Radical-Induced Low-Field ¹H Relaxation in Solid Pyruvic Acid Doped with Trityl-OX063

Area of Science:

• Nuclear Magnetic Resonance (NMR) Spectroscopy
• Solid-state Physics
• Chemical Physics

Background:

• Dynamic nuclear polarization (DNP) enhances NMR sensitivity using polarizing agents like trityl radicals.
• In low magnetic fields, the radical's electron spin reservoir can unexpectedly absorb nuclear spin polarization, acting as a sink.
• Understanding this polarization sink is crucial for optimizing DNP protocols, especially during polarization transfer.

Purpose of the Study:

• To investigate proton spin relaxation in pyruvic acid doped with trityl radicals.
• To quantify the role of the radical Non-Zeeman reservoir as a polarization sink.
• To validate a theoretical model for nuclear spin relaxation across a wide range of magnetic fields.

Main Methods:

• Low-temperature proton spin relaxation measurements were performed on trityl-doped pyruvic acid.
• Experiments covered a magnetic field range from 5 millitesla (mT) to 2 Tesla (T).
• The heat capacity of the radical Non-Zeeman reservoir was experimentally determined.

Main Results:

• Proton spin relaxation rates were measured across the specified magnetic field range.
• Experimental data allowed for the estimation of the radical Non-Zeeman reservoir's heat capacity.
• The Wenckebach formalism provided a quantitative, parameter-free model for the observed relaxation behavior.

Conclusions:

• Trityl radicals, while sources of polarization, can act as significant polarization sinks in low fields.
• The heat capacity of the radical Non-Zeeman reservoir plays a key role in this phenomenon.
• The Wenckebach formalism successfully models nuclear spin relaxation in DNP systems over a broad field range.