Bonding analysis of the effect of strain on trigger bonds in organic-cage energetic materials

Abstract

Wiberg bond index and natural bond orbital analyses were used to examine the effect of strain on trigger bonds for a series of organic-cage molecules. In substituted cage hydrocarbons, weakening of the interior C–C bonds and strengthening of exterior C–X bonds are explained in terms of the contributions of the carbon 2s and 2p atomic orbitals to the bonds. The interior C–C, rather than C–NO2, bonds are expected to break to initiate explosive decomposition in nitro-substituted tetrahedranes, prismanes, and cubanes. Activation of C–C bonds in cage molecules increases with nitro substitution, but persubstituted cages are not necessarily the most activated. For example, heptanitrocubane is predicted to be more sensitive than octanitrocubane, consistent with experimental studies. In contrast, for a series of hypothetical nitroester substituted prismanes, the strengthening of the exterior C–ONO2 bond weakens the O–NO2 bond more than the interior C–C bonds. In CL-20 and TEX, cage strain as determined by WBI analysis in the fused five- and six-membered rings is less significant than for the cage hydrocarbon. In these known energetic materials, the N–NO2 trigger bonds are activated by the orientation of the nitro groups.