Determination of the Reaction Rate Constants of Antioxidants with the Hydroxyl Radical by a Rapid-Flow ESR Method

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The reaction rate constants of antioxidants with the hydroxyl radical (HO•) were determined by a rapid-flow ESR method. ESR spectra of the antioxidant radical formed by a reaction with HO• generated from the Ti3+ + H2O2 system were measured. When an antioxidant and ethanol were mixed with HO•, a superposed spectrum of the 1-hydroxyethyl radical and antioxidant radical was obtained. The intensity ratio of the signals of these radicals was calculated from the doubly integrated curve, and then the ratio of the reaction-rate constant of the antioxidant with HO• to that of ethanol was obtained. The ratios of pyrogallol, gallic acid, catechol, phloroglucinol, resorcinol, and methanol were 19, 17, 11, 1.5, 1.2, and 0.56, respectively. The dissociation energies of the bonds in antioxidant molecules were obtained by MO calculations, which demonstrated that phenoxy, hydroxymethyl, and 1-hydroxylethyl radicals were formed by the reactions with HO•, as expected from their spectra. The relationship between the relative activation energies obtained from the rate constants and the bond-dissociation energies showed that the Evans–Polanyi equation holds in polyphenol series, but the line was shifted from that of alcohols. This suggested that the structures of the transition state of alcohols were stabilized by a polar effect.
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