High-resolution indirect detection of spin-3/2 quadrupolar nuclei in solids using multiple-quantum-filtered...

Area of Science:

• Solid-state Nuclear Magnetic Resonance (NMR) Spectroscopy
• Quantum Coherence Transfer Mechanisms
• Materials Characterization

Background:

• Through-space heteronuclear correlation NMR experiments under magic-angle spinning (MAS) provide insights into inter-atomic proximities.
• Dipolar-mediated heteronuclear multiple-quantum correlation (D-HMQC) experiments offer sensitivity for detecting quadrupolar nuclei via protons.
• Second-order quadrupolar broadening often limits resolution in the indirect dimension of D-HMQC spectra.

Purpose of the Study:

• To develop a novel NMR pulse sequence for high-resolution heteronuclear correlation.
• To overcome the resolution limitations imposed by second-order quadrupolar broadening in D-HMQC experiments.
• To enable sensitive indirect detection of spin-3/2 quadrupolar nuclei via protons.

Main Methods:

• Incorporation of a multiple-quantum MAS (MQMAS) quadrupolar filter into the D-HMQC sequence, creating the D-HMQC-MQ sequence.
• Utilizing cosine-modulated long-pulses synchronized with sample rotation to excite and reconvert triple-quantum coherences, avoiding spinning sidebands.
• Employing two nested cogwheel phase cycles with 56 steps for coherence transfer pathway selection.

Main Results:

• Demonstrated high-resolution heteronuclear correlation for indirect detection of 11B, 23Na, and 35Cl via protons.
• Successfully applied the D-HMQC-MQ technique at high magnetic fields (up to 28.2 T).
• Showcased applicability for protons with chemical shift anisotropies exceeding 20 ppm.

Conclusions:

• The D-HMQC-MQ sequence effectively enhances spectral resolution for quadrupolar nuclei detection.
• This technique provides a valuable tool for studying materials containing spin-3/2 quadrupolar nuclei.
• The method's sensitivity and resolution are maintained at high magnetic fields, with a caveat regarding MAS stability.