Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 115
CHAPTER IV
Intellectual Frontiers in
Chemistry
A remarkable bounty of benefits has
been shown to flow from chemistry. This
chapter will provide abundant evidence
that these benefits will increase greatly in
the years to come. The basis for this
optimistic expectation is that this is a time
of special opportunity for intellectual ad-
vances in chemistry. The opportunity
comes from our developing ability to in-
vestigate the elemental steps of chemical
change and the ability to deal with ex-
treme molecular complexity.
His
OCR for page 116
The Time It Takes to Wag a Tail
When your pet dog sniffs a bone, instantly his tail begins to wag. But it must take
some tune for the northernmost canine extremity to send the news all the way south
where enthusiasm can be registered! How long does it take for that delicious aroma to
lead to the happy response at the other end? Chemists are now asking questions much
like this about their pet molecules! If one end of a molecule is excited, how long does
it take for the other end to share in the excitement? That time may determine whether
the excitation will result in a chemical reaction in the part of the molecule where the
energy was Projected, somewhere else, or nowhere at all.
For the canine expenment, we need a hungry dog, a quick hand with the bone, and a
quick eye to read the stopwatch. For molecules, it's much harder. Only within the last few
years has it been possible to measure the rate of energy movement within a molecule. But
chemists now have pulsed lasers gong bursts of light with durations as short as a millionth
Of a millionth of a second (a "picosecond''). Comparing a chemical change that takes place
one picosecond to a one-second tail-wag delay involves the same speed-up as a
l~second instant replay of all historical events since the pyramids were built.
The aLa~yl benzenes provide an example. Each of these molecules has a nod benzene
Iing at one end and a flexible alkyl group at the other. At room temperature, this flexible
"tail" vibrates and bends under thermal excitation. But to act like our hungry dog, the
molecules must be cooled to cryogenic temperatures, while avoiding condensation.
Supersonic jet expansion makes this possible. When a gas mixture cows through a jet
nozzle into a high vacuum, the molecules can be cooled almost to absolute zero. An alkyl
C'
~ :~/C~1 1
benzene molecule camed along in such a stream loses all its vibrational energy, thus
relaxing the molecular tail. Then, the cold molecules intersect a brief pulse of light with
color that is absorbed by the benzene ring. With careful "color-tun~ng,~' extra vibrational
energy can be placed in the head without any v~b~onal excitation In the tail. Then we
must watch the molecule to see how long it takes for the tail to wag. Fluorescence lets us
do this.
When a molecule In a vacuum absorbs light, the only way it can get rid of the energy is
to reelect light; Such fluorescence can be recorded with a fast-response detection system to
give a spectrum that carries a tell-tale pattern showing where We extra energy was at the
instant the light was emitted. Those molecules that happen to emit right away after
excitation show the molecule head vibrating and the tail still cold. Those Mat emit later
have an emission spectrum Mat shows that the tail is wagging. In this way, we have learned
that the time it takes for the aLky! benzene ~ to begin to wag depends on how long the tad]
is. Su~pnsingly, the longer the aLkyl, the faster the movement out of the nng. The result
shows what detentes energy flow within molecules (the "density of sagest. Such
information might one day charm combustion and help us make fine chemicals out of coal.
~6
r
OCR for page 117
IV-A. CONTROL OF CHEMICAL REACTIONS
IV-A. Control of Chemical Reactions
Ultimately, success in responding to society's needs depends upon the ability to
control chemical change, a control made possible by our understanding of chemical
reactivity. Today, this understanding is being broadened and deepened at an
astonishing pace because of an array of powerful new instrumental techniques.
These instruments permit us to pose and answer fundamental questions about how
reactions take place, questions that were beyond reach only a decade ago. They
account for the recent acceleration of progress in the most basic aspects of
chemical change.
MOLECULAR DYNAMICS
Chemistry is the science concerned with the changes that occur around us when
one set of chemicals turns into another set of chemicals. Such a change, a chemical
reaction, is understood at the atomic level in terms of one set of molecules
rearranging into another set of molecules. The study of these rearrangements is
called molecular dynamics and it encompasses:
· molecular structure, the stable geometries of the reactant and product mole-
cules;
· chemical thermodynamics, the energy effects that accompany the change; and
· chemical kinetics, the time it takes for the reaction to occur.
The theory behind all chemical behavior rests in quantum mechanics. Quantum
mechanics is the mathematical description of atoms and molecules devised by
Erwin Schroedinger in 1926. It is based upon a wave-picture of the atom that has
the potential for explaining all of the chemistry of that atom. Though this has been
known for over 50 years, most of the predictive power of quantum mechanics has
been out of reach because the mathematics has been too difficult to solve. In
contrast, experimental progress on stable molecules has been extremely rapid. This
is evident in the fact that chemists have prepared more than ~ million compounds,
95 percent of them since 1965. On the other hand, our understanding of the speed
aspects of chemical change has been limited by reaction steps too fast to be
observed.
Now a new era has begun. Chemical theory, supported by the power of modern
computers, has emerged from empirical modeling. At the same time, we have
expenmental techniques that open the way to understanding the time dimension of
chemical change. Over the next three decades we will see advances in our
understandings of chemical kinetics that will match the advances in molecular
structures over the last three decacles.
Fast Chemical Processes
A chemical reaction begins with mixing reactants and ends with formation of final
products. In between, there may be a succession of steps, some extremely rapid.
To understand the reaction completely, we must cIanfy all the steps between
beginning and end, including identification of all of the intermediate molecules that
are involved in the steps.
~7
OCR for page 118
118
INTELLECTUAL FRONTIERS IN CHEMISTRY
Fifteen years ago, we could track intermediate molecules only if they hung
around at least as long as a millionth of a second. The many interesting studies on
this time scale only increased the chemist's curiosities because it became clear that
a whole world of processes took place too rapidly to be detected at that limit.
Nowhere was that more apparent than in the centuries-old desire to understand
combustion, perhaps the most important type of reaction known.
Laser light sources have spectacularly expanded these experimental horizons
over the last decade. One of their unique capabilities is to provide short-duration
light pulses with which to investigate chemical processes that occur in less than a
millionth of a second all the way down to a millionth of a millionth of a second (i.e.,
down to a picosecond, 10- ~2 see). At the state of the art, physicists are learning how
to shorten these pulses even more; pulses as short as 0.01 picoseconds (10
femtoseconds) have been measured, and kinetic studies are beginning in the
0.1-picosecond range. At one-tenth of a picosecond frequency accuracy is limited
~ ~ . ~ . . . . · , · · . .. .. . · , ~ · · .
to about 50 cm-l by a fundamental physical principle the Uncertainty Principle
(See Section V-A). These developments imply that chemists can now investigate a
reacting mixture on a time scale that is short compared with the lifetime of any
intermediate molecular species involved. The exploitation of this remarkable
capability has only just begun.
The absorption of visible or ultraviolet light by a molecule adds enough energy to
redistribute the bonding electrons, to weaken chemical bonds, and to produce new
molecular geometries. The outcome might be a high-energy molecular structure
difficult to reach by chemical reactions stimulated by heat. So the excited electronic
states reached by absorption of light furnish a new chemical world that we have
only begun to understand and put to practical use.
