Vibrational spectrum of crystalline potassium hydroxide

The infrared spectrum of KOH has been obtained in the region 4000-300 cm^-1. At 23°C the spectrum shows two broad bands, one at 3600 cm^-1 and the other at 643 cm^-1. At -180°C the O - H stretching band near 3600 cm^-1 appears as a doublet. Splitting is shown to arise from coupling between the two OH^- ions in the unit cell. Evidence is presented that indicates a small difference in the strengths of the hydrogen bonds in KOH and KOD. The first overtone of the O - H stretching band is found at 6868 cm^-1 and from this a value of ω_ex_e of 110±11 cm^-1 is derived for KOH at -180°C. The 643 cm^-1 band of KOH at 23°C is identified as a librational fundamental. At -180°C this band appears at 709 cm^-1 together with bands at 840 and 671 cm^-1; these latter two bands are also identified as librational modes. These librational frequencies lead to a considerably higher barrier to rotation of the OH" ions in KOH than in Mg(OH)² or Ca (OH)₂. This higher barrier is probably the result of the weak hydrogen bonding in KOH. Spectra of KOH-KOD mixtures are interpreted in terms of relatively small coupling between the librational modes of the hydroxyl ions and between the librational and translational modes.