13 C NMR spectroscopic and AM1 study of the intramolecular proton transfer in anils of salicylaldehyde and 2-hydroxynaphthalene-1-carbaldehyde

Tautomeric equilibria in intramolecularly hydrogen bonded Schiff bases is studied on the basis of solution 13C NMR chemical shifts. Variable temperature NMR spectroscopic data and comparison with appropriate models show that while anils of salicaldehyde exist as phenolic tautomers, those of 2-hydroxynaphthalene-1-carbaldehyde exist as equilibrium mixtures containing appreciable amounts of both enol and keto tautomers, the latter being slightly predominant. AM1 results concerning the relative stability of tautomers are in good agreement with the NMR spectroscopic information.