First electron transfer effect

If the substituents are, however, electron-donating, the first electron transfer must take place at the double bond bearing the substituents. Hence, it is impossible to observe the transannular effect in this case. 

There are two mains aspects of the role of dimerization of intermediates on the electrochemical responses that are worth investigating in some detail. One concerns the effect of dimerization on the primary intermediate on the current-potential curves that corresponds to the first electron transfer step, the one along which the first intermediate is generated. Analysis of this effect allows the determination of the dimerization mechanism (radical-radical vs. radical-substrate). It is the object of the remainder of this section. 

Standard potential of the second electron transfer more cathodic than that of the first electron transfer (AE0 negative). One can consider the case where the formal electrode potential of the second couple is more cathodic, by at least 180 mV, with respect to the first couple (which has, for example, E01 = 0.00 V). If kf is low (compared to the intervention times of cyclic voltammetry i.e. if k[< n F- v/R T), the response will be due to the first electron transfer process, without complications caused by the following chemical reaction. As increases, the second process will have increasing effect up to the limiting case in which kt >n-F-v/R-T. In this limiting case the voltammogram will display two forward peaks, but only the second electron transfer will exhibit a return peak. 

The influence of p-toluidine [68], 1,5-diaminonaphthalene (DAN), and N,N -diphenylthiourea (DFTU) [69] in water-organic mixtures on the two-step electroreduction of Zn(II) was also examined. The presence of p-toluidine accelerated the first electron transfer in water —90 vol % DMF and water —91 vol % methanol [68]. DAN and DFTU had no effect on Zn(II) electroreduction in aqueous solutions but they also catalyzed this process in water-methanol mixtures [69]. 

The catalytic influence of several organic substances on the electrode process of the system Zn(II)/Zn(Hg) was investigated intensively. In the presence of anthranilic and thiosalicylic acids [78], the rate constant of the first electron transfer k increased with increasing surface concentration of acids, while the rate constant of the second electron transfer decreased. The catalytic effect of thiosalicyhc acid is higher than that of anthranilic acid. 

The catalytic effect of N,N -dia -kylthioureas on Zn(II) electroreduction was also observed by Dalmata [81], In this case, the rate constant of the first electron transfer increased with increasing concentration of M,Al -dialkylthioureas, whereas the rate constant of the second electron transfer was largely dependent on the double-layer effect. 

The studies on catalytic influence of di-aminotoluene isomers on zinc reaction have shown the highest accelerating effect for 3,4-diaminotoluene [82-85]. The difference in catalytic activity of these isomers resulted from the difference in complex formation with Zn(II) ions because the adsorption properties of these isomers on the mercury electrode are similar [82, 84]. Two steps of Zn(II) electroreduction were postulated in the presence of diaminotoluene isomers, with the first electron transfer as a rate-determining step. The influence of diaminotoluene isomers and pH on this electrode process was studied in acetate buffers [86]. 

When the first electron transfer is rate limiting, it has been suggested that the thermodynamic argument for gating by substrate binding overlooked the effect of O2 binding. The reduction potential of the P450 enzymes (equation 7) are measured under anaerobic conditions. Under turnover conditions, O2 is present and rapidly binds to the heme Fe(II) center (equation 8). 

Despite the abovementioned quasi-equilibrium character of the cyclic voltam-metric curves, a pronounced hysteresis (i.e., a considerable difference between the anodic and cathodic peak potentials) appears. Slow heterogeneous electron transfer, effects of local rearrangements of polymer chains, slow mutual transformations of various electronic species, a first-order phase transition due to an S-shaped energy diagram (e.g., due to attractive interactions between the electronic and ionic... 

The main effect of changing the cationic component of the ioific liquid was found to be its effect on the solvent viscosity, as the diffusion coefficient (D) of each species was found to be inversely proportional to the viscosity across the series of ionic liquids, in accordance with Stake s equation [16,17], The only deviation from this relationship arose for the case when [P6,6,6,i4][N(Tf)2] was used as the solvent [17]. Here, though relatively normal voltammetry was observed for the oxidation and immediate re-reduction of xxi (first electron transfer in Eq. 15.22), sweeping the potential at a more positive value to promote the formation of the dication, resulted in an anomalous wave shape. This behavior was rationalized in terms of a dimerization reaction between the dication and neutral xxi (Eq. 15.23), with the further dimer oxidation at a potential more positive than that for the first oxidation of the monomer (Eq. 15.24) [17]. 

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