Crystal Packing of Cubane and Its Nitryl-derivatives: A Case of the Discrete Dependence of Packing Densities on Substituent Quantities
The discrete crystal density (dc)-nitro group amount dependence of a series of cubane and its nitro derivatives are understood by analogically analyzing their crystal packing. That is, dc does not increase successively with the increase in nitro groups replacing hydrogen atoms because the intermolecular interactions that strongly influence crystal packing vary from H…H to H…O and O…O interactions, with weak to strong and weak strength in turn. The relatively low dc of octanitrocubane (less than that of 1,2,3,4,5,6,8-heptanitrocubane) is thought to be a necessary result of the replacement of all hydrogen atoms by nitro groups, rendering O…O interactions to be completely predominant intermolecular interactions and unfavorable to closer packing, i.e., 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 crystal packing and provide a foundation for designing new CHNO explosives in which H…O and O…O interactions are usually prevalent. Moreover, it is proposed that a specific quantity of hydrogen atoms is usually indispensable for a high dc value for CHNO energetic crystals.