Identifying the Molecular Properties that Drive Explosive Sensitivity in a Series of Nitrate Esters.

Energetic materials undergo hundreds of chemical reactions during exothermic runaway, generally beginning with the breaking of the weakest chemical bond, the "trigger linkage." Herein we report the syntheses of a series of pentaerythritol tetranitrate (PETN) derivatives in which the energetic nitrate ester groups are systematically substituted by hydroxyl groups. Because all the PETN derivatives have the same nitrate ester-based trigger linkages, quantum molecular dynamics (QMD) simulations show very similar Arrhenius kinetics for the first reactions. However, handling sensitivity testing conducted using drop weight impact indicates that sensitivity decreases precipitously as nitrate esters are replaced by hydroxyl groups. These experimental results are supported by QMD simulations that show systematic decreases in the final temperatures of the products and the energy release as the nitrate ester functional groups are removed. To better interpret these results, we derive a simple model based only on the specific enthalpy of explosion and the kinetics of trigger linkage rupture that accounts qualitatively for the decrease in sensitivity as nitrate ester groups are removed.

Mass spectrometry of the soot left after ethylene oxide explosion answers some questions on the crash of Polish Air Force Flight 101.

The Polish TU 154M plane, Polish Air Force Flight 101, had crashed near Smolensk on 10th of April 2010. The crash was investigated by The Interstate Aviation Committee, whose conclusions were questioned by a number of Polish scientists. The cause of the crash still appears to be incompletely documented and requires additional evidence. In this paper, investigations of a solid material eluted from a piece of cloth of one of the victims of the crash are described. High resolution mass spectrometry was applied to analyze the soot left after controlled ethylene oxide (EO) explosions, performed under different conditions. These included electric ignition of EO vapors in a large volume steel container, and explosions of glass tubes filled with liquid EO, stimulated by thermally initiated explosions of pentaerythritol tetranitrate (PETN). One of these explosions was conducted in the vessel used for the electric ignition of EO and the other in a hermetically locked, small volume container. It was shown that the soot comprises a set of C2 H4 O homopolymers and copolymers whose characteristic MS patterns are condition-dependent. The MS spectrum of the postcrash sample referred to above reveals a number of polymers that are also present in the soot obtained in PETN-initiated ethylene oxide explosions. It can be concluded that the piece of cloth was subjected to an EO explosion initiated by an explosion of energetic material, possibly PETN. Similar control experiments with ethylene glycol (EG) showed that the polymers identified in the investigated postcrash sample could not originate from exploding EG.

Selective colorimetric detection of pentaerythritol tetranitrate (PETN) using arginine-mediated aggregation of gold nanoparticles.

Detection of pentaerythritol tetranitrate (PETN) as an explosive has been of great interest because of public safety and military concerns. Here, we have presented a simple, selective and sensitive colorimetric method for direct detection of PETN. The gold nanoparticles (AuNPs) were first exposed to arginine which has primary amines in its structure. Electron deficient -NH2 groups from arginine could strongly interact with -NO2 groups of PETN as electron donors. Hydrogen bonding happens between the -NO2 group of PETN and -NH2 group of arginine molecules. Therefore, selective aggregation of AuNPs happened because of the donor-acceptor and hydrogen bonding interactions. Due to the aggregation, the color of reddish AuNPs turned to blue or purple depend on PETN concentration. A good linear relationship was achieved between the aggregation signal (absorbance ratio of A650/A520) of the probe and the concentration of PETN with a limit of detection of 0.169 µmol L-1. Furthermore, we have found that the developed probe can detect PETN in complex matrices of groundwater and soil samples.

Energetic Metal and Nitrogen-Rich Salts of the Pentaerythritol Tetranitrate Analogue Pentaerythritol Tetranitrocarbamate.

