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

1.2 Time scales for chemical reactions 4
• Other fast electron transfer processes:
— Autoionization of a doubly excited state, by one electron falling to a lower
orbital, while the other is excited into the ionization continuum.
• But “chemical” intra- and intermolecular − -transfer reactions are again slower:
— The electron jump itself is very fast (≈ 10 −18 s,asabove).
— However, in the Franck-Condon limit, the reactions usually require substantial
molecular rearrangements due to the structural differences between the
reactants and products (⇒ Marcus theory, Section 11.4). The net reaction
rate is thus again limited by the time scale of vibrational motion.
I
Conclusions:
• The time scales of chemical reactions span the enormous range of some 30 orders
of magnitude, from 1fs=10 −15 s up to 10 9 y=10 16 s!
• In view of this huge span, a factor-to-two experimental uncertainty in an experimental
measurement or a theoretical prediction of a rate constant really means
rather little.
• The enormous range of reaction rates, their complex dependencies on temperature,
pressure, and species concentrations, and the question of the underlying
reaction mechanisms that determine the reaction rates, make the field of chemical
reaction kinetics and dynamics very attractive and rewarding!