High-resolution 1H NMR in solids with frequency-switched multiple-pulse sequences

Area of Science:

• Solid-state Nuclear Magnetic Resonance (NMR) Spectroscopy
• Magnetic Resonance Imaging (MRI) techniques
• Quantum Control in NMR

Background:

• Abundant spins in solids present challenges for high-resolution NMR due to strong dipolar interactions.
• Existing pulse sequences like MREV8 and BR24 offer partial suppression of these interactions.
• There is a need for advanced pulse sequences to improve spectral resolution and sensitivity in solid-state NMR.

Purpose of the Study:

• To introduce and demonstrate a new pulse sequence, FSLG240W, for abundant-spin NMR in solids.
• To evaluate the performance of FSLG240W in terms of resolution and efficiency.
• To compare FSLG240W with existing state-of-the-art pulse sequences.

Main Methods:

• The FSLG240W pulse sequence utilizes phase-coherent frequency-switching of radiofrequency (rf) irradiation.
• It induces opposite rotations around the magic-angle axis in the rotating frame, employing a series of 4 pi/3 rotations.
• Observation windows are incorporated to separate rotation periods.

Main Results:

• The FSLG240W sequence effectively cancels the average dipolar Hamiltonian, first-order correction terms, and significant second-order terms.
• It exhibits a short cycle time and a high scaling factor (kappa = 1/sqrt(3)).
• Slightly improved resolution was demonstrated compared to MREV8 and BR24 on an L-alanine sample.

Conclusions:

• The FSLG240W pulse sequence is a promising new method for achieving high-resolution solid-state NMR.
• It offers advantages in canceling dominant spin interactions, leading to better spectral clarity.
• Further optimization may be needed to address sensitivity to rf inhomogeneity.