Pi-stacked interactions in explosive crystals: buffers against external mechanical stimuli

The pi-stacked interactions in some explosive crystal packing are discussed. Taking a typical pi-stacked explosive 2,4,6-trinitrobenzene-1,3,5-triamine (TATB) as a sample and using molecular simulations, we investigated the nature of the pi-stacked interactions versus the external mechanical stimuli causing possible slide and compression of explosives. As a result, between the neighbor layers in the TATB unit cell, the electrostatic attraction decreases with a little decrease of vdW attraction when its top layer slides, whereas the vdW attraction increases with a decrease of electrostatic attraction when TATB crystal is compressed along its c axis. Meanwhile, we studied the correlation between the pi-stacked structures and the impact sensitivities of explosives by means of three representatives including TATB with typical planar pi-stacked structures, 2,2-dinitroethylene-1,1-diamine (Fox-7) with wavelike pi-stacked structures, and 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX) without pi-stacked structure. The results showed that pi-stacked structures, particularly planar layers, can effectively buffer against external mechanical stimuli. That is, pi-stacked structures can partly convert the mechanical energy acting on them into their intermolecular interaction energy, to avoid the increase of the molecular vibration resulting in the explosive decomposition, the formation of hot spots, and the final detonation. This is another reason for the low mechanical sensitivity of pi-stacked explosives besides their stable conjugated molecular structures.     

The very far-infrared spectra of energetic materials and possible confusion materials using terahertz pulsed spectroscopy

We have measured the terahertz absorption spectra of 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), pentaerythritol tetranitrate (PETN), 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane (HMX), 2,4,6-trinitrotoluene (TNT), the plastic explosives Semtex H, SX2, and Metabel, and a number of confusion materials using terahertz pulsed transmission spectroscopy. Spectral fingerprints were obtained from 3 to 133 cm⁻¹. The spectra of the plastic explosives are dominated by the spectral signatures of their explosive components due to low frequency vibrations and crystalline phonon modes. Importantly, the terahertz spectra of the confusion materials show no resemblance to the explosives spectra. The refractive indices obtained for the plastic explosives and confusion materials allowed us to derive reflectance spectra, which appear distinct and so suggest that terahertz reflection spectroscopy is a suitable tool for the detection of concealed explosives in security applications.     

Theoretical studies of energy transfer rates of secondary explosives

Understanding the mechanism of shock-induced chemical reaction in secondary explosives is necessary to pursue the development and the safe use of new explosives having high performance and low sensitivity. In an effort to understand the mechanism, the energy transfer rates of such secondary explosives as PETN(I), PETN(II), delta-HMX, alpha-HMX, beta-HMX, RDX, ANTA, DMN, and NM have been evaluated based on the formula derived by Fried and Ruggiero [Fried, L. E.; Ruggiero, A. J. J. Phys. Chem. 1994, 98, 9786]. The energy transfer rates were determined in terms of the density of vibrational states and the unharmonic vibron-phonon coupling term, which were calculated by using a flexible potential containing both intra- and intermolecular terms. For the secondary explosives, a good correlation was found between the energy transfer rates and the impact sensitivity. The energy transfer rates are several times faster for the explosives with higher sensitivity such as PETN, HMX, and RDX than those with lower sensitivity such as ANTA, DMN, and NM. The calculations presented suggest the energy transfer rate in secondary explosive crystals is a significant factor in their sensitivity and introduction of double bond, or hydrogen bonds, or caged structure into secondary explosives is expected to decrease the sensitivity.     

Note on the use of the vacuum stability test in the study of initiation reactivity of attractive cyclic nitramines in Formex P1 matrix

The zero-order reaction rates (specific rate constants) of isothermal decomposition at 120 °C of plastic bonded explosives (PBXs) were measured by means of the Czech vacuum stability test, STABIL. The PBXs are based on 1,3,5-trinitro-1,3,5-triazinane (RDX), 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX), cis-1,3,4,6-tetranitro-octahydroimidazo-[4,5-d]imidazole (BCHMX), and ε 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (ε-HNIW, ε-CL-20) with 13 wt% of the Formex P1 type matrix, i.e., a matrix of the explosive with pentaerythritol tetranitrate (PETN) bound by 13 wt% of a mixture of 25 wt% of styrene–butadiene rubber and 75 wt% of an oily material. Dependencies were found between the specific rate constants mentioned and the detonation velocities of PBXs, and consequently between these constants and the impact and electric spark sensitivities of pure explosive fillers, i.e., RDX, HMX, HNIW, BCHMX, and PETN. It is stated that the higher impact or electric spark sensitivity of their pure explosive fillers corresponds to the higher thermal reactivity of the given PBXs.     

