Multinuclear Solid-State NMR Investigation of Hexaniobate and Hexatantalate Compounds

作者：Deblonde Gauthier J P^*; Coelho Diogo Cristina; Chagnes Alexandre; Cote Gerard; Smith Mark E; Hanna John V; Iuga Dinu; Bonhomme Christian

来源：Inorganic Chemistry, 2016, 55(12): 5946-5956.

DOI：10.1021/acs.inorgchem.6b00345

This work determines the potential of solidstate NMR techniques to probe proton, alkali, and niobium environments in Lindqvist salts. Na7HNb6O19 center dot 15H(2)O (1), K8Nb6O19 center dot 16H(2)O (2), and Na8Ta6O19 center dot 24 center dot 5H(2)O (3) have been studied by solid-state static and magic angle spinning (MAS) NMR at high and ultrahigh magnetic field (16.4 and 19.9 T). H-1 MAS NMR was found to be a convenient and straightforward tool to discriminate between protonated and nonprotonated dusters A(x)H(8-x)M(6)O(19)center dot nH(2)O (A = alkali ion; M = Nb, Ta). Nb-93 MAS NMR studies at different fields and MAS rotation frequencies have been performed on 1. For the first time, the contributions of NbO5O mu H-2 sites were dearly distinguished from those assigned to NbO6 sites in the hexaniobate duster. The strong broadening of the resonances obtained under MAS was interpreted by combining chemical shift anisotropy (CSA) with quadrupolar effects and by using extensive fitting of the line shapes. In order to obtain the highest accuracy for all NMR parameters (CSA and quadrupolar), Nb-93 WURST QCPMG spectra in the static mode were recorded at 16.4 T for sample 1. The Nb-93 NMR spectra were interpreted in connection with the XRD data available in the literature (i.e., fractional occupancies of the NbO5O mu 2H sites). 1D Na-23 MAS and 2D Na-23 3QMAS NMR studies of 1 revealed several distinct sodium sites. The multiplicity of the sites was again compared to structural details previously obtained by single-crystal X-ray diffraction (XRD) studies. The Na-23 MAS NMR study of 3 confirmed the presence of a much larger distribution of sodium sites in accordance with the 10 sodium sites predicted by XRD. Finally, the effect of Nb/Ta substitutions in 1 was also probed by multinudear MAS NMR (H-1, Na-23, and Nb-93).

• 出版日期2016-6-20