Research Article  Open Access
Douglas L. Strout, "Effect of C–O Bonding on the Stability and Energetics of HighEnergy NitrogenCarbon Molecules N_{10}C_{2} and N_{16}C_{2}", Advances in Chemistry, vol. 2014, Article ID 175384, 7 pages, 2014. https://doi.org/10.1155/2014/175384
Effect of C–O Bonding on the Stability and Energetics of HighEnergy NitrogenCarbon Molecules N_{10}C_{2} and N_{16}C_{2}
Abstract
Molecules consisting of nitrogen have been the subject of much attention due to their potential as highenergy materials. Complex molecules consisting entirely of nitrogen can be subject to rapid decomposition, and therefore other atoms are incorporated into the structure to enhance stability. Previous studies have explored the incorporation of carbon atoms into otherwise allnitrogen cages molecules. The current study involves two such cages, N_{10}C_{2} and N_{16}C_{2}, whose structures are derived from N_{12} and N_{18}, respectively. The N_{10}C_{2} and N_{16}C_{2} cages in this study are modified by bonding groups O_{3} and CO_{3} to determine the effect on the relative energies between the isomers and on the thermodynamic energy release properties. Energetic trends for N_{10}C_{2} and N_{16}C_{2} are calculated and discussed.
1. Introduction
Molecules consisting entirely or predominantly of nitrogen have been the subject of much research because of their potential as highenergy materials. Decomposition reactions of the type can be exothermic by up to 50 kcal/mol per nitrogen atom (approximately 3.5 kilocalories per gram of material). Experimental synthetic successes in highenergy nitrogen materials include the and ions [1–4] as well as various azido compounds [5–9] and even a network polymer of nitrogen [10]. Additionally, nitrogenrich salts [11] and the N_{7}O^{+} ion [12] have been achieved experimentally. The production of such a diverse group of nitrogen systems demonstrates the potential for such materials as novel highenergy molecules. Nitrogenbased energetic systems have also been the subject of much theoretical research. Theoretical studies of highenergy nitrogen include cyclic and acyclic compounds [13–20], as well as nitrogen cages [21–27]. Structures and thermodynamics of energetic nitrogen systems have been calculated for both small molecules and larger structures with up to seventytwo atoms.
Theoretical studies [28] of cage isomers of N_{24}, N_{30}, and N_{36} showed that the most stable isomers are narrow cylindrical structures consisting of bands of hexagons capped by trianglepentagon endcaps in either or point group symmetry. A previous study [29] of molecules of N_{22}C_{2} showed that the most stable isomer has a C_{2} parallel to the long axis of the molecule, which allows the C_{2} unit and its C=C double bond the most planar, ethylenelike environment. The least stable isomers have the C_{2} unit in proximity to the triangular endcaps, where angle strain around the C=C double bond becomes a destabilizing factor. In the current study, the smaller analogues N_{10}C_{2} and N_{16}C_{2} are considered, with the bonding groups O_{3} and CO_{3} added to the C=C double bond. The addition of O_{3} to C=C double bonds in cage molecules such as fullerenes is already well known [30], and the CO_{3} bonding group could be achieved by reaction with the metastable carbon trioxide molecule [31] or other means involving carbon dioxide and oxygen. These bonding groups are chosen to determine the effects of C–O bonding on nitrogencarbon molecules. The effects of these bonding groups on the relative isomer energies and on the heat of formation of the molecules are calculated and discussed.
2. Computational Methods
Geometries for all molecules in this study are optimized by the HartreeFock method, and HartreeFock vibrational frequencies are used to confirm each structure as a local minimum. Single energy points are calculated with coupledcluster theory [32, 33] (CCSD(T)). HartreeFock method is chosen for geometry optimization for two reasons: (1) previous calculations on nitrogen cages have shown that energy results are insensitive to the choice of optimized geometry and (2) other optimization methods, such as density functional theory, have been shown to be dissociative for certain nitrogen cages. The correlationconsistent ccpVDZ atomic orbital basis set [34] of Dunning is used for all calculations in this study. Calculations have been carried out using the Gaussian 09 computational chemistry software [35].
3. Results and Discussion
3.1. N_{10}C_{2}
The N_{10}C_{2} cage has three isomers, which are shown in Figure 1. These isomers are labeled A, B, and C and represent the three symmetryindependent substitutions of a C_{2} unit into the structure of the N_{12} cage in symmetry. The O_{3} and CO_{3} adducts of each of these isomers are shown in Figure 2. Energies and vibrational frequencies have been calculated for each molecule, and the energies are shown in Table 1. For N_{10}C_{2}, isomer C has the lowest energy, mainly because of angle strain in the triangular endcaps since isomers A and B have at least one carbon atom in the triangle. However, the application of either O_{3} or CO_{3} bonding group causes a reversal, and isomer B becomes most stable. This is because the addition of O_{3} or CO_{3} converts the sp^{2}hybridized carbon to sp^{3}, which relieves the ring strain to some extent. It is likely that the isomer A adducts also benefit from the adduct stabilization, but this cannot be confirmed since isomer A of N_{10}C_{2} is not a local minimum on the potential energy surface.

