Ring Stacking as a Mechanism to Strengthen Halogen Bonding.

The effects of stacking an aromatic ring over another that bears a halogen atom is considered via quantum chemical calculations. Benzene, furan, cytosine, and phenol slightly weaken the CI···N halogen bond between iodobenzene and NH3, while strengthening is associated with dinitrobenzene, pyrimidine, thymine, and imidazole. A more substantial halogen bond magnification occurs when the external ring is rotated so as to donate a proton in forming a H-bond with the π system of iodobenzene in a T-shaped complex. In most cases, this external ring acts as a sink for some of the electron density donated to iodobenzene by the NH3 nucleophile.