Herman Ammon

Structural chemistry, predictions for explosives and propellants, lattice energy calculations, crystal packing analysis, crystal structure predictions

The discovery of new energetic materials can be facilitated, accelerated and made more cost effective with computer modeling and simulations for the identification of compounds that have significant advantages over materials currently in use. The quantitative estimation of properties such as the heat of formation, solid state density, detonation velocity and pressure and sensitivity can readily screen potential energetic candidates for possible synthesis and subsequent evaluation.

Our procedures for solid state structure prediction attempt to replicate the most common packing motifs observed in thousands of experimental crystal structures. The key codes are MOLPAK (molecular packing), ROTPAK (rotational packing) and PMIN (lattice energy optimization). MOLPAK functions with rigid molecules while ROTPAK handles conformational changes concomitant with crystal packing.

1 = hexanitrohexaaza-isowurtzitane (CL-20)
2 = hexanitrohexaaza-wurtzitane
3 = hexanitrohexaaza-adamantane

Only one of the structures shown (CL-20, 1) is known. The 2.07 g/cm³ experimental density of the e-polymorph is high for a molecule containing only C, H, N and O. Predicted solid state structures and performance characteristics for the currently unknown structural isomers 2 and 3 place these in the same high-energy category as 1. The conformational flexibility of the C₂-N-NO₂ groups in nitramines present a challenge for solid state predictions which must allow multiple structural changes concomitant with crystal packing. Note, for example, the N conformations indicated by the arrows in 1 and 2, can be described as equatorial and axial, respectively.