Article pubs.acs.org/crystal
Reduction of Mechanical Sensitivity in Alkyl Nitrate Explosives through Efficient Crystal Packing Thomas W. Myers,* Christopher J. Snyder, and Virginia W. Manner* Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States S Supporting Information *
ABSTRACT: We report the nitration of tris(hydroxymethyl)aminomethane to tris(nitromethyl)-aminomethane (2) and its corresponding hydrogen chloride (3) and hydrogen bromide (4) salts. While both 3 and 4 conglomerate into tetramers, there are significant differences in the packing of those tetrameric units. In both 3 and 4 the tetrameric units arrange in a hexagonal pattern, but in 3 the units adopt alternating orientations leading to severe interlocking of the alkyl nitrate groups. This ultimately leads to a 2-fold decrease in the mechanical sensitivity of 4 relative to 3 and suggests that utilizing anion size may lead to more efficient packing arrangements and reduced mechanical sensitivity in energetic salts.
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INTRODUCTION Understanding and controlling the mechanical sensitivity of explosives remains a challenge due to the multitude of variables that influence each material.1−3 Certain functional groups are known to have reduced sensitivity relative to others.4−8 An example of this is the series of E-NO2 groups where E is an aromatic carbon, an aliphatic carbon, a nitrogen atom, or an oxygen atom. In this series the mechanical sensitivity increases as the E-NO2 bond strength decreases.4,7,8 Additionally, the arrangement of the functional groups within a molecule also contributes to its sensitivity. Arrangements that reduce the overall charge imbalance in a molecule often reduce the sensitivity of the material as well.9−14 For example, isomers of nitro-aromatic compounds often have differing sensitivity.15 Additionally, introduction of amine groups vicinal to nitro groups has proven to be an effective strategy leading to several prominent high explosives with low mechanical sensitivity including TATB16,17 (TATB = 2,4,6-triamino-1,3,5-trinitrobenzene), DAAF18,19 (DAAF = 3,3′-diamino-4,4′-azoxyfurazan), FOX-720 (FOX-7 = 1,1-diamino-2,2-dinitroethene), and LLM10521,22 (LLM-105 = 2,6-diamino-3,5-dinitropyrazine-1-oxide). However, this approach is limited by the lack of synthetic routes to a wide variety of vicinal amino-nitro explosives. A second approach to reducing mechanical sensitivity has focused on intermolecular interactions. Both hydrogen bonding and π-stacking interactions provide additional vibrations and rotational modes that aid in energy dissipation, resulting in reduced sensitivity.1,23 Recently, it has been discovered that solid state packing can play a critical role in mechanical sensitivity.24−26 Specifically, explosives that pack in planar, graphite-like sheets often have lower sensitivities relative to those that pack in herringbone or interlocking arrangements. While this effect can be significant, it is often difficult to predict © 2017 American Chemical Society
and control the crystal packing in explosive materials. There has been limited success in achieving graphite-like sheets by targeting planar molecules with potential for significant hydrogen bonding and π-stacking interactions, but these restrictions exclude a vast majority of explosive materials. We sought to control the packing of nonplanar energetic salts in order to reduce mechanical sensitivity. Specifically, we targeted salts of alkyl nitrate explosives with halide anions that could result in various solid state structures. Through varying the size of the halide counteranion we postulated that it would be possible to access a variety of packing motifs with a range of mechanical sensitivity.
