Kinetics of oxygen exchange between sites in the GaO4Al12(OH)24(H2O)127+(aq) molecule and aqueous solution

Rates of steady exchange of oxygens between bulk solution and sites in the GaO4Al12(OH)(12)(7+)(aq) (GaAl12) aqueous complex were determined over the temperature range of 301 to 317 K and 4.1 < pH < 4.9 using O-17-nuclear magnetic resonance (NMR). The GaAl12 molecule, like the AlO4Al12(OH)(24)(H2O)(12)(7+)(aq) (Al-13) molecule studied previously, has 12 equivalent bonded water molecules (eta (1)-OH2 sites), two structurally distinct sets of 12 hydroxyl bridges (mu (2)-OH; mu (2)-OH' sites), and four four-coordinated oxo gropups (mu (4)-O sites). The GaAl12 molecule is much less reactive than the Al-13 molecule, and this decreased reactivity is not associated with any clear changes in the structural chemistry at the sites of exchange. The rate coefficients for exchange of the water molecules bonded to the complex with bulk water are as follow: k(ex)(298) = 227(+/-40) s(-1), DeltaH(double dagger) = 63(+/-7) kJ mol(-1), and DeltaS(double dagger) = 13(+/-21) J mol(-1) K-1. The rate at 298 K is approximate to 5 times slower than the corresponding exchange reaction on the Al-13 molecule, but it falls within the range measured for dissolved aluminum monomers. These data support our earlier speculation that rates of exchange of eta (1)-OH2 sites at fully charged aluminum (hydr)oxide mineral surfaces are similar to rates for aqueous aluminum complexes in acids. The rates of isotopic exchange of the two hydroxyl bridges in the GaAl12 complex differ from one another, as we also observed for the Al-13 complex, but to a much smaller extent. Likewise, the activation parameters for exchange at the two sites are much more similar to one another in the GaAl12 molecule than in the Al-13. The rate coefficients for exchange of the more reactive hydroxyl bridge are as follows: k(ex)(298) = 1.8(+/-0.1) . 10(-5) s(-1), DeltaH(1)(double dagger) = 98(+/-3) kJ mol(-1), DeltaS(1)(double dagger) = -8(+/-9) J mol(-1) K-1, and for the less labile bridge, they are k(ex)(298) = 4.1(+/-0.2) . 10(-7) s(-)1, with DeltaH(2)(double dagger) = 125(+/-4) kJ mol(-1) and DeltaS(2)(double dagger) = 54(+/-12) J mol(-1) K-1. There is no strong pH dependence to rates. Copyright (C) 2001 Elsevier Science Ltd.