Crystal Structure, Packing Analysis, and Structural-Sensitivity

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Crystal Structure, Packing Analysis, and Structural-Sensitivity Correlations of Erythritol Tetranitrate Virginia W. Manner,* Bryce C. Tappan,* Brian L. Scott, Daniel N. Preston, and Geoffrey W. Brown Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States S Supporting Information *

ABSTRACT: The explosive erythritol tetranitrate (ETN) has been known since 1849 and has applications as a vasodilator; however, little is known about its structure and bonding. Here we present the X-ray crystal structure of erythritol tetranitrate (ETN), along with characterization by nuclear magnetic resonance (NMR), infrared spectroscopy (IR), elemental analysis, and X-ray diffraction (XRD). Crystal packing and morphology are discussed in relation to explosive handling sensitivity (impact, spark, and friction testing). We compare the structure and property relationship to a closely related common nitrate ester, pentaerythritol tetranitrate (PETN).

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INTRODUCTION Erythritol tetranitrate (ETN; Figure 1) is an explosive that was first prepared in 1849.1 Its explosive properties are similar to

Figure 1. Erythritol tetranitrate (ETN).

pentaerythritol tetranitrate (PETN), a commonly used explosive in military and commercial applications. ETN is melt-castable, has impressive performance, and is not difficult to prepare, which increases the necessity for understanding its properties from a law enforcement or threat determination perspective.2 It is important to note that, due to its sensitivity as an explosive, ETN has been involved in recent accidents3 and should not be handled outside of a dedicated explosives facility. Like nitroglycerin, ETN is a nitrate ester4,5 that has applications as a vasodilator6 and has been prepared by pharmaceutical companies. Surprisingly, ETN has not been well characterized. Although many procedures call for recrystallization of the material,2,7,8 we have found no reported crystal structure information. We report here the first X-ray crystal structure of ETN and discuss the influence of crystal packing9 on the sensitivity of the material. © 2014 American Chemical Society

EXPERIMENTAL SECTION

Erythritol marketed by Now Foods was used as received. Sulfuric acid (98%) was purchased from Sigma-Aldrich and nitric acid (70%) was purchased from Amresco. PETN used for nuclear magnetic resonance (NMR) spectra was purchased from DuPont in 1986, Lot # 298. NMR solvents were purchased from Cambridge Isotope Laboratories, Inc. Solvents were purchased from Fisher. All 14N NMR spectra were taken on a Bruker 400 spectrometer and referenced to an internal nitromethane standard. Synthesis of Erythritol Tetranitrate. CAUTION: Erythritol tetranitrate is a very sensitive and dangerous explosive that has been involved in several recent explosives accidents.3 It should only be prepared and handled in an explosives facility. Mixed Acid Procedure. Following an established procedure,7 a mixture of 36.4 g of 70% nitric acid and 93.6 g of 98% sulfuric acid was cooled in an ice bath. Erythritol (10 g, 8.2 mmol) was dissolved in 36 g of sulfuric acid, and the solution was added slowly to the mixture of nitric and sulfuric acid. The solution was kept at 5−10 °C during addition. After stirring for ∼1 h, the solution was poured into 620 mL of cold (ice) water, and a white precipitate was collected. The solid was washed with a solution of sodium bicarbonate until the water coming through the filter was close to neutral pH. The resulting solid was recrystallized in a mixture of ethanol and acetone, and 9 g (2.8 mmol, 34% yield) was collected. X-ray quality crystals were obtained through slow evaporation in a solution of methanol. In order to investigate the possibility of polymorphism, crystals were collected from slow evaporation in ethanol, slow diffusion of water into ETN/ ethanol, slow precipitation with acetonitrile layered with water, and fast precipitation with acetonitrile/water. Crystal sheets were obtained when crystals were grown quickly, as with fast evaporation of a solution of ethanol, acetone, or a mixture of the two. Placing a room temperature solution of ETN (in acetone, ethanol, or diethyl ether) in a low-temperature freezer, or heating ETN in ethanol at ∼50 °C, Received: September 10, 2014 Revised: October 3, 2014 Published: October 9, 2014 6154

