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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.
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Dynamic Nuclear Polarization Using Electron Spin Cluster
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.
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- Award ID(s):
- 2411584
- PAR ID:
- 10534363
- Publisher / Repository:
- Journal of Physical Chemistry Letters
- Date Published:
- Journal Name:
- The Journal of Physical Chemistry Letters
- Volume:
- 15
- Issue:
- 20
- ISSN:
- 1948-7185
- Page Range / eLocation ID:
- 5366 to 5375
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Understanding the spatial distribution of the P1 centers is crucial for diamond-based sensors and quantum devices. P1 centers serve as polarization sources for dynamic nuclear polarization (DNP) quantum sensing and play a significant role in the relaxation of nitrogen vacancy (NV) centers. Additionally, the distribution of NV centers correlates with the distribution of P1 centers, as NV centers are formed through the conversion of P1 centers. We utilized DNP and pulsed electron paramagnetic resonance (EPR) techniques that revealed strong clustering of a significant population of P1 centers that exhibit exchange coupling and produce asymmetric line shapes. The 13C DNP frequency profile at a high magnetic field revealed a pattern that requires an asymmetric EPR line shape of the P1 clusters with electron–electron (e–e) coupling strengths exceeding the 13C nuclear Larmor frequency. EPR and DNP characterization at high magnetic fields was necessary to resolve energy contributions from different e–e couplings. We employed a two-frequency pump–probe pulsed electron double resonance technique to show cross-talk between the isolated and clustered P1 centers. This finding implies that the clustered P1 centers affect all of the P1 populations. Direct observation of clustered P1 centers and their asymmetric line shape offers a novel and crucial insight into understanding magnetic noise sources for quantum information applications of diamonds and for designing diamond-based polarizing agents with optimized DNP efficiency for 13C and other nuclear spins of analytes. We propose that room temperature 13C DNP at a high field, achievable through straightforward modifications to existing solution-state NMR systems, is a potent tool for evaluating and controlling diamond defects.more » « less
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The dielectric properties of materials play a crucial role in the propagation and absorption of microwave beams employed in Magic Angle Spinning − Dynamic Nuclear Polarization (MAS-DNP) NMR experiments. Despite ongoing optimization efforts in sample preparation, routine MAS-DNP NMR applications often fall short of theoretical sensitivity limits. Offering a different perspective, we report the refractive indices and extinction coefficients of diverse materials used in MAS-DNP NMR experiments, spanning a frequency range from 70 to 960 GHz. Knowledge of their dielectric properties enables the accurate simulation of electron nutation frequencies, thereby guiding the design of more efficient hardware and sample preparation of biological or material samples. This is illustrated experimentally for four different rotor materials (sapphire, yttria-stabilized zirconia (YSZ), aluminum nitride (AlN), and SiAlON ceramics) used for DNP at 395 GHz/1H 600 MHz. Finally, electromagnetic simulations and state-of-the-art MAS-DNP numerical simulations provide a rational explanation for the observed magnetic field dependence of the enhancement when using nitroxide biradicals, offering insights that will improve MAS-DNP NMR at high magnetic fields.more » « less
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Overhauser dynamic nuclear polarization (ODNP) NMR of solutions at high fields is usually mediated by scalar couplings that polarize the nuclei of heavier, electron-rich atoms. This leaves 1H-detected NMR outside the realm of such studies. This study presents experiments that deliver 1H-detected NMR experiments on relatively large liquid volumes (60 ∼ 100 μL) and at high fields (14.1 T), while relying on ODNP enhancements. To this end 13C NMR polarizations were first enhanced by relying on a mechanism that utilizes e--13C scalar coupling interactions; the nuclear spin alignment thus achieved was then passed on to neighboring 1H for observation, by a reverse INEPT scheme relying on one-bond JCH-couplings. Such 13C 1H polarization transfer ported the 13C ODNP gains into the 1H, permitting detection at higher frequencies and with higher potential sensitivities. For a model solution of labeled 13CHCl3 comixed with a nitroxide-based TEMPO derivative as polarizing agent, an ODNP enhancement factor of ca. 5x could thus be imparted to the 1H signal. When applied to bigger organic molecules like 2-13C-phenylacetylene and 13C8-indole, ODNP enhancements in the 1.2-3x range were obtained. Thus, although handicapped by the lower γ of the 13C, enhancements could be imparted on the 1H thermal acquisitions in all cases. We also find that conventional 1H–13C nuclear Overhauser enhancements (NOEs) are largely absent in these solutions due to the presence of co-dissolved radicals, adding negligible gains and playing negligible roles on the scalar e-→13C ODNP transfer. Potential rationalizations of these effects as well as extensions of these experiments, are briefly discussed.more » « less
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Abstract Dynamic nuclear polarization (DNP) is a powerful tool to enhance the NMR signals of molecules by transferring polarization from unpaired electron spins to nuclei through microwave irradiation. The resulting signal enhancements can enable the analysis of samples that have previously been intractable by NMR spectroscopy, including proteins, nucleic acids, and metabolites in cells. To carry out DNP, the sample is doped with a polarization agent, a biradical containing two nitroxide moieties. DNP applications in cells, however, present significant challenges as nitroxides are often susceptible to the reducing cellular environment. Here, we introduce a novel polarization agent, POPAPOL, that exhibits increased lifetimes under reducing conditions. We also compare its bioresistance and DNP performance with three popular, commercially available polarization agents. Our work indicates that pyrrolidine‐based nitroxides can outperform piperidine‐based nitroxides in cellular environments, and that future polarization agent designs must carefully balance DNP performance and stability for cellular applications.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/acs.jpclett.4c00182
