High-pressure vibrational spectroscopy of energetic materials: hexahydro-1,3,5-trinitro-1,3,5-triazine

Vibrational spectroscopy has been used to investigate the room-temperature high-pressure phases of the energetic material hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). The pressure-induced alterations in the spectral profiles were studied in a compression sequence to 30.2 GPa using Raman spectroscopy and to 26.6 GPa using far-infrared spectroscopy. At pressures near 4.0 GPa, several changes become immediately apparent in the Raman spectrum, such as large frequency shifts, mode splittings, and intensity changes, which are associated with a phase transition from alpha-RDX to gamma-RDX. Our study extends the kinetic stability of gamma-RDX to pressures near 18.0 GPa. Evidence for a new phase was found at pressures between 17.8 and 18.8 GPa and is based on the appearance of new vibrational bands and associated changes in intensity patterns. The new phase has vibrational characteristics that are similar to those of beta-RDX, suggesting the two polymorphs share a related crystal structure.     

Computational insight into energetic cage derivatives based on hexahydro-1,3,5-trinitro-1,3,5-triazine

Four novel cage compounds were designed by introducing –N(NO₂)CH₂–, –N(NO₂)O–, –N(NO₂)N(NO₂)–, and –N=N– linkages into the RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) skeleton. Their molecular geometry, electronic structure, heat of formation, and detonation properties were systematically studied using density functional theory (DFT). In addition, the most stable dimers of the four compounds were constructed to further investigate their stability based on intermolecular interactions. It is found that the unconventional CH⋯O interactions would be the dominant driving force when the title compounds form crystals. Compared with the traditional explosives, the compounds with higher detonation properties and lower impact sensitivity will be considered as promising candidates for high energy density compounds. Our results indicate that our innovative design strategy is extremely useful for developing novel energetic compounds.     

Crystal structure of the high-pressure phase of hexahydro-1,3,5-trinitro-1,3,5-triazine (gamma-RDX)

The crystal structure of the high-pressure phase of hexahydro-1,3,5-trinitro-1,3,5-triazine (gamma-RDX), which is stable above 4 GPa at room temperature, was investigated by using infrared spectroscopy and powder X-ray diffraction measurements followed by Rietveld refinements using a diamond anvil cell (DAC). Although gamma and alpha phases were found to belong to the same space group Pbca, they exhibited a different crystal packing. The molecular structure of the gamma phase exhibited the same conformation as that of the alpha phase; however, the torsion angles of N-NO2 changed marginally.     

High pressure Raman spectroscopy of single crystals of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX)

To gain insight into the high-pressure polymorphism of RDX, an energetic crystal, Raman spectroscopy results were obtained for hydrostatic (up to 15 GPa) and non-hydrostatic (up to 22 GPa) compressions. Several distinct changes in the spectra were found at 4.0 +/- 0.3 GPa, confirming the alpha-gamma phase transition previously observed in polycrystalline samples. Detailed analyses of pressure-induced changes in the internal and external (lattice) modes revealed several features above 4 GPa: (i) splitting of both the A' and A' ' internal modes, (ii) a significant increase in the pressure dependence of the Raman shift for NO2 modes, and (iii) no apparent change in the number of external modes. It is proposed that the alpha-gamma phase transition leads to a rearrangement between the RDX molecules, which in turn significantly changes the intermolecular interaction experienced by the N-O bonds. Symmetry correlation analyses indicate that the gamma-polymorph may assume one of the three orthorhombic structures: D2h, C2v, or D2. On the basis of the available X-ray data, the D2h factor group is favored over the other structures, and it is proposed that gamma-phase RDX has a space group isomorphous with a point group D2h with eight molecules occupying the C1 symmetry sites, similar to the alpha-phase. It is believed that the factor group splitting can account for the observed increase in the number of modes in the gamma-phase. Spatial mapping of Raman modes in a non-hydrostatically compressed crystal up to 22 GPa revealed a large difference in mode position indicating a pressure gradient across the crystal. No apparent irreversible changes in the Raman spectra were observed under non-hydrostatic compression.     