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following by cooling, also resulted in fast crystal sheet growth. 1H NMR: (CD3CN, 300 MHz) δ 4.76 (dd, 2H), 4.97 (dd, 2H), 5.71 (m, 2H); ((CD3)2CO, 400 MHz) δ 5.02 (dd, 2H), 5.26 (dd, 2H), 6.02 (m, 2H). 13C NMR: (CD3CN, 400 MHz) δ 69.30, 76.77; ((CD3)2CO, 400 MHz) δ 69.42, 77.11. 14N NMR: ((CD3)2CO, 400 MHz) δ −44.3, −48.5. IR spectroscopy:10 2980.5 and 2913.0 cm−1 (CH2 stretch), 1667.2 and 1630.5 cm−1 (NO2 stretch), 1279.6 cm−1 (NO2 symm. stretch), 878.4 and 834.1 cm−1 (ester O−N stretch). Elemental analysis of X-ray quality crystals grown slowly in methanol: Calculated for C4H6N4O12, C, 15.90; H, 2.00; N, 18.55; found, C, 15.58; H, 1.89; N, 18.75. Nitrate Salt Procedure. Following an established procedure,11 22 g (22 mmol) of potassium nitrate was dissolved in sulfuric acid (28 mL) with mild heating (∼30 °C). The solution was cooled with an ice bath, and erythritol (3.75 g, 3.1 mmol) was added slowly, while keeping the temperature below ∼30 °C. After stirring for ∼1 h, the solution was poured into 250 mL of cold (ice) water, and a white precipitate was collected. The solid was washed with a solution of sodium bicarbonate until the water coming through the filter was close to neutral pH. The resulting solid was dissolved in ethanol with heating at ∼50 °C and poured into cold water. A white powder (2.8 g, 0.93 mmol, 30% yield) was collected and characterized by NMR. PETN NMR Spectra. 1H NMR: (CD3)2CO, 400 MHz) δ 4.86 (s, 8H). 13C NMR: (CD3)2CO, 400 MHz) δ 42.24, 70.88. 14N NMR: ((CD3)2CO, 400 MHz) δ −43.9. X-ray Diffraction. The crystal was mounted in a nylon cryoloop using paratone-n oil (140 K) or epoxy (room temperature). The data were collected on a Bruker SMART APEX II charge-coupled-device (CCD) diffractometer, with KRYO-FLEX liquid nitrogen vapor cooling device. The instrument was equipped with graphite monochromatized MoKα X-ray source (λ = 0.71073 Å), with MonoCap X-ray source optics. A hemisphere of data was collected using ω scans, with 10 s frame exposures and 0.3° frame widths. Data collection and initial indexing and cell refinement were handled using APEX II12 software. Frame integration, including Lorentz-polarization corrections, and final cell parameter calculations were carried out using SAINT+13 software. The data were corrected for absorption using the SADABS14 program. Decay of reflection intensity was monitored via analysis of redundant frames. The structure was solved using direct methods and difference Fourier techniques. All hydrogen atom positions were idealized, and rode on the atom they were attached to. The final refinement included anisotropic temperature factors on all non-hydrogen atoms. Structure solution, refinement, graphics, and creation of publication materials were performed using SHELXTL.15 Powder XRD. Powder X-ray diffraction was collected on powders of ETN crystallized under four different solvent conditions. The powders were not ground due to sensitivity issues and were suspended in paratone-n oil to prevent dispersal and placed on silicon zero background plates for data collection. Data were collected on a Bruker A25 DaVinci diffractometer, with copper radiation (Kα1 = 1.54059 Å, Kα2 = 1.54442 Å) and a Lynxeye detector. All data manipulation and plotting were performed using JADE software.16 Sensitivity Testing. For impact sensitivity testing, ERL Type 12 drop hammer equipment was used, including a 2.5 kg drop weight, a 0.8 kg striker, an anvil, and sound level detection. For each drop, a 40 mg sample weighed on 150 grit paper was placed on the anvil surface and impacted by the drop weight from different heights. The drop hammer Go criterion was defined as the average of two sound level meter measurements greater than 120 dB. The parameter reported is DH50, the height from which dropping the weight produces reaction 50% of the time. DH50 was calculated using the Neyer D-Optimal method. The higher the DH50 value, the lower the impact sensitivity. A BAM friction sensitivity test machine was used to determine the friction sensitivity. The system uses a fixed porcelain pin and a movable porcelain plate that executes a reciprocating motion at a rate of 141 r/m with a stroke length of 10 mm. Weight affixed to a torsion arm allows for a variation in applied force from 5 N (0.5 kg) to 360 N (36.0 kg). The lower the load values, the higher the friction sensitivity. Sample size is typically 2−5 mg, and a Go is acoustically detected by the operator. The parameter reported is F50, the force that produces

