Outer sphere electron transfer
Outer sphere or nonbonded electron transfer refers to an electron transfer (ET) event that occurs between chemicals that remain separate species before, during, and after the ET event. This is opposed to inner sphere electron transfer in which the two sites undergoing ET are connected by a chemical bridge. Because the ET in outer sphere electron transfer occurs between two non-connected species, the electron is forced to move through space from one redox center to the other. Outer sphere electron transfer can occur between different chemicals or between two chemicals that differ only in their oxidation state. The latter is termed "self-exchange".
Outer sphere electron transfer is generally enthalpically less favorable than inner sphere electron transfer because the interaction through space between the redox centers in outersphere electron transfer is weaker than the interaction through the chemical bridge present in the inner sphere mechanism. By the same token, outer sphere electron transfer is usually entropically more favorable than inner sphere electron transfer as the two sites involved do not have to go through the ordering processes associated with the formation of a bridge.
The first generally accepted theory of electron transfer was developed to explain outer-sphere electron transfer. It was developed by Rudolph A. Marcus in the 1950's. Marcus theory is still the most widely accepted theory of outer-sphere electron transfer. The dependence of the electron transfer rate on the thermodynamic driving force (difference in the redox potentials of the electron-exchanging sites), including the so-called “Marcus inverted region”, has also been interpreted by the catastrophe theory (Xu, F. Application of catastrophe theory to the ∆G≠ to -∆G relationship in electron transfer reactions. Outer sphere electron transfer is the most commonly known type of electron transfer. It is by far the more common form of electron transfer biochemistry, where the sterics of large proteins inhibit the formation of a chemical bridge between their redox centers. In biochemistry, there are two main types of outer sphere ET: ET between two biological molecules or fixed distance ET, in which the electron transfers within a single biomolecule (e.g. intraprotein).