| Abstract
| - The pKa's of 17 species from −10 to 50 were calculated using the ab initio MP2/6-311+G(2df,2p) level oftheory and inclusion of solvent effects by the cluster−continuum model, a hybrid approach that combinesgas-phase clustering by explicit solvent molecules and solvation of the cluster by the dielectric continuum. Inaddition, the pure continuum methods SM5.42R and PCM were also used for comparison purposes. Speciessuch as alcohols, carboxylic acids, phenol, acetaldehyde and its hydrate, thiols, hydrochloric acid, amines,and ethane were included. Our results show that the cluster−continuum model yields much better agreementwith experiment than do the above-mentioned pure continuum methods, with a rms error of 2.2 pKa units asopposed to 7 pKa units for the SM5.42R and PCM methods. The good performance of the cluster−continuummodel can be attributed to the introduction of strong and specific solute−solvent interactions with the moleculesin the first solvation shell of ions. This feature decreases the dielectric continuum contribution to the differencein the solvation free energy between ions, making the method less susceptible to error because of the continuumcontribution to solvation. Because the method is not based on extensive parametrizations and it is shown tofare well for several functional groups, the present results suggest that this method could be used as a generalapproach for predicting reliable pKa values.
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