Kinetic effects of hydrogen bonds on proton-coupled electron transfer from phenols

Martin Sjödin, Tania Irebo, Josefin E. Utas, Johan Lind, Gabor Merényi, Björn Åkermark, Leif Hammarström

Research output: Contribution to journalArticle

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Abstract

The kinetics and mechanism of proton-coupled electron transfer (PCET) from a series of phenols to a laser flash generated [Ru(bpy)3] 3+ oxidant in aqueous solution was investigated. The reaction followed a concerted electron-proton transfer mechanism (CEP), both for the substituted phenols with an intramolecular hydrogen bond to a carboxylate group and for those where the proton was directly transferred to water. Without internal hydrogen bonds the concerted mechanism gave a characteristic pH-dependent rate for the phenol form that followed a Marcus free energy dependence, first reported for an intramolecular PCET in Sjödin, M. et al. J. Am. Chem. Soc. 2000, 122, 3932-3962 and now demonstrated also for a bimolecular oxidation of unsubstituted phenol. With internal hydrogen bonds instead, the rate was no longer pH-dependent, because the proton was transferred to the carboxylate base. The results suggest that while a concerted reaction has a relatively high reorganization energy (λ), this may be significantly reduced by the hydrogen bonds, allowing for a lower barrier reaction path. It is further suggested that this is a general mechanism by which proton-coupled electron transfer in radical enzymes and model complexes may be promoted by hydrogen bonding. This is different from, and possibly in addition to, the generally suggested effect of hydrogen bonds on PCET in enhancing the proton vibrational wave function overlap between the reactant and donor states. In addition we demonstrate how the mechanism for phenol oxidation changes from a stepwise electron transfer-proton transfer with a stronger oxidant to a CEP with a weaker oxidant, for the same series of phenols. The hydrogen bonded CEP reaction may thus allow for a low energy barrier path that can operate efficiently at low driving forces, which is ideal for PCET reactions in biological systems.

Original languageEnglish
Pages (from-to)13076-13083
Number of pages8
JournalJournal of the American Chemical Society
Volume128
Issue number40
DOIs
Publication statusPublished - Oct 11 2006

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Phenols
Protons
Hydrogen
Hydrogen bonds
Electrons
Kinetics
Phenol
Oxidants
Proton transfer
Oxidation
Energy barriers
Biological systems
Wave functions
Free energy
Hydrogen Bonding
Enzymes
Lasers
Water

ASJC Scopus subject areas

  • Chemistry(all)

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Kinetic effects of hydrogen bonds on proton-coupled electron transfer from phenols. / Sjödin, Martin; Irebo, Tania; Utas, Josefin E.; Lind, Johan; Merényi, Gabor; Åkermark, Björn; Hammarström, Leif.

In: Journal of the American Chemical Society, Vol. 128, No. 40, 11.10.2006, p. 13076-13083.

Research output: Contribution to journalArticle

Sjödin M, Irebo T, Utas JE, Lind J, Merényi G, Åkermark B et al. Kinetic effects of hydrogen bonds on proton-coupled electron transfer from phenols. Journal of the American Chemical Society. 2006 Oct 11;128(40):13076-13083. https://doi.org/10.1021/ja063264f
Sjödin, Martin ; Irebo, Tania ; Utas, Josefin E. ; Lind, Johan ; Merényi, Gabor ; Åkermark, Björn ; Hammarström, Leif. / Kinetic effects of hydrogen bonds on proton-coupled electron transfer from phenols. In: Journal of the American Chemical Society. 2006 ; Vol. 128, No. 40. pp. 13076-13083.
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AB - The kinetics and mechanism of proton-coupled electron transfer (PCET) from a series of phenols to a laser flash generated [Ru(bpy)3] 3+ oxidant in aqueous solution was investigated. The reaction followed a concerted electron-proton transfer mechanism (CEP), both for the substituted phenols with an intramolecular hydrogen bond to a carboxylate group and for those where the proton was directly transferred to water. Without internal hydrogen bonds the concerted mechanism gave a characteristic pH-dependent rate for the phenol form that followed a Marcus free energy dependence, first reported for an intramolecular PCET in Sjödin, M. et al. J. Am. Chem. Soc. 2000, 122, 3932-3962 and now demonstrated also for a bimolecular oxidation of unsubstituted phenol. With internal hydrogen bonds instead, the rate was no longer pH-dependent, because the proton was transferred to the carboxylate base. The results suggest that while a concerted reaction has a relatively high reorganization energy (λ), this may be significantly reduced by the hydrogen bonds, allowing for a lower barrier reaction path. It is further suggested that this is a general mechanism by which proton-coupled electron transfer in radical enzymes and model complexes may be promoted by hydrogen bonding. This is different from, and possibly in addition to, the generally suggested effect of hydrogen bonds on PCET in enhancing the proton vibrational wave function overlap between the reactant and donor states. In addition we demonstrate how the mechanism for phenol oxidation changes from a stepwise electron transfer-proton transfer with a stronger oxidant to a CEP with a weaker oxidant, for the same series of phenols. The hydrogen bonded CEP reaction may thus allow for a low energy barrier path that can operate efficiently at low driving forces, which is ideal for PCET reactions in biological systems.

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