Examining the chemical and structural properties that influence the sensitivity of energetic nitrate esters

Abstract

The sensitivity of explosives is controlled by factors that span from intrinsic chemical reactivity and chemical intramolecular effects to mesoscale structure and defects, and has been a topic of extensive study for over 50 years. Due to these complex competing chemical and physical elements, a unifying relationship between molecular framework, crystal structure, and sensitivity has yet to be developed. In order to move towards this goal, ideally experimental studies should be performed on systems with small, systematic structural modifications, with modeling utilized to interpret experimental results. Pentaerythritol tetranitrate (PETN) is a common nitrate ester explosive that has been widely studied due to its use in military and commercial explosives. We have synthesized PETN derivatives with modified sensitivity characteristics by substituting one -CCH₂ONO₂ moiety with other substituents, including -CH, -CNH₂, -CNH₃X, -CCH₃, and -PO. We relate the handling sensitivity properties of each PETN derivative to its structural properties, and discuss the potential roles of thermodynamic properties such as heat capacity and heat of formation, thermal stability, crystal structure, compressibility, and inter- and intramolecular hydrogen bonding on impact sensitivity. Reactive molecular dynamics (MD) simulations of the C/H/N/O-based PETN-derivatives have been performed under cook-off conditions that mimic those accessed in impact tests. These simulations infer how changes in chemistry affect the subsequent decomposition pathways.

Fig. 1. Structure of PETN and a list of the PETN-R derivatives examined in this study.

Fig. 2. DSC kinetics plots, showing ramp rates at 0.5 °C, 1 °C, 5 °C, 10 °C, and 20 °C per minute, for (a) PETN–CH, (b) PETN–PO, and (c) PETN–CNH₃Cl.

Fig. 3. Solid state structure of PETN–CH (left) and PETN–PO (right). Orange, red, blue, grey and white ellipsoids represent P, O, N, C and H atoms respectively. Ellipsoids at 50% probability.

Fig. 4. Evolution of the (a) temperature and (b) pressure in liquid PETN–CH, PETN–CMe, and PETN–CNH₂ initialized at a temperature of 1400 K. The pressure data were smoothed using a 50 point running average.

Fig. 5. Species count for the three PETN derivatives, showing N[OO], C[HCO], and C[HHO] for (a) PETN–CNH₂, (b) PETN–CH, (c) PETN–CMe, and N[O], O[HH], C[OO], and N[N] for (d) PETN–CNH₂, (e) PETN–CH, and (f) PETN–CMe.

Fig. 6. Proposed mechanisms for some of the likely first reactions for PETN derivatives including initial intermediate products, where R = NH₂ [PETN–CNH₂; mechanism (a)], H [PETN–CH; mechanism (b)], or CH₃ [PETN–CMe; mechanisms (a) and (b)].

Fig. 7. Solid state packing in PETN–PO, PETN, PETN–CH, PETN–CNH₃Cl, and PETN–CNH₃Br, viewed along the a, b, and c crystallographic axes. Each material has the impact sensitivity listed below it (in cm; decreases from top to bottom), with a summary of whether hydrogen bonding and planes exist.

Fig. 8. Calculated compressibility vs. impact height (cm).