An evaluation of nitro derivatives of cubane using ab initio and density functional theories

A novel method for judging the energy output of energetic compounds has been deduced from the conservation of energy condition. On the basis of B3LYP/6-31++G**fully optimized geometries, the enthalpy of formation, crystal density, detonation velocity and pressure for polynitrocubanes have been calculated using various theoretical methods. It has been observed that for polynitrocubanes the introduction of -NH₂ group onto the skeleton results in the destabilization of the neighboring C-C bonds on the skeleton. The C-C and C-NO₂ bonds of octanitrocubane (ONC) are stronger than those of partly nitrated cubanes, implying that the shock stability of ONC is superior to that of partly nitrated cubanes. For polynitrocubanes the calculated crystal density by the Karfunkel-Gdanitz ab initio method is within 0.07 g/cm³ of experimental crystal density, being more accurate than by the group additivity method. The detonation velocity, the detonation pressure, and the energy output all increase from tetranitrocubane to ONC. The detonation velocity and pressure of ONC are predicted to reach 9.58 km s^-1 and 60.0 Gpa, respectively. It is first indicated that the energy output for 1, 2, 3, 5, 8-pentanitrocuban is close to that of the widely used high explosive HMX and for ONC is about 80% larger than that of HMX.