Crystal packing of cubane and its nitryl-derivatives: a case of the discrete dependence of packing densities on substituent quantities

Abstract

The dependence of discrete crystal density (d_c) of a series of cubane and its nitro derivatives on the nitro group amount was understood via analogically analyzing their crystal packing. It was found that d_c does not successively increase with the increase in the hydrogen atoms replacement by nitro groups because the intermolecular interactions that strongly influence the crystal packing vary from H⋯H to H⋯O and O⋯O interactions, with weak to strong and weak strength, respectively. The relatively low d_c of octanitrocubane (less than that of 1,2,3,4,5,6,8-heptanitrocubane) is thought to be clearly due to the replacement of all the hydrogen atoms by nitro groups, rendering O⋯O interactions to be completely predominant and unfavorable for closer packing, i.e., they have a lower packing coefficient. This deduction from the crystal packing of a series of analogues of nitrocubane derivatives to understand the observed d_c can be seen as a prototype to understand the crystal packing and provide a foundation for designing new CHNO explosives, in which H⋯O and O⋯O interactions are usually prevalent. Moreover, it was proposed that a specific quantity of hydrogen atoms is usually indispensable to achieve a high d_c value for CHNO energetic crystals.