Longitudinal Pulsed Dynamic Nuclear Polarization Transfer via Periodic Optimal Control.

Review: 194 refs.

Dynamic nuclear polarization (DNP) was first demonstrated in the early days of NMR spectroscopy. The technique involves the transfer of large spin polarization from unpaired electron spins to neighboring nuclei using microwave irradiation close to the electron Larmor frequency. Over the years, although a number of applications in solid-state NMR have been demonstrated, the use of DNP in liquid-state experiments has been more limited. Modern technological advances now mean that DNP is undergoing a rejuvenation as a technique to vastly improve the sensitivity of NMR. This article describes the current state of the art in regard to the liquid-state applications of DNP, including sensitivity enhancement in heteronuclear NMR spectroscopy.

Dynamic nuclear polarization (DNP) is a nuclear magnetic resonance (NMR) hyperpolarization technique that mediates polarization transfer from unpaired electrons with large thermal polarization to NMR-active nuclei via microwave (mw) irradiation. The ability to generate arbitrarily shaped mw pulses using arbitrary waveform generators allows for remarkable improvement of the robustness and versatility of DNP. We present here novel design principles based on single-spin vector effective Hamiltonian theory to develop new broadband DNP pulse sequences, namely, an adiabatic version of XiX (X–inverse X)–DNP and a broadband excitation by amplitude modulation (BEAM)–DNP experiment. We demonstrate that the adiabatic BEAM-DNP pulse sequence may achieve a 1H enhancement factor of ∼360, which is better than ramped-amplitude NOVEL (nuclear spin orientation via electron spin locking) at ∼0.35 T and 80 K in static solids doped with trityl radicals. In addition, the bandwidth of the BEAM-DNP experiments (~50 MHz) is about three times the 1H Larmor frequency. The generality of our theoretical approach will be helpful in the development of new pulsed DNP sequences.

The coherent transfer of electron spin polarization to nuclei by means of a microwave pulse sequence is a promising new approach to enhancing the sensitivity of solid-state nuclear magnetic resonance (NMR). The development of pulse sequences for dynamic nuclear polarization (DNP) of bulk nuclei is far from complete, as is the understanding of what makes a good DNP sequence. In this context, we introduce a new sequence, termed Two-Pulse Phase Modulation (TPPM) DNP. We provide a general theoretical description for electron-proton polarization transfer by periodic DNP pulse sequences and find it in excellent agreement with numerical simulations. In experiments at 1.2T, TPPM DNP generates a higher gain in sensitivity than existing sequences XiX (X-inverse-X) and TOP (Time-Optimized Pulsed) DNP but does so at relatively high nutation frequencies. In contrast, we find that the XiX sequence performs very well at nutation frequencies as low as 7MHz. A combination of theoretical analysis and experimental investigation makes clear that fast electron-proton polarization transfer, due to a well-preserved dipolar coupling in the effective Hamiltonian, correlates with a short build-up time of the dynamic nuclear polarization of the bulk. Experiments further show that the performances of XiX and TOP DNP are affected differently by the concentration of the polarizing agent. These results constitute important reference points for the development of new and better DNP sequences.

Dynamic nuclear polarization (DNP) of molecules in a solution at room\ntemperature has potential to revolutionize nuclear magnetic resonance\nspectroscopy and imaging. The prevalent methods for achieving DNP in solutions\nare typically most effective in the regime of small interaction correlation\ntimes between the electron and nuclear spins, limiting the size of accessible\nmolecules. To solve this limitation, we design a mechanism for DNP in the\nliquid phase that is applicable for large interaction correlation times.\nImportantly, while this mechanism makes use of a resonance condition similar to\nsolid-state DNP, the polarization transfer is robust to a relatively large\ndetuning from the resonance due to molecular motion. We combine this scheme\nwith optically polarized nitrogen vacancy (NV) center spins in nanodiamonds to\ndesign a setup that employs optical pumping and is therefore not limited by\nroom temperature electron thermal polarisation. We illustrate numerically the\neffectiveness of the model in a flow cell containing nanodiamonds immobilized\nin a hydrogel, polarising flowing water molecules 4700-fold above thermal\npolarisation in a magnetic field of 0.35 T, in volumes detectable by current\nNMR scanners.\n

