Marcus-type driving force correlations reveal the mechanism of proton-coupled electron transfer for phenols and [Ru(bpy)3 ]3+ in water at low pH

Chemical Science
Janne SoetbeerLeif Hammarström

Abstract

Proton-coupled electron transfer (PCET) from tyrosine and other phenol derivatives in water is an important elementary reaction in chemistry and biology. We examined PCET between a series of phenol derivatives and photogenerated [Ru(bpy)3]3+ in low pH (≤4) water using the laser flash-quench technique. From an analysis of the kinetic data using a Marcus-type free energy relationship, we propose that our model system follows a stepwise electron transfer-proton transfer (ETPT) pathway with a pH independent rate constant at low pH in water. This is in contrast to the concerted or proton-first (PTET) mechanisms that often dominate at higher pH and/or with buffers as primary proton acceptors. The stepwise mechanism remains competitive despite a significant change in the pKa and redox potential of the phenols which leads to a span of rate constants from 1 × 105 to 2 × 109 M-1 s-1. These results support our previous studies which revealed separate mechanistic regions for PCET reactions and also assigned phenol oxidation by [Ru(bpy)3]3+ at low pH to a stepwise PCET mechanism.

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Citations

Sep 24, 2016·Proceedings of the National Academy of Sciences of the United States of America·Christopher J GagliardiDavid W Thompson
Dec 6, 2017·Physical Chemistry Chemical Physics : PCCP·Flor Rodríguez-PrietoManuel Mosquera
Jun 7, 2018·Chemphyschem : a European Journal of Chemical Physics and Physical Chemistry·Rahul V KhadeAvinash S Kumbhar
Apr 10, 2018·Chemical Science·Senthil Kumar ThiyagarajanPerumal Ramamurthy
Apr 15, 2021·Chemphyschem : a European Journal of Chemical Physics and Physical Chemistry·Francesco RigodanzaAndrea Sartorel
Jun 13, 2018·Inorganic Chemistry·Kai-Ti ChuMing-Hsi Chiang

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Methods Mentioned

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PCET

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