Magic-angle-spinning 31P NMR spectra of solid dihydrogen phosphates. Comparison of ordered and dynamically disordered compounds.

High-resolution 31P NMR spectra are reported for several dihydrogen phosphates in the solid state. Shielding tensor components were retrieved by analysis of the sideband intensities. The results, together with previously published data, can be grouped according to the presence and type of proton exchange occurring in the crystals. Static MH2PO4 samples (M = Na, Li) show a shielding anisotropy delta sigma of approximately 120 ppm. In the case of M = Cs (one static P-OH bond and two oxygens attached to "half" hydrogens on the average) delta sigma is smaller (approximately 75 ppm) and can be understood in terms of the accepted, proton-exchange model. For highly disordered samples (M = K, NH4, Rb), where all four oxygens have nearby hydrogens with 50% occupancy, delta sigma is even smaller (approximately 30 ppm) but not zero. In the last-mentioned cases, 31P NMR information suggests that local PO4 environments may not have the symmetry that could be expected on the basis of the known crystal structures.