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A comprehensive experimental study was carried out by measuring the relative strengths of parallel π-stacking interactions of N-heterocycles with nonheterocycles. A versatile and rigid model system was developed, which was in equilibrium between a "closed" conformation that forms an intramolecular π-stacking interaction and an "open" conformation that cannot form the interaction. First, the formation and geometries of the intramolecular N-heterocyclic π-stacking interactions were verified by X-ray crystallography. Next, the closed/open ratios were measured in solution via integration of the (1)H NMR spectra, providing an accurate comparison of the N-heterocyclic π-stacking interactions. The synthetic versatility of this model system enabled the systematic and comprehensive comparison of the influences of position, charge, and substituent effects of the nitrogen atom of the N-heterocycles within a single model system. The π-stacking interactions of the neutral N-heterocyclic rings were slightly stronger than that of nonheterocyclic rings. Cationic N-heterocycles formed significantly stronger π-stacking interactions than neutral N-heterocycles. The position of the nitrogen atom also had a strong influence on the stability of N-heterocyclic π-stacking complexes. Interestingly, opposite stability trends were observed for neutral and cationic N-heterocycles. For neural N-heterocycles, geometries with the nitrogen away from the π-face of the opposing ring were the more stable. For cationic N-heterocycles, geometries with the nitrogen close to the π-face of the opposing ring were the more stable. Finally, N-methylated heterocycles consistently formed stronger π-stacking interactions than N-protonated heterocycles.
Li et al. (Wed,) studied this question.