Searching for Low-Sensitivity Cast-Melt High-Energy-Density

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Searching for Low-Sensitivity Cast-Melt High-Energy-Density Materials: Synthesis, Characterization, and Decomposition Kinetics of 3,4-Bis(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-oxadiazole-2-oxide Roman Tsyshevsky,† Philip Pagoria,‡ Maoxi Zhang,‡ Ana Racoveanu,‡ Alan DeHope,‡ Damon Parrish,§ and Maija M. Kuklja*,† †

Materials Science and Engineering Department, University of Maryland College Park, College Park, Maryland 20742-2115, United States ‡ Energetic Materials Center, Lawrence Livermore National Laboratory, 7000 East Avenue, L-282, Livermore, California 94550, United States § Naval Research Laboratory, 4555 Overlook Aveue, Code 6030, Washington, D.C. 20375, United States S Supporting Information *

ABSTRACT: The most comprehensive approach to analyze and characterize energetic materials is suggested and applied to enable rational, rigorous design of novel materials and targeted improvements of existing materials to achieve desired properties. We report synthesis, characterization of the structure and sensitivity, and modeling of thermal and electronic stability of the energetic, heterocyclic compound, 3,4-bis(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-oxadiazole-2-oxide (BNFF). The proposed novel, relatively simple synthesis of BNFF in excellent yields allows for an efficient scale up. Performing careful characterization indicates that these materials offer an unusual combination of properties and exhibit a relatively high energy density, high and controllable stability against decomposition, low melting temperature, and low sensitivity to initiation of detonation. First-principles calculations of activation barriers and reaction rate constants reveal the decomposition scenarios that govern the thermal stability and chemical behavior of BNFF, which appreciably differ from conventional nitro compounds. Details of the electronic structure and calculated electronic properties suggest that BNFF is an excellent candidate energetic material on its own and an attractive ingredient of modern energetic formulations to improve their stability and enable highly controllable chemical decomposition. There are recent successes in making new explosives2−5 that outperform other commonly used high-energy-density materials and satisfy many of the desired criteria. However, our understanding of microscale behavior of energetic materials remains unachieved, and details of the structure−property− function interplay have yet to be established for both existing and newly emerging candidates.1 In the search for lowsensitivity explosives some authors go to such an extreme as to fabricate a nanocomposite diamond coating to reduce reactivity of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX).6 Nitro-substituted heterocyclic compounds have been of interest as energetic materials for many years. In general, heterocycles have the advantage of higher density and higher heat of formation over their carbon-substituted analogues, important parameters for maximizing the explosive power of crystalline explosives. Since it was reported in 2002,7,8 3,4bis(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-oxadiazole-2-oxide (BNFF, Figure 1a−1d) caused a lot of excitement in the

I. INTRODUCTION The continuous search for safe (insensitive, highly tolerant to various external perturbations) and powerful (releasing a lot of energy on demand) energetic materials is an exciting and challenging area of physical chemistry.1 Both the challenge and excitement are related to and defined by the lack of general microscale theory of initiation of chemistry and of a fundamental understanding of the earliest stages of materials decomposition, which eventually lead to combustion or explosion. Many contradictory requirements to those materials further complicate the situation. Among specified requirements, for example, are high density, high performance, high thermal stability, low sensitivity, low ecological toxicity and benign environmental behavior, low cost and relative simplicity of synthesis, safety in handling and transportation, and predictable response to external stimuli (such as heat, friction, mechanical impact, spark, irradiation, or electrostatic discharge), etc. In many cases, high performance and low sensitivity to mechanical and thermal stimuli appear to be mutually exclusive. Materials with sufficiently large energy content are often too sensitive to find a practical use, and many energetic materials with adequate stability do not meet the performance requirements. © 2015 American Chemical Society

Received: November 25, 2014 Revised: January 23, 2015 Published: January 23, 2015 3509

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Figure 1. (a) Schematic representation of BNFF, (b) molecular structure of BNFF, (c) structure of a BNFF crystal, and (d) slab fragment of the (001) surface of BNFF used in model calculations.

