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.