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.