The Molecular & Crystal Structures of Polycyclic Energetic Materials

Even before the ‘Buckey-ball’ (or fullerene) era, organic chemists were interested in, and attracted to, the synthesis of compounds containing many rings fused together - especially the symmetric ones. By virtue of their balanced linkages, such compounds may be kinetically stable despite inclusion of massive deviations from normal bond distances and angles. These distortions, in compounds such as cubanes, tetrahedranes, dodecahedranes and prismanes, comprise a storehouse of chemical potential energy known to the chemist as ‘strain energy’ (unrelated to overall crystal strain discussed in many materials studies).Besides the obvious benefit of including more potential energy in an energetic material, the arrangement of atoms in ring assemblies (rather than chains) leads to a material with a high intrinsic crystal density - a sine qua non for an explosive or propellant. It has long been recognized that the next generation of energetics - better in performance and/or safety - will contain many polycyclic compounds. Hundreds of model compounds were synthesized in the last decade, and their molecular structures identified or verified at the Naval Research Lab. Some of the most promising, beautiful, and unusual ones, including many energetic cubanes, will be discussed.