Nature of a Hydride–Halogen Bond. A SAPT-, QTAIM-, and NBO-Based Study

The nature of a hydride–halogen bond is investigated by means of high-level quantum mechanical calculations expended with symmetry-adapted perturbation theory (SAPT), quantum theory of atoms in molecules (QTAIM), and natural bond orbital (NBO) methods. As model hydride–halogen bonded systems complexes between either LiH or HBeH and either XCF₃ or XCCH (X = F, Cl, Br, I) are used. It is shown that the formation of a hydride–halogen bond leads to the elongation of the R^δ+–H^δ− hydride bond, which is accompanied by the blue shift of the ν_R–H stretching vibration frequency and the increase of the IR intensity of this mode. All these effects, although untypical in the case of, e.g., hydrogen bonds, can be considered as rather typical for hydride–halogen bonded systems. The decomposition of the interaction energy based on the SAPT method clearly indicates the dominant role of the induction term, thus the inductive nature of a hydride–halogen bond in opposition to previous findings. NBO-based analysis indicates the charge transfer from the hydride molecule to the more remote parts of the halogen donor and that the elongation of the R–H bond is caused by the charge outflow from the σ_RH bonding orbital.