Halogen bonding in (Z)-2-iodocinnamaldehyde.

Based on the bulkiness of the iodine atom, a non-planar conformation was expected for the title compound. Instead, its molecular structure is planar, as experimentally determined using single crystal X-ray diffraction, and confirmed theoretically by DFT calculations on the single molecule and the halogen pair paired molecules, therefore ruling out crystal packing forces as a principal factor leading to planarity. Indeed, planarity is ascribed to the carbonyl double bond, as when this bond is saturated on forming the related alcohol derivative, the molecule loses planarity. The X-ray molecular structure shows an intermolecular separation between the iodine and the oxygen of the carbonyl shorter than the corresponding van der Waals distance suggesting a weak halogen bond interaction. DFT minimization of this 2-molecule arrangement shows the iodine--oxygen distance much shorter than that observed in the crystal interaction and confirming its stronger halogen bond nature. A trend between increasing I•••O(carbonyl) separation and decreasing C-I•••O(carbonyl) angle is demonstrated, further confirming the existence of a halogen bond.