The kinetic effect of internal hydrogen bonds on proton-coupled electron transfer from phenols: A theoretical analysis with modeling of experimental data

Linus O. Johannissen, Tania Irebo, Martin Sjödin, Olof Johansson, Leif Hammarström*

*この研究の対応する著者

研究成果: Article査読

48 被引用数 (Scopus)

抄録

Proton-coupled electron transfer (PCET) was studied in two biomimetic covalently linked Ru(bpy)3-tyrosine complexes with the phenolic proton hydrogen-bonded to an internal carboxylate group. The phenolic group is either a salicylic acid (o-hydroxybenzoic acid, SA) or an o-hydroxyphenyl-acetic acid (PA), where the former gives a resonance-assisted hydrogen bond. Transient absorption data allowed direct determination of the rate constant for these intramolecular, bidirectional, and concerted PCET (CEP) reactions, as a function of temperature and H/D isotope. We found, unexpectedly, that the hydrogen bond in SA is in fact weaker than the hydrogen bond in the complex with PA, which forced us to reassess an earlier hypothesis that the proton coupling term for CEP with SA is increased by a stronger hydrogen bond. Consequently, the kinetic data was modeled numerically using a quantum mechanical rate expression. Sufficient experimentally determined observables were available to give robust and well-determined parameter values. This analysis, coupled with DFT/B3LYP and MP2 calculations and MD simulations, gave a detailed insight into the parameters that control the CEP reactions, and the effect of internal hydrogen bonds. We observed that a model with a static proton-tunneling distance is unable to describe the reaction correctly, requiring unrealistic values for the equilibrium proton-tunneling distances. Instead, when promoting vibrations that modulate the proton donor-acceptor distance were included, satisfactory fits to the experimental data were obtained, with parameter values that agree with DFT calculations and MD simulations. According to these results, it is in fact the weaker hydrogen bond of SA which increases the proton coupling. The inner reorganization energy of the phenolic groups is a significant factor contributing to the CEP barriers, but this is reduced by the hydrogen bonds to 0.35 and 0.50 eV for the two complexes. The promoting vibrations increase the rate of CEP by over 2 orders of magnitude, and dramatically reduce the kinetic isotope effect from ca. 40 for the static case to a modest value of 2-3.

本文言語English
ページ(範囲)16214-16225
ページ数12
ジャーナルJournal of Physical Chemistry B
113
50
DOI
出版ステータスPublished - 2009 12月 17
外部発表はい

ASJC Scopus subject areas

  • 物理化学および理論化学
  • 材料化学
  • 表面、皮膜および薄膜

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