科研之友
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摘要
A series of calcium and strontium complexes featuring aryl carboxylate ligands has been prepared and characterized by alkaline-earth (Ca-43 and Sr-87) solid-state NMR experiments in a magnetic field of 21.1 T. In the 11 compounds studied as part of this work, a range of coordination motifs are observed including nitrogen atom binding to Ca2+ and Sr2+, a binding mode which has not been investigated previously by Ca-43 or Sr-87 solid-state NMR. Ca-43 isotopic enrichment has enabled the full characterization of the Ca-43 electric field gradient (EFG) and chemical shift tensors of the two calcium sites in calcium p-aminosalicylate (Ca(pams)), where both NMR interactions are affected by the presence of a nitrogen atom in the first coordination sphere of one of the metal sites. The Ca-43 isotropic chemical shift is sensitive to the Ca-N. distance as exemplified by the NMR parameters of a second form of Ca(pams) and density functional theory (DFT) calculations. Studies of the strontium analogue, Sr(pams), confirm a similar sensitivity of the Sr-87 EFG tensor to the presence or absence of nitrogen in the first coordination sphere. To our knowledge, this is the first systematic Sr-87 NMR study of strontium complexes featuring organic ligands. The vertical bar C-Q(Sr-87)vertical bar values are found to be sensitive to the coordination number about Sr2+. In general, this work has also established a larger data set of reliable experimental vertical bar C-Q(Ca-43)vertical bar values which correlate well with those obtained using gauge-including projector-augmented-wave (GIPAW) DFT calculations. It is found that the use of a recently recommended quadrupole moment for Ca-43, -44.4 mbarn, improves the agreement with experimental values. This contribution lays the groundwork for the interpretation of Ca-43 and Sr-87 NMR spectra of more challenging systems, particularly where nitrogen alkaline earth metal bonding is occurring.
- 出版日期2014-1-6
