Infrared identification of proton-bound rare-gas dimers (XeHXe)⁺, (KrHKr)⁺, and (KrHXe)⁺ and their deuterated species in solid hydrogen

Proton-bound rare-gas dimer (RgHRg)⁺, in which Rg represents a rare-gas atom, serves as a prototypical system for proton solvation by inert-gas atoms. Until now, only centrosymmetric species with Rg = Ar, Kr, or Xe have been identified with infrared spectra. We employed electron bombardment during deposition of a mixture of Xe (or Kr) in p-H₂ at 3.2 K to prepare (RgHRg)⁺. Lines at 847.0 and 972.1 cm^-1 are assigned as the Rg-H-Rg antisymmetric stretching (μ₃) mode and its combination with the Rg-H-Rg symmetric stretching (μ₁ + μ₃) mode of (XeHXe)⁺ in solid p-H₂, respectively. Lines at 871.1 and 974.0 cm^-1 are assigned as the μ₃ and μ₁ + μ₃ modes of (KrHKr)⁺ in solid p-H₂, respectively. Slightly shifted and broadened lines were observed for these species in solid n-H₂. These results agree satisfactorily with reported experimental values of (XeHXe)⁺ and (KrHKr)⁺ in solid Xe, Kr, and Ar, and with the quantum-chemically predicted anharmonic vibrational wavenumbers of these species in the gaseous phase; the significant spectral shifts in various matrixes are rationalized with the proton affinities of the hosts. When a mixture of Xe and Kr in p-H₂ was used, an additional broad feature at 1284 cm^-1 was observed and assigned as the μ₃ mode of (KrHXe)⁺ in solid p-H₂. This line shifted to 1280 cm^-1 in solid n-H₂ and the corresponding line of (KrDXe)⁺ was observed at 954 cm^-1 in n-D₂. The observations of these lines are new; the wavenumbers significantly blue shifted from those of the centrosymmetric (RgHRg)⁺ agree with the quantum-chemically predicted anharmonic vibrational wavenumbers of 1279 cm^-1 for (KrHXe)⁺ and 916 cm^-1 for (KrDXe)⁺. Analysis of the computational results shows that electronic correlation effects play a much greater role for the asymmetric than for the symmetric species. An interpretation for this is provided.