Multiple electron-transfer reactions

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. 

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. 

Mn and Mn " on the surface of Mn02 can be oxidized by the Mn02 in an autocatalytic inner sphere process before detachment the original Mn and Mn then become Mn and Mn02 which are reduced by the oxalate. The complexity of the rate law (eq. 4) is consistent with multiple electron transfer reaction pathways for eq. 6. Our data do not support a buildup of the Mn(III)-oxalate complex, [Mn(ox)2(H20)2] , in solution which has been observed in sulfuric acid solutions of permanganate, Mn and oxalate at pH < 2 (30). 

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... 

The interconversion between different spin states is closely related to the intersystem crossing process in excited states of transition-metal complexes. Hence, much of the interest in the rates of spin-state transitions arises from their relevance to a better understanding of intersystem crossing phenomena. The spin-state change can alternatively be described as an intramolecular electron transfer reaction [34], Therefore, rates of spin-state transitions may be employed to assess the effect of spin multiplicity changes on electron transfer rates. These aspects have been covered in some detail elsewhere [30]. 

One expects the impact of the electronic matrix element, eqs 1 and 2, on electron-transfer reactions to be manifested in a variation in the reaction rate constant with (1) donor-acceptor separation (2) changes in spin multiplicity between reactants and products (3) differences in donor and acceptor orbital symmetry etc. However, simple electron-transfer reactions tend to be dominated by Franck-Condon factors over most of the normally accessible temperature range. Even for outer-... 

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. 

In Chapter 7 general kinetics of electrode reactions is presented with kinetic parameters such as stoichiometric number, reaction order, and activation energy. In most cases the affinity of reactions is distributed in multiple steps rather than in a single particular rate step. Chapter 8 discusses the kinetics of electron transfer reactions across the electrode interfaces. Electron transfer proceeds through a quantum mechanical tunneling from an occupied electron level to a vacant electron level. Complexation and adsorption of redox particles influence the rate of electron transfer by shifting the electron level of redox particles. Chapter 9 discusses the kinetics of ion transfer reactions which are based upon activation processes of Boltzmann particles. 

The electron waiting-line problem is hence clear. In a particular multistep electron-transfer reaction, the step with the lowest servicing rate or conductivity produces the largest queue and, indeed, the total queue is virtually a simple multiple of the queue at the rds. In other words, in the steady state, all n steps proceed at the rate of the rate-determining step ir, [cf. Eq. (9.4)], and the total net current is... 

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 compound bis-(4,4 -dimethylaminophenyl)-sulfone (DMAPS) and related compounds show multiple fluorescences in polar solvents due to excited state charge transfer (Rettig and Chandross [144]). Su and Simon [84,85] have examined the intramolecular electron transfer reaction in DMAPS, in alcohol solution over the temperature range from — 50°C to + 30°C. They observe that the decay of the local excited state is nonexponential and significantly faster than the longitudinal relaxation time of the solvent. In addition, they observed that the emission spectrum of the TICT state... 

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. 

As most of us recall from our struggles with balancing redox equations in our beginning chemistry courses, many electron-transfer reactions involve hydrogen ions and hydroxide ions. The standard potentials for these reactions therefore refer to the pH, either 0 or 14, at which the appropriate ion has unit activity. Because multiple numbers of H+ or OH- ions are often involved, the potentials given by the Nernst equation can vary greatly with the pH. 

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