Unusual Protonation of the Hydroxylammonium Cation Leading to the Low Thermal Stability of Hydroxylammonium-Based Salts

Abstract

Energetic ionic salts (EISs) are a class of thriving and promising energetic materials (EMs) that can possess excellent properties and performances comparable to common conventional EMs composed of neutral molecules. As EMs, their response mechanisms to external stimuli are strongly responsible for their safety and thus are highly concerned about. Nevertheless, insight into these mechanisms remains still lacking. We find in the present work a bimolecular reaction between two same sign charged ions during heating dihydroxylammonium 5,5′-bistetrazole-1,1′-diolate (TKX-50), a typical EIS that are attracting increasing attention with a high potential of practical applications. That is, the protonation of NH<sub>3</sub>OH<sup>+</sup>, or a reaction between two cations, occurs and serves as a dominant initial step in the thermal decay of TKX-50. This is a rare case as a bimolecular reaction can usually hardly take place between two ions with same sign charges (two anions or two cations), due to their electrostatic repulsion preventing their sufficient approaching each other to induce the reaction. The protonation proceeds by a H<sup>+</sup> transfer from a NH<sub>3</sub>OH<sup>+</sup> to its neighboring one, and subsequently decompose NH<sub>3</sub>OH<sup>+</sup> to the final stable products of NH<sub>3</sub> and H<sub>2</sub>O simultaneously to collapse the crystal lattice of TKX-50. This new finding can well explain the experimental observations of the prior decay of NH<sub>3</sub>OH<sup>+</sup> to the bistetrazole-1,1′-diolate anion when TKX-50 heated at a constant temperature of 190 °C and the relatively low thermal stability of NH<sub>3</sub>OH<sup>+</sup> based EISs relative to others. Thereby, we propose a strategy to avoid a ready proton transfer and subsequent decomposition to enhance the thermal stability of EISs. This work is hopefully to richen the insight into both the decay mechanism of EISs and the mechanism of the reactions between same sign charged ions

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