title: Comprehensive experimental study of N-heterocyclic π-stacking interactions of neutral and cationic pyridines.
abstract: 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.
authors.affiliation: Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States.
authors.forename: Ping
authors.initials: P
authors.lastname: Li
authors.forename: Chen
authors.initials: C
authors.lastname: Zhao
authors.forename: Mark D
authors.initials: MD
authors.lastname: Smith
authors.forename: Ken D
authors.initials: KD
authors.lastname: Shimizu
mesh_terms: D002412:Cations / Q000737:chemistry; D018360:Crystallography, X-Ray; D006571:Heterocyclic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D011725:Pyridines / Q000138:chemical synthesis / Q000737:chemistry*
chemical_list: D002412:Cations; D006571:Heterocyclic Compounds; D011725:Pyridines