Moderate MAS enhances local (1)H spin exchange and spin diffusion

Area of Science:

• Solid-state Nuclear Magnetic Resonance (NMR) Spectroscopy
• Materials Science
• Physical Chemistry

Background:

• Proton NMR spin-diffusion experiments are crucial for characterizing molecular structures in solid samples.
• Magic-angle spinning (MAS) is commonly employed to improve spectral resolution in solid-state NMR.
• Previous studies suggested a spin diffusion rate of 0.8 nm²/ms under static conditions.

Purpose of the Study:

• To investigate the effect of low-speed magic-angle spinning (MAS) on proton spin diffusion rates in solid samples.
• To re-evaluate the spin diffusion rate under static conditions.
• To elucidate the underlying mechanisms responsible for the observed changes in spin diffusion.

Main Methods:

• Proton NMR spin-diffusion experiments were conducted under varying magic-angle spinning (MAS) rates, including static and low-speed (<10 kHz) conditions.
• Density matrix simulations were employed to model and explain the experimental observations.
• Analysis focused on the influence of orientation-dependent dipolar couplings, transient level crossings, and dipolar truncation.

Main Results:

• Local proton spin diffusion was observed to be faster at low MAS rates (<10 kHz) compared to static conditions.
• The spin diffusion rate under static conditions can be slower than the previously reported 0.8 nm²/ms.
• The enhancement of spin diffusion by slow MAS is attributed to the modulation of dipolar couplings and associated phenomena.

Conclusions:

• Slow MAS significantly enhances proton spin diffusion in solid-state NMR.
• The findings necessitate a revision of the accepted spin diffusion rate under static conditions.
• Accurate modeling of spin diffusion requires considering interactions beyond simple spin pairs, involving at least three spins.