摘要:

AbstractThe influence of substituents on1H and13C NMR chemical shifts of 2‐substituted 1,3‐dimethylbenzimidazolium perchlorates in different solvents has been investigated. It has been shown that the transmission of the electronic effects of the substituents in benzimidazoles and their quaternary salts are almost the same. The nature of the solvent influences only the intensity of the transmission of the electronic effects of substituents in 2‐substituted 1,3‐dimethylbenzimidazolium perch

ISSN:0030-4921    

DOI:10.1002/mrc.1270170102

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY

摘要:

Abstract13C NMR spectra of 11 di‐, tri‐ and tetra‐tert‐butyl‐substituted cyclopentadienones are discussed with respect to charge distribution and substituent effects. The chemical shifts of the unsubstituted cyclopentadienone, calculated from substituent increments, are in good agreement with published CNDO calculations and can be rationalized by the inductive effect of the carbonyl group. In addition, the ESR signals shown by some of the cyclopentadienones are described, and possible reasons for their appearance are

ISSN:0030-4921    

DOI:10.1002/mrc.1270170103

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY

摘要:

AbstractThe assignment of the signals in the13C and1H NMR spectra ofN‐phenyl‐2,4‐dimethylbuta‐1,3‐diene‐1,4‐sultam is difficult for the signal pairs C‐2 and C‐4, C‐1 and C‐3, (C‐1)H, (C‐2)CH3and (C‐4)CH3. The13C NMR spectrum recorded under gated decoupling conditions provide long‐range couplings which make possible an unambiguous assignment of the13C NMR signal pairs. Application of the1H CW off‐resonance decoupling technique in recording the13C NMR spectra enables the assignment information from the13C NMR spectrum to

ISSN:0030-4921    

DOI:10.1002/mrc.1270170104

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY

摘要:

AbstractZoapatanol and montanol are biologically active compounds isolated from the leaves ofMontanoa tomentosa(zoapatle), and represent an unusual diterpenoid structural type. The13C NMR of these compounds, and of several of their chemical derivatives, have been assigned by the use of off‐resonance decoupled experiments, internal comparisons within this series of related compounds and also by the use of model compound

ISSN:0030-4921    

DOI:10.1002/mrc.1270170105

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY

摘要:

AbstractCarbon‐13 NMR spectra of the 18 monohydroxylated‐ and of the 18 monochlorinated‐p‐menthanes have been recorded. The data of these complete sets of isomeric compounds allow a comparison of the efficiency of the various empirical methods for the calculation of chemical shifts. The application to conformational analysis is p

ISSN:0030-4921    

DOI:10.1002/mrc.1270170106

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY

摘要:

AbstractThe influence of substituents on the199Hg chemical shifts of some Ar2Hg and ArHgCl species in 1:1 CDCl3: CH2Cl2and in CDCl3: DMSO solutions is reported. The effects are substantial and are in the direction of electron withdrawal producing increased shielding (i.e. shifts towards higher field).

ISSN:0030-4921    

DOI:10.1002/mrc.1270170107

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY

摘要:

AbstractThe hydroxyl protons of 23 monosaccharides in their pyranoid from (α or β) are examined by proton NMR at 250 MHz in DMSO‐d6solutions. Intramolecular hydrogen bonding in sugar anions seems to be the major factor in accounting for the lack of parallelism between the pK values and the chemical shifts of the anomeric protons. Another important factor is the possibility of steric hindrance to the solvation of the acidic hydroxyls by the basic DMSO solvent molecules. Equatorial hydroxyls have, as a general rule, lower field chemical shifts (with some exceptions) and larger vicinal HOCH coupling constants,3J, than axial hydroxyls. Moreover, equatorial hydroxyls are sensitive to the equatorial or axial position of their neighbours, in sharp contrast with axial hydroxyls. Long‐range4Jcouplings are observed whenever a zig‐zag arrangement but a necessary condition is a simultaneous axial position of the OH and CH bonds in the above fragment. In this case, the combined use of vicinal (Karplus‐type equation) and long‐range coupling constants allows the description of the stereochemistry of the OH bond in a sem

ISSN:0030-4921    

DOI:10.1002/mrc.1270170108

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY

摘要:

AbstractThe use of quaternary carbon relaxation data, including the effects of deuteration thereon, to obtain information about chemical structures is considered. It is shown that a simple qualitative analysis of the effects of deuteration is subject to a number of potential errors. Structural inferences should be based only on a rigorous quantitative analysis of the data. The effects of deuterium substitution on the relaxation behaviour of benzamide in solution have been measured. The data are accounted for satisfactorily by treating the molecule as an axially symmetric ellipsoid. Furthermore, it is found that deuteration has no significant effect on the rotational diffusion of benzamide in solution.

ISSN:0030-4921    

DOI:10.1002/mrc.1270170109

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY

摘要:

AbstractThe rates of interconversion of thecisandtransrotational isomers ofN‐trifluoroacetyl‐N‐methylbenzylamine have been determined by a variety of NMR lineshape experiments and by several double resonance methods. Comparison of the results for the slow exchange region suggests that a Fourier transform version of the transfer of saturation method of Forsen and Hoffman, as well as transfer of magnetization after selective 180° inversion of the resonance of one rotamer, give rate constants nearly as reliable as lineshape methods. Use of the rate of signal recovery after saturation of the resonance of one rotamer gave results which were less accurate. The available data produced the following activation parameters for amide rotation in this compound: ΔG‡, 18.9 kcal mol−1; ΔH‡, 22.3 kcal mol−1and ΔS‡, 9.8 e.u. when the solute was 2 M in 1,1,2,2

ISSN:0030-4921    

DOI:10.1002/mrc.1270170110

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY

摘要:

AbstractThe first successful observation of the vicinal15N,13C spin‐coupling constants in a series of amino acids, comprising Val, Ile, Leu and Thr, in which the α‐nitrogens are fully replaced with15N, is described. A Karplus‐type dihedral‐angle dependence was noted for the coupling constants between the α‐nitrogen and γ‐carbon nuclei which may, in turn, be applied to the conformational analysis of other amino acids and small peptides. The potential usefulness and limitation of this new parameter in such applications is bri

ISSN:0030-4921    

DOI:10.1002/mrc.1270170111

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY