The Mechanism for Unimolecular Decomposition of RDX (1,3,5-Trinitro-1,3,5-triazine), an ab Initio Study Debashis Chakraborty, Richard P. Muller, Siddharth Dasgupta, and William A. Goddard, III* Materials and Process Simulation Center, Beckman Institute (139-74), DiVision of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125 ReceiVed: October 18, 1999 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 2 NNO 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 2 NNO 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). I. Introduction The cyclic nitramines hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and octahydro-1,3,5,7- tetranitro-1,3,5,7-tetrazocine (HMX), are important energetic ingredients for various propel- lants and explosives since they release a large amount of energy in bulk decomposition. Thermal decomposition of these ener- getic materials has been observed to form very simple final product molecules, such as HCN, NO, N 2 O, NO 2 , CO, CO 2 , H 2 O, H 2 CO, etc. Understanding the underlying complex chemi- cal processes is essential to obtain an improved model for combustion or detonation of these energetic materials. A number of experimental studies 1-20 have been directed toward elucidating the mechanistic details of the thermal decomposition of RDX, and various plausible reaction pathways have been proposed. Many experiments dealt with bulk phase materials, 1-13 including decomposition in the condensed phase (solid or liquid) and the gas-phase flame structure near the burning RDX surface. The results reported have varied widely depending on the experimental conditions. To determine the initial steps of decomposition for these condensed phase studies, we focused on the gas-phase pyrolysis. I.A. Experimental Studies. In an effort to understand the molecular level decomposition mechanism of RDX, Zhao, Hinsta, and Lee (ZHL), studied the infrared multiphoton dissociation (IRMPD) of RDX in a molecular beam using a time-of-flight velocity spectra (TOFVS). 16 They detected mass fragments of 120, 102, 81, 74, 56, 46, 44, 42, 26-30, and 12- 17 as the major products from laser photolysis (which includes mass fragments of both primary and secondary decomposition products). ZHL concluded that the dominant primary channel in the RDX decomposition is a concerted symmetric triple fission of RDX ring. Based on the reported heat of formation of 33.6 kcal/mol for methylenenitramine (MN) by Melius and Binkley, 21 ZHL estimated the endothermicity to be 80 kcal/ mol for the concerted ring fission (which compares with the N-N bond dissociation energy of 48 kcal/mol calculated by Melius and Binkley). ZHL estimated that the laser pulse in their experiments deposited a total internal energy of 80 kcal/mol. Thus, they concluded “the concerted ring breaking is energeti- cally accessible for a large fraction of the RDX in this experiment, even though the endoergicity derived from the heat of formation of MN given by Melius and Binkley seems unreasonably high”. On the other hand, more recent UV photolysis experiments 18-20 observed N-NO 2 homolysis as the primary decomposition reaction. Behrens and Bulusu 2 using simultaneous thermogravimetry modulated beam mass spectrometry measurements, TOFVS * Corresponding author. 2261 J. Phys. Chem. A 2000, 104, 2261-2272 10.1021/jp9936953 CCC: $19.00 © 2000 American Chemical Society Published on Web 02/23/2000