Editing Hydroxide (section)

===Vibrational spectra===
The [[infrared spectrum|infrared spectra]] of compounds containing the OH [[functional group]] have strong [[spectral line|absorption bands]] in the region centered around 3500&nbsp;cm<sup>−1</sup>.<ref name=nakamoto>{{cite book|last=Nakamoto|first=K.|title=Infrared and Raman spectra of Inorganic and Coordination compounds|edition=5th|series=Part A|year=1997|publisher=Wiley|isbn=978-0-471-16394-7}}</ref> The high frequency of [[molecular vibration]] is a consequence of the small mass of the hydrogen atom as compared to the mass of the oxygen atom, and this makes detection of hydroxyl groups by [[infrared spectroscopy]] relatively easy. A band due to an OH group tends to be sharp. However, the [[spectral linewidth|band width]] increases when the OH group is involved in hydrogen bonding. A water molecule has an HOH bending mode at about 1600&nbsp;cm<sup>−1</sup>, so the absence of this band can be used to distinguish an OH group from a water molecule.

When the OH group is bound to a metal ion in a [[coordination complex]], an M−OH<!-- WP:MOSCHEM#Skeletal_formulas --> bending mode can be observed. For example, in [Sn(OH)<sub>6</sub>]<sup>2−</sup> it occurs at 1065&nbsp;cm<sup>−1</sup>. The bending mode for a bridging hydroxide tends to be at a lower frequency as in [([[bipyridine]])Cu(OH)<sub>2</sub>Cu([[bipyridine]])]<sup>2+</sup> (955&nbsp;cm<sup>−1</sup>).<ref>Nakamoto, Part B, p.&nbsp;57</ref> M−OH stretching vibrations occur below about 600&nbsp;cm<sup>−1</sup>. For example, the [[tetrahedron|tetrahedral]] ion [Zn(OH)<sub>4</sub>]<sup>2−</sup> has bands at 470&nbsp;cm<sup>−1</sup> ([[Raman spectroscopy|Raman]]-active, polarized) and 420&nbsp;cm<sup>−1</sup> (infrared). The same ion has a (HO)–Zn–(OH) bending vibration at 300&nbsp;cm<sup>−1</sup>.<ref>{{cite book|last=Adams|first=D.M.|title=Metal–Ligand and Related Vibrations|year=1967|publisher=Edward Arnold|location=London}} Chapter 5.</ref>