Estimation of empirically fitted parameters for calculating pKa values of thiols in a fast and reliable way

Two empirically fitted parameters (m and C0) for the calculation of pKa values for thiols are provided for the first time, at 74 levels of theory. The coefficients were obtained by least-squares fits of the difference in Gibbs energy between each acid and its conjugated base versus experimental pKa values. The reliability of this fitted parameters approach (FPA) was confirmed using an independent test set of molecules. It was found that deviations from experiments are systematically lower than 0.5 pKa units, in terms of mean unsigned errors. In addition, all the tested levels of theory produced maximum absolute errors lower than 1 pKa unit. The parameters estimated here are expected to facilitate pKa calculations, using electronic structure-based strategies, with uncertainties close to the experimental ones. Albeit the present study deals only with molecules of modest complexity, i.e., the reliability of the FPA for more complex systems remains to be tested, it seems to be a promising approach for obtaining pKa values of thiols in a fast and reliable way.