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RESULTS AND DISCUSSION Synthesis. We targeted various salts of the TrisTN (TrisTN = tris(nitroxymethyl)methylamine) in place of salts of NH2− PETN (NH2−PETN = 2,2,2-tris(nitroxymethyl)ethylamine) due to the ease of synthesis of TrisTN. Following the procedure of Myers and Winthrop27 we nitrated Tris (Tris = tris(hydroxymethyl)methylamine) with 90% nitric acid in the presence of acetic acid and acetic anhydride (Scheme 1). Adjusting the pH to 10 allowed for the extraction of TrisTN (2) as a pale yellow oil. The oil was stable at room temperature for approximately 1 day or at −10 °C for up to 3 days, but decomposed to unidentified solid products after that. Protonation of 2 proved more problematic than anticipated. When aqueous acids were employed no product could be isolated from the reaction mixture, presumably due to decomposition. However, addition of anhydrous hydrogen Received: February 1, 2017 Revised: March 30, 2017 Published: March 31, 2017 3204
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Scheme 1. Synthesis of 2−4 from 1
chloride in diethyl ether to an ethereal solution of 2 led to immediate precipitation of TrisTN·HCl (3) as a white solid in 93% yield.26 Since other hydrogen halides are not stable in ethereal solution, alternative anhydrous reagents were sought for the protonation of 2. Anhydrous hydrogen bromide gas, generated from dropping concentrated sulfuric acid on potassium bromide, was bubbled through a solution of 2 in ether leading to the precipitation of TrisTN·HBr (4) in 77% yield. Crystal Structure. Crystals of 3 and 4 suitable for single crystal X-ray diffraction were grown from the slow evaporation of a concentrated acetone solution of 3 over 3 days and the diffusion of hexane into a concentrated ether solution of 4, respectively (Figure 1). Compound 3 crystallized in the
Table 1. Average Bond Lengths and Angles in 3, 4, and PETN average bond lengths (Å) bond type C−C C−Namine C−O Nnitro-Obridge Nnitro-Oterminal
3
PETN28
4
1.529(2) 1.529(2) 1.487(2) 1.567(2) 1.431(2) 1.451(2) 1.396(2) 1.386(2) 1.195(2) 1.210(2) average angle (deg)
1.532 N/A 1.446 1.401 1.199
angle type
3
4
PETN
C−C−Namine C−C−Obridge C−Obridge-Nnitro Obridge−N-Oterminal Oterminal-Nn-Oterminal
109.4(3) 108.8(3) 112.8(3) 114.9(3) 129.7(3)
107.7(2) 108.0(2) 113.6(2) 115.1(2) 129.7(2)
N/A 106.8 113.0 115.2 129.5
Both compounds 3 and 4 display intermolecular hydrogen bonding (Figure 2). Both 3 and 4 adopt tetrameric structures
Figure 1. Solid state structures of [(O2NOCH2)3CNH3][Cl] in 3 (left) and [(O2NOCH2)3CNH3][Br] in 4 (right). Dark green, green, red, blue, gray, and white ellipsoids represent Br, Cl, O, N, C, and H atoms, respectively. Ellipsoids at 50% probability. H atoms except N− H omitted for clarity.
orthorhombic space group Aba2 [lattice constants a = 19.800(5); b = 17.048(5); c = 13.549(3); α = β = γ = 90°; V = 4574(2)], while compound 4 crystallized in the trigonal space group P3 [lattice constants a = b = 19.243(5); c = 10.924(3); α = β = 90°; γ = 120°; V = 3503(2)]. The unit cell of 3 contains two independent molecules. Four of the six alkyl nitrate groups in 3 are disordered over two possible orientations. The unit cell of 4 contains six independent molecules with three occupying special positions on 3-fold rotation axes. At −100 °C compound 3 has a density of 1.694 g/cm3, while the density of compound 4 at −155 °C is 1.917 g/cm3. In order to remove the effect of the difference in atomic mass between Cl− and Br− we calculated the density of the CHNO components (dCHNO) by multiplying the density by the mass fraction of CHNO (ωCHNO, eq 1). The dCHNO for 3 and 4 are very similar, 1.493 and 1.462 g/cm3, respectively, in spite of differences in packing, and differences in atomic mass and ionic radii for Cl− and Br−. m + mH + mN + mO dCHNO = dωCHNOωCHNO = C (1) MM The bond lengths and angles in 3 and 4 are all within expected values for alkyl nitrate explosives (Table 1). The one noticeable difference between 3 and 4 is the C−Namine bond length, which is shorter in 3 (1.487(2) Å) than in 4 (1.567(2) Å).
