Roles of Small Molecules in the Stability and Sensitivity of CL-20-Based Host-Guest Explosives under Electric Fields: A Reactive Molecular Dynamics Study.

Host-guest inclusion, constructed by inserting small molecules into voids of energetic crystals, is a novel strategy for creating new energetic materials (EMs) with desired energy and safety. To provide an atomistic-level insight into the fact that small guest molecules can effectively regulate the stability and sensitivity of CL-20, we conducted ReaxFF-lg reactive molecular dynamics simulations on electric-field (EF)-induced decomposition of two typical host-guest EMs, CL-20/HO and CL-20/NO, and compared it to that of α-CL-20 and ε-CL-20. Our findings show that the sensitivity order of the CL-20-based EMs under EFs, α-CL-20/HO > ε-CL-20 > α-CL-20 > α-CL-20/NO, agrees well with the sensitivity obtained from the experiment (ε-CL-20 > α-CL-20 > α-CL-20/NO). Different effects of HO and NO molecules were found responsible for the distinct stability and sensitivity of these materials toward EFs. On the one hand, HO accelerate(s) the structural transformation of CL-20 and thus increases the sensitivity, because the wobbling NO group reduces the stability of CL-20 by weakening its adjacent C-N bonds, whereas NO makes this transition less likely, resulting in low sensitivity of α-CL-20/NO. On the other hand, HO and its decomposition intermediate OH radical can promote destruction of CL-20's cage structure and produce a large amount of water molecules to release heat, making CL-20/HO to decompose faster than ε-CL-20. NO molecules rarely react with CL-20 molecules but absorb heat from the surrounding decomposed CL-20 and thus slow down CL-20's decomposition, resulting in low sensitivity of α-CL-20/NO, as confirmed by transition-state calculations. The results provide a comprehensive understanding of the stability and sensitivity of CL-20-based host-guest explosives under EFs.