Showing papers by "Praveen K. Thallapally published in 2003"

Five New Pseudopolymorphs of sym-Trinitrobenzene

Abstract: 1,3,5-Trinitrobenzene (TNB) was recrystd. from about 30 solvents and five mixed solvents in around 50 expts. to yield five new solvates. All the solvates lose their solvent readily, and special precautions were required to det. their crystal structures. In all cases, the solvents that gave solvates are electron-rich aroms. leading to the conjecture that p...p donor-acceptor stacking interactions are important. This was confirmed from the crystal structures. Addnl., these structures are composed of hexagonal sheets in which TNB and solvent mols. 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-Me 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 mol. complex is a subjective matter, and also that a better definition for the term pseudopolymorph may be needed, esp. because it occurs frequently in the pharmaceutical literature.

Coupling octupoles in crystals: The case of the 1,3,5-trinitrobenzene-triphenylene 1:1 molecular co-crystal

Abstract: A new supramolecular crystal engineering approach toward quadratic nonlinear optics is inferred from an acentric cocrystalline template made of two different bi-dimensional octupolar molecules: tr...

C-H···O hydrogen bonds in molecular complexes of 1,3,5-trinitrobenzene with some N-heterocycles

Abstract: The crystal structures of the molecular complexes of sym-trinitrobenzene (TNB) with acridine, 1,10-phenanthroline and phenazine are discussed. In all three cases, the structures are held together by C–H⋯O and π⋯π interactions, and TNB forms hydrogen bonded dimers and tapes that are not found in its native crystal structure. Acridine and 1,10-phenanthroline yield 1 ∶ 1 complexes that have very similar structures. The related molecule phenazine, however, gives a different 2 ∶ 3 complex in which guest exchange, as seen in other recently reported examples, is not observed. A case is made for the reporting of such ‘low yield’ supramolecular reactions because they still provide valuable information about the packing characteristics of organic molecules.

Square networks based on the Br...NO2 supramolecular synthon

Abstract: Six polysubstituted bromonitrobenzenes have been prepared by nitration of the three isomeric dibromobenzenes and their crystal structures analysed in terms of network architectures. The dominant intermolecular interactions in these structures are the polarization-induced Br...NO T and the C-H...O bridges that are promoted by the activation of the C-H groups in these electron-deficient systems. The Br...O 2 interactions act as one-dimensional connectors and, in this sense, may be termed as supramolecular synthons. A networking of these polyfunctional molecules leads to two-dimensional square and hexagonal networks. In one favourable case, the overall crystal packing is in a non-centrosymmetric space group and the alignment of chromophores leads to significant SHG properties. Even for small, simple molecules of the type studied here, crystal structure prediction is difficult and no two compounds have exactly the same crystal structure.