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

Electron spectral diffusion (eSD) plays an important role in solid state, static DNP with polarizers having in-homogeneously 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 14N Solid Effect (SE) has not yet been taken into account in this theoretical framework and is the subject of the present work. The 14N 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 14N and 1H 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 1H and 14N 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 14N SE mechanism improved the agreement with the experimental spectra. For the 10 mM solution, simulations of the ELDOR spectra recorded along the gz position gave a lower quality fit than for spectra recorded in the center of the EPR spectrum, suggesting that the simple approach used to the SE of the 14N contribution, when its contribution is high, is lacking as the anisotropy of its magnetic interactions has not been considered explicitly.