Dependence of tautomerism on substituent type in o-hydroxy Schiff bases

Abstract

Quantum computational methods were used to elucidate the structures of the o-hydroxy Schiff bases with different substituents. It is possible for a Schiff base to have different tautomeric structures depending on intramolecular proton transfer from the phenolic oxygen atom to the nitrogen atom. Proton transfer results in two tautomeric structures known as the phenol-imine and keto-amine forms. To explain the substituent effect on the proton transfer process in five o-hydroxy-Schiff bases, possible geometric structures in gas phase were optimized using density functional theory (DFT) at the B3LYP/6-311G(d,p) level. To describe tautomerism including intramolecular proton transfer, potential energy surface (PES) scans were performed starting from the optimized geometry of the phenol-imine form. HOMA indices were calculated in order to estimate p-electron delocalization. In addition, the substituent effect on the tautomerization rate was examined using Hammett substituent constants and calculating the activation energies.

Keywords

• schiff base
• tautomerism
• phenol-imine
• keto-amine
• dft
• pes