Five new pseudopolymorphs of sym-trinitrobenzene

1,3,5-Trinitrobenzene (TNB) was recrystallized from about 30 solvents and five mixed solvents in around 50 experiments to yield five new solvates. All the solvates lose their solvent readily, and special precautions were required to determine their crystal structures. In all cases, the solvents that gave solvates are electron-rich aromatics leading to the conjecture that pi...pi donor-acceptor stacking interactions are important. This was confirmed from the crystal structures. Additionally, these structures are composed of hexagonal sheets in which TNB and solvent molecules are linked with weak C-H...O hydrogen bonds. The benzene solvate contains an unusual finite TNB motif not hitherto seen. The toluene and chlorobenzene solvates are not isostructural, and C-H...Cl interactions in the latter are unusually short and linear, accounting thereby for the failure of chloro-methyl structural exchange. The m-xylene and mesitylene solvates are similar to the toluene solvate. This study indicates that the use of terms such as solvate, pseudopolymorph, donor-acceptor complex, and molecular complex is a subjective matter, and also that a better definition for the term pseudopolymorph may be needed, especially because it occurs frequently in the pharmaceutical literature.