Category Archives: Theory

Prediction of flow effects in quantitative NMR measurements #DNPNMR

Friebel, Anne, Thomas Specht, Erik von Harbou, Kerstin Münnemann, and Hans Hasse. “Prediction of Flow Effects in Quantitative NMR Measurements.” Journal of Magnetic Resonance 312 (March 2020): 106683.

https://doi.org/10.1016/j.jmr.2020.106683.

A method for the prediction of the magnetization in flow NMR experiments is presented, which can be applied to mixtures. It enables a quantitative evaluation of NMR spectra of flowing liquid samples even in cases in which the magnetization is limited by the flow. A transport model of the nuclei’s magnetization, which is based on the Bloch-equations, is introduced into a computational fluid dynamics (CFD) code. This code predicts the velocity field and relative magnetization of different nuclei for any chosen flow cell geometry, fluid and flow rate. The prediction of relative magnetization is used to correct the observed reduction of signal intensity caused by incomplete premagnetization in fast flowing liquids. By means of the model, quantitative NMR measurements at high flow rates are possible. The method is predictive and enables calculating correction factors for any flow cell design and operating condition based on simple static T1 time measurements. This makes time-consuming calibration measurements for assessing the influence of flow effects obsolete, which otherwise would have to be carried out for each studied condition. The new method is especially interesting for flow measurements with compact medium field NMR spectrometers, which have small premagnetization volumes. In the present work, experiments with three different flow cells in a medium field NMR spectrometer were carried out. Acetonitrile, water, and mixtures of these components were used as model fluids. The experimental results for the magnetization were compared to the predictions from the CFD model and good agreement was observed.

Optimizing nitroxide biradicals for cross-effect MAS-DNP: the role of g-tensors’ distance #DNPNMR

Mentink-Vigier, Frédéric. “Optimizing Nitroxide Biradicals for Cross-Effect MAS-DNP: The Role of g-Tensors’ Distance.” Physical Chemistry Chemical Physics 22, no. 6 (2020): 3643–52.

https://doi.org/10.1039/C9CP06201G

Nitroxide biradicals are common polarizing agents used to enhance the sensitivity of solid-state NMR experiments via Magic Angle Spinning Dynamic Nuclear Polarization (MAS-DNP). These biradicals are used to increase the polarization of protons through the cross-effect mechanism, which requires two unpaired electrons with a Larmor frequency difference greater than that of the protons. From their early conception, the relative orientation of the nitroxide rings has been identified as a critical factor determining their MAS-DNP performance. However, the MAS leads to a complex DNP mechanism with time dependent energy level anti-crossings making it difficult to pinpoint the role of relative g-tensor orientation. In this article, a single parameter called “g-tensors’ distance” is introduced to characterize the relative orientation\’s impact on the MAS-DNP field profiles. It is demonstrated for the first time how the g-tensors’ distance determines the nuclear hyperpolarization and depolarization properties of a given biradical. This provides a new critical parameter that paves the way for more efficient bis-nitroxides for MAS-DNP.

Optically-generated Overhauser dynamic nuclear polarization: A numerical analysis #ODNP #DNPNMR

Cheney, Daniel J., and Christopher J. Wedge. “Optically-Generated Overhauser Dynamic Nuclear Polarization: A Numerical Analysis.” The Journal of Chemical Physics 152, no. 3 (January 21, 2020): 034202.

