6-Azidotetrazolo[5,1-a]phthalazine (ATPH) is a nitrogen-rich compound of surprisingly broad interest. It is purported to be a natural product, yet it is closely related to substances developed as explosives and is highly polymorphic despite having a nearly planar structure with little flexibility. Seven solid forms of ATPH have been characterized by single-crystal X-ray diffraction. The structures show diverse patterns of molecular organization, including both stacked sheets and herringbone packing. In all cases, N•••N and C−H•••N interactions play key roles in ensuring molecular cohesion. The high polymorphism of ATPH appears to arise in part from the ability of virtually every atom of nitrogen and hydrogen in the molecule to take part in close N•••N and C−H•••N contacts. As a result, adjacent molecules can adopt many different relative orientations that are energetically similar, thereby generating a polymorphic landscape with an unusually high density of potential structures. This landscape has been explored in detail by the computational prediction of crystal structures. Studying ATPH has provided insights into the field of energetic materials, where access to multiple polymorphs can be used to improve performance and clarify how it depends on molecular packing. In addition, our work with ATPH shows how valuable insights into molecular crystallization, often gleaned from statistical analyses of structural databases, can also come from indepth empirical and theoretical studies of single compounds that show distinctive behavior.
The synthesis and experimental testing of energetic materials can be hazardous, but their many industrial and military applications necessitate their constant research and development. We evaluate computational methods for predicting the crystal structures of energetic molecular organic crystals from their molecular structure as a first step in computationally evaluating materials, which could guide experimental work. Crystal structure prediction (CSP) is evaluated on a test set of ten energetic materials with known crystal structures, initially using a rigid-molecule, anisotropic atom-atom force field approach, followed by re-optimization of predicted crystal structures using dispersion-corrected solid state density functional theory (DFT). CSP using the force field was found to provide good results for some molecules, whose known crystal structures are reproduced by one of the lowest energy predictions, but are more variable than for other small organic molecules. Re-optimization of predicted crystal structures using solid state DFT leads to reliable predictions, demonstrating CSP as a approach that can be applied in the area of energetic materials discovery and development.
The synthesis and experimental testing of energetic materials can be hazardous, but their many industrial and military applications necessitate their constant research and development. We evaluate computational methods for predicting the crystal structures of energetic molecular organic crystals from their molecular structure as a first step in computationally evaluating materials, which could guide experimental work. Crystal structure prediction (CSP) is evaluated on a test set of 10 energetic materials with known crystal structures, initially using a rigid-molecule, anisotropic atom–atom force-field approach, followed by reoptimization of predicted crystal structures using dispersion-corrected solid-state density functional theory (DFT). CSP using the force field was found to provide good results for some molecules, whose known crystal structures are reproduced by one of the lowest-energy predictions, but are more variable than typical results for other small organic molecules. Reoptimization of predicted crystal structures using solid-state DFT leads to reliable predictions, demonstrating CSP as an approach that can be applied in the area of energetic materials discovery and development.
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