The Pople theory of chemical shift has been employed to present a theoretical interpretation for observed nitrogen chemical shifts. In the absence of lowhyphen;lyingnrarr;pgr;*transitions which give rise to a large paramagnetic shift, the shielding of nitrogen, as predicted by theory, arises primarily from: (1) the electron density on nitrogen; (2) the meanhyphen;excitation energy; and (3) the effect of multiple bonds of nitrogen. A theoretical expression was also derived for calculations of nitrogen chemical shifts in heteroaromatic molecules. It was found that the chemical shifts of the diazines depend on both the pihyphen; and sigmahyphen;electronic distributions on the nitrogen atoms. The variations in the nitrogen sigmahyphen;electronic densities estimated from chemical shift data are in reasonable agreement with those observed in nuclear quadrupole resonance studies.
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