Ab Initio Molecular Dynamics of High-Temperature Unimolecular Dissociation of Gas-Phase RDX and Its Dissociation Products

Unimolecular dynamics of gas-phase hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and its dissociation products were simulated using density functional theory (DFT) at the M06-L level. The simulations of RDX at 2000 K showed that dissociation proceeds from multiple conformers, mostly via homolytic fission of an N-N bond with a minor contribution from elimination of HONO, in agreement with previous transition state theory calculations. However, the simulations of the fission and elimination products revealed that secondary N-N fission is facile and, at the simulated temperature of 1750 K, dominant over other mechanisms. The simulations of the resulting intermediates revealed a number of new unimolecular pathways that have not been previously considered. The transition structures and minimal energy paths were calculated for all reactions to confirm these observations. Based on these findings, a revised set of the unimolecular reactions contributing to gas-phase RDX decomposition is proposed.