is a critical
ingredient in most explosivesthink of TNT (trinitrotoluene),
the ammonium nitrate used in the Oklahoma City bombing, and the
Department of Energys most sophisticated high explosive in
nuclear weapons, TATB (1,3,5-triamino-2,4,6-trinitrobenzene).
Huge amounts of energy are
released when the tight bonds of a typical metastable (readily changed)
energetic molecule are broken and the molecule reforms into smaller
ones. A molecule composed solely of nitrogen atoms will release
even more. For example, the tiny nitrogen anion N3
is a propellant used in automobile air bags. Because of the nature
of nitrogen bonding, the explosive power of a molecule 20 times
larger than N3
would be stunning.
Theoretical chemist Riad
Manaa of Livermore may have found this unusual nitrogen molecule.
His computer simulations show that it might be possible to join
six 10-atom nitrogen molecules into a soccer-ball-shaped molecule
known as a buckminsterfullerenefullerene, for short.
the only fullerenes are large carbon molecules, with from 32 to
as many as 600 atoms. A nitrogen fullerene would truly be an oddity,
because the only forms of nitrogen known outside the laboratory
are N2, the most abundant element
in our atmosphere, and the highly explosive N3.
In the laboratory, other forms of N3
as well as N4+
have been created successfully. However, of these, all but N5+
were short lived.
The first fullerene, a 60-atom
carbon molecule, was created in the laboratory in 1985, winning
its discoverers a Nobel Prize in 1996. Says Manaa, With their
remarkable, perfect symmetry, fullerenes continue to create enormous
excitement among scientists. Fullerenes are named for R. Buckminster
Fuller, whose popular geodesic dome is structurally similar to a
fullerene molecule. Also known as buckyballs, the closed, hollow
carbon fullerenes have been produced in bulk quantities in the laboratory
and show promise for use as superconductors and molecular containers.
Their cage shape also makes them excellent building blocks for carbon-based
With their high energy density,
large nitrogen molecules would be prime candidates for new high
explosives or perhaps for a novel propellant. Supersonic transport
vehicles, for example, must achieve extremely high speeds. A new
propellant that incorporated nitrogen fullerenes could generate
the high thrust (energy release) needed to attain those speeds.
combination of six molecular units of N10
form the nitrogen fullerene, N60.
Search Is On
the U.S. Air Force and the Department of Defenses Defense
Advanced Research Projects Administration (DARPA) have funded research
at Livermore and elsewhere to find a way to destabilize the strong
N2 triple bond, the second strongest covalent
bond in all of nature. The goal is to find polymeric forms of nitrogen
with single and double bonds, which are significantly weaker. Experiments
with a diamond-anvil cell that pressurized N2
up to almost 200 gigapascals failed to find a polymeric form of
nitrogen. Shock compression experiments at high temperatures and
pressures were equally unsuccessful. Only by direct synthesis in
the laboratory have scientists been able to create any new polynitrogen
Thus far, the N3,
molecules created in the laboratory had a linear structure and were
unstable because of their weak bonding. Only the N5+
molecule demonstrated long-term stability. Extensive research continues,
however, on such exotic species as a tetrahedral N4
and a cubic N8. To date, quantum-chemistry-based
computational studies predict that they will be at least metastable.
Because nitrogen atoms so
clearly like to be triple bonded, no one had previously examined
the possibility of creating a nitrogen fullerene. Manaa was thus
the first to suggest that a super-high-energy molecule N60
was a possibility. He has shown that N60 could
be formed from six units of bicyclic N10
molecules, which are themselves formed from two units of N5.
Using several quantum-chemical methods, he determined the structure
and spectroscopy of the N10 molecule.
Simulations based on first-principles
quantum chemistry accurately predict the chemical properties of
atoms and molecules. The technique uses quantum mechanics to determine
the distribution of electrons around each atom. From this electron
distribution, any chemical property can be determined, including
the structures and energies of the molecules.
Such simulations showed that
N10, or dipentazole, would contain a mixture
of single and double bonds and would be metastable. Each of its
two pentazole ions (N5+
would be flat, long lived, and connected perpendicularly to one
another. The bridging bond between the ions appears to be strong
and yet flexible enough to allow movement between them.
Bringing six such molecules
together into a 60-atom buckyball would be tricky. Says Manaa, It
would likely have to be prepared under extreme conditions, such
as high pressure.
The resulting molecule would
be purely single bonded. Breaking those bondssplitting N60
into 30 triple-bonded N2 moleculeswould
release 50 percent more energy than can be released by CL-20, the
best performing explosive currently known.
Manaa now has several studies
under way to examine the possibility of creating N60
and the stability of the resulting molecule. He has also begun to
look at a possible boron fullerene.
This work is part of research
on the properties of energetic materials for the Department of Energys
Stockpile Stewardship Program, which uses the supercomputing capabilities
of the Accelerated Strategic Computing Initiative (ASCI). Manaa
notes that the use of extensive and rigorous computational tools
coupled with the relatively large size of these molecules renders
the use of massively parallel platformssuch as ASCI Blueof
two pentazole ions (N5+
that constitute dipentazole (N10)
are flat and connected perpendicularly to one another.
a Long and Winding Road Ahead
Manaas simulated nitrogen
fullerene and other polynitrogen molecules currently under study
are still a long way from practical use. To create a propellant
for supersonic transport vehicles, the material being considered
must first have a high energy density. It must also be reactive
and must release large amounts of energy while increasing the number
of particlesfor example, one N4 molecule
reacting and releasing two N2 molecules.
The reaction must be controllable, and the material must be easily
Polynitrogens are certainly
high-energy-density materials and highly reactive. Of the new ones
under study, only N5 shows stability.
N60 still exists only in ASCI simulations.
So Manaa is quick to note that these new forms of nitrogen are still
Accelerated Strategic Computing Initiative (ASCI), fullerenes, high-energy-density
information contact Riad Manaa (925) 423-8668 (firstname.lastname@example.org).