Determination of the ionic charge of semiquinones by pulsed conductivity and ESR techniques

Microsecond pulsed conductivity and ESR techniques have been used for unambiguous determination of the ionic charge on semiquinones derived from one-electron reduction of hydroxy-quinones such as juglone, naphthazarin, sodium quinizarin-2-sulphonate and adriamycin. Irrespective of the presence of other charges elsewhere in the molecule, the radical centres of all the semiquinones have been shown to be mono-anionic at around neutral pH. The importance and significance of strong intramolecular H- bonding have been highlighted. Results on one-electron oxidation of naphthazarin have also been briefly discussed. (author)