Spectres Raman de Résonance de Dérivés Substitués de l'Aminoazobenzéne en Solution Acide

The resonance Raman spectra of aminoazobenzene derivatives in acidic solution are investigated. The marked spectral changes observed on decreasing pH are related to transformations between azo and quinoid forms. The pKa value of 4‐amidino, 4′‐diméthylaminoazobenzene obtained from intensity measurements is in good agreement with the value obtained from visible absorption measurements. Comparison of resonance Raman spectra of some substituted aminoazobenzene derivatives and the quinoid form of an hydroxyazobenzene derivative enable a more complete assignment of the bands observed between 900 and 1700 cm−1 to be made. All the spectra are rather similar and the various ring substituents are not resonance Raman active. The influence of the NHN  vibration on resonance Raman spectra of the quinoid forms is demonstrated using deuterium substitution. In strong acid solution a second protonation occurs; a tautomeric equilibrium between azo and quinoid forms of aminoazobenzene derivatives is demonstrated using different exciting laser lines.