Tautomerism in 2‐hydroxy‐1‐naphthaldehyde schiff bases in solution and the solid state investigated using 13C NMR spectroscopy

Abstract 13C and 1H NMR spectra of six Schiff bases formed from 2‐hydroxy‐1‐naphthaldehyde and substituted anilines were measured and assigned in a variety of solvents. Based on the 13C chemical shifts and 3J(CH,NH), the ratio of the keto‐amine and phenol‐imine tautomers was derived using both 13C chemical shifts and 1H−1H coupling constants, the two methods giving comparable results. The solid‐state 13C NMR spectra of two of these Schiff bases were also measured and interpreted. Based on chemical shift assignments from solution NMR, which showed a relationship between certain shifts and the proportions of the tautomers, it has been shown that these compounds exist essentially as the keto‐amine form in the solid, the tautomer previously shown to be the more thermodynamically stable.