Thermal behavior and decomposition kinetics of Formex-bonded explosives containing different cyclic nitramines

Thermal behavior and decomposition kinetics of Formex-bonded PBXs based on some attractive cyclic nitramines, such as 1,3,5-trinitro-1,3,5-triazinane (RDX) and 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX). Actually, cis-1,3,4,6-tetranitrooctahy droimidazo-[4,5-d]imidazole (BCHMX) and 2,4,6,8,10,12-hexanitro-2,4,6,8,10, 12-hexaazaisowurtzitane (CL-20), was investigated by means of nonisothermal thermogravimetry (TG) and differential scanning calorimetry (DSC). It was found that the mass loss rate of PBXs involved in this research depends greatly on heating rate and the residue of the decomposition of these PBXs decreases with the heating rate. The onset of the exotherms was noticed at 215.4, 278.7, 231.2 and 233.7 °C with the peak maximum at 235.1, 279.0, 231.2 and 233.7 °C for RDX-Formex, HMX-Formex, CL-20-Formex, and BCHMX-Formex, respectively. Their corresponding exothermic changes were 1788, 1237, 691, and 1583 J g^?1. It was also observed that the dependence on the heating rate for onset temperatures of HMX- and BCHMX-based PBXs was almost the same due to their similar molecular structure. In addition, based on nonisothermal TG data, the kinetic parameters for thermal decomposition of these PBXs were calculated by isoconversional methods. It was shown that the Formex base has great effects on the activation energy distribution of nitramines. It was further found that the kinetic compensation effects occurred during the thermal decomposition of nitramine-based PBXs, and they almost have the same compensation effects due to similar decomposition mechanism.     

The determination of nitroaromatics and nitramines in ground and drinking water by wide-bore capillary gas chromatography

A method has been developed to determine the concentration of nitroaromatics and nitramines in drinking water at levels below those previously achieved by gas chromatography. The nitroaromatics and nitramines are extracted from water using toluene and isoamyl acetate, respectively. The extracts are analyzed via a gas chromatograph equipped with a DB-1301 widebore fused-silica capillary column and an electron capture detector. Method detection limits of 0.003 micrograms/L for 2,6-dinitrotoluene (2,6-DNT), 0.04 micrograms/L for 2,4-dinitrotoluene (2,4-DNT), 0.06 micrograms/L for 2,4,6-trinitrotoluene (TNT), 0.3 micrograms/L for cyclotrimethylenetrinitramine (RDX), and 6.0 micrograms/L for cyclotetramethylenetetranitramine (HMX) have been obtained using this method.     

Theoretical predictions of chemical degradation reaction mechanisms of RDX and other cyclic nitramines derived from their molecular structures

Analysis of environmental degradation pathways of contaminants is aided by predictions of likely reaction mechanisms and intermediate products derived from computational models of molecular structure. Quantum mechanical methods and force-field molecular mechanics were used to characterize cyclic nitramines. Likely degradation mechanisms for hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) include hydroxylation utilizing addition of hydroxide ions to initiate proton abstraction via 2nd order rate elimination (E2) or via nucleophilic substitution of nitro groups, reductive chemical and biochemical degradation, and free radical oxidation. Due to structural similarities, it is predicted that, under homologous circumstances, certain RDX environmental degradation pathways should also be effective for octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and similar cyclic nitramines. Computational models provided a theoretical framework whereby likely transformation mechanisms and transformation products of cyclic nitramines were predicted and used to elucidate in situ degradation pathways.     

Decomposition of nitramine energetic materials in excited electronic states: RDX and HMX

Ultraviolet excitation (8-ns duration) is employed to study the decomposition of RDX (1,3,5-trinitro-1,3,5-triazacyclohexane) and HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane) from their first excited electronic states. Isolated RDX and HMX are generated in the gas phase utilizing a combination of matrix-assisted laser desorption and supersonic jet expansion techniques. The NO molecule is observed as one of the initial dissociation products by both time-of-flight mass spectroscopy and laser-induced fluorescence spectroscopy. Four different vibronic transitions of NO are observed: A (2)Sigma(v(') = 0)<--X (2)Pi(v(") = 0,1,2,3). Simulations of the NO rovibronic intensities for the A<--X transitions show that dissociated NO from RDX and HMX is rotationally cold (approximately 20 K) and vibrationally hot (approximately 1800 K). Another potential initial product of RDX and HMX excited state dissociation could be OH, generated along with NO, perhaps from a HONO intermediate species. The OH radical is not observed in fluorescence even though its transition intensity is calculated to be 1.5 times that found for NO per radical generated. The HONO intermediate is thereby found not to be an important pathway for the excited electronic state decomposition of these cyclic nitramines.     