1,3,5-trinitro-1,3,5-triazine decomposition and chemisorption on Al(111) surface: first-principles molecular dynamics study

We have investigated the decomposition and chemisorption of a 1,3,5-trinitro-1,3,5-triazine (RDX) molecule on Al(111) surface using molecular dynamics simulations, in which interatomic forces are computed quantum mechanically in the framework of the density functional theory (DFT). The real-space DFT calculations are based on higher-order finite difference and norm-conserving pseudopotential methods. Strong attractive forces between oxygen and aluminum atoms break N-O and N-N bonds in the RDX and, subsequently, the dissociated oxygen atoms and NO molecules oxidize the Al surface. In addition to these Al surface-assisted decompositions, ring cleavage of the RDX molecule is also observed. These reactions occur spontaneously without potential barriers and result in the attachment of the rest of the RDX molecule to the surface. This opens up the possibility of coating Al nanoparticles with RDX molecules to avoid the detrimental effect of oxidation in high energy density material applications.     

环三甲撑三硝胺(RDX)结构和性质的DFT研究

环三甲撑三硝胺(RDX)是综合性能好、使用极广的高能炸药。本文用密度泛函理论(DFT)B3LYP方法,取6-31G*基组,求得其分子几何构型、电子结构、IR谱和热力学性质。全优化几何构型和电子结构均具有C3V对称性。在相邻原子之间以NNO2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。计算所得298-1200K的热力学性质,对进一步深入研究RDX的反应和性质有助。     

Study on the 1,3,5-triazine gamma-radiolysis

Triazines are a class of molecules which have been found in meteorites such as Orgueil meteorite. Despite their poor resistance to UV radiation, these molecules survived millions of years inside a meteorite. The present work is dedicated to the examination of the radiation resistance of the simplest sym-triazine: 1,3,5-triazine. The crystals of this molecule have been irradiated with γ-radiation at 50 and 350 kGy and were studied by electronic absorption spectroscopy, liquid chromatography, FT-IR spectroscopy and differential scanning calorimetry (DSC). All the data suggest the relatively low stability of this molecule to high energy radiation. The resulting products from radiolysis are formamidine together with triazine dimers and oligomers. Other radiolysis products are H₂, CH₄, HCN and other gases.     

Determination of N-nitroso derivatives of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) in soils by pressurized liquid extraction and liquid chromatography-electrospray ionization mass spectrometry

To aid in the evaluation of the potential toxicity of N-nitroso derivatives of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), we describe a pressurized liquid extraction (PLE) followed by liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS) method for determination of RDX and its N-nitroso derivatives: hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine (MNX), hexahydro-1,3-dinitroso-5-nitro-1,3,5-triazine (DNX), and hexahydro-1,3,5-trinitroso-1,3,5-triazine (TNX) in soils. Sandy loam soil was spiked with RDX and its N-nitroso derivatives (MNX, DNX, and TNX). Acetonitrile was used as the PLE extraction solvent at 100 degrees C and 1500 psi for 15 min. Florisil was used to cleanup extracts following PLE. Instrumental analysis employed LC-ESI-MS, in which 1mM acetic acid was added to the mobile phase to facilitate formation of acetate adduct ions [M+CH(3)COO](-). The method detection limits (MDLs) for RDX, MNX, DNX, and TNX were 1.46, 1.46, 1.69, and 1.93 ng/g, respectively. High recovery (91.1-108.3%), good precision (RSD: 3.2-12.4%), and reproducibility were achieved. This method proved effective and was applied to monitor the reductive biotransformation of MNX in soils with the presence of earthworms (Eisenia fetida).     

Geometric structure of molecule of 1,3,5-trinitro-1,3,5-triazacyclohexane in the gas phase

M. V. Lomonosov Moscow State University. Translated from Zhurmal Strukturnoi Khimii, Vol. 33, No. 1, pp. 41–45, January–February, 1992.     

Mass spectral fragmentation pathways of N-acetylnitramines: 1-acetylhexahydro-3,5-dinitro-1,3,5-triazine and 1-acetyloctahydro-3,5,7-trinitro-1,3,5,7-tetrazocine

Mass spectra of the N-acetylnitramines l-acetylhexahydro-3,5-dinitro-l,3,5-triazine (TAX) and 1-acetyloctahydro-3,5,7-trinitro-l,3,5,7-tetrazocine (SEX) were recorded in electron impact (EI) and positive and negative chemical ionization (PCI and NCI) modes, and the fragmentation pathways were compared with those of other nitramines which have been well documented and characterized. Unexpectedly, for both acetylnitramines in the EI mode (and in the PCI mode) proton adducts were the only molecular ion species observed; in neither mode was there evidence for higher adducts. In contrast, for TAX in the NCI mode the [M + NO₂]^? adduct was the second most abundant ion (70%); relatively small amounts of the [M + NO]^? adduct (2%) and the hydrogen adduct [MH]^? (3%) were observed. Under identical NCI conditions no molecular ion species or adduct ions were detected for SEX; the ion of highest m/z corresponded to loss of NO₂ or HNO₂ from a molecular ion species. The findings of collision-induced dissociation experiments are also discussed.     

