Electron-transfer volume profile

Let us see now what happens in a similar linear scan voltammetric experiment, but utilizing a stirred solution. Under these conditions, the bulk concentration (C0(b, t)) is maintained at a distance S by the stilling. It is not influenced by the surface electron transfer reaction (as long as the ratio of electrode area to solution volume is small). The slope of the concentration-distance profile [(CQ(b, t) — Co(0, /))/r)] is thus determined solely by the change in the surface concentration (Co(0, /)). Hence, the decrease in Co(0, t) duiing the potential scan (around E°) results in a sharp rise in the current. When a potential more negative than E by 118 mV is reached, Co(0, t) approaches zero, and a limiting current (if) is achieved ... 

It has in general been the objective of many mechanistic studies dealing with inorganic electron-transfer reactions to distinguish between outer- and inner-sphere mechanisms. Along these lines high-pressure kinetic methods and the construction of reaction volume profiles have also been employed to contribute toward a better understanding of the intimate mechanisms involved in such processes. The differentiation between outer- and inner-sphere mechanisms depends... 

A challenging question concerns the feasibility of the application of high-pressure kinetic and thermodynamic techniques in the study of such reactions. Do long-distance electron-transfer processes exhibit a characteristic pressure dependence and to what extent can a volume profile analysis reveal information on the intimate mechanism ... 

Recent investigations on a series of intramolecular electron transfer reactions, closely related to the series of intermolecular reactions described above, revealed nonsymmetrical volume profiles (159). Reactions of the type... 

Fe +aq reacts with chloranilic acid to give iron(II) chloranilate. " Fe " aq reacts with promazine, (204), to give the promazine radical cation complex of Fe. The volume profile for this combined substitution and electron transfer reaction has been established. The activation volumes for the forward and reverse reactions are —6.2cm mol and — 12.5cm mol the respective activa-... 

Figure 2. Volume profile for the intermolecular electron-transfer reaction [16] Co(phen)3+ + Cyt c" iCo(phen) + + Cyt cm.

The typical results reported in this chapter, clearly demonstrate how the lifetime of excited states and the low-spin/high-spin character of such states can be tuned by pressure. Furthermore, photochemical bond formation and cleavage processes are accelerated or decelerated by pressure, respectively, in a similar way as found for the corresponding thermal reactions. As a result of this, the associative or dissociative nature of such substitution reactions can be characterized. A further characterization of the intimate nature of the reaction mechanism can also be obtained for photochemical isomerization and electron-transfer reactions as reported in Sections V and VI, respectively. The same applies to photoinduced thermal reactions, where the interpretation of the pressure dependence is not complicated by photophysical relaxation processes. The results for the subsequent thermal reactions can be compared with a wealth of information available for such processes [1-6]. Especially the construction of reaction volume profiles has turned out to be a powerful tool in the elucidation of such reaction mechanisms. 

The available results demonstrate readily the complementarity of the kinetic and thermodynamic data obtained from stopped-flow, UV-Vis, electrochemical and density measurements, and yield a mutually consistent set of trends allowing further interpretation of the data. The overall reaction volumes determined in four different ways are surprisingly similar and underline the validity of the different methods employed. The volume profile in Fig. 1.20 illustrates the symmetric nature of the intrinsic and solvational reorganization in order to reach the transition state of the electron-transfer process. In these systems the volume profile is controlled by effects on the redox parmer of cytochrome c, but this does not necessarily always have to be the case. The location of the transition state on a volume basis is informative regarding the early or late nature of the transition state, and therefore details of the actual electron-transfer route followed. 

Reactions of the type shown in Eq. (1.11), where L = isonicotinamide, 4-ethyl-pyridine, 3,5-lutidine, or pyridine, all exhibited volumes of activation for the forward reaction of between +3 and +7 cm mol compared to overall reaction volumes of between +19 and +26 cm mol . This indicates that electron transfer from Fe to Ru is characterized by an early transition state in terms of volume changes along the reaction coordinate (see Fig. 1.21). The overall volume changes could be accoimted for in terms of electrostriction effects centered around the ammine ligands on the ruthenium center. A number of possible explanations in terms of the effect of pressure on electronic and nuclear factors were offered to account for the asymmetrical nature of the volume profile 67]. 

Fig. 1.21. Volume profile for the reversible electron-transfer reaction (NH3)4(L)Ru "-(His33)-Cyt c" (NH3)4 L)Ru"-(His33)-Cyt c"...

The examples presented in this chapter clearly demonstrate that electron transfer reactions exhibit a characteristic pressure dependence that can be employed to gain further insight into the mechanism of the electron transfer process. The pressure dependence of self-exchange reactions can be used to develop the theoretical interpretation of the observed volumes of activation because the overall reactions involve no net volume change. In the case of nonsymmetrical reactions, the volume profile treatment can reveal information regarding the reorganization involved in going from the reactant to the transition and product... 

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