Accurate gas phase acidities of carboxylic acids estimated by scaling the vibrational contribution of ab initio Gibbs free energies.

The gas phase Gibbs free energies deltaG(T) of dissociation reaction of 14 carboxylic acids were calculated on the SCF, as well as G3 and CBS-Q levels. Corresponding accuracies were critically compared with experimental data. Since all of the results suffer from systematic errors, the procedure of scaling of thermal contribution to Gibbs free energy was applied for minimizing differences between theoretical and experimental values of deltaG(T). Two parameters were adjusted, namely the scaling of thermal contribution to Gibbs free energy of neutral and anionic forms. The presented results suggest the great effectiveness of such a procedure since for all applied basis sets within the SCF framework the achieved accuracy was below the experimental error. Besides, the proposed low-cost approximation method leads to precision comparable to or even exceeding the quality offered by more sophisticated composite quantum chemistry methods. The extension of the set of training molecules up to 82 has an insignificant impact on the overall quality of deltaG(T) estimation, which suggests that a wisely chosen set of reference data may be used for the characteristics of the whole class of compounds. There is a straightforward way for the analysis of acidities/basicities of other classes of chemicals such as DNA bases, alcohols, phenols, amines, amino acids, etc.