Abstract
Co-crystallization is an elegant technique to tune the physical properties of crystalline solids. In the field of energetic materials, co-crystallization is currently playing an important role in the engineering of crystals with improved performance. Here, based on an analysis of the structural features of the green primary explosive, tetramethylammonium salt of 7-oxo-5-(trinitromethyl)-4,5,6,7-tetrahydrotetrazolo[1,5-a][1,3,5]triazin-5-ide (1), a co-former such as the powerful secondary explosive, benzotrifuroxan (BTF, 2), has been proposed to improve it. Compared to the original 1, its co-crystal with BTF has a higher detonation pressure and velocity, as well as an initiating ability, while the impact sensitivity and thermal stability remained at about the same level. Both co-formers, 1 and 2, and co-crystal 3 were characterized by single-crystal X-ray diffraction and their crystal packing was analyzed in detail by the set of approaches, including periodic calculations. In the co-crystal 3, all intermolecular interactions were significantly redistributed. However, no new types of intermolecular interactions were formed during co-crystallization. Moreover, the interaction energies of structural units in crystals before and after co-crystallization were approximately the same. A similar trend was observed for the volumes occupied by structural units and their densifications. The similar nature of the organization of the crystals of the co-formers and the co-crystal gives grounds to assert that the selected co-formers are an ideal pair for co-crystallization, and the invariability of the organization of the crystals was probably responsible for the preservation of some of their properties.
Highlights
A key aspiration of crystal engineering is to create functional materials with taskspecific physical and/or chemical properties
Based on visual inspection of the close and shortened intermolecular contacts, one can define the type of interaction of the central molecule in a crystal with its closest environment. Such a qualitative analysis can be supplemented by an estimation of the energy between the central molecule and each molecule from its closest environment. The latter can be done in terms of atom–atom potentials, quantum chemical calculations of dimers at different levels of theory, semi-empirical methods based on electron density distribution, such as PIXEL or CE-B3LYP, “Atoms in molecules” (AIM) topological theory [43,44,45,46,47,48,49]
We made an attempt to improve those properties of the salt by its co-crystallization with some appropriate co-former
Summary
A key aspiration of crystal engineering is to create functional materials with taskspecific physical and/or chemical properties. In order to be able to purposefully influence the specific properties of co-crystals, it is important to understand at least the main factors that determine the supramolecular assembly of the principal molecule with a co-former through the intermolecular interactions. The co-crystallization technology, providing access to various molecular complexes, is under evolution, most studies focus on modifications of the pharmaceuticals [9,10,11,12,13]. In pharmaceutical co-crystals, one component is an API, while the
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