Solvent electron transfer kinetics

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 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. 

Experimental data for the interligand electron transfer kinetics following photoexcitation of [Os(bpy)3] " " are in agreement with a reaction/diffusion model measurements were made in a range of solvents. The variable parameters in the model are interligand electronic coupling and solvent polarization barrier height. 

It is commonly assumed, therefore, that solvent reorganization will dominate electron transfer kinetics and that the reorganization energy in the same medium will be constant within a series of closely related redox partners. With a value of 2.4 kcal/mole for solvent reorganization (as obtained by Rehm and Weller (7) for fluorescence quenching of a series of arenes by substituted anilines in a polar medium) the curve shown in Fig. 2 is obtained. It is clear that substantial solvent-dependent barrier to electron exchange can be encountered. 

Further work by Flowers examined the role of solvent polarity in the electron transfer process.30 Inner-sphere electron transfer kinetics show a weak dependence on solvent polarity due to the considerable orbital overlap of the donor-acceptor pair in the transition state. In an outer-sphere process, changes in solvent polarity alter the energetics of electron transfer. The addition of excess HMPA, beyond that required to saturate Sml2, resulted in a linear correlation to the rate of reduction for alkyl iodides, whereas no impact was observed on the rate of ketone reduction.30 Thus the experiments showed a striking difference in the electron transfer mechanism for the substrate classes, which is consistent with the operation of an outer-sphere-type process for the reduction of alkyl iodides and an inner-sphere-type mechanism for the reduction of ketones.30 These findings are consistent with the observations of Daasbjerg and Skrydstrup.28,29... 

Amperometric sensors monitor current flow, at a selected, fixed potential, between the working electrode and the reference electrode. In amperometric biosensors, the two-electrode configuration is often employed. However, when operating in media of poor conductivity (hydroalcoholic solutions, organic solvents), a three-electrode system is best (29). The amperometric sensor exhibits a linear response versus the concentration of the substrate. In these enzyme electrodes, either the reactant or the product of the enzymatic reaction must be electroactive (oxidizable or reducible) at the electrode surface. Optimization of amperometric sensors, with regard to stability, low background currents, and fast electron-transfer kinetics, constitutes a complete task. 

Due to concerns of through-space and through-solvent pathway contributions to the electron-transfer kinetics of 26, a more rigid system, 27 (R = SiMe2 Bu)... 

It is commonly assumed that solvent reorganization will dominate electron transfer kinetically. Depending on the thermodynamics of the electron exchange, it is possible to quantitatively predict a relationship between the free energy of activation for electron transfer and the free energy associated with solvent reorganization based on Marcus theory. 

H2O. Moreover, the half-reactions in aqueous solution exhibit a pH dependence corresponding to the addition of one proton in conjunction with each electron. Thus, under the conditions of Fig. 12, the [Mn2 O2] center (1) is converted to an [Mn2 O(OH)]5+ center (2) in the first reduction and to an [Mn2 (OH)2] center (3) in the second. Electrode half-reactions, Nernst equations, and pA a values for the phen complex and for several related systems are summarized in Table 5. Precise data are somewhat difficult to obtain for these systems owing to the slow electron-transfer kinetics (fcgj, 10 cm s or less), the need to activate the working electrode surface to achieve a useful response, and the limited stability of some species in the presence of coordinating buffers. Nonetheless, pAa values for the bpy complex obtained from the Pourbaix diagram in Ref. 93 lead to pH-independent potentials of +1.1 and +0.5 V, respectively, for [Mn2 O(OH)]3+ and [Mn2 O2] + reduction, values that compare reasonably well to the results in Table 4, given the difference in solvent environments. 

The previous sections dealt with a generalized theory of heterogeneous electron-transfer kinetics based on macroscopic concepts, in which the rate of the reaction was expressed in terms of the phenomenological parameters, and a. While useful in helping to organize the results of experimental studies and in providing information about reaction mechanisms, such an approach cannot be employed to predict how the kinetics are affected by such factors as the nature and structure of the reacting species, the solvent, the electrode material, and adsorbed layers on the electrode. To obtain such information, one needs a microscopic theory that describes how molecular structure and environment affect the electron-transfer process. 

The response times of neat SAMs in the absence of a redox moiety Our theoretical analysis for the ILIT response has presumed that dC /dt is infinitely fast—if that is not the case, extracting meaningful values of will be difficult, if not impossible. We have already mentioned evidence of a slow response of SAMs formed from 11-mercaptoundecanoic acid [113] (Sec. VII). We expect that the results of these types of experiments will be critically dependent upon the choice of the SAM constituent, the SAM preparation, substrate metal, temperature, the solvent, and the electrolyte ions (the less hydrophobic, the better). A thorough study of the potential of zero response would be important and informative. Establishing which films exhibit the fast responses will be critical for any meaningful studies of fast interfacial electron-transfer kinetics. 

Electron-transfer kinetics using SAMs in nonaqueous solvent systems with obvious concern about the stability of SAMs in these solvents) The decrease in the dielectric constant of the solvent coupled with a likely decrease in the reorganization energy and a likely increase in the accessible temperature range should yield some valuable information. 

This donor number scale is widely referenced in relation to thermodynamic properties, as well as electron-transfer kinetics and photochemical properties. It has been criticized because of the neglect of solvent effects and side reactions that contribute to and because a one-parameter scale can never be entirely adequate. Ambiguities can arise for solvents which have more than one donor site, such as the formamide and sulfoxide derivatives. Recent measurements with BFj as the acid have provided some points of comparison and criticism for the original donor numbers. Recently, Linert et al. have used the solvatochromic shifts of a Cu(II) complex to define donor numbers for anions in dichloromethane. They also have suggested how these values can be converted for use in other solvents through a correlation with the acceptor number of the solvent. Linert et al. have reviewed the area and provided an extensive compilation of donor numbers from calorimetric and solvatochromic shift measurements. Some anion donor numbers in dichloromethane are included in Table 3.4, and the values for anions in water are 21 kcal moH smaller than those given. 

We have attempted to support this explanation for slow electron transfer kinetics by comparing experimental and calculated inner-shell barriers for Mn (TPP) Cl reduction. Table 2 contains data from temperature-dependent electrochemical measurements [11] in several non-aqueous solvents. The experimental barrier is obtained by determining AH real from the temperature dependence of the standard heterogeneous rate constant [12] and subtracting from this quantity the value of AG os calculated by equation 4. This difference, AG is, E. is to be compared with AG is calculated by equation 3. For the... 

The commonly used pretreatment protocols for activating solid electrodes are reviewed in this chapter. Specifically, the pietreatment of carbon, metal, and semiconductor electrodes (thin conducting oxides) is discussed. Details of how the different electrode materials are produced, how the particular pretreatment works, and what effect it has on electron-transfer kinetics and voltammetric background current are given, since these factors determine the electroanalytical utility of an electrode. Issues associated with cell design and electrode placement (Chapter 2), solvent and electrolyte purity (Chapter 3), and uncompensated ohmic resistance (Chapter 1) are discussed elsewhere in this book. This... 

---