π-Stacked Interactions in Explosive Crystals: Buffers against External Mechanical Stimuli

The π-stacked interactions in some explosive crystal packing are discussed. Taking a typical π-stacked explosive 2,4,6-trinitrobenzene-1,3,5-triamine (TATB) as a sample and using molecular simulations, we investigated the nature of the π-stacked interactions versus the external mechanical stimuli causing possible slide and compression of explosives. As a result, between the neighbor layers in the TATB unit cell, the electrostatic attraction decreases with a little decrease of vdW attraction when its top layer slides, whereas the vdW attraction increases with a decrease of electrostatic attraction when TATB crystal is compressed along its c axis. Meanwhile, we studied the correlation between the π-stacked structures and the impact sensitivities of explosives by means of three representatives including TATB with typical planar π-stacked structures, 2,2-dinitroethylene-1,1-diamine (Fox-7) with wavelike π-stacked structures, and 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX) without π-stacked structure. The results showed that π-stacked structures, particularly planar layers, can effectively buffer against external mechanical stimuli. That is, π-stacked structures can partly convert the mechanical energy acting on them into their intermolecular interaction energy, to avoid the increase of the molecular vibration resulting in the explosive decomposition, the formation of hot spots, and the final detonation. This is another reason for the low mechanical sensitivity of π-stacked explosives besides their stable conjugated molecular structures.