Absolute pKa determination for carboxylic acids using density functional theory and the polarizable continuum model

The absolute pKas of eight carboxylic acids have been computed using a Born–Haber cycle, the most recent experimental value for the proton solvation energy and completely ab initio structures, energies, and harmonic frequencies of acids and conjugated bases both in vacuo and in aqueous solution. The proton affinities computed by the PBE0 hybrid functional are in very good agreement with experimental values and with the results of the most sophisticate (and expensive) quantum mechanical models (G2, G3, and CBS). The Gibbs energies in aqueous solution have been computed by our last version of the polarizable continuum model, which takes into the proper account escaped charge effects in a very effective computational implementation. The encouraging results obtained for this training set and the linear scaling both of the electronic and solvation models pave the route for the evaluation of reliable pK values for the large systems of interest in biological or material science frameworks.