Minimum energy crossing point electron transfer

Figure 24. Electron-transfer rate versus electronic coupling strength. The temperature is T = 500 K. Solid line with circle-present results from Eq. (126) with the transition probability averaged over the seam surface. Solid line with square-present results with the transition probability taken at the minimum energy crossing point (MECP). Dashed line-Bixon-Jortner theory Ref. [109]. Dotted line-Marcus s high temperature theory. Taken from Ref. [28].

The stepwise transfer of electrons from cytochrome c to cytochrome via cytochrome a is kinetically favorable due to a substantial decrease in the medium reorganization energy for direct electron transfer from cytochrome c to cytochrome a. The redox potential of FOc may not be smaller, but even greater than the redox potential of FOg. It is essential that only the minimum of the intermediate term on the reaction energy diagram be below the cross-point of the initial and final terms. 

In the cases treated in the present paper, we do not have a reorganization energy because, for example as shown in Figures 5 and 10, the two diabatic states between which electron transfer occurs (e.g., the SS a and excited-Rydberg states) cross so close (i.e., within the zero-point vibrational motion of the SS bond) to the minimum on the Rydberg-state surface as to render A essentially zero. In more traditional electron-transfer events, A contains contributions from the... 

Fig. 12. PET can be studied on the basis of intersecting harmonic potential-energy curves. In the approach of Marcus, the free energy of a reacting system is represented as a function of nuclear geometry on the horizontal axis. During excitation, there is a vertical transition (Franck-Condon) to a point on the excited-state surface, followed by vibrational relaxation. Electron transfer takes place at the crossing of the excited-state and ionic potential-energy curves. The transition-state energy, AGe, corresponds to the energy difference between the minimum on the excited-state surface and the point of intersection...

Fig. 20. Schematic representation of the reaction coordinate for tryptophan fluorescence quenching induced by hydrogen transfer and aborted decarboxylation. The electronic nature of the Si surface changes character along the Si path due to two avoided crossings between jSi and S2 The first one occurs between the covalent state and the ionic La state along the reaction coordinate that interconverts the i9i-Min and. Si-Exc minima. The second one occurs between the ionic La state and the biradical Bi, state along the tautomerization coordinate that leads to the excited-state tautomerized form S -Taut. This point does not corresponds to a minimum on the potential-energy surface and it is found that evolution along a decarboxylation coordinate leads to a -Si /-So conical intersection, where efficient radiationless decay to the ground state takes place. The values of the relevant structural parameters are given in A. Data from Ref. 102.

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