Rudolph A. Marcus
Rudolph A. Marcus BY NeedHelp2333333 Rudolph A. Marcus was born on July 21, 1923, in Montreal, Quebec, Canada. He earned a B. Sc. in 1943 and a Ph. D. in 1946 from McGill University. In 1949, he worked on postdoctoral research at the University of North Carolina at Chapel Hill. From 1956 to 1965 Marcus published a series of papers on electron transfer reactions. His work led to the solution of the problem of greatly varying reaction rates. Marcus made two assumptions about the reacting molecules. First, they had to be very loosely bonded to each other during the course of the reaction for classical hysical-chemical theory to apply.
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Secondly, he assumed that it is the solvent molecules in the immediate vicinity that change their positions, thus increasing the energy in the molecular system. The electron can only Jump between two states that have the same energy, and this condition can be fulfilled only by increasing the energy for both molecules. Marcus found a simple mathematical formula for calculating this energy change and was thus also able to calculate the size of the energy barrier. Somewhat later he extended the theory to include the energy ssociated with changes in the bonds of the reacting molecules.
In addition, Marcus further developed his model by showing that energy barriers could be calculated as a sum of two terms characterizing each of the two components of the reaction. Lastly, he derived a general connection between electron transfer speed and the free energy change of the reaction, its “driving force”. The general equation is quadratic and describes a parabola (see figure). The formula has the interesting consequence, unexpected by the chemist’s intuition, that, for a ufficiently large driving force, the reaction ought to take place more slowly the larger the driving force becomes.
This area even received a special name, “the inverted region”. In the 1960s this prediction ran completely counter to chemists’ expectations and, in addition, it was difficult to study reactions of this type experimentally. Marcus himself proposed in 1965 that chemiluminescence reactions of a certain type ought to represent the inverted region, but it was not until the end of the 1980s that other, more convincing, experimental verifications could be made.