Abstract
The magnetic microwave field strength and its detailed spatial distribution in magic-angle spinning (MAS) nuclear magnetic resonance (NMR) probes capable of dynamic nuclear polarization (DNP) is investigated by numerical simulations with the objective to augment the magnetic microwave amplitude by structuring the sample in the mm and sub-mm range and by improving the coupling of the incident microwave beam to the sample. As it will be shown experimentally, both measures lead to an increase of the microwave efficiency in DNP MAS NMR.
Highlights
The first strategy relies on embedding dielectric particles into the sample [1,2], the former with a dielectric constant higher than the sample material
The last line in Tab. 1 indicates the attainable magnetic field amplitude in an electron paramagnetic resonance (EPR) cavity at 263 GHz, which is much higher because of the optimum resonating structure. Such a more ideal structure is hard to achieve for magic-angle spinning (MAS) dynamic nuclear polarization (DNP) probes for the reasons of the various hardware boundary conditions for MAS DNP as compared to pure EPR
Magnetic microwave amplitudes without and with waveguide coupler compared with the amplitude achievable in a fundamental mode EPR cavity (Q = 500)
Summary
The first strategy relies on embedding dielectric particles (e.g., potassium bromide, KBr, grains) into the sample [1,2], the former with a dielectric constant higher than the sample material (tetrachloroethane, TCE, plus low concentration biradicals, TEKPOL). The second strategy [3] aims at an improved coupling of the incident microwave beam to the sample by uti-
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