Diarylthallium hydroxides react very smoothly in organic solvents with hydroquinones to stable paramagnetic complexes. This reaction has been investigated with two ligands and several organometallic compounds in 8 different solvents. In basic solutions clearly two paramagnetic species can be detected by ESR spectroscopy. One component exhibits a large doublet splitting assigned to the thallium counterion and the second radical does not show any metal coupling. The relative concentrations of both radicals depend strongly on the solvent and the temperature. Therefore, a slow equilibrium betw een a contact ion pair and a solvent separated ion pair is assumed. The thermodynamic parameters are determined to ⊿H = 0.8 kcal/mol, ⊿G293 = -0.1 kcal/mol, ⊿S = 3.04 cl/mol. The proton and thallium coupling constants as well as the g-value of the contact ion pair show a rem arkable temperature dependence. The signs of proton splitting constants are determined by ENDOR-TRIPLE investigations. The change of the electronic structure of the radical anion by formation of a contact ion pair is discussed. For interpretation of these results we assumed a fast equilibrium between different conformations of the contact ion pair according to behaviour of comparable aroxyl radicals. Therefore, the systems investigated allow a simultaneous observation of solvent separated and different contact ion pairs.
