Oxidation of spin-traps by chlorine dioxide (ClO 2) radical in aqueous solutions: First ESR evidence of formation of new nitroxide radicals

Abstract

The reactivities of the chlorine dioxide (ClO 2), which is a stable free radical towards some watersoluble spin-traps were investigated in aqueous solutions by an electron spin resonance (ESR) spectroscopy. The C10 2 radical was generated from the redox reaction of Ti 3+ with potassium chlorate (KC10 3) in aqueous solutions. When one of the spin-traps, 5,5-dimethyl-1-pyrroline N-oxide (DMPO), was included in the Ti 3+-KC10 3 reaction system, ESR spectrum due to the ClO 2 radical completely disappeared and a new ESR spectrum [a N(1) = 0.72 mT, a H(2) = 0.41 mT], which is different from that of DMPO-C1O 2 adduct, was observed. The ESR parameters of this new ESR signal was identical to those of 5,5-dimethylpyrrolidone-(2)-oxyl-(1) (DMPOX), suggesting the radical species giving the new ESR spectrum is assignable to DMPOX. The similar ESR spectrum consisting of a triplet [a N(1) = 0.69mT] was observed when the derivative of DMPO, 3,3,5,5-tetramethyl-l-pyrroline N-oxide (M 4PO) was included in the Ti 3+-KClO 3 reaction system. This radical species is attributed to the oxidation product of M 4PO, 3,3,5,5-tetramethylpyrrolidone-(2)-oxyl-(1) (M 4POX). When another nitrone spin-trap, α-(4-pyridyl-l-oxide)- N-t-butylnitrone (POBN) was used as a spin-trap, the ESR signal intensity due to the C10 2 radical decreased and a new ESR signal consisting of a triplet [a N(1) = 0.76 mT] was observed. The similar ESR spectrum was observed when N-t-butyl-α-nitrone (PBN) was used as a spin-trap. This ESR parameter [a N(1) = 0.85 mT] was identical to the oxidation product of PBN, PBNX. Thus, the new ESR signal observed from POBN may be assigned to the oxidation product of POBN, POBNX. These results suggest that the ClO 2 radical does not form the stable spin adducts with nitrone spin-traps, but oxidizes these spin-traps to give the corresponding nitroxyl radicals. On the other hand, nitroso spin-traps, 5,5-dibromo-4-nitrosobenzenesulfonate (DBNBS), and 2-methyl-2-nitrosopropane (MNP) did not trap the ClO 2 radical. This result indicates that an unpaired electron of the ClO 2 radical is localized on oxygen atom, because nitroso spin-traps cannot form the stable spin adduct with oxygen-centered radical.