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. 2008 Jun 18;130(24):7546-7.
doi: 10.1021/ja802055t. Epub 2008 May 28.

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

Jeffrey J Warren  1 , James M Mayer
Affiliations
Free PMC article

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

Jeffrey J Warren et al. J Am Chem Soc. .
Free PMC article

Abstract

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.

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Figures

Figure 1
Figure 1
(a) UV-Visible spectrum and (b) X-band EPR spectrum of iAsc’“ in CH3CN at 298K, generated via reaction (1)
Scheme 1
Scheme 1
Thermochemistry of the AscH2 system in MeCN
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