Crystal Packing of Low-Sensitivity and High-Energy Explosives

Low-sensitivity and high-energy explosives (LSHEs) are highly desired for their comprehensive superiority of safety and energy. Crystal packing is crucial to both the safety and energy, and therefore becomes of interest in energetic crystal engineering. This work carries out systemic analyses on the crystal packing of 11 existing LSHEs with both energy and safety close or superior to TNT. As a result, we find that the LSHE crystals wholly feature pi-pi stacking with the aid of intermolecular hydrogen bonding. Each LSHE molecule is pi-bonded with a big conjugated structure composed of all non-hydrogen atoms in the entire molecule. Intramolecular hydrogen bonding exists in most LSHE molecules with strongly active hydrogen bond (HB) donors of amino and hydroxyl groups, and various strength. These big pi-conjugated structures and intramolecular HBs lead to planar molecules with high stability, settling a base of pi-pi stacking in crystals. With the help of intermolecular HBs, the pi-pi stacking holding the LSHE crystals appears in four modes. Among them, the face-to-face stacking (always offset) gives rationally the smallest steric hindrance when interlayer slide occurs in crystal, which is the reason for very low impact sensitivity. This work suggests that the planar conjugated molecular structure and intermolecular hydrogen bonding supporting the pi-pi stacking are necessary to the crystal engineering of ISHEs.