Ab initio calculation of aqueous aluminum and aluminum-carboxylate complex energetics and 27Al NMR chemical shifts

Al3+ hydrolysis in aqueous solution was modeled with ab initio calculations. Structural changes surrounding the cation as protons are removed from the initial Al3+(H2O)(6) molecular cluster were predicted. A correlation of the model energy changes and experimental equilibrium constants for these reactions was also found. Calculations of the Al-27 NMR chemical shift between the species Al3+(H2O)(6) and [Al(OH)(4)](-) were performed to test the feasibility of predicting Al-27 NMR chemical shifts in aqueous solution with gas-phase molecular orbital calculations on small clusters. Energetics of Al3+-carboxylic acid complex formation in solution were also calculated using the self-consistent isodensity polarized continuum model (SCIPCM) to account for long-range solvation effects. Comparisons of calculated Al-27 NMR chemical shifts in model Al3+-carboxylate complexes to experimentally assigned values were made to test this methodology and previous peak assignments in Al-27 NMR spectra of Al3+-carboxylic acid solutions. Results suggest that NMR peaks observed in acidic solutions of carboxylic acids should be re-interpreted in terms of monodentate or protonated bidentate species. Peaks observed as solution pH increases are likely due to formation of aluminum oligomers complexing with ligands and not bidentate complexes with isolated Al3+ cations as previously interpreted.