Site-selective QPASS for the isolation of large quadrupolar coupling environments

Area of Science:

• Solid-state Nuclear Magnetic Resonance (NMR) spectroscopy.
• Materials characterization.
• Solid-state chemistry.

Background:

• Quadrupolar nuclei, like 93Nb, exhibit complex NMR spectra due to quadrupolar interactions.
• High spinning rates in solid-state NMR can lead to overlapping powder patterns, complicating spectral analysis.
• Distinguishing between crystallographically distinct sites in materials is crucial for understanding their properties.

Purpose of the Study:

• To develop a spectral editing technique for simplifying complex quadrupolar powder patterns in solid-state NMR.
• To selectively enhance signals from specific crystallographic sites in materials.
• To accurately determine NMR parameters for different sites within a material.

Main Methods:

• Coupling the Quadrupolar Polskiej Acquisition and Cycling experiment (QPASS) with a selective pi/2-Resonance Assignment Pulse Transfer (RAPT) enhancement sequence.
• Implementing the pi/2-RAPT-QPASS sequence for spectral editing of high spinning rate quadrupolar powder patterns.
• Applying the method to analyze the layered material KCa2Nb3O10.

Main Results:

• The pi/2-RAPT-QPASS sequence effectively reduced spectral overlap by emphasizing powder patterns from sites with large quadrupolar couplings.
• Isotropic chemical shifts and quadrupolar coupling parameters for two distinct niobium sites in KCa2Nb3O10 were determined.
• Selective enhancement of an asymmetric surface site was achieved, allowing for straightforward fitting to a second-order quadrupolar powder pattern.

Conclusions:

• The developed pi/2-RAPT-QPASS sequence is a powerful tool for spectral editing of quadrupolar nuclei in solid-state NMR.
• This method significantly improves the ability to analyze materials with multiple crystallographically inequivalent sites.
• Accurate characterization of specific sites, including surface sites, is now more readily achievable.