A new model for Overhauser enhanced nuclear magnetic resonance using nitroxide radicals

Area of Science:

• Magnetic Resonance Spectroscopy
• Chemical Physics

Background:

• Nitroxide free radicals are crucial for dynamic nuclear polarization (DNP) enhanced nuclear magnetic resonance (NMR) and electron spin resonance (ESR) spectroscopy.
• The coupling factor is key to DNP enhancement, but its measurement in nitroxide radicals has been inconsistent.
• Existing models fail to explain experimental data due to overlooking ESR transition characteristics.

Purpose of the Study:

• To develop a new model for quantifying Overhauser enhancements in DNP experiments using nitroxide free radicals.
• To account for the mixing of ESR hyperfine states caused by spin exchange and nuclear spin relaxation.
• To provide a more accurate method for determining the coupling factor in DNP studies.

Main Methods:

• Developed a novel model incorporating intermolecular Heisenberg spin exchange and intramolecular nitrogen nuclear spin relaxation.
• Investigated the effect of these interactions on ESR transitions and Overhauser enhancements.
• Conducted DNP experiments using (14)N and (15)N isotope-enriched nitroxide radicals to validate the model.

Main Results:

• The new model accurately quantifies Overhauser enhancements by considering ESR hyperfine state mixing.
• Nitrogen nuclear spin relaxation significantly increases the maximum saturation factor, especially at low radical concentrations.
• Inconsistency in coupling factor measurements is resolved by accounting for ESR transition characteristics.

Conclusions:

• A refined model for Overhauser enhancements in DNP is presented, improving the accuracy of coupling factor determination.
• The study highlights the importance of considering both spin exchange and nuclear spin relaxation for precise DNP quantification.
• Accurate coupling factor determination requires measuring maximum enhancement across varying concentrations and extrapolating to infinite concentration.