Abstract
Electron spectral diffusion (eSD) plays an important role in solid-state, static dynamic nuclear polarization (DNP) with polarizers that have inhomogeneously broadened EPR spectra, such as nitroxide radicals. It affects the electron spin polarization gradient within the EPR spectrum during microwave irradiation and thereby determines the effectiveness of the DNP process via the so-called indirect cross-effect (iCE) mechanism. The electron depolarization profile can be measured by electron-electron double resonance (ELDOR) experiments, and a theoretical framework for deriving eSD parameters from ELDOR spectra and employing them to calculate DNP profiles has been developed. The inclusion of electron depolarization arising from the solid effect (SE) has not yet been taken into account in this theoretical framework and is the subject of the present work. The SE depolarization was studied using W-band ELDOR of a 0.5 mM TEMPOL solution, where eSD is negligible, taking into account the hyperfine interaction of both and nuclei, the long microwave irradiation applied under DNP conditions, and electron and nuclear relaxation. The results of this analysis were then used in simulations of ELDOR spectra of 10 and 20 mM TEMPOL solutions, where eSD is significant using the eSD model and the SE contributions were added ad hoc employing the and frequencies and their combinations, as found from the analysis of the 0.5 mM sample. This approach worked well for the 20 mM solution, where a good fit for all ELDOR spectra recorded along the EPR spectrum was obtained and the inclusion of the SE mechanism improved the agreement with the experimental spectra. For the 10 mM solution, simulations of the ELDOR spectra recorded along the position gave a lower-quality fit than for spectra recorded in the center of the EPR spectrum. This indicates that the simple approach we used to describe the SE is limited when its contribution is relatively high as the anisotropy of its magnetic interactions was not considered explicitly.
Highlights
It has been recently recognized that electron spectral diffusion plays a significant role in dynamic nuclear polarization (DNP) under static conditions (Hovav et al, 2015a; Leavesley et al, 2017)
It was demonstrated that once the polarization gradient within the EPR spectrum has been determined via the electron spectral diffusion (eSD) model simulations, the lineshape of the associated DNP spectrum could be reproduced taking into account the polarization differences between all electron pairs satisfying the cross effect (CE) condition (Hovav et al, 2015a)
Electron spin-lattice relaxation times T1e were measured at different positions within the EPR spectrum by saturation recovery experiments with a long MW saturation pulse of 30 ms and echo pulses of 300 ns each, which is typical for DNP MW power
Summary
It has been recently recognized that electron spectral diffusion (eSD) plays a significant role in dynamic nuclear polarization (DNP) under static conditions (Hovav et al, 2015a; Leavesley et al, 2017). It was demonstrated that once the polarization gradient within the EPR spectrum has been determined via the eSD model simulations, the lineshape of the associated DNP spectrum could be reproduced taking into account the polarization differences between all electron pairs satisfying the cross effect (CE) condition (Hovav et al, 2015a) This approach was implemented by Leavesley et al (2017) when they explored the eSD process and its influence on the DNP efficiency at a magnetic field of 7 T. Kundu et al (2018b) used the eSD model to quantify the dependence of the electron polarization exchange parameter eSD on radical concentration and temperature
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