J-Driven dynamic nuclear polarization for sensitizing high field solution state NMR

Abstract

Dynamic nuclear polarization (DNP) is widely used to enhance solid state nuclear magnetic resonance (NMR) sensitivity. Its efficiency as a generic signal-enhancing approach for liquid state NMR, however, decays rapidly...

Microwave-freeJ-driven dynamic nuclear polarization: A proposal for enhancing the sensitivity of solution-state NMR

“…Spin population operators corresponding to the  and  nuclear components of these singlet and the triplet states were considered, and described using Vega's fictitious operators notation. [30,48,49] Three-and four-spin systems were considered, encompassing in all cases the aforementioned two-electron biradical plus protons. One of the protons was always assigned to a fluid medium, that would dynamically diffuse around the biradical as described below; this is the "solvent" 1 H whose polarization enhancement MF-JDNP is seeking, and which was assumed to interact with the electrons solely through dipolar (aka anisotropic hyperfine) couplings.…”

“…[53] According to Redfield's relaxation theory, and for the conditions mentioned above, the singlet and triplet relaxation rates of a biradical with identical g-tensors will be dominated by the dipolar hyperfine interactions between the electrons and the surrounding protons. [30] The lifetimes (T1) for the ,/ T   states will then vary strongly with the distance between the electrons and the protons: when 1 Hs do not approach the biradical electrons to distance closer than 5 Å, these T1s extend into the ms range; [30] in the presence of "radical" protons sited ≤5 Å away from either of the electrons, these T1s drop to ≈100 μs (see Supplemental material for the relaxation rates predicted by Redfield theory as a function of these and other parameters [47]). This in turn posed the issue of how to estimate the relaxation behavior expected from two-electrons interacting with "solvent" protons, that can take a number of distances from the electrons.…”

“…[26][27][28][29] We have recently discussed a possible way to bypass these solution-state limitations, based on what we denominate the J-driven DNP (JDNP) effect. [30] JDNP requires stable biradicals with identical monomers and an inter-electron exchange coupling Jex close to the electron Larmor frequency E. As the JDNP condition Jex≈±E is fulfilled, a difference between the relaxation rates for the two-electron singlet and triplet states which are dipolar hyperfine-coupled to nuclear  or  states can lead, upon electron irradiation, to a transient imbalance between these nuclear populations.…”

“…The enhancement predicted by this repeated shuttling is relatively isotropic [30]; it is maximized each time the JDNP condition is fulfilled, but begins to decay as the sample departs from this condition. This explains the oscillations displayed by Z N with the shuttling; oscillations which are magnified further when considering the equilibrium nuclear polarization at each field (Fig.…”

Large31P-NMR enhancements in liquid state dynamic nuclear polarization through radical/target molecule non-covalent interaction

“…12 Those findings sparked new interest in the method, and now several groups are committed to tackle the open challenges to make DNP in the liquid state applicable to routine NMR spectroscopy. [13][14][15][16][17][18][19][20][21][22] Within this context, it is important to design targets and PAs whose properties are specifically tailored to maximize the attainable NMR signal enhancements. To this aim, it is essential to identify the physical mechanisms that make the polarization transfer particularly effective.…”