Electron transfer, ITIES

There are in principle two types of charge-transfer processes at ITIES, a single ion and a single electron transfer reaction. The first one can be described as the transfer of an ion Xf< with charge number z. ... 

Electron transfer processes leading to a product adsorbed in the interfacial region o are of practical interest. These processes include the deposition of a metal such as Cu or Pd at ITIES, the preparation of colloidal metal particles with catalytic properties for homogeneous organic reactions, or electropolymerization. 

The interpretation of phenomenological electron-transfer kinetics in terms of fundamental models based on transition state theory [1,3-6,10] has been hindered by our primitive understanding of the interfacial structure and potential distribution across ITIES. The structure of ITIES was initially studied by electrochemical and thermodynamic analyses, and more recently by computer simulations and interfacial spectroscopy. Classical electrochemical analysis based on differential capacitance and surface tension measurements has been extensively discussed in the literature [11-18]. The picture that emerged from... 

These spectroscopic and theoretical developments have stimulated the recent advances on electron-transfer dynamics at ITIES. In addition to the correlation between structure and dynamics of charge transfer, fundamental problems in connection with the energetics of ET reactions remain to be fully addressed. We shall consider these problems primarily before discussing kinetic aspects in full detail. 

FIG. 7 Simplified equivalent circuit for charge-transfer processes at externally biased ITIES. The parallel arrangement of double layer capacitance (Cdi), impedance of base electrolyte transfer (Zj,) and electron-transfer impedance (Zf) is coupled in series with the uncompensated resistance (R ) between the reference electrodes. (Reprinted from Ref. 74 with permission from Elsevier Science.)... 

Although the correlation between ket and the driving force determined by Eq. (14) has been confirmed by various experimental approaches, the effect of the Galvani potential difference remains to be fully understood. The elegant theoretical description by Schmickler seems to be in conflict with a great deal of experimental results. Even clearer evidence of the k t dependence on A 0 has been presented by Fermin et al. for photo-induced electron-transfer processes involving water-soluble porphyrins [50,83]. As discussed in the next section, the rationalization of the potential dependence of ket iti these systems is complicated by perturbations of the interfacial potential associated with the specific adsorption of the ionic dye. 

A hanging electrolyte drop has also been applied to determine ionic species in solution using differential-pulse-stripping voltammetry procedures [69]. Particular emphasis was given to assessing the selectivity and sensitivity of the method. The technique of current-scan polarography has also been applied in the study of electron-transfer [70] and coupled electron-transfer-ion-transfer [71,72] reactions at the ITIES in this configuration. 

MEMED has also been used to investigate the nature of coupled ion-transfer processes involved in spontaneous electron transfer at ITIES [80]. In this application, a key strength of MEMED is that all of the reactants and products involved in the reaction can be measured, as shown in Figs. 19 and 20. The redox reaction studied involved the oxidation of either ferrocene (Fc) or decamethylferrocene (DMFc) in a DCE phase (denoted by Fcdce) by either IrCle or Fe(CN)g in the aqueous phase (denoted by Ox ) ... 

The use of micropipette electrodes for quantitative voltammetric measurements of ion transfer (IT) and electron transfer (ET) reactions at the ITIES requires knowledge of geometry of the liquid interface. For the micrometer-sized micropipettes, both the orifice radius and the thickness of the pipette wall can be measured microscopically. A typical error of the microscopic determination of a radius was estimated to be 0.5/am for a micropipette and 1 /am for a microhole [24]. 

Studies of electron transfer (ET) at micro-ITIES are scarce. Solomon and Bard first observed the ET between TCNQ (in DCE) and ferrocyanide (in water) at a micro-ITIES supported by micropipettes [5]. The pipette was used as a SECM probe for electrochemical imaging. The current was controlled by the rate of the bimolecular ET reaction at the micro-ITIES... 

The voltammograms at the microhole-supported ITIES were analyzed using the Tomes criterion [34], which predicts ii3/4 — iii/4l = 56.4/n mV (where n is the number of electrons transferred and E- i and 1/4 refer to the three-quarter and one-quarter potentials, respectively) for a reversible ET reaction. An attempt was made to use the deviations from the reversible behavior to estimate kinetic parameters using the method previously developed for UMEs [21,27]. However, the shape of measured voltammograms was imperfect, and the slope of the semilogarithmic plot observed was much lower than expected from the theory. It was concluded that voltammetry at micro-ITIES is not suitable for ET kinetic measurements because of insufficient accuracy and repeatability [16]. Those experiments may have been affected by reactions involving the supporting electrolytes, ion transfers, and interfacial precipitation. It is also possible that the data was at variance with the Butler-Volmer model because the overall reaction rate was only weakly potential-dependent [35] and/or limited by the precursor complex formation at the interface [33b]. 

