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

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.

SUBMITTER: Warren JJ

PROVIDER: S-EPMC2556879 | biostudies-other | 2008 Jun

REPOSITORIES: biostudies-other

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

Warren Jeffrey J JJ Mayer James M JM

Journal of the American Chemical Society 20080528 24

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 nee ...[more]

PMID: 18505256

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Project description:Photo-oxidations of hydrogen-bonded phenols using excited-state polyarenes are described to derive fundamental understanding of multiple-site concerted proton-electron transfer reactions (MS-CPET). Experiments have examined phenol bases having -CPh(2)NH(2), -Py, and -CH(2)Py groups ortho to the phenol hydroxyl group and tert-butyl groups in the 4,6-positions for stability (HOAr-NH(2), HOAr-Py, and HOAr-CH(2)Py, respectively; Py = pyridyl; Ph = phenyl). The photo-oxidations proceed by intramolecular proton transfer from the phenol to the pendent base concerted with electron transfer to the excited polyarene. For comparison, 2,4,6-(t)Bu(3)C(6)H(2)OH, a phenol without a pendent base and tert-butyl groups in the 2,4,6-positions, has also been examined. Many of these bimolecular reactions are fast, with rate constants near the diffusion limit. Combining the photochemical k(CPET) values with those from prior thermal stopped-flow kinetic studies gives data sets for the oxidations of HOAr-NH(2) and HOAr-CH(2)Py that span over 10(7) in k(CPET) and nearly 0.9 eV in driving force (?G(o)'). Plots of log(k(CPET)) vs ?G(o)', including both excited-state anthracenes and ground state aminium radical cations, define a single Marcus parabola in each case. These two data sets are thus well described by semiclassical Marcus theory, providing a strong validation of the use of this theory for MS-CPET. The parabolas give ?(CPET) ? 1.15-1.2 eV and H(ab) ? 20-30 cm(-1). These experiments represent the most direct measurements of H(ab) for MS-CPET reactions to date. Although rate constants are available only up to the diffusion limit, the parabolas clearly peak well below the adiabatic limit of ca. 6 × 10(12) s(-1). Thus, this is a very clear demonstration that the reactions are nonadiabatic. The nonadiabatic character slows the reactions by a factor of ~45. Results for the oxidation of HOAr-Py, in which the phenol and base are conjugated, and for oxidation of 2,4,6-(t)Bu(3)C(6)H(2)OH, which lacks a base, show that both have substantially lower ? and larger pre-exponential terms. The implications of these results for MS-CPET reactions are discussed.

Dataset Information

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

Publications

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

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