Self-exchange ET reactions

Figure 2 Dependence of the mean square electronic coupling on distance between two porphyrin rings in the cytochrome self-exchange ET reaction (41). For each distance, system conformations were sampled using MD and the coupling was computed for each conformation at the extended Huckel level. The black line marked XEI(P, W) shows the water-mediated coupling for comparison, the red line marked XEI(P) shows the coupling computed for the same protein conformation in vacuum. Conformational snapshots typical for the three coupling regimes are shown.

TABLE 17.13 Rate constants and activation parameters for selected self-exchange ET reactions at 25 °C. 

The rate constants for some self-exchange ET reactions are listed in Table 17.13. Notice that the entropies of activation are strongly negative, implying an associative mechanism. 

For the relation between the rate constant for homogeneous self-exchange ET process (kex) and the standard rate constant of the corresponding electrode reaction (kg), see 4) in Chapter 9. 

Ky and ky are the equilibrium constant and cross reaction rate constant for Eq. 2, kii and kjj are the self-exchange ET rate constants, and Z is a preexponential factor usually set at 10 (results are quite insensitive to its value). Because Ky can be calculated from the difference in formal oxidation potentials for the components, Eq. 2 states that ky only depends upon the formal oxidation potential and intrinsic (AG° = 0, or self-ET) rate constant for each couple involved. 

A powerful application of outer-sphere electron transfer theory relates the ET rate between D and A to the rates of self exchange for the individual species. Self-exchange rates correspond to electron transfer in D/D (/cjj) and A/A (/c22)- These rates are related through the cross-relation to the D/A electron transfer reaction by the expression... 

The cross relation has proven valuable to estimate ET rates of interest from data tliat might be more readily available for individual reaction partners. Simple application of tire cross-relation is, of course, limited if tire electronic coupling interactions associated with tire self exchange processes are drastically different from tliose for tire cross reaction. This is a particular concern in protein/protein ET reactions where tire coupling may vary drastically as a function of docking geometry. 

Figure C3.2.11. Log of the ET rate (A) against (l/s p-l/E ) for tire bis(biphenyl) cliromium self-exchange reaction. From 1341.

Esr spectroscopy has also been used to study pure solvent dynamics in electron self-exchange reactions (Grampp et al., 1990a Grampp and Jaenicke, 1984a,b). When the systems are not linked by a spacer (i.e. TCNQ- /TCNQ (TCNQ = tetracyanoquinodimethane), the homogeneous bimolecular rate constants /chom are given by (10), with fcA the association constant and kET... 

The reader is also referred to the innovative nonphotochemical electron transfer studies of Weaver et al. [147], These authors have been exploring dynamical solvent effects on ground state self-exchange kinetics for or-ganometallic compounds. This work has explored many aspects of solvent control on intermediate barrier electron transfer reactions, including the effect on a distribution of solvation times. The experimental C(t) data on various solvents have been incorporated into the theoretical modeling of the ground state electron transfer reactions studied by Weaver et al. [147]. 

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... 

The success of the Marcus model is connected to the consistent description of self-exchange reactions and later to ET reactions with non-zero free energy. Using the easily measured free energy of reaction (-AGe) in the PES diagram, gives the Arrhenius rate ... 

The reorganization energy for the (cross) ET reactions of NO+/arene pairs is about 2.5 eV (as evaluated from the self-exchange rates of aromatic donors and the NO+ acceptor) [28],... 

With new developments in technology, increasingly more rapid reactions can be investigated and many chemical, electrochemical and biological systems were studied. Consequently, the ET field developed in many directions as depicted in Fig. 1.4 [2]. Isotopic exchange reactions or, as they are now more generally termed, self-exchange reactions (since non-... 

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