Chemical Shifts and Electron Densities in Nitrogen Heterocyclic Molecules

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Abstract
The chemical shifts of a series of 11 nitrogen heterocyclic molecules have been measured and, after appropriate correction, compared to calculated π-electron densities. This study includes a comparison of the azines and diazines with their ``benzologs,'' analogous molecules in which a benzene ring has been fused to the original heteroaromatic ring. The chemical shifts and calculated π-electron densities were found to have only a rough correlation. The chemical shifts of protons bonded to carbon atoms adjacent to nitrogen atoms seem especially anomalous. The effect of irregular molecular geometry on the calculated electron densities, of the magnetic anisotropy due to lone-pair π-orbital promotion on the nitrogen, and of σ-π-electron interaction all appear to make significant contributions. The proton chemical shifts appear to be a somewhat unreliable measure of π-electron densities in the nitrogen heterocycles, especially in positions adjacent to a nitrogen atom.