Three-spin solid effect and the spin diffusion barrier in amorphous solids.

Area of Science:

• Solid-state Nuclear Magnetic Resonance (NMR) spectroscopy
• Electron Paramagnetic Resonance (EPR) spectroscopy
• Dynamic Nuclear Polarization (DNP)

Background:

• Dynamic nuclear polarization (DNP) significantly amplifies Nuclear Magnetic Resonance (NMR) signals.
• The precise mechanisms and spatial localization of electron polarization transfer to surrounding nuclei in DNP remain incompletely understood.
• Understanding these processes is crucial for advancing applications in chemistry, biology, and physics.

Purpose of the Study:

• To elucidate the detailed mechanism of electron polarization transfer in DNP.
• To determine the spatial proximity of nuclei involved in polarization transfer relative to the polarizing agent.
• To quantify the extent of the spin diffusion barrier around the polarizing agent.

Main Methods:

• Theoretical analysis of the three-spin solid effect, sensitive to electron-nuclear distances.
• Experimental determination of electron-nuclear distances using the sensitivity of the solid effect.
• Proton Electron Nuclear Double Resonance (¹H ENDOR) experiments to track polarization transfer pathways.

Main Results:

• The three-spin solid effect analysis revealed high sensitivity to electron-nuclear distances.
• The spin diffusion barrier around the trityl radical in a glycerol-water matrix was determined to be less than 6 Å.
• Protons on the trityl molecule itself were identified as the primary sites for initial polarization transfer.

Conclusions:

• The study precisely maps the initial steps of DNP polarization transfer.
• The findings indicate that polarization is first transferred to protons on the trityl radical.
• Subsequent polarization transfer to surrounding glycerol molecules occurs through molecules in close contact with the trityl radical.