Proton-Coupled Electron Transfer

Proton-coupled electron transfer (PCET) is emerging as an important new class of reactions and, over the past decade, great strides have been made in our understanding of them. PCET reactions are studied in many branches of chemistry and are omnipresent in biological processes. This book covers recent developments from both the theoretical and experimental points of view. It concentrates on the importance of PCET in biological systems and for bioenergetic conversion. The oxidation of water in Photosystem II to produce oxygen, and the reduction of protons to hydrogen by hydrogenase, for energy storage gets particular emphasis. Chemical reactivity is currently explained in terms of several scientific principles. One of them is the bond-breaking-bond-forming process and is conceptually based on potential energy surfaces. Another incorporates the role of Franck-Condon factors resulting from the overlap of vibrational wavefunctions. A third, the so-called solvent reorganization, involves solvent configuration around a charged species. PCET brings together such concepts and links them to quantum mechanical tunnelling of the electron particle. This book uses personal accounts of experimental examples to provide additional insight on this important topic. It starts by presenting a general overview of the main theoretical approaches and experimental applications. The chapters then go on to cover topics including: the application of the Marcus Cross Relation; the solvation of ionic systems; experimental approaches in biological redox systems; metal ion-coupled electron transfer, and electrochemical concerted proton-electron transfers.

Application of the Marcus Cross Relation to Hydrogen Atom Transfer/Proton-Coupled Electron Transfer Reactions	p. 1
Introduction	p. 1
An Introduction to Marcus Theory	p. 2
Predicting Organic Hydrogen Atom Transfer Rate Constants	p. 5
Obtaining Self-Exchange Rate Constants and Equilibrium Constants	p. 6
Tests of the Cross Relation for Organic HAT Reactions	p. 7
Solvent Effects on HAT Rate and Equilibrium Constants	p. 11
A Test Case: Reactions of Bulky Phenoxyl Radicals with TEMPOH	p. 12
Tests of the Cross Relation using KSE-Corrected Self-Exchange Rate Constants	p. 13
Predicting HAT Rate Constants for Transition Metal Complexes	p. 14
Applying the Cross Relation as a Function of Temperature; the Importance of Using Free Energies	p. 16
Applying the Cross Relation to Oxidations by [Ru^IV(O)(bpy)₂(py)]²⁺	p. 17
Precursor and Successor Complexes for HAT	p. 18
Applying the Cross Relation for Transition Metal HAT	p. 20
Transition Metal Systems that Deviate from the Cross Relation	p. 21
Conclusions: Implications and Limitations of the Cross Relation for Hydrogen Atom Transfer Reactions	p. 23
References	p. 27
A Transition-State Perspective of Proton-Coupled Electron Transfers	p. 32
Introduction	p. 32
Theory	p. 34
Hydrogen Atom Transfers	p. 34
Proton Transfers in Hydrogen-Bonded Systems	p. 37
Electron Transfers	p. 40
Concerted Proton-Electron Transfers	p. 47
Applications	p. 48
HAT in the Benzyl/Toluene Self-Exchange	p. 48
PCET in the Phenoxyl/Phenol Self-Exchange	p. 50
CPET in Soybean Lipoxygenase-1	p. 52
Conclusions	p. 53
References	p. 54
Experimental Approaches Towards Proton-Coupled Electron Transfer Reactions in Biological Redox Systems	p. 57
Introduction	p. 57
Definitions	p. 57
Thermodynamics of PCET Reactions	p. 61
Kinetics of PCET Reactions	p. 64
Experimental Kinetic Approaches to Analyse PCET Reactions	p. 69
Case Studies	p. 73
PCET in Nitrite Reductase	p. 73
Hydride Transfer Reactions in Old Yellow Enzymes	p. 76
Concluding Remarks	p. 79
References	p. 79
Metal Ion-Coupled and Proton-Coupled Electron Transfer in Catalytic Reduction of Dioxygen	p. 89
Introduction	p. 89
PCET from Electron Donors to O₂	p. 91
MCET from Electron Donors to O₂	p. 97
MCET from O₂•-Mⁿ⁺ to p-Benzoquinones	p. 99
Catalytic Two-Electron Reduction of O₂ via MCET and PCET	p. 103
Catalytic Four-Electron Reduction of O₂	p. 109
Cofacial Dicobalt Porphyrin and Porphyrin-Corrole Dyads	p. 109
Mononuclear Cu Complexes	p. 115
A Heterodinuclear Indium-Ruthenium Complex	p. 118
Mononuclear Mn Complexes	p. 119
Summary and Conclusions	p. 121
Acknowledgements	p. 121
References	p. 121
Proton-Coupled Electron Transfer in Natural and Artificial Photosynthesis	p. 126
Introduction	p. 126
Proton-Coupled Electron Transfer Reactions	p. 128
Interfacial PCET	p. 129
Thermodynamics of Water Splitting and CO₂ Reduction	p. 130
Natural Photosynthesis	p. 132
Structure and Mechanism of Photosystem II	p. 133
PCET in Photosystem II	p. 136
Artificial Photosynthesis	p. 137
Model Systems for Photosystem II	p. 138
Water Oxidation Catalysts	p. 139
Proton and CO2 Reduction	p. 144
Concluding Remarks	p. 145
References	p. 145
Subject Index	p. 152
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Proton-Coupled Electron Transfer

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