When a molecule absorbs light, it gains energy. One of the ways it can dispose
of the energy is to reemit light, generally of a different color than the absorbed light.
If this emission occurs quickly, it is called fluorescence. "Quickly" can mean
anywhere from within a microsecond to a picosecond. The blue light emitted by a
Bunsen burner flame and the spectacular display of the Northern Lights are
examples of fluorescence. If the light emission occurs more slowly, it is called
phosphorescence. "Slowly" can mean anywhere from a millisecond to several
seconds or even minutes. Some clock dials that glow in the dark and the blue glow
of evening ocean tides are examples of phosphorescence.
We have some basic understandings about the differences that cause these two
behaviors. When two electrons are shared in a chemical bond, they must have
opposite magnetic spins (as expressed in the Pauli Principle). But if absorption of
light adds enough energy to move one of these electrons to another part of the
molecule, the Pauli Principle no longer limits the electron spins. Then they can be
oriented opposed to each other, like two magnets whose fields cancel each other,
to give a "singlet" state. But they can also be oriented parallel so that the two
magnetic fields add together. This is called a "triplet" state. We have learned to
associate fluorescence with light emission processes that begin and end in singlet
states. Phosphorescence, however, requires moving from a triplet to a singlet state
(or the reverse). Apparently, the need to change the electron spin makes the
emission much more difficult, so it occurs more slowly.
OCR for page 119
lV-A. CONTROL OF CHEMICAL REACTIONS
There has been a spectacular increase in our ability to clarify what is going on
in these excited states since lasers have come into the chemistry laboratory. We
can now excite particular states (by control of the laser color, or wavelength),
and we can measure the time it takes for reemission to occur (by use of laser
pulses of very short duration). Even for the fastest fluorescent processes, we can
measure the radiative lifetimes, and by measuring the wavelength of the light
emitted (spectral analysis) we can see how rapidly energy moves within the
molecule and where it goes. Thus, we are beginning to map and understand the
high-energy electronic states of molecules so that they can be used to open new
reaction pathways.
Benzophenone is a substance that demonstrates how lasers are being used to
probe these high energy states. When benzophenone in ethanol solution absorbs
ultraviolet light at a 316-nm wavelength, it reemits light at two different colors, at
wavelengths of 410 and 450 nm. If the exciting light (316 nm) is delivered in a laser
pulse of 10 picoseconds duration, "prompt" emission is seen at 410 nm, with
intensity that decreases with a 50-picosecond half-life. This fluorescence is fol-
lowed, however, by weaker emission, still at 410 nm, but with a longer half-life (a
microsecond). This slower fluorescence disappears at lowered temperatures and is
replaced by longer-wavelength
phosphorescence at 450 nm
with an even longer lifetime (a
millisecond).
Photochemists have been
able to interpret these clues
about the excited states of ben-
zophenone. Absorption at 316
nm reaches a singlet state (S~)
but with extra energy placed in
the vibrational motions of the
benzophenone. This vibra-
tional excitation is lost so
quickly in liquids (warming the
solvent) that even the
"prompt" fluorescence back
to the ground state (S0) occurs
at longer wavelengths (410 nary). On the other hand, the low-temperature behavior
shows that benzophenone also has an excited triplet state (T~),that can be reached
via So. Once occupied, To emits phosphorescent light with the characteristic long
lifetime of a triplet-singlet transition (T~ > Ski. The temperature dependence of the
delayed fluorescence shows that To is lower in energy than So and by how much.
The set of processes clarified here have lifetimes that range from 50 picoseconds
to a millisecond, a difference of 20 million. The observations reveal the excited
states of benzophenone and the rates of movement between them. These under-
standings are of extreme significance because they can all be applied to natural
photosynthesis, a process scientists would like very much to master. There are
many other types of laser-based, real-time studies of rapid chemical reactions now
o
FLUORESCENCE (3
_ _
~ - 'A
_ _ S 1 S I _ ~ T I
'PROMPT' 'DELAYED'
he he' he he/'
50 psec 1 User
_ _ _ _ _ ~
_ _ so sow _
50-10-12 sec 1. 10-6 sac
PHOSPHORESCEN CE
~3
_ ~
. ~
T1
~ LOW T
he ~ he"
_ / I msec
_ ~
_
_ Hi=
so-
1~10-3 sec
EXCITED BENZOPHENONE EM ITS LIGHT
WITH TWO COLORS AND THREE CLOCKS
119
OCR for page 120
120
HOT RING
COLD TAIL
_
COLD RING
COLD TAIL
INTELLECTUAL FRONTIERS IN CHEMISTRY
being made, including chemical isomerizations, proton transfers, and photodisso-
ciations. Some of the phenomena to follow also depend upon use of short-puIse
laser excitation instrumentation.
Energy Transfer and Movement
In all chemical changes, the pathways for energy movement are determining
factors. Competition among these pathways determir~es the product yields, the
product state distributions, and the rate at which reaction proceeds. This
competition is highly important in stable flame fronts (as in Bunsen burners, jet
engines, and rocket engines) and in explosions, shock waves, and photochemical
processes.
When two gas phase molecules collide, vibrational energy can be transferred
from one molecule to another. Thus, a vibrationally "cold" molecule might be
heater! up and caused to react or a vibrationally "hot'' molecule might be cooled off
so it cannot react. These transfers of vibrational energy between and within
molecules as a result of collisions between them have long been recognized as
central to determining reaction behavior ire flames. But progress has been slow
because the processes have been too fast to measure. Now a variety of tech-
niques almost all based on laser methods—has opened the way to providing
critical data related to the pathways and rates of energy flow. These data, in turn,
furnish a basis for the develop-
ment of useful theory. As
much has been learned about
vibrational energy movement
in the last 15 years as was
learned in the preceding half-
century.
As tuned lasers became
available they were used to ex-
cite particular vibrations in a
molecule. Then, experiments
were devised to permit us to
watch this carefully placed en-
ergy move into other parts of
the molecule or into another
molecule if collisions occurred.
Fluorescence provides one
way to follow this energy
movement. The light reemitted
during fluorescence carries a
spectral signature that shows
what part of the molecule is
vibrating at the moment of emission.
A clear-cut example is provided by recent studies of the alkyd benzenes,
C6Hs-(CH2)nCH3 with n from 1 to 6. This molecule has a structure like that of a
tadpole, where n determines the length of its tail. Tuned-laser excitation allows
- FLUORESCEN CE
REVEALS
INTRAMOLECULAR
VIBRATIONAL
REDISTRIBUTION
_ _
_ _
_ _
_ _
_ _
_ _
_ _
_ ~ ~
a
~ c=
l-
~~'
COLD RING
HOT TAIL
OCR for page 121
IV-A. CONTROL OF CHEMICAL REACTIONS
us to deposit prescribed amounts of vibrational energy in the benzene end of the
cold molecule (in the head of the tadpole). When this energy is reradiated, its
spectral signature displays its vibrational excitation at the instant of radiation.
