Spin-state changes, self-exchange rates

The electron transfer can only take place when the M-L IxHid distances in the M(II) and M(III) states are the same, i.e. the bonds in [MLe] must be compressed and those in [MLs] must be elongated (Fig. 26.10). This is described as a Franck-Condon restriction. The activation energy required to reach these vibrational excited states varies according to the system, and hence the self-exchange rate constants vary. In the case of [Fe(bpy)3] and [Fe(bpy)3] ", both complexes are low-spin, and the Fe-N bond distances are 197 and 196 pm, respectively. Electron transfer involves only a change from ia/ to hg (Fe " and vice versa. 

Such self-exchange reactions cannot be followed by optical spectroscopy since no chemical change is involved. However, ESR spectroscopy provides a unique capability to do so because the spin states of the protons in the radical effectively label a particular radical. The reaction leads to broadening of the ESR lines, which is detectable at rates of transfer >10 s. From the dependence of line width on phenolate concentration it was calculated that the rate constant for this self-exchange reaction is 1.9 x 10 ... 

---