Theoretical calculation of pKas of phosphoric (V) acid in the polarisable continuum and cluster-continuum models

The accuracy of the polarisable continuum model (PCM) and the mixed cluster-continuum model in the prediction of the absolute values of the three consecutive pKas of phosphoric (V) acid has been checked. PCM calculations at the MP2/6-31+G(d,p) level reproduce the first pKa with an acceptable error. However, they fail significantly for the next two pKas and increasing the level of theory to G3B3 compound method does not provide any improvement. On the other hand, cluster-continuum calculations at the same MP2 level adequately predict all three dissociation constants of H3PO4. The number of necessary solvating water molecules depends on the polarising power of the anion and increases with the charge of the phosphate group. The obtained results indicate the validity of the cluster-continuum approach for the prediction of reliable pKa values of polyprotic inorganic acids.