Effect of C–O Bonding on the Stability and Energetics of High-Energy Nitrogen-Carbon Molecules N10C2 and N16C2

Strout, Douglas L.

doi:https://doi.org/10.1155/2014/175384

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Volume 2014 | Article ID 175384 | https://doi.org/10.1155/2014/175384

Effect of C–O Bonding on the Stability and Energetics of High-Energy Nitrogen-Carbon Molecules N₁₀C₂ and N₁₆C₂

Douglas L. Strout¹

Academic Editor: Mohamed Sarakha

Received19 May 2014

Accepted04 Dec 2014

Published25 Dec 2014

Abstract

Molecules consisting of nitrogen have been the subject of much attention due to their potential as high-energy 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 all-nitrogen cages molecules. The current study involves two such cages, N₁₀C₂ and N₁₆C₂, whose structures are derived from N₁₂ and N₁₈, respectively. The N₁₀C₂ and N₁₆C₂ cages in this study are modified by bonding groups O₃ and CO₃ to determine the effect on the relative energies between the isomers and on the thermodynamic energy release properties. Energetic trends for N₁₀C₂ and N₁₆C₂ 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 high-energy 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 high-energy 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, nitrogen-rich salts [11] and the N₇O⁺ ion [12] have been achieved experimentally. The production of such a diverse group of nitrogen systems demonstrates the potential for such materials as novel high-energy molecules. Nitrogen-based energetic systems have also been the subject of much theoretical research. Theoretical studies of high-energy 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 seventy-two atoms.

Theoretical studies [28] of cage isomers of N₂₄, N₃₀, and N₃₆ showed that the most stable isomers are narrow cylindrical structures consisting of bands of hexagons capped by triangle-pentagon endcaps in either or point group symmetry. A previous study [29] of molecules of N₂₂C₂ showed that the most stable isomer has a C₂ parallel to the long axis of the molecule, which allows the C₂ unit and its C=C double bond the most planar, ethylene-like environment. The least stable isomers have the C₂ 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 N₁₀C₂ and N₁₆C₂ are considered, with the bonding groups O₃ and CO₃ added to the C=C double bond. The addition of O₃ to C=C double bonds in cage molecules such as fullerenes is already well known [30], and the CO₃ 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 nitrogen-carbon 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 Hartree-Fock method, and Hartree-Fock vibrational frequencies are used to confirm each structure as a local minimum. Single energy points are calculated with coupled-cluster theory [32, 33] (CCSD(T)). Hartree-Fock 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 correlation-consistent cc-pVDZ 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. N₁₀C₂

The N₁₀C₂ cage has three isomers, which are shown in Figure 1. These isomers are labeled A, B, and C and represent the three symmetry-independent substitutions of a C₂ unit into the structure of the N₁₂ cage in symmetry. The O₃ and CO₃ 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 N₁₀C₂, 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 O₃ or CO₃ bonding group causes a reversal, and isomer B becomes most stable. This is because the addition of O₃ or CO₃ converts the sp²-hybridized carbon to sp³, 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 N₁₀C₂ is not a local minimum on the potential energy surface.

(a)

(b)

(c)

(a)

(b)

(c)

(d)

(e)

(f)

What effect does the addition of O₃ or CO₃ have on the overall energetic properties of the molecules? Nitrogen-carbon 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 N₁₀C₂ and its O₃ and CO₃ adducts are shown in Table 2 and have been calculated using CCSD(T)/cc-pVDZ energies for the following chemical reactions: The results in Table 2 show that N₁₀C₂ 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 O₃ bonding group lowers the heat of formation to about 2.1 kilocalories per gram, and CO₃ 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. N₁₆C₂

The N₁₆C₂ cage has four isomers, which are shown in Figure 3. These isomers are labeled A, B, C, and D and represent the four symmetry-independent substitutions of a C₂ unit into the structure of the N₁₈ cage in symmetry. The O₃ and CO₃ 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 N₁₆C₂, isomer D is easily the most stable. Isomer D has a C₂ unit parallel to the C₃ axis of the molecule, and the arrangement of the four nitrogen atoms around the C₂ unit in isomer D provides an environment much closer to the planar “ethylene-like” geometry preferred by sp²-hybridized carbon atoms. Isomers A, B, and C all have a C₂ unit with significant angle strain or significant twisting of the four nitrogen atoms around the C₂. By contrast, N₁₀C₂ does not have this very stable isomer, which explains why N₁₀C₂ has isomer energies much closer together than does N₁₆C₂. N₁₆C₂ isomers show a rapid increase in energy as the C₂ is located closer to the triangular endcaps. In a manner similar to N₁₀C₂, 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.

(a)

(b)

(c)

(d)

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Enthalpies of formation for N₁₆C₂ and its O₃ and CO₃ adducts are shown in Table 4 and have been calculated using CCSD(T)/cc-pVDZ energies for the following chemical reactions: The first interesting result shown in Table 4 is that the N₁₆C₂ isomers have enthalpies of formation that are no higher than the corresponding isomers of N₁₀C₂ (about 3.0 kilocalories per gram), despite the fact that N₁₆C₂ 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 N₁₆C₂ does have an impact on the heat of formation of the N₁₆C₂ adducts. Whereas N₁₀C₂ and N₁₆C₂ have about the same heat of formation, N₁₆C₂O₃ and N₁₆C₂CO₃ have much higher heat of formation than their smaller counterparts. N₁₆C₂O₃ isomers have a heat of formation of 2.3–2.5 kilocalories per gram, and N₁₆C₂CO₃ has heat of formation of 1.9–2.0 kilocalories per gram. As with N₁₀C₂, N₁₆C₂ shows a variability of energetic properties depending on the nature of the bonding group added to the C=C double bond.

4. Conclusion

N₁₀C₂ and N₁₆C₂ are high-energy molecules whose properties can be varied by the addition of various bonding groups to the C=C double bond. Adducts of both N₁₀C₂ and N₁₆C₂ demonstrate changes in energetic properties that reflect both relief of ring strain with the structure and the stabilizing influence of the carbon-oxygen bond. The stability and energy-release 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 C₂ units in the structure and additional choices of adduct bonding group should reveal additional molecules with potential as high-energy 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/HBCU-UP Grant 0505872). This work was also supported by the National Institutes of Health (NIH/NCMHD 1P20MD000547-01) and the Petroleum Research Fund, administered by the American Chemical Society (PRF 43798-B6). 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.

Supplementary Material

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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.

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Advances in Chemistry

Effect of C–O Bonding on the Stability and Energetics of High-Energy Nitrogen-Carbon Molecules N10C2 and N16C2

Abstract

1. Introduction

2. Computational Methods

3. Results and Discussion

3.1. N10C2

3.2. N16C2

4. Conclusion

Conflict of Interests

Acknowledgments

Supplementary Materials

References

Copyright

Effect of C–O Bonding on the Stability and Energetics of High-Energy Nitrogen-Carbon Molecules N₁₀C₂ and N₁₆C₂

3.1. N₁₀C₂

3.2. N₁₆C₂