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-tBu3C6H2O. and by excess TEMPO. to give the corresponding 5,6-isopropylidene ascorbyl radical anion (iAsc.-), which persists for hours at 298 K in dry MeCN solution. The stability of iAsc.- 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, pKa 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 67.8 +/- 1.2 kcal mol-1. These values are much lower than the aqueous BDFE for ascorbate of 74.0 +/- 1.5 kcal mol-1 derived from reported data.
Warren
Jeffrey J
JJ
Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195, USA.
Mayer
James M
JM
D000097:Acetonitriles / Q000737:chemistry*; D001205:Ascorbic Acid / Q000031:analogs & derivatives* / Q000737:chemistry; D004579:Electron Transport; D005609:Free Radicals / Q000032:analysis; D006358:Hot Temperature; D010084:Oxidation-Reduction; D011522:Protons; D012997:Solvents / Q000737:chemistry; D013696:Temperature
C530008:5,6-isopropylidene ascorbate; D000097:Acetonitriles; D005609:Free Radicals; D011522:Protons; D012997:Solvents; D001205:Ascorbic Acid; C032159:acetonitrile