Press Release: The 1998 Nobel Prize in Chemistry
KUNGL. VETENSKAPSAKADEMIEN
THE ROYAL SWEDISH ACADEMY OF SCIENCES
13 October 1998
The Royal Swedish Academy of Sciences
has awarded the 1998 Nobel Prize in Chemistry to
Professor Walter Kohn, University of California at Santa Barbara,
USA and,
Professor John A. Pople, Northwestern University, Evanston, Illinois,
USA (British citizen).
The Laureates have each made pioneering contributions in developing methods
that can be used for theoretical studies of the properties of molecules
and the chemical processes in which they are involved.
Citation:
"to Walter Kohn for his development of the density-functional
theory and to John Pople for his development of computational methods
in quantum chemistry."
Development of computational methods in chemistry awarded
Researchers have long sought methods for understanding how bonds between
the atoms in molecules function. With such methods it would be possible
to calculate the properties of molecules and the interplay between them.
The growth of quantum mechanics in physics at the beginning of the 1900s
opened new possibilities, but applications within chemistry were long
in coming. It was not practically possible to handle the complicated mathematical
relations of quantum mechanics for such complex systems as molecules.
One of the founders of quantum physics, Dirac, expressed the problem
in 1929 as follows: "The fundamental laws necessary for the mathematical
treatment of large parts of physics and the whole of chemistry are thus
fully known, and the difficulty lies only in the fact that application
of these laws leads to equations that are too complex to be solved.
Things began to move at the beginning of the 1960s when computers came
into use for solving these equations and quantum chemistry (the application
of quantum mechanics to chemical problems) emerged as a new branch of
chemistry. As we approach the end of the 1990s we are seeing the result
of an enormous theoretical and computational development, and the consequences
are revolutionising the whole of chemistry. Walter Kohn and John
Pople are the two most prominent figures in this process. W.Kohn's
theoretical work has formed the basis for simplifying the mathematics
in descriptions of the bonding of atoms, a prerequisite for many of today's
calculations. J. Pople developed the entire quantum-chemical methodology
now used in various branches of chemistry.
Computer-based calculations are now used generally to supplement experimental
technics. For several decades they have been developed and refined so that
it is now possible to analyse the structure and properties of matter in
detail. Conventional calculation of the properties of molecules is based
on a description of the motion of individual electrons. For this reason,
such methods are mathematically very complicated. Walter Kohn showed
that it is not necessary to consider the motion of each individual electron:
it suffices to know the average number of electrons located at any one point
in space. This has led to a computationally simpler method, the density-functional
theory. The simplicity of the method makes it possible to study very
large molecules. Today, for example, calculations can be used to explain
how enzymatic reactions occur. It has taken more than thirty years for a
large number of researchers to render these calculations practicable, and
the method is now one of the most widely used in quantum chemistry.
John Pople is rewarded for developing computational methods making
possible the theoretical study of molecules, their properties and how
they act together in chemical reactions. These methods are based on the
fundamental laws of quantum mechanics as defined by, among others, the
physicist E. Schrödinger. A computer is fed with particulars of a
molecule or a chemical reaction and the output is a description of the
properties of that molecule or how a chemical reaction may take place.
The result is often used to illustrate or explain the results of different
kinds of experiment. Pople made his computational technics easily accessible
to researchers by designing the GAUSSIAN computer program. The first version
was published in 1970. The program has since been developed and is now
used by thousands of chemists in universities and commercial companies
the world over.
Quantum chemistry - a background
The laws of quantum mechanics as formulated more than 70 years ago make
it theoretically possible to understand and calculate how electrons and
atomic nuclei interact to build up matter in all its forms. The task of
quantum chemistry is to exploit this knowledge to describe the molecular
system. This has proved easier said than done. It was not until the beginning
of the 1960s that development really started, when two events became decisive.
One was the development of an entirely new theory for describing the spatial
distribution of electrons, and the other was the use of the increasing potential
offered by the computer. Walter Kohn showed in 1964 that the total
energy for a system described by the laws of quantum mechanics can be theoretically
calculated if the electrons' spatial distribution (electron density)
is known. The question is only how the energy depends on the density. Kohn
gave important clues based on what this dependence looked like in an imaginary
system with free electrons. It was to take several decades and contributions
from many researchers, however, before the equation for determining the
energy was sufficiently accurately mapped to permit large-scale studies
of molecular systems. This has taken place partly through the adaptation
of a small number of variables to experimental data. The method Kohn introduced
came to be known as the density-functional theory. It is now used
in studies of numerous chemical problem areas, from calculating the geometrical
structure of molecules (i.e. bonding distance and angles) to mapping chemical
reactions.
During the 1960s many research groups in Europe and the USA started feverishly
to exploit the great potential of the computer. New methods of computation
were developed and refined. John Pople was a leading figure in this
field. He realised that if theoretical methods were to gain any significance
within chemistry it was necessary to know how accurate the results are in
any given case. In addition, they must be easy to use and not too demanding
of resources. Through significant improvements in the theoretical methodology
at the end of the 1960s Pople designed a computer program which at a number
of decisive points was superior to others' efforts. The prerequisites mentioned
above could now be fulfilled and GAUSSIAN-70, as the program was called,
very soon became widely used. Pople continued during the 1970s and 1980s
to refine the methodology, at the same time building up a well-documented
model chemistry. Here he was able at the beginning of the 1990s to
include Kohn's density-functional theory. By these means new possibilities
opened up for analysing ever-more complex molecules.
