Theoretical and experimental studies of the zwitterion reversible arrow neutral form equilibrium of ampholytes in pure solvents and mixtures

The equilibrium ratios for the zwitterionic and neutral forms of the nicotinic and isonicotinic acids (3-and 4-COOH pyridines) have been determined by spectrophotometry in mixtures of water with methanol, tetrahydrofuran and dioxane. The zwitterionic form of the solute is the prevalent structure in mixtures with at least 60% water. In pure organic solvents, the neutral form was found in almost 100%. Ab initio calculations at the MP2/6-311++G**//MP2/6-31G* level predict that the zwitterion is higher in free energy by 33-35 kcal/mol than the neutral tautomer at T = 298 K and p = 1 atm in the gas phase. In contrast, relative solvation free energies, as calculated by the free energy perturbation method in Monte Carlo simulations, are -36, -27, and -15 kcal/mol in water, methanol, and tetrahydrofuran, respectively. The solvent effects for overbalancing the relative internal free energy of the zwitterion and stabilizing this tautomer are large enough only in water and in aqueous solution with water fraction above 60%, Predominance of the neutral form in pure organic solvents is due to considerably lower internal energy of this structure as compared to that of the zwitterion.