A systematic tandem mass spectrometric study of anion attachment for improved detection and acidity evaluation of nitrogen‐rich energetic compounds

The development of rapid, efficient, and reliable detection methods for the characterization of energetic compounds is of high importance to security forces concerned with terrorist threats. With a mass spectrometric approach, characteristic ions can be produced by attaching anions to analyte molecules in the negative ion mode of electrospray ionization mass spectrometry (ESI‐MS). Under optimized conditions, formed anionic adducts can be detected with higher sensitivities as compared with the deprotonated molecules. Fundamental aspects pertaining to the formation of anionic adducts of 1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocane (HMX), 1,3,5‐trinitro‐1,3,5‐triazinane (RDX), pentaerythritol tetranitrate (PETN), nitroglycerin (NG), and 1,3,5‐trinitroso‐1,3,5‐triazinane energetic (R‐salt) compounds using various anions have been systematically studied by ESI‐MS and ESI tandem mass spectrometry (collision‐induced dissociation) experiments. Bracketing method results show that the gas‐phase acidities of PETN, RDX, and HMX fall between those of HF and acetic acid. Moreover, PETN and RDX are each less acidic than HMX in the gas phase. Nitroglycerin was found to be the most acidic among the nitrogen‐rich explosives studied. The ensemble of bracketing results allows the construction of the following ranking of gas‐phase acidities: PETN (1530‐1458 kJ/mol) > RDX (approximately 1458 kJ/mol) > HMX (approximately 1433 kJ/mol) > nitroglycerin (1427‐1327.8 kJ/mol).     

Fabric analysis by ambient mass spectrometry for explosives and drugs

Desorption electrospray ionization (DESI) is applied to the rapid, in-situ, direct qualitative and quantitative analysis of mixtures of explosives and drugs from a variety of fabrics, including cotton, silk, denim, polyester, rayon, spandex, leather and their blends. The compounds analyzed were explosives: trinitrohexahydro-1,3,5-triazine (RDX), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), 2,4,6-trinitrotoluene (TNT), pentaerythritol tetranitrate (PETN) and the drugs of abuse: heroin, cocaine, and methamphetamine. Limits of detection are in the picogram range. DESI analyses were performed without sample preparation and carried out in the presence of common interfering chemical matrices, such as insect repellant, urine, and topical lotions. Spatial and depth profiling was investigated to examine the depth of penetration and lateral resolution. DESI was also used to examine cotton transfer swabs used for travel security sample collection in the screening process. High throughput quantitative analysis of fabric surfaces for targeted analytes is also reported.     

Micellar extraction and high performance liquid chromatography-ultra violet determination of some explosives in water samples

An analytical method based on the cloud point extraction combined with high performance liquid chromatography is used for the extraction, separation and determination of four explosives; octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazine (HMX), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), 2,4,6-trinitrotoluene (TNT) and pentaerythritol tetranitrate (PETN). These compounds are extracted by using of Triton X-114 and cetyl-trimethyl ammonium bromide (CTAB). After extraction, the samples were analyzed using a HPLC-UV system. The parameters affecting extraction efficiency (such as Triton X-114 and CTAB concentrations, amount of Na₂SO₄, temperature, incubation and centrifuge times) were evaluated and optimized. Under the optimum conditions, the preconcentration factor was 40 and the improvement factors of 34, 29, 61 and 42 with detection limits of 0.09, 0.14, 0.08 and 0.40 (μg L⁻¹) were obtained for HMX, RDX, TNT and PETN, respectively. The proposed method was successfully applied to the determination of these compounds in water samples and showed recovery percentages of 97-102% with RSD values of 2.13-4.92%.     

