Abstract
Computational chemistry is an important tool for signal assignment of 27Al nuclear magnetic resonance spectra in order to elucidate the species of aluminum(III) in aqueous solutions. The accuracy of the popular theoretical models for computing the 27Al chemical shifts was evaluated by comparing the calculated and experimental chemical shifts in more than one hundred aluminum(III) complexes. In order to differentiate the error due to the chemical shielding tensor calculation from that due to the inadequacy of the molecular geometry prediction, single-crystal X-ray diffraction determined structures were used to build the isolated molecule models for calculating the chemical shifts. The results were compared with those obtained using the calculated geometries at the B3LYP/6-31G(d) level. The isotropic chemical shielding constants computed at different levels have strong linear correlations even though the absolute values differ in tens of ppm. The root-mean-square difference between the experimental chemical shifts and the calculated values is approximately 5 ppm for the calculations based on the X-ray structures, but more than 10 ppm for the calculations based on the computed geometries. The result indicates that the popular theoretical models are adequate in calculating the chemical shifts while an accurate molecular geometry is more critical.
Highlights
Studies with aluminum(III) (Al(III)) complexes are important in bioinorganic chemistry, geochemistry, environmental science, and material science due to the toxic effects of many aluminum compounds and their industrial usage as catalysts and coagulation agents
Complex species were obtained from the Cambridge Crystal Database. (There is more than one crystal structure available for some complexes.) In most species, Al(III) is found in an octahedral-coordination environment binding with six ligands or functional groups
The main purpose of this work is to give a quantitative evaluation of the errors involved in the calculation of 27Al chemical shifts
Summary
Studies with aluminum(III) (Al(III)) complexes are important in bioinorganic chemistry, geochemistry, environmental science, and material science due to the toxic effects of many aluminum compounds and their industrial usage as catalysts and coagulation agents. Because various kinds of species of Al(III) might differ significantly in their toxic effects, speciation of aqueous Al(III). Nuclear magnetic resonance spectroscopy (NMR) has been routinely used in studying the coordination chemistry and speciation of Al(III) in aqueous solutions due to its noninvasive character [3]. Solution-state 27Al NMR spectroscopy has been a very powerful tool to characterize the structures of Al(III) complexes, to monitor the hydrolysis of Al(III), to identify
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