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

Douglas Strout, Alabama State University

During the period 9/1/07-8/31/08, research continued in the area of high-energy carbon-nitrogen molecules.  Many of the molecules in the previous year’s research only contained roughly 50% nitrogen by mass, so the emphasis for the current year was on molecules that are richer in nitrogen and therefore more energetic.
One study involved cage isomers of N₁₈C₆H₆.  Two issues were explored: 1) Stability with respect to the structure of the cage, and 2) stability with respect to placement of the carbon atoms.  The two structures under consideration were a hexagon-capped structure analogous to the C₂₄ fullerene, and the other was a more prolate structure capped by nitrogen triangles.  Three significant conclusions arose from the study.  First, the triangle-capped structures are generally more stable than the hexagonal ones.  Secondly, carbon atoms directly adjacent to the triangle nitrogen were a destabilizing feature of the triangle-capped isomers.  Thirdly, the CNCNCN fully alternated hexagon was shown to be a stabilizing feature, most likely because of the interactions among the polar C-N bonds.
A second study involved cages of N₈C₄H₄ and N₁₂C₄H₄, which are nitrogen-enriched versions of the previously-studied N₆C₆H₆ and N₈C₈H₈ cages.  The major result was that the N₈C₄H₄ has high dissociation energies in all of its bonds and is therefore a stable nitrogen-enriched cage.  However, the N₁₂C₄H₄ cage was shown to have at least one low-energy path to dissociation and may not have the stability to be a practical high-energy material.
The nitrogen-enrichment issue was also explored for small open chains.  In a study of open-chain N₄C₂ (70% nitrogen by mass), all nine distinct isomers were calculated to determine thermodynamic stability.  The study determined two trends: 1) Isomers with carbon on the end of the chain tended to be less stable, and 2) isomers with the two carbon atoms adjacent to each other tended to be less stable.  An isomer with both features was shown to be the least stable of all.  The three most stable isomers were subjected to a more detailed bond-breaking study to determine kinetic stability.  The isomer with the greatest resistance to dissociation is an open chain with the two carbon atoms at the second position with respect to each end of the chain (i.e. positions 2 and 5 from one end of the chain).
Further studies of both cages and open chains will be carried out during the 2008-2009 research year.

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