Hydrogen bonding: VII. Low temperature infrared spectral studies of potassium dihydrogen trifluoride,trihydrogen tetrafluoride,and tetrahydrogen pentafluoride; structure of the H3F−4 ION

We have recorded the infrared spectra of crystalline K⁺H₂F₃^?, K⁺H₃F₄^?, and K⁺H₄F₅^? at room temperature and 12–18 K. The broad (4̃00 cm^?1) anion F-H stretching bands for these three salts are not resolved at low temperature; however, in each case the F-H-F bending region is resolved into the maximum number of bands predicted by theory. The complexity of the F-H-F bending mode region shows the H₄F₅^? ion to be of lower symmetry than predicted by X-ray studies. The H₃F₄^? ion has three hydrogen fluorides hydrogen bonded to a central fluoride ion, rather than a chain type structure. The F-H stretching bands for the three ions have very similar frequencies (1750–1800 cm^?1), which shows the hydrogen bonds in the three ions to be of about the same strength, and to be significantly weaker than the hydrogen bond in the HF₂^? ion.