Spectroscopic Study and Semi-empirical Calculations of Tautomeric Forms of Schiff Bases Derived from 2-hydroxy- 1-naphthaldehyde and Substituted 2-aminothiophene

New Schiff bases derivative were synthesized by reaction of 2-aminothiophene derivatives and 2-hydroxy-1-naphthaldehyde. The compounds exhibit intra-molecular hydrogen bonding and keto-enamine tautomerism were conformed by using IR and UV-Visible spectral data. The UV-Visible spectra of these compounds have been investigated in different solvents mainly acetonitrile and toulene. The compounds were in tautomeric equilibrium (enol-imine O–H…N, keto-amine O…H–N forms) in polar and nonpolar solvents. The keto-amine form was observed in both solutions of toluene and acetonitrile for some derivative. 1H-NMR and IR results showed that all Schiff bases studied favor the enol-imine form over the keto form in a weakly polar solvent such as deuterochloroform solution. The molecular structures of Schiff bases 4a–d enol and 5a–d keto semi-empirical AM1, PM3, MNDO and ab initio quantum mechanical (RHF/3–21G* and RHF/6–31G* basis set) was calculated in the gas phase. The relative energy of the keto-enol tautomeric equilibrium in the most methods of calculation in the gas phase is shifted toward the ketone form.