Self-exchange electron transfer

Figure 9-6. The consequences of a self-exchange electron transfer between a ground state cobalt(ii) and a ground state cobalt(iii) complex.

Figure 9-7. The self-exchange electron transfer reaction between vibrationally excited cobalt(ii) and cobalt(iii) complexes.

Outer-sphere electron transfer reactions involving the [Co(NH3)6]3+/2+ couple have been thoroughly studied. A corrected [Co(NH3)6]3+/2+ self-exchange electron transfer rate (8 x 10-6M-1s-1 for the triflate salt) has also been reported,588 which is considerably faster than an earlier report. A variety of [Co(NH3)g]3+/2+ electron transfer cross reactions with simple coordination compounds,589 organic radicals,590,591 metalloproteins,592 and positronium particles (electron/ positron pairs)593 as redox partners have been reported. 

It has been recognized that sulfur donors aid the stabilization of Cu(i) in aqueous solution (Patterson Holm, 1975). In a substantial study, the Cu(ii)/Cu(i) potentials and self-exchange electron transfer rate constants have been investigated for a number of copper complexes of cyclic poly-thioether ligands (Rorabacher et al., 1983). In all cases, these macrocycles produced the expected stabilization of the Cu(i) ion in aqueous solution. For a range of macrocyclic S4-donor complexes of type... 

Fig. 15. Plot of AV (observed) versus AV (calculated) for a series of self-exchange electron-transfer reactions.

Another treatment of the problem, more adapted to this situation, thus consisted of dividing the film in successive monolayers and describing electron transport as self-exchange electron transfer between the reduced and oxidized forms of the redox couple.14 The variation of the concentration with time, t, and distance from the electrode, x, are thus depicted by equations (4.22) and (4.33), as established in Section 6.4.3.15... 

Ey2 reversible half-wave potential, k, vxl> experimental standard rate constant, a transfer coefficient, (outer-Helmholtz plane, L ron standard rate constant after correction for the double-layer effect (see 4)), lcex rate constant for the homogeneous self-exchange electron-transfer (see 4) in Chapter 9) obtained with a HMDE in DMF-0.5 M BU4NCIO4 at 22 2°C, except the last two obtained with a DME in DMF-0.1 M Bu4NI at 30°C. 

Another interesting result obtained by Shalev and Evans [55] is that the rate constant for the homogeneous self-exchange electron transfer, log fcex, between various Q and Q in DMF decreases linearly with the increase in AGgV(Q/Q -). This is because log k is mainly governed by the solvent reorganiza-... 

Combination of Electrochemical and Non-Electrochemical Techniques 279 Studies of Self-Exchange Electron Transfer in the Solution... 

If the solution contains both the parent compound (Q) and its radical ion (Q or Q +), the self-exchange electron transfer reaction occurs between them (Q + Q <= Q + Q or Q + Q - Q + + Q) and the line width of the ESR spectrum increases with the increase... 

The self-exchange electron-transfer (SEET) process, in which a radical is trapped by the parent molecule, has been studied using the intersecting-state model (ISM).91 Absolute rate constants of SEET for a number organic molecules from ISM show a significant improvement over classical Marcus theory92-94 in the ability to predict experimental SEET values. A combination of Marcus theory and the Rips and Jortner approach was applied to the estimation of the amount of charge transferred in the intramolecular ET reactions of isodisubstituted aromatic compounds.95... 

When ZnTPP (TPP2- = teteraphenylporphyrin dianion 5.0 X 10-4 M) is oxidized by exactly 1 equiv. of Ru(bpy)3+ (bpy = 2,2 -bipyridine 5.0 X 10 4 M), ZnTPP+ is produced without leaving any neutral species. In such a case, no self-exchange electron transfer of ZnTPP+ with ZnTPP occurs when the ESR spectrum of ZnTPP+ detected at 233 K exhibits well-resolved hyperfine structures as shown in Fig. 13.2a. The hyperfine coupling constants (hfc) are determined by comparison of the observed spectrum with the computer simulation spectrum as shown in Fig. 13.2b. 