Figure 2. Synthesis of BNFF

sives,9−13 oxidizers for rocket propellant formulations,11 possibly melt-castable explosive/detonator materials,14 and a multifunctional building block for energetic composites.15 Despite certain enthusiasm among researchers, BNFF and its derivatives are only touched upon with a few fragmentary studies, and their declared promising properties are neither confirmed nor understood. The complexity of the structure and

community as an attractive heterocyclic energetic compound with good thermal stability, low melting point (110 °C), high density (1.937 g/cm3), and moderate sensitivity to various insults. This compound along with the structurally similar 3,4bis(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-oxadiazole (LLM-172, BNFF-1), both multicyclic heterocycles, were announced as attractive candidate materials for use as secondary explo3510

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structure was solved and refined with the aid of the programs in the SHELXTL-plus [v2008/4] system of programs.20 The full-matrix least-squares refinement on F2 included atomic coordinates and anisotropic thermal parameters for all nonhydrogen atoms. The hydrogen atoms were included by using a riding model. Commercial reagents were employed in the syntheses, and compounds were identified by a combination of NMR, IR, mass spectroscopy, and single-crystal X-ray diffraction. NMR spectra were recorded either on a Varian 90 MHz NMR spectrometer or on a Bruker RDX-600 NMR instrument. Deuterated solvent was used as a reference of chemical shift. Infrared spectra were determined in KBr pellets on a Nicolet IR 100 spectrometer. Trifluoroacetic acid was purchased from Spectrum Chemical Co. and used as purchased. The melting point was determined on a Mel-Temp apparatus and is reported uncorrected. An Explosives Research Laboratory type 12 drop weight apparatus (Drop Hammer) was used to determine the impact sensitivity relative to the primary calibration materials PETN, RDX, and Comp B-3. The apparatus was equipped with a Type 12A tool and a 2.5 kg weight. The 35 mg ± 2 mg powder sample was impacted on a carborundum “fine” (120-grit) flint paper. A sample population of 15 is generally used. The mean height for “go” events, called the 50% impact height or Dh50, was determined using the Bruceton up−down method. Friction sensitivity is evaluated using a B.A.M. high-friction sensitivity tester. The tester employs a fixed porcelain pin and a moveable porcelain plate that executes a reciprocating motion. Weight affixed to a torsion arm allows for a variation in applied force between 0.5 and 36 kg. The infrared spectra were obtained using a Nicolet 730 FTIR spectrophotometer. Elemental analysis was performed by Midwest Microlab LLC, Indianapolis, Indiana. 2.3. Computational Details. Decomposition reactions in the gas phase were studied within density functional theory (DFT)21,22 in the generalized gradient approximation (GGA) with the Perdew−Burke−Ernzerhof (PBE),23 and M0624 functionals as implemented in the GAUSSIAN code.25 All molecular calculations were performed by using the double-ζ cc-pVDZ basis set.26 The periodic surface calculations were performed in the GGA approximation with the PBE exchange correlation functional23 and projector-augmented wave (PAW) pseudopotentials27 as implemented in the plane-wave code VASP.28−30 In simulating an ideal BNFF crystal (Figure 1c), we used the 3 × 2 × 2 Monkhorst−Pack k-point mesh, and the kinetic energy cutoff was set to 800 eV. Atomic coordinates and lattice constants were allowed to simultaneously relax without any symmetry constraints. The convergence criterion for electronic steps was set to 10−4 eV, and the maximum force acting on any atom was set not to exceed 0.02 eV/Å. Surface reactions were simulated in a slab model, in which the supercell consisted of the three-molecule-thick crystalline layer, which was cut out to expose the (001) surface (Figure 1d). A large size of the supercell and a 10 Å thick vacuum layer, placed on top of the slab, ensured that spurious interactions between reactants and reaction products in neighbor cells are minimal. Surface supercell calculations were performed at the gamma point only, with kinetic energy cutoff set to 600 eV. The convergence criteria for electronic and ionic steps were set to 10−4 eV and 0.03 Å/eV, respectively.

