Source of Cooperativity in Halogen-Bonded Haloamine Tetramers

Inspired by the isostructural motif in α-bromoacetophenone oxime crystals, we investigated halogen–halogen bonding in haloamine quartets. Our Kohn–Sham molecular orbital and energy decomposition analysis reveal a synergy that can be traced to a charge-transfer interaction in the halogen-bonded tetramers. The halogen lone-pair orbital on one monomer donates electrons into the unoccupied σ*N−X orbital on the perpendicular N−X bond of the neighboring monomer. This interaction has local σ symmetry. Interestingly, we discovered a second, somewhat weaker donor–acceptor interaction of local π symmetry, which partially counteracts the aforementioned regular σ-symmetric halogen-bonding orbital interaction. The halogen–halogen interaction in haloamines is the first known example of a halogen bond in which back donation takes place. We also find that this cooperativity in halogen bonds results from the reduction of the donor–acceptor orbital-energy gap that occurs every time a monomer is added to the aggregate.