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

Research output: Contribution to journalArticle

40 Citations (Scopus)

Abstract

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.

Original languageEnglish
Pages (from-to)16214-16225
Number of pages12
JournalJournal of Physical Chemistry B
Volume113
Issue number50
DOIs
Publication statusPublished - 2009 Dec 17
Externally publishedYes

Fingerprint

Phenols
phenols
Protons
Hydrogen
Hydrogen bonds
electron transfer
Electrons
hydrogen bonds
Kinetics
protons
kinetics
Salicylic Acid
Vibration
Discrete Fourier transforms
Isotopes
Salicylic acid
vibration
Biomimetics
acids
tyrosine

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Materials Chemistry
  • Surfaces, Coatings and Films

Cite this

The kinetic effect of internal hydrogen bonds on proton-coupled electron transfer from phenols : A theoretical analysis with modeling of experimental data. / Johannissen, Linus O.; Irebo, Tania; Sjödin, Martin; Johansson, Olof; Hammarström, Leif.

In: Journal of Physical Chemistry B, Vol. 113, No. 50, 17.12.2009, p. 16214-16225.

Research output: Contribution to journalArticle

Johannissen, Linus O. ; Irebo, Tania ; Sjödin, Martin ; Johansson, Olof ; Hammarström, Leif. / The kinetic effect of internal hydrogen bonds on proton-coupled electron transfer from phenols : A theoretical analysis with modeling of experimental data. In: Journal of Physical Chemistry B. 2009 ; Vol. 113, No. 50. pp. 16214-16225.
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