Synthesis of 2,4,6-triphenyl-1,3,5-triazine

Conclusions A study was made of the cyclotrimerization of benzonitrile in chlorosulfonic acid and the optimum conditions were ascertained for the formation of 2,4,6-triphenyl-1,3,5-triazine.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2105, September, 1972.     

A study of the sensitivity and decomposition of 1,3,5-trinitro-2-oxo-1,3,5-triazacyclo-hexane

The thermal decomposition and thermal stability of 1,3,5-trinitro-2-oxo-1,3,5-triazacyclohexane (keto-RDX or K-6) was studied. The keto-RDX synthesis is described, mass spectra (electron impact (70 eV) and chemical ionization) similar to RDX spectra registered under identical conditions are presented, and mass spectroscopy fragmentation paths are proposed. The LI-MS (laser induced/mass spectroscopic) results imply that the first step in the decomposition of keto-RDX is the elimination of NO₂ or HONO and subsequent breakdown of the triazacyclohexane ring. The thermal stability, activation energy (E_a = 140 kJ mol⁻¹), and frequency factor (K₀ = 9 × 10⁹ s⁻¹) in the temperature interval 90-120°C were measured using chemiluminescence (NO detection only). The activation energy was also determined from DSC data using the ASTM method E 698-79, and was found to be 280 kJ mol⁻¹ with a frequency factor of 7.0 × 10²⁹ s⁻¹ in the temperature interval 175-200°C. Microcalorimetry, drop-weight test, friction test, and ignition temperature (Wood's metal bath) measurements were also conducted. Quantum mechanical calculations (semi-empirical method with PM3 set at the unrestricted Hartree-Fock level) were conducted to correlate the sensitivity and thermal decomposition with those of RDX. No significant differences in bond-breaking energies for RDX and keto-RDX were found. Conclusions drawn from the experiments are that the decomposition of keto-RDX is auto-catalytic, and that the sensitivity of keto-RDX is not connected with the initial bond-breaking step. More than one method for measuring the risk involved in handling an explosive is necessary since the sensitivity depends on different stages in the decomposition.     

Molecular dynamics simulations of melting of perfect crystalline hexahydro-1,3,5-trinitro-1,3,5-s-triazine

The melting mechanism of superheated perfect crystalline hexahydro-1,3,5-trinitro-1,3,5-s-triazine (alpha-RDX) has been investigated using molecular dynamics simulations with the fully flexible force field developed by Smith and Bharadwaj [J. Phys. Chem. B 103, 3570 (1999)]. Sequential 50 ps equilibration simulations of the constant stress-constant temperature ensemble were performed at 10 K intervals over the range of 300-650 K, corresponding to a heating rate of 2.0 x 10(11) Ks. A solid-solid phase transition is observed between 480 and 490 K, followed by melting, which occurs between 500 and 510 K. The solid-solid phase transition, both displacive and rotational, is characterized by an abrupt decrease in the lengths of the unit cell edges a and b and an increase of the length of edge c. The molecular conformation in the new phase is AAE, although the axial nitro groups have different changes: one shift is more axial and the other is more equatorial. Phases other than alpha-RDX have been observed experimentally, however, there are insufficient data for comparisons to ascertain that the new phase observed here corresponds to a real phase. At the high heating rate (2.0 x 10(11) Ks) used in the simulations, the melted RDX reaches full orientational disorder at about 540 K and translational freedom at around 580 K. If the simulation at the melting temperature (510 K) is run sufficiently long complete rotational freedom is achieved in a few hundreds of picoseconds, while complete translational freedom requires much longer. These results show that given a sufficiently high heating rate, the system can exist for significant periods of time in a near-liquid state in which the molecules are not as free to rotate and diffuse as in the true liquid state. The bond lengths and bond angles undergo little change upon melting, while there are significant changes in the dihedral angles. The molecular conformation of RDX changes from AAE to EEE upon melting. The ramification of this for formulating force fields that accurately describe melting is that it is important that the torsional motions are accurately described.     

