Abstract
Thurber and Tycko recently described a 'bleaching effect' that occurs in magnetic resonance when solid samples that are doped with paramagnetic agents are subjected to rotation by magic angle spinning (MAS) in a static magnetic field with a rotation period comparable to the longitudinal relaxation time T1e of the electron spins. The bleaching effect has been investigated by Thurber and Tycko in samples spinning at temperatures near 20 K in a field of 9.4 T and by Corzilius et al. near 80 K in a field of 4.9 T. In our experience, the bleaching effect is not very severe at temperatures near 100 K in a field of 9.4 T at spinning frequencies up to 12 kHz. Bleaching can partly cancel DNP enhancements that are normally evaluated by comparing signal intensities with and without microwave irradiation. The signal attenuation due to doping and sample rotation is usually not taken into consideration when defining enhancement factors. In this paper, we describe a novel observation that the rotation of glassy samples doped with lanthanides spinning at frequencies as low as 0.1 kHz can lead to a significant reduction of the spin-lattice relaxation times T1((1)H) of protons. This effect, which bears similarities with the so-called spin refrigerators, may contribute to the success of 'brute force polarization' at sample temperatures in the mK range. The acceleration of longitudinal proton relaxation also allows one to improve the signal-to-noise ratio per unit time.
Highlights
A decrease of nuclear polarization due to the rotation of glassy samples doped with paramagnetic impurities has been observed[17] and evaluated for various radicals at 4.9 T and 80 K and at 9.4 T and 18 K.19 This decrease tends to cancel some of the benefits of Dynamic nuclear polarization (DNP)
The marked reduction of T1(1H) in samples doped with lanthanides such as Dy3+ or Ho3+ at spinning frequencies as low as nrot = 0.1 kHz and in the absence of spinning-induced bleaching is consistent with the principles of the proton spin refrigerator discussed above for lanthanides such as Yb3+, Ce3+ or Dy3+ and at low sample spinning frequencies
RAS-electron paramagnetic resonance (EPR) could play an important role in determining contributions of different mechanisms to the perturbation of the nuclear magnetic polarization in samples doped with paramagnetic agents that are subject to rotation
Summary
A decrease of nuclear polarization due to the rotation of glassy samples doped with paramagnetic impurities has been observed[17] and evaluated for various radicals at 4.9 T and 80 K (ref. 18) and at 9.4 T and 18 K.19 This decrease tends to cancel some of the benefits of DNP. A decrease of nuclear polarization due to the rotation of glassy samples doped with paramagnetic impurities has been observed[17] and evaluated for various radicals at 4.9 T and 80 K The enhancement factors have to be carefully measured to evaluate the balance between the positive and negative contributions to the nuclear polarization that can arise from doping with different paramagnetic agents such as nitroxides and lanthanides, microwave irradiation, sample spinning, low temperatures, and high magnetic fields. We shall briefly discuss mechanisms that can drive the nuclear polarization away from thermal equilibrium through mechanical rotation of samples doped with paramagnetic agents. The use of the lanthanide Dy3+ as a doping agent in either static or rotating glassy samples leads to a welcome reduction of the longitudinal relaxation times T1(1H) in solids.[14,19] An NMR study of samples
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