Electron self-exchange, reorganization energy

The nucleophilicity of superoxide toward different alkyl halides was estimated from kinetic measurements [255]. In particular substitution kinetics toward alkyl halides were compared to electron transfer rates with the same halides. Thus, the reactivity of aromatic radical anion A (a donor having the some potential as O2") was compared to that of superoxide. The influence of sub/ ET on the difference in self-exchange reorganization energy A(0) between O2/O2 and X/A was discussed. 

Marcus theory of electron transfer (Eq. 4) [91] to the rate of electron transfer from ferrocyanide to HRP compound I (8 x 10 M s ) [105], An even larger reorganization energy (2 = 78.0 kcal mol ) [104] was derived from the electron self-exchange rate between HRP compound II and ferric HRP (4.9 x 10 m s ) [104], The extremely large 2 value (78.0 kcal mol ) for the metal-centered electron-exchange between HRP compound II (Fe ) and ferric HRP (Fe ) is consistent with the large... 

It is called the cross-relation because it is algebraically derived from expressions for the two related electron self-exchange reactions shown inEquations 1.21 and 1.22.. Associated with these reactions are two self-exchange rate constants k and k22 and reorganization energies Xu and 22-... 

Hence, by measuring the rate of electron self-exchange, one can readily use the Marcus model to calculate the reorganization energy X. To evaluate this, a collision frequency ofZ 10nM s is generally used, and the temperature is given in Kelvin. 

Electron self-exchange reaction between O2 and 02 was then discussed, and developments before and after an experimentally determined rate constant for this reaction was published, were also summarized. Related to this, the problem of size differences between O2 or 02 and their typical metal-complex electron donors or acceptors was recently solved quantitatively by addition of a single experimentally accessible parameter, A, which corrected the outer-sphere reorganization energy used in the Marcus cross relation. When this was done, it was found that rate constants for one electron oxidations of the superoxide radical anion, 02 , by typical outer-sphere oxidants are successfiiUy described by the Marcus model for adiabatic outer-sphere electron transfer. 

It has been shown so far that internal and external factors can be combined in the control of the electron-transfer rate. Although in most cases a simple theoretical treatment, e.g. by the Marcus approach, is prevented by the coincidence of these factors, it is clear that the observed features for the isoenergetic self-exchange differ by the electronic coupling and the free energy of activation. Then it is also difficult to separate the inner- and outer-sphere reorganization energies. 

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