Correlations between the Basicity and Proton Affinity of Substituted Anilines

The enthalpies and free energies of gas-phase proton affinity (PA) of aniline and 62 derivatives with different electron donor and electron acceptor substituents in the aromatic ring and at the nitrogen atom were calculated by the PM3 technique. Linear correlations between the shapes of pK_a and PA have been found. The deviation of data from the above dependences for ortho-substituted anilines are discussed in the light of possible hydrophobic hydration of molecular fragments near the protonation reaction center.