High-Resolution NMR of S = 3/2 Quadrupole Nuclei by Detection of Double-Quantum Satellite Transitions via Protons.

Indirect NMR detection via protons under fast magic-angle spinning can help overcome the low sensitivity and resolution of low-γ quadrupole nuclei such as 35Cl. A robust and efficient method is presented for indirectly acquiring the double-quantum satellite-transition (DQ-ST) spectra of quadrupole nuclei. For a spin S = 3/2, the DQ-STs have a much smaller second-order quadrupolar broadening, one-ninth compared to that of the central transition. Thus, they can provide a factor of up to 18 in resolution enhancement. The indirect detection of DQ-STs via protons is carried out using the heteronuclear multiple-quantum coherence (HMQC) experiment with the transfer of populations in double-resonance (TRAPDOR) recoupling mechanism. The resolution enhancement by detecting DQ-STs and the high efficiency of the TRAPDOR-HMQC experiment are demonstrated by 35Cl NMR of several active pharmaceutical ingredients (APIs).