C–H ⋯ O interactions and the adoption of 4 Å short-axis crystal structures by oxygenated aromatic compounds
Oxygen atoms, pendant as substituents or occurring within the ring systems of planar aromatic molecules, have a pronounced tendency to direct crystallisation patterns of such compounds to the β-structure, characterised by a 4 Å short axis. They seem to perform this function by stabilising a critical number of hydrogen atoms, covalently bonded to carbon, through short and directionally specific intermolecular C–H ⋯ O hydrogen bonds. Consequently, the number of ‘free’ hydrogen atoms which contribute to crystal stabilisation through non-β steering C ⋯ H interactions is reduced. Both these factors result in the formation of C–H ⋯ O stabilised two-dimensional entities such as sheets and ribbons. Such entities may be stacked at 4 Å translational separation to generate the entire structure. These concepts are illustrated for some methylenedioxy and alkoxy aromatic systems, quinones, and heterocycles. However, both intra- and inter-sheet C–H ⋯ O interactions may sometimes be present and the unusual crystal structure of 7-acetoxycoumarin (5) shows how a significantly non-planar molecule may still adopt the β-structure if it is particularly well suited for the formation of C–H ⋯ O bonds. Yet 4-acetoxycinnamic acid (6), the crystal structure of which was determined in this work and which has almost the same C:H:O ratio as (5), adopts a non-β structure because the number of oxygen atoms available for C–H ⋯ O bond formation is greatly reduced. The crucial role of the number of such ‘available’ oxygen atoms and ‘H-bonded’ and ‘free’(C–)H atoms vis-à-vis the carbon content is exemplified by the β-steering behaviour of oxygen in some large fused-ring quinones and heterocycles. These trends may also be extended to nitrogen and sulphur heterocycles.