difficulty in synthesis of obtaining the material evidently contribute to this state of matter. In this article, aimed at analyzing the structure and evaluating bond strengths of those new molecular materials, we present the synthesis procedure combined with the structural characterization, sensitivity tests, and modeling of decomposition channels of BNFF. We propose a novel, relatively straightforward, and simple method of synthesis of BNFF in excellent yields that allows for an efficient scale up. First-principles calculations of activation barriers and reaction rate constants reveal main decomposition channels of the gas-phase molecules and crystals. Performed careful characterization indicates that these materials offer an unusual combination of properties and exhibit a relatively high energy density, high and controllable stability against decomposition, low melting temperature, and low sensitivity to initiation of detonation. We discuss how the thermal stability and decomposition scenarios that govern the chemical behavior of BNFF differ from traditional nitro compounds. We compare density, friction, drop hammer tests, and melting point temperature of BNFF with other well-known energetic materials. We propose to expand this conventional set of sensitivity parameters by augmenting it with activation barriers and kinetics of chemical decomposition reactions, as a measure of thermal stability, and with optical energy gaps, electronic affinity, and ionization potentials, as a measure of electronic stability. Such an expanded set provides a more holistic description of stability of energetic materials than traditionally accepted and serves as a better characterization of sensitivity to detonation initiation by various external perturbations, including mechanical, thermal, optical, or electronic.

II. METHODS 2.1. Synthesis. The synthesis of BNFF is a four-step process from commercially available malononitrile in 25% overall yield (Figure 2). The synthesis involves initially a onepot synthesis of 4-amino-1,2,5-oxadiazole-3-carboxamidoxime (AFCAO) by allowing malononitrile to react with aqueous nitrous acid to give the intermediate, 2-hydroximinomalononitrile. The reaction mixture is neutralized to pH = 10, and the mixture is treated with 50% aqueous hydroxylamine and heated to reflux to give AFCAO in 84% yield. Treatment of AFCAO with sodium nitrite in aqueous HCl gave 4-amino-1,2,5oxadiazole carbohydroximoyl chloride (AFCHC) in 80% yield, which is converted to 3,4-bis(4-amino-1,2,5-oxadiazol-3yl)-1,2,5-oxadiazole-2-oxide (BAFF, DATF) in 80% yield by treatment with silver carbonate in THF. BAFF is oxidized with a mixture of trifluoroacetic acid and 70% hydrogen peroxide (trifluoroacetic peracid) to give BNFF in 70% yield. 2.2. Characterization. X-ray crystallography: Colorless crystals were mounted on α MiteGen MicroMesh by using a small amount of Cargille Immersion oil. Data were collected on a Bruker three-circle platform diffractometer equipped with a SMART APEX II CCD detector. The crystals were irradiated by using graphite monochromated MoKα radiation (λ = 0.71073). An Oxford Cobra low-temperature device was used to maintain the crystals at a constant 150(2) K during data collection. Data collection was performed, and the unit cell was initially refined by using APEX2 [v2010.3-0].16 Data reduction was performed using SAINT [v7.68 A]17 and XPREP [v2008/ 2].18 Corrections were applied for Lorentz, polarization, and absorption effects by using SADABS [v2008/1].19 The 3511

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that underwent a dipolar [2 + 3] cycloaddition reaction to give BAFF (Figure 2) in 66% yield (23% overall yield), containing 5−10% of the side product, 3,6-bis(4-amino-1,2,5-oxadiazol-3yl)-1,4,2,5-dioxadiazene (BAODD).41,42 BAODD was difficult to remove from the crude product by recrystallization. The procedures described were repeated in our laboratories and found to be sufficient for the preparation of BAFF at HMX > RDX > PETN. Interestingly, there is an inverse correlation between the thermal stability and optical band gap in a series PETN > RDX > HMX > TNT > TATB > BNFF. The trend is consistent with early experiments in which it has been found that the energy of “shake-up transitions” observed in X-ray photoelectron spectra of the structurally similar family of explosives, TATB, DATB, picramide, and TNB, shows a linear relation with the explosive sensitivity and inverse relation with optical absorption maxima.88 At this point, the correlation serves as an observation only and does not necessarily mean that PETN is thermally most sensitive and optically most stable in the considered series

