Electron Transfer Reactions: TheoryBased in part on the article Electron Transfer Reactions: Theory by Roderick D. Cannon which appeared in theEncyclopedia of Inorganic Chemistry, First Edition.

Abstract

AbstractThis article discusses the progress in the field of electron‐transfer theory over the period of about 50 years since early 50s marked by the development of theories of radiationless transitions in molecules in the gas phase and the Marcus theory of electronic transitions between ions in solution. The article starts with the classification of electron‐transfer reactions in terms of reaction mechanisms and the extent of electronic delocalization. This is followed by the outline of the picture of intersecting parabolas due to Marcus, which defines the activation barrier for nonadiabatic electron transfer. The rate of nonadiabatic electron transfer strongly depends on electronic coupling between the donor and acceptor states. The Mulliken‐Hush theory, along with its recent generalizations, provides coupling from spectroscopic observables. The success of electron‐transfer theories has largely been supported by the connection between thermal rates and optical spectroscopy. The activation barrier can be obtained from optical spectra by using the band‐shape analysis discussed next. This is followed by the overview of the inverted region of electron transfer and the consequences of introducing nonparabolic free energy surfaces of electron transfer in polarizable systems. The advent of ultrafast spectroscopic techniques opened up a new avenue in electron‐transfer research related to the effects of polarization dynamics on the reaction rate. Measurements at varying pressure allow one to introduce an additional thermodynamic variable probing the activation barrier. The article concludes with a brief account of magnetic effects in electron transfer.