Halogen bonding interactions in ion pairs versus conventional charge-assisted and neutral halogen bonds: a theoretical study based on imidazolium species

Halogen bonds in imidazolium-based ion pairs have attracted recent research interest, due to their importance in the fields of anion recognition and ionic liquids. According to our survey of the Cambridge Structural Database (CSD), a number of crystal structures involving these specific halogen bonds were extracted. In this work, three different types of halogen bonding interactions, i.e. ion-pair halogen bonds, charge-assisted and neutral halogen bonds, in a series of dimeric complexes of imidazolium species were systematically studied at the M06-2x and B3LYP levels of theory. Ion-pair halogen bonds, despite being considerably stronger, show similar structural and energetic characteristics to common charge-assisted and neutral halogen bonds. To gain a deeper understanding of these interactions, the atoms in molecules (AIM), noncovalent interaction index (NCI), and energy decomposition analysis (EDA) calculations were undertaken. Most halogen bonds in imidazolium-based ion pairs have some covalent character, while the other two kinds of halogen bonds are weak electrostatic interactions. The attraction of ion-pair halogen bonds arises dominantly from electrostatic forces, while dispersion interaction plays a minor role. These two terms, however, contribute almost equally to the attraction of neutral halogen bonds. In addition to electrostatic attraction, induction interaction, which corresponds to charge transfer and mixing terms, also plays an important role in ion-pair and charge-assisted halogen bonds. The results presented should assist in the development of potent imidazolium-based anion receptors and novel halogenated ionic liquids with promising properties.