A study of the tautomers of N-salicylidene-p-X-aniline compounds in methanol

Abstract

We have studied the enol-imine→keto-amine tautomeric equilibrium of N-salicylidene-p-X-aniline compounds with X = Me, OMe, NMe₂ as electron donor substituents and X = COMe, CN and NO₂ as electron acceptor substituents. The equilibrium constants (K^°_tau) and standard thermodynamic properties ΔG^°_tau, ΔH^°_tau and ΔS^°_tau were measured and calculated in methanol solution at various temperatures, by means of excitation fluorescence spectroscopy. We have analyzed the p-phenylaniline substitution effect on K^°_tau and the thermodynamic properties through the Hammett parameters σ. We have performed molecular orbital calculations at the semiempirical AM1 and ab initio HF/3-21G levels to interpret the experimental results, explicitly including the solute–solvent interaction through the formation of an intermolecular hydrogen bond between the salicylidene and methanol molecules. These computational results show a good correlation with the experimental values. An interpretation of the experimental values of TΔS°, based on changes in the molecular structure produced in the enol-imine→keto-amine tautomeric reaction, is proposed.