Electron-transfer reactions at ITIES resemble electron-transfer reactions across biological membranes, which adds a special interest. Also, in contrast to homogeneous electron-transfer reactions, they allow a separation of the reaction products. So it is disappointing to report that only very few experimental investigations of electron-transfer reactions at ITIES have been performed. This is mainly due to the fact that it is difficult to find systems where the reactants do not cross the interface after the reaction in addition, side reactions with the supporting electrolyte can be a problem. 

Figure 12.5 Electron transfer at ITIES. A is the total drop in the inner potential, Aeff is the part that is effective in the reaction.

In a number of cases ITIES can be used to separate the products of a photoinduced electron-transfer reaction. An early example is the work by Willner et al. [7] at the water/toluene interface, who studied the photooxidation of [Ru(bpy)3]2+ in the aqueous phase. The excited state was quenched by hexadecyl- 4,4 bipyridinium, which becomes hydrophobic on reduction and crosses to the toluene phase. There are other examples and mechanisms at the present time their main interest resides in their chemistry, and in the separation of products that can be achieved at the interface. 

Ion transfer across ITIES is easier to study than electron transfer so there is a greater body of experimental data. However, their interpretation is just as difficult. At the present time we can safely state ... 

Cyclopropyl carbanions are capable of maintaining their configuration whereas the CT-radical has been shown to reach inversion equlibrium with a rate constant of lO" s". ITie cyclopropyl bromide 13, and the corresponding iodide, are reduced in a single two-electron polarographic wave and the S +)-isomer yields the R(-)-hydrocarbon with 26% enantiomeric excess [67, 68]. Such a substantial retention of configuration during reduction of the carbon-bromine bond indicates a very fast second electron transfer process. Results from reduction of the cyclopropyl bro-... 

The electrodes used in conventional polarography and voltammetry are electronic conductors such as metals, carbons or semiconductors. In an electrode reaction, an electron transfer occurs at the electrode/solution interface. Recently, however, it has become possible to measure both ion transfer and electron transfer at the interface between two immiscible electrolyte solutions (ITIES) by means of polarography and voltammetry [16]. Typical examples of the immiscible liquid-liquid interface are water/nitrobenzene (NB) and water/l,2-dichloroethane (DCE). 

An example of electron transfer at ITIES can be obtained if NB contains ferrocene (Fc) and water contains [Fe(CN)6]3. The reaction at the interface is... 

A is the sum of the solvent and intramolecular reorganization energies, and AG = F(A 0 - A 0J) is the standard electrochemical Gibbs energy of the electron transfer from x = a to x = b. Parabolic dependence of AG on AG was demonstrated [viii]. Electrochemical behavior including the kinetic analysis of various ET systems was reviewed [ix]. A special type of the ET reaction is the deposition of a metal at ITIES, e.g., the deposition of Au particles by the interfacial reaction between AUCI4 in 1,2-dichloroethane and Fe(CN)6 in water [x]. 

Another approach to heterogeneous electron transfer relies on photoexcitable dye molecules adsorbed at the polarizable ITIES [xi]. General mechanism for a pho-toinduced electron transfer between a water-soluble dye sensitiser and a redox quencher located in the organic... 

Dynamic solvent effect — is a phenomenon typical for adiabatic -> electron transfer and -> proton transfer reactions. This effect is responsible for a dependence of the reaction rate on solvent relaxation parameters. The initial search for a dynamic solvent effect (conventionally assumed to be a feature of reaction adiabatic-ity) consisted in checking the viscosity effect. However, this approach can lead to controversial conclusions because the viscosity cannot be varied without changing the -> permittivity, i.e. a dynamic solvent effect cannot be unambiguously separated from a -> static solvent effect [i]. Typically a slower solvent relaxation goes along with a higher permittivity, and the interplay of the two solvents effects can easily look as if either of them is weaker. The problems of theoretical treatment of the dynamic solvent effect of solvents having several relaxation times are considered in refs, [ii-iii]. 

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