Abstract:
Rates of steady exchange of oxygens between bulk solution and sites in the GaO4Al12(OH)24(H2O)127+(aq) (GaAl12) aqueous complex were determined over the temperature range of 301 to 317 K and 4.1 < pH < 4.9 using 17O-nuclear magnetic resonance (NMR). The GaAl12 molecule, like the AlO4Al12(OH)24(H2O)127+(aq) (Al13) molecule studied previously, has 12 equivalent bonded water molecules (η1-OH2 sites), two structurally distinct sets of 12 hydroxyl bridges (μ2-OH; μ2-OH' sites), and four four-coordinated oxo groups (μ4-O sites).The GaAl12 molecule is much less reactive than the Al13 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 follows: kex298 = 227(+/-40) s-1, ΔH# = 63(+/-7) kJ mol-1, and ΔS# = 13(+/-21) J mol-1 K-1. The rate at 298 K is ~5 times slower than the corresponding exchange reaction on the Al13 molecule, but it falls within the range measured for dissolved aluminum monomers. These data support our earlier speculation that rates of exchange of η1-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 Al13 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 Al13. The rate coefficients for exchange of the more reactive hydroxyl bridge are as follows: kex298 = 1.8(+/-0.1) . 10-5 s-1, ΔH1# = 98(+/-3) kJ mol-1, ΔS1# = -8(+/-9) J mol-1 K-1, and for the less labile bridge, they are kex298 = 4.1(+/-0.2) . 10-7 s-1, with ΔH2# = 125(+/-4) kJ mol-1 and ΔS2# = 54(+/-12) J mol-1 K-1. There is no strong pH dependence to rates.
