The infrared spectrum of isotopically diluted potassium hydrogen bifluoride. Doubling of the HF stretching and of the v₃ + v₁ band

Abstract

Infrared spectra (of mulls in Nujol and Fluorolube and discs in potasium iodide) have been obtained for crystalline KDF₂ of higher deuterium content (c. 97.5%) than used by previous workers. This made possible an unambiguous identification of the bands of KHF₂ as a solid solute in KDF₂. HF stretching (v₃) is split into a doublet with separation c. 52 cm^-1, and the combination band v₃ + v₁ has a doublet separation of c. 64 cm_-1. For 1.5% KDF₂ in KHF₂ the splitting of v₃ of DF₂^- is too small to produce two separate bands. The splitting is attributed to the interconversion doubling which Ketelaar erroneously believed to have detected in 1941. Arguments previously put forward against the F^-...HF structure are shown not to take into account the increasing tendency of the anionic dipoles to adopt an ordered arrangement as the temperature is lowered.     KHF₂ and KDF₂ have also been examined in solid solution in potassium halides and in water. Narrow v₃ bands are obtained in KCl and KBr, but broad ones, as in crystalline KHF₂, in KI and in water.