Dynamic nuclear polarization with a cyclotron resonance maser at 5 T

Abstract

DNP (dynamic nuclear polarization) experiments at 5 T are reported, in which a cyclotron resonance maser (gyrotron) is utilized as a 20 W, 140 6Hz microwave source to perform the polarization. MAS (magic angle spinning) NMR spectroscopy with DNP has been performed on samples of polystyrene doped with the free radical BDPA (a, y-bisdiphenylene-P-phenylallyl) at room temperature. Maximal DNP enhancements of -10 for 'H and -40 for ' C are observed and are considerably larger than expected. The DNP and spin relaxation mechanisms that lead to these enhancements at 5 T are discussed.PACS numbers: 76.70.Fz DNP (dynamic nuclear polarization) is a magnetic resonance technique utilized to enhance the polarization of nuclei in samples containing paramagnetic centers [1,2] through irradiation of the electron spins with microwaves in the neighborhood of their Larmor frequency. In studies on solids, this technique has been primarily employed to produce polarized targets for neutron scattering experiments, and to explore magnetic ordering at pK tempera-More recently, DNP has been utilized in high-resolution solid-state NMR (nuclear magnetic resonance) experiments. In MAS (magic angle spinning) NMR spectra, signal enhancements of greater than an order of magnitude have been regularly obtained at room temperature [3][4][5]. Moreover, with high eSciency microwave cavities and with small static samples, enhancements of 50-200 have been achieved [6]. Such enhancements clearly enable significant reductions in the amount of sample or the acquisition time needed to perform an NMR experiment and are therefore of great interest.The utility of these experiments has, however, been limited to date, since they have been performed only at a relatively low field -specifically 1.4 T, corresponding to Larmor frequencies for 'H and electron of coH/2tr= 60 MHz and to, /2tr=40 GHz, respectively. Consequently, the inherent sensitivity and spectral resolution routinely available in high field NMR has not been accessed in DNP-NMR experiments. The restriction to low fields arises mainly from the diSculty in procuring adequate high-power, high-frequency microwave sources. The primary millimeter wave sources, such as the EIO (extended interaction oscillator) or BWO (backward wave oscillator), rely on fragile slow-wave structures to generate microwave radiation, and thus at the high power levels required for DNP experiments have limited operating lifetimes.In the experiments described here, we have surmounted this problem by introducing a cyclotron resonance maser, or gyrotron, as the microwave source. Gyrotrons are presently employed extensively in plasma physics [7]. In the gyrotron, a continuous magnetic field replaces the slow wave structure, and the device is thus capable of generating high microwave powers over a lifetime that is considerably longer than that of an EIO or BWO [8].The microwave frequency of a gyrotron is determined by the applied magnetic field strength and accelerating voltage: gyrotron operation has been demonstrated from 8-600 GHz...

On the role of electron–nucleus contact and microwave saturation in thermal mixing DNP

“…4) by repeating the computation for our reference sample at higher field (parameters were set as follow, according to reference [22]: B 0 = 4.64 T, T = 1.2 K, T 1e = 1 s, ∆ω e = 63 MHz). The extension of our calculations to a model representing different radicals and eventually higher temperatures, for comparison with the experimental observations achieved under such conditions [23][24][25][26], will be faced in a next dedicated study.…”

250GHz CW gyrotron oscillator for dynamic nuclear polarization in biological solid state NMR

“…spin reservoirs via cross polarization. We note that it is also possible to polarize the low-γ spins directly [27,28,87], but, because of the smaller magnetic moment, the transfer process is slower and the slower spin diffusion among the low-γ nuclei limits the sphere of polarized nuclei to the vicinity of the paramagnetic center. Nevertheless, direct polarization of low γ spins presents interesting scientific opportunities that have not yet been fully exploited.…”

“…To satisfy these requirements, we turned to gyrotrons, a type of cyclotron resonance maser, as microwave sources for DNP experiments [28][29][30]. This choice was motivated by the fact that gyrotrons are fast wave devices [31], with interaction structures whose dimensions are large compared to the operating wavelength, and as such are capable of generating high powers (10-100 watts CW) for the extended periods typical of multidimensional NMR experiments.…”

“…In addition, the 140 GHz system permitted us to develop and refine a number of experimental techniques, for example MAS at 90 K and lower temperatures [42,43]. Finally, over the last few years, research with the 140 GHz system led to increases in DNP enhancements in MAS experiments from ~10 to ~300 [28,35]. However, it remains that this spectrometer is operating at a 5 T field, and as such is a low frequency instrument by present day NMR standards.…”

Sub-THz technology for dynamic nuclear polarization in nuclear magnetic resonance (DNP NMR): transverse confinement of microwave propagation through heterogeneous solid DNP samples

“…We study specific examples of low-temperature (100 K) DNP samples, characterized by a high degree of spatial heterogeneity on a sub-mm scale. We report on our work in progress investi-gating the local microwave propagation modified through transverse confinement in DNP samples with possible consequences for the DNP spin transfer mechanism.DNP NMR [1] on solids samples, in particular, under magic-angle spinning (MAS) has become an extremely useful technique in solid-state NMR to enhance the weak NMR signal by one or two orders of magnitude [2][3][4]. Application examples range from the study of biomacromolecules [5-6] to material samples like zeolites or mesoporous silica with small molecules or catalytic compounds adsorbed at the inner surfaces of the mesoporous material [7][8][9].…”