An ESR Investigation of the Photolysis of Benzene Polycarboxylate Anions in Aqueous Solution
The radicals produced by continuous uv photolysis of agueous solutions of the anions of benzene polycarboxylic acids have been detected and identified by means of their ESR spectra. The behavior of terephthalate (1,4-benzenedicarboxylate) under basic conditions (pH 13) is typical. With terephthalate alone, only the lines of the trianion radical are seen. In the presence of 1.3 M 2-propanol, the intensity of these lines is greatly enhanced and joined by those of a new radical, which can be identified from its spectrum to be the cyclohexadienyl radical formed by H addition at a ring carbon atom bearing a carboxyl group. The parameters of this radical (in gauss) are a(CHCO2−)=42.93, a(ortho)=8.15, a(meta)=2.34 G and g factor 2.00305. Addition of acetone further enhances the intensities of the lines of these radicals. The chemical mechanism proposed to account for the radical formation is quenching of the triplet state of terephthalate by the alcohol with competitive H transfers to a carboxylate oxygen and to the substituted ring position. Optical laser photolysis studies support this mechanism in that triplet state guenching by the alcohol is observed along with corresponding formation of the trianion radical . Analogous ESR results were found with many of the other benzene carboxylates up to the hexacarboxylate. (With phthalate only the anion is seen. ) In the two cases where the carboxyl groups are not opposite each other (1,3-benzenedicarboxylate and 1,3,5-benzenetricarboxylate) H addition occurs opposite one of the carboxyl groups. In 1,2,4-benzenetricarboxylate two isomers are seen with addition at either 1 or 4 positions.