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Fig. 2.4 The Franck-Condon principle in action in one of the simplest chemical reactions known,
i.e., the one-electron redox reaction of the iron ions. ( Lower ) Due to the greater charge density
around the ferric ion (Fe +3 ), as compared with that around the ferrous ion (Fe +2 ), water dipoles
(depicted as crossed arrows ) are more strongly attracted to the former than to the latter, forming
smaller and tighter primary hydration shell around Fe +3 than around Fe +2 .( Upper ) The electron
transfer process is much faster than the nuclear rearrangements accompanying hydration shell
changes (due to the proton being ~2,000 times more massive than the electron). The hydration shells
around the Fe +3 and Fe +2 ions contract and expand (i.e., “breath”) periodically as a consequence of
thermal fluctuations or Brownian motions (not shown) (Drawn by Julie Bianchini, 2008)
2.2.2 Franck-Condon Principle in Chemistry
It is well established in inorganic electron transfer reactions that electron transfer
processes must be preceded by the reorganization of the solvation (also called
hydration) shells surrounding reactants (Reynolds and Lumry 1966). It was Libby
(1952) who accounted for this phenomenon based on the Franck-Condon principle,
suggesting that, before the fast electron transfer can occur, the slower nuclear
rearrangements of water molecules in the hydration shells must take place . This is
schematically illustrated in Fig. 2.4 . The overall reaction involves the transfer of
one electron from the ferrous ion, Fe +2 , to the ferric ion, Fe +3 . Due to the charge
difference, the hydration shell around the ferric ion is more compact than the
hydration shell around the ferrous ion. Despite this, there is a finite probability
that the two hydration shells assume similar sizes at some time points (as the result
of thermal fluctuations) as depicted by the two identically sized spheres partially
overlapping in the upper portion of Fig. 2.4 . Such a transient, metastable state is
known as the Franck - Condon state or the transitions state , and it is only in this
state that one electron can be transferred from Fe +2 to Fe +3 resulting in the electron
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