reaction 50% of the time. F50 was calculated using the Neyer DOptimal method. A SMS ABL electrostatic discharge (ESD) machine was used to evaluate the ESD hazard. A small sample, typically 5−10 mg, was placed into a sample holder and placed on a grounded pedestal. Scotch tape is placed over the sample holder lightly confining the sample. If the sample evolves enough gas to break the tape during the ESD event, the trial is recorded as a Go. High voltage is applied and discharged to the sample through an approaching needle. Capacitance is adjusted to achieve the desired stimulus energy. ESD results are expressed as a threshold initiation level (TIL) corresponding to the highest spark energy at which 20 consecutive tests produce no reaction with at least one recorded reaction out of 20 at the next highest level. Differential scanning calorimetry (DSC) was used to determine the thermal stability of the sample. A small sample of the explosive was heated at 10 °C/min under a flow of nitrogen gas, and the temperature of the exotherm was noted. An endotherm was observed at the melting temperature. An onset temperature corresponding to an exothermic heat flow exceeding 0.01 W/g/C is also reported. Atomic Force Microscopy. Atomic force microscopy (AFM) was used to examine the surfaces of the particles of ETN crystallized from ethanol (heated) and methanol. The microscope was a Veeco CP-II with a 90 μm scanner and Veeco 300 kHz RTESPA-CP silicon tips. Images were acquired in noncontact tapping mode. Samples were prepared by lightly dropping ETN particles onto the surface of doublesided tape mounted on a stainless steel sample platen. Because of the scanning geometry of the AFM, images were always acquired on the large, flat surfaces of the particles.

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RESULTS AND DISCUSSION Crystal Structure. Crystals of ETN were obtained using either slow evaporation of methanol, slow evaporation of ethanol, or slow diffusion of water into ETN/ethanol. Despite the different solvents and conditions, all methods produced crystals with the monoclinic space group P2(1)/c. Crystal sheets were obtained when crystals were grown quickly, as with fast evaporation of a solution of ethanol, acetone, or a mixture of the two. Placing a room temperature solution of ETN (in acetone, ethanol, or diethyl ether) in a low-temperature freezer, or heating ETN in ethanol at ∼50 °C, followed by cooling, also resulted in fast crystal sheet growth. X-ray quality crystals obtained through slow evaporation in a solution of methanol were used for the crystal structure analysis. The density of ETN crystals with this crystal structure is 1.851 g/cm3 (at 140 K), which is higher than that of PETN, 1.845 g/cm3 (at 100 K).9 At room temperature, the measured ETN density is significantly lower: 1.773 g/cm3 [lattice constants: a = 16.132(6), b = 5.314(2), c = 14.789(6), β = 116.78(4), V = 1132(1)]. Select crystal data for ETN and PETN are given in Table 1. The PETN molecule is much more Table 1. Bond Distances and Angles for ETN and PETN bonds

ETN bonds (Å)

PETN bond(s) (Å)a

C−C C−O N−Obridging N−Oterminal angles

1.5142(16)−1.541(2) 1.4466(14)−1.4481(14) 1.3968(13)−1.4246(13) 1.1902(13)−1.2080(14) ETN angles (deg)

