Using gradient-corrected density functional theory, we have investigated various competing mechanisms involved in the early stages of the decomposition of a “push−pull” (containing both electron-donating and -withdrawing groups) aromatic (1,3,5-triamino-2,4,6-trinitrobeznene, TATB), and particularly how hydrogen transfer affects the competition between ring closure and single bond scission. On the basis of the obtained energetics, we found several previously suggested mechanisms to be energetically disfavored. These mechanisms include direct N−H, N−O, C−NH2 bond dissociation, carbon ring cleavage, and the production of NO2-1 anion. Our results indicate that the rate-limiting step is ring closure mediated by intramolecular hydrogen transfer. This mechanism is predicted to have a barrier height of 47.5 kcal/mol. The hydrogen motion forms a biradical state which we suggest is the precursor to further decomposition products, such as benzofurazans.

Ring Closure Mediated by Intramolecular Hydrogen Transfer in the Decomposition of a Push−Pull Nitroaromatic: TATB

Study of initiation reactivity of some plastic explosives by vacuum stability test and non-isothermal differential thermal analysis

The kinetics of the thermal decomposition of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) in condensed state has been investigated by high temperature infrared spectroscopy (IR) and thermogravimetry (TG) in conjunction with pyrolysis gas analysis, differential thermal analysis (DTA) and hot stage microscopy. The decomposition proceeds in two main stages under isothermal conditions and the initial stage involving about 24% loss in weight obeys Avrami-Erofe'ev equation (n= 1), and is governed by an activation energy (E) of 150.58 kJ·mol−1 and log(A in s−1) 12.06. The second stage corresponding to 24 to 90% loss in weight gave best fit for Avrami-Erofe'ev equation,n=2, withE=239.56 kJ·mol−1 and log(A in s−1) 19.88 by isothermal TG. The effect of additives, on the initial thermolysis of TATB has also been studied. Evolved gas analysis by IR showed that NH3, CO2, NO2, HCN and H2O are produced in the initial stage of decomposition. The decomposition in KBr matrix in the temperature range 272 to 311.5°C shows relative preferential loss in the -NH2 to -NO2 band intensity which indicates that the rupture of C-NH2 bond, weakened also by the interaction of the NH2 with the neighbouring NO2 group, appears to be the primary step in the thermolysis of TATB.

Spectroscopic and thermal studies on the decomposition of l,3,5-triamino-2,4,6-trinitrobenzene (TATB)

The insensitive explosive 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) has been recrystallized from dimethylsulfoxide (DMSO) with particle sizes ranging up to 2 mm diameter. Crystal growth was controlled by varying the heating and cooling rates and the solution agitation speed. The role of additives and “green” u.v. radiation-induced impurity in affecting crystal morphology, size, and colour was also studied. To provide the thermal concentration data needed for the crystallization study, the solubility curve of TATB in DMSO was determined gravimetrically. The solubility data spans the range of 0.47±0.23 gl−1 at 21.2 °C to 6.80±1.13 gl−1 at 145.5 °C.

Recrystallization and solubility of 1,3,5-triamino-2,4,6-trinitrobenzene in dimethyl sulfoxide

An attempt was made to develop a general description of impact sensitivity. For this purpose a set of 24 well-known, as well as recently synthesized, C–H–N–O–Cl explosives covering the wide range of impact sensitivity (h50 = 9–320 cm) was studied using first-principles calculations at different external pressures. To quantify impact sensitivity, a theoretical approach was developed based on the solid-state derived criteria, which include triggering pressure, average number of electrons per atom, crystal morphology, energy content and melting temperature. These criteria follow from the theoretical consideration of the crystal compression caused by an impact event. Apart of the compression, the influence of crystal habit shapes and energy content are also discussed. The main idea is in the electron flow probability from valence to conduction bands in a solid. To support the developed theoretical background, the corresponding numerical illustration is presented in the paper. The obtained empirical correlatio...

Quantification of Impact Sensitivity Based on Solid-State Derived Criteria

TATB (1,3,5-triamino-2,4,6-trinitrobenzene) forms an unusually stable free radical upon irradiation with ultraviolet light. Extraction with dimethylsulfoxide yields a liquid solution ESR spectrum which is, apparently, due to a TATB-H adduct. The g-values, spectral widths, and hyperfine assignments for the solution radical are given.

Free Radicales of TATB

Efforts to generate detectable concentrations of free radicals in explosives, binders, and their mixtures are described. Radicals were readily produced in polycaprolactone and polyethylene glycol binders at liquid nitrogen temperature using stresses as low as 0.4 kbar. These radicals were all of the peroxy type, and presumably formed by reaction of mechano-radicals with oxygen present in the polymer. No mechano-radicals were observed from HMX or RDX using samples cooled to liquid nitrogen temperature and applied stresses up to 4 kbar. In neither impacted samples that failed to explode nor the residues remaining after a partial explosion were radicals detected by ESR.

Low temperature γ-irradiation of these materials was also carried out. Free radical signals originating in both the polymer and the explosive could be identified. The reactivity of NO2 radicals from γ-irradiated HMX is enhanced in the presence of binder. In γ-irradiated HMX/polycaprolactone mixtures, the NO2 radical anneals rapidly at 150 K, versus 240 K for HMX alone.

Evidence is presented to suggest that the relatively stable NO2 radical (such as produced by γ-irradiation) in HMX does not play a dominant role in mechanical initiation.

Generation of Free Radicals in RDX and HMX Compositions

Impact of thin layers of HMX at room temperature and near liquid nitrogen temperature were performed. Polycrystalline powders of HMX irradiated with 7.5 megarad in evacuated tubes at liquid nitrogen temperature showed significant enhanced sensitivity. With 10-megarad irradiations of HMX in evacuated tubes at liquid nitrogen temperature frequent spontaneous explosions in HMX have occurred. ESR measurements on impacted HMX and RDX powders at room temperature and near liquid nitrogen temperature at energies of less than 1 kg · m have not produced detectable mechanically generated free radicals. A significant increase in sensitivity of vacuum irradiated HMX was observed but efforts to correlate this with the dominant frozen-in NO2 γ-radical concentration were unsuccessful. It is suggested that the NO2 radical is a chain terminating radical in explosive systems.

W. B. Moniz

Papers

Free Radicales of TATB

Generation of Free Radicals in RDX and HMX Compositions

Impact Sensitivity of γ-Irradiated HMX