Electron transfer irreversible

Reversible chemical reaction preceding an irreversible electron transfer, CrEi mechanism ... 

Irreversible electron transfer followed by an irreversible regeneration of starting material ... 

Case III Irreversible Electron Transfer or Insoluble Products. Steady-state currents due to one-dimensional flux between the STM tip and substrate will not exist if either, 1) the products at both electrodes are insoluble, or, 2) the electron transfer reactions at... 

In addition, all complexes display a reversible, one-electron reduction at a very negative potential Em —1.70 to -1.90 V vs Fc+/Fc, which is metal centered and nearly invariant with respect to the substitution pattern of the coordinated pheno-lates. It demonstrates the enormous stabilization of the high-spin ferric state by three phenolato ligands. The electrochemistry also nicely shows that unprotected ortho- or para positions of these phenolates lead to irreversible electron-transfer waves on the time scale of a cyclic voltammogram and that methyl substituents are inefficient protecting groups. 

First-order chemical reaction preceding an irreversible electron transfer. The case in which a chemical reaction precedes an irreversible electron transfer can be depicted by the scheme ... 

Also in this case, if the kinetics of the preceding chemical reaction are very slow (kf + kT < a na-F-v/R-T), the process appears as a simple irreversible electron transfer. The peak height of the process depends on the equilibrium constant because, as mentioned previously, the concentration of the active species C0x is a fraction of the amount C (= Cox + Cy) put in solution ... 

Diagnostic criteria to identify a chemical reaction preceding an irreversible electron transfer. Other than the particular case of an S-like shaped voltammogram (not accompanied by a return peak), there are no criteria which define this case except for the decrease of the current function ipf/v1/2 with increasing v. 

Catalytic regeneration of the reagent following an irreversible electron transfer. When the catalytic reaction follows an irreversible electron transfer, the process can be represented as follows ... 

In this case, the cyclic voltammetric response is essentially similar to the preceding case, with the difference that, given the irreversibility of the electron transfer, the return peak is missing. Thus, if kf is low, the response is that of a simple irreversible electron transfer. As k increases, the greater the potential scan rate the higher the peak current (compared to simple irreversible electron transfer). This continues up to a maximum value at which the response assumes a S-like shape. 

An irreversible chemical reaction interposed between a reversible and an irreversible electron transfer (case R-I). The ErQEi mechanism, involving one-electron transfers, can be written as ... 

A plausible explanation of the observed irreversibility may lie in the following. Given that the addition of one electron generates the Cr(II) ion which has a greater ionic radius than the original Cr(III) (0.89 A vs. 0.63 A), in those cases (like the original) in which the molecular assembly is such that there are severe constraints at the metal site environment, any increase in the metal size would result in a rupturing of the molecule. It is evident that such a situation will lead to irreversible electron transfers. 

REDOX TRANSFORMATIONS FOLLOWING IRREVERSIBLE ELECTRON-TRANSFER PATHWAYS... 

We turn to the chemical behavior of cycloalkane holes. Several classes of reactions were observed for these holes (1) fast irreversible electron-transfer reactions with solutes that have low adiabatic IPs (ionization potentials) and vertical IPs (such as polycyclic aromatic molecules) (2) slow reversible electron-transfer reactions with solutes that have low adiabatic and high vertical IPs (3) fast proton-transfer reactions (4) slow proton-transfer reactions that occur through the formation of metastable complexes and (5) very slow reactions with high-IP, low-PA (proton affinity) solutes. 

The Pu +/Pu + couple for a series of Pu(IV)/(EDTA ) based complexes, where EDTA = ethylenediaminetetraacetate, has been studied as a function of pH and EDTA concentration [118]. The voltammetry was also studied with citrate and carbonate ions present in solution. At a relatively low pH of 2.3 and equimolar Pu +/EDTA concentrations a quasi-reversible one-electron reduction is observed for Pu(EDTA) at E /2 = 0.342 V versus SHE. The quasireversibility of this process remains as the pH is raised to 4.6. Additional voltammetry studies are discussed in the paper for the higher coordinate Pu + species, Pu(EDTA)-L (where L = EDTA, carbonate, citrato), all of which show irreversible electron-transfer behavior. 

The system is reversible in the absence of an added electron donor but undergoes irreversible reaction at the reduced rhenium bipyridine center in the presence of added triethylamine. The observation of reaction at the rhenium site upon excitation in the absorption band of the metalloporphyrin site is compatible with an ultrafast back electron transfer, provided that the triethylamine coordinated to the magnesium prior to absorption and that the electron transfer from the metalloporphyrin to the bipyridine was followed rapidly by irreversible electron transfer from the triethylamine to the metalloporphyrin. The experiments graphically demonstrated the benefits of the incorporation of carbonyl ligands at the electron acceptor as they allowed a tracking of the sequence of charge separation and back electron transfer via time-resolved IR data . ... 

Figure 1. Energy diagram of irreversible electron transfer from an electron donor (D) to an acceptor (A).

Irreversible electron transfers such as those shown in Fig. 6.12 are rarely observed in organic electrochemistry. The radical ions resulting from slow electron transfer reactions... 

So, a totally irreversible process could be mistaken for a quasi-reversible one with a 0.5 (Fig. 7.17f). In order to discriminate the reversibility degree of the electrochemical reaction, it is necessary to take into account that for a quasi-reversible process the peak corresponding to more cathodic potentials in the second scan (denoted as RC by [29]) is higher than that located at more anodic ones (denoted as RA by [29]) when a 3> 0.5, whereas the opposite is observed for a fully irreversible electron transfer for any value of a (see also Table insert, Fig. 7.20). 

For irreversible systems the peak potential of a reduction process is shifted toward more negative potentials by about 0.030 V for a decade increase in the scan rate [Eq. (3.43)]. By analogy, a peak of an anodic process is shifted toward more positive potentials. The most characteristic feature of a cyclic voltammogram of a totally irreversible system is the absence of a reverse peak. However, it does not necessarily imply an irreversible electron transfer but could be due to a fast following chemical reaction. 

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