Presentation Speech by Professor Lennart Eberson of the Royal Swedish Academy of Sciences
Translation from the Swedish text
Your Majesties, Your Royal Highnesses, Ladies and Gentlemen,
The 1992 Nobel Prize in Chemistry is being awarded to Professor Rudolph Marcus for his contributions to the theory of electron transfer reactions in chemical systems. To understand the background of his achievements, we must transport ourselves back to the period around 1950, when chemistry looked completely different than it does today. In those days, it was still difficult to determine the structure of chemical compounds, and even more difficult to make theoretical calculations of the rate of chemical reactions.
Reaction rate is a fundamental concept in chemistry. A mixture of chemical compounds undergoes changes, or chemical reactions, at different rates. Today we can measure reaction rates using virtually any time scale from quadrillionths of a second to thousands of years. By the late 19th century, Sweden’s Svante Arrhenius;. later a Nobel Laureate, had shown that the rate of a chemical reaction can be described in terms of the requirement for a reacting system to cross an energy barrier. The size of this barrier was easy to determine experimentally. Calculating it was a formidable problem.
In the years after 1945, a new technique for determining reaction rates had been developed: the radioactive tracer technique. By substituting a radioactive isotope for a given atom in a molecule, new types of reactions could be studied. One such reaction was the transfer of an electron between metal ions in different states of oxidation, for example between a bivalent and a trivalent iron ion in an aqueous solution. This turned out to be a slow reaction, that is, it took place over a period of hours, something highly unexpected by the chemists of that day. Compared with an atomic nucleus, an electron is a very light particle. How could the slowness of its movement between iron ions be explained?
This problem led to lively discussion around 1950. Marcus became interested when he happened to read through some papers from a symposium on electron transfer reactions, where the American chemist Willard Libby had suggested that a well-known spectroscopic principle known as the Franck-Condon principle might apply to the movement of an electron between two molecules. Marcus realized that this ought to create an energy barrier, which might explain the slow electron transfer between bivalent and trivalent iron in an aqueous solution. To enable the two iron ions to exchange an electron, a number of water molecules in their surroundings must be rearranged. This increases the energy of the system temporarily, and at some point the electron can jump without violating the restrictions of the Franck-Condon principle.
In 1956, Marcus published a mathematical model for this type of reaction, based on classic theories of’ physical chemistry. He was able to calculate the size of the energy barrier, using simple quantities such as ionic radii and ionic charges. He later extended the theory to cover electron transfer between different kinds of molecules and derived simple mathematical expressions known as “the quadratic equation” and “the cross-equation.” These could be tested empirically and led to new experimental programs in all branches of chemistry. The Marcus theory greatly contributed to our understanding of such widely varying phenomena as the capture of light energy in green plants, electron transfer in biological systems, inorganic and organic oxidation and reduction processes and photochemical electron transfer.
The quadratic equation predicts that electron transfer reactions will occur more slowly the larger the driving force of the reaction is. This phenomenon received its own name, “the inverted region.” To a chemist, the phenomenon is just as unexpected as when a skier finds himself gliding more slowly down a slope the steeper it is. In 1965, Marcus himself suggested that certain chemiluminescent reactions (“cold light”) might serve as an example of the inverted region. Only after 1985, however, could further examples of such reactions be demonstrated. The most improbable prediction in his theory had thereby been verified.
In the space of a few minutes, I have tried to trace and explain the origins of the theory of electron transfer that carries your name. Your theory is a unifying factor in chemistry, promoting understanding of electron transfer reactions of biochemical, photochemical, inorganic and organic nature and thereby contributing to science as a whole. It has led to the development of many new research programs, demonstrating the lasting impact of your work. In recognition of your contribution to chemistry, the Royal Swedish Academy of Sciences has decided to confer upon you this year’s Nobel Prize in Chemistry.
Professor Marcus, I have the honor and pleasure to extend to you the congratulations of the Royal Swedish Academy of Sciences and to ask you to receive your Prize from the hands of His Majesty the King.