Benzene radical ion in equilibrium with solvated electrons

Radical anions of benzene (C6H6) and its monofluoro and -methyl derivatives are found to exist in equilibrium with solvated electrons in THF solution. At 298 K C6H6.- is less stable than the solvated electron with K-eq= 0.22 M-1, but at lower temperatures C6H6.- is favored, K-eq reaching 65 M-1 at 218 K. The energetics (DeltaHdegrees = -26 kJ/mol; DeltaSdegrees = -101 J/deg mol) are remarkable because the same reaction is endoergic by 1.17 eV in the gas phase. For fluorobenzene, K-eq =1.7 x 10(2) M-1 at 298 K and is also strongly temperature-dependent with a similar, large negative entropy change (DeltaHdegrees = -49 kJ/mol; DeltaSdegrees = - 121 J/deg mol). The energetics can be understood by taking into account a substantial (similar to1 eV) electronic energy of the solvated electron. The large apparent "entropy changes" may include changes that are not entropic, but arise instead from temperature and density dependent enthalpy changes. The optical absorption spectrum Of C6H6.- is similar to that reported at 77 K. As would be expected for an ion of small size, C6H6.- forms strongly bound ion pairs with Na+; the dissociation constant for (C6H6.-,Na+) ion pairs is 4.5 x 10(-10) M. This strong ion pairing stabilizes C6H6.- when a counterion is available, explaining why the ion-pair, but not the free ion of benzene, is readily observed.