Modeling of explosives: 1,4,2,3,5,6-dioxatetrazinane as a new green energetic material with enhanced performance

Highlights

• Two novel carbon-free crystalline energetic compounds are predicted and characterized.

• One of them represents a powerful green explosive, yielding only N₂ and H₂O as detonation products.

• Its detonation properties exceed those of most explosives hitherto synthesized or predicted.

• The other compound is a very effective solid oxidant, which outperforms gaseous oxygen.

Abstract

Crystal structure prediction and characterization data have been computed for two novel carbon-free energetic compounds, namely 1,4,2,3,5,6-dioxatetrazinane (DOTZ) and its 2,5-dinitro derivative (DNDOTZ). Dynamic and mechanical stability of these crystalline materials at ambient pressure has been confirmed. Ab initio molecular dynamics simulations confirmed thermal stability of DOTZ at 298 K and of DNDOTZ at 77 K. A comprehensive spectral characterization (IR, Raman, UV/Vis, NMR) has been performed to provide information for future experimental identification. The parent molecule exhibits powerful detonation properties, which exceed those of most compounds hitherto obtained experimentally or predicted theoretically. Meanwhile, DNDOTZ is predicted to be an effective solid oxidant for common explosives with negative oxygen balance, such as TNT, TNB, DATB, and TATB. Such mixtures demonstrate better propulsive properties than corresponding mixtures of these explosives with gaseous oxygen. By virtue of its zero oxygen balance, the only detonation products of DOTZ are environmentally friendly molecular nitrogen and water. Therefore, this compound is of great potential interest for further energetic applications.

Introduction

According to an analysis of the explosives market by Global Industry Analytics, Inc., the global market for explosives is projected to reach 30 million metric tons in 2024 [1]. Since most explosive formulations contain a significant amount of carbon, a huge mass of carbon oxides is released into the atmosphere, exacerbating the greenhouse effect. In this context, carbon-free nitrogen-rich explosives are highly desirable, since these would release environmentally friendly molecular dinitrogen as the primary detonation product. Ideally, pure nitrogen allotropes other than N₂ [[2], [3], [4]] could replace conventional C–H–N–O explosives, but, to date, only three allotropes of nitrogen, both molecular and polymeric (cg-N, LP-N, and N₈), have been characterized experimentally [[5], [6], [7]]. We should stress that these allotropes have been obtained only in very small quantities under extreme conditions; thus, their practical application is still unachievable.

Meanwhile, various H–N–O compounds have attracted close attention due to their superior performances as high-energy-density materials (HEDM). Among these compounds, the family of ammonium salts with various nitrogen-rich carbon-free anions occupies a special place [[8], [9], [10], [11], [12]]. For example, the burning rate of ammonium dinitramide is about 10 times faster than those of ammonium perchlorate, RDX, and HMX [13]. The most widely used cations and anions, as components of these salts, are illustrated in Chart 1. The latter not only have high nitrogen contents, but also display high enthalpies of formation, favorable detonation properties, and remarkable insensitivities [8]. The most promising species among those presented in Chart 1 is the long-sought pentazolate anion cyclo-N₅^– [14]. Though this anion was first observed in 2002, its synthesis in the form of an ammonium salt was only reported in 2017 [15,16]. A synthetic route to various salts of the cyclo-N₅^– anion has now been developed, making this hitherto elusive species much more readily available [9]. We should stress that the crystal structure of NH₄⁺ cyclo-N₅^– was predicted earlier using an evolutionary algorithm, and it was calculated that this salt should be thermodynamically stable at pressures above 30 GPa [17]. Despite the wrong space group, the predicted crystalline environment is impressively close to that determined experimentally [9]. This clearly demonstrates the predictive force of modern theoretical methods.

Steele and Oleynik performed an extensive study of ternary C–N–O and H–N–O systems at 50 and 200 GPa using an evolutionary algorithm, and obtained the corresponding phase diagrams [[18], [19], [20], [21]]. A few interesting compounds, namely hydrazinium hydroxide (N₂H₅)(HO)–P2₁/m, diammonium oxide (NH₄)₂O-Cmcm, and nitric acid (HNO₃)–P2₁/m, were found to be stable at 200 GPa [18]. At 50 GPa, six ternary compounds were located on the convex hull: H₁₀N₂O–C2/m, H₈N₄O-$R\bar{3}m$ , HNO₃–P2₁/m, H₆N₂O₈-$P\bar{1}$ , H₈N₂O-$P\bar{3}m1$ , and H₁₂N₂O₃–Cm [18]. Bogdanova et al. [22] calculated detonation parameters of different condensed high explosives of the general composition H_xN_yO_z using a multiphase model of detonation products based on the equations of state.

The aforementioned studies inspired us to model a molecular form of an H_xN_yO_z system having zero oxygen balance (H₂O)_xN_y, thus releasing only H₂O and N₂, relatively high nitrogen content (>50 wt%), existing as relatively small molecules, having a high positive enthalpy of formation and crystal density, while maintaining dynamic, mechanical, and thermal stability. Moreover, as we have recently found, the novel energetic polymeric material CarNit4, poly(1,5-tetrazolediyl), demonstrates excellent potential as an explosive, but suffers from a lack of internal oxidant [23]. Thus, we have also modeled a positive oxygen balance material of the composition H_xN_yO_z for potential application as a solid oxidant in explosive formulations or multi-component propellants, alternatives to various heterocyclic compounds [24], or strained nitro-triaziridine derivatives [[25], [26], [27]]. Herein, we present our crystal structure prediction and characterization with respect to dynamic, mechanical, and thermal stability of two crystalline materials, 1,4,2,3,5,6-dioxatetrazinane (DOTZ) and its 2,5-dinitro derivative (DNDOTZ), as well as information for their future spectral identification.