The tetravalent pentaerythritol tetranitrocarbamate (PETNC) is deprotonated by nitrogen-rich, alkaline, alkaline earth metal, and silver bases to form the corresponding salts. Thorough analysis and characterization by multinuclear NMR, vibrational spectroscopy, elemental analysis, thermoanalytical techniques, and single crystal X-ray diffraction was performed. Furthermore, the energies of formation for the nitrogen-rich salts were calculated utilizing the Gaussian program package. The detonation performances were calculated with the Explo5 (V6.03) computer code, and the sensitivities toward impact and friction were determined and compared to the neutral PETNC and pentaerythritol tetranitrate (PETN). Ecotoxicological studies of the ammonium and guanidinium salt using Vibrio fischeri bacteria complete this study.

Identification of peptide sequences that selectively bind to pentaerythritol trinitrate hemisuccinate-a surrogate of PETN, via phage display technology.

The present research investigates the identification of amino acid sequences that selectively bind to a pentaerythritol tetranitrate (PETN) explosive surrogate. Through the use of a phage display technique and enzyme-linked immunosorbent assays (ELISA), a peptide library was tested against pentaerythritol trinitrate hemisuccinate (PETNH), a surrogate of PETN, to screen for those with amino acids having affinity toward the explosive. The results suggest that the library contains peptides selective to PETNH. Following three rounds of panning, clones were picked and tested for specificity toward PETNH. ELISA results from these samples show that each phage clone has some level of selectivity for binding to PETNH. The peptides from these clones have been sequenced and shown to contain certain common amino acid segments among them. This work represents a technological platform for identifying amino-acid sequences selective toward any bio-chem analyte of interest.

Photochemistry of the α-Al2O3-PETN Interface.

Optical absorption measurements are combined with electronic structure calculations to explore photochemistry of an α-Al2O3-PETN interface formed by a nitroester (pentaerythritol tetranitrate, PETN, C5H8N4O12) and a wide band gap aluminum oxide (α-Al2O3) substrate. The first principles modeling is used to deconstruct and interpret the α-Al2O3-PETN absorption spectrum that has distinct peaks attributed to surface F°-centers and surface-PETN transitions. We predict the low energy α-Al2O3 F°-center-PETN transition, producing the excited triplet state, and α-Al2O3 F°-center-PETN charge transfer, generating the PETN anion radical. This implies that irradiation by commonly used lasers can easily initiate photodecomposition of both excited and charged PETN at the interface. The feasible mechanism of the photodecomposition is proposed.

Observations and sources of carbon and nitrogen isotope ratio variation of pentaerythritol tetranitrate (PETN).

Isotope ratio analysis allows forensic investigators to discriminate materials that are chemically identical but differ in their isotope ratios. Here we focused on the discrimination of pentaerythritol tetranitrate (PETN), an explosive with military and civilian applications, using carbon (δ(13)C) and nitrogen (δ(15)N) isotope ratios. Our goal was to understand some of the factors influencing the isotope ratios of commercially manufactured PETN. PETN was isolated from bulk explosives using preparative HPLC, which reduced chemical and isotopic within-sample variability. We observed isotope ratio variation in a survey of 175 PETN samples from 22 manufacturing facilities, with δ(13)C values ranging from -49.7‰ to -28.0‰ and δ(15)N values ranging from -48.6‰ to +6.2‰. Both within-sample variability and variation of PETN within an explosive block were much smaller than between-sample variations. Isotopic ratios of PETN were shown to discriminate explosive blocks from the same manufacturer, whereas explosive component composition measurements by HPLC were not able to do so. Using samples collected from three industrial PETN manufacturers, we investigated the isotopic relationship between PETN and its reactants, pentaerythritol (PE) and nitric acid. Our observations showed that δ(13)C values of PETN were indistinguishable from that of the reactant pentaerythritol. Isotopic separation between nitric acid and PETN was consistent within each sampled manufacturer but differed among manufacturers, and was likely dependent upon reaction conditions. These data indicate that δ(13)C variation in PETN is dependent on δ(13)C variation of PE supplies, while δ(15)N variation in PETN is due to both nitric acid δ(15)N and reaction conditions.

Accurate quantitation of pentaerythritol tetranitrate and its degradation products using liquid chromatography-atmospheric pressure chemical ionization-mass spectrometry.

