Effects electron transfer kinetics

R.L. McCreery, Carbon Electrodes Structural Effects on Electron Transfer Kinetics, in A.J. Bard, Ed., Electroanalytical Chemistry, Vol 18, Marcel Dekker, New York, 1991. 

The quantitative effects of steric encumbrance on the electron-transfer kinetics reinforce the notion that the inner-sphere character of the contact ion pair D+, A- is critical to the electron-transfer paradigm in Scheme 1. Charge-transfer bonding as established in the encounter complex (see above) is doubtless an important consideration in the quantitative treatment of the energetics. None the less, the successful application of the electron-transfer paradigm to the... 

Ion pairing in aqueous solution often plays a minor role in electron-transfer kinetics, but in low-dielectric solvents such as dichloromethane the effects can be much stronger. Wherland s group has recently uncovered a fascinating case where ion pairing drastically inhibits the rates of outer-sphere electron transfer, even though one of the reactants is uncharged (5). 

In all preceding cases, the double-layer effect on electron transfer kinetics has not been taken into account explicitly. Doing so requires that we replace... 

Double-layer charging current and ohmic drop are likely to interfere at high scan rates. The procedures for extracting the Faradaic component of the current and correcting the potential axis from the effect of ohmic drop described earlier (see Sections 1.3.2 and 1.4.3) should then be applied. The same is true for the double-layer effect on the electron transfer kinetics (Section 1.4.2). 

As part of a subsequent study concerning primarily second-site revertant yeast iso-l-cytochrome c variants, Hazzard et al. evaluated the effect of converting Lys-72 to an aspartyl residue by site-directed mutagenesis on the electron transfer kinetics of the cytochrome c-cytochrome c peroxidase complex [136]. Lys-72 was of interest for this purpose, because it is involved in the hypothetical model for the complex formed by these two proteins that was proposed by Poulos and Kraut on the basis of molecular graphics docking [106]. In these... 

In a recent upsurge of studies on electron transfer kinetics, importance was placed on the outer shell solvent continuum, and the solvent was replaced by an effective model potential or a continuum medium with an effective dielectric constant. Studies in which the electronic and molecular structure of the solvent molecules are explicitly considered are still very rare. No further modem quantum mechanical studies were made to advance the original molecular and quantum mechanical approach of Gurney on electron and proton (ion) transfer reactions at an electrode. 

In the case of a solution follow-up reaction (47) so fast that the thickness of the reaction layer is of the same order of magnitude as the double layer, the competition between the electron transfer step (46) and the follow-up reaction (47) can be described in a similar way with a somewhat different effect of the double layer on the electron-transfer kinetics (Saveant, 1980b, 1983). 

Cyclic voltammetry is generally considered to be of limited use in ultratrace electrochemical analysis. This is because the high double layercharging currents observed at a macroelectrode make the signal-to-back-ground ratio low. The voltammograms in Eig. 9B clearly show that at the NEEs, cyclic voltammetry can be a very powerful electroanalytical technique. There is, however, a caveat. Because the NEEs are more sensitive to electron transfer kinetics, the enhancement in detection limit that is, in principle, possible could be lost for couples with low values of the heterogeneous rate constant. This is because one effect of slow electron transfer kinetics at the NEE is to lower the measured Faradaic currents (e.g.. Fig. 8). 

A number of studies were done in order to determine which of these various factors contribute to the large peak separations observed here. First, it is well known that the effects of resistive elements can be obviated by applying positive feedback [132]. When positive feedback was applied to a thin-film confrol elecfrode similar fo fhaf described in Fig. 27, the peak separation decreased from 0.8 to —0.35 V (Fig. 28). These data show that resistance does, indeed, contribute to the large AEp values observed here. However, the fact that —0.35 V of this peak splitting cannot be removed by applying positive feedback clearly indicates that slow electron transfer kinetics also contribute to AEp. ... 

The decreased contribution due to slow electron transfer kinetics for the microtubular electrode is also attributable to the higher underlying surface area of the tubular current collector. Because the surface area is higher, the effective current density for the microtubular TiS2 is less than for the thin film TiS2, which has a conventional planar current collector. The decreased contributions of film resistance and slow electron transfer kinetics also account for the higher peak current density of the microtubular electrodes (Fig. 27). 

Diamond surfaces after anodic oxidation treatment involve oxygen-containing surface functional groups. The electron-transfer kinetics for ions and polar molecules are expected to be quite different. Fe(CN)l /4 was highly sensitive to the surface termination of diamond. For an anionic reactant, there was an inhibition of the electron transfer for the oxygen-terminated diamond electrodes compared with the hydrogen-terminated diamond electrodes, and there was also an acceleration of the electron transfer for oxygen-terminated diamond for some cationic reactants such as Ru(NH3) +/3+ and Fe2+/3+. These results can be explained by electrostatic effects, which interact between the ionic... 

In a related study [62], similar effects on conductivity of SWCNTs were reported, but here a comparison was also made between the effects of nitric acid reflux and air plasma treatment, and an attempt was made to relate the changes observed to the creation of defect sites. The authors did not offer a more concrete proposal regarding the nature of the sites involved in these treatments. After the acid treatment, Raman microscopy results indicated a dramatic change in SWNT electronic structure, and both treatments enhanced the electron transfer kinetics for the oxidation of inner-sphere dopamine. By contrast, both treatments had a negligible effect on the voltammetric response of a simple outer-sphere electron-transfer redox process Ru(NH3)63+/2+. ... 

McCreery, R. L. Carbon electrodes structural effects on electron transfer kinetics, in Bard, A. L. (ed.), Electroanalytical Chemistry, Vol 17, 1991, New York Marcel Dekker, Inc., pp. 221-374. 

---