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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.