11B[27Al] and 27Al[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 27Al, are reported, to demonstrate boron-aluminum connectivities in a model aluminoborate glass. A detailed study of the spin-lock behavior of 11B and 27Al proves to be a prerequisite for successful CP/MAS experiments. Under MAS conditions, two distinct boron sites are observed, corresponding to tetrahedral BO4/2- sites (nuclear electric quadrupole coupling constant near 0.3 MHz) and trigonal BO3/2 sites (nuclear electric quadrupole coupling constant near 2.7 MHz). The BO4/2- sites are most successfully spin-locked in the adiabatic regime at high radio frequency (RF) field strengths, whereas for the BO3/2 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 T1p 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.