Press
Release: The 1999 Nobel Prize in Chemistry
KUNGL. VETENSKAPSAKADEMIEN
THE ROYAL SWEDISH ACADEMY OF SCIENCES
12 October 1999
The Royal Swedish Academy of Sciences has awarded the 1999 Nobel
Prize in Chemistry to
Professor Ahmed H. Zewail, California Institute of Technology,
Pasadena, USA
for showing that it is possible with rapid laser technique to
see how atoms in a molecule move during a chemical reaction.
The Academy’s citation:
For his studies of the transition states of chemical reactions using
femtosecond spectroscopy.
This year’s laureate in Chemistry is being rewarded for his
pioneering investigation of fundamental chemical reactions, using
ultra-short laser flashes, on the time scale on which the reactions
actually occur. Professor Zewail’s contributions have brought
about a revolution in chemistry and adjacent sciences, since this
type of investigation allows us to understand and predict important
reactions.
Development of femtochemistry rewarded
What would a football match on TV be without “slow motion” revealing
afterwards the movements of the players and the ball when a goal
is scored? Chemical reactions are a similar case. The chemists’ eagerness
to be able to follow chemical reactions in the greatest detail has
prompted increasingly advanced technology. This years laureate in
Chemistry, Ahmed H. Zewail, has studied atoms and molecules in “slow
motion” during a reaction and seen what actually happens when
chemical bonds break and new ones are created.
Zewail’s technique uses what may be described as the world’s
fastest camera. This uses laser flashes of such short duration that
we are down to the time scale on which the reactions actually happen
- femtoseconds (fs). One femtosecond is 10-15 seconds, that is,
0.000000000000001 seconds, which is to a second as a second is to
32 million years. This area of physical chemistry has been named
femtochemistry.
Femtochemistry enables us to understand why certain chemical reactions
take place but not others. We can also explain why the speed and
yield of reactions depend on temperature. Scientists the world over
are studying processes with femtosecond spectroscopy in gases, in
fluids and in solids, on surfaces and in polymers. Applications
range from how catalysts function and how molecular electronic components
must be designed, to the most delicate mechanisms in life processes
and how the medicines of the future should be produced.
How fast are chemical reactions?
Chemical reactions can, as we all know, take place at very varying
velocities - compare a rusting nail and exploding dynamite! Common
to most reactions is that their velocity increases as temperature
rises, i.e. when molecular motion becomes more violent.
For this reason researchers long believed that a molecule first
needs to be activated, ‘kicked’ over a barrier, if it
is to react. When two molecules collide, nothing normally happens,
they just bounce apart. But when the temperature is high enough
the collision is so violent that they react with one another and
new molecules form. Once a molecule has been given a sufficiently
strong ‘temperature kick’ it reacts incredibly fast,
whereupon chemical bonds break and new ones form. This also applies
to the reactions that appear to be slow (e.g. the rusting nail).
The difference is only that the ‘temperature kicks’ occur
more seldom in a slow reaction than in a fast one.
The barrier is determined by the forces that hold atoms together
in the molecule (the chemical bonds) roughly like the gravitational
barrier that a moon rocket from Earth must surmount before it is
captured by the Moon’s force field. But until very recently
little was known about the molecule’s path up over the barrier
and what the molecule really looks like when it is exactly at the
top, its ‘transition state’.
Hundred years of research
Svante Arrhenius (Nobel laureate in Chemistry 1903), inspired by
van’t Hoff (the first Nobel laureate in Chemistry, 1901) presented
just over a hundred years ago a simple formula for reaction speed
as a function of temperature. But this referred to many molecules
at once (macroscopic systems) and relatively long times. It was
not until the 1930s that H. Eyring and M. Polanyi formulated a theory
based on reactions in microscopic systems of individual molecules.
The theoretical assumption was that the transition state was crossed
very rapidly, on the time scale that applies to molecular vibrations.
That it would ever be possible to perform experiments over such
short times was something no-one dreamed of.
But this is exactly what Zewail set out to do. At the end of the
1980s he performed a series of experiments that were to lead to
the birth of the research area called femtochemistry. This involves
using a high-speed camera to image molecules in the actual course
of chemical reactions and trying to capture pictures of them just
in the transition state. The camera was based on new laser technology
with light flashes of some tens of femtoseconds. The time it takes
for the atoms in a molecule to perform one vibration is typically
10-100 fs. That chemical reactions should take place on the same
time scale as when the atoms oscillate in the molecules may be compared
to two trapeze artists “reacting” with each other on
the same time scale as that on which their trapezes swing back and
forth.
