Halogen Bonding to the π-Systems of Polycyclic Aromatics

The propensity of the pi-electron system lying above a polycyclic aromatic system to engage in a halogen bond is examined by DFT calculations. Prototype Lewis acid CF3I is placed above the planes of benzene, naphthalene, anthracene, phenanthrene, naphthacene, chrysene, triphenyl, pyrene, and coronene. The I atom positions itself some 3.3-3.4 & Aring; above the polycyclic plane, and the associated interaction energy is about 4 kcal/mol. This quantity is a little smaller for benzene, but is roughly equal for the larger polycyclics. The energy only oscillates a little as the Lewis acid slides across the face of the polycyclic, preferring regions of higher pi-electron density over minima of the electrostatic potential. The binding is dominated by dispersion which contributes half of the total interaction energy. A fairly strong CI-pi halogen bond is formed between CF3I and any of a set of polycyclic aromatic systems, varying from the single ring of benzene to the seven rings of coronene. The interaction is controlled largely by dispersion, with only a minor electrostatic component. The interaction energy changes only slightly as the donor is swept across the face of the polycyclic. image