Figure 2. Hydrogen bonding tetramer in 3 (left) and 4 (right). Dashed blue lines indicate hydrogen bonding interaction. Alkyl nitrate groups omitted for clarity.
with four ammonium and four halide anions occupying alternating vertexes of a cube. The average Cl−Namine distance in 3 (3.151 Å) is significantly longer than the average Br−Namine distance in 4 (3.313 Å) corresponding to the difference in ionic radii between Cl− and Br−. In this tetrameric arrangement each ammonium N−H is able to hydrogen bond with a halide anion. While compounds 3 and 4 adopt nearly identical hydrogen bonding arrangements, the two materials adopt very different packing arrangements. The tetramers in 3 adopt a simple cubic packing arrangement, while the tetramers in 4 adopt a hexagonal packing arrangement (Figure 3). When viewed along the a,c diagonal crystallographic axis, the packing in 3 can be seen to have a similar hexagonal arrangement. However, the tetrameric units in 3 adopt alternating orientations leading to severe interlocking of the alkyl nitrate groups, while the tetrameric units in 4 adopt a repeating orientation preventing this interlocking interaction (Figure 4). The degree of interlocking in energetic materials has previously been shown to significantly impact the sensitivity of the materials, where more interlocking results in more sensitive materials.29−31 It is 3205
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Figure 3. Solid-state packing in 3 and 4 viewed along the a, b, and c crystallographic axes. Dashed blue lines indicate hydrogen bonding, while solid black lines indicate slip planes.
Table 2. Decompositions Temperatures, Densities, and Oxygen Balances in 2−4 and PETN 2 3 4 PETN
To (°C)a
Tp (°C)b
ρ (g/cm3)
Ωd (%)
104.6 110.1 106.3 164.8
147.3 123.4 118.6 206.4
1.69c 1.92c 1.845c
−18.8 −19.1 −16.6 −10.1
a Onset of decomposition temperature from DSC. b Peak of decomposition temperature from DSC. cX-ray crystal density of 3 at 173 K and 4 at 118 K (this work); PETN at 100 K (ref 34). d Calculated oxygen balance to CO2,Ω = −1600[2(# of C atoms) + (#H atoms)/2 − (# of O atoms)]/MW.
Figure 4. Hexagonal packing in 3 (viewed along a,c diagonal crystallographic axis) with tetrameric units in alternating orientation (left) and hexagonal packing in 4 with tetrameric units in repeating orientation (right).
significantly higher at 1.92 g/cm3 due to the presence of the bromide anions. In order to determine the handling sensitivity of 2−4, we performed impact, friction, and electrostatic discharge (ESD) tests on the materials (Table 3). Small-scale sensitivity tests
likely that the smaller size of the chloride anion prevents 3 from adopting the more favorable repeating orientation in 4. In addition to altering how the layers can slide past each other, the difference in packing geometry also affects the amount of void space within the unit cells of 3 and 4. It has been observed that materials that pack with less void space in the unit cell often have reduced impact sensitivities.32 One measure of the void space within the unit cell is the Kitaigorodskii Packing Index, which is defined as the volume of the molecules in the unit cell divided by the unit-cell volume in a crystal.33 The packing index is 69.5% for 3 and 71.4% for 4, indicating that 4 packs more efficiently with less void space than 3. Sensitivity Testing. Sensitivity tests were performed to determine the mechanical and thermal stabilities of 2, 3, and 4 (Table 2). Differential scanning calorimetry (DSC) measurements confirmed that 2−4 are all less thermally stable than PETN with onsets of decomposition between 104.6 and 110.1 °C, consistent with what has been previously observed for explosives with alkyl nitrate groups beta to amine groups.35 The replacement of a CH2ONO2 group with a NH2 or NH3+ group reduces the oxygen balance of explosives 2−4 slightly to between −16.6% and −19.1%. Lastly the densities of 2−4 are comparable to PETN with the exception of 4, which is
Table 3. Mechanical Sensitivity Properties of 2−4 and PETN 2 3 4 PETN
impact (cm)a
friction (N)b
ESD (J)c
± ± ± ±
>360 >360 >360 81 ± 16
0.0625 0.125 0.0625 0.0625
35.8 30.3 62.6 12.0
3.4 4.4 14.4 1.4
a LANL type 12, 50% drop height, 2.5 kg. bBAM 50% load. cABL spark 3.4% threshold initiation level (TIL).
