Calculation of Ionization Constants of Monoprotic Weak Acids in Aqueous Solutions

Motivation. The ionization constants are fundamental to explain the occurrence of numerous and varied processes of great importance in Chemistry and Biology. With the specific aim of contributing to the development of theoretical procedures to study the acidity constants in this work we show the feasibility of a DFT method that makes use of Tomasi's model to calculate the ionization constants of monoprotic weak acids in aqueous solutions. Method. The B3LYP/6-311++G(d,p) and HF/6-311++G(d,p) methods were used for performing the calculations that permitted to describe the molecular conformations and solute-solvent interactions of the acids and their corresponding conjugate bases. Results. Several ionization reactions and equilibria in protic solvents, which possess a high hydrogen-bond-donor capability, were proposed. It was considered that the solvation of the hydrogen and hydroxyl ions, as wells as of conjugate bases occurs by means of intermolecular hydrogen bonds that involve one molecule of the ion and one molecule of water. The calculated ionization constants show a good agreement with the experimental values reported in the literature. Conclusions. The very good relationship that exists between the calculated pKa values and those obtained experimentally constitutes a good support for the reactions and equilibria of ionization proposed in this paper