USE OF THE HAMMETT CORRELATION AND SIGMA+ FOR CALCULATION OF ONE-ELECTRON REDOX POTENTIALS OF ANTIOXIDANTS

Spectral, acid-base, and redox properties of methoxyphenoxyl radicals were determined in aqueous solutions by pulse radiolysis. Except for 2,6-dimethoxyphenoxyl, with lambda(max) < 300 nm, methoxyphenoxyl radicals absorb in the visible spectrum, with high molar absorptivities. From the effect of ionic strength on the rate of radical decay, a pK(a) < 2 was estimated for the deprotonation of phenolic radical cations. The oxidation potentials of methoxyphenols were determined from the electron-transfer equilibria with inorganic redox standards. The largest effect of methoxy groups on oxidation potential was observed for 2,6-dimethoxyphenol, E7 = 0.58 V vs NHE, whereas the effect of meta substitution was minimal for 3,5-dimethoxyphenol, E7 = 0.85 V, yet this was 0.12 V lower than the measured E7 of 0.97 V for unsubstituted phenol. The reduction potential E7 of methoxyphenoxyl radicals satisfies the Hammett correlation using Brown sigma+ substituent constants, as do other monosubstituted phenoxyls if the additivity of sigma+ values is assumed and the state of protonation of the substituents is taken into consideration. The derived sigma+(CH3O)m value of -0.14, in contrast to reported positive values, indicates the contribution of the methoxy group, even in the meta position, to charge delocalization and resonant stabilization of the phenoxyl radical. The contribution of the ring structure to more negative sigma+ values was observed, as expected. Use of the Hammett correlation is suggested for rapid estimation and accurate calculation of the redox potentials of polysubstituted phenolic antioxidants.