Indirect detection of wideline magic angle spinning solid-state NMR spectra of spin-5/2, -7/2, and -9/2 half-integer...

Quadrupolar nuclei with a nuclear spin I > 1/2 account for ∼73 % of all NMR-active nuclei. The quadrupolar interaction broadens solid-state NMR spectra, frequently resulting in low resolution and poor sensitivity. Here, we present a theoretical and experimental investigation of the use of magic angle spinning (MAS) ¹H{X} double-echo resonance-echo saturation-pulse double-resonance (DE-RESPDOR) pulse sequences for the indirect detection of NMR spectra of half-integer quadrupolar nuclei with spin >3/2 (spin 5/2, 7/2, or 9/2 nuclei). In these experiments, a dephasing profile for the quadrupolar nucleus is created by plotting the observed dephasing of the detected spin as a function of the transmitter offset of the indirectly detected spin. Simulating the dephasing profile allows the quadrupolar coupling constant (C_Q) and the EFG tensor asymmetry parameter (η_Q) to be estimated. The achievable dephasing levels and the lineshapes of dephasing profiles of the indirectly detected nuclei were predicted using numerical simulations. We demonstrate ¹H detection of ¹²⁷I (I = 5/2), ¹³⁹La (I = 7/2), and ¹¹⁵In (I = 9/2) nuclei in BaI₂.2H₂O (barium iodide dihydrate), La(BH₄)₃(THF)₃ (tris(borohydride)tris(tetrahydrofuran)lanthanum(III)), and In(OH)₃ (indium(III) hydroxide), respectively. The observed improvements or reductions in sensitivity with indirect detection are related to the proportion of ¹H T₁ to quadrupolar nucleus T₁, alongside the quadrupolar nucleus's spin quantum number and gyromagnetic ratio (γ). Additionally, the indirect detection experiments confirm the existence of dipolar or scalar couplings between the detected nucleus and the quadrupolar nucleus of interest, providing important structural information. Numerical simulations suggest these methods are also potentially applicable to quadrupolar nuclei having C_Q larger than 100 MHz.