After an explosion of pentaerythritol tetranitrate (PETN), its degradation products pentaerythritol trinitrate (PETriN), dinitrate (PEDiN) and mononitrate (PEMN) were detected using liquid chromatography-atmospheric-pressure chemical-ionization-mass spectrometry (LC-APCI-MS). Discrimination between post-explosion and naturally degraded PETN could be achieved based on the relative amounts of the degradation products. This information can be used as evidence when investigating a possible relationship between a suspect and a post-explosion crime scene. The present work focuses on accurate quantitation of PETN and its degradation products, using PETriN, PEDiN and PEMN standards specifically synthesized for this purpose. With the use of these standards, the ionization behavior of these compounds was studied, and a quantitative method was developed. Quantitation of PETN and trace levels of its degradation products was shown to be possible with accuracy between 85.7% and 103.7% and a precision ranging from 1.3% to 11.5%. The custom-made standards resulted in a more robust and reliable method to discriminate between post-explosion and naturally-degraded PETN.

Tip induced crystallization lithography.

We demonstrate a new technique for efficiently fabricating large-area organic crystal arrays on substrates using tip induced crystallization lithography (TICL). This technique depends on coating an amorphous organic thin film on a substrate and then inducing crystallization of the thin film using an atomic force microscope tip. After the noncrystalline materials are removed from the substrate by heating or washing, the organic crystal arrays are stable on the substrate. In this communication, the size of the smallest feature made using TICL technique is less than 1 µm.

Molecular recognition using receptor-free nanomechanical infrared spectroscopy based on a quantum cascade laser.

Speciation of complex mixtures of trace explosives presents a formidable challenge for sensors that rely on chemoselective interfaces due to the unspecific nature of weak intermolecular interactions. Nanomechanical infrared (IR) spectroscopy provides higher selectivity in molecular detection without using chemoselective interfaces by measuring the photothermal effect of adsorbed molecules on a thermally sensitive microcantilever. In addition, unlike conventional IR spectroscopy, the detection sensitivity is drastically enhanced by increasing the IR laser power, since the photothermal signal comes from the absorption of IR photons and nonradiative decay processes. By using a broadly tunable quantum cascade laser for the resonant excitation of molecules, we increased the detection sensitivity by one order of magnitude compared to the use of a conventional IR monochromator. Here, we demonstrate the successful speciation and quantification of picogram levels of ternary mixtures of similar explosives (trinitrotoluene (TNT), cyclotrimethylene trinitramine (RDX), and pentaerythritol tetranitrate (PETN)) using nanomechanical IR spectroscopy.

Embossing of organic thin films using a surfactant assisted lift-off technique.

A simple technique for patterning organic materials using a surfactant assisted lift-off method is proposed. Thin films of various organic materials are prepared, and areas in contact with a surfactant coated poly(dimethylsiloxane) (PDMS) stamp are selectively removed. The general applicability of this technique is shown for materials containing nitrate, amine, and carboxylic acid functional groups. This technique provides a new methodology for fabricating patterns with vertical dimensions ranging from 30 nm up to 3 µm on organic thin films with specific functional groups.

Biodegradation of pentaerythritol tetranitrate (PETN) by anaerobic consortia from a contaminated site.

This study examined the role of denitrifying and sulfate-reducing bacteria in biodegradation of pentaerythritol tetranitrate (PETN). Microbial inocula were obtained from a PETN-contaminated soil. PETN degradation was evaluated using nitrate and/or sulfate as electron acceptors and acetate as a carbon source. Results showed that under different electron acceptor conditions tested, PETN was sequentially reduced to pentaerythritol via the intermediary formation of tri-, di- and mononitrate pentaerythritol (PETriN, PEDN and PEMN). The addition of nitrate enhanced the degradation rate of PETN by stimulating greater microbial activity and growth of nitrite reducing bacteria that were responsible for degrading PETN. However, a high concentration of nitrite (350mgL(-1)) accumulated from nitrate reduction, consequently caused self-inhibition and temporarily delayed PETN biodegradation. In contrast, PETN degraded at very similar rates in the presence and absence of sulfate, while PETN inhibited sulfate reduction. It is apparent that denitrifying bacteria possessing nitrite reductase were capable of using PETN and its intermediates as terminal electron acceptors in a preferential utilization sequence of PETN, PETriN, PEDN and PEMN, while sulfate-reducing bacteria were not involved in PETN biodegradation. This study demonstrated that under anaerobic conditions and with sufficient carbon source, PETN can be effectively biotransformed by indigenous denitrifying bacteria, providing a viable means of treatment for PETN-containing wastewaters and PETN-contaminated soils.

