Resonance raman spectra and structure of p-aminoazobenzene dyes in diprotonated form

Resonance Raman spectra of methyl orange and its three ring deuterated and three ¹⁵N-azo derivatives have been measured in solutions in concentrated sulfuric acid and sulfuric acid-d₂. From the isotope shifts of the observed Raman bands, methylorange is confirmed to take the dicationic azo form protonated at the dimethylamino and the α-azo nitrogen atoms. The resonance Raman spectrum of tropaeolin OO is entirely different from that of methyl orange in concentrated sulfuric acid and is well elucidated on the basis of the protonated hydrazone-form, -NH-N⁺H =.