Franck Condon Factors
Electron-transfer processes must satisfy the Frank-Condon restrictions, i.e.
the act of electron transfer is much shorter than atomic motion. The
consequences are that no angular momentum can be transfered to or from the
transition state during electron transfer, there is also restrictions in
changes in spin.
electron-transfer timescale ~ 10-15 sec nuclear motion ~10-13sec
Has an outer sphere electron transfer rate of 1.1 M-1
sec-1, simply requires the transfer of electron from eg orbital of
one Co to the other
Is very slow k = 10-9 M-1 sec-1 requires a
change of spin multiplicity as well.
In addition the oxidant and reductant must structurally reorganise themselves
before electron transfer so that the energies of their transition states are
equal. For example:
If electron transfer took place at their equilibrium positions, then we would
have compressed Fe(II) ion and a stretched Fe(III) ion. These are in
vibrationally excited states and would release energy, contravening
Thus the reagents must match their energies before electron transfer can
Theory: Eact ~ 25 kJ Mol-1
Expt: Eact ~ 44.1 kJ Mol-1