Anisotropic relaxation effects are well know and understood in EPR spectroscopy and have long served as measures to understand the motion (libration) of paramagnetic co-factors (quinones, nitroxide radicals etc.) in biological system. In this study the authors investigate the effect of anisotropic relaxation effects in DNP experiments.
To find more about anisotropic relaxation effects studied by EPR take a look at the work by Sergei Dzuba or the Eatons:
- Dzuba, S.A., Librational motion of guest spin probe molecules in glassy media. Physics Letters A, 1996. 213(1-2): p. 77-84.
- Dzuba, S.A., Echo-induced EPR spectra of nitroxides: Study of molecular librations. Pur. Appl. Chem., 1992. 64(6): p. 825-831.
- Du, J.L., G.R. Eaton, and S.S. Eaton, Temperature, Orientation, and Solvent Dependence of Electron Spin-Lattice Relaxation Rates for Nitroxyl Radicals in Glassy Solvents and Doped Solids. J. Magn. Reson. Ser. A, 1995. 115(2): p. 213-221.
Weber, E.M.M., et al., Anisotropic longitudinal electronic relaxation affects DNP at cryogenic temperatures. Phys. Chem. Chem. Phys., 2017. 19(24): p. 16087-16094.
We report the observation of anisotropic longitudinal electronic relaxation in nitroxide radicals under typical dynamic nuclear polarization conditions. This anisotropy affects the efficiency of dynamic nuclear polarization at cryogenic temperatures of 4 K and high magnetic fields of 6.7 T. Under our experimental conditions, the electron paramagnetic resonance spectrum of nitroxides such as TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl) is only partly averaged by electronic spectral diffusion, so that the relaxation times T1e([small omega]) vary across the spectrum. We demonstrate how the anisotropy of T1e([small omega]) can be taken into account in simple DNP models.