2,4,6-Tris(isopropylamino)-1,3,5-trinitrobenzene : Probing the Impact of Crystal Packing Forces in the Dimorphs and the Cocrystals–Experimental and Theoretical Study

Abstract

We attempted to obtain a large amount of solvates of 2,4,6-tris(isopropylamino)-1,3,5-trinitrobezene 1 and tried to embark on a systematic study (both experimentally and theoretically) of conformational effects of crystal packing. Here we describe their investigation by X-ray diffraction, thermal analysis, and computational methods (force field and ab-initio). The crystal and molecular structures were determined by X-ray diffraction. For thermal analysis investigations a combination of Differential Scanning Calorimetry (DSC) and Thermogravimetry (TG) measurements were done. In our work we have calculated the electrostatic potential (ESP) charges and the molecular energies of the experimental molecular structures after the correction of the hydrogen atomic positions, using the Density Functional Theory (DFT) electronic structure program DMOL3 (2.2), with DNP basis set and GGA-BLYP functional. The molecular structures with the calculated charges then have been used to calculate: the crystal, the lattice and individual interaction energies in each structure, within the molecular simulation program Discover, using COMPASS force field. A wide range of small molecules were found to be incorporated into crystals of 1.The crystal and molecular structures of the first dimorph 1a of 1 has been found previously. Six different packing arrangements, which have been divided into four groups, for the cosolvates of 1 were found. For the conformations of the six-membered ring of 1 in the different cosolvates, we have found that: three different conformations can be adopted by the host molecule 1: boat form with two short and four long C—C bonds in the six-membered ring (quinonoid character), twist form with two long and four short C—C bonds (cyanine character) and intermediate twisted-boat form. The following results were obtained from both DSC experiments and the theoretical calculations of the lattice energy: 1.Substantially high dissolution energy Ediss (obtained from DSC measurements) and lattice energy/molecule ELM (obtained from force field calculations) were found for the solvates that have host-guest intermolecular hydrogen bonds, for solvates that have methyl group in the para position and for inclusion crystals with benzonitrile and nitroethane. 2.Form 1a, which has chain character of the intermolecular hydrogen bonds system, is more stable than 1b, which have a dimer character of the intermolecular hydrogen bonds system. 3.The stabilities of the structures with isomeric inclusion molecules are in the following sequence: cosolvates with para > ortho > meta substitution pattern. 4.The close packing structure is always more stable then the other structure. For every structure powder diffraction measurements have been performed at room temperature and at temperature higher than the guest dissolution temperature. All the cosolvate structures changes to structure 1a after the evaporation of the solvent, except that of anisole which change to structure 1b. Detailed calculations of the interaction energies between the different molecules in each structure have been done to understand the factors affecting the crystal packing forces. The following results have been found: ·In the structures without solvent (the dimorphs) the largest interaction in the crystal has been calculated to be between the pair of molecules that connected with intermolecular hydrogen bonds. ·For the cosolvate structures in group 2, there are a dimer (two neighbouring molecules that are connected with intermolecular hydrogen bonds) or a pair (two neighbouring molecules sitting in the analogue position as the dimer but not connected with intermolecular hydrogen bonds) character structure of the host molecules. The largest interaction has been calculated to be between the host molecules of different dimers or pairs, which are not connected with intermolecular hydrogen bonds. This is because the intermolecular hydrogen bonds within the dimer are of week type, their lengths (NH—O) range between 3.20 A and 3.43 A. On the other hand, strong dispersion interactions in addition to considerable electrostatic interactions, have been calculated between the molecules of the different dimers or pairs, which can be attributed to the many close contacts between the methyl…methyl, nitro…methyl and nitro…nitro groups of these molecules. The nitro group free of intramolecular hydrogen bond and the methyl groups of the neighbouring isopropylamino groups are generally the groups that responsible for this interaction. ·The twist or the twisted boat six-membered ring conformations of the host 1, which are the forms adopted by 1 in the most cosolvates structures, are very suitable for the generation of many van der Waals interactions between the different layers within the packing patterns of our cosolvate structures.