An 17O-NMR Study of the Exchange of Water on AlOH(H2O)52+(aq)
- 1 September 1998
- journal article
- research article
- Published by American Chemical Society (ACS) in Inorganic Chemistry
- Vol. 37 (19) , 4760-4763
- https://doi.org/10.1021/ic980333l
Abstract
The pH-dependence of the rate of water exchange between hydrated aluminum complexes and bulk aqueous solution is reported from variable-temperature dynamic 17O-NMR measurements. Over the temperature range 298−348 K, the pseudo-first-order rate coefficient for exchange at any given temperature, , is described by = + [H+]-1, where is the rate coefficient for exchange of inner-sphere water molecules with bulk solution for the Al(H2O)63+(aq) complex at that temperature, and [H+]-1 is the pH-dependent contribution to from the first hydrolysis product: AlOH(H2O)52+(aq). The rate parameters are = 1.3 s-1, ΔH1⧧ = 84.7 kJ·mol-1, ΔS1⧧ = 41.6 J·mol-1·K-1 (from 6); and = 7.2 × 10-2 M·s-1, ΔH2⧧ = 90.4 kJ·mol-1, ΔS2⧧ = 36.6 J·mol-1·K-1 (this work). From this result we obtained the activation parameters for water exchange on the AlOH(H2O)52+(aq) complex: = 3.1 × 104 s-1, ΔH⧧ = 36.4 kJ·mol-1, ΔS⧧ = −36.4 J·mol-1·K-1. Thus, deprotonation of an inner-sphere water in Al(H2O)63+(aq) enhances by a factor of ≈104 the lability of other inner-sphere oxygens. This labilizing effect of hydroxyl is much larger than for other geochemically important ligands, including fluoride.Keywords
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