Improved double-quantum NMR correlation spectroscopy of dipolar-coupled quadrupolar spins

We show that symmetry-based zero-quantum dipolar recoupling sequences sandwiched between central transition selective π/2-pulses may be used to excite double-quantum coherences (2QC) between the central transitions of homonuclear half-integer quadrupolar spins undergoing magic-angle spinning. The 2QC excitation is explored experimentally by 23Na and 27Al NMR on Na2SO3 and α-Al2O3 model compounds. When incorporated into 2Q–1Q 2D correlation protocols, the new recoupling method established all expected 27Al-27Al connectivities over 6 Å in the open-framework aluminophosphate AlPO–CJ19 [(NH4)2Al4(PO4)4HPO4 · H2O] and the layer mineral chlorite [Mg9Al6Si5O20(OH)16].