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

Abstract

Solid‐state NMR of low‐γ nuclides is often characterized by low sensitivity and 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, in systems with a sufficiently long dipolar relaxation time, T1D. 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.