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Nuclear relaxation restores the equilibrium population imbalance and can occur via spin–lattice or spin–spin mechanisms, which are first-order exponential decay processes.
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In the absence of an external magnetic field, nuclear spin states are degenerate and randomly oriented. When a magnetic field is applied, the spins begin to precess and orient themselves along (lower energy) or against (higher energy) the direction of the field. At equilibrium, a slight excess population of spins exists in the lower energy state. Because the direction of the magnetic field is fixed as the z-axis,  the precessing magnetic moments are randomly oriented around the z-axis.
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Spin-spin interaction and relaxation in two trityl-nitroxide diradicals.

Whylder Moore1, Ru Yao2, Yangping Liu2

  • 1Department of Chemistry and Biochemistry, University of Denver, Denver, CO 80210, United States.

Journal of Magnetic Resonance (San Diego, Calif. : 1997)
|October 14, 2021
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Summary

Spin-spin interactions in trityl-nitroxides significantly impact relaxation times, crucial for solid-state dynamic nuclear polarization. Understanding these interactions optimizes polarizing agent performance.

Keywords:
Dynamic nuclear polarizationElectron spin exchangeElectron spin relaxationNitroxideTrityl

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Area of Science:

  • Chemical Physics
  • Magnetic Resonance Spectroscopy
  • Materials Science

Background:

  • Trityl-nitroxides are promising polarizing agents for solid-state dynamic nuclear polarization (DNP).
  • Optimizing DNP performance requires understanding how spin-spin interactions affect diradical relaxation times.

Purpose of the Study:

  • To investigate the impact of exchange interaction (J) strength on electron spin relaxation in trityl-nitroxides.
  • To analyze the influence of molecular conformation on spin-spin interactions and relaxation properties.

Main Methods:

  • Electron paramagnetic resonance (EPR) spectroscopy (CW, X-band, Q-band) was used to study two trityl-nitroxides.
  • Analysis of EPR spectra involved overlapping AB spin-spin splitting patterns.
  • Electron spin relaxation rates (1/T1 and 1/Tm) were measured at various temperatures.

Main Results:

  • Two distinct conformations with varying exchange interaction strengths (J) were observed for each diradical in solution.
  • Exchange interaction strength (J) was found to be dependent on through-bond orbital interactions and molecular conformation.
  • Spin relaxation rates for trityl-nitroxides were comparable to nitroxide monoradicals and faster than trityl radicals, indicating nitroxide's effectiveness in enhancing trityl relaxation.

Conclusions:

  • The exchange interaction (J) significantly influences EPR spectral features and relaxation dynamics in trityl-nitroxides.
  • Conformational flexibility plays a key role in modulating spin-spin interactions and their impact on DNP performance.
  • Trityl-nitroxides demonstrate efficient relaxation enhancement, making them suitable for solid-state DNP applications.