Symthons reveal how fluorine disrupts π–π stacking in halobenzene crystal structures†

Symmetry-forming closest approaches of aromatic rings; ‘Symthons’, are usually the strongest interactions in halobenzene crystal structures. Where they combine to create π–π stacking, they are as strong as hydrogen bonds. This π–π stacking is disrupted when fluorine is present, as revealed in this analysis of all 290 closest aromatic approaches in the crystal structures of halobenzenes. Closest aromatic approaches involving fluorine show fewer examples of ‘Symthon I’ π–π stacking, and more examples of offset translations with larger displacements. Edge⋯face approaches are also more common in the presence of fluorine, frequently accommodating fluorine atoms in the ‘edges’. Some edge⋯face approaches do not embody any symmetry. These findings are consistent with lower melting points and favourable connections between fluorine and positively charged carbon atoms.