Initial Decomposition of HMX Energetic Material from Quantum Molecular Dynamics and the Molecular Structure Transition of beta-HMX to delta-HMX

We demonstrate the use of quantum molecular dynamics to identify the beta- to delta-molecular structure transition in bulk-phase HMX, which has been considered as the primary reason for the increased sensitivity in the thermal decomposition of HMX. Both physical and chemical changes accompany this transition, but no previous study has shown conclusively which specific change, or set of changes, is responsible. We find that the initial decomposition mechanism of HMX can explain this sensitivity issue. Our DFT simulations of the periodic system followed by detailed finite cluster calculations of the transition states find two distinct initial unimolecular reaction pathways in beta-HMX that operate simultaneously. (1) For the HONO release reaction, beta-HMX first transformed to an intermediate, in which one parallel N-NO2 group transitions from chair to boat conformations with a low +1.2 kcal/mol barrier, followed by unimolecular HONO release (+42.8 kcal/mol barrier, rate-determining step). (2) For the NO2 cleavage reaction, beta-HMX first transforms to the delta-HMX structure in two steps, with low barriers of +1.9 and +7.6 kcal/mol for each step, followed by unimolecular NO2 release (+31.3 kcal/mol barrier). Starting with delta-HMX, we find an initial unimolecular NO2 cleavage and then an independent HONO release reaction with the barriers of +31.6 kcal/mol (NO2 cleavage) and +38.9 kcal/mol (HONO release). We find that the constant proportional simulated initial structure transition temperature is 453 K, which is consistent with the experimental results (466 K).