Electron transfer reactions in solution

The theoretical method employed is based on and largely similar to the theory of electron-transfer reactions in solution [123,124,125]. Thus the intramolecular spin conversion may be described as a transition between an initial manifold of states [f((r, qc)ZK,( c)[Pg.94]

The theory of homogeneous electron transfer reactions in solution has been formulated in terms of models in which the transferring electron is localized at a donor site in the reactant and at an acceptor site in the... 

Note that all cations are initially in solution and will certainly be solvated to some extent. In addition, notice that the symbol for the electron is again subscripted to show that charge comes from an electrode rather than from a homogeneous electron-transfer reaction in solution (cf. the potentiometric titrations we discussed in the previous chapter). 

Bimolecular electron-transfer reactions in solutions frequently have rates limited by the diffusion of the donor and acceptor molecules, because one or both of the reactant species is usually at a low concentration relative to the solvent. To obtain a detailed mechanistic and kinetic understanding of electron-transfer reactions in solutions, chemists have devised ingenious schemes in which the two reactants, the donor and acceptor, are held in a fixed distance and orientation so that diffusion will not complicate the study of the intrinsic electron-transfer rates. Recent developments, however, have led to theoretical models in which the orientation and the distance are changeable (see Rubtsov et al. 1999). 

The above model has been further explored to account for reaction efficiencies in terms of a scheme where nucleophilicities and leaving group abilities can be rationalized by a structure-reactivity pattern. Pellerite and Brau-man (1980, 1983) have proposed that the central energy barrier for an exothermic reaction (see Fig. 3) can be analysed in terms of a thermodynamic driving force, due to the exothermicity of the reaction, and an intrinsic energy barrier. The separation between these two components has been carried out by extending to SN2 reactions the theory developed by Marcus for electron transfer reactions in solutions (Marcus, 1964). While the validity of the Marcus theory to atom and group transfer is open to criticism, the basic assumption of the proposed model is that the intrinsic barrier of reaction (38)... 

Chapter 17 emphasizes the principles associated with obtaining electrical energy from electron-transfer reactions in solution. This chapter emphasizes what happens when electrical energy is applied to solutions in the operation of electrolytic cells. The oxidation and reduction processes that take place in an electrolytic cell are called electrolysis. We focus on determining what products are obtained and how much energy is required. 

Henry Taube, Canadian-born US chemist (1915-2005) winner of the 1983 Nobel Prize for chemistry for his work on the kinetics and mechanism of inorganic electron transfer reactions in solution. 

To fully understand the relaxation pathways for photoinduced charge-transfer reactions in solutions we need to take solvent effects into account. For that reason it is necessary to recall some basic principles of the classical Marcus Theory for electron-transfer reactions in solution. 

Sutin, N. (1999) Electron transfer reaction in solution a historical perspective, in Jortner, J., Bixon. M. (eds.), Advances in Chemical Physics. 107, Part 1, John Wiley Sons. NY. Pp. 7-33. 

Simulations of Electron-Transfer Reactions in Solution and in Photosynthetic Reaction Centers. 

The efficiency of electron-transfer reduction of Cgo can be expressed by the selfexchange rates between Coo and the radical anion (Ceo ), which is the most fundamental property of electron-transfer reactions in solution. In fact, an electrochemical study on Ceo has indicated that the electron transfer of Ceo is fast, as one would expect for a large spherical reactant. This conclusion is based on the electroreduction kinetics of Ceo in a benzonitrile solution of tetrabutylammonium perchlorate at ultramicroelectrodes by applying the ac admittance technique [29]. The reported standard rate constant for the electroreduction of Ceo (0.3 cm s ) is comparable with that known for the ferricenium ion (0.2 cm s l) [22], whereas the self-exchange rate constant of ferrocene in acetonitrile is reported as 5.3 x 10 s , far smaller than the diffusion limit [30, 31]. 

Hence there is no gas-phase experiment yet which fully encompasses all aspects of an electron-transfer reaction in solution. In solution, the solvent acts first as a polarization medium, which affects the energetics of direct transfers from the donor to the acceptor. It can also act as a transport medium for indirect electron transfers. The first aspect has been addressed in various cluster experiments [276]. The second aspect was addressed more recently by considering the femtosecond dynamics of iodide-(water) anion clusters, as reviewed below [277]. Finally, clusters present the advantage of isolating one reaction pair free from secondary collisions, except those, which are desired, with the solvent molecules (or atoms). The latter consideration motivated the cluster isolated chemical reaction (CICR) technique reviewed in Section 2.8.3. 

