Ab Initio Calculation of Aqueous Aluminum and Aluminum−Carboxylate Complex Energetics and ²⁷Al NMR Chemical Shifts

Abstract

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 27Al NMR chemical shift between the species Al3+(H2O)6 and [Al(OH)4]- were performed to test the feasibility of predicting 27Al 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 27Al NMR chemical shifts in model Al3+−carboxylate complexes to experimentally assigned values were made to test this methodology and previous peak assignments in 27Al 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.

Theoretical and 27Al CPMAS NMR investigation of aluminum coordination changes during aluminosilicate dissolution

“…Isotropic chemical shifts for 27 Al were calculated relative to the isotropic chemical shielding for Al 3ϩ · 6(H 2 O) that represents the experimental 27 Al NMR standard of Al (aq) 3ϩ . This method has been shown to provide accurate results for 27 Al chemical shifts in silicate glasses (Sykes et al, 1997) and aqueous solutions (Kubicki et al, 1999a).…”

Computational Spectroscopy in Environmental Chemistry

“…In addition, the B3LYP [87,88] density functional methods are included in both the energy minimization and NMR calculations. Although the current methods should be much more accurate than those employed in Kubicki et al [92], the results for the calculated d 27 Al values are similar. For example, the previous d 27 Al values for the monodentate and bidentate complexes were 6 and 16 ppm, respectively.…”

“…Kubicki et al [92] created molecular models of the various possible complexes, performed energy minimizations and frequency calculations, and then calculated 27 27 Al values consistent with observation within computational accuracy (i.e., 6 and 7 ppm, respectively). However, the latter complex with the phenol group protonated resulted in much poorer correlations to the observed vibrational frequencies of this complex [93].…”

A Correlation for Establishing Solvolysis Rates of Aqueous Al(III) Complexes: A Possible Strategy for Colloids and Nanoparticles

“…2 well. The value of the average Al-O bond length that we report is for the lowest energy geometry investigated, although alternate lowest-energy geometries have been previously reported for this complex (3,44,45). We also investigated AlOH 2+ as a hexacoordinated aluminum complex because there is no experimental evidence that the aluminum is a fiveor four-coordinated species (18,44,46).…”