Modulating the Competition between Different Atoms to Form Halogen Bonds.

I and Br atoms are placed on opposite ends of a -butyl group, with each allowed to form a halogen bond (XB) with NH. DFT calculations show that the intrinsic preference of the nucleophile for the heavier I over Br can be reversed by the proper placement of substituents on the alkyl chain. A similar reversal occurs for NH and OH groups on the alkyl chain, where substituents make the O a better electron donor than N in an XB to an electrophilic ICCH. The highly mobile π-electron cloud of an aromatic ring makes such reversals much more difficult when the pair of competing atoms are placed on, or within, such a ring.