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

We have studied the enol-imine-->keto-amine tautomeric equilibrium of N-sal icylidene-p-X-aniline compounds with X=Me, OMe, NMe2 as electron donor subs tituents and X=COMe, CN and NO2 as electron acceptor substituents. The equi librium constants (K-tau(o)) and standard thermodynamic properties DeltaG(t au)(o), DeltaH(tau)(o) and DeltaS(tau)(o) were measured and calculated in m ethanol solution at various temperatures, by means of excitation fluorescen ce spectroscopy. We have analyzed the p-phenylaniline substitution effect o n K-tau(o) and the thermodynamic properties through the Hammett parameters sigma. We have performed molecular orbital calculations at the semiempirica l AM1 and ab initio HF/3-21G levels to interpret the experimental results, explicitly including the solute-solvent interaction through the formation o f an intermolecular hydrogen bond between the salicylidene and methanol mol ecules. These computational results show a good correlation with the experi mental values. An interpretation of the experimental values of T DeltaS(o), based on changes in the molecular structure produced in the enol-imine-->k eto-amine tautomeric reaction, is proposed.