Quantitative Multiple-Quantum Magic-Angle-Spinning NMR Spectroscopy of Quadrupolar Nuclei in Solids

We describe a new approach for observation of multiple-quantum (MQ) NMR spectra of S = ³/₂ nuclei with magic-angle spinning (MAS). The new method employs the Rotation-Induced Adiabatic Coherence Transfer (RIACT) that occurs between triple-quantum (3Q) and central-transition (1Q) coherences in S = ³/₂ systems. In contrast to currently available coherence-transfer techniques, RIACT is relatively insensitive to the magnitude of the quadrupole interaction for e²qQ/h ≤ 4 MHz for both 3Q excitation and 3Q-to-1Q conversion. Thus, RIACT provides a means of extracting quantitative information about site populations from isotropic MQ NMR spectra. We illustrate the utility of the approach with ²³Na (S = ³/₂) MQ NMR spectra of a series of sodium salts exhibiting crystallographically distinct sites. The spectra provide quantitative measurements of quadrupolar parameters, chemical shifts, and relative site populations for each of the crystallographically distinct sodium sites.