Nitrogen and carbon CPMAS NMR investigations of keto–enol tautomerism in asymmetric o-hydroxy Schiff bases

The five Schiff bases obtained by condensation of dehydroacetic acid, p-phenylenediamine and derivatives of salicylaldehyde were investigated by 13C and 15N CPMAS NMR methods to find the structure of intramolecular hydrogen bridges. Additionally the 15N NMR spectra in CDCl3 were done. The results obtained in the solid state and in solution were compared with the X-ray previously published for some of investigated compounds. The relatively small influence of substituent in salicylaldehyde unit on proton position was found as well as only small difference in the hydrogen bridges structure in both phases, solution and solid state, which is in contrast with results acquired for Schiff bases obtained from simple aliphatic amines.

► We synthesized five o-hydroxy Schiff bases from dehydroacetic acid.
► We performed combined solution and solid state 13C and 15N NMR spectra.
► The positions of tautomeric equilibria of Schiff bases were defined.
► The comparison of X-ray and NMR data has been done.