Since this light emission is a time-dependent process, we can monitor the
movement of energy from the original location of excitation into the rest of the
molecule. This movement in absence of collisions is called Intramolecular
Vibrational Redistribution (IVR). Light emitted in the first picoseconds shows
that the energy has not yet left the benzene unit where it was absorbed. The time
scale for appearance of vibrational excitation in the alky! tail depends upon the
tad] length. For n - 4, vibrational energy moves out into the tail in 2 to 100
picoseconds. In contrast, for n = 1 (ethy~benzene), it is a thousand times slower,
it takes 100 nanoseconds or more. Thus, we have direct evidence about the
. . .. . . .. · .
factors that determine IVR energy movement in an isolated molecule.
State-to-State Chemistry
When two gaseous reactants A and B are mixed and react to form products C and
D, the outcome is determined by statistical probabilities. The different encounters that
may happen between A and B include all the possible energy contents, specific
different types of excitation, and all the ways molecules may be oriented in space at the
moment of collision. Not ad of these collisions are favorable for reaction most
collisions have too little energy, or the energy is in the wrong place, or the collisions
are at an awkward geometry. If we are to understand finely the factors that permit
chemical reactions to occur, we should control the energy content of each reactant,
i.e., control the "state" of each reactant. Then we could systematically vary the
amount and type of energy available for reaction. Finally, we would like to see how the
available energy is lodged in the products. Such an experiment is ceded a "state-to-
state" study of reaction dynamics, and 20 years ago it was beyond all reach. Now, with
modern instrumentation, chemists are realizing this goal.
The earliest efforts, based upon chemiluminescence, revealed a part of the
picture: the energy distribution among the products. For example, when a
gaseous hydrogen atom and a chlorine molecule react they form hydrogen
chloride and a chlorine atom. These reaction products emit infrared light.
Analysis of the spectra from that light shows that the energy released in the
reaction is not randomly distributed between the final products. Instead, a large
fraction of it (39 percent) is initially located in the vibration of the hydrogen
chloride product. Discoveries like this won John Polanyi (University of Toronto)
a share of the 1986 Nobel Prize in Chemistry. This measurement led directly to
the demonstration of the first chemical laser a laser that derived its energy from
the hydrogen/chlorine explosion. Chemical lasers differ from conventional lasers
in that the energy to produce their light comes from a chemical reaction instead
of an electrical source. These beginnings led to the discovery of dozens of
chemical lasers, including two sufficiently powerful to be considered for possible
initiation of nuclear fusion (the iodine laser) and for possible military use in the
"Star Wars" program (the hydrogen fluoride laser).
"Molecular beams" move even closer toward "state-to-state" investigations. A
molecular beam is a stream of molecules produced by a suitably hot oven. A
121
OCR for page 122
122
Cot
c
Cat
INTELLECTUAL FRONTIERS lN CHEMISTRY
CHEMICAL LASERS REVEAL
THE PRODUCT ENERGY
DISTRIBUT ION
F+ H2
~~k ~
ko:
~ = 0
HF~ H
RERCTIQn COORDInRTE
substance is placed in this oven, and when it melts
and vaporizes the vapor is directed out a tiny hole
to form a unidirectional beam of molecules. Out-
side the oven the pressure is kept extremely low-
so low that no molecular collisions occur. The
molecular beam can then be directed toward reac-
tants. In such experiments, the reactants collide at
such low pressures—10-~° atmospheres that
each reactant molecule has at most one collisional
opportunity to react, and the products have none.
These sophisticated instruments depend upon ul-
tra-high vacuum equipment, high-intensity super-
sonic beam sources, sensitive mass spectrometers
for detectors, and electronic timing circuitry for
time-of-flight measurements. With this incredible
control it has become possible to predetermine the
energy state of each reactant molecule and then to
measure both the probability of a certain reaction
and the energy distribution in the products. For bringing such elegant experi-
ments into chemistry, Yuan-Tseh Lee (University of California, Berkeley) and
DudIey Herschbach (Harvard University) shared in the 1986 Nobel Prize in
Chemistry.
For example, a current study has explained a key reaction in the combustion of
ethylene. These molecular beam experiments show that the initial reaction of
oxygen atoms with ethylene produces the unexpected short-lived molecule
CRECHE. With this starting point, calculations have confirmed that a hydrogen
atom can be knocked out of an ethylene molecule by a reacting oxygen atom more
easily than that atom can be moved about within the molecule. This combustion
example illustrates the intimate detail with which we can now hope to understand
chemical reactions.
Multiphoton and Multiple Photon Excitation
Photochemistry has traditionally been concerned with what happens when a
single photon is absorbed by an atom or a molecule. This productive field accounts
for the energy storage in photosynthesis, the ultimate source of all life on this
planet. Photochemistry also provides us with new ways to synthesize organic
compounds and, through photodissociation, to produce a variety of short-lived
molecules that play critical roles in flames and as intermediates in reactions.
Now lasers give us optical powers lO,OOO times higher at a given frequency than
even the largest flashiamps ever built. Clearly, these devices do not simply extend
the boundaries of conventional light sources, they open doors to new processes as
molecules interact with such intense photon fields. For example, at normal light
intensities, the simultaneous absorption of two photons by a single molecule takes
place so rarely that it cannot be detected. However, the probability of this
happening increases with the square of the light intensity. Thus, if a laser increases
OCR for page 123
lV-A. CONTROL OF CHEMICAL REACTIONS
light intensity by a factor of 10,000, then the chance of two-photon absorption
increases by four orders of magnitude over the chance of one-photon absorption.
This lets us do experiments in which we can prepare molecular states that cannot
be reached with a single photon. Furthermore, the total energy absorbed can be
enough to produce ions. This opens a new avenue to the chemistry of ions, a field
of rapidly rising interest because of the discovery of interstellar ion-molecule
reactions and because ions are major species in the plasmas (glow discharges) of
nuclear fusion. Two-photon ionization has been used to detect specific molecules in
difficult environments, like those found in explosions and in flames. Thus, nitric
oxide, NO, which is an ingredient in smog, can be easily measured in a flame by
counting the ions produced by a finely tuned laser probe. The probe is tuned so
carefully that only the desired molecule, NO, can absorb light energy.
However, the most spectacular instance of multiphoton excitation came with the
development of extremely high-power CO2 infrared lasers. One of the most
surprising scientific discoveries of the 1970s was that an isolated molecule whose
vibrational adsorptions are in close vibrational harmony (near resonance) with the
laser frequency could absorb not two or three but dozens and dozens of photons.