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What effect does the addition of O_{3} or CO_{3} have on the overall energetic properties of the molecules? Nitrogencarbon cages tend to be highly energetic and have high heat of formation, but molecules with C−O bonds tend to have lower heat of formation (the heat of formation [36] of carbon dioxide, e.g., is −94.0 kcal/mol). Enthalpies of formation for N_{10}C_{2} and its O_{3} and CO_{3} adducts are shown in Table 2 and have been calculated using CCSD(T)/ccpVDZ energies for the following chemical reactions: The results in Table 2 show that N_{10}C_{2} is highly energetic, with a heat of formation above 3.0 kilocalories per gram of material. Such molecules would release a large amount of energy upon their decomposition. Introduction of the O_{3} bonding group lowers the heat of formation to about 2.1 kilocalories per gram, and CO_{3} lowers the energy even further, down to about 1.5 kilocalories per gram, which is still greater than the energy densities [37] of conventional explosives such as RDX and HMX, which are approximately 0.5−1.0 kcal/g. By the choice of adduct bonding group, the energetic properties of these molecules are essentially tunable, and the tradeoff between stability and energy release may be a significant consideration in the synthesis of these molecules.

3.2. N_{16}C_{2}
The N_{16}C_{2} cage has four isomers, which are shown in Figure 3. These isomers are labeled A, B, C, and D and represent the four symmetryindependent substitutions of a C_{2} unit into the structure of the N_{18} cage in symmetry. The O_{3} and CO_{3} adducts of the four isomers are shown in Figure 4. Energies and vibrational frequencies have been calculated for each molecule, and the energies are shown in Table 3. For N_{16}C_{2}, isomer D is easily the most stable. Isomer D has a C_{2} unit parallel to the C_{3} axis of the molecule, and the arrangement of the four nitrogen atoms around the C_{2} unit in isomer D provides an environment much closer to the planar “ethylenelike” geometry preferred by sp^{2}hybridized carbon atoms. Isomers A, B, and C all have a C_{2} unit with significant angle strain or significant twisting of the four nitrogen atoms around the C_{2}. By contrast, N_{10}C_{2} does not have this very stable isomer, which explains why N_{10}C_{2} has isomer energies much closer together than does N_{16}C_{2}. N_{16}C_{2} isomers show a rapid increase in energy as the C_{2} is located closer to the triangular endcaps. In a manner similar to N_{10}C_{2}, the adduct energies in Table 3 show a substantial narrowing of the energy gaps between isomers, because of the relief of angle strain in isomers A and B. However, owing to the special stability of isomer D, the relief of ring strain does not cause a reversal of the ordering of the isomer energies.

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Enthalpies of formation for N_{16}C_{2} and its O_{3} and CO_{3} adducts are shown in Table 4 and have been calculated using CCSD(T)/ccpVDZ energies for the following chemical reactions: The first interesting result shown in Table 4 is that the N_{16}C_{2} isomers have enthalpies of formation that are no higher than the corresponding isomers of N_{10}C_{2} (about 3.0 kilocalories per gram), despite the fact that N_{16}C_{2} has a higher proportionate nitrogen content. It appears that the increased size of the molecule, and correspondingly greater distance between the triangular endcaps, provides for a more relaxed, less strained structure that stabilizes the molecule. On the other hand, the higher nitrogen content of N_{16}C_{2} does have an impact on the heat of formation of the N_{16}C_{2} adducts. Whereas N_{10}C_{2} and N_{16}C_{2} have about the same heat of formation, N_{16}C_{2}O_{3} and N_{16}C_{2}CO_{3} have much higher heat of formation than their smaller counterparts. N_{16}C_{2}O_{3} isomers have a heat of formation of 2.3–2.5 kilocalories per gram, and N_{16}C_{2}CO_{3} has heat of formation of 1.9–2.0 kilocalories per gram. As with N_{10}C_{2}, N_{16}C_{2} shows a variability of energetic properties depending on the nature of the bonding group added to the C=C double bond.

4. Conclusion
N_{10}C_{2} and N_{16}C_{2} are highenergy molecules whose properties can be varied by the addition of various bonding groups to the C=C double bond. Adducts of both N_{10}C_{2} and N_{16}C_{2} demonstrate changes in energetic properties that reflect both relief of ring strain with the structure and the stabilizing influence of the carbonoxygen bond. The stability and energyrelease properties of these molecules can be adjusted by the appropriate choice of adduct bonding group, which should provide a number of potential synthetic targets. Further studies involving multiple C_{2} units in the structure and additional choices of adduct bonding group should reveal additional molecules with potential as highenergy materials.
Conflict of Interests
The author declares that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
The Alabama Supercomputer Authority is gratefully acknowledged for a grant of computer time on the SGI Ultraviolet in Huntsville, AL. This work was supported by the National Science Foundation (NSF/HBCUUP Grant 0505872). This work was also supported by the National Institutes of Health (NIH/NCMHD 1P20MD00054701) and the Petroleum Research Fund, administered by the American Chemical Society (PRF 43798B6). The taxpayers of the state of Alabama in particular and the United States in general are gratefully acknowledged.
Supplementary Materials
HF geometries and CCSD(T) energies for all molecules in this study are available as Supporting Information.
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Copyright
Copyright © 2014 Douglas L. Strout. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.