Theoretical Study of Mechanisms, Thermodynamics, and Kinetics of the Decomposition of Gas-Phase r-HMX (Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) Shaowen Zhang, Hung N. Nguyen, and Thanh N. Truong* Henry Eyring Center for Theoretical Chemistry, Department of Chemistry, UniVersity of Utah, 315 S 1400 E, Room 2020, Salt Lake City, Utah 84112 ReceiVed: January 10, 2003 We present a theoretical study of the decomposition mechanism of gas-phase R-HMX. Four distinct channels were studied using the B3LYP/cc-pVDZ level of theory. These are as follows: (i) HMX first loses an NO 2 to form HMR which further breaks a C-N bond to form a chain structure and then later loses three methylenenitramines (MN, H 2 CNNO 2 ) successively; (ii) the chain structure forms a 10-member ring via a rung closure step before undergoing further decomposition; (iii) HMX first eliminates an HONO, then loses two MN, and eliminates an HONO successively; (iv) HMX eliminates two HONO successively, then loses an MN, and finally eliminates an HONO. The rate constants of each elementary reaction have been calculated using the transition-state theory. The thermodynamics properties were also calculated for the stable species by employing a standard statistical thermodynamics method. Channel i was found to be the preferred decomposition pathway on the basis of the analysis of rate constants of the elementary reactions. 1. Introduction The cyclic nitramines octahydro-1,3,5,7-tetranitro- 1,3,5,7- tetrazocine (HMX) and hexhydro-1,3,5-trinitro-1,3,5-triazine (RDX) have been widely used in various propellants and explosives due to their physical properties. Understanding the fundamental chemical mechanism and kinetics of the combus- tion or detonation processes of these materials is important for further improvement in the use of these materials. However, due to the energetic nature of these materials, the decompositions are so fast and complex 1,2 that it is difficult to experimentally explore the chemical details of these processes. In recent years, with the advance in computer technology and computational quantum chemistry methods, especially the development of the density functional theory, it is possible to predict the energetic nature of these reactions with high accuracy for larger molecules such as HMX. In the past several years, theoretical studies of nitramines mainly concentrated on smaller molecules and RDX. 3-9 Few theoretical results have been reported concerning the decom- position mechanism of HMX. 2,10-14 Melius studied the gas- phase decomposition mechanism using the empirically corrected ab initio quantum chemistry method BAC-MP4. 12 He concluded that a N-NO 2 bond fission reaction occurs at high temperature and HONO elimination occurs at low temperature. Lewis and co-workers 11 calculated the initial steps of four possible decomposition pathways of HMX using BLYP and B3LYP DFT methods with the 6-311G(d,p) basis set. The four pathways are N-NO 2 bond fission, HONO elimination, C-N bond scission of the ring, and the concerted ring fission. According to their results, the N-NO 2 bond fission path is the dominant initial step of the decomposition of gas-phase HMX. Chakraborty and co-workers 10 calculated the unimolecular decomposition mech- anism of -HMX using the B3LYP/6-31G(d) method. They identified three distinct channels: (1) a N-NO 2 bond fission reaction to form NO 2 and HMR, which subsequently decom- poses to various products through several subsequent pathways; (2) successive HONO elimination to give four HONO plus a stable intermediate; and (3) oxygen migration from one of the NO 2 groups of HMX to a neighboring carbon atom followed by decomposition steps. Among these three pathways, the N-NO 2 bond fission path has the lowest barrier in the initial step. We have in fact performed an accurate direct dynamics study on the kinetics of the N-NO 2 bond fission reaction using the microcanonical variational transition-state theory with the B3LYP/cc-pVDZ potential energy surface. The predicted ther- mal rate constants are in agreement with experimental results. 13 We also studied the branching ratio and pressure-dependent rate constants of the N-NO 2 bond fission path and HONO elimina- tion path with the master equation method and found that the N-NO 2 bond fission path dominates the reaction at high- pressure limits. 14 Experimental studies of HMX were mainly concentrated on the condensed phase. Various species (for example m/e ) 250, 249, 222, 205, 176, 175, 148, 128, 120, 102, 97, 81, 75, 74, 70, 56, 54, 47, 46, 45, 43, 42, 32, 30, 28) have been identified in different decomposition experiments of HMX using mass spectra techniques. 15-22 These results indicate that the decomposition of HMX is very complicated and may undergo different mechanisms in different conditions. Brill 23 has suggested two global pathways in the thermal decomposition of HMX in the condensed phase: Tang and co-workers also concluded 22 that a multistep mech- anism might be more realistic in explaining the experimental data in their laser-assisted self-burning experiment of the condensed-phase HMX. On the basis of his pyrolysis results of HMX determined by simultaneous thermogravimetric modulated beam mass spectrometry technique, Behrens proposed a mech- anism 24 where HMX first decomposes via the N-N bond HMX f 4(HOHO + HCN) HMX f 4(H 2 CO + NNO) 2981 J. Phys. Chem. A 2003, 107, 2981-2989 10.1021/jp030032j CCC: $25.00 © 2003 American Chemical Society Published on Web 04/01/2003