Reversibility of the Hydrogen Transfer in TKX-50 and Its Influence on Impact Sensitivity: An Exceptional Case from Common Energetic Materials

Abstract

Hydrogen transfer (HT) has been confirmed to possibly serve as a crucial step to initiate and control the decay of common energetic materials (EMs). Nevertheless, the case of currently thriving energetic ionic salts (EISs), as well as the difference in the HT influence on the properties and performances of common EMs and EISs, is poorly known. In this work, we carry out a comparative study of the HTs of dihydroxylammonium-5,5′-bistetrazole-1,1′-diolate (TKX-50) and β-1,3,5,7-tetranitro-1,3,5,7-tetrazocane (β-HMX), as the candidates of EISs and common EMs, respectively, by means of ab initio molecular dynamics simulations and climbing image nudged elastic band calculations, as well as powder X-ray diffraction (PXRD) detections. As a result, the HT in TKX-50 is found to be significantly different from that in β-HMX in many respects. The HT in TKX-50 can occur reversibly between the adjacent cations and anions as an intermolecular reaction. The HT is in fact a proton transfer and is energetically preferred as a first-step reaction with a relatively low energy barrier, instead of a rate-determining one for the entire decay of TKX-50. In comparison, with respect to HMX, the intermolecular HT unlikely takes place, while the intramolecular one, with a neutral H atom and a relatively high energy barrier, occurs as one of the possible rate-controlling steps toward the entire decomposition. More importantly, the reversible HT in TKX-50 implies a completely novel impact sensitivity mechanism for EMs; i.e., the external impact energy can partly be converted into the chemical energy stored first (when heating to approaching ignition) and dissipated by the chemical energy release subsequently (when cooling), which contributes to low impact sensitivity. This work provides an exception of a prior reaction favoring high impact safety of EMs, relative to those that mostly result in irreversible and disastrous consequences. Thus, this study hopefully extends the fields of both HT and EMs.