Relationships Between Aqueous Acidities of Benzene Polycarboxylic Acids and Computed Surface-electrostatic Potentials and Charges

A correlation between calculated surface-electrostatic potential of benzene polycarboxylic acids and their conjugate bases on one side and experimental pKa values on the other side was examined. It was found out that pKa values for consecutive dissociation steps of polyprotic carboxylic acids are linearly dependent on the minimum of the surface-electrostatic potential of the conjugate bases obtained in the ionization step. Such a relation could be used in evaluation of pKa values of a polyprotic acid from the titration data. The minimum of the surface-electrostatic potential of the conjugate base and the number of dissociated protons can be used also as two descriptors in the multiple regression model that predicts pKa values for the whole family of benzene polycarboxylic acids. Replacing the surface-electrostatic potential of the conjugate base with the number of carboxyl groups in the molecule gives a multiple regression model whose results are even better correlated to experimental pKa values but its use is less universal and, among other things, ignores occurrence of the intramolecular hydrogen bond. Lack of trustworthy experimental data was identified to be one of the bottlenecks in obtaining more reliable pKa prediction models for polyprotic acids.