15N Chemical Shift Principal Values in Nitrogen Heterocycles

This paper presents data on the 15N chemical shift tensor principal values in a series of 15N-enriched heterocycles. Compounds that are liquids at room temperature were frozen, and the principal values of all compounds studied were measured from static powder patterns. Four different types of nitrogen tensors are described, consisting of protonated and nonprotonated nitrogens in both five- and six-membered rings. The principal values were oriented on the molecular frame using the DFT quantum mechanical calculations of the 15N chemical shielding tensors. The agreement between the calculated and experimental principal values is adequate to make these assignments, but the relative scatters are greater than those observed in similar 13C chemical shift calculations. The largest shift component, δ11, is always oriented in the radial direction to the ring for substituted nitrogens but is tangential to the ring for the nonsubstituted nitrogens. The large variations observed in the nitrogen chemical shift tensors upon changing the nitrogen hybridization can be explained using qualitative arguments on the localization of the smallest bonding-antibonding or HOMO−LUMO gap in the molecule. The orientation of the largest shift component is always found in the plane of the molecule and is approximately perpendicular to the plane containing the bonding−antibonding or HOMO−LUMO pair of orbitals with the smallest energy gap.