https://doi.org/10.1063/1.5133408

Recently, an alternative approach to dynamic nuclear polarization (DNP) in the liquid state was introduced using optical illumination instead of microwave pumping. By exciting a suitable dye to the triplet state which undergoes a diffusive encounter with a persistent radical forming a quartet-doublet pair in the encounter complex, dynamic electron polarization (DEP) is generated via the radical-triplet pair mechanism. Subsequent cross-relaxation generates nuclear polarization without the need for microwave saturation of the electronic transitions. Here, we present a theoretical justification for the initial experimental results by means of numerical simulations. These allow investigation of the effects of various experimental parameters, such as radical and dye concentrations, sample geometry, and laser power, on the DNP enhancement factors, providing targets for experimental optimization. It is predicted that reducing the sample volume will result in larger enhancements by permitting a higher concentration of triplets in a sample of increased optical density. We also explore the effects of the pulsed laser rather than continuous-wave illumination, rationalizing the failure to observe the optical DNP effect under illumination conditions common to DEP experiments. Examining the influence of the illumination duty cycle, the conditions necessary to permit the use of pulsed illumination without compromising signal enhancement are determined, which may reduce undesirable laser heating effects. This first simulation of the optical DNP method therefore underpins the further development of the technology.

DNP Mechanisms #DNPNMR

Kundu, Krishnendu, Frédéric Mentink‐Vigier, Akiva Feintuch, and Shimon Vega. “DNP Mechanisms.” In EMagRes, 295–338. American Cancer Society, 2019.

https://doi.org/10.1002/9780470034590.emrstm1550.

This article presents a comprehensive description of the spin dynamics underlying the main DNP mechanisms leading to nuclear signal enhancements in glassy amorphous solids containing free radicals. The emphasis of the article to derive quantum mechanically based formalisms that enable us to analyze experimental DNP data. After a short review of the history of DNP, rate equations of the eigenstate populations of static coupled electron–nuclear spin systems are introduced, based on their spin-Hamiltonians and including spin-lattice and cross-relaxation mechanisms. They are used to simulate the dynamics of small spin systems under microwave (MW) irradiation and the basic Solid Effect (SE), Cross Effect (CE), and Overhauser DNP (O-DNP) enhancement mechanisms are presented. These calculations are then extended to systems containing up to 10 spins and are used to calculate EPR, ELDOR, and DNP spectra. Plots showing the population of the eigenstates vs energy are used to demonstrate the conditions for the thermal mixing mechanism and the corresponding EPR and ELDOR spectra are discussed. Following these calculations, the electron spectral diffusion (eSD) and the indirect Cross Effect (iCE) numerical models are introduced and used to analyze EPR and DNP spectra of real samples. In the last section, the basic theory of magic angle spinning (MAS) DNP on small spin systems is summarized and the influence of the rotor events on the quasiperiodic steady-state DNP enhancements discussed. The origins of depolarization effects occurring when no MW is applied are described. Finally, the nuclear spin diffusion process inside the diffusion barrier is studied using multielectron and multinuclear calculations.

Determination of the thermal, oxidative and photochemical degradation rates of scintillator liquid by fluorescence EEM spectroscopy #DNPNMR

Andrews, N. L. P., J. Z. Fan, R. L. Forward, M. C. Chen, and H.-P. Loock. “Determination of the Thermal, Oxidative and Photochemical Degradation Rates of Scintillator Liquid by Fluorescence EEM Spectroscopy.” Physical Chemistry Chemical Physics 19, no. 1 (2017): 73–81.

https://doi.org/10.1039/C6CP06015C.

We present observations of an NMR maser (microwave amplification by stimulated emission of radiation) of hyperpolarized 1H nuclei by dynamic nuclear polarization (DNP) at 1.2 K and in a magnetic field of 6.7 T. The sustained maser pulses originate from the interplay between radiation damping (RD) due to the large 1H magnetization, and the remagnetization to a negative value by the DNP process. NMR signals lasting for several tens of seconds are thus observed on an ensemble of dipolar-coupled nuclear spins. Magnetization dynamics are analyzed in terms of the combined Bloch-Maxwell and Provotorov (BMP) equations for RD and DNP. Insight into the long time evolution of the magnetization is provided by a theoretical analysis of this nonlinear dynamical system, and by fitting the NMR signal to a simplified version of the BMP equations.

