Single-pulse MAS, selective Hahn echo MAS, and 3QMAS NMR studies of the mineral zoisite at 400, 500, 600, and 800 MHz. Exploring the limits of Al NMR detectability

Major advances in instrumentation have made it feasible to begin studying the highly spectroscopically challenging mineral zoisite, which has octahedral aluminum sites with quadrupole coupling constants of 8.0 and 18.5 MHz. Initial results are presented from single-pulse excitation MAS, selective Hahn echo MAS, and two different types of 3QMAS studies. The selective Hahn echo spectrum can be deconvoluted into two components of the proper relative intensity. The selective Hahn echo and the single-pulse excitation spectra provide nearly identical values for the isotropic chemical shift, quadrupole coupling constant, and asymmetry parameter for each AlO6 site. In the 3QMAS work, using double frequency sweeps generated by amplitude-modulated pulses for more efficient conversion of triple-quantum coherence to single-quantum coherence results in a higher signal-to-noise ratio compared to the frequently used z-filtered, three-pulse 3QMAS experiment. Zoisite appears to be a particularly good sample for testing a variety of solid-state NMR techniques and advances in instrumentation. Such testing is of particular interest for determining the limits beyond which "NMR-invisible" aluminum would occur in Bronsted acid and nonframework aluminum sites of some zeolites.