5. Conclusions
The azo-hydrazone tautomeric equilibrium of EBB was investi-
gated by using UV–visible absorption spectroscopy and DFT calcu-
lations. The calculated results of relative energies and mole
fraction for three tautomers were showed that the azo tautomer
was the only stable form of EBB in gas phase, and only one of the
hydrazone tautomeric forms was found to be a lower rate (ca.
10%) in solution phase. The pH-dependence of the UV–vis. absorp-
tion spectra indicated that the EBB compound existed in acid–base,
azo-hydrazone and aggregation equilibria in aqueous solution.
Although, DMF and water solutions had different dielectric con-
stants (36.71 and 78.39), azo-hydrazone tautomer formation was
observed in these solutions. This behavior indicated that specific
solute solvent interactions such as H-bonding and basicity effects
played major role in the tautomerization and aggregation of EBB.
The observed changes with increasing temperature were reversible
by cooling to the initial temperature. At low concentrations, the
less stable intermolecular hydrogen bonding is converted to
the intramolecular hydrogen bonding.