Theoretical Treatment of Pulsed DNP Experiments: Effects of Spectral Exchange #DNPNMR

Nasibulov, Egor A., Konstantin L. Ivanov, and Renad Z. Sagdeev. “Theoretical Treatment of Pulsed DNP Experiments: Effects of Spectral Exchange.” Applied Magnetic Resonance 50, no. 10 (October 2019): 1233–40.

https://doi.org/10.1007/s00723-019-01144-6

In the present work, we provide a theoretical treatment of pulsed Overhauser-type dynamic nuclear polarization (DNP) in the presence of spectral exchange, namely, Heisenberg exchange. The expression for the DNP enhancement of a nuclear magnetic resonance (NMR) signal is generalized by redefining the “deviation factor”, expressing the deviation of the electron spin polarization from its equilibrium value, averaged over the period of the pulse sequence. We can demonstrate that spectral exchange significantly increases the deviation factor and, thus, the NMR enhancement. The present treatment allows one to determine the optimal pumping frequency at different exchange rates.

The influence of Ho3+ doping on 13C DNP in the presence of BDPA #DNPNMR

Khattri, Ram B., Ali A. Sirusi, Eul Hyun Suh, Zoltan Kovacs, and Matthew E. Merritt. “The Influence of Ho3+ Doping on 13C DNP in the Presence of BDPA.” Physical Chemistry Chemical Physics 21, no. 34 (2019): 18629–35.

https://doi.org/10.1039/C9CP03717A

Polarization transfer from unpaired electron radicals to nuclear spins at low-temperature is achieved using microwave irradiation by a process broadly termed dynamic nuclear polarization (DNP). The resulting signal enhancement can easily exceed factors of 104 when paired with cryogenic cooling of the sample. Dissolution-DNP couples low temperature polarization methods with a rapid dissolution step, resulting in a highly polarized solution that can be used for metabolically sensitive magnetic resonance imaging (MRI). Hyperpolarized [1-13C]pyruvate is a powerful metabolic imaging agent for investigation of in vitro and in vivo cellular metabolism by means of NMR spectroscopy and MRI. Radicals (trityl OX063 and BDPA) with narrower EPR linewidths typically produce higher nuclear polarizations when carbon-13 is the target nucleus. Increased solid-state polarization is observed when narrow line radicals are doped with lanthanide ions such as Gd3+, Ho3+, Dy3+, and Tb3+. Earlier results have demonstrated an incongruence between DNP experiments with trityl and BDPA, where the optimal concentrations for polarization transfer are disparate despite similar electron spin resonance linewidths. Here, the effects of Ho-DOTA on the solid-state polarization of [1-13C]pyruvic acid were compared for 3.35 T (1.4 K) and 5 T (1.2 K) systems using BDPA as a radical. Multiple concentrations of BDPA were doped with variable concentrations of Ho-DOTA (0, 0.2, 0.5, 1, and 2 mM), and dissolved in 1 : 1 (v/v) of [1-13C] pyruvic acid/sulfolane mixture. Our results reveal that addition of small amounts of Ho-DOTA in the sample preparation increases the solid-state polarization for [1-13C] pyruvic acid, with the optimum Ho-DOTA concentration of 0.2 mM. Without Ho-DOTA doping, the optimum BDPA concentration found for 3.35 T (1.4 K) is 40 mM, and for 5 T (1.2 K) system it is about 60 mM. In both systems, inclusion of Ho-DOTA in the 13C DNP sample leads to a change in the breadth (ΔDNP) of the extrema between the P(+) and P(−) frequencies in microwave spectra. At no combination of BDPA and Ho3+ did polarizations reach those achievable with trityl. Simplified analysis of increased polarization as a function of decreased electron T1e used to explain results in trityl are insufficient to describe DNP with BDPA.

