Oral Presentation The 35th Biennial Conference of the Society of Crystallographers in Australia and New Zealand 2024 (Crystal 35)

High-pressure co-crystallisation of acetonitrile and trimethylacetonitrile (109036)

Rebecca M Blake 1 , Isabelle M Jones 1 , James R Brookes 1 , Nicholas D Stapleton 1 , Gemma F Turner 1 , Stephanie A Boer 2 , Alan Riboldi-Tunnicliffe 2 , Rachel M Williamson 2 , Rosemary M Young 2 , Helen E Maynard-Casely 3 , Dino Spagnoli 1 , Stephen A Moggach 1
  1. The University of Western Australia, Crawley, WA, Australia
  2. MX Beamlines, Australian Synchrotron, Clayton, Victoria, Australia
  3. Australian Nuclear Science and Technology Organisation, Sydney, New South Wales, Australia

Co-crystallisation is broadly utilised in the fields of pharmaceuticals and crystal engineering to design materials with tuneable solid-state properties1, yet these systems are seldom studied at high-pressure where their bulk properties may be studied. Molecular co-crystals are also likely to constitute a class of molecular minerals on the surface of Saturn’s largest moon, Titan2. Small nitrile-containing molecules are thought to comprise a group of surface compounds on the moon, formed via photochemical processes in the nitrogen-hydrocarbon atmosphere. Moreover, 2-3% of the Cambridge Structural Database3 contains a small nitrile molecule as part of a multicomponent system, making nitriles prime candidates for investigations of co-crystallisation. To-date, nine co-crystal candidates for Titan-relevant minerals have been identified, two of which contain acetonitrile4.

Here we have used high-pressure X-ray diffraction and Density Functional Theory (DFT) to study a trimethylacetonitrile-acetonitrile system up to 5.90 GPa. Initial increase of pressure to 0.29 GPa leads to the formation of a single component trimethylacetonitrile crystal structure, while melting and reforming the solid above 2 GPa leads to the formation a new, previously unreported 1:1 trimethylacetonitrile-acetonitrile co-crystal. The structure was elucidated in the space group P21/m and shows characteristic alternating layers of each chemical species. In the co-crystal, trimethylacetonitrile molecules arrange similarly to the single-component high pressure phase which also crystallises in the space group P21/m. DFT calculations have revealed that two possible arrangements of the acetonitrile hydrogen atoms yield local energy minima, implying disorder within the co-crystal structure. Compression behaviour is driven by the strength and directionality of the stabilising intermolecular C-H···N and C-H···pi interactions, as well as steric clashes between hydrogen atoms. The results of this study highlight common packing motifs in crystal structures containing small nitrile molecules and the differences in physical properties between co-crystals and single-component phases at high-pressure.

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  2. H. E. Maynard-Casely et al, Am Mineral, 2018, 103, 343-349.
  3. C. R. Groom, I. J. Bruno, M. P. Lightfoot and S. C. Ward, Acta Crystallogr Sect B Struct Sci Cryst Eng Mater, 2016, 72, 171-179.
  4. E. C. Czaplinski et al, ACS Earth Space Chem, 2023, 7, 597-608.