11B{27Al} and Al{11B} double resonance experiments on a glassy sodium aluminoborate

Cross-polarization/magic-angle spinning (CP/MAS) and rotational echo double resonance (REDOR) experiments involving two half-integer quadrupolar nuclei, 11B and 27A1, are reported, to demonstrate boron-aluminum connectivities in a model aluminoborate glass. A detailed study of the spin-lock behavior of 11B and 27A1 proves to be a prerequisite for successful CP/MAS experiments. Under MAS conditions, two distinct boron sites are observed, corresponding to tetrahedral BO−42 sites (nuclear electric quadrupole coupling constant near 0.3 MHz) and trigonal BO32 sites (nuclear electric quadrupole coupling constant near 2.7 MHz). The BO−42 sites are most successfully spin-locked in the adiabatic regime at high radio frequency (RF) field strengths, whereas for the BO32 sites optimum spin-lock conditions are achieved in the sudden regime (low RF field strengths). These differences can be exploited for spectral editing purposes in REDOR experiments. Using corresponding T1π filters, it becomes possible to measure individual REDOR dephasing curves for both types of boron sites. The results illustrate the possible utility of heteronuclear X-Y double resonance techniques in unravelling the intermediate range order in amorphous systems containing quadrupolar nuclei.