Ionization of Organic Acids in Dimethyl Sulfoxide Solution:  A Theoretical Ab Initio Calculation of the pKa Using a New Parametrization of the Polarizable Continuum Model

The pKa values of over 41 organic acids in dimethyl sulfoxide (DMSO) solution were calculated using ab initio electronic structure methods at MP2 and MP4 levels of electron correlation and including basis set of 6-31+G(d) and 6-311+G(2df,2p) quality. The solvation was included through the polarizable continuum model (PCM), using the recent parametrization of Pliego and Riveros. The root-mean-square (RMS) error over this set of molecules having different functional groups is only 2.2 units. A linear fit on this data set decreases this error by only 0.2 units, indicating that this empirical correction is not necessary. The major error in the calculated pKa value was −5.3 units for the CH3SO3H solute. Halogenated carboxylic acids have also presented a high deviation (∼4 units). An explanation for these high deviations is the possibility of strong hydrogen-bond formation involving the neutral acid molecule and DMSO. The pKa values were also calculated using a combination of theoretical solvation data with experimental gas-phase data. In this case, the RMS error increased to 2.3 units for a set of 36 acids. Our results show that the performance of the PCM model with a fixed atomic radius in DMSO solution is very superior to its performance in aqueous solution, which is a behavior that can be attributed to the presence of strong and specific solute−solvent interactions of ionic solutes with water molecules. In addition, no extensive parametrization of the PCM model is needed to describe the solvation of anions in DMSO solution.