Structural Evolution of Trimethylacetonitrile under Pressure: A Combined X-ray Diffraction and Computational Study

Three high-pressure phases of trimethylacetonitrile, a compound of potential interest in the context of Titan's atmospheric chemistry, have been investigated using single-crystal X-ray diffraction, periodic density functional theory, and CrystalExplorer intermolecular energy calculations. A disordered tetragonal P4/nmm phase is formed between 0.07 and 0.29 GPa (denoted hp-I). Compression to 0.43 GPa forms an ordered orthorhombic Pnma phase (hp-II), which transforms to a monoclinic P2(1)/m phase (hp-III) at 1.52 GPa. The hp-III phase persists to at least 3.34 GPa. Phase transitions are driven by densification of the crystal and facilitated by rearrangement of the supramolecular hydrogen-bonding network, with 180 degrees reorientation of half the molecules. Compression of each phase is associated with slight shortening of the intermolecular hydrogen bonds, with gradual destabilization of the cohesive energy to 3.34 GPa.