MS31-P13 Insights into Weak C-H…F-C Interactions in C6F6:C6H6−nMen Co-crystals using a Combination of Powder Neutron and X-ray Diffraction, Single-Crystal Diffraction, and DSCPowder X-ray and neutron diffraction (PXRD and PND) are ideal tools for the study of solid-solid phase transitions, especially when complemented by DSC measurements. Similarly, single-crystal X-ray diffraction (SXD) is the method of choice for structure determination, especially given the capabilities of modern X-ray equipment (focussed micro-source X-rays with 2D detectors) combined with state-of-the-art data processing (CrysAlisPro) and analysis (Olex2) software. In addition, variable temperature (VT) crystallography can provide a more detailed understanding of the interactions between molecules than the static one-shot single temperature approach. Combining all of these methods has improved our understanding of the co-crystal system C6F6:C6H6−nMen for n=0 to 3.
The structure of phase IV of the prototype material C6F6:C6H6 was solved from combined neutron and synchrotron X-ray powder diffraction a quarter of a century ago, but a detailed understanding of the other 3 phases has remained elusive until now: new laboratory PXRD and SXD data has resulted in the determination of the crystal structures of the unknown phases and a better understanding of the transitions, particularly when combined with DSC measurements. Furthermore, due to the combined use of PXRD, DSC and SXD techniques, the origins of the phase transitions in the related system C6F6:C6H3Me3 co-crystal system have now been fully understood. More recent studies of C6F6:C6H4Me2 (p-xylene) using a similar approach has reveal subtleties in the structure of this adduct as a function of temperature which have not previously been seen; it exhibits similarities in structural behaviour to that of the C6F6:C6H3Me3 adduct. Finally, a preliminary PXRD study combined with low temperature DSC on C6F6:C6H5Me (toluene) shows that this system is more similar to the prototype material. In each case, an understanding of the changes in symmetry as a function of temperature is essential to solving the structures.
Variable temperature crystallography provides a tool for studying the competing intermolecular interactions in these systems. In all cases, there is a very strong electrostatic interaction from the equal and opposite quadrupolar force between the C6F6 and C6H6−nMen rings leading to columns of molecules stacked like dinner plates, but subtle changes in the C-H…F-C interactions between columns of molecules as a function of temperature lead to a variety of phase transformations.References:
 Williams, J. H., Cockcroft, J. K. & Fitch, A. N. (1992) Angew. Chem. Ind. Ed. Engl. 31, 1655-1657.
 Cockcroft, J. K., Rosu-Finsen, A., Fitch, A. N. & Williams, J. H. (2018) for submission in May.
 Cockcroft, J. K., Ghosh, R. E., Shephard, J.J., Singh, A. & Williams, J.H. (2017) CrystEngComm, 2017, 19, 1019-1023. Keywords: Variable temperature powder diffraction, C-H…F-C Interactions, Co-crystals