Rotation-induced recovery and bleaching in magnetic resonance

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.

Nuclear depolarization and absolute sensitivity in magic-angle spinning cross effect dynamic nuclear polarization

“…4 presents the data measured for the Trityl-doped sample, for which the DNP enhancement e on/off is found to be MAS independent (black curve), in agreement with previous theoretical description of the SE. 37,38,42 Nonetheless, the behavior of I S,off for the Trityl-doped sample as a function of MAS is significantly different than those observed for the biradicals, showing clear differences between the CE and SE mechanisms. For the doped sample, I S,off is slightly lower than for the undoped sample by B15%, as in the static case for the biradicals.…”

“…Note that for all radical-containing samples, the same value of 1H T 1,n was found in the absence and in the presence of mw irradiation. 37,38 It is interesting to note that despite the fact that 1H T 1,n are similar for AMUPOL and TOTAPOL samples under static conditions (which is expected at identical electron concentration), they significantly differ under MAS, suggesting strongly that the dominant mechanism behind the measured values of 1H T 1,n under MAS may not be anymore paramagnetic relaxation. 55 The introduction of nitroxide biradical species in the frozen solution leads to a strong decrease of the apparent 1H T 1,n saturation-recovery time constant.…”

“…During the time for the sample (placed in a so-called rotor) to complete one cycle under MAS (a rotor period), the frequencies of the electronic spins are moving across the entire EPR line, allowing for different crossings or anti-crossings of energy levels. 38 In addition to the depolarization phenomenon, the presence of paramagnetic radicals also directly affects the intensity of the detectable NMR signal as it leads to shortening of nuclear coherence lifetimes and potentially broadens NMR resonances of close-by nuclei beyond detection. The difference lies in the fact that the forbidden transition is irradiated periodically and not continuously.…”

“…For the SE, the mechanism remains similar to the static case, and relies on the irradiation of a forbidden transition. 22,23,[38][39][40][41][42][43][44][45][46][47][48] Despite the fact that depolarization and bleaching effects have been evidenced experimentally, many studies still rely on the use of the DNP enhancement factor e on/off to reflect the absolute sensitivity gain, although it is now clear that this simple parameter is not suited to this. For the CE, these theoretical contributions have significantly improved the description of nitroxide radicalbased MAS-DNP experiments through the development of full quantum mechanical simulations for small spin systems while including relaxation.…”

Theoretical aspects of Magic Angle Spinning - Dynamic Nuclear Polarization

“…Recently it has been shown [46,36,47] that the signal intensities of these rotating samples without the presence of MW are smaller than the equilibrium signals obtained from the same samples when they are not rotating. This depolarization effect enhances the experimental enhancement factors.…”