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

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 in the Reaction of 3,4-Dihydroxyphenylpyruvic Acid with Reactive Species in Various Media

International Journal of Chemical Physics ◽

10.1155/2015/835707 ◽

2015 ◽

Vol 2015 ◽

pp. 1-13 ◽

Cited By ~ 4

Author(s):

J. J. Fifen ◽

Z. Dhaouadi ◽

M. Nsangou ◽

O. Holtomo ◽

N. Jaidane

Keyword(s):

Electron Transfer ◽

Proton Transfer ◽

Activation Barrier ◽

Bond Cleavage ◽

Hydrogen Atom Transfer ◽

Free Energies ◽

Solvent Interactions ◽

Proton Coupled Electron Transfer ◽

Nonpolar Solvents ◽

One Step

The distinction of concerted proton-coupled electron transfer (CPCET) from sequential one as well as proton transfer-electron transfer (PT-ET) from electron transfer-proton transfer (ET-PT) in the O–H bond cleavage reactions in various media has always been a difficult task. In this work, the activation barrier of the CPCET mechanism, its rate constants, and reaction free energies related to ET-PT and PT-ET involving coreactive species were presented as good parameters to attempt the problem. DFT calculations were carried out studying the described pathways subsequent to the scavenging of OH• and OBr- by the 3,4-DHPPA in various media. The solvation was described in a hybrid manner using IEF-PCM model conjointly with a model that takes into account some solute-solvent interactions. As a result, we found that the scavenging of hydroxyl radical by 3,4-DHPPA is thermodynamically governed by a one-step hydrogen atom transfer (CPCET) from the acid to the radical in all media. In kinetic viewpoint, CPCET still dominates in the vacuum and in nonpolar solvents, but in polar solvents it could compete strongly with the ET-PT mechanism so that the latter could slightly dominate.

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

Inorganic Chemistry ◽

10.1021/acs.inorgchem.0c02382 ◽

2020 ◽

Vol 59 (20) ◽

pp. 15394-15401

Author(s):

Grant W. Margulieux ◽

Sangmin Kim ◽

Paul J. Chirik

Keyword(s):

Electron Transfer ◽

Free Energy ◽

Molybdenum Complex ◽

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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