EE mechanism

Many anodic oxidations involve an ECE pathway. For example, the neurotransmitter epinephrine can be oxidized to its quinone, which proceeds via cyclization to leukoadrenochrome. The latter can rapidly undergo electron transfer to form adrenochrome (5). The electrochemical oxidation of aniline is another classical example of an ECE pathway (6). The cation radical thus formed rapidly undergoes a dimerization reaction to yield an easily oxidized p-aminodiphenylamine product. Another example (of industrial relevance) is the reductive coupling of activated olefins to yield a radical anion, which reacts with the parent olefin to give a reducible dimer (7). If the chemical step is very fast (in comparison to the electron-transfer process), the system will behave as an EE mechanism (of two successive charge-transfer steps). Table 2-1 summarizes common electrochemical mechanisms involving coupled chemical reactions. Powerful cyclic voltammetric computational simulators, exploring the behavior of virtually any user-specific mechanism, have... 

Eriksson EE. Mechanisms of leukocyte recruitment to atherosclerotic lesions future prospects. Curr Opin Lipidol 2004 15(5) 553—558. 

Two reversible one-electron transfers. The simplest case to treat consists of two successive one-electron transfers (EE mechanism). 

It should be recalled that in the case of the EE mechanism, in solution one can have the comproportionation reaction ... 

In the theory of SWV, two different types of surface EE mechanisms have been treated [91,92], O Dea et al. [91] considered a mechanism in which the first redox step was chemically reversible, whereas the second one was a totally irreversible process. In the succeeding study [91], a more general case has been treated consisting of two quasireversible redox transformations, as indicated by (2.129) ... 

Cotton, effect of oxidants on, 462,687 Cotton fiber, ozone damage to, 665 Coulombmetry. See Amperometric analyzers <>i ee mechanism, for liquid-phase ozone-olefin reaction, 72-74, 76 Cydic olefins, 4,60,76 aerosols from, 70-72,83,88 importance of, 104... 

The double-layer effect in the electrode kinetics of the amalgam formation reactions was discussed [67]. The dependences on the potential of two reduction (EE) mechanisms of divalent cations at mercury electrode, and ion transfer-adsorption (lA) were compared. It was suggested that a study of temperature dependence of the course of these reactions would be helpful to differentiate these two mechanisms. 

Constant current chronopotentiometry has been utilized by Honeychurch [156] to study reduction of methylene blue adsorbed at HMDE. The obtained results were interpreted in terms of a two consecutive electron transfers (EE) mechanism and the interfacial potential distribution model. 

Figure 30(a) concerns the EE mechanism for the reaction O + 2 e = R. The solid curve represents the standard free energy profile pertaining to the standard potential E° of the redox couple O/R. In this case, the energy levels of the initial and the final state are equal by definition. Well... 

Fig. 30. Schematic representation of energy vs. reaction coordinate diagrams for (a) an EE mechanism, (b) a CECEC mechanism (see text). ------, E = E° ------, E < E° ...

Strictly, this is a logical extension of the EE mechanism with stable intermediate, considered in Sect. 5. The reduction product of the first electrode reaction, Z, is supposed to be non-reducible itself, but can react to yield a compound X that can be reduced to R. 

Many electrode reactions, particularly those of organic compounds, take place in two or more steps, exemplified by the EE mechanism... 

The theoretical description of any mechanism is most complex at the DME. Solutions using the stationary plane, expanding plane and the expanding sphere models have been obtained and exhibit some differences, the last being, of course, the most rigorous [168—170]. The EEE mechanism at stationary and expanding planes has also been discussed [171, 172]. Immobilization or adsorption of the intermediate in the EE mechanism is a realistic possibility [173]. 

Figure 23.9 EE mechanism with all parameters same as Figure 23.8, except that E° = -1.00 V, E°2 = -1.05 V.

There are a number of molecules for which two- or more electron transfers can be detected (i.e., stepwise processes). For molecules capable of giving two-electron transfer reactions (EE mechanism see reaction scheme (3.II)), the addition of the second electron occurs, in the more typical case, with greater difficulty than the first, so the single pulse voltammogram presents two well-separated waves because of the difference between the two formal potentials defined as... 

In this section, the electrochemical behavior of an EE mechanism with two reversible electron transfer reactions will be studied. It will also be shown that for this electrode process (given in reaction scheme (3.II)) in both cases, i.e., normal ordering and potential inversion, the disproportionation/comproportionation reaction (3) can take place in the diffusion layer. 

Equation (3.140) shows that the electrochemical response of an EE mechanism depends on the difference between the formal potentials t Ef and is not influenced by the homogeneous reaction. This behavior is shown in Fig. 3.16 where the current... 