In a time short compared with
collision times, so many pho- ~ ~ ~
tons can be absorbed that ~ E _
chemical bonds can be broken QUASI ~ I
entirely with vibrational exci- CONTINUUM ~
Ml is: S _~
ration. This unpredicted be- ~ --Gus''' _ ~ ~
RESONANT ~_L'~!~~r;~. ~
havlor 1S commonly called mul- ABSORPTION_ ~ --,.~; is. DISSOCIATIVE
tiple photon excitation to dis- - .~. - - CONTINUUM
tinguish it from two-photon ' T
(multiphoton) excitation. ~ ~ -'- ~ '' - . -
This behavior stunulated a 32
large group of studies on energy 34s~6 hV 3~`
flow within excited polyatomic SF6 ~ hV ~ SF6
molecules. Many un~molecu- ~ hV 34sF ,,
tar breakdowns and rearrange-
ments have been triggered using
multiple photon excitation. Yet,
the understanding gained from
this phenomena may be over-
shadowed by the importance of
its practical uses. Infrared absorption depends upon vibrational movements whose
frequencies are quite sensitive to atomic mass. As a result, the tuned laser can be used
to break up just those molecules containing particular isotopes, leaving behind the
others—a new method for isotope separation. For example, deuterium is present at
0.02 percent in natural hydrogen. Yet, by multiple photon excitation, this tiny
percentage can be extracted using trifluoromethane, CF3H. The process has been
shown to have a 10,000-fold preference for exciting CF3D over CF3H. This could be of
considerable importance as a source of deutenum since '`heavy water," D2O, is used
in large quantities in some nuclear reactors.
ISOTOPE SEPARATION
THROUGH
MUTT I PHOTON
ho 34SF $$* EXCITATION
nhy 3.SF`,nt 1' 3 SF5 ~ F
123
OCR for page 124
124
INTELLECTUAL FRONTIERS lN CHEMISTRY
Even restore significant is sulfur isotope separation through excitation of sulfur
hexafluoride, SF6. This gaseous compound gave the first convincing evidence that
multiple photon excitation really occurred so rapidly that collisional energy transfer
could be avoided. The successful use of SF6 for sulfur isotope separation could
have heavy significance in human history. The gaseous substance that has always
been used in the difficult processes used to separate uranium isotopes is uranium
hexafluoride, UFO. Because SF6 and UFO have identical molecular structures, they
have similar vibrational patterns. Thus, multiple photon excitation might offer a
new and simpler approach to isolation of the uranium isotopes that undergo nuclear
fission. It depends, of course, upon finding a sufficiently powerful and efficient laser
at the lower frequencies absorbed by UFO. It will bring more general access to the
critical ingredients of nuclear energy and, unfortunately, nuclear bombs as well.
The dangers of increased proliferation of nuclear weaponry can only be increased
by such access.
Mode-Selective Chemistry
When two molecules collide with each other, the violence of the collision may
cause their atoms to rearrange to form two new molecules (i.e., a reaction may
occur). Such an outcome al-
most always requires that the
molecular collision involve
some minimum energy-
enough to break some of the
bonds in the reactants in order
to form the new bonds in the
products. This minimum en-
ergy, the activation energy, de-
termines the rate of the reac-
tion and it accounts for the
dramatic effect of temperature
on reaction rates.
However, the question of
whether a reaction will result
from a molecular collision
turns out to involve more than
just whether there was enough
energy. There is also a ques-
tion of whether the collisional
energy is in the right form. To
understand what this means,
consider a bedspring thrown
against a wall. As it bounces
off, it has energy of several
types. It will be moving
through space, which is energy
of the old-fashioned kinetic en-
a!
~ VIBRATION
STRETCH I NO
$ BEND I GIG
H
TRANSLATION ~
C H
H C
\ '
H
H
ROTATI ON . - ~i,~
.. ' ,,. ' ,,. ' ,,.. ,, ,,.—,,~, ~ . ,,
7 .. 7 ,^~ ~ i< ~ ~~ ~ ,.
.. ~= ~ i' ,. ~ ,,. ~ ~ ,.
/",~,,r,~t " Z
4, ~ ,. ~ ,. ~~~, ~ ,. " , " ,. ~
, ,"' ," '
j'C ~,~
H7-C
~ H_
~ "
C `.
r
~ an
H ~ C
_ _ \~
H
MOLECULES TRANSLATE ROTATE AND VIBRATE
LIKE A BEDSPRING THROWN AGAINST A WALL
OCR for page 125
~-A. CONTROL OF CHEMICAL REACTIONS
ergy type. This is called translational energy. In addition, the bedspring will be
tumbling in space. This, too, is a form of kinetic energy called rotational energy.
Then, the spring will be twisting and vibrating to and fro. This vibrational energy
consists of both potential and kinetic energy. Molecules carry energy in exactly the
same ways. Whether we are talking of bedsprings or molecules, the directions of
translational motion, the axes of rotation, and the spring connections (in molecules,
the bonds) are called degrees of freedom. The total energy in a collision is the sum
of all of these forms of energy translational, rotational, and vibrational from both
molecules.
Chemists have long wondered whether it matters which degree of freedom
carries the energy in a reactive collision. If all of the energy is in translational
energy, the molecules are near each other only a short time. If the same amount of
energy is brought to the collision mostly as vibration, the molecules move toward
each other slowly, but now the bonds that must be broken are vibrating rapidly. Is
this more or less effective?
Only since chemists have acquired lasers has it been possible to seek an answer to
this fundamental question. With high-power, sharply tunable lasers, we can excite one
particular degree of freedom for many molecules in a bulk sample. As long as this
situation persists, such molecules react as if this particular degree of freedom is at a
very high temperature while all the rest of the molecular degrees of freedom are cold.
The chemistry of such molecules has the potential to show us the impomnce of that
particular degree of freedom in causing reaction. This is called mod~e-selective
chemistry.
Both unimolecular reactions and molecular beam studies of bimolecular (two-
molecule) reactions escape this problem. Unimolecular reactions involve only one
molecule, so collisions are not required. At sufficiently low pressures, the effects of
selective excitation on reactivity can be studied. The beam experiments sidestep
the problem by giving each excited molecule only one chance for collision and by
noticing only those collisions which result in a reaction. Nevertheless, mode-
selective reactions are not readily coming from such experiments. Apparently the
problem is that vibrational redistribution takes place within molecules even without
collisions (IVR). This problem is of such basic importance to molecular dynamics
that it will be one of the most important study topics for the next decade.
There is, however, evidence for two-molecule mode-selective chemistry in certain
solid inert-gas environments. In this situation the environment is so cold (IOK) that the
reactive molecules are held immobilized. They are "frozen" in a prolonged, cold
collision and rotational movement has been halted. For example, fluonne, F2, and
ethylene, C2H4, suspended in solid argon at lOK do not react until one of the
vibrational motions of ethylene is excited with a resonantly tuned laser. Then it is
found that the most efficient vibrational motions are those that distort the molecular
plananty. This is plausible because this type of distortion changes the molecular shape
"toward'' the nonplanar, ethane-like structure of the final product.
Theoretical Calculations of Reaction Surfaces
Schroedinger~s wave equations of quantum mechanics have long been known to
describe all chemical events. Yet quantum mechanics has been used in chemistry
125
OCR for page 156
156
INTEr~r ACTUAL FRONTIERS IN CHEMISTRY
Molecules in liquids can be highly efficient agents for storing or transferring energy.
The very structure of liquid water determines our planetary environment and
influences the course and nature of all biochemical processes essential to life.
The structure and dynamics
of a wide range of fluids, from
liquid hydrogen to molten sili-
cates, can be investigated by a
number of spectroscopic tech-
niques, such as X-ray and
neutron diffraction, nuclear
magnetic resonance, and laser
Raman and light scattering.
