The Effect of Pressure on Halogen Bonding in 4-Iodobenzonitrile

The crystal structure of 4-iodobenzonitrile, which is monoclinic (space group I2/a) under ambientconditions, contains chains of molecules linked through C≡N···I halogen-bonds. The chains interactthrough CH···I, CH···N and π-stacking contacts. The crystal structure remains in the same phase upto 5.0 GPa, the b axis compressing by 3.3%, and the a and c axes by 12.3 and 10.9 %. Since the chainsare exactly aligned with the crystallographic b axisthese data characterise the compressibility of theI···N interaction relative to the inter-chain interactions, and indicate that the halogen bond is themost robust intermolecular interaction in the structure, shortening from 3.168(4) at ambientpressure to 2.840(1) Å at 5.0 GPa. The π···π contacts are most sensitive to pressure, and in one casethe perpendicular stacking distance shortens from 3.6420(8) to 3.139(4) Å. Packing energycalculations (PIXEL) indicate that the π···π interactions have been distorted into a destabilisingregion of their potentials at 5.0 GPa. The structure undergoes a transition to a triclinic (P−1) phaseat 5.5 GPa. Over the course of the transition, the initially colourless and transparent crystal darkenson account of formation of microscopic cracks. The resistance drops by 10% and the opticaltransmittance drops by almost two orders of magnitude. The I···N bond increases in length to2.928(10) Å and become less linear [<C−I···N = 166.2(5)°]; the energy stabilises by 2.5 kJ mol−1 andthe mixed C-I/I..N stretching frequency observed by Raman spectroscopy increases from 249 to 252cm−1. The driving force of the transition is shown to be relief of strain built-up in the π···πinteractions rather than minimisation of the molar volume. The triclinic phase persists up to 8.1 GPa.