Kinetic investigations of the laser-induced photolysis of sodium rhodizonate in aqueous solutions

Article abstract of DOI:10.1016/S1386-1425(96)01773-8

The laser-induced photolysis of rhodizonate and tetrahydroxy-p-benzoquinone(4^-) (THQ(4 - )) has been studied with particular consideration of the photoreactions in de-aerated solutions. Both rhodizonate and THQ(4 - ) can be converted to croconate. The quantum yields using rhodizonate as the initial compound are very small ( < 10^-4) and depend on the applied irradiation technique. Compared with that finding, the photooxidation of rhodizonate in aerated solutions exhibits large quantum yields ( ≈ 0.05) which are independent of the excitation wavelength and of the laser power. By increasing the local photon density in the irradiation of de-aerated solutions, a hitherto unknown photoproduct could be detected and was identified as the ring-enlarged species C₇O^2-₇. The seven-membered ring compound was characterized by UV/visible and Raman transitions in combination with theoretical investigations concerning the structure and vibrations of oxocarbons. This new aromatic oxocarbon decays with a half-life of about 20 min. Additionally, the alkaline hydrolysis of rhodizonate was reinvestigated at various temperatures. At room temperature the uniform reaction yielding THQ(4 - ) was quantified by the first order rate constant. At higher temperatures the quinone represents the only intermediate during the formation of croconate. This consecutive reaction was found to be independent of the presence of oxidizing agents. Therefore, the rearrangement mechanism suggested previously has to be revised.