Assessment of pKa Determination for Monocarboxylic Acids with an Accurate Theoretical Composite Method: G4CEP

For 22 monoprotic acids, pKa values were calculated using the G4CEP composite theory. The solvation effect was included considering the continuous SMD solvation model; SMD and one explicit water molecule; and SMD, one water molecule, and linear correction with respect to the experimental pKa values. The three tests provided mean absolute errors equal to 0.83, 0.51, and 0.30 pKa units, respectively, indicating excellent performance of the G4CEP method. Comparison with density functional theory at the B3LYP and BMK levels showed that these results are quickly obtained but with a significant error. The best performance of the functionals was obtained from the combination of SMD, one explicit water molecule, linear regression correction, and basis set including diffuse functions. However, the dispersion of the results with DFT can lead to deviations of up to two pKa units, whereas for G4CEP the largest deviations seldom exceed one pKa unit.