Electronic Structure and Properties of Trihalogen Cations X₃⁺ and XY₂⁺ (X, Y = F, Cl, Br, I)

Electronic structures, charge distributions, geometries, valence force constants, and vibrational frequencies of the homoatomic clusters F₃⁺, Cl₃⁺, Br₃⁺, and I₃⁺ and of the heteroatomic clusters ClF₂⁺, BrF₂⁺, IF₂⁺, BrCl₂⁺, ICl₂⁺, and IBr₂⁺ were determined. The self-consistent field approach extended by MP2-correlation energy or density-functional corrections was applied using various basis sets. It was found that d- and f-type polarization functions play a crucial role as in some other halogen compounds. The MP2 approach yields the most satisfactory results. The effect of the crystalline environment surrounding the Cl₃⁺, Br₃⁺, and I₃⁺ species is successfully simulated by a Madelung potential. Frequencies calculated in the crystal field are in reasonable agreement with the more reliable ones among the experimental results. Coupling force constants were determined. They are not consistent with some empirical rules. Bonding and charge distributions of the formally mixed-valence systems X⁺Y₂⁰ are discussed. X⁺ behaves like a divalent chalcogen with high electronegativity. Each of the X⁺-Y bonds in XY₂⁺ is very similar to the bond in X-Y. We predict the experimentally unknown F₃⁺ to be stable in vacuum but not in the solid state. Structures and frequencies of XY₂⁺ species, which are as yet unknown, are also predicted.