Improved efficiency for better resolved solid-state NMR studies of Aluminum sites in materials
Year: 2026
Type: preprint
Abstract: In solid-state nuclear magnetic resonance (NMR) spectroscopy of materials, a multiple-quantum magic angle spinning (MQMAS) experiment is routinely employed to characterize quadrupolar nuclei. It provides high-resolution isotropic spectra correlated with anisotropic quadrupolar lineshapes that contain valuable structural information. For aluminum (27Al, spin 5/2), a key constituent of active catalytic sites in zeolites, quintuple-quantum magic angle spinning (5QMAS) NMR spectroscopy provides substantially increased frequency dispersion in the isotropic dimension relative to its conventional triple-quantum (3QMAS) counterpart. However, the enhanced resolution of 5QMAS remains largely underexplored due to prohibitively low sensitivity for most samples. To overcome this limitation, optimal control (OC) method was used to design highly efficient excitation and reconversion pulses for both 3QMAS and 5QMAS sequences. Using Al(acac)3 reference sample, experimental 3QMAS and 5QMAS spectra acquired with specifically tailored OC pulses yielded 3.4- and 18.5-fold higher signal intensities relative to conventional rectangular pulses, respectively, at magnetic field strength of 9.4 T and MAS frequency of 12.5 kHz. Analogous measurements conducted at 14.1 T and MAS frequency of 55.555 kHz provided corresponding signal enhancements of 5.2- and 50-fold. Although broadband OC pulses optimized over broader distributions of quadrupolar coupling parameters and chemical shifts exhibit lower relative signal enhancements, OC-5QMAS for a hydrated MFI zeolite sample yielded a 6-fold intensity enhancement relative to the conventional 5QMAS experiment. Furthermore, the OC-5QMAS spectra exhibited a signal-to-noise ratio superior to that of traditional 3QMAS spectra, thereby enabling routine 27Al solid-state NMR characterization of tetrahedral sites with both improved sensitivity and larger frequency dispersion to potentially resolve crystallographically distinct framework positions.
Source: ChemRxiv
Authors: Andrej Šmelko, Piotr Paluch, Zdeněk Tošner
Language: English
Subfield: Spectroscopy
Field: Chemistry
Domain: Physical Sciences
Open Access status: green
Awards: G5960638817, G6165425573, G8227998441
OpenAlex ID: W7211865784