Stacking Interactions between Nitrogen-Containing Six-Membered Heterocyclic Aromatic Rings and Substituted Benzene: Studies in Solution and in the Solid State

The stacking interactions between an aromatic ring and a pyridine or a pyrimidine ring are studied by
using a series of triptycene-derived scaffolds. The indicative ratios of the syn and anti conformers were
determined by variable-temperature NMR spectroscopy. The syn conformer aligns the attached aromatic
ring and the heterocycle in a parallel-displaced orientation while the anti conformer sets the two rings
apart from each other. Comparing to the corresponding control compounds where a benzene ring is in
the position of the heterocycle, higher attractive interactions are observed as indicated by the higher
syn/anti ratios. In general, the attractive interactions are much less sensitive to the substituent effects
than the corresponding nonheterocycles. The greatest attractive interactions were observed between a
pyrimidine ring and a N,N-dimethylaminobenzene, consistent with a predominant donor−acceptor
interaction. The interactions between a pyridine ring and a substituted benzene ring show that the pyridine
is comparable to that of a NO2- or a CN-substituted benzene ring except for the unpredictable substituent
effects.