Surprisingly long-lived ascorbyl radicals in acetonitrile: Concerted proton-electron transfer reactions and thermochemistry

Proton-coupled electron transfer (PCET) reactions and thermochemistry of 5,6-isopropylidene ascorbate (iAscH(-)) have been examined in acetonitrile solvent. iAscH(-) is oxidized by 2,4,6-tBu(3)C(6)H(2)O center dot and by excess TEMPO center dot to give the corresponding 5,6-isopropylidene ascorbyl radical anion (iAsc(center dot-)), which persists for hours at 298 K in dry MeCN solution. The stability of iAsc(center dot-) is surprising in light of the transience of the ascorbyl radical in aqueous solutions and is due to the lack of the protons needed for radical disproportionation. A concerted proton-electron transfer (CPET) mechanism is indicated for the reactions of iAscH(-). Redox potential, pK(a) and equilibrium measurements define the thermochemical landscape for 5,6-isopropylidene ascorbic acid and its derivatives in MeCN. These measurements give an O-H bond dissociation free energy (BDFE) for iAscH(-) of 65.4 +/- 1.5 kcal mol(-1) in MeCN. Similar studies on underivatized ascorbate indicate a BDFE of 6708 +/- 1.2 kcal mol(-1). These values are much lower than the aqueous BDFE fir ascorbate of 74.0 +/- 1.5 kcal mol(-1) derived from reported data.