Energy decomposition of intermolecular interactions in energetic co-crystals

Energetic co-crystals (ECCs) are now thriving and becoming alternatives to energetic materials. Thereby, it is important to understand the intermolecular interactions present in ECCs to obtain knowledge about ECC engineering. However, the physical sources of the interactions remain unclear, even though the interactions have already been understood as the three conventional basic interaction kinds, or the three main traditional engineering motifs of organic crystals, namely hydrogen bonding, pi-stacking and halogen bonding. Twelve typical molecular pairs extracted from five observed EECs covering all the three interaction kinds are selected to partition the intermolecular interaction energy and discuss the physical sources, by density functional theory calculations and block located wavefunction energy decomposition analyses (BLW-EDA). We find that, after carefully examining all observed ECCs, each conventional interaction motif in energetic-energetic molecular pairs is always weak, and dominated by a frozen effect, i.e., van der Waals and electrostatic interactions. The rather strong hydrogen bonding exists in the molecular pairs with one non-energetic molecule at least, and is predominated by the polarization and charge transfer effects. Meanwhile, we find small bond order variations caused by the crystal packing of energetic-energetic cocrystals (EECCs), thereby showing small molecular stability variation. It suggests that it is difficult to increase the molecular stability of the energetics by cocrystallization to improve safety; while the safety of EECCs will benefit from the enhanced intermolecular interactions and the improved crystal packing mode favoring ready shear slip.