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Dynamic Nuclear Polarization (DNP) utilizing Electron Spin Clusters to achieve resonance matching with the nucleus and to generate an Asymmetric Polarization Elevation (ESCAPE-DNP, or ESC-DNP for short) by monochromatic microwave irradiation at a select frequency is debuted as a promising mechanism to achieve NMR signal enhancements with a wide design scope requiring low microwave power at high magnetic field. In this paper, we present the design for a trityl-based tetra-radical (TetraTrityl) to achieve DNP for 1H NMR at 7 Tesla, supported by experimental data and quantum mechanical simulations. A slow relaxing (T1e ≈ 1 ms) four electron spin cluster is found to require at least two electron pairs with e-e distances of 8 Å or below to yield any meaningful 1H ESC-DNP NMR enhancement, while squeezing the rest of the e-e distances to 12 Å or below gives rise to near maximum 1H ESC-DNP-NMR enhancements. For the more common case of a fast-relaxing spin cluster (T1e ≈ 1 μs), efficient ESC-DNP is found to require an asymmetric ESC that contains a cluster of strongly coupled narrow-line radicals coexisting with a weakly coupled narrow-line radical acting as a sensitizer to extract polarization from the cluster. This study highlights the untapped potential of utilizing strong coupling of narrow-line radical clusters to achieve microwave power-efficient DNP that extends design options beyond what is available today and offers great tunability at high magnetic field.more » « less
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