Physical Chemistry 3: — Chemical Kinetics — - Christian-Albrechts ...

11.4 Electron transfer reactions (Marcus theory) 235
11.4 Electron transfer reactions (Marcus theory)
Electron transfer is what happens in oxidation-reduction reactions. Hence, electron
transfer reactions are extremely important. They are strongly affected by the solvent
because the change of the charge distribution results in a large reorganization energy.
I Figure 11.5:
I Marcus theory: For a simplified derivation of ( ) we refer to Fig. 11.5.
(1) The energy of the donor and acceptor molecules will generally depend on all 3−6
internal coordinates. We consider the dependence on an effective coordinate ,
which describes the minimum energy path from reactant to product (Fig. 11.5).
(2) The exergonicity of the ET reaction ∆ ª is negative for an exergonic reaction
(convention; cf. Fig. 11.5).
(3) For the donor (D) and the acceptor (A), the Gibbs energy profiles () and
() shall be of parabolic form, which we may write as
() = 2 (11.43)
and
() =( − ) 2 + ∆ ª . (11.44)
The origin =0has been placed at the minimum of the donor; the acceptor has
its minimum at = . 50
50 We assume that ∝ 2 , taking any proportionality constant into account by proper rescaling of the
abscissa.