Determination of the overlapping pKa values of chrysin using UV-vis spectroscopy and ab initio methods

The overlapping pKa values of 5,7-dihydroxyflavone (chrysin) in EtOH-water solutions were determined by means of a UV-vis spectroscopic method that uses absorbance diagrams, at constant ionic strength (0.050 M) and temperature (25.0 ± 0.1 °C). It was observed that the pKa values increase when the polarity-polarizability and solvation abilities of the reaction medium decrease. In order to calculate the pKa1 and pKa2 of chrysin in pure water, various relationships between the determined pKa and properties of solvents (relative permittivity, α-parameter of Taft and parameter Acity), are proposed. Moreover, with the aim of explaining the first pKa1 value obtained, the molecular conformations and solute-solvent interactions of the 7(O−)chrysinate monoanion were also investigated, using ab initio methods. Several ionization reactions and equilibria in water, which possesses a high hydrogen-bond-donor ability, are proposed. These reactions and equilibria constituted the necessary theoretical basis to calculate the first acidity constant of chrysin. The HF/6-31G(d) and HF/6-31 + G(d) methods were used for calculations. Tomasi's method was used to analyze the formation of intermolecular hydrogen bonds between the 7(O−)chrysinate monoanion and water molecules. It was proposed that in alkaline aqueous solutions the monoanion of chrysin is solvated with one water molecule. The agreement between the experimental and theoretical pKa1 values provides good support for the acid-base reactions proposed in this paper.