Investigations in the infra‐red. I: The absorption and reflection spectra of KHF₂, KDF₂ and RbHF₂ in relation to the constitution of the bifluoride ion

An arrangement is described for the analysis of the infra‐red spectrum using a prism spectrometer and thermo‐electric radiation receivers. Experimental details are given especially of a self‐constructed telluriumconstantan thermo‐element. With this arrangement the absorption and reflection spectra were investigated of crystals of KHF 2 , KDF 2 and of RbHF 2 in the quartz and in the rock‐salt region from 1‐17 μ. The spectrum of RbHF 2 is similar to that of the isomorphous potassium compound except for a small shift of a few cm −1 toward lower frequencies. Two fundamental frequencies are observed for KHF 2 at 1222 cm −1 (8.18 μ) and at 1450 cm −1 (6.90 μ) in absorption, and at 1238 cm −1 (8.08 μ) and at 1490 cm −1 (7.08 μ) in reflection. For KDF 2 the corresponding frequencies are 891 cm −1 and 1046 cm −1 in reflection. It is shown, that both frequencies belong to the fundamental asymmetric valence frequency ν 3 (the “hydrogen” frequency) of the bifluoride ion. It is further shown, that this doubling of the hydrogen frequency is due to the presence of a double minimum in the potential curve for the hydrogen atom, analogous to the doubling observed in the parallel bands of the NH 3 ‐molecule. The phenomenon of protonic resonance is discussed. The distance between the two levels of the zero pair for KHF 2 is estimated to be 25 cm −1 , that for the first pair is equal to 185 cm −1 . A potential curve is drawn, and the height of the potential hump separating the two minima is estimated as 2570 cm −1 or 7.3 kcal/mol. The analysis of the large number of bands observed shows that all are due to the double “hydrogen” frequency ν 3 and higher harmonics in combination with the inactive symmetrical frequency ν 1 at 580‐550 cm −1 . The stationary state of the HF 2− group is a resonance hybride between the two equivalent asymmetric structures. The resonance consists in the migration of the hydrogen ion between the two minima of potential energy, one near each of the two fluorine atoms. However the resonance energy is negligible compared with the total bond energy of the HF 2 ‐group.