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| - First-Principles Calculation of the 17O NMR Parameters in CaOxide and Ca Aluminosilicates: the Partially Covalent Natureof the Ca−O Bond, a Challenge for Density Functional Theory
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| - We apply density functional theory (DFT) to the calculation of the 17O NMR parameters in Caand Mg oxides and aluminosilicates. We study the accuracy of the Perdew, Burke, and Ernzerhof (PBE)generalized-gradient approximation to DFT in the description of these systems and the origin of theexperimentally observed large dependence of the 17O chemical shift on the alkaline earth ion. We find that(i) the partially covalent nature of the Ca−O bond has a huge impact on the O chemical shifts. The Ca−Ocovalence alone explains why in Ca oxides and aluminosilicates the 17O chemical shifts are much moredeshielded than those of the corresponding Mg compounds. (ii) The Ca−O covalence is overestimated bythe PBE functional. Thus PBE-DFT is not able to reproduce the measured 17O NMR parameters in Caoxide and Ca aluminosilicates. (iii) It is possible to correct for the PBE-DFT deficiency in a simple andtransferable way and to predict very accurate 17O NMR parameters. Such accuracy allows us to assignthe 17O NMR spectra of two important model systems: the grossite aluminate (CaAl4O7) and the wollastonite(CaSiO3) silicate.
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