Heterogenous electron transfer outer sphere

Figure 1 Schematic representation of an heterogeneous electron transfer taking place through an outer-sphere mechanism at a negatively charged electrode...

The protein environment also provides a challenge in investigating redox processes since it acts as a barrier to electron flux in and out of the iron centre. This is due to the steric bulk of the folded polypeptide chain that can also act as an electronic insulator. As there is distance dependence in the outer-sphere electron transfer at the protein/solid electrode interface, these heterogeneous electron-transfer rates tend to be slow. This problem is overcome by the use of a small-molecule mediator as discussed in Section 2.2.3. 

The previous chapters dealt with a number of electrochemical techniques and the responses obtained when the electroactive species (O) is converted in a heterogeneous electron-transfer reaction to the product (R). This reaction is often a simple one-electron transfer, such as an outer-sphere reaction where no chemical bonds in species O are broken and no new bonds are formed. Typical reactions of this type are... 

In the simplest cases, it is possible to compare rates for homogeneous and heterogeneous electron-transfer reactions. According to the Marcus theory for homogeneous outer-sphere reactions [10],... 

The inner-sphere X can be calculated from the measured force constants of the redox molecule in each oxidation state. It is possible to have distinctly different Aig values for each oxidation state. The outer sphere X for heterogeneous electron transfer can be estimated from the dielectric continuum theory (Eq. 20) [238] ... 

Clegg AD, Rees NV, Klymenko OV, Coles BA, Compton RG (2004) Marcus theory of outer-sphere heterogeneous electron transfer reactions dependence of the standard electrochemical rate constant on the hydrodynamic radius from high precision measurements of the oxidation of anthracene and its derivatives in nonaqueous solvents using the high-speed channel electrode. J Am Chem Soc 126(19) 6185-6192... 

The one-electron electroreduction of hexacyanoferrate(III) to hexacyanoferrate(Il) (or ferricyanide to ferrocyanide) is a classic example of a quasi-reversible process. Despite being initially regarded as an outer-sphere adiabatic charge-transfer process [44], the electrode kinetics of the [Fe(CN)6] transformation exhibits a pronounced dependence on the properties of the electrode, and reproducible results can be achieved only when the working electrode surface is reproducibly pretreated and cleaned, either by polishing or electrochemically [47,48]. Metal electrodes provide notably faster heterogeneous electron transfer kinetics of [Fe(CN)6] as compared to carbon surfaces, possibly due to differences in the density of electronic states (DOS) for these materials, which can impose a notable influence on the rate of adiabatic charge transfer [49]. 

Outer-Sphere Electron Transfer The minimal interpenetration of the coordination spheres of the reactants is inherent in any mechanistic formulation of the outer-sphere process for electron transfer. As such, steric effects provide a basic experimental criterion to establish this mechanism. Therefore we wish to employ the series of structurally related donors possessing the finely graded steric and polar properties described in the foregoing section for the study of both homogeneous and heterogeneous processes for electron transfer. 

Electron Transfer Far From Equilibrium. We have shown how the Marcus Theory of electron transfer provides a quantitative means of analysis of outer-sphere mechanisms in both homogeneous and heterogeneous systems. It is particularly useful for predicting electron transfer rates near the equilibrium potential,... 

In heterogeneous redox reactions similar reaction sequences are observed usually an encounter (outer-sphere or inner-sphere) surface complex is formed to facilitate the subsequent electron transfer. 

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