An (17)O-NMR Study of the Exchange of Water on AlOH(H(2)O)(5)(2+)(aq).

The pH-dependence of the rate of water exchange between hydrated aluminum complexes and bulk aqueous solution is reported from variable-temperature dynamic (17)O-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(H(2)O)(6)(3+)(aq) complex at that temperature, and [H(+)](-1) is the pH-dependent contribution to from the first hydrolysis product: AlOH(H(2)O)(5)(2+)(aq). The rate parameters are = 1.3 s(-1), DeltaH(1)() = 84.7 kJ.mol(-1), DeltaS(1)() = 41.6 J.mol(-1).K(-1) (from 6); and = 7.2 x 10(-2) M.s(-1), DeltaH(2)() = 90.4 kJ.mol(-1), DeltaS(2)() = 36.6 J.mol(-1).K(-1) (this work). From this result we obtained the activation parameters for water exchange on the AlOH(H(2)O)(5)(2+)(aq) complex: = 3.1 x 10(4) s(-1), DeltaH() = 36.4 kJ.mol(-1), DeltaS() = -36.4 J.mol(-1).K(-1). Thus, deprotonation of an inner-sphere water in Al(H(2)O)(6)(3+)(aq) enhances by a factor of approximately 10(4) the lability of other inner-sphere oxygens. This labilizing effect of hydroxyl is much larger than for other geochemically important ligands, including fluoride.