Computer Simulations and Analysis of Structural and Energetic Features of Some Crystalline Energetic Materials

A database of 43 literature X-ray crystal structure determinations for compounds with known, or possible,
energetic properties has been collected along with some sublimation enthalpies. A statistical study of these
crystal structures, when compared to a sample of general organic crystals, reveals a population of anomalously
short intermolecular oxygen−oxygen separations with an average crystal packing coefficient of 0.77 that
differs significantly from 0.70 found for the general population. For the calculation of lattice energies, three
atom−atom potential energy schemes and the semiempirical SCDS-PIXEL scheme are compared. The nature
of the packing forces in these energetic materials is further analyzed by a study of the dispersive versus
Coulombic contributions to overall lattice energies and to molecule−molecule energies in pairs of near
neighbors in the crystals, a partitioning made possible by the unique features of the SCDS-PIXEL scheme.
It is shown that dispersion forces are stronger than Coulombic forces, contrary to common belief. The low
abundance of hydrogen atoms in these molecules, the close oxygen−oxygen contacts, and the high packing
coefficients explain the observation that, for these energetic materials, crystal densities are anomalously high
compared to those of most organic materials. However, an understanding, not to mention prediction or control,
of the deeper mechanisms for the explosive power of these crystalline materials, such as the role of lattice
defects, remains beyond present capabilities.