Benzene Radical Ion in Equilibrium with Solvated Electrons

Abstract

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 Keq= 0.22 M-1, but at lower temperatures C6H6•- is favored, Keq reaching 65 M-1 at 218 K. The energetics (ΔH° = −26 kJ/mol; ΔS° = −101 J/deg mol) are remarkable because the same reaction is endoergic by 1.17 eV in the gas phase. For fluorobenzene, Keq =1.7 × 102 M-1 at 298 K and is also strongly temperature-dependent with a similar, large negative entropy change (ΔH° = −49 kJ/mol; ΔS° = −121 J/deg mol). The energetics can be understood by taking into account a substantial (∼1 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 si...