Synergistic effects of red- and blue-shifting hydrogen bonds in CL-20 host–guest systems: counteracting hyperconjugation to enhance stability

The host–guest inclusion strategy is a promising approach for solving the contradiction between sensitivity and energy in energetic materials, which is applied to CL-20. In this study, a series of CL-20 based host–guest systems, anhydrous a-CL-20 (denoted as CL-20/0), CL-20/O₂, CL-20/F₂, CL-20/H₂O (a-CL-20), CL-20/HF, and CL-20/2HF, were designed and optimized by density functional theory (DFT) and the molecular dynamics (MD) method. Their structures, electronic properties, and intramolecular weak interactions between the host and guest molecules were systematically investigated. Our research reveals that blue-shifting hydrogen bonds (bHBs) and red-shifting hydrogen bonds (rHBs) both exist in the systems with polar guest molecules and dominate the intermolecular weak interactions. The bHBs facilitate the flow of electrons from the guest molecule to CL-20, and rHBs allow electrons to flow from the cage structure of CL-20 to the nitro groups. Their synergistic effect breaks the intramolecular hyperconjugation effect inside CL-20 and increases the electron density of N–NO₂ bonds, which helps enhance the strength of the N–NO₂ bonds and stability of CL-20 guest–host systems. This study provides novel insights into the competitive and cooperative roles of hydrogen bonding and hyperconjugation in CL-20 host–guest systems, offering valuable guidance for designing novel energetic materials with both improved safety and performance.