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 theGaO₄Al₁₂(OH)₂₄ (H₂O)₁₂⁷⁺(aq) (GaAl₁₂) complex were measured by variable-pressure ¹⁷O 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 ΔV^‡ = +7 ± 1 cm³/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-pressure techniques employed here. The exchange of bound-water molecules on the GaAl₁₂ was observed at P = 350 MPa using the ¹⁷O-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 ΔV^‡ = +3(± 1) cm³ /mol. This activation volume is smaller than the value for the Al (H₂O)₆³⁺ complex, suggesting that water exchange on the larger GaAl₁₂ complex has less dissociative character although the average charge density is lower.