B-11{Al-27} and Al-27{B-11} 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, B-11 and Al-27, are reported, to demonstrate boron-aluminum connectivities in a model aluminoborate glass. A detailed study of the spin-lock behavior of B-11 and Al-27 proves to be a prerequisite for successful CP/MAS experiments. Under MAS conditions, two distinct boron sites are to 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 T-1 rho 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.