The Nobel Prize in Chemistry 1999. " for his studies of the transition states of chemical reactions using femtosecond spectroscopy". Ahmed H. Zewail. Egypt and USA . California Institute of Technology Pasadena, CA, USA . b. 1946.
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"for his studies of the transition states of chemical reactions using femtosecond spectroscopy"
Ahmed H. Zewail
Egypt and USA
California Institute of Technology Pasadena, CA, USA
The "shutter speed" of such a camera must be extremely high since molecules are very small (about 10-9m) and move extremely rapidly (1 000 m/s).
A chemical reaction - up hill and down daleLike everything in nature, molecules strive to reach the lowest possible energy state. This makes it practical to describe reactions using energy surfaces. A molecule on an energy surface tries, like a child in a water-slide, to reach the lowest point. You need enough speed (high energy) to get up over the crest.The picture below shows the ring opening of a cyclo-butane molecule to form two ethylene molecules. Zewail studied this reaction by exciting cyclopentanone molecules with a femtosecond pulse. He could show that this reaction occurs via a transition state living a few hundred femtoseconds. This experiment settled an old argument over whether the reaction takes place in one step with simultaneous breaking of both bonds or in two steps, one bond breaking before the other.
Further readingInformation of the 1999 Nobel Prize in Chemistry (press release): The Royal Swedish Academy of SciencesThe Femtoland web page: www.its.caltech.edu/~femtoThe Birth of Molecules, A.H. Zewail, Scientific American, Vol. 262, Dec. 1990, pp.40-46Laser Femtochemistry, A.H. Zewail, Science, Vol. 242 (1988), pp. 1645-1653Femtochemistry: Recent progress in studies of Dynamics and Control of Reactions and their Transition States, A.H. Zewail, J. Phys. Chem. (Centennial Issue), Vol. 100 (1996) 12701-12Femtochemistry: Ultrafast Dynamics of the Chemical Bond, Vol 1-2, A.H. Zewail. World Scientific 1994 pp.915, ISBN 9810219407Femtosecond Chemistry, Vol 1-2, Eds. J. Manz, L. Wöste. VCH 1995 pp. 916, ISBN 3-527-29062-1Femtochemistry and Femtobiology: Ultrafast Reaction Dynamics at Atomic-Scale Resolution, Nobel Symposium 101.Ed.V. Sundström.World Scientific, Singapore 1997. ISBN 1-86094-039-0The World's Fastest Camera, V.K. Jain. The World and I, October 1995, pp. 156-163Freezing Time - in a Femtosecond, J.S. Baskin and A.H. Zewail, Science Spectra, Issue 14 (1998) pp. 62-71Ten years of Femtochemistry: Time Resolution of Physical, Chemical and Biological Dynamics, A.W. Castleman and V. Sundström (eds.), J. Phys. Chem. - Special Issue (1998), pp. 4021.
During the 1950's chemists were able to establish that
reactions consisting of a single electron transfer, have a welldefined
rate. From one reaction to another the difference may
be as great as that between a snail and an express train. Why?
Rudolph Marcus solved this riddle by considering all the details
of the reaction. Although no bonds are broken during the
reaction, there are still small changes in structure when
electrons are added or removed. The lengths of the chemical
bonds are changed and the molecules of the solvent are thrown
about. Such structural changes require the reorganization
In fig 1, λ is the energy difference between the bottom of
the left parabola (representing the equilibrium positions before
the reaction) and a point vertically above on the upper part of
the other curve (representing the reaction products A/A- before
any atomic positions have been changed).
Marcus was the first to calculate λ. He also realized that the
leap of the electron only requires the energy λ/4 via the
crossing point of the parabolas. The activation energy is thus
Ea = λ/4. One may then, according to well-known principles,
Reaction rate = ν exp(-Ea/kT)
where k is the Boltzmann constant and T the absolute
temperature. The preexponential factor (ν) is the vibration
frequency of the atoms around their equilibrium positions. The
reorganization energy λ, and hence Ea, varies greatly from one
reaction to another. In this way one may explain the great
variations in reaction rate.
the Marcus model to moving the energy curve of the products
(marked D+/A-) down by the amount E. As is evident from fig
2 the activation energy (Ea) is decreased and is instead Ea =
(E-λ)2/4λ. The dependence of the reaction rate on E is then
parabolic, and this theoretical prediction agrees very well with
If the liberated energy E is equal to λ, then Ea will be equal
to zero and the reaction is very fast.