Stability of the nitroxide biradical AMUPol in intact and lysed mammalian cells

Molecular orientation in amorphous organic semiconducting thin-film devices is an important issue affecting device performance. However, to date it has not been possible to analyze the "distribution" of the orientations. Although solid-state NMR (ssNMR) spectroscopy can provide information on the "distribution" of molecular orientations, the technique is limited because of the small amount of sample in the device and the low sensitivity of ssNMR. Here, we report the first application of dynamic nuclear polarization enhanced ssNMR (DNP-ssNMR) spectroscopy for the orientational analysis of amorphous phenyldi(pyren-1-yl)phosphine oxide (POPy2 ). The 31 P DNP-ssNMR spectra exhibited a sufficient signal-to-noise ratio to quantify the distribution of molecular orientations in amorphous films: the P=O axis of the vacuum-deposited and drop-cast POPy2 shows anisotropic and isotropic distribution, respectively. The different molecular orientations reflect the molecular origin of the different charge transport behaviors.

The sensitivity of NMR spectroscopy is considerably enhanced by dynamic nuclear polarization (DNP). In DNP polarization is transferred from unpaired electrons of a polarizing agent to nearby proton spins. In solids, this transfer is followed by the transport of hyperpolarization to the bulk via 1 H-1 H spin diffusion. The efficiency of these steps is critical to obtain high sensitivity gains, but the pathways for polarization transfer in the region near the unpaired electron spins are unclear. Here we report a series of seven deuterated and one fluorinated TEKPol biradicals to probe the effect of deprotonation on MAS DNP at 9.4 T. The experimental results are interpreted with numerical simulations, and our findings support that strong hyperfine couplings to nearby protons determine high transfer rates across the spin diffusion barrier to achieve short build-up times and high enhancements. Specifically, 1 H DNP build-up times increase substantially with TEKPol isotopologues that have fewer hydrogen atoms in the phenyl rings, suggesting that these protons play a crucial role transferring the polarization to the bulk. Based on this new understanding, we have designed a new biradical, NaphPol, which yields significantly increased NMR sensitivity, making it the best performing DNP polarizing agent in organic solvents to date.

Dynamic nuclear polarization via the cross effect and thermal mixing: A. The role of triple spin flips.

Dynamic nuclear polarization (DNP) with a 131.5 GHz gyrotron (Gyrotron FU CW IV) has achieved enhancement factor of around 30 by using stable mono-radical 4-amino-TEMPO for 200 MHz high-resolution solid-state NMR spectroscopy for the analysis of polymer surface. Dependencies of the enhancement factor on irradiation time of the gyrotron output, the power of irradiation, cooling time, and irradiation frequency are examined. Application of the 200 MHz DNP NMR spectroscopy to the analysis of polymer surface is also presented briefly.

Frequency-swept dynamic nuclear polarization

Impact of non-polar solvents in dynamic nuclear polarization at high magnetic fields.

Dynamic nuclear polarization (DNP) is a nuclear magnetic resonance (NMR) hyperpolarization technique that mediates polarization transfer from unpaired electrons to nuclear spins. DNP performance can vary significantly depending on the types of polarizing agents employed, and the criteria for optimum DNP efficiency are not fully understood. Thus, a better understanding of the structure, electron paramagnetic resonance (EPR) line widths, and relaxation properties would aid in designing more efficient DNP polarizing agents. However, EPR characterizations of the polarizing agents are typically performed in different environments (e.g., strength of magnetic field and microwave power) than typical DNP experiments. Here, we demonstrate a low-cost and home-built setup that enables in situ EPR detection in a dual resonance DNP-NMR/EPR probe using an optical chopper. The optical chopper modulates the microwave irradiation, thereby modulating the longitudinal magnetization (Mz) of the electron spins, which results in a LOngitudinal-Detected (LOD) EPR spectrum. The EPR spectrum acquired for TEMPOL at 6.7 T/188 GHz using a mechanical modulation showed a good agreement with the spectrum acquired using an electrical modulation. Hence, our work provides a proof-of-principle setup that enables an optical chopper EPR spectrometer.

Quasistationary and High Frequency Periodic Control of Continuous Processes

Biological Applications of Hyperpolarized 13C NMR