1.53169(13) 1.44634(18) 1.40088(17) 1.19941(19), 1.2057(2) PETN angles (deg)a

C−C−C C−O−N O−C−C Oterm−N−Oterm Obridge−N−Oterm a

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114.10(12)−114.16(12) 113.04(9)−114.54(8) 102.37(11)−104.38(9) 129.30(11)−130.35(11) 111.33(9)−118.59(9)

107.998(5), 112.46(1) 112.95(1) 106.774(8) 129.53(2) 118.01(1), 112.46(1)

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hydrogen bond distances and angles are in line with literature values.9 However, for ETN the distances are shorter than normally reported, and the C−H···O angles are small, ranging from 100.0° to 125.0°. These short distances and small angles are a strong indication that these are not true hydrogen bonds and may even serve to destabilize the crystal structure in ETN.17 The combination of hydrogen bonding and closed shell contacts result in a three-dimensional intermolecular network of chemical bonds in both the ETN (Figure 3) and PETN

symmetrical than ETN and occupies a site of 4 crystallographic symmetry.9 There are two independent ETN molecules in the unit cell, and each occupies an inversion center (Figure 2).

Figure 2. Thermal ellipsoid plot of ETN with ellipsoids drawn at the 50% probability level. There is a center of inversion at the midpoint of the C1−C1a bond. This is one of two independent molecules in the unit cell.

The bond distances and angles observed in ETN are in line with those reported for PETN (Table 1), but the bond angles do indicate more strain in the ETN molecule. The carbon angles are significantly more strained in ETN [114.10(12)− 114.16(12)°] than in PETN [107.998(5)°, 112.46(1)°]. Also, the O−C−C angles are more strained in ETN [102.37(11)− 104.38(9)°] when compared to PETN [106.774°]. The ETN and PETN molecules have numerous intermolecular contacts in the crystal including C−H···O hydrogen bonds, and O···O and O···N closed shell interactions. The intermolecular bonding information is given in Tables 2 and 3. Table 2. Closed Shell Intermolecular Contacts for ETN and PETN

a

bonds

ETN bonds (Å)

PETN bonds (Å)a

O···O O···N

2.7718−3.2200 3.0425

3.1467−3.2278 2.8042−3.0293

Reference 9.

Table 3. Hydrogen Bonds for ETN and PETN bonds C−H···O, tert C−H···O, sec a

ETN (C···O, Å) 2.8433, 2.8435 3.1799, 3.2483

PETN (C···O, Å)a

ETN (C−H···O angle, deg)

Figure 3. Crystal packing in ETN viewed down the b axis. The ETN molecules are linked into sheets in the bc-plane via a network of hydrogen bonding and closed shell interactions; a representative sheet of ETN molecules is colored gray in the top pane. The closed shell network of N−O and O−O bonds are shown in the top pane, and the hydrogen bonding network of C−H−O hydrogen bonds is shown in the bottom pane. A weaker intermolecular bonding network serves to link the sheets together in stacks along the a-axis.

PETN (C−H··· O angle, deg)a

100.00 3.4100, 3.4217

124.0, 125.0

149.0, 179.0

Reference 9.

materials. In Figure 3 it can be seen that there is some anisotropy in the hydrogen bonding network along the a-axis and that a pseudosheet-like structure is observed. Important data collection and refinement parameters are given in Table 4. Powder X-ray Diffraction. Powder X-ray diffraction was performed for samples of ETN prepared using different purification methods: ETN was dissolved in acetone/ethanol and allowed to evaporate over the course of a day (Acetone/

The O···O closed shell contact distances are significantly stronger in ETN than in PETN, while the opposite is true of the O···N distances. The only mechanism for hydrogen bonding for these structures is weak C−H···O hydrogen bonds. ETN has both secondary and tertiary C−H donors, while PETN has only secondary C−H donors. In all cases these hydrogen donors pair up with oxygen acceptors. For PETN the 6156

