Institut für Organische Chemie, Freie Universität Berlin, D-1000 Berlin 33.
Reversed-micellar solutions, i.e. "water in oil" microemulsions, have proved to be very suitable media for ESR and ENDOR investigations of semiquinone radical anions. [1,2] Advantages are higher stability of radicals as compared to alcoholic solutions and decreased exchange interaction due to encapsulation of single semiquinone molecules in any one reversed micelle. Although the semiquinones reside in an aqueous (hence physiological) medium, the adverse properties of water are avoided, which are a consequence of its strong microwave absorption. Moreover, a particular advantage for ENDOR measurements is the relatively high viscosity of the solutions, yielding good ENDOR enhancements.
The semiquinone radical anions do not float freely within the water pool of the reversed micelles, but are anchored at the water-surfactant interface, probably by hydrophobic interaction of an apolar alkyl side chain with the hydrocarbon chains of the amphiphilic molecules making up the interface. Experimentally, significantly broader lines as compared to purely aqueous or alcoholic solutions are observed. Moreover, different molecular positions are affected to a different extent because the motion of the probe molecules is anisotropic. The M_I dependent line broadening of the ESR spectra was studied in detail by means of computer simulations.
Various substituted p-benzosemiquinone radical anions, inter alia ubisemiquinone and derivatives, have been investigated. Unsymmetrical semiquinones, with respect to the oxygen atoms, experience remarkable hyperfine shifts depending on the medium. This effect even allows differentiation between stereoisomers. In the case of 2-cyclohexyl-3-methyl-1,4-benzosemiquinones, mixtures of two species (conformers) have been observed. Apparently the internal rotation of the cyclohexyl moiety is severely hindered owing to steric interference with the neighbouring methyl group and solvation effects.
 D. Niethammer, B. Kirste and H. Kurreck, J. Chem. Soc. Faraday Trans. 86, 3191 (1990).
 B. Kirste, D. Niethammer, P. Tian and H. Kurreck, Appl. Magn. Reson., in the press.