The compound tetranitrotetraazabicyclooctane (bicyclo-HMX) has been calculated by using the density functional theory method at the B3LYP/6-311G* level. There are two optimized molecular geometries: α (1H,5H-cis) and β (1H,5H-trans) conformations. Based on the comparison of molecular symmetry, intramolecular hydrogen bonds, ring strain, total energies and frontier orbital energies, it was found that α conformation is more stable than β. The bond lengths of N—N are longer and Mulliken population of N—N is smaller than the other bonds in bicyclo-HMX, which means that the N—N may be the initial bond in pyrolysis and explosion. Normal-mode analyses were used to characterize the stable point and to determine the harmonic vibrational frequencies. Thermodynamic properties at 200～1000 K were provided using statistical thermodynamic method. Pyrolysis mechanism was investigated using unrestricted Hatree-Fock model of semi-empirical PM3 method, getting each transition state and activation energy, and finding that the rupture of side N—NO₂ bond is preferential. And the density, detonation velocity and pressure of bicyclo-HMX were predicted, which will set a foundation to look for high energy density materials.

Key words: tetranitrotetraazabicyclooctane, density functional theory, pyrolysis mechanism, detonation property