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Fig. 1. Electron density in the amino acid cystein
calculated using a quantum-chemistry computer program. The picture
shows the surface where the electron density is 0.002 electrons/Å3
(meaning that nearly all electrons are inside the surface). The
grey scale shows the electrostatic potential at this surface,
darker portions representing negative potential.
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Applications of quantum chemistry
Quantum chemistry is today used within all branches of chemistry and molecular
physics. As well as producing quantitative information on molecules and
their interactions, the theory also affords deeper understanding of molecular
processes that cannot be obtained from experiments alone. Theory and experimentation
combine today in the search for understanding of the inner structure of
matter. How is then a quantum-chemical calculation carried out?
Let us take the example of the amino acid cystein, illustrated above.
How do we produce that image? We sit in front of the computer and start
the quantum chemistry program. From the menu we select a molecule in which
a carbon atom (C) is bound to a hydrogen atom (H), an amino group (NH2),
a thiolathomethyl group CH2SH) and a
carboxyl group (COOH). The computer draws a rough picture of the molecule
on the screen. We now instruct the computer to determine the geometry
of the molecule with a quantum-chemical calculation. This can take a minute
or so if we are content with a rough result, but up to a day if we desire
high accuracy. The screen picture gradually changes towards greater accuracy
up to a predetermined level. When this operation is finished we can ask
the computer to calculate different properties for the system. In the
illustration above we have calculated a surface with constant electron
density. The surface is coloured according to the value of the electrostatic
potential. This can be used, for example, to predict how the molecule
interacts with other molecules and charges in its environment. Such information
may be used to study how proteins (which are built up of amino acids)
interact with different substrates, e.g. in pharmaceuticals.
Another example may be taken from the universe, in which, apart from
stars and planets, there are great quantities of interstellar matter,
often collected in vast clouds. What does this matter consist of? It can
be studied from the Earth through the radiation the molecules emit. The
radiation occurs because the molecules rotate. Hence it is possible using
the frequency spectrum of the radiation to determine the composition and
appearance of the molecules. This, however, is an immensely difficult
task, particularly since these molecules cannot always be produced in
the laboratory so as to obtain material for comparative studies. Quantum
chemistry, however, does not suffer from such limitations. Calculations
based on assumed structures can give information on radio emission frequencies
that can be directly compared with data collected by the radio telescope.
In this way, theory and measurement together can give information on the
molecular composition of interstellar matter.
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Fig 2. High up in the atmosphere, CF2Cl2
molecules (freon, left in picture) are destroyed by ultraviolet
light. Free chlorine atoms are formed, which react with O3
molecules (ozone, right in picture) and destroy them. The process
can be studied using quantum-chemical calculations.
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Another example. High up in the atmosphere there is a thin layer of ozone
molecules that protect us from ultra-violet radiation from the sun. Substances
that we release into the atmosphere (e.g. freons) can lead to the destruction
of the ozone layer. How does it happen? Which chemical reactions are involved?
With quantum-chemical computation we can describe them in detail and thus
understand them. This knowledge may help us to take steps to make our
atmosphere cleaner.
Quantum chemistry is used nowadays in practically all branches of chemistry,
always with the aim of increasing our knowledge of the inner structure
of matter. The scientific work of Walter Kohn and John Pople has been
crucial for the development of this new field of research.
Further reading
Additional background material on the Nobel
Prize in Chemistry 1998, The Royal Swedish Academy of Sciences.
E.K. Wilson, Theoretical Chemistry Expands and Diversifies Across Chemical
Disciplines, Chemical & Engineering News, August 19, 1996.
J.H. Krieger, Computational Chemistry Impact, Chemical & Engineering
News, May 12, 1997.
W.J. Hehre, L. Radom, P. v. R. Schleyer och J.A. Pople, Ab Initio Molecular
Orbital Theory, John Wiley & Sons, New York, 1986.
R.G. Parr and W. Yang, Density-Functional Theory of Atoms and Molecules,
Oxford Science, Oxford, 1989.
Encyclopaedia of Computational Chemistry (ed. Paul v. R. Schleyer),
John Wiley & Sons,
New York, 1998.
Walter Kohn was born in Vienna, Austria, in 1923. He was a professor
at the Carnegie Institute of Technology in Pittsburgh, USA between 1950
and 1960 and at the University of California in San Diego from 1960 to
1979. He was Director of the Institute for Theoretical Physics in Santa
Barbara, where he is still active, from 1979-1984.
Professor Walter Kohn
Department of Physics
University of California at Santa Barbara
CA 93106, USA
John A. Pople was born in Burnham-on-Sea in Somerset, U.K. in 1925.
British citizen. He became Ph.D. in Mathematics at Cambridge, U.K., in
1951. In 1964 he became Professor of Chemical Physics at Carnegie-Mellon
University, Pittsburgh, USA and subsequently Professor of Chemistry at
Northwestern University, USA, in 1986, where he is still active.
Professor John A. Pople
Northwestern University
Department of Chemistry
2145 Sheridan Road
Evanston, IL 60208, USA
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