Ab initio calculation of aqueous aluminum and aluminum-carboxylate complexenergetics and Al-27 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 th e model energy changes and experimental equilibrium constants for these rea ctions 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 fea sibility of predicting Al-27 NMR chemical shifts in aqueous solution with g as-phase molecular orbital calculations on small clusters. Energetics of Al 3+-carboxylic acid complex formation in solution were also calculated using the self-consistent isodensity polarized continuum model (SCIPCM) to accou nt for long-range solvation effects. Comparisons of calculated Al-27 NMR ch emical shifts in model Al3+-carboxylate complexes to experimentally assigne d values were made to test this methodology and previous peak assignments i n Al-27 NMR spectra of Al3+-carboxylic acid solutions. Results suggest that NMR peaks observed in acidic solutions of carboxylic acids should be re-in terpreted in terms of monodentate or protonated bidentate species. Peaks ob served as solution pH increases are likely due to formation of aluminum oli gomers complexing with ligands and not bidentate complexes with isolated Al 3+ cations as previously interpreted.