Azo—hydrazone tautomerism in azo dyes. II. Raman, NMR and mass spectrometric investigations of 1-phenylazo-2-naphthylamine and 1-phenylazo-2-naphthol derivatives

Abstract

In a comparative study 1-phenylazo-2-naphtylamines, 1(R) , and 1-phenylazo-2-naphthols, 2(R) , with R = MeO, CN and NO 2 have been investigated by means Raman, NMR and mass spectroscopy in order to elucidate their tautomeric structures. Based on Raman spectra obtained under resonance conditions, the trans azo structure has been inferred for 1(R) with all the substituents under scrutiny. From the naphthol derivatives the hydrazone predominance has been confirmed for 2(CN) and 2(NO 2 ) in agreement with previous reports using different techniques. No useful Raman spectrum could be obtained, however, for 2(MeO) due to the inherent fluorescence of this dye. Here nuclear magnetic resonance proved to be helpful, yielding a K T of 0·7 from both 13C and 1H chemical shift arguments. For the other five dyes the Raman results have been corroborated by the measurement of carbon and proton chemical shifts [δ(C-2), δ(H-3) and δ(H-8)] and the coupling coupling constants J(H-3, H-4). In the mass spectrometer all compounds behaved similarly irrespective of the tautomeric structures in solution or in the solid state with an 'azo-type' fragmentation pattern originating from a primary CN cleavage of the molecular ion. A theoretical discussion is provided to show that this decomposition pathway for the hydrazones is not contradictory to the results obtained above. An initial CN cleavage instead of the generally assumed NN cleavage of the hydrazone molecular ion can occur in certain cases, particularly with electron withdrawing R, thus leading to the observed azo type decomposition. From the present results it became obvious that mass spectrometry is not a generally applicable tool for azo-hydrazone structure discrimination.