Prediction of the pKa of carboxylic acids using the ab initio continuum-solvation model PCM-UAHF

Experimental pK(a) data for 16 aliphatic carboxylic acids are compared with calculated proton-transfer energies in the gas phase and in aqueous solution. The calculations are performed at the SCF and MP2 levels with inclusion of SCF-level entropic and thermochemical corrections to yield free energies of dissociation, using the basis sets 6-31G**, 6-31+G**, 6-311G(2d,2p), and 6-311+G(2d,2p) and the recently parametrized continuum-solvation method PCM-UAHF for the solvation contribution. Relative pK(a) trends are reproduced well with correlation coefficients (adjusted for degrees of freedom) of up to 0.97 and standard errors down to 0.24 log units, while the computational accuracy is not sufficient for predicting absolute proton-transfer energies. The latter is mainly caused by deficiencies of the underlying gas-phase calculations, as is demonstrated by a separate analysis of the gas-phase and solution-phase contributions to pK(a).