Efficiency of the refocused 31P-29Si MAS-J-INEPT NMR experiment for the characterization of silicophosphate crystalline phases and amorphous gels

One- and two-dimensional refocused MAS-J-INEPT NMR experiments in the solid state (through-bond polarization transfer) involving the highly abundant P-31 spin and the rare Si-29 spin are described for the crystalline silicophosphate phase Si5O(PO4)(6) and complex mixtures of SiP2O7 polymorphs. The evaluation of the (2)J(P-O-Si) coupling constants for all Si-29 sites is obtained by the careful analysis of the INEPT build-up curves under fast MAS. The results are in agreement with the crystallographic data, taking into account the various J coupling paths. The efficiency of the experiment is demonstrated by its application to more complex systems such as silicophosphate amorphous gels (obtained by the sol-gel process).