The Nobel Prize in Chemistry 1992 was awarded to Rudolph A. Marcus "for his contributions to the theory of electron transfer reactions in chemical systems".
 

The processes Marcus has studied, the transfer of electrons between molecules in solution, underlie a number of exceptionally important chemical phenomena, and the practical consequences of his theory extend over all areas of chemistry. From 1956 to 1965 Professor Marcus developed his theory for what is perhaps the simplest chemical elementary process, the transfer of an electron between two molecules. Professor Marcus found simple mathematical expressions for how the energy of the molecular system is affected by these changes. With these he was able to calculate and explain the greatly varying rates measured for electron transfer reactions. In the mathematical connection the Marcus theory makes between theoretical and experimental quantities, experimental chemists gained a valuable tool. The theory has proved useful in the interpretation of many chemical phenomena, even though it was initially controversial at some points. Certain predictions turned out to conflict with what the chemists had expected, and were also difficult to confirm experimentally.