Resonance Raman spectroscopic studies of 2-(4′-hydroxyphenylazo)-benzoic acid and some substituted analogs—I. pH effect on spectra

Abstract

The resonance Raman spectrum of 2-(4′-hydroxyphenylazo)-benzoic acid (HABA) is pH dependent; the spectral changes are related to the formation of protonated and deprotonated ionic species of the dye molecule. In addition a tautomeric equilibrium process between azo and hydrazone forms of HABA in neutral aqueous solution has been demonstrated using different excitating laser lines. In alkaline, neutral and weakly acid media HABA exists mainly in the azo form. Resonance Raman spectra between 1000 and 1700 cm −1 of six HABA analogs have been recorded at alkaline, neutral and acid pH. A comparison of these spectra allow us to assign most of the important bands, and to establish some intramolecular coupling effects.