Site-selective QPASS for the isolation of large quadrupolar coupling environments

Spectral editing of high spinning rate quadrupolar powder patterns observed using the QPASS experiment was achieved through the coupling of QPASS with the selective π/2-RAPT enhancement sequence. The resulting π/2-RAPT-QPASS sequence yields spectra that are dominated by the powder patterns form sites with large quadrupolar couplings thus reducing the overlap of patterns from multiple sites of different symmetry in a material. The 93Nb isotropic chemical shifts and quadrupolar coupling parameters were determined for the two niobium crystallographic sites in the layered KCa2Nb3O10. The asymmetric surface site in the structure was selectively enhanced and easily fit to second-order quadrupolar powder pattern with this method.