Sensitivity Enhancement of Ultra‐Wideline NMR by Progressive Saturation of the Proton Reservoir Under Magic‐Angle Spinning

Solid-state NMR of low-γ nuclides is often characterized by low sensitivity and by significant spectral broadenings induced by the quadrupolar and the chemical-shift anisotropy interactions. Herein, we introduce an indirect acquisition method, termed PROgressive Saturation of the Proton Reservoir Under Spinning (PROSPRUS), which could facilitate the acquisition of ultra-wideline NMR spectra under magic-angle spinning (MAS), in systems with a sufficiently long dipolar relaxation time, T_1D. PROSPRUS NMR relies on the generation of so-called second-order dipolar order among abundant protons undergoing MAS, and on the subsequent depletion of this dipolar order by a series of looped cross-polarization events, transferring the proton order into polarization of the low-γ I-nuclei as a function of the latter's offsets. While the spin dynamics of the ensuing experiment is complex, particularly when dealing with narrow I spectral lines, it is shown that PROSPRUS can lead to faithful lineshapes for ultra-wideline spin-1/2 and spin-1 species, providing high sensitivity with extremely low RF power requirements. It is also shown that the ensuing ¹H-detected PROSPRUS experiments can efficiently characterize I-spin lineshapes in excess of 1 MHz without having to retune electronics, while providing improvements in sensitivity per unit time over current broadband direct-detection methods by up to a factor of four.