Hydrogen bonding Part 31. IR and thermodynamic evidence for unusual hydrogen bonding in the higher alkali metal hydroxide monohydrates

There is a significant and progressive change in the IR spectra associated with H2O hydrogen bonding in alkali metal hydroxide monohydrates as cation size increases from Li+ to Cs+. Lithium and sodium hydroxide monohydrates show IR bands typical of normal electrostatic hydrogen bonds; in contrast, the IR spectra of cesium and rubidium hydroxide monohydrates resemble that of potassium dihydrogen trifluoride, and suggest that covalent three-center hydrogen bonds might be present. The spectrum of potassium hydroxide monohydrate shows both types of absorptions. The IR spectra are not in accord with diffraction studies, which support the presence of electrostatic hydrogen bonds in all of these hydrates. Equilibrium dissociation vapor pressure measurements show that the hydrogen bonds in sodium hydroxide monohydrate have a minimum strength of ∼ 15 kcal mol−1, twice as great as those in lithium hydroxide monohydrate. The physical properties of the higher alkali metal monohydrates demonstrate that water is extraordinarily tightly bound in these compounds.