Structure of crystalline phosphates from P-31 double-quantum NMR spectroscopy

P-31 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 P-31 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 Ca2P6O17, which contains Q(2) and Q(3) 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.