Hydroxide ions are molecular ions with the formula OH-, formed by the loss of a proton from a water molecule.
The hydroxide ion is the strongest base that can exist in aqueous solution. The structure and dynamics of the hydroxide ion in water have been reviewed [2628]. The hydration of the hydroxide ion (OH-)d is very important for both biological and non-biological processes. b Unfortunately, hydroxide hydration 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 many hydrogen bonds are broken. Also, the specific effects of solvent separated and contact ion-pairs 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 above 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. The vibrational spectra of aqueous hydroxide ions have been determined using two-dimensional infrared (2DIR) spectroscopy [2151].
OH- has an effective ionic radius of 0.110 nm [1946], somewhat less than that of the H2O molecular radius (0.138 nm). Its molar volume is 1.2 cm3 mol-1 due to electrostriction [1946]. c 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 [698] and at a distance often considered as showing no bond [173]. The O-H stretch vibration behaves as the free hydroxyl group in small gas-phase clusters [461] and both concentrated and more dilute hydroxide solutions [1229]. However, its intensity reduces and wavenumber increases as more water molecules hydrogen bond to its oxygen atom [2229].
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 [1509], 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 generally noted for hydrogen bonds [1735]), and not readily formed [1510]. Even in ab initio calculations, no local minima is found for a water hydrogen-bonded to the OH- proton unless it is held by an extensive (and somewhat unlikely) network of bridging hydrogen bonds from 16 other water molecules [1511], The free hydroxide O-H stretch is found at higher frequency by Fourier transform infrared (FTIR) spectroscopy of HDO isotopically diluted in H2O [1512], and this is indicative of very weak or absent true hydrogen-bonding. It is probable, however, that a fleeting very weak hydrogen bond may facilitate the OH- transport mechanism [650].
The hydrated hydroxide ion (H3O2-)
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') [1648] does not appear to be stable, however. Although evidence for this H3O2- ion has been challenging to find in aqueous solution [1512], it is expected to possess a particularly strong hydrogen bond [102], but infrared spectroscopy indicates that it may only last for 2-3 vibration periods (? 110 fs) [1647].
See http://www1.lsbu.ac.uk/water/ionisoh.html