Determination of the N–H Bond Dissociation Free Energy in a Pyridine(diimine)molybdenum Complex Prepared by Proton-Coupled Electron Transfer

Baird antiaromaticity plays a central role in the photochemistry of proton-coupled electron transfer (PCET) reactions. We recognize that many popular organic chromophores that catalyze photoinduced PCET reactions are Hückel aromatic in the ground state, but gain significant Baird antiaromatic character in the lowest ππ* state, having important barrier-lowering effects for electron transfer. Two examples, 1) the photolytic O–H bond dissociation of phenol and 2) solar water splitting in the pyridine-water complex, are discussed. Contrary to an assumed homolytic O–H bond dissociation, both reactions proceed through loss (and gain) of an electron in the π-system (i.e., antiaromaticity relief), followed by heterolytic cleavage of the polar O–H bond near barrierlessly. Nucleus-independent chemical shifts (NICS), ionization energies (IE), electron affinities (EA), and excited-state PCET energy profiles of selected [4n] and [4n+2] π-systems are presented.

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Iron Complex as a Water-Oxidizing Catalyst: Free-Energy Barriers, Proton-Coupled Electron Transfer, Spin Dynamics, and Role of Water Molecules in the Reaction Mechanism

The Journal of Physical Chemistry C ◽

10.1021/acs.jpcc.9b10378 ◽

2019 ◽

Vol 124 (1) ◽

pp. 205-218 ◽

Cited By ~ 3

Author(s):

Koteswara Rao Gorantla ◽

Bhabani S. Mallik

Keyword(s):

Electron Transfer ◽

Free Energy ◽

Reaction Mechanism ◽

Spin Dynamics ◽

Iron Complex ◽

Water Molecules ◽

Energy Barriers ◽

Catalyst Free ◽

Proton Coupled Electron Transfer

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Electrochemically Determined O–H Bond Dissociation Free Energies of NiO Electrodes Predict Proton-Coupled Electron Transfer Reactivity

Journal of the American Chemical Society ◽

10.1021/jacs.9b07923 ◽

2019 ◽

Vol 141 (38) ◽

pp. 14971-14975 ◽

Cited By ~ 3

Author(s):

Catherine F. Wise ◽

James M. Mayer

Keyword(s):

Electron Transfer ◽

Free Energies ◽

Bond Dissociation ◽

Proton Coupled Electron Transfer

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Barrier-Lowering Effects of Baird Antiaromaticity in Photoinduced Proton-Coupled Electron Transfer (PCET) Reactions

10.26434/chemrxiv.14562069 ◽

2021 ◽

Author(s):

Lucas Karas ◽

Chia-Hua Wu ◽

Judy Wu

Keyword(s):

Electron Transfer ◽

Chemical Shifts ◽

Water Complex ◽

Bond Dissociation ◽

Organic Chromophores ◽

Solar Water Splitting ◽

Proton Coupled Electron Transfer ◽

Energy Profiles ◽

Important Barrier ◽

Antiaromatic Character

Baird antiaromaticity plays a central role in the photochemistry of proton-coupled electron transfer (PCET) reactions. We recognize that many popular organic chromophores that catalyze photoinduced PCET reactions are Hückel aromatic in the ground state, but gain significant Baird antiaromatic character in the lowest ππ* state, having important barrier-lowering effects for electron transfer. Two examples, 1) the photolytic O–H bond dissociation of phenol and 2) solar water splitting in the pyridine-water complex, are discussed. Contrary to an assumed homolytic O–H bond dissociation, both reactions proceed through loss (and gain) of an electron in the π-system (i.e., antiaromaticity relief), followed by heterolytic cleavage of the polar O–H bond near barrierlessly. Nucleus-independent chemical shifts (NICS), ionization energies (IE), electron affinities (EA), and excited-state PCET energy profiles of selected [4n] and [4n+2] π-systems are presented.

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Proton-coupled electron transfer and the role of water molecules in proton pumping by cytochrome c oxidase

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1409543112 ◽

2015 ◽

Vol 112 (7) ◽

pp. 2040-2045 ◽

Cited By ~ 45

Author(s):

Vivek Sharma ◽

Giray Enkavi ◽

Ilpo Vattulainen ◽

Tomasz Róg ◽

Mårten Wikström

Keyword(s):

Electron Transfer ◽

Free Energy ◽

Proton Transfer ◽

Short Circuit ◽

Proton Pumping ◽

Free Energy Calculations ◽

Water Molecules ◽

Transfer Event ◽

Proton Coupled Electron Transfer ◽

Dynamics Simulations

Molecular oxygen acts as the terminal electron sink in the respiratory chains of aerobic organisms. Cytochrome c oxidase in the inner membrane of mitochondria and the plasma membrane of bacteria catalyzes the reduction of oxygen to water, and couples the free energy of the reaction to proton pumping across the membrane. The proton-pumping activity contributes to the proton electrochemical gradient, which drives the synthesis of ATP. Based on kinetic experiments on the O–O bond splitting transition of the catalytic cycle (A → PR), it has been proposed that the electron transfer to the binuclear iron–copper center of O2 reduction initiates the proton pump mechanism. This key electron transfer event is coupled to an internal proton transfer from a conserved glutamic acid to the proton-loading site of the pump. However, the proton may instead be transferred to the binuclear center to complete the oxygen reduction chemistry, which would constitute a short-circuit. Based on atomistic molecular dynamics simulations of cytochrome c oxidase in an explicit membrane–solvent environment, complemented by related free-energy calculations, we propose that this short-circuit is effectively prevented by a redox-state–dependent organization of water molecules within the protein structure that gates the proton transfer pathway.

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