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

We demonstrate the use of quantum molecular dynamics to identify the β- to δ-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 β-HMX that operate simultaneously. (1) For the HONO release reaction, β-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, β-HMX first transforms to the δ-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 δ-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).