Multiple electron transfer

The nature of electrode processes can, of course, be more complex and also involve phase fonnation, homogeneous chemical reactions, adsorption or multiple electron transfer [1, 2, 3 and 4],... 

Multiple electron transfer with intervening chemical reaction—ECE mechanism... 

We have therefore been able to prepare the highly stable tetracation 254+ salt despite the presence of four positive charges in the structure. However, this first example for a cyanine-cyanine hybrid with cyanine units at both two termini did not demonstrate the presumed multiple color changes during electrochemical reduction. However, the tetracation 254+ exhibited multiple-electron transfer as a function of the substituted di(l-azulenyl)methylium units and also showed color change during the electrochemical reduction. 

The activity of these materials was attributed to the ability to promote multiple electron transfer for 02 evolution and the presence of at least partially separated oxidation and reduction sites [108]. Their major drawback is the high optical band-gap, which in several cases is over 4.0 eV (7 < 310 nm). 

Substantial research efforts should be dedicated also to the development of low cost multiple-electron transfer catalysts for oxygen production. Electrochemical losses related to O2 evolution are a considerable part of the overall inefficiencies. 

Low cost multiple electron transfer catalysts for oxygen production... 

Even if the present chapter is essentially devoted to mononuclear complexes, we have already seen in Sections 3.2 and 3.3, that polynuclear complexes may hold great interest in applicative fields, in particular as far as their ability to undergo reversible multiple electron transfers such a property is typical of semiconducting materials. 

Even when forward reactions proceed rapidly at laboratory conditions, as is observed with Se(IV) and Cr(VI) reduction, evidence exists that chemical and isotopic equilibrium are not approached rapidly. Altman and King (1961) studied the kinetics of equilibration between Cr(III) and Cr(Vt) at pH = 2.0 to 2.5 and 94.8°C. Radioactive Cr was used to determine exchange rates, and Cr concentrations were greater than 1 mmol/L. Time scales for equilibration were found to be days to weeks. The mechanism of the reaction was inferred to involve unstable, ephemeral Cr(V) and Cr(IV) intermediates. Altman and King (1961) stated that the slowness of the equilibration was expected because the overall Cr(VI)-Cr(III) transformation involves transfer of three electrons and a change in cooordination (tetrahedral to octahedral). Se redox reactions also involve multiple electron transfers and changes in coordination. 

Figure 6.14 Cyclic voltammogram obtained for a multiple-electron-transfer system, where a thin film of sputtered V2O5 on a platinum working electrode has been immersed in an electrolyte solution of propylene carbonate containing LiCI04 (1.0 mol dm ). From Cogan, S. F., Nguyen, N. M Perrotti, S. J. and Rauh, R. D Electroctromism in sputtered vanadium pentoxide , SPIE, 1016, 57-62 (1989). Reproduced by permission of the International Society for Optical Engineering (SPIE).

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. 

Recent Achievements in Apparently Direct Multiple Electron Transfers... 

An issue that is being considered with attention concerns the possibility to achieve multiple electron transfers on the first wave of POMs. The aim is twofold (1) save energy by favoring those electro-catalytic processes that necessitate several electron to be performed (2) avoid the... 

The reach of cyclic voltammetry is vast. It has been applied to the investigation of simple electron-transfer reactions those with two successive electron transfers (so-called EE reactions) and with multiple electron transfers (EEE) involving electron transfer to and from compounds, say, with several benzene rings. The technique has been applied to complex sequences in which an electron transfer is followed by a chemical reaction step, and then by another electron transfer (ECE reactions), etc. The complexity of some of the reaction sequences investigated by cyclic voltammetry lends itself well to calculations that need computers the classic work of Feldburg in this direction (digital simulation) has been already mentioned (Section 7.5.19.2). 

The rate of flow of electrons from such a charged particle depends on the availability of an accessible site for this transfer. Although it is known that lattice defects provide such sites and that conduction band electrons can trickle down through solid dislocation levels reduction sites for electron accumulation are usually provided by metallization of the semiconductor particle. This can be achieved through photo-platinization or by a number of vapor transfer techniques and the principles relevant to hydrogen evolution on such platinized surfaces have been delineated by Heller The existence of such sites will thus control whether single or multiple electron transfer events can actually take place under steady state illumination. 

As can be seen from Table X, bright eel emission is observed only when both radical-anion and radical-cation are of moderate stability. The mechanism of the eel emission has been studied in some detail. A cation-anion annihilation delivers insufficient energy to reach the excited singlet state directly. Probably, ion-radical aggregates are involved and multiple electron transfer results in sufficient energy accumulation. 

The mixed potential accounts for a large portion of reported artifacts in the unorthodox potentiometric sensors, particularly biosensors, and can be rightfully called evil potential . The physical origin of such artifacts can be illustrated using a simple example. Let us assume that a multiple electron transfer takes place simultaneously at the interface of a lump of Zn immersed in dilute HC1. Because this metal is not externally connected the net current is zero. The redox reactions taking place are as follows. 

Rathore, R., Bruns, C.L and Deselnicu, M.I. (2001) Multiple-electron transfer in a single step Design and synthesis of highly charged cation-radical salts. 

In dissolving-metal ester reduction, the ester carbonyl is believed to accept an electron to form a radical oxyanion 37 (Scheme 12.12). Chelation with a lithium counterion then ensues to produce a tertiary radical 38 which then captures a second electron to become a carbanion. Protonation of 39 next yields 40, whose fate is to collapse to aldehyde 41. Another multiple electron transfer/protonation sequence subsequently yields the product alcohol 46. 

Electrode processes involving multiple electron transfer... 

Electrode processes involving multiple electron transfer 119 are two extreme cases ... 

Finally in this section, we remember that multiple electron transfer has to follow the reaction coordinate and has consecutive steps, even if the first step is rate determining. The possibility of multiple electron transfer reactions without intermediate chemical steps has been questioned, with experimental evidence from, for example, the supposedly relatively simple reduction of Cd(II) and similar ions at mercury electrodes6. This is because solvation and interaction with the environment, adsorption, etc. are different for each oxidation state. 

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