The force-field derivation and atomistic simulation of HMX–fluoropolymer mixture explosives

We get ab initio-based force field between octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and fluoropolymer. The HMX is a high-energy explosive, and fluoropolymer is a binder. By using this force field, the mechanical properties of mixture explosives are investigated. Nine kinds of polymers are considered: polyvinylidene fluoride, polychlorotrifluoroethene, polytetrafluoroethene, polyhexafluoropropene, F2311, F2312, F2313, F2314, and Viton-A. The deformation processes of explosives are simulated, the structure evolution and energy variation are calculated, and the coating and plasticizing properties of binders to HMX are obtained.     

Optimized thermal desorption for improved sensitivity in trace explosives detection by ion mobility spectrometry

In this work we evaluate the influence of thermal desorber temperature on the analytical response of a swipe-based thermal desorption ion mobility spectrometer (IMS) for detection of trace explosives. IMS response for several common high explosives ranging from 0.1 ng to 100 ng was measured over a thermal desorber temperature range from 60 °C to 280 °C. Most of the explosives examined demonstrated a well-defined maximum IMS signal response at a temperature slightly below the melting point. Optimal temperatures, giving the highest IMS peak intensity, were 80 °C for trinitrotoluene (TNT), 100 °C for pentaerythritol tetranitrate (PETN), 160 °C for cyclotrimethylenetrinitramine (RDX) and 200 °C for cyclotetramethylenetetranitramine (HMX). By modifying the desorber temperature, we were able to increase cumulative IMS signal by a factor of 5 for TNT and HMX, and by a factor of 10 for RDX and PETN. Similar signal enhancements were observed for the same compounds formulated as plastic-bonded explosives (Composition 4 (C-4), Detasheet, and Semtex). In addition, mixtures of the explosives exhibited similar enhancements in analyte peak intensities. The increases in sensitivity were obtained at the expense of increased analysis times of up to 20 seconds. A slow sample heating rate as well as slower vapor-phase analyte introduction rate caused by low-temperature desorption enhanced the analytical sensitivity of individual explosives, plastic-bonded explosives, and explosives mixtures by IMS. Several possible mechanisms that can affect IMS signal response were investigated such as thermal degradation of the analytes, ionization efficiency, competitive ionization from background, and aerosol emission.     

Radiation-induced decomposition of PETN and TATB under extreme conditions

We conducted a series of experiments investigating decomposition of secondary explosives PETN and TATB at varying static pressures and temperatures using synchrotron radiation. As seen in our earlier work, the decomposition rate of TATB at ambient temperature slows systematically with increasing pressure up to at least 26 GPa but varies little with pressure in PETN at ambient temperature up to 15.7 GPa, yielding important information pertaining to the activation complex volume in both cases. We also investigated the radiation-induced decomposition rate as a function of temperature at ambient pressure and 26 GPa for TATB up to 403 K, observing that the decomposition rate increases with increasing temperature as expected. The activation energy for the TATB reaction at ambient temperature was experimentally determined to be 16 +/- 3 kJ/mol.     

环四甲撑四硝胺/1,3,5-三氨基-2,4,6-三硝基苯共晶炸药的分子模拟研究

构建环四甲撑四硝胺 (HMX) /1,3,5-三氨基-2,4,6三硝基苯 (TATB)不同的共晶结构模型,用分子动力学(MD)模拟得到其平衡结构。基于平衡结构进行X射线粉末衍射（XRD）图谱模拟和能量计算。结果表明,与纯组分相比,HMX/TATB共晶结构的X射线粉末衍射图与主成分HMX相似,并均有新峰出现;TATB在HMX表面自由能最高、生长速率最慢的 (0 1 1) 晶面上发生取代后的能量最低,结构最稳定,据此推测在制备HMX/TATB共晶炸药过程中,TATB分子更容易进入HMX自由能高的晶面,得到结构稳定的共晶而使HMX变得更为钝感。     

Temperature-dependent far-infrared spectra of single crystals of high explosives using terahertz time-domain spectroscopy

Survey spectra of single-crystal HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine), and PETN (pentaerythritol tetranitrate) were acquired in the region from 10 to 80 cm(-1) using terahertz time-domain spectroscopy. The spectra were taken at temperatures ranging from 8.4 to 300 K. Generally, the spectra show multiple absorption peaks in the range 50-80 cm(-1), with PETN (110) showing strong absorption features at room temperature. RDX (210) is the most notable in the region 10-40 cm(-1), showing multiple spectral features, while HMX (010) shows a very broad absorption at 47.8 cm(-1) with a fwhm of 37.3 cm(-1). Future plans include polarization-dependent investigations for multiple crystallographic orientations over an increased spectral range and higher-level theoretical calculations.     