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be more susceptible to early deformation under pressure than the more three-dimensional PETN network. Atomic Force Microscopy. Atomic force microscopy (AFM) was used to examine the surfaces of the particles of ETN crystallized from methanol and heated ethanol. For both of these types of ETN samples, the AFM images showed stepped surfaces with smooth terraces and some debris that may be ETN but which cannot be identified. One example is shown in Figure 5 (inset), which is a 4 μm by 4 μm image of a

Table 4. Crystallographic Data for ETN and PETN

a

crystal data

ETN

PETNa

formula temperature (K) MW (g mol−1) crystal system space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z μ (mm−1) ρcalc (g cm−3) R (I > 2s) Rw GOF

C4H6N4O12 140(1) 302.13 monoclinic P21/c 15.893(6) 5.1595(19) 14.731(5) 116.161(3) 1084.2(7) 4 0.19 1.851 0.0357 0.0426 1.210

C5H8O12N4 100 (1) 316.14 tetragonal P4̅21c 9.2759(8) 6.6127(4) 568.97 2 0.19 1.845 0.0104 0.0125 1.168

ref 9.

EtOH), dissolved in acetonitrile and layered with water for slow diffusion (MeCN/H2O), dissolved in acetonitrile and precipitated quickly in excess water (MeCN/H2O/shake), or dissolved in heated ethanol followed by cooling over a period of a few hours (heated EtOH). The powder X-ray diffraction patterns are shown in Figure 4. All peaks in the observed

Figure 5. Inset: 4 μm by 4 μm AFM image of the surface of an ETN particle with terrace morphology. Graph: Height versus surface length for a 5 μm surface section taken from lower left to upper right in the inset. The section captures the step heights of four steps on the surface. Each appears to be about 15 Å tall (corresponding to the 15.893 Å a-lattice constant).

particle from the sample recrystallized from heated ethanol. The direction of the steps, the density of the steps, and correspondingly the width of the terraces, varied randomly with position on all particles. Very flat or very stepped regions could be found easily by positioning the tip in smoother or rougher areas, respectively, as observed through the CP-II optical microscope. The smallest step height routinely measured in the AFM images was approximately 15 Å. Figure 5 shows a graph of height versus surface length for a 5 μm surface section (taken diagonally from lower left to upper right of the AFM image shown in the figure inset). Each of the four steps is about 15 Å tall, which corresponds to the 100 direction (Figure 4; 15.893 Å a-lattice constant). Sensitivity Testing. Sensitivity testing, or safety testing, is necessary to determine conditions under which an explosive may be handled safely. Small-scale sensitivity testing has been performed to determine material response to various stimuli including impact, friction, and static spark. These tests provide parameters for safety in handling, but results may vary from one laboratory to another and reported values should thus be considered approximate. PETN values are included with the reported data to aid in comparing results to an accepted standard material used in other laboratories. Table 5 shows the results of sensitivity testing on six samples of ETN, and Figure 6 shows the larger crystals grown from slow evaporation of methanol. All samples have significantly greater impact sensitivity than PETN (measured ETN DH50 values average to 6.1 ± 2 cm), consistent with a fairly sensitive explosive. Friction sensitivity for ETN is higher than PETN (average 57 ± 14; Table 5), but still within the measurement uncertainty. The measured values for impact, spark, and friction for all ETN samples fell within the limits of instrument uncertainty, which is

Figure 4. Powder X-ray diffraction pattern of ETN. The blue sticks at the bottom of the pattern were calculated from the single crystal X-ray structure.

patterns match those of ETN calculated from the single crystal structure. A high degree of preferred orientation is observed in the heated EtOH sample, which is caused by the plate-like morphology. From the heated ethanol pattern it can be seen that platelet faces are normal to the 100 direction in the crystal. X-ray crystallography and powder diffraction collectively indicate that the morphology of the crystals does not appear to change under different preparation conditions. The ETN structure shows a pseudosheet-like structure, with intermolecular bonds being more plentiful within the sheets than between. This sheet like structure is consistent with the crystalline plates observed under the heated ethanol crystallization conditions, and the faces of the plates do indeed correspond to the bc-plane of the unit cell. This weaker bonding between sheets may also 6157

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Table 5. Results of Sensitivity Testing for ETN Prepared from Various Methods

a

Temperature at melt, and onset and peak decomposition (°C). bGreen bar inset is 500 μm.

