The mechanism for unimolecular decomposition of RDX (1,3,5-trinitro-1,3,5-triazine), an ab initio study

Gas phase hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) is a relatively sta ble molecule which releases a large amount of energy upon decomposition. Al though gas-phase unimolecular decomposition experiments suggest at least tw o major pathways, there is no mechanistic understanding of the reactions in volving RDX or other energetic molecules (such as HMX and TATB), used in ap plications ranging from automobile air bags to rocket propellants. For the unimolecular decomposition of RDX, we find three pathways: (i) concerted de composition of the ring to form three CH2NNO2 (M = 74) molecules, and (ii) homolytic cleavage of an NN bond to form NO2 (M = 46) plus RDR (M = 176), w hich subsequently decomposes to form various products. Experimental studies suggest that the concerted pathway is dominant while theoretical calculati ons have suggested that the homolytic pathway might require significantly l ess energy. We report here a third pathway: (iii) successive HONO eliminati on to form 3 HONO (M = 47) plus stable 1,3,5-triazine (TAZ) (M = 81) with s ubsequent decomposition of HONO to HO (M = 17) and NO (M = 30) and at highe r energies of TAZ into three HCN (M = 27). We examined all three pathways u sing first principles quantum mechanics (B3LYP, density functional theory), including the barriers for all low-lying products. We find: A threshold at similar to 40 kcal/mol for which HONO elimination leads to TAZ. plus 3 MON O, while NN homolytic cleavage leads to RDR plus NO2, and the concerted pat hway is not allowed; above similar to 52 kcal/mol the TAZ of the HONO elimi nation pathway can decompose into 3 HCN while the HONO can decompose into H O + NO; above similar to 60 kcal/mol the concerted pathway opens to form CH 2NNO2; at a threshold of similar to 65 kcal/mol the RDR of the NN homolytic pathway can decompose into other products. These predictions are roughly c onsistent with previous experimental results and should be testable with ne w experiments. This should aid the development of a kinetic scheme to under stand combustion and decomposition of solid-phase RDX and related energetic compounds (e.g., HMX).