provide parameters for safe handling of explosives, but reported values will vary from one laboratory to another and should always be performed with an accepted standard such as PETN for relative comparison. Sensitivity tests were performed on microcrystalline samples of 3 and 4 to ensure that the structure and packing observed in the single crystal samples of 3 and 4 were present in the tested samples. Relative to PETN, all three compounds have reduced sensitivity toward friction. This is likely due to 2 being a liquid and the presence of hydrogen bonding in 2−4, which has been previously shown to reduce sensitivity in explosive materials.1,22 Additionally, 2 and 4 have 3206
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corrected for Lorentz polarization effects using SAINT37 and were corrected for absorption effects using SADABS 2.3.38 Space group assignments were based upon systematic absences, E statistics, and successful refinement of the structures. Structures were solved by direct methods with the aid of successive difference Fourier maps and were refined against all data using the SHELXTL 5.0 software package.39 Thermal parameters for all non-hydrogen atoms were refined anisotropically. Hydrogen atoms, where added, were assigned to ideal positions and refined using a riding model with an isotropic thermal parameter 1.2 times that of the attached carbon atom (1.5 times for methyl hydrogens). Sensitivity Tests. Quantitative impact sensitivity tests were performed on a LANL type 12 instrument with a 50% drop height and a 2.5 kg weight using the Neyer D-Optimal method. Quantitative friction sensitivity tests were performed with a BAM friction instrument determining the 50% load using the Neyer D-Optimal method. Quantitative electrostatic discharge sensitivity testing was performed on an ABL ESD instrument. Preparation of Compounds. Deuterated solvents were purchased from Cambridge Isotopes Laboratories, Inc. and used without purification. All other reagents were purchased from commercial vendors and used without further purification. TrisTN (2). TrisTN was synthesized by modifying the procedure of Myers and Winthrop.26 A solution of Tris (1.21 g, 10 mmol) in acetic acid (5 mL) and acetic anhydride (5 mL) was cooled to 0 °C in an ice bath. To this stirred solution 90% HNO3 (3 mL) was added dropwise maintaining the temperature below 10 °C throughout. The solution was stirred for 15 min at 10 °C before being warmed to room temperature and stirred for an additional 30 min. The solution was quenched in ice water (50 mL) and adjusted to pH 10 with NaHCO3. The solution was extracted with dichloromethane (3 × 10 mL), and the organic layer was washed with brine (10 mL) and dried with sodium sulfate. Removal of solvent in vacuo furnished TrisTN (2, 2.11 g, 82%) as a yellow oil. 1H NMR (400 MHz, d6-DMSO) 4.83 (s, 6H, CH2), 8.87 (s, 2H, NH2) δ. 13C NMR (100 MHz, d6-DMSO) 56.16, 58.58 δ. TrisTN·HCl (3). To a solution of 2 (1.29 g, 5 mmol) in diethyl ether (15 mL) was added an 1 M ethereal solution of anhydrous hydrogen chloride (5.5 mL, 5.5 mmol) leading to the immediate precipitation of a white solid. TrisTN·HCl (3, 1.38 g, 93%) was collected via filtration and washed with cold ether (10 mL). Single crystals suitable for single crystal X-ray diffraction were obtained from the slow evaporation of a concentrated acetone solution, while polycrystalline materials for mechanical sensitivity measurements were obtained by the rapid evaporation of a concentrated acetone solution. 1H NMR (400 MHz, d6-DMSO) 4.54 (s, 6H, CH2) δ. 13C NMR (100 MHz, d6-DMSO) 54.20, 69.59 δ. TrisTN·HBr (4). Anhydrous hydrogen bromide gas was bubbled through a solution of 2 (1.29 g, 5 mmol) in diethyl ether (15 mL) for 15 min leading to the precipitation of a white solid. TrisTN·HBr (4, 1.30 g, 77%) was collected via filtration and washed with cold ether (10 mL). Single crystals suitable for single crystal X-ray diffraction were obtained from the slow diffusion of hexane into a concentrated ether solution, while polycrystalline materials for mechanical sensitivity measurements were obtained by the rapid evaporation of a concentrated acetone solution over 3 days. 1H NMR (400 MHz, d6DMSO) 4.86 δ. 13C NMR (100 MHz, d6-DMSO) 54.80, 70.19 δ.
