n-π Stacking in Energetic Crystals

n-pi stacking refers to the molecular stacking through the lone-pair electron (n)-pi-structure interaction. As an important stacking mode supplementary to hydrogen bonding (HB) and pi-pi stacking, which are usually dominant in the intermolecular interaction, it has generally been ignored in energetic crystals. This work extracts typical n-pi stacked energetic single-component crystals and cocrystals from CSD and categorizes them into three forms, including the contacts of the NO2 benzene ring, NO2 heterocycle, and furazan ring-furazan ring. The n-pi stacking in energetic crystals has the nature of weak electrostatic attraction because each n-pi couple always possesses a weakly positively charged pi and a weakly negatively charged n. This attraction ranges from 6.3 to 20.0 kJ/mol in energy and is generally weaker than intermolecular HB and pi-pi stacking. The n-pi stacking exhibits a mode of T-shape and can hardly occur alone for planar molecules with strong HB acceptors and donors. Hopefully, this work is expected to enrich and perfect the knowledge of the intermolecular interactions in energetic crystals and pave the way for a comprehensive understanding of molecular stacking therein.