WHY do atoms have a propensity
to bond in certain configurations? That mystery continues to puzzle
the scientists who study molecules and their isomers—molecules
that have the same atomic weight as the original but a different
structure. A better understanding of the rules governing molecular
structure would help them predict which forms of a substance would
be most useful.
In 1985, scientists were excited by the discovery of a new form
of carbon. This molecule, called a buckyball or a fullerene, contains
60 carbon atoms (C60). Its molecular structure resembles a soccer
ball or the geodesic dome designed by architect Buckminster Fuller,
for whom the molecule is named.
on that discovery, Livermore scientists are developing computer
models to study buckyballs that feature other atoms, such
as nitrogen and boron, in place of some of the carbon atoms in
C60. (See S&TR, June 2001, This
Nitrogen Molecule Really Packs Heat.) In particular, the research has focused
on how to predict the most
of these new compounds and on how other atoms bond to one another
to create unique structures.
One team, led by theoretical chemist Riad Manaa, is studying nitrogen
fullerenes, especially C48N12. Nitrogen-doped fullerenes offer
an impressive range of potential applications, from orthopedic
implants to new pharmaceuticals to high explosives.
“These fullerenes are interesting to study,” says Manaa,
who works for the Chemical Engineering Division in Livermore’s
Chemistry and Materials Science Directorate. “Their hollow,
cagelike shape and their extreme stability at high temperature
and pressure allow them to retain their spherical structure when
they interact with other atoms and molecules. By understanding
how the carbon and nitrogen bonds come together, we can study the
properties and predict how other atoms will interact with the fullerene
to form new compounds.”
nitrogen fullerene has properties that differ from the more commonly
known carbon fullerene. All electron shells in the C60 molecule
are filled, so C60 is inert. However, when some of the
carbon atoms are replaced with nitrogen atoms, the new molecule
acts as an electron donor. Nitrogen also carries much more energy
than carbon, so nitrogen fullerenes might be useful in developing
new high-explosive formulations. Computer simulations indicate
that other elements could be added to the molecule to form compounds
for a range of applications.
The molecular structure of C48N12 with its electronic cloud.
Computing the Possibilities
The team’s search to find the most stable forms of nitrogen
fullerenes began as a teaching project between Manaa and a group
of summer interns. Manaa, who investigates energetic materials
and conducts simulations of these materials in extreme conditions,
worked with the interns to study various forms of the C48N12 molecule,
which has a high energy content.
When C48N12 was first synthesized several years ago, the electron
microscopy and energy-loss spectroscopic analysis showed that its
structure corresponds to that of a buckyball. At that time, researchers
believed the most stable form of this nitrogen-substituted fullerene
had 12 pentagons with evenly spaced nitrogen atoms, one in each
pentagon. Nitrogen atoms tend to repel each other and destabilize
the structure, so if the molecule is stable, they must be separated.
In the 12-pentagon model, every nitrogen atom is separated by two
carbon atoms. The remainder of each pentagon is composed of carbon
atoms. That molecule also has two all-carbon hexagons, or benzenelike
rings. Benzene rings are very stable, so having two benzene rings
and at least two carbon atoms between each nitrogen atom provided
the molecule’s stability.
Predicting the most energetically stable structure of a molecule
is a formidable task for computational scientists, especially when
they must determine the various configurations for as many as 60
atoms. It’s time-consuming work to examine the many possibilities
of distributing the 12 nitrogen atoms in C48N12 among
the 20 hexagons and 12 pentagons of a buckyball structure. Even
supercomputers, such as ASCI Blue, must process calculations day
and night for weeks to model all the configurations.
According to Manaa, the team’s original goal was to find
stable molecular structures with subunits of nitrogen–nitrogen
(N–N) bonding, which have high energy content. The team used
quantum-chemical methods to predict the stable structures of these
fullerenes, which have a radius of 0.35 nanometer. The computer
code calculates the distribution of electrons around each atom,
which then determines the chemical property of a molecule.
we were studying the high-energy, fullerene-analog structures of
C48N12 with 6N2, 4N3, and 2N6 subunits,” says Manaa, “we
also found the energetically most stable structure of this molecule.
This finding allows us to predict the chemical and physical properties
of the material.”
new molecule has eight highly stable all-carbon hexagons. Although
the nitrogen atoms are separated by only one carbon atom, it has
six additional benzenelike rings, which more than make up for any
possible repulsion between the nitrogen atoms. Thus, the new C48N12 structure is more stable because the molecule’s resonance
energy is maximized and the repulsive force from the N–N
bonds is minimized. In fact, the team’s calculations showed
that this structure is much more stable (as much as 13.1 kilocalories
per mole) than the most stable structure reported for the first
The geometry of the minimum energy
(stable) structure of C48N12 (nitrogen
in blue), indicating the distribution of nitrogen atoms.
Front view of the symmetric C48N12 structure, showing the position of the nitrogen atoms.
Seven all-carbon hexagons are visible, and the eighth is
superimposed on the central ring.
Designing New Molecules
the C60 molecule—that is, substituting some of the
molecule’s carbon atoms with other atoms—changes the
structural, electronic, chemical, and physical properties of the
parent fullerene. For example, when some of the carbon atoms on
the buckyball cage are replaced with nitrogen, the molecule’s
electronic properties change to match those of a semiconductor.
Other doped fullerenes are ideal candidates for phototonic devices,
such as optical switches, eye protectors, and sensors, and some
are being considered as therapeutic agents.
group also examined a fullerene molecule doped with boron. Their
results showed that C48B12 has the same stability
as C48N12. This same molecular structure
as C48N12 decisively confirmed the overall
stability of the C48X12 molecules
(where X can be boron, nitrogen, or silicon). Recent calculations
show that, even though
C48N12 and C48B12 have
similar structures, they have opposite properties. While C48N12 acts
as an electron donor, the charge distribution
in C48B12 makes it an electron acceptor.
Thus, when these molecules are combined, C48N12 and
a donor–acceptor pair
for molecular electronic building blocks. Potential applications
combining the two molecules include circuits for electronic switches
and nanocircuits for data exchange.
efforts are now directed toward building carbon structures with
other combinations of nitrogen and boron, such
as C48B6N6. Materials made from
this combination would be harder than C60.
Most of the fullerene research to date has been conducted as part
of the hardware tests and code development work for the National
Nuclear Security Administration’s (NNSA’s) Advanced
Simulation and Computing Program, which is an integral component
of the NNSA’s Stockpile Stewardship Program. However, according
to Manaa, the team is also interested in collaborating with groups
outside the Laboratory to build on Livermore’s expertise
in the computational research of fullerenes.
“So much of the progress in synthesizing new forms of nitrogen fullerenes
and other molecules requires a thorough understanding of their
structure and properties,” he says. “Understanding
why molecules take the form they do adds to the predictive possibilities
scientists can make about new molecules for all kinds of applications.”
Key Words: Advanced Simulation and Computing Program,
ASCI Blue, buckyball, carbon, fullerenes, nitrogen.
For further information contact Riad Manaa (925) 423-8668
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