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

The dependence of discrete crystal density (dc) 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 dc 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 center dot center dot center dot H to H center dot center dot center dot O and O center dot center dot center dot 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 dc can be seen as a prototype to understand the crystal packing and provide a foundation for designing new CHNO explosives, in which H center dot center dot center dot O and O center dot center dot center dot 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.