An ab Initio Study of the Effect of Substituents on the n → π* Interactions between 7-Azaindole and 2,6-Difluorosubstituted Pyridines.

The n → π* interaction is a weak but important noncovalent interaction present in biomolecules and other compounds. Complexes between 7-azaindole and 2,6-difluorinated pyridines were demonstrated earlier to interact not only via an expected strong hydrogen bond but also by a weaker and unexpected n → π* interaction between the nucleophilic nitrogen atom of the 7-azaindole and the electrophilic π-system of the pyridine ring. This system provides a unique and convenient framework upon which to investigate the effect that distal substitution on the 7-azaindole ring has on the strength of the n → π* interaction. Herein we describe our thorough analysis of these effects by applying a variety of diverse methods including NBO, ETS-NOCV, and AIM. Very good agreement in trends was observed among all these diverse methods of analysis. Substitution at the position para to the nucleophilic nitrogen atom of the 7-azaindole ring with electron-donating groups weakened the hydrogen bond interaction with the 2,6-difluoropyridine but enhanced the n → π* interaction. Substitution with electron-withdrawing groups had the opposite effect. In addition, good correlation of the results of the calculations with the substituents' Hammett σp values was observed. Energy decomposition analysis (EDA) corroborated the conclusions derived by the other methods of analysis.