Mass spectral fragmentation pathways in RDX and HMX. A mass analyzed ion kinetic energy spectrometric/collisional induced dissociation study

A collisional induced dissociation study of 1,3,5-trinitro-1,3,5 triazacyclohexane (RDX) and 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane (HMX) was carried out using mass analyzed kinetic energy spectrometry. High resolution mass spectra and mass analyzed ion kinetic energy/collisional induced dissociation spectra of RDX and HMX were recorded in the electron impact, chemical ionization and negative ion chemical ionization modes. Fragmentation pathways of the compounds investigated were determined in all three modes of ionization. It was found that a major part of the fragment ions in RDX and HMX originate from formation of the aduct ions [M+NO]⁺ and [M+NO₂]⁺ in electron impact and chemical ionization, and from [M+NO]^? and [M+NO₂]^? in negative chemical ionization, followed by dissociation.     

Characterization of cyclotetramethylenetetranitramine (HMX) thermal degradation by isotope analyses with analytical pyrolysis-atmospheric pressure ionization tandem mass spectrometry

Analytical pyrolysis-atmospberic pressure ionization (Py-API) tandem mass Spectrometry was used in the structure elucidation of the oxidalive and non-oxidative thermal decomposition products of cyclotetramethylenetetranitramine (HMX). The [¹⁵NO₂]-, [¹⁵N₈]- and [²H₈]-HMX isotope preparations provided fundamental information in the determination of the identities of the various pyrolyzate species. All RDX pyrolysis product ions that were identified by Py-API tandem mass Spectrometry, i.e. m/z 44, 60, 74, 75, 85 and 98, were present in the pyrolyzate of HMX. In both RDX and HMX investigations, these ions provided identical mass spectral daughter ion analyses. HMX, however, provided additional ions at m/z 30, 58, 69, 71, 83 and 141. Of all thirteen ions identified in the Py-APJ mass spectrum of HMX, only that at m/z 75 contained a nitrogen atom that originated from the NO₂ group. Standards analysis confirmed the identities of the ions at m/z 69, 71 and 141 as methyleneaminoacetonitrile, methylaminoacetonitrile and the caged compound hoxamethylenetetraamine, respectively. Isotopic analyses provided a high degree of confidence on the structural assignments of the ions at m/z 30 and 58 as methyleneimine and methyleneformamide; the ion at m/z 83, however, appeared to be a heterocyclic compound with daughter ion mass spectral elements similar to but not identical with that of 1-methylimidazole and 3-methylpyrazole.     

Relation between the N-NO2 bond length and stability of the secondary nitramines

The fact of the constancy of activation entropy of N-NO₂ bond homolysis in a series of secondary nitramines was utilized for correction of the experimental values of activation energy E of this process proceeding from the reliable data for the rate constants of the nitramines decomposition in solutions. When comparing the refined values of E (kJ mol^?1) with the N-N bond length d _N-N (Å) the following correlations were obtained: for cyclic and framework nitramines E = 663 ? 356d _N-N, and for the aromatic nitramines E = 1810 ? 1227d _N-N. A linear relationship between E and d is observed in the series of similar compounds. It depends on the electronic and steric effects of substituents.     

New theoretically predicted RDX‐ and β‐HMX‐based high‐energy‐density molecules

Theoretically new high‐energy‐density materials (HEDM) in which the hydrogens on RDX and β‐HMX (hexahydro‐1,3,5‐trinitro‐1,3,5‐triazine and octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine, respectively) were sequentially replaced by (N NO₂)x functional groups were designed and evaluated using density functional theory calculations in combination with the Kamlet–Jacobs equations and an atoms‐in‐molecules (AIM) analysis. Improved detonation properties and reduced sensitivity compared to RDX and β‐HMX were predicted. Interestingly, the RDX and β‐HMX derivatives having one attached N NO₂ group [RDX‐(NNO₂)1 and HMX‐(NNO₂)1] showed excellent detonation properties (detonation velocities: 9.529 and 9.575 km·s⁻¹, and detonation pressures: 40.818 and 41.570 GPa, respectively), which were superior to the parent compounds. Sensitivity estimations obtained by calculating impact sensitivities and HOMO‐LUMO gaps indicated that RDX‐(NNO₂)1 and HMX‐(NNO₂)1 were less stable than RDX and HMX but more stable than any of the other derivatives. This method of sequential NNO₂ group attachment on conventional HEDMs offers a firm basis for further studies on the design of new explosives. Furthermore, the newly found structures may be promising candidates for better HEDMs.     

