Alison M. Funston, Sergei V. Lymar, Barbara Saunders-Price, Gidon Czapski, and John R. Miller

Abstract:

Aryl radical anions created in liquid alcohols decay on the microsecond time scale by transfer of protons from the solvent.^1,2 This paper reports a 4.5 decade range of rate constants for proton transfer from a single weak acid, ethanol, to a series of unsubstituted aryl radical anions, Ar^•-. The rate constants correlate with free energy change, ΔG^o, despite wide variations in the two factors that contribute to ΔG^o: (a) the reduction potentials of the aryls and (b) the Ar-H^• bond strengths in the product radicals. For aryl radical anions containing CH₂OH substituents, such as 2,2'-biphenyldimethanol^•- which is protonated with a rate constant of 3 × 10⁹ s^-1, the faster rates do not fit well in the free energy correlation, suggesting a change in mechanism.