Comparative pulse radiolysis studies of alkyl- and methoxy-substituted semiquinones formed from quinones and hydroquinones

Abstract

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 N₃ as an oxidant) of the corresponding hydroquinones (QH₂) were determined by pulse radiolysis in 50 mM sodium phosphate buffer, pH 7.40 at room temperature. Both spectral and kinetic characteristics of Q⁻ only moderately depended on whether Q⁻ was produced from Q or QH₂. Spectra of benzosemiquinones display two peaks with maximum at 310–320 nm and ca. 430 nm with the ratio of about 2–2.5. Molar absorption coefficients were determined. Rate constants for Q⁻ disproportionation (2k₁) were correlated with the nature of substituents. While 2k₁ was scarcely affected by methyl substitution, Q⁻ containing isopropyl, tert-butyl and methoxy substituents were visibly more stable than non-substituted and methyl-substituted Q⁻.