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].