On the influence of weak intermolecular interactions on the molecular crystal density of 1,3,5-triazine trinitroalkyl derivatives

It is well known that the densest molecular crystals formed by molecules consisting of C, H, N, O, and F atoms are those of polynitro compounds that do not and cannot participate in strong intermolecular interactions. At the same time, many “typical” organic crystals are also stabilized by weak intermolecular contacts, such as C–H···O(N), O(N)···π, H···H, and so forth. However, the density of such crystals is significantly lower. In this work, we report the synthesis and properties of 1,3,5-triazine derivatives containing both trinitromethyl and/or trinitroethoxy, as well as isopropoxy groups. It was shown by X-ray structural analysis and quantum chemistry calculations that the crystal structures of the obtained 1,3,5-triazine derivatives were stabilized by weak interactions only, such as NO2···NO2, O···π, C–H···O, C–H···N, and H···H contacts. The effects from various types of weak intermolecular contacts on crystal density were evaluated. No strong correlation was found between the energy of interactions, decrease of system volume caused by such interactions, and an increase in crystal density. It was demonstrated that crystal packing can be studied effectively by a recently proposed method via the analysis of ΔOED criterion describing the density increase of molecule and its fragments upon crystal formation from isolated molecules.