 The hydroxide ion appears to rotate freely in crystals of the heavier alkali metal hydroxides at higher temperatures so as to present itself as a spherical ion, with an effective ionic radius of about 153 pm.Thus, the high-temperature forms of KOH and NaOH have the sodium chloride structure, which gradually freezes in a monoclinically distorted sodium chloride structure at temperatures below about 300 °C.The OH groups still rotate even at room temperature around their symmetry axes and, therefore, cannot be detected by X-ray diffraction.