Infra-red spectra and the solid state. III. Potassium bifluoride

Abstract

The infra-red spectrum of potassium bifluoride has been determined between 2 and 8$\mu $ at temperatures between 293 and 90 degrees K. On passing from room temperature to that of liquid air, some of the absorption bands sharpen and split. The results have been analyzed with the object of assigning fundamental vibration frequencies to the (HF$_{2}$)$^{-}$ ion, and also of deciding whether this ion has a symmetrical or unsymmetrical structure. Values assigned to the stretching vibration frequencies are 600 and 1450 cm.$^{-1}$, and to the deformation a pair at 1225/1274 cm.$^{-1}$. The spectrum can then be satisfactorily explained in terms of the selection rules which would be expected for a symmetrical (HF$_{2}$)$^{-}$ ion lying in the particular crystal field suggested by X-ray work. Whereas the bending vibrations appear to involve little anharmonic character, there is an apparently large negative anharmonic coefficient for the overtone and combinations of the stretching vibration frequencies. This might be expected if the potential energy function for the antisymmetrical stretching vibration involved a higher power of the displacement co-ordinate than the second, and strengthens the interpretation in terms of a symmetrical ion. The molecular environment of the ion in the crystal state leads to a removal of the degeneracy of the deformational vibration, the latter being found to split, although one component is not noticed in absorption when the crystal is viewed along the c-axis.