What did the chemists see as the time resolution was successively
improved? The first success was the discovery of substances formed
along the way from the original one to the final product, substances
termed intermediates. To begin with these were relatively stable
molecules or molecule fragments. Each improvement of the time resolution
led to new links in a reaction chain, in the form of increasingly
short-lived intermediates, being fitted into the puzzle of understanding
how the reaction mechanism worked.
The contribution for which Zewail is to receive the Nobel Prize
means that we have reached the end of the road: no chemical reactions
take place faster than this. With femtosecond spectroscopy we can
for the first time observe in ‘slow motion’ what happens
as the reaction barrier is crossed and hence also understand the
mechanistic background to Arrhenius’ formula for temperature
dependence and to the formulae for which van’t Hoff was awarded
his Nobel Prize.
Femtochemistry in practice
In femtosecond spectroscopy the original substances are mixed as
beams of molecules in a vacuum chamber. An ultrafast laser then
injects two pulses: first a powerful pump pulse that strikes the
molecule and excites it to a higher energy state, and then a weaker
probe pulse at a wavelength chosen to detect the original molecule
or an altered form of this. The pump pulse is the starting signal
for the reaction while the probe pulse examines what is happening.
By varying the time interval between the two pulses it is possible
to see how quickly the original molecule is transformed. The new
shapes the molecule takes when it is excited - perhaps going through
one or more transition states - have spectra that may serve as fingerprints.
The time interval between the pulses can be varied simply by causing
the probe pulse to make a detour via mirrors. Not a long detour:
the light covers the distance of 0.03 mm in 100 fs!
To better understand what happens, the fingerprint and the time
elapsing are then compared with theoretical simulations based on
results of quantum chemical calculations (Nobel Prize in Chemistry
1998) of spectra and energies for the molecules in their various
states.
The first experiments
In his first experiments Zewail studied the disintegration of iodocyanide:
ICN -->I + CN. His team were able to observe a transition state
exactly when the I-C bond was about to break: the whole reaction
takes place in 200 femtoseconds.
In another important experiment Zewail studied the dissociation
of sodium iodide (NaI): NaI --> Na + I. The pump pulse excites
the ion pair Na+ I– which has an equilibrium distance of 2.8 Å between
nuclei (Fig. 1) to an activated form [NaI]* which then assumes covalent
bonding. However, its properties change when the molecules vibrate;
when the nuclei are at their outer turning points, 10-15 Å apart,
the electron structure is ionic, while at short distances it is
covalent. At a certain point on the vibration cycle, just when the
nuclei are 6.9 Å apart, there is a great probability that
the molecule will fall back to its ground state or decay into sodium
and iodine atoms.
Figure
1
Potential energy curves showing ground state and excited state for
NaI. The upper curve shows the molecule vibrations in excited NaI.
When the distance between the sodium nucleus and the iodine nucleus
is short the covalent bond dominates, while the ion bond dominates
at a greater distance. The vibrations may be compared to those of
a marble rolling back and forth in a dish. As the 6.9 Å point
is passed there is a chance that the marble will roll down to the
lower curve. There it may end up in the pit to the left (return to
ground state) or fly out to the right (decay into sodium and iodine
atoms respectively).
Zewail also studied the reaction between hydrogen and carbon dioxide:
H + CO2 --> CO + OH a reaction that takes place in the atmosphere
and in combustion. He showed that the reaction crosses a relatively
long state of HOCO (1 000 fs).
A question that has occupied many chemists is why certain chemical
bonds are more reactive than others and what happens if there are
two equivalent bonds in one molecule: will they break simultaneously
or one at a time? To answer this kind of question Zewail and his co-workers
studied the disassociation of tetrafluordiiodethane (C2I2F4) into
tetrafluorethylene (C2F4) and two iodine atoms (I):
They discovered that the two C-I bonds, despite their equivalence
in the original molecule, break one at a time.
Research
is extra interesting when the results are unexpected. Zewail studied
what may be thought the simple reaction between benzene, a ring
of six carbon atoms (C6H6) and iodine (I2), a molecule consisting
of two iodine atoms. When the two molecules become sufficiently
close together they form a complex. The laser flash causes an electron
to be shot from the benzene molecule into the iodine molecule. This
then becomes negatively charged while the benzene molecule becomes
positively charged. The negative and positive charges cause the
benzene and the nearest iodine atom to be rapidly drawn to one another.
The bond between the two iodine atoms is stretched when one of them
is sucked in towards the benzene, whereupon the other atom breaks
free and flies away. All this happens within 750 fs. Zewail found,
however, that this is not the only way individual iodine atoms can
be formed: sometimes the electron falls back onto benzene. But it
is already too late for the iodine atoms: like a stretched rubber
band breaking, the bond between the two atoms breaks and they fly
apart.
