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The presence of electron-donating, electron-withdrawing, or conjugating groups adjacent to a radical center, imparts electronic stabilization to the radicals. Examples of such electronically-stabilized radicals are triphenylmethyl, tetramethylpiperidine‐N‐oxide, and 2,2‐diphenyl‐1‐picrylhydrazyl. These radicals are remarkably stable and are known as persistent radicals. Some of the persistent radicals can even be isolated and purified.
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Radicals, the highly reactive species, gain stability by undergoing three different reactions. The first reaction involves a radical-radical coupling, in which a radical combines with another radical, forming a spin‐paired molecule. The second reaction is between a radical and a spin‐paired molecule, generating a new radical and a new spin‐paired molecule. The third reaction is radical decomposition in a unimolecular reaction, forming a new radical and a spin‐paired...
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Hyperpolarized 13C Metabolic Magnetic Resonance Spectroscopy and Imaging
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Dynamic nuclear polarization with trityl radicals.

Ravi Shankar Palani1, Michael Mardini1, Yifan Quan1

  • 1Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, United States.

Journal of Magnetic Resonance (San Diego, Calif. : 1997)
|March 9, 2023
PubMed
Summary

This study investigates dynamic nuclear polarization (DNP) mechanisms using trityl radicals. Researchers propose a new "resonant mixing" mechanism to explain observed DNP frequency profiles, challenging existing theories.

Keywords:
Dynamic nuclear polarizationResonant mixingSolid state NMRThermal mixingTrityl radical

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Area of Science:

  • Magnetic Resonance
  • Chemical Physics

Background:

  • Dynamic Nuclear Polarization (DNP) enhances nuclear spin sensitivity for applications in biology and materials science.
  • Unresolved questions persist regarding the precise mechanisms driving DNP, particularly the origin of observed frequency profiles.

Purpose of the Study:

  • To investigate the Zeeman DNP frequency profiles of trityl radicals (OX063 and OX071) in glycerol and DMSO.
  • To elucidate the underlying mechanism responsible for the dispersive DNP field profile observed.
  • To challenge existing DNP mechanisms like thermal mixing (TM) and propose a novel explanation.

Main Methods:

  • Experimental DNP measurements using trityl radicals OX063 and OX071 in glycerol and DMSO.
  • Microwave irradiation near the electron paramagnetic resonance (EPR) transition.
  • Direct DNP observations on 13C and 2H nuclei to probe spin interactions.

Main Results:

  • A dispersive shape was observed in the 1H Zeeman field profiles upon microwave irradiation, with larger effects in DMSO.
  • A weak nuclear Overhauser effect between 1H and 13C was detected.
  • Irradiation at the positive 1H solid effect (SE) condition led to a negative enhancement of 13C spins, inconsistent with TM.

Conclusions:

  • The observed 1H DNP Zeeman frequency profile is not explained by thermal mixing (TM).
  • A new mechanism, termed 'resonant mixing,' is proposed, involving the mixing of nuclear and electron spin states.
  • This resonant mixing mechanism offers a potential explanation for the dispersive DNP field profile without requiring electron-electron dipolar interactions.