The Marcus-Hush Model of Electron Transfer

In a classical picture the reaction from R to P takes place when the system reaches the saddle point X at the intersection of the wells the activation energy is then A G, the reaction free energy being A G°. If the energy wells 

This external or outer-sphere parameter A0 can be expressed according to a modified form of the Born equation to take into account only the solvent s orientation polarization. (As in the theory of solvatochromic shifts, it is assumed that the induction polarization can follow electronic motion.) 

There is no such simple expression for the internal reorganization energy Aj, which must be given as a sum of terms related to the vibrational states of the reactants and products 

The actual AG° of the primary process would then be greater by the ion s excited state energy, so that the reaction would not reach the inverted region. This explanation is quite reasonable since it is well known that open-shell species such as radical ions have generally excited states of very low energy. 

Other complexes are made up of many molecules, like the organometallic complexes that are considered in section 4.6.3. The important thing is that 

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