Research Explosion
A much studied model reaction in organic chemistry is the ring opening
of cyclobutane to yield ethylene or the reverse, the combining of
two ethylene molecules to form cyclobutane. The reaction may thus
go directly via one transition state with a simple activation barrier
as shown schematically on the left in Figure 2. Alternatively, it
may proceed through a two-stage mechanism (right) so that first one
bond breaks and tetramethylene is formed as an intermediate. After
crossing another activation barrier the tetramethylene in turn is
converted to the final product. Zewail and his co-workers showed with
femtosecond spectroscopy that the intermediate product was in fact
formed, and had a lifetime of 700 fs.
Figure
2
How does the reaction from the cyclobutane molecule to two ethylene
molecules actually proceed? The left-hand figure shows how the state
energy varies if both bonds are stretched and broken simultaneously.
The right-hand figure shows the case where one bond at a time breaks.
Another type of reaction studied with femtosecond technology is the
light-induced conversion of a molecule from one structure to another,
photoisomerisation. The conversion of the stilbene molecule, which
includes two benzene rings, between the cis- and trans- forms was
observed by Zewail and his co-workers.
They
concluded that during the process the two benzene rings turn synchronously
in relation to one another. Similar behaviour has also recently
been observed for the retinal molecule, which is the colour substance
in rodopsin, the pigment in the rods of the eye. The primary photochemical
step, when we perceive light, is a cis-trans conversion around a
double bond in retinal. With femtosecond spectroscopy other researchers
have found that the process takes 200 fs and that a certain amount
of vibration remains in the product of the reaction. The speed of
the reaction suggests that energy from the absorbed photon is not
first redistributed but is localised directly to the relevant double
bond. This would explain the high efficiency (70%) and hence the
eye’s good night vision. Another biologically important example
where femtochemistry has explained efficient energy conversion is
in chlorophyll molecules, which capture light in photosynthesis.
Femtosecond studies following Zewail’s work are being performed
intensively the world over, using not only molecular beams but also
processes on surfaces (e.g. to understand and improve catalysts),
in liquids and solvents (to understand mechanisms of the dissolving
of and reactions between substances in solution) and in polymers (e.g.
to develop new material for use in electronics). Another important
research field is studies of biological systems. Knowledge of the
mechanisms of chemical reactions is also important for our ability
to control the reactions. A desired chemical reaction is often accompanied
by a series of unwanted, competing reactions that lead to a mixture
of products and hence the need for separation and cleansing. If the
reaction can be controlled by initiating reactivity in selected bonds,
this could be avoided.
Femtochemistry has fundamentally changed our view of chemical reactions.
From a phenomenon described in relatively vague metaphors such as ‘activation’ and ‘transition
state’, we can now see the movements of individual atoms as
we imagine them. They are no longer invisible. Here lies the reason
why the femtochemistry research initiated by this year’s Nobel
Laureate has led to explosive development. With the world’s
fastest camera available, only the imagination sets bounds for new
problems to tackle.
Further
reading
Extended version in English by
Prof. Bengt Nordén.
Ultrafast Processes
in Chemistry and Photobiology, eds. M. A. El-Sayed, I. Tanaka,
Y. Molin, IUPAC, Blackwell Science, Cambridge, 1995.
S. Pedersen, J. L. Herek and A. H. Zewail, The Validity of the
Diradical Hypothesis: Direct Femtosecond Studies of the Transition-State
Structures, Science 266, 1359 (1994).
A. H. Zewail, The Birth of
Molecules Sci. Am. 263, 76 (1990).
V. K. Jain, The World's Fastest Camera, The World
and I, October 1995, p. 156.
Femtochemistry and Femtobiology: Ultrafast Reaction Dynamics at Atomic-Scale
Resolution, ed. V. Sundström, Imperial College Press, London, 1997.
Ahmed
Zewail was born in 1946 in Egypt where he grew up and studied
at the University of Alexandria. After continued studies in the
U.S.A. he graduated for PhD in 1974 at the University of Pennsylvania.
After two years at the University of California at Berkeley he
was employed at Caltech where he has the Linus Pauling Chair of
Chemical Physics since 1990. Zewail is Egyptian and American citizen.
Professor Ahmed H. Zewail
California Institute of Technology
Arthur Amos Noyes Laboratory of Chemical Physics
Mail Code 127-72
Pasadena, California 91125
USA
The amount of the Nobel Prize Award is
SEK 7, 900, 000.
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