Optical and thermal electron transfer

Fig. 1 also leads to some relatively straightforward conclusions about the relationships between optical and thermal electron transfer. For a chemically symmetrical system (e.g., eq 1), the energy of the optical transition should be related to the classical vibrational barrier (x/4) as in eq 3. Eq 3 includes the separation of the vibrational barrier into intra-... 

Eqs 3-5 provide a relatively simple theoretical basis for understanding optical charge transfer and the relationship between optical and thermal electron transfer. One implication... 

The apparent ability of simple absorption band measurements to provide direct information about kinetic barriers to electron transfer constitutes a significant demonstration of the predicted relationship between optical and thermal electron transfer. Turning the argument around, it is possible to use variations in EoP with solvent, which are easily measured, to gain insight into solvent effects in thermal electron transfer. For example, equation (70) in the form of equation (74) has... 

This relation relies on the fact that the entropy is unchanged in the Franck-Condon transition, so that the energy difference measured by optical transitions can be connected to the free energy difference for the activations barrier. The conclnsion, that the rate of a thermal electron-transfer reaction can be estimated from relatively simple spectroscopic data, is remarkable. But it has been pointed out that cases in which an optical and thermal electron transfer have actually been measured on precisely the same system, free of donbts over the assigmnents, are still very few. ... 

From data in Table 2, once they are corrected as indicated above, the free energy of the thermal electron transfer has been calculated by using eq. 1 and the in ref. S. A value of H =3.68 kJ mol, that is about 75% of the value obtained from the band, was used in this eq. This difference arises because the optical and thermal electron transfers are produced at different points along the reaction coordinate . The calculated values of AG as well as the experimental AG values (obtained from the rate constants with s ) are... 

The influence of solvent reorganization on optical and thermal electron-transfer processes in clathrochelate complexes is theoretically calculated in Ref 322. 

Brunschwig B. S., Ehrenson S. and Sutin N. (1986), Solvent reorganization in optical and thermal electron-transfer processes , J. Phys. Chem. 90, 3657-3668. Branschwig B. S. and Sutin N. (1999), Energy surfaces, reorganization energies, and coupling elements in electron transfer . Coord. Chem. Rev. 187, 233-254. 

Hush N. S. (1968), Homogeneous and heterogeneous optical and thermal electron transfer , Electrochim. Acta 13, 1005- 1023. 

Postscript Energy Terms in Optical and Thermal Electron Transfer... 

Measurements and calculations of this sort are of interest in providing a check on the validity of equations (14) and of diagrams such as Figures 2 and 3. Already, however, there are indications that the calculations of Fa from optical data may not be generally valid. Morrison and Hendrickson have published a detailed study of the electron-transfer process in the partially oxidized dinuclear ferrocene-type complexes (5)—(8) (/i=l) based on new magnetic resonance, susceptibility, electronic, and Mossbauer spectra and have reviewed earlier work from their own and other laboratories. Only the main conclusions relevant to optical and thermal electron transfer can be summarized here. 

Penfield KW, Miller JR, Paddon-Row MN, Cotsaris E, Oliver AM and Hush NS. Optical and Thermal Electron-Transfer in Rigid Difunctional Molecules of Fixed Distance and Orientation. J. Am. Chem. Soc. 1987 109 5061-5065. 

The nonadiabatic treatment of Hopfield has been elaborated to include the possibility that Hp may vary with the binding energy or the transferring electron. Model calculations are given for both optical and thermal electron transfer (cf. below, p. 12) and for barriers of different shapes, including square energy wells. 

Figure 1.1. Optical and thermal electron transfer for a thermodynamically unfavorable change.

Here we review mainly experimental work, with emphasis on the determination of the extent of electron delocalization between metal ion centers (i.e., the placing of systems in the Robin and Day classification), measurements of rates of internal electron transfer, and the relationship between optical and thermal electron transfer processes. 

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