(1923 - )
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 1951 to 1964, Marcus worked at the Polytechnic
Institute of Brooklyn. In 1951 and 1952, he published RRKM theory papers
(Rice-Ramsperger-Kassel-Marcus), utilizing statistical ideas of the
original RRK theory with the transition state theory of the 1930s to
explain solution reaction rates. In 1955, Marcus began his research
on electron transfers utilizing his knowledge in electrostatics.
Marcus explored the function of enfolding solvent molecules
in solution to resolve the speed of reactions. He concluded that because
slight adjustments occur in the molecular structure of the reactants
and molecules, it is more difficult for electrons to move about. Furthermore,
Marcus determined that the parabola depicts the correlation between
the electron-transfer reaction and the reaction rate.
In 1968, he joined the faculty at the University of
Illinois. From 1976 until 1977, Marcus traveled to Europe where he was
first a Visiting Professor at the University of Oxford, then a Humboldt
Awardee at the Technical University of Munich. It was in Munich that
Marcus became fascinated by photosynthesis and electron transfers. In
1978, Marcus moved to the California Institute of Technology as the
Arthur Amos Noyes Professor of Chemistry.
Marcus received the Nobel
Prize for Chemistry in 1992 for advancing the theory of electron-transfer
reactions in chemical systems. The Marcus theory revealed information
on such common phenomenon as photosynthesis and corrosion.
Marcus has also been awarded numerous additional honors,
including the National Medal of Science in 1989 and the Irving Langmair
and the Peter Debye Awards of the American Chemical Society in 1978
and 1988. Marcus was elected to the National Academy of Sciences in
1970 and the American Academy of Arts and Sciences in 1973.
The following press release
from the Royal Swedish Academy of Sciences
describes Marcus's work:
Professor Rudolph A. Marcus
is being rewarded for his theoretical work
on electron transfer - work which has greatly
stimulated experimental developments in
chemistry. 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. The Marcus theory
describes, and makes predictions concerning,
such widely differing phenomena as the
fixation of light energy by green plants,
photochemical production of fuel, chemiluminescence
the conductivity of electrically conducting
polymers, corrosion, the methodology of
electrochemical synthesis and analysis,
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. No chemical bonds are broken in
such a reaction, but changes take place in the molecular structure of
the reacting molecules and their nearest neighbours. This molecular
change enables the electrons to jump between the 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. We had to wait for the final
experimental confirmation until the latter part of the 1980s.
When two molecules in a solution exchange one or more
electrons, there is a reduction/oxidation process (redox process) in
which one molecule accepts the electrons (reduction) and the other loses
them (oxidation). Several different mechanisms can underlie such reactions.
The simplest is the transfer of one single electron from one molecule
to another. Changes take place in the structure, both in the reacting
molecules and in those of the solution medium. Because of all these
changes the energy of the molecular system rises temporarily and enables
the electron to jump between the molecules. Energy must thus be supplied
for the electron to be able to cross an energy barrier. The size of
the energy barrier determines the speed of the reaction. An electron
transfer of this kind is the simplest chemical elementary process, and
is eminently suitable for theoretical studies.
At the beginning of the 1950s it was possible to determine
the speed of a number of electron transfers between inorganic ions.
Some of the reactions turned out to be very slow, which was surprising
in view of the fact that only one electron changed places. It was considered
at the time that such an insignificant change should not give rise to
any large energy barrier.
The prizewinner's contributions
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 physical-chemical theory to apply. 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 associated with changes in the bonds of the reacting
In addition, Marcus further
developed his model by showing that energy
barriers could be calculated as a sum of
two terms characterising 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 sufficiently 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.
A. Marcus Autobiography"; britannica;
Press release: "The
1992 Nobel Prize in Chemistry";
Photo courtesy of ICTP Photo Archives,