Vibrational Spectrum of Crystalline Potassium Hydroxide

Abstract

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 ωexe 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.