NMR study of proton transfer equilibrium in Schiff bases derived from 2-hydroxy-1-naphthaldehyde and 1-hydroxy-2-acetonaphthone.: Deuterium isotope effects on 13C and 15N chemical shifts

The proton transfer equilibrium in series of Schiff bases derived from 2-hydroxy-1-naphthaldehyde and 1-hydroxy-2-acetonaphthone were measured by means of H-1, C-13 and N-15 NMR spectra and deuterium isotope effects on C-13 and N-15 chemical shifts at variable temperature and in different solvents. The latter made it possible to investigate a broad range of equilibrium positions. All compounds exist mainly as the NH tautomer over the full range of temperatures in CDCl3 solution. The position of the equilibrium was estimated using (1)J(N-15,H-1) and (3)J((NH)-N-15,H-1) coupling constants. The correlation between deuterium isotope effects (n)Delta C-13-2(XD) or (n)Delta N-15(XD), where X = O or N, and the mole fraction of the NH form show the characteristic, non-monotonic function as for other intramolecular hydrogen bonded systems. The position of the minimum depends on substituents and the maximal negative values of (n)Delta C-13-2(XD) varies with substituents and solvents. Copyright (C) 2001 John Wiley & Sons, Ltd.