Studies of Tautomerism in 2-Hydroxynaphthaldehyde Schiff Bases by Multinuclear Magnetic Resonance

Abstract

The tautomerism in a Schiff base formed from 2-hydroxynaphthaldehyde and p-methyl aniline has been investigated using multinuclear magnetic resonance techniques. ¹H and ¹³C resonances have been assigned and chemical shifts tabulated. The proportions of the tautomers have been obtained from ¹J_NH both from direct ¹⁵N NMR and from ¹H detected ¹⁵N spectroscopy and from ³J_CH-NH coupling constants and these have been supported by UV spectroscopic measurements. The temperature and solvent dependence of the equilibrium have been investigated and the ketoamine tautomer shown to have a lower free energy than the phenolimine tautomer in CDCI₃ solution by approximately 1 KJ.mol⁻¹ at 273K. ΔH and ΔS have been determined from the temperature dependence of the equilibrium constant. The ¹³C chemical shifts of some of the carbons are very dependent on the proportions of the tautomers and can be used to estimate tautomeric ratios in solvents where no couplings are observed because of exchange processes.