Among the newer experimen-
tal approaches, pulsed laser
excitation techniques are par-
ticularly powerful. On a pico-
second time scale (10- ~2
seconds), we can sense the
freedom of movement of a sol-
ute molecule held in its solvent
cage. Now we can watch fun-
damental chemical events as
they take place: how two io-
dine atoms combine in a liquid
to produce an iodine molecule; how electrons released in liquid water become
trapped, or solvated; how energy placed in a solute molecule like nitrogen or
benzene is transferred to its solvent environment.
Quite a different opportunity area is connected with the melting of small clusters
of metal atoms. We have a variety of new experimental methods for producing and
studying small metal clusters, as well as the theoretical tools with which to interpret
the results. We can look ahead to an understanding of how the change from the fluid
liquid state to the rigid solid state emerges as cluster size increases toward bulk
amounts. Furthermore, the computer can keep track of the energy and randomness
associated with each arrangement, so thermodynamic data can be calculated for
comparison with experiments, and then for predictions under conditions out of
reach expenmentally.
~ fOI03 ~ ~ &
To
LASERS LET US MEASURE FAST CHANGES
IN THE SOLVENT CAGE
Critical Phenomena
For any fluid, there is a characteristic temperature and pressure above which the
liquid and gaseous states are identical. Fluid behavior under these "critical
conditions" can differ markedly from normal behavior and give rise to new
phenomena. The past 20 years have seen a revolution in our understanding of such
critical phenomena. Undoubtedly, the most important single theoretical advance in
our understanding in the last 15 years has been the development of the new
mathematical technique called the "renormalization group" approach. It has
OCR for page 157
IV<. NATIONAL WELL-BEING
shown promise for quantitative description of fluid properties and their dependence
upon molecular shapes and forces.
The past 15 years have seen the beneficial use of critical phenomena in a variety
of applications. Critical point drying is now a standard sample preparation method
in electron microscopy. Further, there are remarkable changes in the solvent power
of a liquid near its critical point. These are at work, for example, in the removal of
caffeine from coffee for cadeine-free instant coffee and in the extraction of perfume
essences. In addition, there are valuable research applications in liquid chroma-
tography.
Chemistry of the Terrestrial and Extraterrestrial Materials
The Earth's geochemical phenomena involve complex mixtures, frequently with
a number of crystalline and glassy (amorphous) phases, and they may take place at
extremely high pressures and temperatures. Recent advances in high-pressure
technology have made studies possible that duplicate conditions near the earth's
core. In recent years many earth scientists have studied the "geochemical cycles"
of elements that is, the changing chemical and physical environment of a given
element during such natural processes as crystallization, partial dissolving, change
of mineral structure (metamorphism), and weathering. These processes may lead to
concentration (e.g., ore deposits) or dispersion of an element. The geochemical
cycle of carbon has provided a focus for the reawakened field of organic
geochemistry. Research on the stability, conformation, and decomposition reac-
tions of fossil organic molecules has led to greater understanding of the origin and
composition of coal and other organic deposits. Such knowledge has obvious value
that extends from guiding our exploration for new fossil fuel deposits to helping us
decide how to use the ones we have.
Meteontes are of considerable chemical interest because they include the oldest
solar system materials available for research and they provide samples of a wide
range of parent bodies some primitive, some highly evolved. Meteorites carry
records of certain solar and galactic events and yield data otherwise unobtainable
about the genesis, evolution, and composition of the Earth and other planets,
satellites, asteroids, and the Sun. Unusual isotopic percentages of many metals and
gaseous elements, and compositional data particularly trace elements—have shed
light on stages of the formation, evolution, and destruction of the original parent
body or asteroid where the meteorite originated.
Within the last decade, the study of meteorites has been dramatically advanced
by the recognition that if these projectiles from outer space land on the Antarctic
ice sheet, they are immediately entombed in an inert environment and permanently
refrigerated, stopping chemical changes. The question, of course, is how does one
find these meteorites in the wide and forbidding spaces of this hostile region?
Nature provides an astonishingly convenient answer. The Antarctic ice sheet is a
vast glacier, so it gradually flows northward, carrying the meteorites with it. Over
thousands of years, snow that fell near the South Pole finally reaches the end of the
glacier where the ice begins to evaporate. Here at the glacier edges, the meteorites
are dropped in great numbers, essentially never having been exposed to terrestrial
life forms, erosion, or weathenng. Since this discovery, more meteorites have been
157
OCR for page 158
158
.
METEORITES: AN ANTARCTIC TREASURE TROVE
I7vTEr r ~.cTuAL FRONTIERS IN CHEMISTRY
collected (in the last decade)
than over all of history before.
The chemical and physical
analysis of this meteorite trea-
sure trove has only just begun.
ANALYTICAL
CHEMISTRY
Characterization of atomic
and molecular species their
structures, compositions, etc.,
is called qualitative analytical
chemistry. The measurement
of the relative amount of each
atomic and molecular species is called quantitative analytical chemistry. Both
areas contribute to and benefit from the current rapid progress in science. Basic
discoveries from physics, chemistry, and biology are providing new methods of
analysis. In return, these new abilities are central to research progress in chemis-
try, other sciences, and medicine, as well as to a wide range of applications in
environmental monitoring, industrial control, health, geology, agriculture, defense,
and law enforcement. Further, the lO-fold growth of the analytical instrumentation
industry to $3 billion in sales worldwide has been led by the United States with its
nearly $1 billion positive balance of trade in this area.
A key factor in this growth has been the incorporation of computers into
analytical instrumentation. The benefits here are circular; modern computers have
evolved through advances in solid-state technology. In turn, these advances have
critically depended upon the ability to analyze quantitatively the concentrations of
trace impurities in silicon, the key element in current computer technology. Now
microprobe analyzers using computer imaging techniques are answering questions
critical to making microcircuitry even smaller, which will produce computers that
are faster, more reliable, and cheaper.
Analytical Separations
Analyses of some complex mixtures are possible only after separation of the
mixture into its components. Then, a variety of identification and quantitative
measurement schemes become effective that would be confusing or impossible if
applied to the unseparated mixture. Hence, devising new separations for use in an
analytical context is an active field of research.
There is no single technique more effective and generally applicable than the
chromatographic method. The basic principle depends upon the fact that each
molecular species, whether gaseous or in solution, has its own characteristic
strength of attachment to, and ease of detachment from, any surface it encoun-
ters. The differences in these attachment strengths can furnish a basis for
separation. The differences can depend upon heat of adsorption, volatility,
interaction with the solvent, molecular shape (including stereogeometry),
OCR for page 159
Rae. NATIONAL WEr r-BEING
charge, charge distribution, and even functional chemistry. Great ingenuity has
made it possible to use the whole range of molecular properties for analytical
separations that can require only tiny amounts of material.
The different instrumental methods of chromatography will be discussed in
Section V-C. For this discussion, a few illustrative examples will show the
potential. In liquid chromatography, a solution of the mixture of interest passes
through a column loaded with a suitable particulate material. For example, if an
aqueous solution of pigments (such as those contained in carrot juice) is slowly
passed through a tube containing small lumps of a suitable resin, the various
pigments pass through the tube at different rates. The pigments that attach most
weakly to the resin wash through fastest, and the ones that attach most strongly
come out last. This provides a vivid example because we can actually see the
different colors of the carrot juice pigments once they are separated. Of course, the
method works to separate all sorts of compounds, whether colored or not. Under
the best conditions, liquid chromatography can separate and reveal the presence of
as little as lo-~2 grams of a substance in a mixture. For gaseous samples, the
technique can separate literally thousands of components such as are found in
flavors, insect communication chemicals (pheromones), and petroleum samples. It
is even possible to separate compounds that differ only in isotopic composition
(e.g., deuterium instead of hydrogen!) by this method.