Detection of explosives on skin using ambient ionization mass spectrometry

Single nanogram amounts of the explosives TNT, RDX, HMX, PETN and their mixtures were detected and identified in a few seconds on the surface of human skin without any sample preparation by desorption electrospray ionization (DESI) using a spray solution of methanol-water doped with sodium chloride to form the chloride adducts with RDX, HMX, and PETN while TNT was examined as the radical anion and tandem mass spectrometry was used to confirm the identifications.     

Initial chemical events in shocked octahydro-1,3,5,7-tetranitro-1,3,5,7- tetrazocine: a new initiation decomposition mechanism

We have performed ab initio molecular dynamics simulations in conjunction with the multiscale shock technique to study the initial chemical processes of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) under shock wave loading. The results show that the initial decomposition of shocked HMX is triggered by the N-O bond breaking and the ring opening. This will initiate many decomposition reactions and lead to the production of many small radicals at a moment. As the shock compression continues, these small radicals recombine to produce many large radicals and further form ring-shaped radicals. Then, these radicals begin to further decompose. It is also found that the system transiently produces a large number of metallic states under the shock compression. Our simulations thus suggest a new mechanism for the initial chemical processes of shocked HMX and provide fundamental insight into the initial mechanism at the atomistic level, which is of important implication for understanding and development of energetic materials.     

环四甲撑四硝胺(HMX)结构和性质的DFT研究

用密度泛函理论(DFT)B3LYP方法,取6-31G基组,求得环四甲撑四硝胺分子的几何构型、电子结构、 IR谱和298～1200 K的热力学性质.全优化几何构型和电子结构均具有Ci对称性.在相邻原子之间以N-NO2键的Mulliken集居数最小,表明其间电子分布较少,预示其为热解和起爆的引发键.IR谱与实验结果良好相符.     

Direct identification of HMX via guest-induced fluorescence turn-on of molecular cage

Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) is one of the most widely used powerful explosives. The direct and selective detection of HMX, without the requirement of specialized equipment, remains a great challenge due to its extremely low volatility, unfavorable reduction potential and lack of aromatic rings. Here, we report the first chemical probe of direct identification of HMX at ppb sensitivity based on a designed metal-organic cage (MOC). The cage features two unsaturated dicopper units and four electron donating amino groups inside the cavity, providing multiple binding sites to selectively enhance host-guest events. It was found that compared to other explosive molecules the capture of HMX inside the cavity would strongly modulate the emissive behavior of the host cage, resulting in highly induced fluorescence “turn-on” (160 folds). Based on the density functional theory (DFT) simulation, the mutual fit of both size and binding sites between host and guest leads to the synergistic effects that perturb the ligand-to-metal charge-transfer (LMCT) process, which is probably the origin of such selective HMX-induced turn-on behavior.     

Predicting the shock sensitivity of cyclotrimethylene-trinitramine

We studied the surface and thermal properties of seven different varieties of cyclotrimethylene-trinitramine (RDX) crystalline explosives from five manufacturers using differential scanning calorimetry (DSC) and atomic force microscopy (AFM). The specific varieties of the RDX studied were acquired from the previous Reduced Sensitivity RDX Round Robin program. They were chosen because intensive characterization of the materials already existed including shock sensitivity and cyclotetramethylene-tetranitramine (HMX) impurity levels. AFM scans revealed a diversity of surface defects. To quantify the surface defects on the crystalline surface of the RDX particles, surface roughness measurements were acquired. Statistical analysis was undertaken to correlate the observed surface, HMX impurity levels, and DSC thermal curve properties with the known shock sensitivities of the material. It was determined that a statistically significant relationship exists between surface roughness and the shock sensitivity of the material while no relationship was observed between the DSC thermal properties and either surface roughness or shock sensitivity. The HMX content greatly affected the thermal properties of RDX but was uncorrelated with the shock sensitivity.     