As mentioned above, the formation of excited states in chemical reactions may be understood in the context of an electron transfer model for chemiluminescence, first proposed by Marcus [2]. According to this model the formation of excited states is competitive with the formation of the ground state, even though the latter is strongly favored thermodynamically. Thus, understanding the factors that determine the electron transfer rate is of considerable importance. The theory of electron transfer reactions in solution has been summarized and reviewed in many reviews (e.g., [30-36]). Therefore, in this chapter the relevant ideas and equations are only briefly summarized, to serve as a basis for description of the ECL experiments. 

However it turned out that the structural, chemical and dynamical details are essential for complex descriptions of long-range proton transport. These parameters appear to be distinctly different for different families of compounds, preventing proton conduction processes from being described by a single model or concept as is the case for electron transfer reactions in solutions (described within Marcus theory [23]) or hydrogen diffusion in metals (incoherent phonon assisted tunneling [24]). 

In summary, the key predictions of Eqs (5.5)-(5.8) are (1) that AVj is usually the dominant part of and (2) that AV (and hence AV ) will be negative for simple outer-sphere electron transfer reactions in solution, regardless of whether electron transfer is fully adiabatic - with the caveat that predictions of the magnitude of are unlikely to be reliable for solvents of low e. 

The existence of the inverted region accounts for the phenomenon of electrogenerated chemiluminescence (Chapter 18) and has also been seen by other means for several electron-transfer reactions in solution. 

Topics, which have formed the subjects of reviews this year, include the luminescence kinetics of metal complexes in solution, photochemical rearrangements of co-ordination compounds, photochromic complexes of heavy metals with diphenylthiocarbazone derivatives, the photochemistry of actinides, actinide separation processes, and light-induced electron-transfer reactions in solution and organized assemblies. A discussion has also appeared on assigning excited states in inorganic photochemistry. ... 

A review of the theory of electron transfer reactions in solution and how it pertains to reactions of chlorophyll a can be found in Ref. 59. A brief summary of the salient features will be presented. During an electron transfer reaction in solution, the excited singlet donor (D ) and the acceptor (A) approach one another to form an encounter complex [(AD) ]. 

Eirst Principles Thermochemistry of Electron, Proton, and Proton-Coupled Electron Transfer Reactions in Solution... 

A. Warshel and W. W. Parson, Annu. Rev. Phys. Chem., 42, 279 (1991). Computer Simulations of Electron-Transfer Reactions in Solution and in Photosynthetic Reaction Centers. 

D. A. Zichi, G. Ciccotti, J. T. Hynes, and M. Ferrario, Molecular dynamics simulations of electron-transfer reaction in solution. J. Phys. Chem., 93 (1989), 6261-6265. 

Abstract. This paper provides a retrospective overview of the title paper written by Marcus around the middle of the twentieth century. A description of the history that led to this work, the basic features of the theory of electron-transfer reactions in solution developed in it, and a comment on its huge influence on succeeding developments are presented. 

Electron transfer reactions in solutions have been studied by pulse radiolysis, in which spectral changes taking place at different wavelengths due to loss of a reactant or formation of a product are recorded. The formation of ascorbyl radical can be studied by following the spectral changes occurring at 360 nm (Swartz and Dodd, 1981). These studies indicate that in the overall one-electron reduction of carbon-centered free radicals, thiyl radicals, or tocopheryl radicals, ascorbate acts as a hydrogen atom donor (Dunster and Willson, 1990). 

In this section we review most of the new experimental data on metal-metal electron transfer reactions in solution, in which the rate law for the primary process (19) can be expressed in the second-order form (20), or in related forms explicable in terms of... 

The most important theoretical ideas concerning adiabatic outer sphere electron transfer reactions in solution are summarized. The kinetics of the reduction of a series of different tris-1,10,-phenanthroline complexes of Fe(III) by Fe(CN) were measured in order to test the influence of the redox-potential on these reactions. The lectron exchange rate of the complexes Fe(dipy), Ru(dipy) and 0 (dipy) was derived from the study of their reduction by Fe(CN). Using the edox reaction between the anionic complexes of Fe(CN) and the effect of added... 

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