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One- and two-dimensional ³¹P MAS NMR experiments on inorganic phosphates are important to obtain short-to-mid-range distance information, including network connectivities. While it is straightforward to obtain simple one-dimensional ³¹P MAS spectra of such systems, in-depth understanding of these spectra and extraction of wanted parameters present a considerable challenge in terms of NMR spectroscopy: the ³¹P spin systems in inorganic phosphates can be treated neither as collections of isolated spins nor as isolated spin pairs, and the ³¹P spins are not sufficiently tightly (dipolar) coupled to qualify as an abundant spin bath. This intermediate regime of fairly strongly interacting, fairly abundant ³¹P nuclei can only be tackled by employing a wide range of different one- and two-dimensional ³¹P MAS NMR techniques. In addition, one-dimensional ³¹P MAS spectra of such compounds may be of deceptive simplicity.

As a starting point we have chosen the compounds SiP₂O₇ and TiP₂O₇ which, to some extent, are isostructural. A second set of compounds includes cadmium phosphates such as Cd₃(PO₄)₂. For the latter and for SiP₂O₇, ³¹P MAS investigations are also necessary as a preparation step towards future ³¹P-²⁹Si, ¹¹³Cd double resonance experiments, ultimately with the aim of using such techniques for the investigation of amorphous phosphate systems.

The (known) complicated superstructure of SiP₂O₇ with a multitude of similar P-sites is equally reflected in a multitude of ³¹P resonances (see Fig. 3.3-9). Two-dimensional ³¹P MAS experiments which are either sensitive to geometric distances (EXSY-type experiments) or tailored to reveal connectivities via homonuclear J-coupling (COSY-type experiments), together can be used to construct maps of connectivities from ³¹P NMR. A ³¹P MAS COSY experiment on SiP₂O₇ is also shown in Fig. 3.3-9. Some unsolved problems with the single-crystal X-ray structure of SiP₂O₇ have so far remained, orbiting around the question of whether some linear O₃P-O-PO₃ units do exist in these structures or not. ³¹P MAS spectra of TiP₂O₇ as a function of the external magnetic field strength B_o and of the MAS frequency, show characteristic changes which can be taken as evidence for the presence of (some) linear P-O-P links. This is shown for two selected ³¹P MAS spectra of TiP₂O₇ in Fig. 3.3-10. In summary, ³¹P MAS NMR methods should enable us to complete our understanding of the crystal structures of SiP₂O₇ and TiP₂O₇, while X-ray diffraction results are necessary to better understand the ³¹P NMR parameters of inorganic phosphates once these parameters have been extracted from a multitude of NMR experiments.

Cd₃(PO₄)₂ has an important role to play in this learning process. Also this compound displays a fairly complicated crystal structure, but all crystallographically distinct sites are resolved in the ³¹P and ¹¹³Cd MAS spectra, respectively. This is illustrated in Fig. 3.3-11. Based on the information from the single-crystal X-ray structure, it is known that different sites display a range of Cd ... P distances which makes this compound ideal to explore the application of ³¹P-¹¹³Cd CP/MAS experiments. Also, given the resolution of all crystallographic sites in the ³¹P and ¹¹³Cd MAS spectra, two-dimensional NMR methods can now be used to obtain heteronuclear Cd-P correlations, as well as to obtain ³¹P and ¹¹³Cd shielding tensor components for the different crystallographic sites. First preliminary ³¹P-¹¹³Cd double resonance experiments have been carried out as described in 3.3-10.
 

Fig. 3.3-9: 31P MAS NMR of SiP2O7; top: one-dimensional 31P MAS; bottom: 31P MAS COSY experiment where off-diagonal peaks reveal connectivities of sites by chemical bonds P-O-P.

Fig. 3.3-10: 31P MAS spectra of TiP2O7 at different MAS frequencies (top: 6.0 kHz, bottom: 2.5 kHz). The marked resonance displays typical MAS frequency dependent splitting for an (isolated) pair of chemically equivalent 31P spins, such as in a linear P2O7-unit.

Fig. 3.3-11: MAS NMR spectra of Cd3(PO4)2; left: 113Cd MAS, right: 31P MAS. Note the spectroscopic resolution of the 9 Cd and 6 P crystallographic sites, which renders this compound an ideal test-case for the application of 31P-113Cd double resonance NMR techniques.