Improved accuracy in high-temperature conversion elemental analyzer δ18O measurements of nitrogen-rich organics.

RATIONALE: The use of high-temperature conversion (HTC) reduction systems interfaced with isotope ratio mass spectrometers for δ(18)O measurements of nitrogen-containing organic materials is complicated by isobaric interference from (14)N(16)O(+). This ion is produced in the ion source when N(2) reacts with trace oxygen shifting the m/z 30 baseline prior to elution of CO. METHODS: We compared adaptations to a typical HTC system (TC/EA) to determine the best method to measure the δ(18)O values of nitrogen-rich organic substrates including: (1) 0.6 and 1.5 m 5 Šmolecular sieve GC columns; (2) reduction of N(2) peak via He dilution; and (3) diversion of N(2) to waste via an automated four-port valve. These methods were applied to caffeine (IAEA-600), glycine, 4-nitroacetanilide, pentaerythritol tetranitrate (PETN) and cyclotrimethylene trinitramine (RDX), as well as pure and sodium azide-doped benzoic acid (IAEA-601) and sucrose (IAEA-CH6). RESULTS: The efficiency of N(2) production in the HTC interface was highly variable among these compounds. Both the longer column and the dilutor improved, but did not eliminate, the adverse effects of nitrogen. CONCLUSIONS: The diversion of N(2) adequately addressed the nitrogen-induced problems as indicated by: (1) consistent m/z 30 background offset between reference and sample CO for both N-free and N-rich materials; (2) production of the highest δ(18)O values; and (3) high correlation between the increase in the δ(18)O values relative to the GC-only measurements and the N(2) peak area. Additional validation would require N-rich oxygen isotope standards for inter-laboratory comparisons. Further, more stringent methodology may improve the poor inter-laboratory δ(18)O reproducibility of IAEA-600.

Terahertz normal mode relaxation in pentaerythritol tetranitrate.

Normal vibrational modes for a three-dimensional defect-free crystal of the high explosive pentaerythritol tetranitrate were obtained in the framework of classical mechanics using a previously published unreactive potential-energy surface [J. Phys. Chem. B 112, 734 (2008)]. Using these results the vibrational density of states was obtained for the entire vibrational frequency range. Relaxation of selectively excited terahertz-active modes was studied using isochoric-isoergic (NVE) molecular dynamics simulations for energy and density conditions corresponding to room temperature and atmospheric pressure. Dependence of the relaxation time on the initial modal excitation was considered for five excitation energies between 10 and 500 kT and shown to be relatively weak. The terahertz absorption spectrum was constructed directly using linewidths obtained from the relaxation times of the excited modes for the case of 10 kT excitation. The spectrum shows reasonably good agreement with experimental results. Dynamics of redistribution of the excited mode energy among the other normal modes was also studied. The results indicate that, for the four terahertz-active initially excited modes considered, there is a small subset of zero wave vector (k = 0) modes that preferentially absorb the energy on a few-picosecond time scale. The majority of the excitation energy, however, is transferred nonspecifically to the bath modes of the system.

Dissociative electron attachment to pentaerythritol tetranitrate: significant fragmentation near 0 eV.

Gas phase dissociative electron attachment (DEA) measurements to pentaerythritol tetranitrate (PETN) are performed in a crossed electron-molecular beam experiment at high-energy resolution and high sensitivity. DEA is operative at very low energies close to approximately 0 eV showing unique features corresponding to a variety of fragment anions being formed. There is no evidence of the parent anion formation. The fragmentation yields are also observed for higher electron energies and are operative via several resonant features in the range of 0-12 eV. In contrast to nitroaromatic compounds, PETN decays more rapidly upon electron attachment and preferentially low-mass anions are formed. The dominant fragment ion formed through DEA is assigned to the nitrogen trioxide NO(3)(-) and represents about 80% of the total anion yield. Further intense ion signals are due to NO(2)(-) (11%) and O(-) (2.5%). The significant instability of PETN after attachment of an electron with virtually no kinetic energy confers a highly explosive nature to this compound.