For E>λ a so called inverted region is reached (see fig 3),
where the activation energy Ea increases with E. The reaction is
slower the greater the energy liberated. This rather remarkable
result has been experimentally confirmed.
An important part of the reorganization energy, λ, is
dependent on the solvent. The higher the dielectric constant of
the latter the larger the value obtained for λ. Water with a high
dielectric constant is thus a poor solvent if fast electron transfer reactions are desired.
When the rod is bent a glass ampoule is broken, the liquids
from the ampoule and the rod are mixed and 'cold light' is
produced. The phenomenon is an example of
chemiluminescence, a complicated process involving electron
transfer steps. Due to the inverted region the total reaction
results in an excited state from which light is emitted. Safety
lights of this kind, which are non-flammable and weatherproof
are used by seamen and divers in emergency.
between two imidazole groups (the five-membered rings) and two yellow sulphur atoms.
"for their studies of extremely fast chemical reactions, effected by disturbing the equlibrium by means of very short pulses of energy"
1/2 of the prize
Federal Republic of GermanyU Max-Planck-Institut für Physikalische Chemie Goettingen, Federal Republic of Germany 1927b. 1897d.
Ronald George Wreyford Norrish
1/4 of the prize
Institute of Physical Chemistry
Cambridge, United Kingdom
b. 1897 d. 1978
1/4 of the prize
Royal Institution of Great Britain Londonb. 1920d. 2002
Your Majesty, Your Royal Highnesses, Ladies and Gentlemen.The chemists of older times were chiefly interested in how to produce substances from natural products which might prove useful; for example, metals from ores and the like. As a matter of course, they were bound to notice that some chemical reactions took place rapidly, while others proceeded much more slowly. However, systematic studies of reaction velocities were hardly undertaken before the mid-19th century. Somewhat later, in 1884, the Dutch chemist, Van 't Hoff, summarized the mathematic laws which chemical reactions often follow. This work, together with other achievements, earned for Van 't Hoff the first Nobel Prize for Chemistry in 1901.Almost all chemical reactions will proceed more rapidly if the mixture is heated. Both Van 't Hoff and Svante Arrhenius, who for other discoveries was awarded the third Nobel Prize for Chemistry in 1903, set up a mathematical formula which describes how the velocity of a reaction increases with temperature. This formula could be interpreted by the assumption that when two molecules collide, they usually part again and nothing happens; but if the collision is sufficiently violent, the molecules disintegrate and their atoms recombine into new molecules. One could also envisage the possibility that the molecules moved towards each other at moderate velocity, but that the atoms in one molecule oscillated violently so that no severe impact would be required for that molecule to disintegrate. It was already then realized that higher temperature implied two things: the molecules moved faster, and the atoms oscillated more violently. It was also realized that when a reaction velocity could be measured, only the merest fraction of the collisions involved really resulted in a reaction.
Professor Dr. Manfred Eigen. Although chemists had long been talking of instantaneous reactions, they had no way of determining the actual reaction rates. There were many very important reactions of this type, such as the neutralization of acids with alkalis. Thanks to you, chemists now have a whole range of methods that can be used to follow these rapid processes, so that this large gap in our chemical knowledge has now been filled.
Professor Ronald George Wreyford Norrish, Professor George Porter. Photo-reactions have been studied by chemists for more than two hundred years, but the detailed knowledge of the behaviour of the activated molecules was meagre and most unsatisfactory. By your flash photolysis method you have provided us with a powerful tool for the study of the various states of molecules and the transfer of energy between them.May I convey to you the warmest congratulations of the Royal Swedish Academy of Sciences.
Professor Eigen. May I ask you to come forward to receive the Nobel Prize for Chemistry from His Majesty the King.
Professor Norrish, Professor Porter. May I request you to receive the Nobel Prize for Chemistry from the hands of His Majesty the King.
From Nobel Lectures, Chemistry 1963-1970, Elsevier Publishing Company, Amsterdam, 1972