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

Benzonitrile phase behaviour under Titan-relevant conditions: insights from experiments and DFT calculations (109057)

Sonya K Emmett 1 , Rebecca M Blake 1 , Aston M Summers 1 , Nicholas D Stapleton 1 , Gemma F Turner 1 , Stephanie A Boer 2 , Rosemary Young 2 , Alan Riboldi-Tunnicliffe 2 , Rachel M Williamson 2 , Dino Spagnoli 1 , Stephen A Moggach 1
  1. School of Molecular Sciences, the University of Western Australia, Perth, WA, Australia
  2. MX Beamlines, ANSTO Australian Synchrotron, Clayton, VIC, Australia

Titan, Saturn’s largest moon, has been the subject of ongoing scientific interest due to its unique compositional and geographic behaviours. These include its Earth-like, dense nitrogen-hydrocarbon atmosphere, and its surface conditions near to the triple point of methane (1.5 bar, 90-94 K) which allow for a methane rain cycle to form geographic features such as hydrocarbon rivers, lakes, and dunes. (1) Titan also possesses an atmospheric haze, which acts as a reaction site that generates a diverse range of organic products from energised atmospheric nitrogen and methane. (1) The molecular products of these reactions are believed to be highly unsaturated and nitrogen rich, containing nitrile and aromatic functional groups. (2) Once created, these organic species condense and cover Titan’s surface, integrating with Titan’s geography and creating mineral formations of great interest. (1)

Benzonitrile, the simplest aromatic nitrile, has been identified as a Titan-relevant molecule through simulation studies of Titan’s atmospheric processes. (1) Previously, a low-temperature tetragonal phase of benzonitrile was identified at 198 K. (3) In this work we have extended the phase space on benzonitrile with respect to both temperature and pressure using differential scanning calorimetry (DSC) and X-ray diffraction (XRD) techniques. Benzonitrile’s known phase has been validated at low-temperature (from near melt at 255 K to 130 K), crystallising between 255 and 260 K in a P43212 space group, in head-to-tail benzonitrile chains. A novel high-pressure orthorhombic phase has been investigated between 0.3 and 5 GPa, crystallising at 0.36 GPa in the Pca21 space group, adopting a structure more akin to zig-zag layers. This work has been complemented with periodic Density Functional Theory (DFT) calculations at the RPBE-D4 level of theory to determine optimised geometries and cohesive energies.

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  2. [2] Cable ML, Hörst SM, Hodyss R, Beauchamp PM, Smith MA, Willis PA. Titan tholins: simulating Titan organic chemistry in the Cassini-Huygens era. Chem. Rev. 2011;112(3):1882-1909.
  3. [3] Fauvet G, Massaux M, Chevalier R. Etude de la forme cristalline du benzonitrile à 198 K. Acta Crystallographica Section B. 1978;34(4):1376-1378.