The influence of pressure on the infrared spectra of hydrogen bonded solids. II. Bihalide salts

Abstract

Infrared measurements have been made of the influence of pressures between 0 and 40 kbar on the asymmetrical stretching frequencies v₃ and bending frequencies v₂ of the hydrogen-bonded ions FHF^- and ClHCl^- in the solid salts NaHF₂, KHF₂, NH₄HF₂, (CH₃)₄NHCl₂ and (C₂H₅)₄NHCl₂ at 25ºC. The behaviour of the symmetrical stretching frequency v₁ for FHF^- in KHF₂ has been deduced indirectly from the shifts of combination bands. Contrary to the behaviour of compounds with weaker, 'normal', hydrogen bonds, the v₃ bands shift to higher frequencies and the v₂ bands shift to lower frequencies with increasing pressure. The v_l band of KHF₂ shifts to higher frequencies. These trends are all in agreement with predictions made in Part I for a simple model of hydrogen bonds. A new high-pressure phase transition has been found to occur in NaHF₂ at about 40 kbar.