COMPARATIVE PULSE-RADIOLYSIS STUDIES OF ALKYL-SUBSTITUTED AND METHOXY-SUBSTITUTED SEMIQUINONES FORMED FROM QUINONES AND HYDROQUINONES

Absorption spectra and rate constants for the disproportionation of 12 alkyl- and methoxy-substituted semiquinone anion free radicals (Q(.-) ) produced by the one-electron reduction (using CO2.- as a reductant) of 1,4-benzoquinones and 1,4-naphthoquinone (Q) as well the oxidation (using N-3(.) as an oxidant) of the corresponding hydroquinones (QH(2) ) were determined by pulse radiolysis in 50 mM sodium phosphate buffer , pH 7.40 at room temperature. Both spectral and kinetic characteristi cs of Q(.-) only moderately depended on whether Q .(-) was produced fr om Q or QH(2). Spectra of benzosemiquinones display two peaks with max imum at 310-320 nm and ca. 430 mm with the ratio of about 2-2.5. Molar absorption coefficients were determined. Rate constants for Q(.-) dis proportionation (2k(1)) were correlated with the nature of substituent s. While 2k(1) was scarcely affected by methyl substitution, Q(.-) con taining isopropyl, tert-butyl and methoxy substituents were visibly mo re stable than non-substituted and methyl-substituted Q(.-).