Two-~unensional chromatography can give additional specificity, resolution, and
sensitivity by coupling with techniques such as electrophoresis, which involves the
movement of substances in the presence of a high electric field. For example,
two~unensional electrophore-
sis can sort 2,000 blood proteins
at once by separating a mixture
spot of the sample linearly under
one set of conditions, and then
using another set of conditions
to separate further the initial line
of spots at right angles. Spot
locations and amounts can be
measured quantitatively with
computenzed scanning based
album]
. . .
—' ~'~ ~CT;;T~
_a01~ps~n
tQnsf~rrin
' ] ~ ,~ ,, ,,,,, .,— ,,
>~ : *I :t :~ - ~
;~- Alga ;~=, ... - ;-
~ _
on National Aeronautics and
Space Administration computer
programs developed for satellite
pictures.
Optical Spectroscopy
The intellectual opportunities in this field, which introduce a variety of valuable
analytical techniques, can be illustrated by two notable achievements of the last
decade: the incorporation of computers as an essential part of most instrumentation,
and the detection of single atoms and molecules. "Smart" commercial instruments
now include microcomputers preprogrammed to carry out a wide variety off expen-
mental procedures and sophisticated data analyses. The more powerful computers of
:~
... .~ .
~ .~.~- i; ~ ; ; ~: Am.. ~ - ~~ -
4 ~ ~ . a ~ , ; ~ : ;~ ~ ;.—,
PROTEIN GEL PATTERN
HU MAN MYALOMA SERUM
5~Q
. . .
159
OCR for page 160
160
INTELl~CTUAL PRONTlERS lN CHEMISTRY
~.Llp,l
w7 '(~L
cC14 CFC13
700 750 800 850
CO2 . ~~r~CO2_
)~-~-'''''"1
~ ~ !r
CF2C12
950 looo lose ~ loo
~ (cm~l)
~N2O .
art_
1150 1200 1250 1300
THE INFRARED SPECTRUM SHOWS
ATMOSPHERIC POLLUTANTS EVEN AT NIGHT
the future will digest huge vol-
umes of data from spectroscopic
methods (especially Fourier
transform and two-dimensional
methods) much more efficiently.
This will further improve resolu-
tion, detection limits, interpreta-
tion, spectral file searching, and
immediate presentation of the
results with three-dimensional
color graphics to permit direct
human interaction with the ex-
periment.
Intense laser light sources are
revolutionizing analytical opti-
cal spectroscopy. An immediate
benefit is increased sensitivity.
In special cases, resonance-en-
hanced two-photon ionization
using tuned lasers has achieved
the ultimate sensitivity: detec-
tion of a single atom (cesium) or
molecule (naphthalene). Achievements in laser-induced fluorescence are approaching
this same incredible limit. Laser remote sensing, such as for atmospheric pollutants, is
effective at distances of over one mile; fluorescence excitation and pulsed laser Raman
are particularly promising. In these latter methods, a laser pulse is emitted in the
direction of the sample, which might be a smokestack plume. Then the time that it
takes the fluorescence or Raman signal to return (at the speed of light) is measured to
determine how far away the sample is. Thus, the signal not only tells us what
substances (pollutants) are in the sample but also permits us to track them as they
move away from the source.
The ability of a laser to emit a precise wavelength means there is the potentiality for
the identification of one component in a mixture (without need for separation). Yet this
selectivity is sometimes defeated because atomic and molecular absolutions can be
much broader in wavelength than the laser line width. However, the resulting overlap
can be eliminated by the wavelength narrowing that occurs at extremely cold,
cryogenic temperatures. This cooling can be achieved for gaseous molecules by
passing them through a nozzle to bring them to supersonic velocities. In an alternate
approach, molecules can be embedded in a cryogenic solid, such as solid argon, at
temperatures near that of liquid helium (a process ceded matrix isolation). These two
complementary techniques me ze interference by rotational and vibrational abso~p-
tions and improve detection sensitivity and diagnostic capability.
Mass Spectrometry
This method involves separation of gaseous charged species according to their
mass (see Section V-B), and it offers unusual analytical advantages of sensitivity,
OCR for page 161
IV-C. NATIONAL WELL-BEING
specificity, and speed (10-2-second response). All of these attributes make for an
ideal marriage to the computer. In the celebrated Viking Mars Probe, mass
spectrometry was the basis for both the upper atmosphere analysis and the search
for organic material in the planetary soil 30 million miles from home. Such sensitive
soil sniffing to detect hydrocarbons might become a fast method for of! exploration.
A special tandem-accelerator/mass spectrometer can detect three atoms of ~4C in
low atoms of TIC, which corresponds to a radiocarbon age of 70,000 years. The
broad applications of mass spectrometry include the analysis of elements, isotopes,
and molecules for the semiconductor, metallurgical, nuclear, chemical, petroleum,
and pharmaceutical industnes.
In tandem mass spectrometry, one mass spectrometer (DISC) feeds ions of a
selected mass into a collisional zone where impacts cause fragmentation into a new
set of fragment ions for analysis in a second mass spectrometer (MS-~. This
technique, abbreviated MS/MS, offers a particularly promising frontier for analysis
of mixtures of large molecules. "Soft" ionization that avoids extensive fragmen-
tation is used first to produce a mixture of molecular ions. From this mixture, one
mass at a time is selected by MS-l, and it is more vigorously fragmented to produce
an MS-~l spectrum that characterizes the structure of that one component. High
speed and molecular specificity are important features of MS/MS. It is a powerful
tool for analysis of groups of compounds sharing common structural features. It is
particularly effective in removing any background signal caused by the contaminant
species usually present in biological samples. It is now possible to determine the
sequence of peptides with up to 20 amino acids and, in some instances, with sample
sizes as small as a few micrograms.
Combined ("Hyphenated") Techniques
There is a growing appreciation for the extra benefits of using these computerized
instruments in combination, such as the mass spectrometer coupled to a chroma-
tograph (gas or liquid, GC/MS or LCtMS) or to another mass spectrometer
(MS/MS), or these coupled with the Fourier transform infrared spectrometer
(GC/IR, GC/IRtMS). High-resolution MS gives one part per trillion (1/10~2)
analyses for the many forms of dioxin (TCDD) to see if the toxic form is present in
human milk and the fatty tissue of Vietnam war veterans. GC/MS is necessary for
the specific detection of 2,3,7,8-TCDD, the most toxic dioxin isomer. GC/MS is
used routinely for detecting halocarbons in drinking water at concentrations far
below the toxic level, polychIorobiphenyIs (PCBs), viny} chloride, nitrosamines,
and for detecting most of the Environmental Protection Agency's list of other
priority pollutants. MS/MS with atmospheric pressure ionization can monitor many
of these contaminants continuously at the parts per billion level, even from a mobile
van or helicopter. The high specificity as well as sensitivity of these methods make
them especially promising for detecting nerve gases, '~yellow rain," and natural
toxins in foodstuffs (10-~ g of vomitoxin in wheat) and plants (Astragalus or "Ioco
weedy. Metabolites found by GC/MS have led to the identification of more than 50
metabolic birth defects in newborn infants where early identification is critical in
preventing severe mental retardation or death. One of the most exciting intellectual
161
OCR for page 162
162
lN~TE.r I EcTu~ FRONTIERS IN CHEMISTRY
frontiers is the possibility that routine profiling of human body fluids can detect
disease states well before external symptoms of those illnesses appear.
