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43798-B6
Role of Heteroatoms in Stabilizing High-Energy Nitrogen Molecules

Douglas Strout, Alabama State University

Carbon atoms have been investigated as stabilizing heteroatoms in nitrogen cages. The primary objects of studied were N6C6H6 and N8C8H8, which are carbon-substituted forms of previously-studied N12 and N16 cages. Hydrogen atoms were incorporated to satisfy the fourth valence of the carbon. Various isomers of these carbon-nitrogen cages were examined by theoretical calculations to determine their thermodynamic and kinetic stability.

Isomers have been compared one to another to determine the most stable isomeric forms. The chief design principle of stable cages is the maximization of the number of carbon-nitrogen bonds. Since the cages studied incorporate pentagons, full alternation of carbon and nitrogen atoms is impossible. Rearranging the structure to all full alternation actually destabilizes, so heteronuclear bonds are not the only stability consideration. Ring strain in the cage polygons also plays a significant role.

Kinetic stability was tested through the calculation of bond-breaking energies in the cage structures. The nitrogen-nitrogen bonds were shown to be the weakest, which is not surprising because of the low bond enthalpy of N-N bonds. However, the nitrogen-nitrogen bond strengths were also shown to be strongly influenced by environmental factors. A nitrogen-nitrogen bond in a cage will be much stronger if the N-N bond is surrounded by carbon atoms.

Several stable high-energy isomers of N6C6H6 and N8C8H8 were identified through these studies, but these molecules are only a little over 50% nitrogen by mass. This reduction in nitrogen causes a corresponding reduction in the energy-release properties of the molecules. Further studies have identified other molecules with comparable stability with 60-90% nitrogen content. The ideal high-energy nitrogen molecule has both good stability and the highest possible nitrogen content.

Undergraduate research has been an integral part of the research program. During the program period 1/1/06-8/31/07, nine undergraduate scholars have contributed to the research program, including four whose summer research was funded directly by the PRF grant.

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