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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. 

Dynamic nuclear polarization (DNP) can amplify the solid-state nuclear magnetic resonance (NMR) signal by several orders of magnitude. The mechanism of DNP utilizing α,γ- Bisdiphenylene-β-phenylallyl (BDPA) variants as Polarizing Agents (PA) has been the subject of lively discussions on account of their remarkable DNP efficiency with low demand for microwave power. We propose that electron spin clustering of SA-BDPA is responsible for its DNP performance, as revealed by the temperature-dependent shape of the central DNP profile and strong electron-electron (e-e) crosstalk seen by Electron Double Resonance. We demonstrate that a multi-electron spin cluster can be modeled with three coupled spins, where electron J (exchange) coupling between one of the e-e pairs matching the NMR Larmor frequency induces the experimentally observed absorptive central DNP profile, and the electron T1e modulated by temperature and magic angle spinning alters the shape between an absorptive and dispersive feature. Understanding the microscopic origin is key to designing new PAs to harness the microwave power-efficient DNP effect observed with BDPA variants.