Two-electron transfer

The preparation of 5-azothiazoles uses the nucleophilic character of C-5 carbon in reaction with the appropriate diazonium salt (402, 586). These 5-azothia2oles form 1 1 complexes with Ag (587). 2-Amino-4-methyl-5-arylazothiazoles give reduction waves involving two-electron transfer the Ej/ values correlate to the angle between the thiazole and phenyl rings (588). 

In the reaction of lead tetraacetate with 1,3- or 1,4-dihydtopetoxides (10) to produce cychc monoperoxides there are two electron transfers to the metal (eq. 14). 

FIGURE 18.19 The structures and redox states of the nicotinamide coenzymes. Hydride ion (H , a proton with two electrons) transfers to NAD to produce NADH. 

Access to three different redox states allows flavin coenzymes to participate in one-electron transfer and two-electron transfer reactions. Partly because of this, flavoproteins catalyze many different reactions in biological systems and work together with many different electron acceptors and donors. These include two-electron acceptor/donors, such as NAD and NADP, one- or two-elec-... 

Note that flavin coenzymes can carry out either one-electron or two-electron transfers. The succinate dehydrogenase reaction represents a net two-electron reduction of FAD. 

The second step involves the transfer of electrons from the reduced [FMNHg] to a series of Fe-S proteins, including both 2Fe-2S and 4Fe-4S clusters (see Figures 20.8 and 20.16). The unique redox properties of the flavin group of FMN are probably important here. NADH is a two-electron donor, whereas the Fe-S proteins are one-electron transfer agents. The flavin of FMN has three redox states—the oxidized, semiquinone, and reduced states. It can act as either a one-electron or a two-electron transfer agent and may serve as a critical link between NADH and the Fe-S proteins. 

A study of the electrochemical oxidation and reduction of certain isoindoles (and isobenzofurans) has been made, using cyclic voltammetry. The reduction wave was found to be twice the height of the oxidation wave, and conventional polarography confirmed that reduction involved a two-electron transfer. Peak potential measurements and electrochemiluminescence intensities (see Section IV, E) are consistent vidth cation radicals as intermediates. The relatively long lifetime of these intermediates is attributed to steric shielding by the phenyl groups rather than electron delocalization (Table VIII). 

Before considering the role of the electrode material in detail, there is one further factor which should be pointed out. The product of an electrode process may be dependent on the timescale of the contact between the electroactive species and the electrode surface, particularly when a chemical reaction is sandwiched between two electron transfers in the overall process. This was first realized when it was found that ir E curves and reaction products at a dropping mercury electrode were not always the same as those at a mercury pool electrode (Zuman, 1967a). For example, the reduction of p-diacetylbenzene at a mercury pool was found to be a four-electron process, giving rise to the dialcohol, while at a dropping mercury electrode the product was formed by a two-electron process where only one keto group was reduced (Kargin et al., 1966). These facts were interpreted in terms of the mechanism... 

Tl(III) < Pb(IV), and this conclusion has been confirmed recently with reference to the oxythallation of olefins 124) and the cleavage of cyclopropanes 127). It is also predictable that oxidations of unsaturated systems by Tl(III) will exhibit characteristics commonly associated with analogous oxidations by Hg(II) and Pb(IV). There is, however, one important difference between Pb(IV) and Tl(III) redox reactions, namely that in the latter case reduction of the metal ion is believed to proceed only by a direct two-electron transfer mechanism (70). Thallium(II) has been detected by y-irradiation 10), pulse radiolysis 17, 107), and flash photolysis 144a) studies, butis completely unstable with respect to Tl(III) and T1(I) the rate constant for the process 2T1(II) Tl(III) + T1(I), 2.3 x 10 liter mole sec , is in fact close to diffusion control of the reaction 17). 

In principle, the oxidation of proceeds at an electrode potential that is more negative by about 0.7 V than the anodic decomposition paths in the above cases however, because of the adsorption shift, it is readily seen that practically there is no energetic advantage compared to CdX dissolution in competing for photogenerated holes. Similar effects are observed with Se and Te electrolytes. As a consequence of specific adsorption and the fact that the X /X couples involve a two-electron transfer, the overall redox process (adsorption/electron trans-fer/desorption) is also slow, which limits the degree of stabilization that can be attained in such systems. In addition, the type of interaction of the X ions with the electrode surface which produces the shifts in the decomposition potentials also favors anion substitution in the lattice and the concomitant degradation of the photoresponse. 

Both reactions follow simple second-order expressions with A 2(nitrate) = 2.19 + 0.13 l.mole .sec and 2(nitrite) = 0.644 + 0.010 l.mole . sec , both at 25 °C ". In the NOJ reduction, NO is considered to enter the inner sphere of the Re(V) complex rapidly to give [ReCl4(ONO)] which then breaks down to products, following an internal two-electron transfer in the slow step. By analogy, NO is considered to enter the Re(V) complex by displacing H2O rapidly in the nitrite reduction slow internal two-electron transfer to give NO" (or HNO) follows the latter is then consumed by Re(V). 

Benzoyl-CoA reductase carries out the two-electron reduction of the aromatic ring dnring the anaerobic degradation of benzoate by Thauera aromatica. This involves two-electron transfer from ferredoxin, and a combination of EPR and Mossbaner spectroscopy showed the presence of three different clusters, while inactivation by oxygen was associated with partial conversion of [4Fe-4S] clnsters to [3Fe-4S] clnsters (Boll et al. 2000). 

Free radicals generally undergo one-electron transfer processes in homogeneous solution. Two-electron transfer processes, in which two radicals participate, are often highly exoergic. Typical examples are... 

As a rule, high quantum yields for two-electron transfer reactions are expected when the mechanism is one-electron/two-hole or two-electron/one-hole. In the cases of twQ-electron/two-hole or one-electron/one-hole efficient back reactions of the intermediates on the colloidal particles or in solution, respectively, will lead to a low yield of the final products. 

A negative current wave appeared in the polarogram as shown in curve 1 of Fig. 5, though the wave was not observed in the absence of NADH in W or CQ in DCE. The logarithmic analysis of the current wave based on the theoretical equation for the electron transfer [42,54,55] indicated that the wave was caused by two-electron transfer at the interface and controlled by the diffusion of NADH in W. 

I Iapiot, P., L. D. Kispert et al. (2001). Single two-electron transfers vs successive one-electron transfers in polyconjugated systems illustrated by the electrochemical oxidation and reduction of carotenoids../. Am. Chem. Soc. 123 6669-6677. 

Upon further contact with a redox reagent or at higher redox potentials, additional electrons can be transferred. After a two-electron transfer, each redox unit can accept one charge with formation of a singlet or triplet dianion, or (less favourably from an electrostatic point of view) both charges can enter one redox unit. Here again, an intramolecular electron-exchange process is possible. 

Since the excited state may act as an electron donor or as an electron acceptor, two electron-transfer quenching pathways, reductive and oxidative, are possible. 

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