ISSN:
0018-019X
Keywords:
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
In a series of 21 m-/p-substituted N-methylpyridinium salts, N-methyl proton, carbon-13 and pyridinium nitrogen-14 chemical shifts were determined by heteronuclear double resonance, and partly in the case of the 14N nucleus and the methyl protons by direct measurement. In a few compounds the quadrupole relaxation times proved to be too short for the 14N coupling to be detected. This problem was overcome by adequately rising the sample temperature. For all three nuclei a marked dependence of the chemical shifts on the nature of the substituent could be established. Increased nitrogen π-electron density shifts the resonances towards higher fields. In the case of the 14N shieldings, this tendency is attributed to changes in the paramagnetic screening term, whereas for 13C and 1H an interpretation is given in terms of a neighbour anisotropy contribution. The latter explanation is based on the observation that the calculated carbon and hydrogen charge densities show no significant variations throughout the series. Excellent shift correlations were obtained with Hammett substituent constants when σ+-values were used for donor substituents. A similar substituents, dependence could be observed for the direct 13CH methyl coupling constants, whose magnitude increases with decreasing nitrogen charge density.
Additional Material:
10 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/hlca.19710540120
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