In this work, calculations of pK a values have been performed on benzoic acid and its para-substituted derivatives and some drugs by using Gaussian 98 software package. Gas-phase energies were calculated with HF/6-31 G** and B3LYP/6-31 G** levels of theory. Free energies of solvation have been computed using the polarizable continuum model (PCM), conductor-like PCM (CPCM), and the integral equation formalism-PCM at the same levels which have been used for geometry determination in the gas-phase. The results that show the calculated pK a values using the B3LYP are better than those using the corresponding HF. In comparison to the other models, the results obtained indicate that the PCM model is a suitable solvation model for calculating pK a values. For the investigated compounds, a good agreement between the experimental and the calculated pK a values was also observed.

In this work, calculations of pK b values have been performed for aniline and its substituted derivatives and sulfonamide drugs by using Gaussian 98 software package. Gas-phase energies were calculated with HF/6-31 G** and B3LYP/6-31 G** levels of theory. Free energies of solvation have been computed using the polarizable continuum model (PCM), conductor-like polarizable continuum model (CPCM) and the integral equation formalism-polarizable continuum model (IEFPCM) at the same levels which have been used for geometry determination in the gas-phase. The results show that the calculated pK b values using the B3LYP/6-31 G** are better than those using the corresponding HF/6-31 G**. At first, the correlation equation was found to determine the pK b values of the investigated anilines. Then, this correlation equation was used to calculate the pK b values of the sulfonamide drugs. The results obtained indicate that the PCM model is a suitable solvation model for calculating pK b values in comparison to the other solvation models. For the investigated compounds a good agreement between the experimental and the calculated pK b values was also observed.

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