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 DSC Jeremy Karl Cockcroft (Department of Chemistry, University College London (UCL), London, United Kingdom)email: 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[1], 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[2].  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[3].  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.

[1] Williams, J. H., Cockcroft, J. K. & Fitch, A. N. (1992) Angew. Chem. Ind. Ed. Engl. 31, 1655-1657.

[2] Cockcroft, J. K., Rosu-Finsen, A., Fitch, A. N. & Williams, J. H. (2018) for submission in May.

[3] 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