Relative importance of electrostatic and intermolecular charge-transfer interactions in halogen bonding depending on the properties analyzed.

A suitable description of intermolecular interactions would preferably be pursued from the viewpoints of both practicality and correct understanding. Herein, based on this idea, the relative importance of electrostatic and intermolecular charge-transfer (CT) interactions in halogen bonding has been examined for the cases of the FCBr⋯X complexes (X = Cl, Br, and I) taken as examples, with an emphasis on the angular dependence of the interaction energies that leads to high directionality (one of the important characteristics of halogen bonding) and the infrared (IR) intensity changes occurring upon complex formation. Analyses are carried out by employing an extra-point model, which correctly describes the anisotropy in the charge distributions of halogen atoms and may be practically usable in molecular dynamics simulations, and by referring to the electron density features for correct understanding of the behavior of electrons. It is shown that intermolecular CT plays a major role in the modulation of the IR intensities of the C-Br stretching and CF symmetric bending modes, while the angular dependence of halogen-bonding interaction energies is largely interpretable by the contributions of electrostatics and exchange repulsion. With regard to the overall depth of the halogen-bonding interaction energies, electrostatic polarization also plays an important role. It is therefore suggested that, even for the same set of systems, the relative importance of the electrostatic and intermolecular CT interactions strongly depends on the properties analyzed.