Initial Decomposition Reactions of Bicyclo-HMX [BCHMX or <i>cis</i>-1,3,4,6-Tetranitrooctahydroimidazo-[4,5‑<i>d</i>]imidazole] from Quantum Molecular Dynamics Simulations

We investigated the initial chemical reactions of BCHMX [<i>cis</i>-1,3,4,6-tetranitrooctahydroimidazo-[4,5-<i>d</i>]­imidazole] with the following procedure. First we used density functional theory molecular dynamics simulations (DFT-MD) on the periodic crystal to discover the initial reaction steps. This allowed us to determine the most important reactions through DFT-MD simulations at high temperatures. Then we started with the midpoint of the reaction (unimolecular or bimolecular) from the DFT-MD and carried out higher quality finite cluster DFT calculations to locate the true transition state of the reaction, followed by calculations along the reaction path to determine the initial and final states. We find that for the noncompressed BCHMX the nitro-aci isomerization reaction occurs earlier than the NO₂-releasing reaction, while for compressed BCHMX intermolecular hydrogen-transfer and bimolecular NO₂-releasing reactions occur earlier than the nitrous acid (HONO)-releasing reaction. At high pressures, the initial reaction involves intermolecular hydrogen transfer rather than intramolecular hydrogen transfer, and the intermolecular hydrogen transfer decreases the reaction barrier for release of NO₂ by ∼7 kcal/mol. Thus, the HONO-releasing reaction takes place more easily in compressed BCHMX. We find that this reaction barrier is 10 kcal/mol lower than the unimolecular NO₂ release and ∼3 kcal/mol lower than the bimolecular NO₂ release. This rationalizes the origin of the higher sensitivity of BCHMX compared to RDX (1,3,5-trinitrohexahydro-1,3,5-triazine) and HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine). We suggest changes in BCHMX that might help decrease the sensitivity by avoiding the intermolecular hydrogen-transfer and HONO-releasing reaction