Mechanism for unimolecular decomposition of HMX (1,3,5,7-tetranitro-1,3,5,7-tetrazocine) an ab initio study

To improve the mechanistic understanding of the possible decomposition in t he gas phase of the energetic material HMX (octahydro-1,3,5,7-tetranitro-1, 3,5,7-tetrazocine), we used ab initio calculations to determine the various unimolecular decomposition channels. We find three distinct mechanisms: (i ) homolytic cleavage of N-N bond to form NO2 (M = 46) and HMR (M = 250) whi ch subsequently decomposes to form various products (ii) successive HONO el iminations to give four HONO (M = 47) plus a stable intermediate (M 108); ( iii) O-migration from one of the NO2 groups of HMX to neighboring C atom fo llowed by the decomposition of intermediate (M = 296) to INT222 (a ring-ope ned RDX structure) and MN-oring (M = 74), which can undergo dissociation to smaller mass fragments. The decomposition scheme for HMX is similar to tha t for RDX presented earlier (J. Phys. Chem. A 2000, 104, 2261), except that concerted decomposition of HMX to four MN (M = 74) molecules is not a favo rable decomposition pathway, whereas this pathway was found in RDX decompos ition (both experimentally and theoretically). The formation of RDR-o in th e N-N homolysis pathway 1 or the formation of INT222 in pathways 1 and 3 pr esents an unified mechanistic scheme for the decomposition of both of these nitramines. The HMX decomposition mechanism correlates with available cond ensed phase experimental results, but detailed comparison of the predicted gas phase energetics is not possible.