Intramolecular hydrogen bond investigations in Schiff bases derivatives of 2-hydroxy-1-naphthaldehyde and 2-hydroxy-1-acetonaphthone in CDCl 3 solution and in the solid state by NMR methods

Abstract

Three Schiff bases: 1-( N-methyl-iminomethyl)-2-naphthol ( I), 1-( N-isopropyl-iminomethyl)-2-naphthol ( II) and 1-( N-isopropyl-α-iminoethyl)-2-naphthol ( III) have been investigated by 15N and 13C NMR spectroscopy in order to study the tautomeric equilibrium in both the solid and liquid phases. For two derivatives of 2-hydroxy-1-naphthaldehyde ( I and II), the NH tautomers have been found to be dominant in both the phases. The Schiff base derived from 2-hydroxy-1-acetonaphthone ( III) exists in the CDCl 3 solution mainly in the NH form but in the solid state only the OH tautomer has been detected. The steric effect of the methyl group at the imine carbon atom seems to be responsible for such exceptional behaviour of the Schiff base investigated. Additionally, the principal components of 15N chemical shift (CS) tensor of the compound III and two model compounds (5-methoxy-salicylidenmethylamine ( IV) and 4-methoxy-salicylidenemethylamine ( V)) have been measured to confirm the electronic structure of III.