Influence of specific interaction effects on the proton-transfer equilibrium in intermolecular hydrogen bonds of carboxylic acids and phenols with amines

Abstract

Specific effects of solvents on the shift of proton-transfer equilibria AH⋯B ⇌ A^–⋯H⁺B in carboxylic acid and phenol–N-base hydrogen bonds have been studied as a function of the solvent, using IR spectroscopy. Specific effects due to CH acidity of the solvents dominate the carboxylic acid systems compared with the non-specific reaction field effect. This fact is also true for solvents with small CH acidities. With increasing CH acidity of the solvent the CO₂H⋯N ⇌ CO^– ₂⋯H⁺N equilibrium is shifted to the right as shown by an increase in the intensity of ν_as(CO^– ₂). In phenol–N-base systems the OH⋯N ⇌ O^–⋯H⁺N equilibria are also shifted to the right as a result of specific interactions of slightly acidic CH groups of the solvents with the O atom and/or the π-electron system of the phenols. In the phenol–N-base systems the size of the shift of the OH⋯N ⇌ O^–⋯H⁺N equilibria to the right by this specific effect is comparable to the size of the reaction field effect (non-specific interaction).