Dynamic Nuclear Polarization with Vanadium(IV) Metal Centers

Jain, Sheetal Kumar; Yu, Chung-Jong; Wilson, C. Blake; Tabassum, Tarnuma; Freedman, Danna E.; Han, Songi

doi:10.1016/j.chempr.2020.10.021

Cited by 28 publications

(26 citation statements)

References 32 publications

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“…Wolfe determined experimentally that proton spins at 0.3 nm to the paramagnetic center were still in strong contact with the bulk spins and, hence, outside of the barrier, according to his definition (for proton spins separated by 0.16 nm in a paramagnetic crystal at 1.4 K and 1.6 T). This approach was used in other recent studies (35,37). It is common to call the spins that lie inside and outside of the spin diffusion barrier core and bulk spins, respectively.…”

Section: Theory and Definitionsmentioning

confidence: 99%

“…The existence of the diffusion barrier (or the slowdown of diffusion) is assessed indirectly based on the ability of the models to reproduce experimental DNP build-up curves. Two recent studies have approached the question experimentally using synthetic chemistry to precisely control the distance between the electron and the closest nuclear spin ( 35 , 37 ). They were able to determine the minimal distance from the electron at which nuclei could contribute to the DNP enhancement of the bulk spins, for their respective samples and experimental conditions.…”

Section: Introductionmentioning

confidence: 99%

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Direct observation of hyperpolarization breaking through the spin diffusion barrier

et al. 2021

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Dynamic nuclear polarization (DNP) is a widely used tool for overcoming the low intrinsic sensitivity of nuclear magnetic resonance spectroscopy and imaging. Its practical applicability is typically bounded, however, by the so-called “spin diffusion barrier,” which relates to the poor efficiency of polarization transfer from highly polarized nuclei close to paramagnetic centers to bulk nuclei. A quantitative assessment of this barrier has been hindered so far by the lack of general methods for studying nuclear polarization flow in the vicinity of paramagnetic centers. Here, we fill this gap and introduce a general set of experiments based on microwave gating that are readily implemented. We demonstrate the versatility of our approach in experiments conducted between 1.2 and 4.2 K in static mode and at 100 K under magic angle spinning (MAS)—conditions typical for dissolution DNP and MAS-DNP—and directly observe the marked dependence of polarization flow on temperature.

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Section: Theory and Definitionsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Direct observation of hyperpolarization breaking through the spin diffusion barrier

et al. 2021

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“…Usually, the figure of merit for the SMMs is the blocking temperature (below this temperature the system behaves like a magnet) while for qubit molecules is the coherence time (which must be longer than clock time of the quantum computer) [2,4] . The molecules with the longest coherence times are mononuclear S =1/2 transition metal complexes, usually V IV and Cu II complexes, with moderate/low magnetic anisotropy that also avoid any electronic and/or nuclear spin coupling [3–6] . Recently, titanium cations have been also proposed as qubits [7] or for quantum sensing devices [8,9] .…”

Section: Introductionmentioning

confidence: 99%

Spin‐Phonon Coupling and Slow‐Magnetic Relaxation in Pristine Ferrocenium

Amoza

Maxwell

Aliaga‐Alcalde

et al. 2021

Chemistry A European J

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We report the spin dynamic properties of non‐substituted ferrocenium complexes. Ferrocenium shows a field‐induced single‐molecule magnet behaviour in DMF solution while cobaltocene lacks slow spin relaxation neither in powder nor in solution. Multireference quantum mechanical calculations give a non‐Aufbau orbital occupation for ferrocenium with small first excitation energy that agrees with the relatively large measured magnetic anisotropy for a transition metal S=1/2 system. The analysis of the spin relaxation shows an important participation of quantum tunnelling, Raman, direct and local‐mode mechanisms which depend on temperature and the external field conditions. The calculation of spin‐phonon coupling constants for the vibrational modes shows that the first vibrational mode, despite having a low spin‐phonon constant, is the most efficient process for the spin relaxation at low temperatures. In such conditions, vibrational modes with higher spin‐phonon coupling constants are not populated. Additionally, the vibrational energy of this first mode is in excellent agreement with the experimental fitted value obtained from the local‐mode mechanism.

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“… 24–27 The finding was recently corroborated by other approaches. 28,29 The observation of the 3SSE prompted us to search for other higher-order transitions, and we report here our recent observation of the four-spin solid effect (4SSE), i.e., a forbidden transition present only in a four-spin system comprised of one electron and three nuclei.…”

Section: Introductionmentioning

confidence: 83%

Observation of a four-spin solid effect

Tan

Griffin

2022

The Journal of Chemical Physics

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The two-spin solid effect (2SSE) is one of the established continuous wave dynamic nuclear polarization (DNP) mechanisms that enables enhancement of NMR signals. It functions via a state-mixing mechanism that mediates the excitation of forbidden transitions in an electron-nuclear spin system. Specifically, microwave irradiation at frequencies ᵱ4;μw~ᵱ4;0S {plus minus} ᵱ4;0I, where ᵱ4;0S and ᵱ4;0I are electron and nuclear Larmor frequencies, respectively, yields enhanced nuclear spin polarization. Following the recent rediscovery of the three-spin solid effect (3SSE) (Tan et al., Sci. Adv, aax2743), where the matching condition is given by ᵱ4;μw = ᵱ4;0S {plus minus} 2ᵱ4;0I,we report here the first direct observation of the four-spin solid effect (4SSE) at ᵱ4;μw = ᵱ4;0S {plus minus} 3ᵱ4;0I . The forbidden double- and quadruple-quantum transitions were observed in samples containing trityl radicals dispersed in a glycerol-water mixture at 0.35 T / 15 MHz / 9.8 GHz and 80 K. We present a derivation of the 4SSE effective Hamiltonian, matching conditions, and transition probabilities. Finally, we show that the experimental observations agree with the results from numerical simulations and analytical theory.

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Dynamic Nuclear Polarization with Vanadium(IV) Metal Centers

Cited by 28 publications

References 32 publications

Direct observation of hyperpolarization breaking through the spin diffusion barrier

Direct observation of hyperpolarization breaking through the spin diffusion barrier

Spin‐Phonon Coupling and Slow‐Magnetic Relaxation in Pristine Ferrocenium

Observation of a four-spin solid effect

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