Structural characterisation of phosphate materials: new insights into the spatial proximities between phosphorus and quadrupolar nuclei using the D-HMQC MAS NMR technique

We show in this article how the spatial proximity between phosphorus and quadrupolar nuclei can be efficiently and easily investigated with the D-HMQC (Dipolar Hetero-nuclear Multiple-Quantum Coherences) NMR technique. Compared to the commonly used CP-HETCOR (Cross-Polarisation HETero-nuclear CORrelation) sequence, the D-HMQC pulse scheme exhibits a higher sensitivity and a better robustness with respect to spinning frequency, electronic shielding and quadrupole interaction, and thus does not require time-consuming and complicated optimisation procedures. The advantages of the D-HMQC are demonstrated in this article through the acquisition of P-31/S through-space two-dimensional correlation NMR spectra providing unreported structural information on (i) a sodium alumino-silicate glass doped with only 3% of P2O5, (ii) a potassium boro-phosphate glass containing BO3 and BO4 groups and (iii) a crystalline zirconium vanado-phosphate. All these systems, representative of the most important mixed phosphate network materials, cannot be correctly investigated with the conventional CP-HETCOR NMR technique.