Dissecting the electric quadrupolar and polarization effects operating in halogen bonding through electron density...

The form of the electron density change (or difference) is usable as a kind of fingerprint of the electronic structural origin or mechanism that gives rise to intermolecular interactions. Here, this method is applied to halogen-bonding brominated systems to dissect the electric quadrupolar effect (arising from the anisotropic distribution of the valence electrons and intrinsic to the s²p_x ²p_y ²p_z electronic configuration) and the polarization effect (induced by a partial negative charge of the halogen-bond accepting atom). It is shown that a suitable location of the "extra point" for placing a partial positive charge to represent the former is crucial and is clearly found from the electron density difference from the spherically isotropic Br^- ion, while the latter consists of the dipolar polarization of the Br atom and the delocalized polarization of the whole molecule. A practical way for application to molecular dynamics simulations, etc., to represent these two factors is discussed.