Bonding Analysis in Homo- and Hetero-Trihalide Species: A Charge Displacement Study

The bonding in homo- (X₃^–) and hetero-trihalides (XYX^–), with X = F, Cl, Br, and I; Y = I, has been analyzed through the charge displacement (CD) method, coupled with energy decomposition analysis (EDA). We focus our attention on how the different bond components vary with the distance between the terminal and the central halogen atoms. The results clearly show that all the homo-halide systems X₃^–feature the same trend of the charge transfer (CT) versus the asymmetry of the three-body system, defined as the ratio between the two bond lengths d_X1–X2and d_X2–X3, whereas in the case of the hetero-trihalides BrIBr^–and ClICl^–, the CT values are similar, but systematically lower with respect to the homo-trihalide case. Comparison with solid-state geometries also allows us to estimate the influence of the crystal lattice (the packing and the interactions with the surrounding fragments) on the geometry. Following the results reported here, the CD method is confirmed to be a useful alternative computational tool to analyze the nature and the origin of weak intermolecular interactions such as halogen and hydrogen bonding.