ESR studies of intramolecular electron transfer in malonic acid radical chelates of cerium(IV)

Abstract

The reduction of Ce(IV) complexed to dicarboxymethyl (malonic acid radical; •CH(COOH),) has been directly investigated by kinetic electron spin resonance spectrometry in aqueous perchloric acid media at 298 K. The corresponding first-order rate constant has a value of k_(11) = (1.5 ± 0.3) x10^3 s^(-1). Free (M_1) and Ce(IV)-chelated (M_2) dicarboxymethyl radicals have slightly different magnetic parameters [g(M_1) = 2.0039, a(H_α)(M_1) = 20.3 G, g(M_2) = 2.0035, a(H_α)(M_2) = 19.6 G] and coexist in equilibrium, the ratio r = [M_2]/[M_1] being a function of both [Ce(IV)] and [H^+]. The stability constant of the radical chelate, K_g = 242 ± 31 M^(-1), is considerably larger than that of malonic acid itself, K_5 = 1.14 M^(-1). The product K_gk_(11), which represents the overall rate constant for dicarboxymethyl radical oxidation by Ce(IV), has a value within the range of those measured for the direct oxidations of substituted secondary alkyl radicals by Ce(IV), Co(III), and Ti(IV).

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