Why Hexanitrobenzene (HNB) Molecules Are Parallelly Stacked along Their Benzene Ring Planes

Abstract

Understanding the molecular structure-crystal packing structure relationship is one of the core objectives of crystal engineering. Hexanitrobenzene (HNB) is a sensitive high-energy compound and presents a surprising molecular stacking structure, i.e., the parallel stacking along their benzene ring planes in crystal seems counterintuitive. HNB molecules each contain six NO₂ and a central frame of benzene ring surrounded by the NO₂ and feature a same sign-charged molecular edge. Given the electrostatic repulsion among the NO₂ groups of adjacent HNB molecules in the crystal, it can hardly make the parallel molecular stacking, as observed experimentally. In the present work, we confirm that the twist of NO₂ and the exposure of the molecular frame around the molecular edge are responsible for the aforementioned stacking pattern. Thereon, six negative and six positive electrostatic potential (ESP) regions alternately occur, belonging to the NO₂ and exposed molecular frame, respectively. It satisfies a necessary requirement of intralayer molecular arrangement, i.e., the alternate distribution of negative and positive ESP regions on the molecular edge. Meanwhile, the n−π interaction of the NO₂·benzene ring governs the interlayer intermolecular interactions. This work is expected to enrich the insight into molecular structure-crystal packing structure relationships.