Activation volumes for oxygen exchange between the GaO4Al12(OH)24(H2O)127+(aq) (GaAl12) polyoxocation and aqueous solution from variable pressure 17O NMR spectroscopy

Activation volumes for exchange of oxygen between bulk aqueous solution and sites in the GaO4Al12(OH)(24)(H2O)(12)(7+) (aq) (GaAl12) Complex were measured by variable-pres sure O-17 NMR techniques. Near 322 K, rates of exchange for the less labile set of bridging hydroxyls in the GaAl12 decrease by a factor of about two with increasing pressure from 0.1 to 350 MPa. These data indicate a substantially positive activation volume of DeltaV(dagger) = +7 +/- 1 cm(3)/Mol, which is the first activation volume measured for a bridging hydroxyl in a polynuclear complex. This result suggests significant bond-lengthening in the activation step. Electrostriction effects should be small because exchange occurs via a pH-independent path under the experimental conditions. The second, more labile set of bridging hydroxyls exchange too rapidly for the variable-pres sure techniques employed here. The exchange of bound-water molecules on the GaAl12 was observed at P = 350 MPa using the O-17-NMR line-broadening technique. Comparison with previous measurements at 0.1 MPa indicates decreasing line width from 0.1 to 350 MPa for temperatures at which exchange dominates, yielding an activation volume of DeltaV(dagger) = + 3(+/- 1) cm(3)/mol. This activation volume is smaller than the value for the Al(H2O)(6)(3+) complex, suggesting that water exchange on the larger GaAl12 complex has less dissociative character although the average charge density is lower. Copyright (C) 2004 Elsevier Ltd.