Raman spectra of shock compressed pentaerythritol tetranitrate single crystals: anisotropic response

To gain insight into the anisotropic sensitivity of shocked pentaerythritol tetranitrate (PETN) single crystals, single-pulse Raman spectroscopy was used to examine the response of crystals shocked along the [100] (insensitive) and [110] (sensitive) orientations. High-resolution Raman spectra revealed several orientation-dependent features under shock compression: (i) substantially different stress dependence of the Raman shift for the CH(2) and NO(2) stretching modes for the two orientations, (ii) discontinuity in the stress dependence of the Raman shift for the CH(2) stretching modes above 4 GPa for the [110] orientation, and (iii) large broadening for the CH(2) and NO(2) asymmetric stretching modes for stresses above 4 GPa for the [110] orientation. The present data in combination with previous static pressure results provide support for conformational changes in PETN molecules for shock compression along the [110] (sensitive) orientation. Implications of the present results for the anisotropic sensitivity of shocked PETN are discussed.     

Low-mass ions observed in plasma desorption mass spectrometry of high explosives

The low-mass ions observed in both positive and negative plasma desorption mass spectrometry (PDMS) of the high explosives HMX, RDX, CL-20, NC, PETN and TNT are reported. Possible identities of the most abundant ions are suggested and their presence or absence in the different spectra is related to the properties of the explosives as matrices in PDMS. The detection of abundant NO+ and NO2- ions for HMX, RDX and CL-20, which are efficient matrices, indicates that explosive decomposition takes place in PDMS of these three substances and that a contribution from the corresponding chemical energy release is possible. The observation of abundant C2H4N+ and CH2N+ ions, which have high protonation properties, might also explain the higher protein charge states observed with these matrices. Also, the observation of NO2-, possibly formed by electron scavenging which increases the survival probability of positively charged protein molecular ions, completes the pattern. TNT does not give any of these ions and it is thereby possible to explain why it does not work as a PDMS matrix. For NC and PETN, decomposition does not seem to be as pronounced as for HMX, RDX and CL-20, and also no particularly abundant ions with high protonation properties are observed. The fact that NC works well as a matrix might be related to other properties of this compound, such as its high adsorption ability.     

Intermolecular interactions,thermodynamic properties,crystal structure,and detonation performance of HMX/NTO cocrystal explosive

Previous studies have shown that the design of cocrystal explosives is one of the most promising approaches to decrease the sensitivity and maintain the detonation performance of existing explosives. As is well‐known, octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine (HMX) is a high energy density material (HEDM). But the application of HMX is limited, due to its high sensitivity. Thus, an insensitive explosive 5‐nitro‐1,2,4‐triazol‐3‐one (NTO) is proposed as a cocrystal former (CCF) to cocrystallize with HMX in the present work. The binding energies, heat of formations (HOFs), thermodynamic properties, atoms in molecules, and natural bond orbital analysis of four HMX/NTO complexes have been calculated using density functional theory methods, including meta‐hybrid functional (M062X) and dispersion‐corrected density functionals (B97D, ωB97XD). In addition, the crystal structure of HMX/NTO cocrystal has been investigated using Monte Carlo simulation and first principles methods. The HMX/NTO cocrystal is most likely to crystallize in triclinic crystal system with P1 space group, and corresponding cell parameters are Z = 2, a = 9.06 Å, b = 8.19 Å, c = 10.27 Å, α = 81.94^°, β = 98.42^°, γ = 82.03^°, and ρ = 1.92 g/cm³. The detonation velocity and detonation pressure of HMX/NTO cocrystal are 8.73 km/s and 35.14 GPa, respectively, a little lower than those of HMX. Finally, bond dissociation energies (BDEs) of the weakest trigger bond in HMX/NTO complexes are calculated. The results show that HMX/NTO complexes are thermally stable and meet the thermal requirement of HEDMs (BDE > 120 kJ/mol). © 2012 Wiley Periodicals, Inc.