Electroanalytical Chemistry
Electrochemistry has a long history of analytical applications, beginning with pH
meters. Today, pulse voltammetric techniques permit detection of picomole
quantities (10~ i2 moles). Solid-state circuitry, microprocessors, miniaturization,
and improved sensitivity have made possible continuous analysis in living single
cells (with electrode areas of a few square microns). Electroanalytical methods are
also useful in such difficult environments as flowing rivers, nonaqueous chemical
process streams, molten salts, and nuclear reactor core fluids.
SEPARATIONS SCIENCES
1.8
1.6
1.2
Separations Chemistry
Separations chemistry is the application of chemical principles, properties, and
techniques to the separation of specific elements and compounds from mixtures
(including mineral ores). It takes
advantage of the differences in
such properties as solubility,
volatility, adsorbability, extract-
ability, stereochemistry, and ion
properties of elements and mol-
ecules. As an example, the rare
earth elements neodymium (Nd)
and praseodymium (Pr), impor-
tant in laser manufacture, must
be separated from a mineral
called monazite. A difficult part
of this extraction is the separa-
tion from cenum, which is
chem~caDy similar. Photochem~-
cal studies show that this sepa-
ration cart be greatly enhanced
by selective excitation to take
advantage of the different chem-
istnes of the elements under
photoexcitation .
The availability of cntical and strategic matenals to U.S. industry and the
military is dependent in many instances on the development of practical, econom-
ical chemical separations methods. Table IV-C-] shows our dependence on imports
for some critical metals and minerals. For example, almost 90 percent of our use of
platinum, in great demand as a catalyst, comes from imports. Mining of the major
platinum source in the United States, in Stillwater, Montana, has not yet begun. A
second important example concerns our access to uranium. About 13 percent of the
nation's electncal energy is denved from nuclear energy, and a much larger
OPTIMUM
~ \
, _ /
O _ ~
J
~5 1.4 _
2 _
~ _/
1.0
INdl/
/1~1
1
| UNDER SELECTIVE ~
/ RADIATIVE EXCITATION \
,,,, I,,,, 1,,,, 1,,,, 1,,
.5 2.0 2.5 3.0
HCI CONCENTRATION
AT ALL MC' CONCENTRATIONS
SELECTIVE EXCITATION FAVORS NEODYMIUM
OCR for page 163
IV'. NATIONAL WELL-BEING
percentage than that is utilized in the
industrialized Northeast. Chemical sepa-
rations are vitally important in the nu-
clear fuel cycle, beginning at the uranium
mill where low-grade uranium ores (typi-
cally only 0.1 to 0.3 percent U3Os) are
treated in selective chemical processes to
produce a concentrate of more than 80
percent U3Os. Then, further refinement,
based on transfer from one solvent to
another (solvent extraction), or formation
of the volatile fluoride, UFO, produces a
uranium product pure enough for use in
nuclear fuel manufacture. Then, after re-
TABLE IV-C-1 U.S. Import
Dependence, Selected Elements
(Imports as Percentage of Apparent
Consumption)
1950
1980
Manganese
Aluminum (bauxite)
Cobalt
Chromium
Platinum
Nickel
Zinc
Tungsten
Iron (ore)
Copper
Lead
77
71
92
100
91
99
37
80
35
59
97
94
93
91
87
73
58
~4
22
14
<10
moval from the reactor, the highly radio-
active fuel is subjected to a selective chemical process to separate uranium and
plutonium from the fission products for recycling or for weapons use. This step
is a remarkable feat of chemistry and chemical engineering because the aim is to
separate two similar elements, uranium and plutonium, from each other and also
from the highly radioactive fission products, which include about half of the
Periodic Table. All of this must be done in a remotely operated plant which, by
robotics, handles tons of materials so radioactive that they cannot be approached
by a human being.
These are only a few examples of the many ways we depend upon separations
chemistry. Future availability of many of the critical elements listed in Table
IV-C-l will depend, sooner or later, upon developing new chemical mining or
separations processes that permit us to use low-grade domestic ores and the salt
solutions (brines) that are found in geothermal wells. These developments will
require research advances across a wide front, mainly focusing on the action of
solvents and all of the properties of the liquid state that affect solvent power.
NUCLEAR CHEMISTRY
Since the days of the Curies, chemists have played a key role in the fundamental
exploration of radioactivity and nuclear properties, as well as in nuclear applica-
tions to other fields. Thus, the 1944 Nobel Prize for the discovery of nuclear fission
went to a chemist, Otto Hahn. Then, the 1951 Nobel Prize for the discovery of the
first elements beyond uranium in the Periodic Table, neptunium and plutonium,
went jointly to a chemist, Glenn Seaborg, and a physicist collaborator, Edward
McMilian. Most of the advances in our understanding of the atomic nucleus have
depended strongly on the complementary skills and approaches of physicists and
chemists. Furthermore, the applications of nuclear techniques and nuclear phe-
nomena to such diverse fields as biology, astronomy, geology, archaeology, and
medicine, as well as various areas of chemistry, have often been, and continue to
be, pioneered by people educated as nuclear chemists. Thus, the impact of nuclear
chemistry is broadly interdisciplinary.
163
OCR for page 164
164
INTEL~iCTUAL FRONTIERS IN CHEMISTRY
Studies of Nuclei and Their Properties
Particularly exciting advances have been made in extending our knowledge of
nuclear and chemical species at the upper end of the Periodic Table. In the last 15
years, elements 104 to 109 have been synthesized and identified, often by ingenious
chemical techniques geared to deal with the very short half-lives of these species
(down to milliseconds). In addition to these new-element discovenes, many new
isotopes of other elements beyond uranium have been found, and the study of their
nuclear properties has played a vital role in advancing our understanding of alpha
decay, nuclear fission, and the factors that govern nuclear stability. Fission
research in particular has been quite fruitful. For example, the "nuclear Periodic
Table" identifies particular stable pro/on-neutron combinations ("closed shells";
one of these is the tin isotope ~32Sn (50 protons, 82 neutrons). Changing this nucleus
by only one nucleon gives a dramatic change in the nuclear fission behavior, both
in the distnbution of fission products obtained and in their kinetic energies.
Furthermore, the study of spontaneously fissioning isomers among the heaviest
elements has led to the important realization that the potential energy surfaces of
these nuclei have two specially stable regions. This, in turn, opened the way to a
new approach to calculating such surfaces the so-called shell correction method.
