Abstract:
Rates of steady exchange of oxygens between bulk solution and the largest known aluminum polyoxocation: Al2O8Al28(OH)56(H2O)26^18+(aq) (Al30) are reported at pH~4.7 and 32-40°C. The Al30 molecule is a useful model for geochemists because it is ~2 nm in length, comparable to the smallest colloidal solids, and it has structural complexity greater than the surfaces of most aluminum (hydr)oxide minerals. The Al30 molecule has 15 distinct hydroxyl sites and eight symmetrically distinct bound waters. Among the hydroxyl bridges are two sets of μ3-OH, which are not present in any of the other aluminum polyoxocations that have yet been studied by NMR methods. Rates of isotopic equilibration of the μ2-OH and μ3-OH hydroxyls and bound water molecules fall within the same range as we have determined for other aluminum solutes, although it is impossible to determine rate laws for exchange at the large number of individual oxygen sites. After injection of 17O-enriched water, growth of the 17O-NMR peak near 37 ppm, which is assigned to μ2-OH and μ3-OH hydroxyl bridges, indicates that these bridges equilibrate within two weeks at temperatures near 35°C. The peak at +22 ppm in the 17O-NMR spectra, assigned to bound water molecules (η-OH2), varies in width with temperature in a similar fashion as for other aluminum solutes, suggesting that most of the η-OH2 sites exchange with bulk solution at rates that fall within the range observed for other aluminum complexes. Signal from one anomalous group of four η-OH2 sites is not observed, indicating that these sites exchange at least a factor of ten more rapidly than the other η-OH2 sites on the Al30.
