Large 31P-NMR enhancements in liquid state dynamic nuclear polarization through radical/target molecule non-covalent interaction

Dynamic nuclear polarization (DNP) is a method to enhance the low sensitivity of nuclear magnetic resonance (NMR) via spin polarization transfer from electron spins to nuclear spins. In the liquid state, this process is mediated by fast modulations of the electron-nuclear hyperfine coupling and its efficiency depends strongly on the applied magnetic field. A peculiar case study is triphenylphosphine (PPh₃) dissolved in benzene and doped with BDPA radical because it gives ³¹P-NMR signal enhancements of two orders of magnitude up to a magnetic field of 14.1 T. Here we show that the large ³¹P enhancements of BDPA/PPh₃ in benzene at 1.2 T (i) decrease when the moieties are dissolved in other organic solvents, (ii) are strongly reduced when using a nitroxide radical, and (iii) vanish with pentavalent ³¹P triphenylphosphine oxide. Those experimental observations are rationalized with numerical calculations based on density functional theory that show the tendency of BDPA and PPh₃ to form a weak complex via non-covalent interaction that leads to large hyperfine couplings to ³¹P (ΔA_iso ≥ 13 MHz). This mechanism is hampered in other investigated systems. The case study of ³¹P-DNP in PPh₃ is an important example that extends the current understanding of DNP in the liquids state: non-covalent interactions between radical and target can be particularly effective to obtain large NMR signal enhancements.

A triple resonance (e, 1H, 13C) probehead for liquid-state DNP experiments at 9.4 Tesla

In DNP experiments, NMR signal intensity is increased by transferring the much larger electron spin polarization to nuclear spins via microwave irradiation. Here we describe the design and performance of a probehead that makes it possible to perform Overhauser DNP experiments at ¹H and ¹³C in liquid samples with a volume of up to 100 nl. We demonstrate on a ¹³C-labeled sodium pyruvate sample in water that proton decoupling under DNP conditions is possible with this new triple-resonance DNP probehead. In addition, the heat dissipation from the sample has been greatly improved with our new probe design. This makes it possible to keep liquid samples at a constant temperature under irradiation with a high-frequency 263 GHz microwave gyrotron with a few watts of output power. This improved performance opens up the possibility to disentangle the role of sample temperature and applied microwave power for DNP efficiency in liquids and to obtain a quantitative determination of EPR saturation by observing the suppression of paramagnetic shift as a function of microwave power.

Nitroxide Derivatives for Dynamic Nuclear Polarization in Liquids: The Role of Rotational Diffusion

Polarization transfer efficiency in liquid-state dynamic nuclear polarization (DNP) depends on the interaction between polarizing agents (PAs) and target nuclei modulated by molecular motions. We show how translational and rotational diffusion differently affect the DNP efficiency. These contributions were disentangled by measuring ¹H-DNP enhancements of toluene and chloroform doped with nitroxide derivatives at 0.34 T as a function of either the temperature or the size of the PA. The results were employed to analyze ¹³C-DNP data at higher fields, where the polarization transfer is also driven by the Fermi contact interaction. In this case, bulky nitroxide PAs perform better than the small TEMPONE radical due to structural fluctuations of the ring conformation. These findings will help in designing PAs with features specifically optimized for liquid-state DNP at various magnetic fields.

Large volume liquid state scalar Overhauser dynamic nuclear polarization at high magnetic field

Dynamic Nuclear Polarization (DNP) can increase the sensitivity of Nuclear Magnetic Resonance (NMR), but it is challenging in the liquid state at high magnetic fields. In this study we demonstrate significant enhancements of NMR signals (up to 70 on 13C) in the liquid state by scalar Overhauser DNP at 14.1 T, with high resolution (∼0.1 ppm) and relatively large sample volume (∼100 μL).