The cage strain energies of high-energy compounds

In this paper the cage strain energies of polynitroadamantanes, polynitrocubanes, hexanitrohexaazaisowurtzitane (HNIW or Cl-20), and some polynitrogen compounds were systematically calculated by density functional theory BOP/TNP method and the designs of homodesmotic reactions. The cage strain energies of polynitroadamantanes and polynitrocubanes increase gradually with the increase of the nitro-substituted number. However, the increase of the cage strain energies of polynitroadamantanes is less drastic than that of polynitrocubanes; on the contrary, the decrease of the bond dissociation energies of C–NO2 bonds in the former compounds is more rapidly than the latter. Accordingly, the cage strain energies and bond dissociation energies of C–NO2 bonds play different roles in determining the stability of high-energy compounds. An increase in the number of nitro groups may be advantageous to the performance and oxygen balance, but disadvantageous to the stability; an increase of cage strain energy may result in a decrease of stability, but more energy will release when a cage opening occurs. Rational number of nitro groups and proper cage strain energy are indispensable for an ideal nitro-containing explosive with high energy but insensitive to external impact. Polynitrogen compounds are high in cage strain energies, this may be why fewer polynitrogen compounds have been synthesized, so far. A proper design may harvest some polynitrogen compounds with low cage strain energies and high stability.