Representing ET reactions by the crossing of two potential-energy curves

1 Representing ET reactions by the crossing of two potential-energy curves 

Electron-transfer reactions, such as the Fe(OH2)6 self-exchange, mechanism 16.1, do not involve any bond breaking. In this specific case the FeHD bonds linked to the 6 water molecules of the first coordination shells of Fe(III) remain intact. However, this does not mean that there is not a substantial distortion of the Fe-O bonds in the electron-transfer process. In fact, such a bond rearrangement is fundamental for the electron-transfer step and allows it to occur. 

Let us assume, for simplicity, that the hydrated iron ions are each bonded to a single water molecule and, therefore, have a single Fe-O bond. 

The same treatment is used to obtain the effective force constant of products, /p. Eq. (16.24) is an effective way to consider the equation of Schane (16.III) as divided by two, since the energy barrier for the reaction is defined for 1 mol of reactants. For consistency, the same averaging procedure is also applied to equihbrium bond lengths. 

We will subsequently represent such lengths in a more simplified manner by and It must be emphasised that eq. (16.21) already accounts for the existence of two reactants (and two products) in the reaction coordinate, because it gives the sum of the bond extensions in the two species. 

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