Trace level analysis of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and its biodegradation intermediates in liquid media by solid-phase extraction and high-pressure liquid chromatography analysis

The use of solid-phase extraction for the analysis of liquid media containing low microg/L levels of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), mononitroso-RDX (MNX), dinitroso-RDX (DNX), and trinitroso-RDX (TNX) is examined. Aqueous samples (100 mL) consisting of water and a microbiological basal medium are spiked with known concentrations of RDX, MNX, DNX, and TNX. The compounds are extracted from the liquid media using a Porapak RDX cartridge and then eluted from the cartridge with 5 mL of acetonitrile. The eluent is concentrated to 1 mL before analysis by high-pressure liquid chromatography (HPLC). The method detection limits for RDX are 0.1 microg/L in water and 0.5 microg/L in the basal medium after a 100-fold concentration. For MNX, DNX, and TNX, the method detection limits are approximately 0.5 microg/L in water and approximately 1 microg/L in the basal medium after a 100-fold concentration. Interferences in the basal medium and a contaminant in the standard made quantitation for MNX and TNX, respectively, is less accurate below the 1 microg/L level. Solid-phase extraction of the liquid media gave good recoveries of nitramines and nitroso intermediates from a microbiological basal medium, allowing HPLC detection of RDX and the nitroso intermediates in the low microg/L (ppb) range.     

Enhanced extraction of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) in the presence of sodium dodecyl sulphate and its application to environmental samples

A method for enhanced extraction of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) from environmental samples is developed with the assistance of sodium dodecyl sulphate (SDS) surfactant. In this study, the concentration of SDS surfactant and other analytical parameters are optimized on a high-performance liquid chromatography-UV system. An isocratic flow of 1.0 mL/min with mobile phase acetonitrile-water; 70:30 (v/v) at 230 nm wavelength on a reverse-phase amide column is used for baseline separation of explosives and making calibration curves. The amount of recovered explosives from spiked soil and water samples are calculated. The limits of detection obtained for HMX and RDX standards are 1.5 and 3.8 ppb (S/N=3), respectively, which are much better than obtained by the Environmental Protection Agency method 8330. The recoveries are found to be enhanced by 1.7 and 1.6-fold with SDS solution as compared to water for HMX and RDX, respectively, from soil samples.     

Michael Reaction of 2,4-Dioxohexahydro-1,3,5-triazine

With the extensive investigation of the Michael reaction, not the methylene compounds activated by electron withdrawing group[1,2] but other activated unsaturated compounds containing such as activate N-H or O-H have been used as Michael acceptor.[3,4] However, the research on the Michael reaction of 2,4-dioxohexahydro-1,3,5-triazine (DHT) has not been reported. After the systematic researches on the substitution reactions of DHT, we first report the Michael reaction of DHT with acrylic esters in this paper.     

Detection of the cyclic nitramine explosives hexahydro-1,3,5-trinitro- 1,3,5-triazine (RDX) and octahydro- 1,3,5,7-tetranitro- 1,3,5,7-tetrazine (HMX) and their degradation products in soil environments

The cyclic nitramine explosives hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazine (HMX) were examined in field and microcosm soil samples to determine their patterns of degradation and environmental fates. A number of analytical techniques, including solid-phase microextraction with on-fiber derivatization, gas chromatography-mass spectrometry, gas chromatography with electron-capture detection, liquid chromatography-mass spectrometry, and micellar electrokinetic chromatography were required for the analyses. Two different classes of intermediates were detected, both of which lead ultimately to the formation of nitrous oxide (N2O) and carbon dioxide (CO2). The first class was identified as the nitroso derivatives formed by the sequential reduction of -NO2 functional groups. The second class of intermediates, which was favored at higher humidities and in the presence of anaerobic sludge amendments, consisted of ring cleavage products including bis-(hydroxymethyl)-nitramine and methylenedinitramine. Rye-grass (Lolium perenne) present in field samples was found to extract and accumulate HMX from soil without further degradation. In all cases (excepting the plant samples), the indigenous microbes or amended domestic anaerobic sludge consortia degraded the cyclic nitramine explosives eventually to produce N2O and CO2.