Aidas K.,Vsl Aukstieji Algoritmai |
Aidas K.,Vilnius University |
Lanevskij K.,Vsl Aukstieji Algoritmai |
Kubilius R.,Vsl Aukstieji Algoritmai |
And 4 more authors.
Journal of Computational Chemistry | Year: 2015
Aqueous pKa of selected primary benzenesulfonamides are predicted in a systematic manner using density functional theory methods and the SMD solvent model together with direct and proton exchange thermodynamic cycles. Some test calculations were also performed using high-level composite CBS-QB3 approach. The direct scheme generally does not yield a satisfactory agreement between calculated and measured acidities due to a severe overestimation of the Gibbs free energy changes of the gas-phase deprotonation reaction by the used exchange-correlation functionals. The relative pKa values calculated using proton exchange method compare to experimental data very well in both qualitative and quantitative terms, with a mean absolute error of about 0.4 pKa units. To achieve this accuracy, we find it mandatory to perform geometry optimization of the neutral and anionic species in the gas and solution phases separately, because different conformations are stabilized in these two cases. We have attempted to evaluate the effect of the conformer-averaged free energies in the pKa predictions, and the general conclusion is that this procedure is highly too costly as compared with the very small improvement we have gained. © 2015 Wiley Periodicals, Inc.