Photoelectrochemical study of the radical anion cleavage rates in aromatic molecules: Halobenzoic acids

The cleavage rates of transient aromatic radical anions of halobenzoic acids (k_c=10³-10⁹ s^-1) have been measured in water using a photoelectrochemical method based on photoelectron injection from a metal into an electrolyte solution. Values of k_c have been determined from the dependence of the photoinduced cathode charge on the concentration of substances studied. Results indicate that k_c depends on the nature of the halogen and its position in the ring relative to the carboxylate group, increasing in the series F<Cl<Br and meta<para<ortho (Cl, Br) and meta<ortho<para (F). The structure of the transient state and the factors determining the cleavage rates of the radical anions are discussed.