Figure 6. ETN crystals grown from slow evaporation in a methanol solution.

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fine powders. Generally, it is assumed that fine explosive powders are less sensitive to mechanical input than large crystals of the same material.23 This study shows that in the case of ETN, when purity and crystal polymorphs are controlled, the particle size of the explosive does not affect its handling sensitivity.

consistent with the fact that all samples are the same polymorph. ETN has a low melting point, which allows for melt-casting of the material in explosives applications. Differential scanning calorimetry measurements (Table 5) indicate that ETN melts at ∼62 °C, significantly lower than the decomposition temperature, as well as the temperature of the PETN melt (∼141 °C). The observed weaker bonding between ETN sheets in the crystal structure (which may allow for the higher sensitivity of ETN relative to PETN) may account for the low melting point of ETN as well.18 The difference in sensitivity between ETN and PETN is unlikely to be significantly influenced by the type of the nitrate ester in each molecule. In general, chemical stability is decreased in secondary nitrate esters (ETN) relative to primary nitrate esters (PETN), and critical temperatures for thermal decomposition are lower for molecules with secondary nitrate esters.5 However, activation energies for decomposition are essentially the same for primary and secondary nitrate esters,5 and sensitivities of liquids containing nitrate esters are independent of primary or secondary nitrate groups.19 Therefore, the higher handling sensitivity of ETN is likely due to crystal packing or other chemical factors such as the slightly positive oxygen balance of ETN versus the slightly negative oxygen balance of PETN.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed cif files. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(V.W.M.) E-mail: [email protected]. *(B.C.T.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Los Alamos National Laboratory is operated by LANS, LLC, for the U.S. Department of Energy under contract DE-AC5206NA25396. The authors acknowledge Campaign 2 (LANL) for support of this work, Stephanie Hagelberg for elemental analysis, Mary Sandstrom for DSC measurements, Kelly L. Parker for the Table of Contents graphic design, G. Kenneth Windler for help with 14N NMR measurements, and Kevin Fleming (Trinity Scientific) for helpful discussions.

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CONCLUSIONS In explosives, it has been proposed that intermolecular interactions (such as O···O and O···H weak interactions) influence sensitivity properties.9 For example, rupture of these weak interactions has been proposed to destabilize the molecule and facilitate initiation of the explosive. Erythritol tetranitrate (ETN) is a powerful high explosive that has undergone recent interest due to its ease of preparation and melt-castable properties. We have prepared crystals of ETN, and for the first time, collected X-ray crystallography information. ETN has been shown to be somewhat more sensitive to impact and friction than PETN, with similar sensitivity to spark initiation (Table 5). The crystal structure suggests that the molecular structure of ETN is more strained, as evidenced by distorted C−C−C and O−C−C angles, when compared to PETN. The hydrogen-bonding network also shows very weak and possibly even destabilizing structures in the ETN crystals. The presence of strong hydrogen bonding has often been correlated with lower sensitivity in energetic materials, and conversely, the lack thereof has been correlated with higher sensitivities.18,20 The increased strain in the ETN molecular structure and crystal packing may be a contributing factor in its higher observed handling sensitivity, although the effect is also likely concomitant with other molecular differences (such as the small difference in oxygen balance between ETN and PETN) as well as the above-mentioned very weak hydrogen-bonding. In PETN, higher shock sensitivity has been observed in crystal planes where the atoms cannot slip past each other easily during impact (the shortest run to detonation occurred when slip was induced on the {100} planes with the most steric hindrance).21 The ETN crystal network clearly shows a sheet-like structure that may result in similar directionality of shock sensitivity as PETN.22 The same polymorph of ETN was obtained using a variety of preparation and crystallization conditions. Sensitivity testing was performed on all samples, and the larger crystal samples exhibit the same impact, spark, and friction sensitivity as the

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