Reversible one-electron process

It means that, for reversible one-electron processes, the peak-to-peak separation assumes different values as a function of the temperature namely ... 

One may suppose, for example, that, as illustrated in Figure 29a, an original complex M(L ) gives a cathodic response consistent with a reversible one-electron process (say at E01 = -0.50 V). 

Figure 35 Comparison between the cyclic voltammetric responses of a reversible one-electron process E° = 0.00 V) complicated by different adsorption phenomena (—) and that of a reversible one-electron process (- -). (a) Weak adsorption of the reagent Ox (b) weak adsorption of the product Red (c) strong adsorption of the reagent Ox (d) strong adsorption of the product Red...

The fact that either the peak-to-peak separation, AEp, somewhat departs from the value of 59 mV or the current function ipJvl/2 is not rigorously constant seems to contrast with the diagnostic criteria (illustrated in Chapter 2, Section 1.1.1) for an electrochemically reversible one-electron process. This can be largely attributed to the non-compensated resistance given by the dichloromethane solution, which is a low conducting solvent. 

Firstly, let us discuss its electrochemical behaviour. As previously illustrated in Chapter 2, Figure 5, the anodic response in dichlorome-thane solution also shows features of chemical reversibility 0pCApa= 1)-The peak-to-peak separation (A p = 76mV) again indicates a slight deviation from the theoretical value of 59 mV expected for an electrochemically reversible one-electron process. 

Figure 14 Cyclic voltammetric responses for a reversible one-electron process expected at (a) a perfectly clean electrode surface (b) an electrode surface covered by a passivating layer...

In aprotic solvents, the first reduction step of dissolved oxygen is also a reversible one-electron process to form superoxide ions (02 + e- O -)- However, in aqueous solu-... 

In some cases, the reduction mechanism of a metal complex varies with solvent. For example, the complex [Fe(bpy)3]2+ (bpy = 2,2 -bipyridine) in aqueous solutions is reduced at a dropping mercury electrode directly to metal iron by a two-electron process [Fe(II) —> Fe(0)]. In aprotic solvents, however, it is reduced in three steps, each corresponding to a reversible one-electron process, and the final product is [Fe(bpy)3], which is relatively stable [5] ... 

E vs. log(id-i)/f which should be linear with a slope of 59.1/n mV at 25 °C if the wave is reversible. This method relies however upon a prior knowledge of n, and if this is not known then the method is not completely reliable as theory predicts that when the electron transfer process itself is slow, so that equilibrium at the electrode between the oxidized and reduced forms of the couple is established slowly and the Nemst equation cannot be applied, then an irreversible wave is obtained and a similar plot will also yield a straight line but of slope 54.2/ana mV at 25 °C (a = transfer coefficient, i.e. the fraction of the applied potential that influences the rate of the electrochemical reaction, usually cu. 0.5 na = the number of electrons transferred in the rate-determining step). Thus a slope of 59.1 mV at 25 °C could be interpreted either as a reversible one-electron process or an irreversible two-electron process with a = 0.45. If the wave is irreversible in DC polarography then it is not possible to obtain the redox potential of the couple. 

Because the electron-transfer reduction of 02 is a reversible one-electron process, the hope for an electron-transfer catalyst is futile. Atom-transfer catalysts [Eqs. (9.54) and (9.55)] can promote more extensive reduction and higher potentials (those that correspond to the reduction of the metal oxide). 

DTBSQ) in the presence of an equivalent of HO-.12 The first reduction step is a reversible one-electron process that is followed by a second one-electron reduction, which can be reversible in rigorously anhydrous media to give catechol dianion (3,5-DTBC2-) ... 

The chromium analogue of 16, [(mesitylene)Cr(CO)3] (17) also undergoes a net two-electron oxidation in MeCN, but the chemistry involved is entirely different for the two complexes. With 17+, attack by solvent leads to rapid loss of arene and CO ligands, accompanied by further oxidation. (At fast scan rates, the 17+/17 couple becomes a reversible one-electron process.) A comparison of the relative reactivity of the 17-electron complexes 16+ and 17+ toward associative attack by MeCN led to the rate order W > Cr (ratio of ca. 104 1). It was suggested that this reactivity order reflects less steric congestion for nucleophilic attack at the... 

Hunig and co-workers have investigated the polarography of 4,4 -bipyrylium, bithiopyrylium, and bipyridinium salts in CHjCN (73LA1036). The process involves two reversible one-electron processes, involving the dication 13, the radical cation 55, and the neutral compound 14, as indicated by Eq. (1). 

For a reversible electron-transfer process, the Tafel relationship corrected for mass transport holds in the central region of the voltammogram (Brett and Oliveira-Brett, 1993). Therefore, for a reversible one-electron process, a plot of - Euz versus logio(/ - Zi7m) will have a slope of 59/n mV per decade at 25°C. 

Fig. 27 Voltammogram obtained for an irreversible one-electron transfer at a hydrodynamic electrode (note a reversible one-electron process is also illustrated with equal half-wave potential for comparison).

Figure 4.30 Schematic representation of three reversible one-electron processes.

Also, in case of the two reported bisulphides, Ph2As(S)(CH2) As(S)Ph2, n= 2,3, cleavage of the As—S bonds leading to Ph2As(CH2) AsPh2, n = 2,3, has been reported . Equation 35 is in agreement with results obtained in reduction of PhjPS (the reduction of PhjPO is a chemically reversible one-electron process in DMF) . 

Compared to the analogous trisubstituted phosphines, the trisubstituted arsines are oxidized at slightly higher potentials (50-100mV) as expected from the decrease in electron-donating ability in the series N, P, As. However, the analogous stibines are oxidized at almost the same potentials as the arsines but differences in the rates of the follow-up reactions may be the cause of this apparent coincidence in irreversible potentials. Trimesitylarsine and trimesitylstibine are oxidized in diffusion-controlled, chemically reversible one-electron processes ", and the reversible potentials for these oxidations show (cf Table 14) that the stibine is 60 m V more difficult to oxidize than the arsine. 

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