Structure Determination of Tetranitromethane, C(NO2)4, in the Solid and Gaseous Phases
Final Report Abstract
For more than 70 years scientists have tried to determine the structure of tetranitromethane (C(NO2)4, TNM) in the solid state and the gas phase. Tetranitromethane is the parent molecule for organic nitro materials, the one with the highest nitration grade, and thus of fundamental interest. In a joint project between the Munich group of Prof. Klapötke (synthesis) and the Bielefeld group of Prof. Mitzel in the core facility GED@BI (Centre of Gas Electron Diffraction and Small Molecule Structures) a new attempt of structure determination was undertaken. We determined electron diffraction patterns of gaseous tetranitromethane and collected X-ray diffraction data from two crystalline phases obtained by in-situ crystallization. For the analysis of the gas diffraction data we had to employ a new method of description of the intramolecular dynamics: the correlated torsional movement of the four nitro groups made it necessary to separate their torsional movement and to describe it by a four-dimensional dynamic model, the first time that such a complex description was undertaken. The result is a dramatically improved fit of this 4-D-dynamic model to the experimental diffraction data. For the crystalline high-temperature phase of tetranitromethane we found a description by disorder and twinning that allows now a satisfactory description of the experimental diffraction data. We could also achieve a well resolved structure determination of tetranitromethane in its low-temperature phase and found independent molecules of low symmetry, but very similar overall geometry. In essence, we can now describe tetranitromethane, an unusually dynamic molecule, in a way that is consistent with diffraction data from two crystalline phases and the gas phase. It is merely the misfit between the local threefold symmetry of the core and the twofold symmetry of the nitro groups which causes their highly correlated torsional movements, that lead to disorder in the solid and dynamics in the gaseous state. In essence our research has clarified structural chemistry questions open for more than 70 years and generated a model case for the future treatment of many other open questions. It also demonstrates that a continuous development and refinement of structural techniques is necessary to answer the sophisticated questions that even seemingly simple molecules may pose to structural chemistry.
Publications
- “Tetranitromethane: A Nightmare of Molecular Flexibility in the Gaseous and Solid States”. Angew. Chem. Int. Ed. 2017, 56, 9619-9623; German version: Angew. Chem. 2017, 129, 99371-9375
Yury Vishnevskiy, Dennis S. Tikhonov, Jan Schwabedissen, Hans-Georg Stammler, Richard Moll, Burkhard Krumm, Thomas M. Klapötke, Norbert W. Mitzel
