Strong environmental effects upon .DELTA.G.degree. of electron transfer between an anion radical and its perdeuterated analog

The free energy change for electron transfer from the anion radical of nitrobenzene to perdeuteriated nitrobenzene (PhNO{sub 2}{sup {sm bullet}{minus}} + PhNO{sub 2}-d{sub 5} = PhNO{sub 2} + PhNO{sub 2}-d{sub 5}{sup {sm bullet}{minus}}) has been determined via solution-phase EPR measurements and physical separation of the neutral nitrobenzenes from the solid anion-radical salts. This free energy is largest in hexamethyl-phosphoramide ({Delta}G{degrees} = 1,400 J/mol) where the anion radical exists free of ion association and hydrogen bonding. In liquid ammonia, where the anion radicals are strongly hydrogen bonded through the NO{sub 2} group, this free energy is only 389 J/mol. The addition of extra sodium cations to the ammonia solution results in a further decrease in {Delta}G{degrees} to 243 J/mol where it is of the same magnitude as in the solid-state anion radical where the interaction between the cation and anion radical is very strong. Ion association and hydrogen bonding pull spin and charge density into the NO{sub 2} group and away from the phenyl ring where isotopic substitution takes place, resulting in a smaller equilibrium isotope effect. In HMPA the isotope effect is even larger than in the gas phase due to stronger solvation of the phenyl moiety than of themore » NO{sub 2} group in this solvent.« less