Table 6. Due to such an ambiguity in an interpretation, an accurate evaluation of kinetic parameters and reaction mechanisms requires that the experimental measurements be augmented by a detailed theoretical modeling. Hence, we included in Table 6 the activation energies calculated for the gas-phase and solid-state decomposition of these materials. It is commonly accepted that the trend in thermal stability is in line with the X−NO2 (X = O, N, or C) bond dissociation energies and associated with the NO2 loss:66,67 nitro-esters, O− NO2, require about 35−40 kcal/mol for initiation of chemical decomposition; nitramines, N−NO2, need about 40−50 kcal/ mol; and nitro-arenes, C−NO2, need about 50−70 kcal/mol. These estimates are rough and oversimplified because the realistic measured range is appreciably wider for each corresponding class, as revealed by experiments,50 and at least two initiating reactions compete at the earliest stages of decomposition, as established by theory (see, for example, refs 36−38 and 68). Nevertheless, the trend in comparative thermal stability is still valid, although characteristic activation barriers and decomposition mechanisms require refinement. Typically, gas-phase activation barriers are less scattered than solid-state barriers for the same material. For example, activation barriers for condensed RDX samples range from 24.7 to 52 kcal/mol,69 and about 30−40 kcal/mol is required to decompose gas-phase RDX molecules.69 Even larger range is reported for HMX, with 13−67 kcal/mol needed to break crystals69 and about 32−53 kcal/mol to initiate molecules.69 First-principles calculations suggest that the activation barrier of an RDX molecule is ∼35 70 −39 kcal/mol, 71 which is comparable to HMX (38.1 kcal/mol),72 as expected from similarities of the molecular structures and the arrangement of chemical functional groups in the molecules. Decomposition of RDX surfaces is found in calculations to require 33.2−35.1 kcal/mol.70 Modeling also offers a consistent interpretation for the large range of HMX decomposition energies and proposes that the low energies (about 20.1 kcal/mol or lower) are likely to be associated with initiation of polar surfaces of the δ-phase73 and that the middle of the range (about 37.4 kcal/mol)72 is needed for dissociation of nonpolar β-phase surfaces. The experimentally obtained activation energy of condensed BNFF (42.3 kcal/mol, Table 6) falls in the typical range depicted for explosives and does not offer a clear judgment of its thermal stability relative to other materials. The data for a gas-phase BNFF molecule are not available. A comparison of the calculated activation energy of isolated molecules suggests 3517

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Another useful characteristic is electronic affinity that tells whether the molecule is electron-withdrawing or electrondonating. All tested molecules are electron-withdrawing as the electron affinities are positive, which means that in an electronrich environment the molecules will trap an electron and gain energy by doing so. The higher the EA, the more energy is gained. BNFF possesses the highest EA (2.3 eV) among all six molecules (Table 7). This observation means that BNFF decomposition can be efficiently controlled by electric current, doping with donor-impurity additives, or electromagnetic fields. In addition, this quality makes BNFF an attractive dopant for other explosive ingredients for modern energetic formulations because it allows for a rich variety of desired and highly controllable properties. Ionization potentials of all six molecules are high and relatively close to each other with the only exception of TATB (Table 7). These parameters indicate that the removal of an electron from those energetic molecules is an energetically costly process.

of molecules and that BNFF is thermally most stable and optically most sensitive. Although optical band gap excitation can, in fact, lead to the bond dissociation through the excited state potential energy surface,54,61,68 it is doubtful that an absolute value of the molecular HOMO−LUMO gap alone may be used as a direct measure of materials (optical) sensitivity to initiation. Instead, a combination of electronic properties, such as the nature of HOMO and LUMO orbitals, energies and localization of electronic singlet−triplet transitions, exciton binding energies, and sensitivity of the HOMO−LUMO gap to an external perturbation should be considered as it provides a wealth of information that is indeed closely relevant to the decomposition of molecules (and solids) through electronic excited states. Illustratively, Figure 6 shows that the BNFF’s HOMO

