Rare gas atomic number dependence of the halogen bond in HRgF-XCCF (Rg = He, Ar, Kr; X = Cl, Br) complex

Quantum chemical calculations have been performed to study the halogen bond in HRgF-XCCF (Rg = He, Ar, Kr; X = Cl, Br) complex at the MP2/aug-cc-pVTZ level. The Rg insertion has a great effect on the strength and properties of the halogen bond. The Br bonding is stronger than the Cl bonding. The associated C-X bond is elongated and the respective stretch exhibits a red shift, while the distant H-Rg bond is contracted and the respective stretch displays a blue shift. The strength and properties of halogen bond show an unmonotonic dependence of the rare gas atomic number. The theories of natural bond orbital and atoms in molecules have been used to unveil the nature of halogen bond and the origin of the frequency shift.