The ionic product, Kw
The hydration of the hydroxide ion (OH-) is very important for biological and non-biological processes.b Unfortunately, it is neither well-known nor simply described. Most experimental structural work on this hydrated ion involves concentrated or very concentrated solutions, containing structure-disruptive cations. Within such experimental environments, the basic tetrahedral structuring of water is destroyed and the specific effects of solvent separated and contact ion pairs are introduced and confuse any results. It is clear, however, that the hydroxide ion is strongly hydrated but the extent of this hydration is less clear.
The hydroxide ion, shown right,a strongly interacts with other water molecules to give clusters and is essentially absent (as such) in aqueous solution. All the occupied molecular
orbitals of OH- are on another page.
Although many recent studies have attempted to determine the preferred hydration of the hydroxide ion in solution, there is no consensus. In particular, the hydrogen bonding capacity utilizing the donated OH- proton, remains in doubt. Studies indicate that any such bond must be very weak, if formed, and may be essentially absent.
OH- has an effective ionic radius of 0.110 nm , somewhat less than that of the H2O molecular radius (0.138 nm). Its molar volume is 1.2 cm3 mol-1 due to electrostriction . The nearest aqueous oxygen atom to the hydroxide proton appears to average about 0.25 nm, almost twice the distance as in the hydroxide ions accepting hydrogen bonds (~0.14 nm), well outside the normal hydrogen-bond signature distance of 0.15-0.21 nm  and at a distance often considered as showing no bond . The O-H stretch vibration behaves as the free hydroxyl group in small gas-phase clusters  and both concentrated and more dilute hydroxide solutions .
In solution, the hydroxide ion must be surrounded by water with orientations governed by the local polarity and the presence of counter ions. Clearly water molecules (rather than cationic counter ions) will reside relatively close to the hydroxide proton and it is not surprising that this can form the fleeting hydrogen bonds described , perhaps encouraged by solvent separated counter ions and contact ion-pairing when in concentrated solution. Such bonds are, however, far weaker than the hydrogen bonds between water molecules, as judged from their bond length and longer wavelength (i.e. blue-shifted rather than the usual red-shift noted generally for hydrogen bonds ), and not readily formed . Even in ab initio calculations no local minima is found for a water hydrogen-bonded to the OH- proton unless held by an extensive (and somewhat unlikely) network of bridging hydrogen bonds from 16 other water molecules  and the free hydroxide O-H stretch was found at higher frequency indicative of very weak or absent true hydrogen bonding, by Fourier transform infrared (FTIR) spectroscopy of HDO isotopically diluted in H2O . It is probable, however, that a fleeting very weak hydrogen bond may facilitate the OH- transport mechanism .
The hydroxide ion (OH-) is a very good acceptor of hydrogen bonds, with the first water molecule binding strongly to form H3O2- (right,
where the proton is off-center, giving rise to a low-barrier
hydrogen bond) hydrated ions.a A centrally-positioned proton within the hydrogen bond (similar to that in the H5O2+ ion, 'Zundel cation') does not appear to be stable, however. Although evidence for this H3O2- ion has been difficult to find in aqueous solution , it is expected to possess a particularly strong hydrogen bond  and infrared spectroscopy indicates that it may last for 2-3 vibration periods (~110 fs) .
The hydrogen atom lies significantly
asymmetrically in H3O2- where
the energy barrier for proton transfer has been calculated
(~0.9 kJ mol-1) to be much lower than the available
vibrational energy, so allowing easy equilibration of the
proton's position  as occurs
with H5O2+. The vibrational
spectrum of H3O2- indicates
that it behaves as a single (vibrationally-averaged) species
with no bend vibration (v2),
both free O-H stretch vibrations being equivalent and a very
strong and sharp band at 697 cm-1, corresponding
to the vibration of the shared proton ;
thus the shared proton hops between the two minimum energy
sites giving a quantum-averaged structure, similar to what may happen with H5O2+, which also shows
a strong sharp peak (at 1090 cm-1) for its shared
proton. As expected, these spectra will be very much broadened,
shifted and poorly resolved in bulk liquid water.
In crystal structures, H3O2- with a symmetrical hydrogen bond may be found (for example, O···O 2.41 Å; O···H
1.205 Å; O-H 0.733 Å ).
occupied molecular orbitals, found using the 6-31G** basis
set, of H3O2- are on another page.
Ab initio studies shows that up to four further water molecules may hydrogen bond around the oxygen atom of the OH-, as it charge density is spread out and not tetrahedrally situated. As the hydration increases, the hydroxide O-H bond becomes shorter, its hydrogen atom more positive and its oxygen atom less negative. The hydrogen bonds become longer and individually weaker whereas the hydrogen bonded water molecules become less polarized.
The O···O distance in H7O4- and H3O2- are slightly greater
(~2.67 Å and ~2.50 (2.467 Å )
respectively) and the O-H slightly shorter (~0.98 Å and ~1.05
(1.125 Å ) respectively)
than in H5O2+. The hydration of
these ions reduce their chemical activity, which may be a factor
in their increased reactivity when subjected to hydrogen-bond disrupting
electromagnetic effects .
The tetrahedral ion H7O4-,
see HO-··(HOH)3 above opposite,
is probably the most stable hydrated hydroxide
being slightly energetically favored over H3O2- (above) . It hydrogen bonds well at the surface of small clusters and even in the gas phase
. It is also possible that four water molecules may coordinate
to the hydroxide ion as HO-(··HOH)4,
(all donating their hydrogen atoms, see below opposite) because the electron
distribution around the hydroxide ion is not directional
. Such an
arrangement has been recently reported using neutron
diffraction, with empirical structure refinement,  and is consistent with X-ray absorption spectroscopy of concentrated solutions , with both studies utilizing concentrated hydroxide solutions. It should be noted, however, that at such high concentrations most, if not all, water molecules must be within the first shell of at least one ion  and the normal tetrahedral clustering of water, as found in more dilute solutions, has been destroyed. Certainly the Raman spectra of hydroxide solutions changes when the solution is diluted below OH-:H2O 1:20 . Also, HO-(··HOH)4 was found to be energetically unfavorable using quasi-chemical theory  and spectroscopic studies indicate the 4th H2O in HO-(··HOH)4 to be preferably hydrogen bonded to the other three forming a second
The strong hydrogen bonding between the hydroxide ion (OH-) and its first shell water molecules is thought responsible for the very large temperature dependence of the hydroxide reorientation, with three-fold increase in activation energy at low temperatures (<290 K) . Although thought possibly due to the presence of hyper-coordinated HO-··(HOH)4 clusters , such an effect could equally well be due to dominant tetrahedral HO-··(HOH)3 clusters at low temperatures, fitting better into the more extensive tetrahedral network of water molecules then present. Certainly, this reorientation effect seems to indicate a changing hydration structuring around the hydroxide ion with temperature. It is clearly not proven that the planar HO-(··HOH)4 ion (bottom opposite) has importance in dilute solutions beyond its, perhaps transient, formation during diffusion.
[Back to Top ]
a The hydroxide ion and small hydrated hydroxide clusters shown on this page were drawn using ab initio calculations using the 6-31G**
basis set. Where not otherwise referenced, bond distances, angles and atomic charges were derived from these calculations. [Back, 2]
b OH- + aq OH- (aq) (ΔG° = -437.6 kJ mol-1, assuming that the standard state of the solvent water is taken as 1.0 M.). ). [Back]