Analysis of intermolecular interactions in homologous molecular crystals of energetic materials

Abstract

Crystal packing modes determine the energy and safety performance of energetic materials (EMs) to some extent. To deepen the understanding of the intermolecular interactions in the crystal packings, the homologous molecular crystals of energetic materials were first defined according to the packing similarities in this study, and four groups of energetic homologous crystals with the same packing structures were found from the large experimentally available data. Several computational approaches were employed to comparatively probe the intermolecular interactions at molecular and crystal levels. The results of the analysis show that the common organic skeleton of two molecular conformations in each homologous packing structure remains the same or becomes mirror-symmetrical with a minor variation of functional groups, resulting in the local change of molecular electrostatic potentials, while the overall distribution of electrostatic potentials can be largely maintained. Compared with the other groups of homologous crystals, interestingly, the differences in intermolecular interactions in Group 4 are relatively large according to two-dimensional fingerprint plots and energy frameworks, indicating that the crystal packing of energetic materials is predominately caused by some types of intermolecular interactions. The results reported here are expected to inspire the constructions of energetic crystals with specific packing modes to improve performances, such as a minor modification to the molecular structure while keeping original crystal packing style. The homologous molecular crystals in energetic materials were first proposed based on the packing similarities to comparatively explore the intermolecular interactions in the same packing structures, with the goal of improving the understanding of crystal construction of energetic materials with specific packing modes.