The rate constants (kex) of the electron exchange reactions between ZnTPP+ and ZnTPP [Eq. (1)] were determined using Eq. (2), where AHms( and AH°msi are the maximum slope linewidths of the ESR spectra in the presence and absence of ZnTPP+, respectively, and P, is a statistical factor [14]. From the linear plots of (AHmsi - Afi°msl) and [ZnTPP] at various temperatures are obtained the self-exchange electron-transfer rate constant (k ). The Arrhenius plots are shown in Fig. 13.3 together with the observed activation enthalpies (AHols ), where the effect of diffusion (kdiff) is taken into account. The AHol/ values are all positive and decrease in order toluene > MeCN > CH2C12 [16],... 

Fig. 13.3 Arrhenius plots of self-exchange electron transfer between ZnTPP+ and ZnTPP in different solvents [16].

It is probably necessary to make a primary operational distinction of reaction classes based on the phase (or phases) of matter involved thus (1) homogeneous, liquid phase (2) gas phase (3) solid phase (4) heterogeneous. A basic subclassification distinguishes between reactions in which the reactants are chemically different from the prodncts, as in equations (1) and (2), and reactions in which the reactants and prodncts involve the same chemical species, as in equations (3) and (4) when (N4) = ( N4). Eqnations (1) and (2) are examples of cross electron-transfer reactions (or cross-reactions), while eqnations (3) and (4) are examples of self-exchange electron-transfer reactions when (N4) = ( N4). More generally, subclassifications of the primary classes are commonly based on energy or free energy considerations such as ... 

Several studies of bimetallic complexes in which the donor and acceptor are linked across aliphatic chains have demonstrated that these are generally weakly coupled systems. " Studies of complexes linked by l,2-bis(2,2 bipyridyl-4-yl)ethane (bb see Figure 5), indicate that these are good models of the precursor complexes for outer-sphere electron-transfer reactions of tris-bipyridyl complexes. A careful comparison of kinetic and spectroscopic data with computational studies has led to an estimate of //rp = 20cm for the [Fe(bb)3pe] + self-exchange electron transfer. In a related cross-reaction, the Ru/bpy MLCT excited state of [(bpy)2Ru(bb)Co(bpy)2] + is efficiently quenched by electron transfer to the cobalt center in several resolved steps, equations (57) and (58). ... 

The self-exchange electron-transfer rate constant for [Cu([15]-aneSs)] " " has been determined as 3 x 10 M sec which is the largest value determined for any low molecular weight Cu(II)/(I) system apart from [CuCU] " (76). 

The flow cell has been used in combination with EPR spectroscopy to study self-exchange electron-transfer reactions between aromatic radical anions in line-... 

Figure 25. Corrected EPR linewidth Lw vs the concentration of 2,5-di-tert-butyl-l,4-dimethoxy-benzene [A ], measured for the self-exchange electron-transfer reaction between the substrate and its radical cation (A" ). The concentration of the latter was in all experiments equal to 0.05 mM in acetonitrile/0.1 M B114NBF4. From the slope, /cet is calculated to be 1.1 x 10 M s" this corresponds to a self-exchange reorganisation energy of 21.2 kcal moC. From D. Jurgen, S. U. Pedersen, and H. Fund, Acta Chem. Scand. 51 161 (1997).

The coordination of copper ion by macrocyclic quadri- and quinque-dentate thi-oethers makes the Cu VCu reduction process easier, and also increases the rate of the redox self-exchange electron transfer process [55]. For example, [Cu(13-aneS4)] undergoes a one-electron reduction at 0.52 V versus NHE [58] (cf. E°(Cu /Cu ) = 0.15V versus NHE for the aquated ion in water) and the rate constant for the corresponding self-exchange electron transfer is 3 x 10 s [59] (to be compared... 

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