Aromaticity-Controlled Tautomerism and Resonance-Assisted Hydrogen Bonding in Heterocyclic Enaminone−Iminoenol Systems

The contribution of aromaticity and intramolecular hydrogen bonding to relative stability, for a set of (1H-azahetero-2-ylidene)-acetaldehyde and 2-azahetero-2-yl-ethanol tautomeric pairs, has been investigated by means of quantum chemical DFT and ab initio methods up to the MP4(SDTQ)/AUG-cc-pVDZ and MP2/AUG-cc-pVTZ levels of theory. It is found that the relative energy of the tautomers is governed by the change in the degree of heterocycle aromaticity upon intramolecular hydrogen transfer. An analysis of geometrical parameters of a hydrogen-bonded system reveals a clear relationship between the aromaticity of the heterocycle, the conjugation in a resonant spacer, and the strengths of the intramolecular hydrogen bonds. This allows the conclusion to be drawn that intramolecular N−H···O and O−H···N hydrogen bonds formed are found to be resonance-assisted and their strength is dependent on the π-donating/accepting properties of the heterocycle. On the basis of the results of the calculations, a simple model describing the mechanism of resonance assistance of hydrogen bonding has been suggested.