Electron paramagnetic resonance and ENDOR study of semiquinones in reversed micelles

Abstract

A series of substituted p-benzosemiquinone radical anions have been investigated in reversed micelles by EPR and ENDOR spectroscopies. Compared with EPR studies in homogeneous aqueous phases or in alcoholic solutions, small but significant changes in g values and hyperfine splittings were observed. Particularly interesting is the observation of asymmetric line-broadening effects in reversed-micellar solutions. Computer simulations of the EPR spectra demonstrate that different molecular positions make distinctly different contributions to this effect, and the ENDOR spectra exhibit a selective broadening of some of the signal pairs. It is concluded that the semiquinone anion radicals are immobilized at the surfactant–water interface. Models for the preferred location and anisotropic motion are discussed. The effect of bulky alkyl groups such as t-butyl on the spin density distribution in semiquinones in polar protic solvents is rationalized in terms of a hydrophobic effect.