Abstract
The investigated co-crystal of 3-chloro-N-methylpyridinium iodide with tetrabromoquinone (3-Cl-N-MePy·I·Br(4)Q) reveals a π-hole interaction between an iodide anion and a quinoid ring involving an n → π* charge transfer. The quinoid ring has a partial negative charge (estimated to be in the range 0.08-0.11e) and a partial radical character, which is related to the black colour of the crystals (crystals of neutral tetrabromoquinone are yellow). A detailed X-ray charge density study revealed two symmetry-independent bond critical points between the iodide anions and carbon atoms of the ring. Their maximum electron density of 0.065 e Å(-3) was reproduced by quantum chemical modelling. The energy of the interaction is estimated to be -11.16 kcal mol(-1), which is comparable to the strength of moderate hydrogen bonding (about -10 kcal mol(-1)); it is dominantly electrostatic in nature, with a considerable dispersion component.