Study of electron spectral diffusion process under DNP conditions by ELDOR spectroscopy focusing on the ¹⁴N solid...

Area of Science:

• Magnetic Resonance Spectroscopy
• Solid-State Physics
• Chemical Physics

Background:

• Electron spectral diffusion (eSD) is crucial for dynamic nuclear polarization (DNP) in systems with inhomogeneously broadened Electron Paramagnetic Resonance (EPR) spectra, like nitroxide radicals.
• eSD influences the electron spin polarization gradient during microwave irradiation, affecting DNP efficiency via the indirect cross-effect (iCE).
• Existing theoretical frameworks for deriving eSD parameters from Electron-Electron Double Resonance (ELDOR) experiments do not account for electron depolarization from the $$ solid effect (SE).

Purpose of the Study:

• To investigate and incorporate the $$ solid effect (SE) into the theoretical framework of electron spectral diffusion (eSD) for dynamic nuclear polarization (DNP).
• To analyze the contribution of $$ SE depolarization in TEMPOL solutions using W-band ELDOR experiments.
• To improve simulations of DNP processes by including $$ SE contributions in eSD models.

Main Methods:

• W-band ELDOR experiments were performed on TEMPOL solutions of varying concentrations (0.5, 10, and 20 mM).
• Analysis of $$ SE depolarization was conducted on a low-concentration (0.5 mM) TEMPOL solution where eSD is negligible.
• Simulations of ELDOR spectra for higher concentration (10 and 20 mM) TEMPOL solutions were performed using an established eSD model, with added $$ SE contributions.

Main Results:

• The $$ SE depolarization was successfully studied in a 0.5 mM TEMPOL solution, considering hyperfine interactions and relaxation.
• Simulations for a 20 mM TEMPOL solution showed good agreement with experimental ELDOR spectra upon inclusion of the $$ SE mechanism.
• Simulations for a 10 mM TEMPOL solution yielded a lower-quality fit, suggesting limitations in the simplified treatment of $$ SE, particularly its anisotropic magnetic interactions.

Conclusions:

• The inclusion of $$ SE depolarization in eSD models enhances the accuracy of DNP simulations, especially for concentrated samples.
• The simplified approach to incorporating $$ SE is effective but has limitations when SE contributions are significant and anisotropy is not explicitly modeled.
• Further development of theoretical frameworks is needed to fully account for anisotropic magnetic interactions in $$ SE for precise eSD and DNP modeling.