Spectroscopic and theoretical studies of the protonation of N-(5-nitrosalicylidene)-ethylamine

The structures of the main tautomers of the Schiff base of salicylaldehyde with ethylamine and its protonated species have been studied by ESI MS, FT-IR as well as 1H, 13C and 15N NMR methods. The spectroscopic methods have provided clear evidence that the Schiff base exists in acetonitrile solution in a low barrier O⋯H⋯N imine–oxo tautomeric form. This form is also energetically favourable as indicated by PM5 calculations. This untypical Schiff base form is stabilized by a resonance effect with the nitro group and the interaction of the intramolecular hydrogen-bonded proton with acetonitrile via a intermolecular hydrogen bond. The protonated Schiff base exists in solution as imine–hydroxy form stabilized by intermolecular hydrogen bonds with nitro groups and solvent molecules.