PETN ignition experiments and models.

Ignition experiments from various sources, including our own laboratory, have been used to develop a simple ignition model for pentaerythritol tetranitrate (PETN). The experiments consist of differential thermal analysis, thermogravimetric analysis, differential scanning calorimetry, beaker tests, one-dimensional time to explosion tests, Sandia's instrumented thermal ignition tests (SITI), and thermal ignition of nonelectrical detonators. The model developed using this data consists of a one-step, first-order, pressure-independent mechanism used to predict pressure, temperature, and time to ignition for various configurations. The model was used to assess the state of the degraded PETN at the onset of ignition. We propose that cookoff violence for PETN can be correlated with the extent of reaction at the onset of ignition. This hypothesis was tested by evaluating metal deformation produced from detonators encased in copper as well as comparing postignition photos of the SITI experiments.

Magnetic field-cycling NMR and (14)N, (17)O quadrupole resonance in the explosive pentaerythritol tetranitrate (PETN).

The explosive pentaerythritol tetranitrate (PETN) C(CH(2)-O-NO(2))(4) has been studied by (1)H NMR and (14)N NQR. The (14)N NQR frequency and spin-lattice relaxation time T(1Q) for the nu(+) line have been measured at temperatures from 255 to 325K. The (1)H NMR spin-lattice relaxation time T(1) has been measured at frequencies from 1.8kHz to 40MHz and at temperatures from 250 to 390K. The observed variations are interpreted as due to hindered rotation of the NO(2) group about the bond to the oxygen atom of the CH(2)-O group, which produces a transient change in the dipolar coupling of the CH(2) protons, generating a step in the (1)H T(1) at frequencies between 2 and 100kHz. The same mechanism could also explain the two minima observed in the temperature variation of the (14)N NQR T(1Q) near 284 and 316K, due in this case to the transient change in the (14)N...(1)H dipolar interaction, the first attributed to hindered rotation of the NO(2) group and the second to an increase in torsional amplitude of the NO(2) group due to molecular distortion of the flexible CH(2)-O-NO(2) chain which produces a 15% increase in the oscillational amplitude of the CH(2) group. The correlation times governing the (1)H T(1) values are approximately 25 times longer than those governing the (14)N NQR T(1Q), explained by the slow spin-lattice cross-coupling between the two spin systems. At higher frequencies, the (1)H T(1) dispersion results show well-resolved dips between 200 and 904kHz assigned to level crossing with (14)N and weaker features between 3 and 5MHz tentatively assigned to level crossing with (17)O.

Calculation and measurement of terahertz active normal modes in crystalline PETN.

The terahertz frequency spectrum of pentaerythritol tetranitrate (PETN) is calculated using Discover with the COMPASS force field, CASTEP and PWscf. The calculations are compared to each other and to terahertz spectra (0.3-3 THz) of crystalline PETN recorded at 4 K. A number of analysis methods are used to characterise the calculated normal modes.

Dual-frequency imaging using an electrically tunable terahertz quantum cascade laser.

We report dual-frequency transmission imaging of polycrystalline materials using an electrically tunable terahertz (THz) frequency quantum cascade laser (QCL). Using our system we are able to obtain images at both 3.05 THz and 3.24 THz in a single two-dimensional scan of a sample. By taking the difference of the natural logarithms of the transmission coefficients obtained at each frequency, the difference-attenuation coefficient is determined, and evaluated for samples of lactose monohydrate, glucose monohydrate, sucrose, and the high explosive PETN. We also demonstrate difference-intensity imaging at these frequencies by combining amplitude modulation of the QCL bias with lock-in detection. Owing to the specific molecular absorption spectra of these materials in the THz frequency range, the samples can be distinguished using our technique.