Electron-topological, energetic and π-electron delocalization analysis of ketoenamine-enolimine tautomeric equilibrium

Abstract

The ketoenamine-enolimine tautometic equilibrium has been studied by the analysis of aromaticity and electron-topological parameters. The influence of substituents on the energy of the transition state and of the tautomeric forms has been investigated for different positions of chelate chain. The quantum theory of atoms in molecules method (QTAIM) has been applied to study changes in the electron-topological parameters of the molecule with respect to the tautomeric equilibrium in intramolecular hydrogen bond. Dependencies of the HOMA aromaticity index and electron density at the critical points defining aromaticity and electronic state of the chelate chain on the transition state (TS), OH and HN tautomeric forms have been obtained.

Aromaticity from the Viewpoint of Molecular Geometry: Application to Planar Systems

“…The tautomerism of Schiff bases can be affected by many intra- and intermolecular factors (π-electron delocalization, intramolecular hydrogen bond, and effect of substituent, polarity of the solvent, temperature). Recently, it was documented that an increase in π-electron delocalization in the chelate ring ( quasi -aromatic ring, Figure , ring B) is associated with a decrease of π-electron delocalization in the adjacent aromatic ring (Figure , ring A) and consequently its increase in the distant aromatic ring (Figure , ring C). − Similar trends were observed for aryl- and alkyl- Schiff bases (Figures and , respectively).…”

“…Among systems with heteronuclear RAHB alkyl and aryl, ,− Schiff bases were the most frequently studied. In their quasi -aromatic rings, a proton transfer from the hydroxyl group to the imine group, through the O–H···N and O···H–N hydrogen bonds, takes place (Figure ).…”

“…Scatter plot of HOMA index versus the OH bond distance, d (OH), for the chelate chain of alkyl Schiff bases; ○, □, and △ correspond to the OH, transition state, and HN forms, respectively. Reprinted with permission from ref . Copyright 2012 Springer Science and Business Media.…”

Quasi-aromaticity—what does it mean?

“…It is worth mentioning that the structural and electron density-topological parameters of H-bridge in malonaldehyde derivatives were found to be relatively well correlated with indications of various aromaticity indices, including those based on structural and electronic properties of the chelate ring. 93,94,98 …”