The Mechanism for Unimolecular Decomposition of RDX (1,3,5-Trinitro-1,3,5-triazine), an ab Initio Study

Abstract

Gas phase hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) is a relatively stable molecule which releases a large amount of energy upon decomposition. Although gas-phase unimolecular decomposition experiments suggest at least two major pathways, there is no mechanistic understanding of the reactions involving RDX or other energetic molecules (such as HMX and TATB), used in applications ranging from automobile air bags to rocket propellants. For the unimolecular decomposition of RDX, we find three pathways:  (i) concerted decomposition of the ring to form three CH_2NNO_2 (M = 74) molecules, and (ii) homolytic cleavage of an NN bond to form NO_2 (M = 46) plus RDR (M = 176), which subsequently decomposes to form various products. Experimental studies suggest that the concerted pathway is dominant while theoretical calculations have suggested that the homolytic pathway might require significantly less energy. We report here a third pathway:  (iii) successive HONO elimination to form 3 HONO (M = 47) plus stable 1,3,5-triazine (TAZ) (M = 81) with subsequent decomposition of HONO to HO (M = 17) and NO (M = 30) and at higher energies of TAZ into three HCN (M = 27). We examined all three pathways using first principles quantum mechanics (B3LYP, density functional theory), including the barriers for all low-lying products. We find:  A threshold at ∼40 kcal/mol for which HONO elimination leads to TAZ plus 3 HONO, while NN homolytic cleavage leads to RDR plus NO_2, and the concerted pathway is not allowed; above ∼52 kcal/mol the TAZ of the HONO elimination pathway can decompose into 3 HCN while the HONO can decompose into HO + NO; above ∼60 kcal/mol the concerted pathway opens to form CH_2NNO_2; at a threshold of ∼65 kcal/mol the RDR of the NN homolytic pathway can decompose into other products. These predictions are roughly consistent with previous experimental results and should be testable with new experiments. This should aid the development of a kinetic scheme to understand combustion and decomposition of solid-phase RDX and related energetic compounds (e.g., HMX).