IV. CONCLUSIONS We proposed and tested a comprehensive approach to synthesize, characterize, and model energetic materials, to catalyze their improvements, and to achieve desired properties. The novel synthesis procedure combined with the structural characterization, sensitivity tests, and density functional theory modeling of decomposition channels and electronic structure properties of BNFF is performed and analyzed. The developed synthesis methodology of BNFF offers a relatively simple method in excellent yields and allows for an efficient scale up. The main decomposition channels of the BNFF gas-phase molecules and crystals are explored and characterized by activation barriers, pre-exponential factors, and reaction rate constants. An analysis of decomposition mechanisms suggests that these novel energetic materials are relatively stable and therefore should exhibit a low sensitivity to initiation of detonation. We discover that the cleavage of the outer oxadiazol ring will be the predominant pathway in the decomposition of condensed BNFF at moderate and low temperatures, whereas the NO2 loss will be the fastest reaction at high temperatures. The gas-phase decomposition of BNFF will be also initiated with those two reactions; however, CONO isomerization may also contribute to some extent. We note that while the thermal stability of BNFF seems to be on par with TATB and TNT the initiating decomposition scenarios are different and determined by the presence of the heterocyclic ring in the BNFF molecule. We propose to augment the conventional matrix of sensitivity parameters with activation barriers and kinetics of chemical decomposition reactions, as a measure of thermal stability, and electronic properties, as a measure of electronic stability. Such an upgraded set represents a comprehensive approach to stability of energetic materials and offers a better characterization of sensitivity to detonation initiation by external stimuli. Applying this holistic approach to BNFF we conclude that in response to a mechanical perturbation BNFF behaves somewhat similar to nitramine explosives (HMX and RDX) as evidenced by the measured density, the drop hammer, and friction tests (Table 5). The calculated decomposition activation barriers suggest that BNFF is relatively stable against thermal heating, and its thermal sensitivity is lower than that of PETN, RDX, and HMX and comparable to or slightly higher

Figure 6. HOMO and LUMO are shown for a BNFF molecule.

orbital is mainly localized on the central oxadiazole-1-oxide ring and formed from 2px atomic functions of carbon, nitrogen, and oxygen atoms with a major contribution of oxide oxygen. The LUMO orbital is localized on one of the outer nitro-oxadiazole rings with large contributions of 2pz atomic functions of carbon, nitrogen, and oxygen atoms. Such a configuration of frontier orbitals suggests that the low-energy optical excitation would rather affect the central ring bonds than the C−NO2 or outer ring bonds. We note that the HOMO−LUMO gaps of RDX and HMX are close to each other (6.76 vs 6.49 eV, Table 7), which is consistent with similar sensitivities of those explosives. This connection is additionally supported by the fact that both orbitals are well localized on N−NO2 groups, which means that the low-energy singlet−triplet transition will involve excitation of the critical chemical bond of the RDX or HMX molecule and may indeed lead to its decomposition. The energy required for such a transition, leading to the bond breaking, is (by ∼2 eV) smaller than the HOMO−LUMO gap due to high exciton binding energy. Noting that the HOMO−LUMO gaps of BNFF and TATB are close to each other, one may want to extrapolate the correlation and suggest that the sensitivity of BNFF and TATB to laser irradiation should be also comparable. The clear understanding, however, can be obtained only after a detailed, explicit modeling of excitonic initiation mechanisms is complete. However, if the revealed inverse correlation (between the thermal stability and optical energy gap) withstands tests for a meaningful justification, it may offer a straightforward method of controlling sensitivity of energetic materials to initiation because it allows one to completely separate the initiation through electronic and thermal channels. 3518

