Formation of Stable Radicals by Mechanochemistry and Their Application for Magic Angle Spinning Dynamic Nuclear...

Area of Science:

• Solid-state Nuclear Magnetic Resonance (SSNMR)
• Dynamic Nuclear Polarization (DNP)
• Materials Science

Background:

• High-field magic angle spinning (MAS) dynamic nuclear polarization (DNP) enhances SSNMR sensitivity by hyperpolarizing nuclear spins.
• Previous work demonstrated gamma irradiation for creating long-lived radicals in solids for MAS DNP.
• A need exists for alternative, accessible methods to generate radicals for DNP applications.

Purpose of the Study:

• To investigate ball milling as a method for generating stable radical species in various materials.
• To characterize the radicals formed by ball milling in quartz and borosilicate glass using electron paramagnetic resonance (EPR).
• To demonstrate the utility of ball milling-generated radicals for solid-effect DNP in quartz.

Main Methods:

• Ball milling of diverse materials including polystyrene, cellulose, borosilicate glass, and quartz.
• High-field electron paramagnetic resonance (EPR) spectroscopy for radical characterization.
• Magic angle spinning (MAS) dynamic nuclear polarization (DNP) experiments on ball-milled quartz.

Main Results:

• Ball milling successfully generated millimolar concentrations of stable radical species in multiple materials.
• EPR analysis provided insights into the nature of radicals in ball-milled quartz and borosilicate glass.
• Solid-effect DNP using radicals from ball-milled quartz yielded significant ²⁹Si enhancements (approx. 114 at 110 K, 33 at room temperature).

Conclusions:

• Ball milling is an effective and versatile technique for producing stable radicals suitable for DNP.
• This method offers a practical alternative to gamma irradiation for radical generation in DNP-enhanced SSNMR.
• Ball-milled quartz serves as an efficient material for achieving high sensitivity in ²⁹Si SSNMR via DNP.