N-15 CHEMICAL-SHIFT PRINCIPAL VALUES IN NITROGEN-HETEROCYCLES

This paper presents data on the N-15 chemical shift tensor principal v alues in a series of N-15-enriched heterocycles. Compounds that are li quids at room temperature were frozen, and the principal values of all compounds studied were measured from static powder patterns. Four dif ferent types of nitrogen tensors are described, consisting of protonat ed and nonprotonated nitrogens in both five-and six-membered rings. Th e principal values were oriented on the molecular frame using the DFT quantum mechanical calculations of the N-15 chemical shielding tensors . The agreement between the calculated and experimental principal valu es is adequate to make these assignments, but the relative scatters ar e greater than those observed in similar C-13 chemical shift calculati ons. The largest shift component, delta(11), is always oriented in the radial direction to the ring for substituted nitrogens but is tangent ial to the ring for the nonsubstituted nitrogens. The large variations observed in the nitrogen chemical shift tensors upon changing the nit rogen hybridization can be explained using qualitative arguments on th e localization of the smallest bonding-antibonding or HOMO-LUMO gap in the molecule. The orientation of the largest shift component is alway s found in the plane of the molecule and is approximately perpendicula r to the plane containing the bonding-antibonding or HOMO-LUMO pair of orbitals with the smallest energy gap.