Experimental and computational probes of the nature of halogen bonding: complexes of bromine-containing molecules with bromide anions.

The nature of halogen bonding is examined via experimental and computational characterizations of a series of associates between electrophilic bromocarbons R-Br (R-Br=CBr3F, CBr3NO2, CBr3COCBr3, CBr3CONH2, CBr3CN, etc.) and bromide anions. The [R-Br, Br(-)] complexes show intense absorption bands in the 200-350 nm range which follow the same Mulliken correlation as those observed for the charge-transfer associates of bromide anions with common organic π-acceptors. For a wide range of the associates, intermolecular R-Br···Br(-) separations decrease and intramolecular C-Br bond lengths increase proportionally to the Br(-)→R-Br charge transfer; and the energies of R-Br···Br(-) bonds are correlated with the linear combination of orbital (charge-transfer) and electrostatic interactions. On the whole, spectral, structural and thermodynamic characteristics of the [R-Br, Br(-)] complexes indicate that besides electrostatics, the orbital (charge-transfer) interactions play a vital role in the R-Br···Br(-) halogen bonding. This indicates that in addition to controlling the geometries of supramolecular assemblies, halogen bonding leads to electronic coupling between interacting species, and thus affects reactivity of halogenated molecules, as well as conducting and magnetic properties of their solid-state materials.