Two electron transfer process

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... 

Figure 24. Analysis of the exchange mechanism for EET in hole transfer (HT) in the correspon ingca g terms of two electron transfer processes, one involving ET in side), the corresponding anion (centre) and the other involving...

Using a typical A value for intramolecular electron transfer of 750 mV, it is possible to use the Dutton s ruler expression to determine the rates of the two electron-transfer processes ... 

Figure 34 Typical cyclic voltammograms for a two-electron transfer process in which one of the steps is irreversible. Case R-I (a) A E° = —180 mV (b) A E° = 0 mV. Case I-R (c) AE01 = -180 mV. Case I-I (d) AE01 = -180 mV...

Reinmuth notation. In the electrochemical world, the sequence of electrode and/or chemical reactions that occur are described by a simple shorthand code. Simple electron-transfer reactions are called E reactions. In the same shorthand system, a multiple electron-transfer reaction such as Fe " Fe " -> Fe is an EE reaction , i.e. the product of an electron-transfer process itself undergoes a second electron-transfer process. (Note that the two electron-transfer processes might occur at the same time, in which case it is merely an E reaction.) The vanadium pentoxide system illustrated in Figure 6.14 is another example of an EE system. 

For a discussion of single electron shifts in two-electron transfer processes see Refs [4d] and... 

In the oxidation of glycolaldehyde in alkali, a two-electron-transfer process is proposed for Pt(IV) but a one-electron-transfer process for Ir(IV). ... 

The photoreduction of polymer pendant viologen by 2-propanol was reported to proceed by the successive two-electron transfer processes between the adjacent viologen units and the propanol which is a two-electron reducing agent44). Preferential formation of a dimeric cation radical of viologen observed was ascribed to the polymeric structure and the two-electron process. These fundamental studies on polymeric electron mediators contribute to the construction of solar energy conversion systems. 

Most successful selective fluorination reactions are carried out under conditions which limit any free radical processes and encourage nucleophilic attack on fluorine either by a one- or two-electron transfer process (see Sect. 3.1.1.3). 

The extent of the above so-called side reaction was determined by considering the decrease in the absorbance of the chloride band. We found that with the stoichiometric amount of chloro-(pyridine)cobaloxime(III), the side reaction was much more prominent than with the catalytic amount. Nevertheless, no appreciable unsaturation (the absence of CF=CF2 around 1790 cm1)39 was indicated in the IR spectra in both cases, implying that the undesired two-electron transfer process was not involved. 

Fig. 3.18 Current density-potential curves for two-electron transfer processes at disc (solid line) and spherical (dashed line) microelectrodes of the same radius. The values of the difference between the formal potentials of the redox centers, AEf (in V), are indicated on the curves. These curves have been calculated with Eq. (3.154) by assuming ra = rd = rs = 5 pm, t = 1 s. D = 10 5 cm2 s-1. T = 298 K...

For the sake of simplicity, only two-electron transfer processes are discussed in this section. The theory corresponding to multielectronic transfer processes can be easily generalized from that presented here and is discussed in detail in [60-62],... 

In this section, the current-potential curves of multi-electron transfer electrode reactions (with special emphasis on the case of a two-electron transfer process or EE mechanism) are analyzed for CSCV and CV. As in the case of single and double pulse potential techniques (discussed in Sects. 3.3 and 4.4, respectively), the equidiffusivity of all electro-active species is assumed, which avoids the consideration of the influence of comproportionation/disproportionation kinetics on the current corresponding to reversible electron transfers. A general treatment is presented and particular situations corresponding to planar and nonplanar diffusion and microelectrodes are discussed later. 

Tetrathionate reductase converts tetrathionate to two equivalents of thiosulfate, with no change in the number of oxygen atoms. Polysulfide reductase catalyzes the conversion of sulfur (in the form of a polysufide) to sulfide (HS ), a two-electron transfer process, through which a sulfur atom is reduc-... 

Both single-sweep and cyclic voltammetry can provide information about the approximate number of electrons transferred in each wave or peak. This is done by comparing the plateau or peak height with that of a known one- or two-electron transfer process under identical conditions (as an example, the oxidation of 9,10-diphenylanthracene to the cation radical is a commonly used reference reaction). 

Nickel catalysis is a very active field in organometallic and organic chemistry (selected reviews [3-7]). Complexes of all oxidation states are active in two-electron transfer processes, such as oxidative addition or reductive elimination as well as in single electron transfer initiating radical reactions. Through these processes, oxidation states from Ni(0) to Ni(III) can be easily accessed under mild conditions. Occasionally, Ni(IV) intermediates were also proposed. Apart from the vast number of Ni(II) complexes, a number of organonickel(I) complexes were characterized by X-ray crystallography and their potency as active species in catalytic cycles tested [8-10]. Either radical or two-electron reactivity was observed. Recently, the structure of some alkylnickel(III) complexes was also structurally elucidated [11]. 

Electron-exchange reactions in the Tl1—Tl111 system have been intensively studied and appear to be two-electron transfer processes various Tl111 complexes participate under appropriate conditions. 

Depending on the relative values of Eh, and ki, the voltammograms associated with an ECE mechanism consists of two resolved one-electron transfer processes (Fig. 12a) or a single overall two-electron transfer process (Fig. 12b) or intermediate situations. In the above oxidative mechanism D could be formed by the homogeneous disproportionation (DISP) step (24)... 

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