Halogen bonds between I2 and ion pairs: Interpreting the ability of ionic liquids in efficient capture of radioactive iodine

To elucidate the efficient removal and reliable storage of radioactive iodine for ionic liquids, halogen bonds in the complexes of I2 with a series of ion pairs were investigated at the M06-2x level of theory in this work. Albeit halogen bonds in these complexes are somewhat weaker than those in the systems of bare anions, these interactions still belong to strong halogen bonds and show some covalent character. The strength of these interactions deceases in the following order: [Cl]− ≈ [Br]− ≈ [I]− > [Ac]− ≈ [NO3]− > [BF4]−, consistent with the experimentally determined trend of the removal efficiency for corresponding ionic liquids. In addition, the electrostatic potential, atoms in molecules, and charge transfer analyses were also undertaken for the complexes under study, to gain a deeper understanding of the nature of these interactions.