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(8) Zhao, F.-Q.; Chen, P.; Hu, R.-Z.; Luo, Y.; Zhang, Z.-Z.; Zhou, Y.S.; Yang, X.-W.; Gao, Y.; Gao, S.-L.; Shi, Q.-Z. Thermochemical Properties and Non-Isothermal Decomposition Reaction Kinetics of 3,4-Dinitrofurazanfuroxan (DNTF). J. Hazard. Mater. 2004, 113, 67− 71. (9) Pagoria, P.; Hope, M.; Lee, G.; Mitchell, A.; Leonard, P. “Green” Energetic Materials Synthesis at LLNL. Proceedings of the 15th International Seminar New Trends in Research of Energetic Materials (NTREM), Pardubice, Czech Republic, April 18−20, 2012. (10) Lim, C. H.; Kim, T. K.; Kim, K. H.; Chung, K.-H. Synthesis and Characterization of Bisnitrofurazanofuroxan. Bull. Korean Chem. Soc. 2010, 31, 1400−1402. (11) Zheng, W.; Wang, J.; Ren, X.; Zhang, L.; Zhou, Y. An Investigation on Thermal Decompoosition of BNFF-CMDB Propellants. Propellants Explos. Pyrotech. 2007, 32, 520−524. (12) Kotomin, A. A.; Kozlov, A. S.; Dushenok, S.A. Detonatability of High-Energy-Density Heterocyclic Compounds. Russ. J. Phys. Chem. 2007, 1, 573−575. (13) Sinditskii, V. P.; Burzhava, A. V.; Sheremetev, A. B.; Aleksandrova, N. S. Thermal and Combustion Properties of 3,4Bis(3-nitrofurazan-4-yl)furoxan (DNTF). Propellants Explos. Pyrotech. 2010, 35, 1−6. (14) Ravi, P.; Badgujar, D. M.; Gore, G. M.; Tewari, S. P.; Sikder, A. K. Review on Melt Cast Explosives. Propellants Explos. Pyrotech. 2011, 36, 393−403. (15) Stepanov, A. I.; Dashko, D. V.; Astrat’ev, A. A. 3, 4-Bis (4′nitrofurazan-3′-yl) furoxan: a Melt Cast Powerful Explosive and a Valuable Building Block in 1, 2, 5-Oxadiazole Chemistry. Cent. Eur. J. Energ. Mater. 2012, 9, 329−342. (16) Bruker, APEX2 v2010.3 − 0; Bruker AXS Inc.: Madison, Wisconsin, USA, 2010. (17) Bruker, SAINT v7.68 A; Bruker AXS Inc.: Madison, Wisconsin, USA, 2009. (18) Bruker, XPREP v2008/2; Bruker AXS Inc.: Madison, Wisconsin, USA, 2008. (19) Bruker, SADABS v2008/1; Bruker AXS Inc.: Madison, Wisconsin, USA, 2008. (20) Bruker, SHELXTL v2008/4; Bruker AXS Inc.: Madison, Wisconsin, USA, 2008. (21) Hohenberg, P.; Kohn, W. Inhomogeneous Electron Gas. Phys. Rev. 1964, 136, B864−B71. (22) Kohn, W.; Sham, L. J. Self-Consistent Equations Including Exchange and Correlation Effects. Phys. Rev. A 1965, 140, A1133− A38. (23) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (24) Zhao, Y.; Truhlar, D. G. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, Noncovalent interactions, Excited states, And transition elements: Two new functionals and systematic testing of four M06-class functionals and 12 other functionals. Theor. Chem. Acc. 2008, 120, 215−241. (25) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al. Gaussian 09, revision B.01; Gaussian, Inc.: Wallingford CT, 2009. (26) Dunning, T. H., Jr. Gaussian Basis Sets for Use in Correlated Molecular Calculations. I. The Atoms Boron Through Neon and Hydrogen. J. Chem. Phys. 1989, 90, 1007−1023. (27) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953−17979. (28) Kresse, G.; Futhmuller, J. Efficiency of Ab-initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (29) Kresse, G.; Furthmuller, F. Efficient Iterative Schemes for Ab initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169−11186. (30) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B 1993, 47, 558−561.

than the benchmark sensitivity of TATB (Table 6). The melting temperature of this novel material is significantly lower than that of RDX, HMX, PETN, and TATB, which makes BNFF attractive as a melt-castable explosive compound. With BNFF electronic properties, resembling TATB, and high electronic affinity, indicating an ability to trap electrons (Table 7), BNFF appears as an excellent candidate on its own or as an ingredient for composite energetic formulations to improve their stability and significantly enhance control over initiation of chemical decomposition reactions.

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ASSOCIATED CONTENT

S Supporting Information *

Details of synthesis procedures, the structure of a BNFF molecule, the structures of considered molecules in their transition states during decomposition, calculated activation barriers to estimate van der Waals interactions, and atomic coordinates of the BNFF molecule in its ground state and in the transition states for each decomposition reaction. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] and [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Work performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. This research is also supported in part by ONR (Grant N00014-12-1-0529) and NSF. We used NSF XSEDE resources (Grant DMR-130077) and DOE NERSC resources (Contract DE-AC0205CH11231). MMK is grateful to the Office of the Director of National Science Foundation for support under the IRD program.

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REFERENCES

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