Thermodynamics of Protonation and Prototropic Equilibria in Simple para- and Di-meta-Substituted Phenols

The systematic ab initio G4 study of para- and di-meta-substituted phenol derivatives is presented for the gas phase and the model water environment. The thermodynamics of proton addition was considered based on calculated proton affinities for unprotonated and monoprotonated compounds. The effect of substitution on the enthalpy balance shows a linear dependence with Hammett constants for O-protonation of the neutral compounds where the aromatic character of the molecules is preserved. Depending on the substitution and the extent of protonation, O-protonation proves to be thermodynamically less favorable than C-protonation in the monoprotonated species. It was demonstrated that the highest exothermic character of protonation was found for the gas-phase and neutral keto tautomers forming under flash photolysis that have the best ability to accept a proton. The theoretical reaction Gibbs free energies were used for the quantification of prototropic tautomerism between C-protonated species and enol-keto equilibria occurring in the suggested reaction scheme. The obtained results indicate a significant change in the tautomeric equilibria between the neutral and protonated compounds. Presented theoretical calculations could be helpful in the identification of intermediates occurring in various chemical reactions of phenols in strong acidic environments.