Proton-Coupled Electron Transfer Guidelines, Fair and Square

Abstract

Proton-coupled electron transfer (PCET) reactions are fundamental to energy transformation reactions in natural and artificial systems and are increasingly recognized in areas such as catalysis and synthetic chemistry. The interdependence of proton and electron transfer brings a mechanistic richness of reactivity, including various sequential and concerted mechanisms. Delineating between different PCET mechanisms and understanding why a particular mechanism dominates are crucial for the design and optimization of reactions that use PCET. This Perspective provides practical guidelines for how to discern between sequential and concerted mechanisms based on interpretations of thermodynamic data with temperature-, pressure-, and isotope-dependent kinetics. We present new PCET-zone diagrams that show how a mechanism can switch or even be eliminated by varying the thermodynamic (Δ G PT ° and Δ G ET ° ) and coupling strengths for a PCET system. We discuss the appropriateness of asynchronous concerted PCET to rationalize observations in organic reactions, and the distinction between hydrogen atom transfer and other concerted PCET reactions. Contemporary issues and future prospects in PCET research are discussed.

Water‐Assisted Concerted Proton‐Electron Transfer at Co(II)‐Aquo Sites in Polyoxotungstates With Photogenerated RuIII(bpy)33+ Oxidant

“…A linear dependence of log( k ) vs pH with slope in the range 0.15–0.50 has been previously observed for light induced, water assisted CPET in covalently linked Ru(bpy) 3 2+ /tyrosine or tryptophan systems [62,67] . Although the origin of such weak pH dependence (α<1) of log( k ) is not clearly understood, [6] this behavior supports the CPET mechanism envisioned herein (Scheme 3). [6] …”

“…Although the origin of such weak pH dependence (α<1) of log( k ) is not clearly understood, [6] this behavior supports the CPET mechanism envisioned herein (Scheme 3). [6] …”

Simulation of Heat Flow Curves of NC‐Based Propellants – Part 1: Determination of Reaction Enthalpies and Other Characteristics of the Reactions of NC and Stabilizer DPA Using Quantum Mechanical Methods

“…The recombination of this unpaired electron with one more moiety of NO 2 is therefore not confined to the amino position. The products of the addition of NO 2 to the phenyl ring are tautomers and precursors to the nitro‐DPA's which can re‐aromatize through a proton migration from the C‐position to the N‐position, maybe similar to one of HAT mechanisms (hydrogen atom transfer mechanism), here in the intramolecular version [39, 40, 41]. Anyway, the driving force for any type of H migration is to retain the resonance energy of the aromatic ring.…”

Photoinduced Electron vs. Concerted Proton Electron Transfer Pathways in SnIV (l‐Tryptophanato)2 Porphyrin Conjugates

“…[38] We would like to stress that small values of KIEs as well as the absence of any KIE are not in disagreement with such a PCET mechanism since the KIE is known to depend on many variables. [6,41] The spectral evolution of the transient absorption of 2 in CH 2 Cl 2 in the presence of 0.13 M pyrrolidine has been then analyzed to gain further insight into the quenching process (Figure 7C). The transient spectrum immediately detected after 0.1 μs corresponds to the spectrum of the triplet excited state of the SnOEP component (absorption maximum at ca.…”

“…[5] PCET reactions involving redox active amino acids may occur either through stepwise ET-PT/PT-ET pathways or via a concerted mechanism (concerted proton electron transfer, CPET). [2,3,6] The latter implies the transfer of both electron and proton in a single, concerted kinetic event. CPET is usually favored on thermodynamic basis as it avoids the formation of high energy intermediates, while it may present kinetic barriers associated to the simultaneous involvement of both electron and proton motion.…”