True Electron Transfer

In a true electron transfer reaction, the electron transfer step is the slowest step in the overall redox reaction such that k3 = kg-p- When such a 

In a coupled electron transfer reaction, the preceding adiabatic reaction step influences the experimentally-determined rate constant even though the electron transfer step is the slowest for the overall redox reaction. This occurs when the relatively fast reaction step which precedes electron transfer is very unfavorable (i.e. Kx (kx/kfix) l)- In Hii case, ks will be influenced by the equilibrium constant for that non-electron transfer process such that ks = kgT Kx (Harris et ah, 1994 Davidson, 1996). It follows that the experimentally-derived X ( lobs) may contain contributions from both the electron transfer event and the preceding reaction step (i.e. obs [ ET. x])- For example, lo sfor interprotein electron transfer reactions may reflect contributions from an intracomplex rearrangement of proteins after binding to achieve an optimum orientation for electron transfer. As with a true electron transfer reaction, k3 will vary with AG° since ks is proportional to ksT, although H b may also be affected by the coupling. 

---

This value is discussed in terms of two-electron transfer when Zn + is reduced to free zinc on Pt(lll) surface with a true electron transfer number of = 2. Also, induced adsorption of OH ions takes place to give OHads in an oxidative process. 

Grove TZ, Kostic NM. Metalloprotein association, self-association, and dynamics governed by hydrophobic interactions simultaneous occurrence of gated and true electron-transfer reactions between cytochrome and cytochrome c6 from Chlamydomonas reinhardii. J Am Chem Soc 2003 125 10598-607. 

Unlike substitution reactions, electron transfer reactions proceed without mass transfer between the reacting species. Sometimes it is not possible to decide a priori whether a reaction is a true electron transfer reaction or rather a mass transfer (substitution) reaction (21). For example, evidence has been presented from kinetic studies that the reaction... 

Osakai, T., S. Ichikawa, H. Hotta, and H. Nagatani, A true electron-transfer reaction between 5,10,15,20-tetraphenylporphyrinato cadmium(II) and the hexacyanoferrate couple at the nitrobenzene/water interface. Anal Sci, Vol. 20, (2004) p. 1567. 

The discussion thus far in this chapter has been centred on classical mechanics. However, in many systems, an explicit quantum treatment is required (not to mention the fact that it is the correct law of physics). This statement is particularly true for proton and electron transfer reactions in chemistry, as well as for reactions involving high-frequency vibrations. 

Selecting a rigorous and convenient quantitahve parameter characterizing the catalyhc achvity, A, is of prime importance when studying electrocatalytic phenomena and processes. The parameter usually selected is the current density, i (in AJan ), at a specified value of electrode poteuhal, E. The current density is referred to the electrode s true working surface area [which can be measured by the Brunauer-Emmett-TeUer (BET) or other methods]. Closely related to this true current density is another parameter, known as the turnover number y (in s ), and indicating the number of elementary reachon acts performed or number of electrons transferred in unit time per surface atom (or catalytic surface site) of the catalyst. 

Electron transfer reactions constitute an ubiquitous class of chemical reactions. This is particularly true in biological systems where these reactions often occur at interfaces, in photosynthesis for instance. It is therefore challenging to use the surface specificity and the time resolution of the SHG technique to investigate these processes. At liquid-liquid interfaces, these processes are mimicked through the following scheme ... 

In an anionic/radical domino process an interim single-electron transfer (SET) from the intermediate of the first anionic reaction must occur. Thus, a radical is generated which can enter into subsequent reactions. Although a SET corresponds to a formal change of the oxidation state, the transformations will be treated as typical radical reactions. To date, only a few true anionic/radical domino transformations have been reported in the literature. However, some interesting examples of related one-pot procedures have been established where formation of the radical occurs after the anionic step by addition of TEMPO or Bu3SnH. A reason for the latter approach are the problems associated with the switch between anionic and radical reaction patterns, which often do not permit the presence of a radical generator until the initial anionic reaction step is finished. 

The opposite is true for the o- and p-dicyanomethylene quinocyclopropenes 118-1257S. The only electron-transfer observed on polarography corresponds to a one-electron oxidation resulting in the radical cation 479. A qualitative explanation can be seen in the transformation of the quinocyclopropene into two Htickel-... 

The first attempt to describe the dynamics of dissociative electron transfer started with the derivation from existing thermochemical data of the standard potential for the dissociative electron transfer reaction, rx r.+x-,12 14 with application of the Butler-Volmer law for electrochemical reactions12 and of the Marcus quadratic equation for a series of homogeneous reactions.1314 Application of the Marcus-Hush model to dissociative electron transfers had little basis in electron transfer theory (the same is true for applications to proton transfer or SN2 reactions). Thus, there was no real justification for the application of the Marcus equation and the contribution of bond breaking to the intrinsic barrier was not established. 

How can these photochemical and electrochemical data be reconciled With the benzylic molecules under discussion, electron transfer may involve the n or the cr orbital, giving rise to stepwise and concerted mechanisms, respectively. This is a typical case where the mechanism is a function of the driving force of the reaction, as evoked earlier. Since the photochemical reactions are strongly down-hill whereas the electrochemical reaction is slightly up-hill at low scan rate, the mechanism may change from stepwise in the first case to concerted in the second. However, regardless of the validity of this interpretation, it is important to address a more fundamental question, namely, whether it is true, from first principles, that a purely dissociative photoinduced electron transfer is necessarily endowed with a unity quantum yield and, more generally, to establish what are the expressions of the quantum yields for concerted and stepwise reactions. 

The analytical usefulness of this reaction, stems mainly from that fact that the electrochemically generated Ru(bpy)33+ species can be reduced by a large number of potential analyte compounds, or their electrochemical derivatives, via high-energy electron transfer reactions, to produce the Ru(bpy)32+ excited species, without the need for an electrochemical reduction step. The converse is also true. The reduction of peroxodisulfate (S2082-) for example, in the presence of Ru(bpy)32+, produces the Ru(bpy)32+ excited species and an ECL emission, from the reaction of Ru(bpy)3+ and S04 [20], Although this latter system has been used for the determination of both Ru(bpy)32+ [21] and S2082- [22], the vast majority of analytical applications use the co-oxidation route. 

At present, new developments challenge previous ideas concerning the role of nitric oxide in oxidative processes. The capacity of nitric oxide to oxidize substrates by a one-electron transfer mechanism was supported by the suggestion that its reduction potential is positive and relatively high. However, recent determinations based on the combination of quantum mechanical calculations, cyclic voltammetry, and chemical experiments suggest that °(NO/ NO-) = —0.8 0.2 V [56]. This new value of the NO reduction potential apparently denies the possibility for NO to react as a one-electron oxidant with biomolecules. However, it should be noted that such reactions are described in several studies. Thus, Sharpe and Cooper [57] showed that nitric oxide oxidized ferrocytochrome c to ferricytochrome c to form nitroxyl anion. These authors also proposed that the nitroxyl anion formed subsequently reacted with dioxygen, yielding peroxynitrite. If it is true, then Reactions (24) and (25) may represent a new pathway of peroxynitrite formation in mitochondria without the participation of superoxide. 

---