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