Further exploration of the limits of nuclear stability is clearly in order, both at the
upper end of the presently known nuclei and on the neutron-nch and neutron-poor
sides of the region of stability defined by the stable nuclei found in nature. Newly
discovered nuclear reaction mechanisms, based upon accelerating heavy nuclei as
bombarding particles, promise to give access to more neutron-nch, and therefore
much longer lived (minutes to hours), isotopes of elements with Z > 100 than have
been available. This should open the way to more detailed investigations of the
chemistry of these interesting elements at the upper end of the actinide series and
beyond. The quest for so-called "superheavy'' elements, i.e., nuclear species in or
near the predicted "island of stability" around atomic number 114 and neutron
number 182, has not been successful so far, but this exciting goal is still being
pursued.
Space Exploration
The wide range of applicability of nuclear techniques is demonstrated in the
exploration of the Moon and our companion planets during the past two decades.
For example, the unmanned Surveyor missions to the Moon provided the first
chemical analyses of the Moon. They employed a newly developed analytical
technique that utilized the synthetic transuranium isotope 242Cm. The analyses
identified and determined the amounts of more than 90 percent of the atoms at three
locations on the lunar surface. These analyses, verified later by work on returned
samples, provided answers to fundamental questions about the composition and
geochemical history of the Moon. Nuclear techniques also played an important role
in the chemical analyses performed by Soviet unmanned missions to the Moon, and
in experiments designed to seek life on the surface of Mars by the U.S. Viking
missions. Similarly, isotopic distributions were important results in the analyses of
OCR for page 165
ILIAC. NATIONAL WELL-BEING
returned lunar samples and of meteorites, making possible clarification of the
history of the Moon and meteorites.
Isotopic Composition
Ever since the discovery of the isotopic composition of the chemical elements, it
has been assumed that this isotopic composition is essentially constant in all
samples, an assumption that provides the basis for assigning atomic weights. The
only exceptions involved elements with long-lived radioactive isotopes. Since 1945,
however, humans have affected the atomic weights of several elements (e.g., Li, B.
U) under some circumstances. More fundamentally, it has been discovered that the
solar system is not composed of an isotopically homogeneous mixture of chemical
elements. Even for an element as abundant as oxygen, variations of the isotopic
abundance have been noted for different parts of the solar system. Such isotopic
variations have now been established for several chemical elements and provide
clues to the processes that gave rise to the chemical elements, as well as to the
conditions that existed at the birth of the solar system.
A startlingly large isotopic variation was discovered in the uranium of ore
samples from the Oklo Mine in Gabon (West Africa) in 1972. Unusually low
isotopic abundances of uranium-235 in these ores led to the astonishing conclusion
that, 1.8 billion years before the first man-made nuclear reactor, nature had
accidentally assembled a uranium fission reactor in Africa! This reactor was made
possible by the higher 23su concentration (~3 percent instead of the present-day 0.7
percent) at the time. Mass spectrometric analyses of various elements in the Oklo
ore proved that isotopic compositions labeled them unmistakably as fission
products. It also made it possible to deduce such characteristics of the reactor as
total neutron flow (] .5 x 102' neutrons cm-2), power level (~20 kW), and duration
of the self-sustaining chain reaction (~106 years). An important practical result of
the Oklo studies is the fact that most fission products, as well as the transuranium
elements produced in the reactor, did not migrate very far in 1.8 billion years. This
has a clear relationship to the possibility of long-term confinement of radioactive
waste products in geologic formations.
Nuclear Chemistry in Medicine
Nearly 20 million nuclear medicine procedures are performed annually in the
United States (radioactive iodine thyroid treatment is one example). Advances in
nuclear medicine depend crucially on research in nuclear and radiochemistry. For
example, great progress in our knowledge of the chemistry of the element
technetium in the past decade will clearly lead to much more elective applications
of radioactive technetium, 99Tc. This is the most widely used radionuclide, because
the chemical properties of technetium compounds give them therapeutic activity.
For example, technetium tends to concentrate in bone and particularly in cancer-
ous bones, providing important diagnostic power.
Another important example is the development of especially rapid ways to
incorporate into molecular structures short-lived isotopes that emit positrons. Two
examples are the carbon isotope ~C, with a 20-minute half-life, and the fluorine
isotope, OFF, with a 110-minute half-life. Both are produced through cyclotron
165
OCR for page 166
166
INTEL r.F`CTUAL FRONTIERS IN CHEMISTRY
bombardment. These nuclei are then placed in such compounds as '~F-2-deoxy-2-
fluoro-D-glucose and 1-~C-palmitic acid in a time short enough to permit their use
in positron emission tomography (PET), which is analagous to X-ray tomography
(CAT scan). The positron technique is finding new clinical applications in studies of
the nervous system and the heart, known as neurology and cardiology.
Stable isotopes, in conjunction with NMR spectroscopy, also have important
applications in medicine. With tic, 2H, '5N, and ~70 tracers, NMR spectroscopy of
humans will allow new insights into the molecular nature of diseases, provide a
noninvasive method for their early detection, and make possible studies of
metabolic processes in living subjects. This has led to one of the most exciting
developments of the last few vears. large object imagine. In this technioue. a
~ , ~ ~ ~
computer stores the NMR signals that result when an object as large as a human is
slowly moved through the magnetic field of the NMR sample space. Then the
computer reconstructs a three-dimensional image of the object, showing the
location and local concentration of the atoms whose NMR is being measured.
Thus, the presence and chemical form of key elements can be mapped in entire
human organs in living patients. These powerful, noninvasive techniques were
literally undreamt of 15 years ago. They have arisen in response to demands for
ability to study via NMR ever larger biomolecules and working biological systems.
SUPPLEMENTARY READING
Chemical & Engineering News
"Vibrational Optical Activity Expands
Bounds of Spectroscopy" by S.C. Stinson
(C.& E.N. staff), vol. 63, pp. 21-33, Nov.
11, 1985.
"Progress Reported in Coupling LC and
MS" (C.& E.N. staff), vol. 63, pp. 38-40,
May 20, 1985.
4'New Chromatography Columns Cut Need
for Sample Preparation" by W. Worthy
(C.& E.N. staff), vol. 63, pp. 47-48, Apr.
29, 1985.
"New Methods for Trace Analysis of Man-
ganese" (C.& E.N. staff), vol. 63, pp.
56-57, Jan. 14, 1985.
"Microsensors Developed for Chemical
Analysis" (C.& E.N. staff), vol. 63, pp.
61-62, Jan. 14, 1985.
"New Laser System Far Surpasses Mass
Spec for Surface Analyses'' by W. Worthy
(C.& E.N. stair, vol. 62, pp. 20-22, Oct. 8,
1984.
"New Detectors for Microcolumn HPLC"
(C.& E.N. staff), vol. 62, pp. 39-42, Sept.
17, 1984.
"New Methods Shed Light on Surface
Chemistry" (C.& E.N. staff), vol. 61, pp.
30-32, Sept. 12, 1983.
"Archeological Chemistry" by P.S. Zurer,
vol. 61, pp. 26 44, Feb. 21, 1983.
Representative terms from entire chapter:
vibrational energy
