CPET concerted proton-electron

This chapter focuses on hydrogen atom transfer (HAT) reactions, which involve concerted transfer of a proton and an electron from a single donor to a single acceptor in one kinetic step (eqn (1.1)). These are one subset of PCET processes and are one type of concerted proton-electron transfer (CPET). ... 

Figure 3.2 Uni-directional (A) and bi-directional (B) PCET. Synonyms for unidirectional PCET are collinear PCET, concerted electron-proton transfer (CEP, ref. 236), electron-proton transfer (EPT, 120 and 237), concerted proton-electron transfer (CPET, 238 and 239) and concerted electron transfer proton transfer (ETPT, ref. 240). Bi-directional PCET is also termed orthogonal PCET, bi-directional concerted electron-proton transfer (CEP, ref. 236) and multisite electron proton transfer (MS-EPT, ref. 237). Adapted from ref. 21.

The rate constants, kn, obtained at all pH values studied, were effectively identical. For anions lie and 2ie, the rate of reactions with O2 showed no significant change as the pH was decreased from 2 to 1. These were the first indications that the reaction is zero-order in [H+], namely, pH-independent. Solvent kinetic-isotope effect experiments were carried out in D2O at D+ concentrations corresponding to pH values of 2 and 7.2. The rate remained unchanged when the solvent H2O was replaced by D2O. That provided a second line of evidence that even at pH 2, well below the pAfa = 4.7 of protonated superoxide (HO2 ), proton transfer (PT) occurs after rate-limiting electron transfer to O2 (ETPT mechanism), rather than via concerted proton-electron transfer (CPET) [61-65], in which an electron and proton are transferred simultaneously in a single elementary step—see Sect. 12.3.2. 

Concerted Proton-Electron Transfer (CPET) to O2 in Water... 

In a recent publication [66], we showed that at low pH values in water, the ETPT mechanism discussed in the previous section is accompanied by a parallel pathway concerted proton-electron transfer (CPET Fig. 12.8) [61-65]. 

Reaction between the one electron reduced POM anions and O2 were then summarized. These studies demonstrated that at mildly acidic to neutral pH values in water, electron transfer from Keggin heteropolytungstates to O2 occurs by an outer-sphere mechanism. In these cases, the effect of Keggin-ion charge on rate constants for the reduction of O2 was shown to be significant, and was attributed to anion-anion repulsion within the successor-complex ion pairs formed between the negatively charged POM anions and 02 . This was followed by the discovery of a concerted proton-electron (CPET) pathway for electron transfer to O2 at lower pH values (< 1). 

The CPET reaction is termolecular, and is facilitated by the unique nature of proton diffusion in water. At the large [H+] at which CPET is observed, protons are present within the aqueous medium within typical proton-diffusion reaction distances from the encounter complexes formed by bimolecular collisions between lie and O2. Rapid proton diffusion in water then increases the probability of CPET occurring by allowing protons to repeatedly diffuse to within a few angstroms of the relatively long-lived (lie,02) encounter complexes, whose lifetimes are estimated at 70 to 200 ps. These findings suggest that concerted proton-electron transfer may be... 

Several parameters can govern the storage mechanism in the EDLC for PILs according to the electrolytes/material couple. First, a reduction of the ammonium cation in the presence of an activated carbon electrode is presumably linked to electronic transfer kinetics. Reactions in which electron and proton transfer is performed have been described either by two distinct steps. Electron Proton Transfer or Proton Electron Transfer (EPT or PET), or in the same concerted step. Concerted Proton Electron Transfer (CPET) [140-142]. Contrary to simple proton or electron transfer, CPET is more complicated and the coupling at the activated carbon /molecule (cation) interface influences the process both thermodynamically and kinetically. 

Fig. 19 A Pourbaix diagram for phenol oxidation in water with concentration regimes for favored concerted proton-coupled electron transfer (CPET), proton-first stepwise PCET (PET), or electron-first stepwise PCET (EPT). Figure reprinted with permission from Proc Nat Acad Sci [153]...

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