Insight into the roles of small molecules in CL-20 based host-guest crystals: a comparative DFT-D study

The host-guest inclusion strategy has emerged as a promising method for developing advanced energetic materials and has been successfully applied to a CL-20 crystal. Based on density functional theory (DFT), the effects of inserted molecules were evaluated by comparatively studying the structural, electronic, energetic, mechanical, and detonation properties of three host-guest crystals (CL-20/H2O, CL-20/N2O, and CL-20/CO2) and an anhydrous CL-20 crystal. The results demonstrate that the insertion of H2O, N2O, and CO2 has little effect on the crystal and molecular structure of CL-20. But they enlarge the lattice cavities and density of states and decrease the band gaps of the host-guest crystals at different degrees compared with the CL-20 crystal. The energetic analysis clarifies and quantifies the stabilization effect of different small molecules on the host-guest crystals. The inclusion molecules interact with the CL-20 molecules through hydrogen bonding and van der Waals forces, which can be verified by the Hirshfeld surface and radial distribution functional. The introduction of the three small molecules increases the bulk modulus and shear modulus of the CL-20 crystals, which indicates that the insertion of the guest molecules is helpful for increasing the stiffness and fracture strength of the CL-20 crystal. The insertion of the host molecules improves the detonation properties of CL-20 to different extents, among which N2O works best. Our results would provide fundamental insight into the roles of the guest molecules in the host-guest energetic crystals.