KINETIC INVESTIGATIONS OF THE LASER-INDUCED PHOTOLYSIS OF SODIUM RHODIZONATE IN AQUEOUS-SOLUTIONS

The laser-induced photolysis of rhodizonate and tetrahydroxy-p-benzoqu inone(4(-)) (THQ(4(-))) has been studied with particular consideration of the photoreactions in de-aerated solutions. Both rhodizonate and T HQ(4(-)) can be converted to croconate. The quantum yields using rhodi zonate as the initial compound are very small (<10(-4)) and depend on the applied irradiation technique. Compared with that finding, the pho tooxidation of rhodizonate in aerated solutions exhibits large quantum yields (approximate to 0.05) which are independent of the excitation wavelength and of the laser power. By increasing the local photon dens ity in the irradiation of de-aerated solutions, a hitherto unknown pho toproduct could be detected and was identified as the ring-enlarged sp ecies C7O72-. The seven-membered ring compound was characterized by UV /visible and Raman transitions in combination with theoretical investi gations concerning the structure and vibrations of oxocarbons. This ne w aromatic oxocarbon decays with a half-life of about 20 min. Addition ally, the alkaline hydrolysis of rhodizonate was reinvestigated at var ious temperatures. At room temperature the uniform reaction yielding T HQ(4-) was quantified by the first order rate constant. At higher temp eratures the quinone represents the only intermediate during the forma tion of croconate. This consecutive reaction was found to be independe nt of the presence of oxidizing agents. Therefore, the rearrangement m echanism suggested previously has to be revised. (C) 1997 Elsevier Sci ence B.V.