ESD sensitivities comparable to PETN at 0.0625 J, while 3 has lower sensitivity to ESD at 0.125 J. Compounds 2 and 3 have similar sensitivity toward impact at 35.8 and 30.3 cm, respectively, while compound 4 has a significant reduction in impact sensitivity at 62.6 cm. This significant difference in impact sensitivity was interesting given the striking similarities between 3 and 4. As previously noted, both compounds adopt the same tetrameric units with similar bond lengths and angles. They have similar oxygen balances as well as similar CHNO densities. The only major difference between the two materials is the packing arrangement with 3 adopting a more interlocked arrangement than 4. Thus, 2-fold decrease in impact sensitivity of 4 relative to 3 is due to the combination of the higher packing efficiency, decreased degree of interlocking, and decrease in resistance in the sliding of layers of molecules past each other in 4 relative to 3.
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CONCLUSION We have synthesized amine and ammonium salt analogues of PETN. All of the materials display improved mechanical sensitivity relative to PETN due to the presence of hydrogen bonding, along with diminished thermal stability due to the presence of an amine/ammonium group in a beta position relative to alkyl nitrate groups. Both TrisTN·HCl (3) and TrisTN·HBr (4) aggregate into tetramers held together by hydrogen bonding. The tetramers in TrisTN·HCl pack in a simple cubic arrangement, while the tetramers in TrisTN·HBr adopt a hexagonal arrangement. Relative to TrisTN·HCl, TrisTN·HBr has a lesser degree of interlocking, a decrease in the resistance to one layer of molecules sliding past another, and a higher packing efficiency. Ultimately, these differences result in a 2-fold decrease in the impact sensitivity of TrisTN· HBr relative to TrisTN·HCl. This decrease in mechanical sensitivity in nonplanar energetic salts materials illustrates that graphite-like sheets are not the only solid state packing phenomenon in energetic materials that can result in reduced mechanical sensitivity. This anion-induced difference in packing and corresponding mechanical sensitivity change will be utilized in the design of new nonplanar energetic materials with decreased mechanical sensitivity.
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EXPERIMENTAL SECTION
Caution! Although no problems have occurred during the synthesis and handling of these complexes, the materials are explosive. Laboratories and personnel should be properly grounded, and safety equipment such as Kevlar gloves, blast shields, and ear plugs are necessary, especially when working with large scale reactions. Physical Measurements. Elemental analysis for 4 was performed using a PerkinElmer series II 2400 CHNS/O analyzer. Elemental analysis of explosive compounds often returns low values for the %N, and materials were purified until satisfactory C% and H% values were obtained.36 1H NMR spectra were recorded at ambient temperature using a Bruker Avance 400 MHz spectrometer. Chemical shifts (δ) were referenced to the residual solvent signal. Differential scanning calorimetry was performed using a TA Instruments 2920 Modulated DSC at 10 °C/min heating ramp rate. X-ray Structure Determinations. X-ray diffraction studies use a Rigaku R-axis IV with an imaging plate detector or a Bruker D8 Quest with a Photon II detector. Measurements were carried out at −100 °C using Mo Kα 0.71073 radiation. Crystals were mounted on a Kaptan loop with paratone-N oil. Initial lattice parameters were obtained from a least-squared analysis of more than 100 centered reflections; these parameters were later refined against all data. Data were integrated and
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b00159. Cif files, NMR data, crystallographic data, and DSC plots (PDF) Accession Codes
CCDC 1530516−1530517 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by 3207
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Thomas W. Myers: 0000-0003-3748-0848 Funding
For financial support of this work, the authors acknowledge Campaign 2 at Los Alamos National Laboratory. Los Alamos National Laboratory is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy (contract DE-AC5206NA25396). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank A. M. Giambra, H. Tian, and M. Sandstrom for assistance with elemental, thermal, and mechanical sensitivity analysis. In addition, we thank K. Ramos and G. K. Windler for assistance with explosive single crystal XRD.
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