Concerted Proton-Electron Transfers

When the proton and electron transfer involve different sites of the reactants, we may expect, in addition to the extra reorganization energy, that the electron transfer may be strongly non-adiabatic (Xei 1) and substantially reduce the effective reaction frequency. Georgievskii and Stuchebrukhov addressed the transition between electronically adiabatic and non-adiabatic CPET and showed that the adiabatic factor that reflects the deviation from adiabaticity has the form  

The theoretical treatments of nonadiabatic transitions and the formulation of hydrogen atom or proton tunnelling, as particular cases of such transitions, motivated various theoretical developments of CPET. In the simplest case, when only one initial and one final vibrational states are involved in the reaction, the CPET rate has been written in a form similar to that given by the nonadiabatic ET theory  

The transition between adiabatic PT and electronically non-adiabatic CPET can also be incorporated in the adiabatic PT rate constant using the electronic nonadiabatic factor Xei, that depends on (XeiVei)/vv in the same way as IkgsI depends on pi x- Additionally, we include in the energy barrier the contributions from the reorganizations of the parts of the molecular system involved in the synchronous proton and electron transfer, and express the rate constant for nonadiabatic CPET as 

Before we compare the experimental and semiempirical EIAT rates of the benzyl/toluene system, it is important to assess the validity of the semiempirical calculations with a set of representative and accurate EIAT barrier heights. Lynch and Truhlar showed that the systems OEI -b CEI4 CH3 -I- H2O, H -I- HO O -I- H2 and H + H2S H2 + HS are very representative of this type of reaction and benchmarked comparisons to these systems. Truhlar and 

The benzyl/toluene self-exehange is diffieult to measure experimentally because the reaetants and produets are identieal. Alternatively, Jaekson and O Neill measured the HAT between benzyl radieal and m-deuteriotoluene in the 124-168 °C range and obtained the Arrhenius equation log (k/(M s )) = 10.5-19.9/0, whereas Franz and eo-workers measured the HAT between 2-allylbenzyl radieal and p-xylene between 130 and 190 °C and obtained log (k/(M s )) = 7.1-13.4/0), 6 = 2.3RT in keal moH ISM calculations give, per equivalent hydrogen atom and for m=l, log (k/(M s )) = 9.4—15.2/0) in the same temperature range. This, again, demonstrates the validity of ISM to calculate HAT rates. 

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Continuing with the first pathway through step 2 and step 3, two identical reactions involving the last two concerted proton-electron transfers transform adsorbed... 

Each net rate in Equation 3.56 has the form of an effective resistance (i) the abundance of charge carriers available to pass the highest energy barrier is represented by the product of K values, and (ii) the mobility of these carriers is represented by the kinetic rate k. In the considered mechanism, three possible uphill steps exist, which all involve concerted proton-electron transfer processes. They correspond to adsorptive formation of OORad via Equation 3.40 with rate constant k, the intermediate step in Equation 3.42 with rate constant k, and desorptive formation of water in Equation 3.45 with rate constant k. ... 

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 importance of PCET and concerted proton electron transfer (EPT) in the activation of ruthenium oxido complexes was initially acknowledged for [Ru(bpy)2(py)OH2], but this complex failed to catalyse water oxidation. 

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]. 

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... 

Bonin J, Costentin C, Robot M, Saveant JM, Tard C (2012) Hydrogen-bond relays in concerted proton-electron transfers. Acc Chem Res 45 372... 

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. 

Oxidation of phenol by tris(l,10-phenanthroline)osmium(III) is second order in Os(III) and phenol and inverse second order in Os(II) and acidity. A mechanism is inferred in which the phenoxyl radical is produced through a rapid proton-coupled electron transfer (PCET) pre-equilibrium, followed by rate-limiting phenoxyl radical coupling. Application of Marcus theory indicated that the rate of electron transfer from phenoxide to osmium(III) is fast enough to account for the rapid PCET preequilibrium, but it did not rule out the intervention of other pathways such as concerted proton-electron transfer or general-base catalysis DFT studies, at B3LYP/LACVP level, of the oxidation of ethylene by osmium tetroxide, osmyl hydroxide, and osmyl chloride indicated that in the reaction of osmium tetroxide, the [3 4- 2] addition pathway leading to a five-membered metallacycle intermediate is more favourable than the [24-2] addition. The reaction with osmyl hydroxide is less favourable. In the reaction with osmyl chloride, the [24-2] addition pathway is more favourable than the [3 4-2] addition. ... 

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