High-Pressure Co-Crystallization of Acetonitrile and Trimethylacetonitrile

The behavior of a coformer mixture of acetonitrile and trimethylacetonitrile has been examined up to 5.90 GPa using in situ high-pressure single crystal X-ray diffraction and periodic Density Functional Theory. A single-component phase of trimethylacetonitrile is formed at 0.29 GPa, which is isostructural to its previously reported high-pressure phase. Between 1.73 and 5.90 GPa, a 1:1 cocrystal of trimethylacetonitrile and acetonitrile formed in space group P2₁/m, dominated by C–H···N and C–H···π interactions. The cocrystal was grown in situ by heat annealing the sample above 1.1 GPa. This is the first ever reported cocrystal of trimethylacetonitrile and acetonitrile. The structure is resistant to compression, with a bulk modulus of 20(1) GPa, due to H-bonding networks and short N···H contacts. The arrangement of trimethylacetonitrile molecules within the cocrystal mirrors their organization in the high-pressure phase of pure trimethylacetonitrile, revealing a supramolecular synthon and highlighting common crystalline packing motifs of small nitrile-containing species.