No abstract

31P multiple-quantum solid-state NMR spectroscopy is introduced as a new approach for elucidating internuclear distances between phosphorus nuclei in phosphates. The typical shorter distance between chemically bound groups allows the determination of the chemical network. The method is superior to 31P exchange experiments, which, in principle, provide similar information. Separation of two crystalline Mg2P2O7 phases is observed by both methods, but the double-quantum experiment gives further information of couplings between sites with equal isotropic chemical shifts. In Ca2P6Ol7, which contains Q2 and Q3 groups with large chemical shift anisotropies, the connectivities can be deduced from the double-quantum experiment due to different cross-peak intensities. Our results are in good agreement with X-ray diffraction measurements and suggest applications to other, more complicated phosphates.

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It is demonstrated on a polycrystalline powder sample of Cd3(PO4)2, given a known single-crystal X-ray structure, that two-dimensional 31P double-quantum single-quantum MAS correlation experiments allow unique assignment of multiple 31P resonances to specific P sites in the structure of such inorganic orthophosphates. The eigenvalues of the 31P and 113Cd shielding tensors of Cd3(PO4)2 are reported, the determination of which requires usage of a 2D MAS spinning sideband separation experiment.