E process but with double height is observed. Finally, for very positive values of A Ef (see curves with AEL° = 200 mV in Fig. 3.16a and b), the response of the EE mechanism is indistinguishable from that obtained for a single charge transfer of two electrons (see dashed blue curve). 

The voltammetric response of the EE mechanism can be compared with those for two independent reversible charge transfers (two reversible independent E processes or E + E mechanism) with identical bulk concentrations of oxidized species (c ) by assuming that the diffusion coefficients of all species are equal, that is,... 

Table 3.1 Analytical expressions for the roots with physical meaning of the second derivative of the current of an EE mechanism given by Eq. (3.141) [43]...

The value A Ef = —35.6 mV has the particular interest of corresponding to a 50 % of character E e-E c and E2c At the average formal potential Ef, the intermediate species reaches half of its maximum value and, hence, at this A Ef species 02 may or may not gain a second electron (and as a direct consequence, for higher AEf it will be considered that the intermediate species is no more stable at the average formal potential). So, this AEf could be considered as the boundary between anti-cooperative and cooperative behavior of both electron transfer reactions [35, 43]. Indeed, it is well known that the voltamogram of an EE mechanism under these conditions is identical to that of an E mechanism multiplied by a factor... 

The current for a reversible EE mechanism can achieve a stationary feature when microelectrodes are used since in these conditions the function fG(t, qa) that appears in Eq. (3.150) transforms into fG,micro given in Table 2.3 of Sect. 2.6. For microelectrode geometries for which fo.micro is constant, the current-potential responses have a stationary character, which for microdiscs and microspheres can be written as [16] ... 

Moreover, the current-potential curves are affected by the disproportionation reaction therefore, other variables (the rate constant for the disproportionation reaction) must be taken into account. Since experimental results for many interesting systems show clear evidence of slow kinetics, ad hoc simulation procedures have typically been used for the analysis of the resulting current-potential curves [31, 38, 41, 48]. As an example, in reference [38], it is reported that a clear compropor-tionation influence is observed for an EE mechanism with normal ordering of potentials and an irreversible second charge transfer step. In this case, the second wave is clearly asymmetric, showing a sharp rise near its base. This result was observed experimentally for the reduction of 7,7,8,8-tetracyanoquinodimethane in acetonitrile at platinum electrodes (see Fig. 3.20). In order to fit the experimental results, a comproportionation rate constant comp = 108 M-1 s-1 should be introduced. 

References [40,41] report the chronoamperometric analysis of the response of an EE mechanism with non-reversible charge transfer processes including the consideration of a fast comproportionation step [40], indicating that strong differences in the diffusion coefficients of the different species are needed to cause a clear influence of the comproportionation process in the electrochemical response. 

As in the case of two electrochemical steps (EE mechanism), the following reaction occurs ... 

AEf = -0.200 V. Dotted lines correspond to simple EE mechanism (calculated from Eq. (3.133))... 

Multi-electronic processes (like those consisting of two-electron transfers, EE mechanism) have been widely treated in the literature, both in their theoretical and applied aspects [4, 10, 56-68]. This high productivity measures in some way the great presence and relevance of these processes in many fields, and hence the importance of understanding them. 

For reversible electrode reactions and no solution phase reactions other than disproportionation, the following reaction scheme (EE mechanism) applies (see also Sect. 3.3) ... 

The dimensionless theoretical ADDPV curves for a reversible EE mechanism at a disc electrode of radius rd = 50 pm, with a pulse amplitude A = 50 mV and different values of the difference between the formal potentials of both electrochemical steps, can be seen in Fig. 4.22. In all cases, ADDPV curves have a center of symmetry at the crossing potential, s given by Eq. (4.186). So, the determination of this point helps to extract the formal potentials accurately. Besides being easier to measure than the potential or width of a peak, the s, value is independent of the pulse amplitude, the electrode size and shape, and the difference between the formal potentials, so this diagnosis criterion is very general. 

Fig. 4.23 Comparison between experimental curves for 1.25 mM pyrazine at an SMDE at pH = 1.0 in HC104 + NaC104 adjusted to ionic strength 1.0 M (dotted lines) and theoretical curves for an EE mechanism with A= —117 mV (see Eq. (4.185) for a spherical electrode, solid lines), for different values of the pulse amplitude lA/il (shown in the Figure). = 1 s, r2 = 0.05 s, / 298.15 K, A = 0.011 cm2, D 0.781 x 10 5 cm2 s 1. Reproduced with permission of...

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