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01/25/2000
Buckyballs and Charcoal Flowers
Carbon is certainly a most important element as far as
humankind is concerned. Organic chemistry is the study of
carbon compounds and we humans depend on some of the most
cleverly arranged organic chemicals, DNA being a prime
example. As far as plain elemental carbon is concerned, until
about 15 years ago there were just a couple of major forms of
crystalline carbon that were familiar to everyone. One was
diamond, the hardest material known. Then there was graphite,
the so-called "lead" in your pencil. We''ve discussed earlier the
fact that the remarkable differences in the properties of these two
materials are due to their crystal structures. In diamond, each
carbon atom is bonded very strongly to four other carbon atoms.
In graphite, the carbon atoms are arranged in layers of hexagonal
units with carbon atoms at the 6 corners of the hexagons. The
bonds between these layers are very weak, which allows the
layers of graphite to slide easily past each other from you pencil
onto your sheet of paper.
The world of carbon changed radically back in 1985. There was
this fellow Sir Harold Kroto in England who was involved in the
field of microwave spectroscopy. I won''t go into the details of
this field but suffice to say that Kroto used the technique to study
gases in giant carbon-rich stars. He found that these stars
contained molecules of compounds of carbon and nitrogen,
compounds that were also present in interstellar gas clouds.
Kroto wanted to study the formation of such compounds and
heard from an acquaintance at Rice University, Robert Curl, that
another Rice professor, Richard Smalley, had built a special
"laser-supersonic cluster beam" apparatus. Well, Kroto must
have been quite taken with this apparatus, quite aside from its
impressive name. With it, Prof. Smalley could vaporize just
about anything. He was interested in cluster chemistry,
particularly in tiny clusters of atoms of metallic elements that
form when you vaporize the metal and then condense the vapor
back into a solid deposit. Of special interest to him were so-
called "magic numbers". Typically, when the sizes of the metal
clusters in a sample were measured, the clusters would vary in
size from large to small. However, in a given sample there
would typically be many more clusters of a certain size. The
"magic number" is the number of atoms in these most numerous
clusters. It was felt that these magic number clusters were
special, with the atoms forming some kind of favored
symmetrical structure.
Well, you can tell Kroto was really enthused when he made the
trip all the way from England to Texas to work with the Rice
crew. These three guys and a couple graduate students got
together to try out Smalley''s fancy instrument on a nonmetal,
carbon. And as they say, the rest is history. They found that
carbon also had magic numbers at 60, as well as 70 carbon atoms
per cluster. In itself, this may not seem so surprising since
metals showed the same effect. What was very surprising and
innovative was the idea they proposed to account for the magic
numbers in the carbon experiments. They suggested that the 60
carbon atoms were arranged in a structure that matches precisely
the pattern of a European football (soccer ball) or, equally, one of
those geodesic domed structures designed by the famed architect
Buckminster Fuller. In fact, they decided to name their new
form of carbon buckminsterfullerene, shortened in the vernacular
to "buckyball" or the more erudite term "fullerene". As with
many innovative scientific discoveries, their suggestion was
greeted with mixed enthusiasm and some outright skepticism by
the scientific community. You might deduce that the skeptics
were wrong if I tell you these three gentlemen shared the 1996
Nobel Prize in Chemistry for their discovery of fullerenes.
Actually, these three were unable to separate out the magic
portion of the rest of their carbon deposit to prove their point. It
wasn''t until five years later that other workers managed to obtain
the pure fullerenes. Now they could run X-ray measurements
and the buckyball structure was confirmed. Surprisingly, to me at
least, these fullerenes turned out to be soluble in organic
solvents. It was this key finding that permitted the buckyball
material to be dissolved and separated from the rest of the sooty
deposits. The soot was formed when the carbon vaporized and
condensed in an inert atmosphere such as helium gas. Indeed,
plain old soot apparently harbors very slight quantities of this
"new" form of carbon, which has actually been around forever,
so to speak. Nobody ever thought to look for it!
The discovery of this new material set off a flurry of activity and
it wasn''t long before other exotic structures even more
complicated than the 60- or 70-atom buckyballs were found. A
whole new field of chemistry was born. For example, metal
atoms could be inserted inside the buckyball cage. The medical
community is interested in the possibility of using such
compounds to deliver radioactive or other elements to targeted
areas of the body, either for treatment or diagnostic purposes.
One example of a possible application in the diagnostic area is in
the field of magnetic resonance imaging (MRI). For improved
contrast and clarity of MRI images, a contrast agent may be used.
One metallic element, gadolinium, is currently used in the form
of certain organic compounds. There appear to be potential
advantages related to enhanced contrast and safety if a "naked"
gadolinium ion inside a fullerene cage can be used.
These days, we all hear about antioxidants and their importance
in our diet. The 60-atom fullerene (call it C60) is a molecule
loaded with what chemists call "double bonds". Crudely, these
double bonds would much rather grab onto something and
become ordinary single bonds. Those nasty hydrogenated fats in
our processed foods are formed when the double bonds in
unsaturated fats are allowed to pick up hydrogen and convert the
bonds to single bonds. Well, with all its double bonds, our C60
is just itching to pick up lots of those pesky free radicals that
promote cancer and other maladies. One study has been done on
mice carrying a defective gene encoding for a compound found
in familial Lou Gehrig''s disease (ALS). Treating the mice with a
fullerene derivative postponed symptoms of ALS for 10 days and
prolonged the lives of the mice by 8 days. A hopeful start.
Another area of possible application is the field of catalysis, now
actively being investigated. Unfortunately, one of the problems
in studying these fullerenes is the fact that there just isn''t all that
much of the stuff and it''s very expensive. The price of a gram of
the empty C60 buckyball material is about $30, almost three
times the price of gold. The same stuff containing gadolinium
inside the C60 cage might cost $1000 for a milligram! Today,
there is a concerted effort to come up with processes for forming
and recovering the fullerenes economically in large quantities.
You might be saying, "What about the form of carbon that I use
for my outdoor barbecue?" There is, of course, charcoal.
However, we''ve been talking about crystalline carbon and
charcoal is not crystalline, but amorphous and not all that pure.
An amorphous substance is one that, at least as far as X-rays are
concerned, does not show any crystal structure. The reason for
bringing it up now is that here in New Jersey, Sayreville to be
exact, there once was a tropical forest with lots of flowers in it
(New York Times Science Times section, December 21, 1999).
However, a big fire burned through the forest, turning these
flowers into charcoal. These weren''t very big flowers, only
about an eighth of an inch or so in diameter. The interesting
thing is that this fire happened 90 million years ago and the
charcoal fossils have only recently been discovered and dated.
The charcoal has preserved the shapes of the flowers amazingly
well. I don''t know how the Cornell University biologists who
did the study happened to be poking around in this Sayreville
vacant lot. The finding purportedly pushes back the beginning of
flowering plants some 30 million years earlier than had been
thought previously. This study also raises the question as to
when the insects and the pollination process developed (another
chicken and the egg problem?).
Finally, carbon has also been in the astronomical news recently.
There seems to be evidence that, on one of the planets (I believe
it''s Neptune), it isn''t raining rain you know, it''s raining.........
diamonds! You have to admit that carbon is one strange and
wonderful element! And I haven''t even touched on nanotubes, a
subject for another column.
Allen F. Bortrum
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