A LC-MS method allowing the analysis of HMX and RDX present at the picogram level in natural aqueous samples without a concentration step

The introduction of chloroform into the nebulising gas of a LC/MS electrospray interface (ESI), in a perfectly controlled way, leads to the formation of intense adducts ([M+Cl]⁻) when a mobile phase containing HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane or octogen) and RDX (1,3,5-trintro-1,3,5-triazacyclohexane or hexogen) is eluted. This LC/MS method allows the direct analysis of aqueous samples containing HMX and RDX at the pictogram level without a concentration step. The method is used to determine HMX and RDX concentrations in ground water samples from a military site.     

Dimethyl ether and dimethyl-d6 ether chemical ionization mass spectrometry of nitramines,nitroaromatics and related compounds

Dimethyl ether (DME) chemical ionization mass spectrometry with introduction by direct exposure desorption was utilized for the characterization of a variety of nitramines, nitroaromatics and related compounds. For the nitramines and for many of the nitroaromatics the most abundant ions were fragment-molecule adduct ions resulting from ion-molecule reactions with the reagent gas. Nitroaromatic positional isomers were readily distinguished by large differences in the abundances of the various adduct ions. For the nitramines, collision-induced dissociations of the prominent methoxymethylene adduct ions were studied and contrasted with those of the corresponding adducts derived from DME-d₆ as reagent gas.     

Far IR and Raman vibrational spectra of nitramines

Vibrational spectra of several nitramines in the long-wave region (50–450 cm⁻¹) were studied. The frequencies of intra- and intermolecular vibrations were separated and a tentative assignment of the frequencies of self-associative complexes was performed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2241–2244, November, 1998.     

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.     

Theoretical studies on the heats of formation, detonation properties, and pyrolysis mechanisms of energetic cyclic nitramines

Density functional theory calculations were performed to find comprehensive relationships between the structures and performance of a series of highly energetic cyclic nitramines. The isodesmic reaction method was employed to estimate the heat of formation. The detonation properties were evaluated by using the Kamlet-Jacobs equations based on the theoretical densities and HOFs. Results indicate the N-NO(2) group and aza N atom are effective substituents for enhancing the detonation performance. All cyclic nitramines except C11 and C21 exhibit better detonation performance than HMX. The decomposition mechanism and thermal stability of these cyclic nitramines were analyzed via the bond dissociation energies. For most of these nitramines, the homolysis of N-NO(2) is the initial step in the thermolysis, and the species with the bridged N-N bond are more sensitive than others. Considering the detonation performance and thermal stability, twelve derivatives may be the promising candidates of high energy density materials (HEDMs). The results of this study may provide basic information for the further study of this kind of compounds and molecular design of novel HEDMs.     

Bond dissociation energies in nitramines

The enthalpies of formation in the standard state and in the gas phase were recommended for a series of secondary nitramines and n-butyldinitramine on the basis of the experimental enthalpies of combustion and vaporization and literature data. An analysis of the main thermochemical values (the enthalpies of formation in the gas phase and the enthalpies of atomization) showed that the energy properties of the nitramine group are independent of the structure of the molecules studied and of the number of functional groups in them. The enthalpies of formation of the alkylnitramine radicals were determined. The values obtained make it possible to calculate the bond dissociation energies in the nitramines and their radicals of different structures.     

Electroreduction of primary nitramines on platinum in MeCN

Reduction of primary nitramines RNHNO₂ (R=Me, Et, Prⁱ, α-pyridyl) in anhydrous MeCN at a Pt cathode was studied by voltammetry and electrolysis. The process includes one-electron transfer and nitramine deprotonation to give the corresponding anions. The products ofN-alkylation of these anions can be obtained only when their potassium salts are used (but not tetrabutylammonium salts). This is due to the effects of ion association, which influence the dual reactivity of the anions under investigation. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 82–85, January, 1999.     

3D Electron-Rich ZIF-67 Coordination Compounds Based on 2-Methylimidazole: Synthesis,Characterization and Effect on Thermal Decomposition of RDX,HMX, CL-20, DAP-4 and AP

