Resonance Raman Spectra of Acid-Base Indicators. I. p-Aminoazobenzene Derivatives

Resonance Raman spectra between 1800 and 1000 cm−1 of methyl orange and a few other p-aminoazobenzene-type acid-base indicators have been investigated at various pH, and a marked spectral change has been observed on the transformation between the acid and the base forms of these indicators. The characteristic Raman frequencies have been assigned by referring to the spectra of related compounds and the deuteration effects. The spectra of the acid form are well elucidated on the basis of the quinoid-type structure.