalar relaxation mechanism. Calculated rotational diffusion constants were found to be in reasonable agreement with values predicted by theJ‐diffusion model of inertially controlled reorientation. The results show that scalar coupling of the second kind is an important mechanism for13C spin–lattice relaxation in this molecule. The data further indicates that dibromomethane may indeed be exhibiting quasi‐symmetric top behavior

ISSN:0749-1581    

DOI:10.1002/mrc.1260280104

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY

摘要:

AbstractThe13C and14N NMR spectra of a variety of highly nitrated aliphatic, aromatic and heterocyclic compounds were obtained. The proton decoupled13C peaks of the nitro‐substituted carbons in most of these compounds were split into the multiplets expected for coupling with spin 1 nuclei with negligible quadrupolar broadening. In addition, the19F spectra of some fluoronitroaliphatic compounds showed that the19F signals were also split into the expected multiplets from coupling with the14N of the nitro group

ISSN:0749-1581    

DOI:10.1002/mrc.1260280105

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY

摘要:

AbstractAn experimental method is proposed which selects from all the inter‐proton distancesrdetermined from two‐dimensional NMR NOESY experiments, those which do not depend on the internal motions of the molecule under investigation. Only these distances are quantitatively reliable for modelling purposes, owing to (1/r6) averaging and the possible correlation betweenrand the orientation of two protons as a consequence of the molecular Brownian internal dynamics. All the other distances must be used only qualitatively. Two examples of the application of this method to the lipopeptide stendomycin are gi

ISSN:0749-1581    

DOI:10.1002/mrc.1260280106

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY

摘要:

AbstractCarbon‐13 spin–lattice relaxation times were measured for some penta‐ and tricarbonylchromium complexes of organic ligands. The chemical shift anisotropy of the carbonyl carbons was estimated to beca.500 ppm. The contributions of dipole–dipole and chemical shift anisotropy mechanisms to the overall relaxation were evaluated and used to determine the reorientational parameters for the ligand and carbonylchromium moieties. It seems that the internal rotation rate about the ligand—chromium bond is governed mainly by intermolecular int

ISSN:0749-1581    

DOI:10.1002/mrc.1260280107

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY

摘要:

AbstractThe one‐bond13C13C coupling constants (1JCC) in all nine possible dithieno[b,d]pyridines were measured at natural isotope abundance by using the INADEQUATE pulse sequence technique. The unambiguous assignments of the13C NMR shifts of these compounds are also reported. Assignments are based on proton decoupled13C spectra, heteronuclear correlation spectra (HETCOR) and 1D‐INADEQUATE13C NMR spectroscopy. The effect of the fusion pattern and the influence of the heteroatoms in the ring skeleton on the NMR parameters are discussed. By a multivariate principal components data analysis, it was shown that the magnitude of1J(CC) of the pyridine subunit could be satisfactorily predicted from shift data, the azomethine CH coupling constant and MNDO bond orders. The last parameter gave only a minor contribution to the coupling vari

ISSN:0749-1581    

DOI:10.1002/mrc.1260280108

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY

摘要:

AbstractAn equation correlating13C NMR chemical shifts and the global van der Waals energy of the carbon atoms (rms deviation δ ≤ 2.6 ppm) in aliphatic hydrocarbons, alcohols and chlorinated derivatives was derived. Molecular mechanics calculations have been used to determine the global van der Waals ener

ISSN:0749-1581    

DOI:10.1002/mrc.1260280109

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY

摘要:

AbstractN,N‐Bis(2‐fluorophenyl)carbamoyl chloride inactivates the serine protease α‐chymotrypsin by stoichiometric carbamoylation, presumably at the active site serine‐195 residue. At sample temperatures above 25°C fluorine NMR spectra of this enzyme derivative reveal the presence of conformations which are interchanged by processes that have ΔG 25≠of about 70 kJ mol−1as estimated by two independent methods. Fluorine NMR spectra of the protein dissolved in 8 M urea were characterized by shift and relaxation parameters which suggest an unfolded state for the protein but with interconversion kinetics similar to those of the native structure. At sample temperatures below 25°C a second kinetic process becomes apparent in the spectra of both the native and denatured protein, although it was not possible to quantitate the rate of this process. These spectral changes are discussed in terms of the likely conformational properties of theN,N‐bis(2‐fluorophenyl) group attached to the native enzyme and are shown to be consistent with one of the 2‐fluorophenyl rings having a high barrier to rotation, and thus rotating slowly, while the other rotates rapidly. Both rings rotate rapidly in the denatured protein. A form of the modified enzyme which had fluorine NMR characteristics similar to those of the denatured enzyme yet was fairly soluble in 0.05 M sodium chloride solution was detected in samples after extended heating; formation of this species was not readily reversible. Spin‐lattice relaxation times (T1) and19F{1H} NOE data for the native and denatured modi

ISSN:0749-1581    

DOI:10.1002/mrc.1260280110

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY

摘要:

AbstractRelative signs of1J(207Pb13C) and2J(207Pb1H) were determined for numerous methyllead derivatives by observing the207Pb satellites in two‐dimensional (2D)13C/1H heteroscalar correlated NMR spectra. The compounds studied include the species [R3Pb]Li (R = Me, 1a, Et 1b), Me3PbSiMe3(2), [Me3PbBH3]Li, several alkenyltri‐methyllead compounds and some heterocyclics. Together with the linear correlation between1J(207Pb13CMe) and1J(119Sn13CMe) for the corresponding tin compounds, this enables the prediction of the absolute sign of1J(207Pb13CMe). An analogous linear correlation1J(207Pb13CC=)/1J(119Sn13CC=) exists which explains some apparently anomalous values reported in the literature, e.g. for [Ph3Pb]Li and for an intramolecular‐base‐stabilized plumbylene. The two coupling constants,1J(207Pb13CMe), for the Me2Pb group in a 2,5‐dihydro‐1,2,5‐azoniaplumbaboratole are of opposite sign, giving the first example of this type of behaviour for any dimethyl element fragment. In the case of 2 the negative sign of1J(207Pb29Si) [reduced coupling constant1K(207Pb29Si)>0] has been deduced from the tilt of the207Pb satellites in the 2D29Si/1H heteroscalar correlated NMR spectrum. A positive sign of1J(207Pb11B) follows from the consistent trend observed for the reduced coupling constants1K(M11B) and1K(M13C) (M =13C,29Si,119Sn,207Pb) in the series of compounds [Me3MBH3]Li and Me4M,

ISSN:0749-1581    

DOI:10.1002/mrc.1260280111

出版商:John Wiley&Sons, Ltd.    

年代:1990

数据来源: WILEY