ZIF-67 is a three-dimensional zeolite imidazole ester framework material with a porous rhombic dodecahedral structure, a large specific surface area and excellent thermal stability. In this paper, the catalytic effect of ZIF-67 on five kinds of energetic materials, including RDX, HMX, CL-20, AP and the new heat-resistant energetic compound DAP-4, was investigated. It was found that when the mass fraction of ZIF-67 was 2%, it showed excellent performance in catalyzing the said compounds. Specifically, ZIF-67 reduced the thermal decomposition peak temperatures of RDX, HMX, CL-20 and DAP-4 by 22.3 °C, 18.8 °C, 4.7 °C and 10.5 °C, respectively. In addition, ZIF-67 lowered the low-temperature and high-temperature thermal decomposition peak temperatures of AP by 27.1 °C and 82.3 °C, respectively. Excitingly, after the addition of ZIF-67, the thermal decomposition temperature of the new heat-resistant high explosive DAP-4 declined by approximately 10.5 °C. In addition, the kinetic parameters of the RDX+ZIF-67, HMX+ZIF-67, CL-20+ZIF-67 and DAP-4+ZIF-67 compounds were analyzed. After the addition of the ZIF-67 catalyst, the activation energy of the four energetic materials decreased, especially HMX+ZIF-67, whose activation energy was approximately 190 kJ·mol⁻¹ lower than that reported previously for HMX. Finally, the catalytic mechanism of ZIF-67 was summarized. ZIF-67 is a potential lead-free, green, insensitive and universal EMOFs-based energetic burning rate catalyst with a bright prospect for application in solid propellants in the future.     

Application of 13C and 15N NMR spectroscopy to structural studies on nitramines

¹⁵N and ¹³C chemical shifts and the ¹J(¹⁵N¹⁵N), ¹J(¹⁵N¹³C) and ¹J(¹³CH) coupling constants have been determined for a number of ¹⁵N-enriched cyclic and acyclic secondary nitramines. The results are interpreted in terms of both electronegativity effects and conformational factors.     

Physico-chemical measurements of CL-20 for environmental applications. Comparison with RDX and HMX

CL-20 is a polycyclic energetic nitramine, which may soon replace the monocyclic nitramines RDX and HMX, because of its superior explosive performance. Therefore, to predict its environmental fate, analytical and physico-chemical data must be made available. An HPLC technique was thus developed to measure CL-20 in soil samples based on the US Environmental Protection Agency method 8330. We found that the soil water content and aging (21 days) had no effect on the recoveries (>92%) of CL-20, provided that the extracts were kept acidic (pH 3). The aqueous solubility of CL-20 was poor (3.6 mg l(-1) at 25 degrees C) and increased with temperature to reach 18.5 mg l(-1) at 60 degrees C. The octanol-water partition coefficient of CL-20 (log KOW = 1.92) was higher than that of RDX (log KOW = 0.90) and HMX (log KOW = 0.16), indicating its higher affinity to organic matter. Finally, CL-20 was found to decompose in non-acidified water upon contact with glass containers to give NO2- (2 equiv.), N2O (2 equiv.), and HCOO- (2 equiv.). The experimental findings suggest that CL-20 should be less persistent in the environment than RDX and HMX.     

Density functional theory studies of effects of boron replacement on the structure and property of RDX and HMX

In this study, based on two model nitramine compounds hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5, 7-tetrazocine (HMX), two series of new energetic molecules were designed by replacing carbon atoms in the ring with different amounts of boron atoms, their structures and performances were investigated theoretically by the density functional theory method. The results showed that the boron replacement could affect the molecular shape and electronic structure of RDX and HMX greatly, and then would do harm to the main performance like the heat of formation, density, and sensitivity. However, the compound RDX-B2 is an exception; it was formed by replacing two boron atoms into the system of RDX and has the symmetric boat-like structure. Its oxygen balance (4.9%), density (1.91 g/cm³), detonation velocity (8.85 km/s), and detonation pressure (36.9 GPa) are all higher than RDX. Furthermore, RDX-B2 has shorter and stronger N NO₂ bonds than RDX, making it possesses lower sensitivity (45 cm) and better thermal stability (the bond dissociation energy for the N NO₂ bond is 204.7 kJ/mol) than RDX. Besides, RDX-B1 and HMX-B4 also have good overall performance; these three new molecules may be regarded as a new potential candidate for high energy density compounds.     

Molecular and crystal structure of polycyclic nitramines

An X-ray structural investigation has been carried out for four polycyclic nitramines with an isowurtzitane structure. These compounds are high-density, high-energy materials: 4,10-dinitro-2,6,8,12-tetraoxa-4,10-diaza-tetracyclo(5.5.0.0^3,11.0^5,9)dodecane (4), 4,8,10,12-tetranitro-2,6-dioxa-4,8,10,12-tetrazatetracyclo (5.5.0.0^3,11.0^5,9)dodecane (5), and 4,6,10,12-tetranitro-2,8-dioxa-4,6,10,12-tetrazatetracyclo(5.5.0.0^3,11.0^5,9)-dodecane (6). Nitramine 5 crystallizes as triclinic (form α, d_calc = 1.966 g/cm³) and trigonal (form β, d_calc = 2.014 g/cm³) modifications. All amine nitrogen atoms have a nonplanar structure; the mean sum of bond angles of the six-membered cycles is 345.2°, and the corresponding mean value for the five-membered cycles is 337.6°. Other features of their intramolecular structure are also discussed.