Calculation of Nitrogen Chemical Shifts

The Pople theory of chemical shift has been employed to present a theoretical interpretation for observed nitrogen chemical shifts. In the absence of low‐lying $\mathrm{n\to \pi *}$ 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 mean‐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 pi‐ and sigma‐electronic distributions on the nitrogen atoms. The variations in the nitrogen sigma‐electronic densities estimated from chemical shift data are in reasonable agreement with those observed in nuclear quadrupole resonance studies.