An 17O-NMR study of the exchange of water on AlOH(H2O)52+(aq)

The pH-dependence of the rate of water exchange between hydrated aluminum complexes and bulk aqueous solution is reported from variable-temperature dynamic O-17-NMR measurements. Over the temperature range 298-348 K, the pseudo-first-order rate coefficient fur exchange at any given temperature, k(ex,obs)(T), is described by k(ex,obs)(T) = k(1)(T) + k(2)(T)[H+](-1), where k(1)(T) is the rate coefficient for exchange of inner-sphere water molecules with bulk solution for the Al(H2O)(6)(3+)(aq) complex at that temperature, and k(2)(T)[H+](-1) is the pH-dependent contribution to k(ex,obs)(T) from the first hydrolysis product: AlOH(H2O)(5)(2+)(aq). The rate parameters are k(1)(298) = 1.3 s(-1), Delta H(1)double dagger = 84.7 kJ.mol(-1), Delta S(1)double dagger = 41.6 J.mol(-1).K-1 (from 6); and k(2)(298) = 7.2 x 10(-2) M s(-1). Delta H(2)double dagger = 90.4 kJ.mol(-1), Delta S(2)double dagger = 36.6 J.mol(-1).K-1 (this work). From this result we obtained the activation parameters for water exchange on the AlOH(H2O)(5)(2+)(aq) complex: k(ex,AlOH(H2O)52+)(298) = 3.1 x 10(4) s(-1), Delta H double dagger = 36.4 kJ.mol(-1), Delta S double dagger = -36.4 J.mol(-1).K-1. Thus, deprotonation of an inner-sphere water in Al(H2O)(6)(3+)(aq) enhances by a factor of approximate to 10(4) the lability of other inner-sphere oxygens. This labilizing effect of hydroxyl is much larger than fur other geochemically important ligands, including fluoride.