Reversible Hydrogen Transfer as New Sensitivity Mechanism for Energetic Materials against External Stimuli: A Case of the Insensitive 2,6-Diamino-3,5-dinitropyrazine-1-oxide

Abstract

2,6-Diamino-3,5-dinitropyrazine-1-oxide (LLM-105) was first synthesized about 20 years ago and is regarded as a representative of the new generation of low-sensitivity energetic materials (EMs). Nevertheless, its thermal decay detail still remains lacking; in particular, the atomistic details of the decomposition of its condensed phase are absent. Thus, this work presents a quantum chemistry based study to reveal the details. Four pathways are found to initiate the primary molecular fission, including the intramolecular H transfer from a NH2 group to its neighboring acyl O atom, the NO2 partition, the acyl O partition, and the O partition from a NO2 group. The dominance of these pathways is strongly temperature-dependent, i.e., the intramolecular H transfer and the NO2 partition govern the initial steps at relatively low and high temperatures, respectively, and both the O partitions each occur once only at high temperature. Furthermore, we find that the intramolecular H transfer takes place always with a low energy barrier, and the H-transferred product can also return back to the original molecules with a low barrier too, exhibiting the reversibility of the transfer. More importantly, such reversibility can partly be responsible for the low sensitivity of LLM-105, as the reversible H transfer can buffer the external stimuli with an energy transfer and a slight structural variation only, while without disastrous and irretrievable consequence of a series of spontaneous reactions to final products. This work verifies that the reversible H transfer can take place not only intermolecularly in energetic ionic salts but also intramolecularly in neutral energetic molecules to buffer against external stimuli to facilitate low-impact sensitivity.