The infrared spectrum of solid sodium hydrogen bifluoride. Doubling of the HF stretching band of dilute NaHF₂ in NaDF₂

Abstract

Infrared spectra (4000-200 cm^-1) of discs in potassium iodide and mulls in paraffin and Fluorolube have been measured for NaHF₂ and NaDF₂ (99%D), and a range of isotopic dilutions. Stretching v₃ of DF₂-, as an isotopic solid solute in NaHF₂, appears as a single band with a half-width of c. 50 cm^-1, but v₃ of HF₂^- dissolved in NaDF₂ has two components, separated by c. 62 cm^-1 (relative intensities c. 5 : 3). For bulk NaHF₂ the frequency intervals in the sequence v₃+nv₁ are much smaller than v₁, and apparently irregular. The splitting of v₃ of HF₂^- is attributed to the F-H...F^- ↔ F^-...H-F interconversion doubling which Ketelaar erroneously believed to have detected (for KHF₂) in 1941. The effect of high isotopic dilution on frequency is straightforward for bending v₂, which is raised, but not for v₃, which is raised (by c. 35 cm^-1) for HF₂^- in NaDF₂, but lowered (by c. 75 cm^-1) for DF₂^- in NaHF₂. This anomalous behaviour by v₃ is indicative of disorder of some kind.