The effect of benzoannulation on the transition state and the proton transfer equilibrium in di(2-pyridyl)methane derivatives

Abstract

The tautomeric properties of di(2-pyridyl)methane and its benzoannulated derivatives were studied using a computational approach (M05/6-31G(2d,p)). Our analysis showed that the degree of cyclic π-electron delocalization in benzene and pyridine rings is directly connected to the effect of resonance present in quasi-rings formed by intramolecular hydrogen bonds of the N–H⋯N type. This direct relation can be explained using two concepts, namely, the concept of Clar's aromatic sextet and the Leffler–Hammond concept originally developed for the explanation of the energy relation between ground-state and transition-state structures corresponding to proton transfer in H-bridges. Application of these two different concepts allows us to explain in detail the role of intramolecular hydrogen bonding in polycyclic aromatic hydrocarbons containing N heteroatoms.