Biradical rotamer states tune electron J coupling and MAS dynamic nuclear polarization enhancement #DNPNMR

Tagami, Kan, Asif Equbal, Ilia Kaminker, Bernard Kirtman, and Songi Han. “Biradical Rotamer States Tune Electron J Coupling and MAS Dynamic Nuclear Polarization Enhancement.” Solid State Nuclear Magnetic Resonance 101 (September 2019): 12–20.

https://doi.org/10.1016/j.ssnmr.2019.04.002

Cross Effect (CE) Dynamic Nuclear Polarization (DNP) relies on the dipolar (D) and exchange (J) coupling interaction between two electron spins. Until recently only the electron spin D coupling was explicitly included in quantifying the DNP mechanism. Recent literature discusses the potential role of J coupling in DNP, but does not provide an account of the distribution and source of electron spin J coupling of commonly used biradicals in DNP. In this study, we quantified the distribution of electron spin J coupling in AMUPol and TOTAPol biradicals using a combination of continuous wave (CW) X-band electron paramagnetic resonance (EPR) lineshape analysis in a series of solvents and at variable temperatures in solution – a state to be vitrified for DNP. We found that both radicals show a temperature dependent distribution of J couplings, and the source of this distribution to be conformational dynamics. To qualify this conformational dependence of J coupling in both molecules we carry out “Broken Symmetry” DFT calculations which show that the biradical rotamer distribution can account for a large distribution of J couplings, with the magnitude of J coupling directly depending on the relative orientation of the electron spin pair. We demonstrate that the electron spin J couplings in both AMUPol and TOTAPol span a much wider distribution than suggested in the literature. We affirm the importance of electron spin J coupling for DNP with density matrix simulations of DNP in Liouville space and under magic angle spinning, showcasing that a rotamer with high J coupling and “optimum” relative g-tensor orientation can significantly boost the DNP performance compared to random orientations of the electron spin pair. We conclude that moderate electron spin J coupling above a threshold value can facilitate DNP enhancements.

EasySpin Academy, Aug 26-28 2019, Seattle

Dear EPR community,

We are excited to announce EasySpin Academy 2019, a 2.5-day workshop for EasySpin, a MATLAB-based software toolbox for simulating and fitting Electron Paramagnetic Resonance (EPR) spectra – see http://easyspin.org.

The workshop will be held at the University of Washington in Seattle. It starts on Monday Aug 26 at 6 pm and ends on Wednesday Aug 28 in the evening. Instructors include Stefan Stoll (creator of EasySpin), Stephan Pribitzer (active developer), and others.

The workshop is open to everyone. Some experience with EPR spectroscopy is required, but no prior experience with either MATLAB or EasySpin are expected. We will be able to accommodate both beginners and users with EasySpin experience.

This workshop is a unique opportunity to learn directly from the core developers of EasySpin, to interact with other EPR researchers, to discuss your EPR simulation projects and needs with EasySpin developers, and to provide feedback about desired features for future releases of EasySpin.

For the workshop, bring your own laptop with MATLAB pre-installed. If you do not have access to a licensed copy of MATLAB, you can download a 30-day trial from http://mathworks.com.

The registration fee is $295 and covers three nights in on-campus double-occupancy dorm rooms (Monday, Tuesday, Wednesday) and all meals during the workshop.

To register, please visit http://easyspin.org/academy. After registration, you will received an email with details about the payment. The registration deadline is July 25.

There are only a limited number of slots available, and they will be allocated on a first-come, first-served basis.

Best regards

Stefan Stoll

Maximizing nuclear hyperpolarization in pulse cooling under MAS #DNPNMR

Björgvinsdóttir, Snædís, Brennan J. Walder, Nicolas Matthey, and Lyndon Emsley. “Maximizing Nuclear Hyperpolarization in Pulse Cooling under MAS.” Journal of Magnetic Resonance 300 (March 2019): 142–48.

https://doi.org/10.1016/j.jmr.2019.01.011

It has recently been shown how dynamic nuclear polarization can be used to hyperpolarize the bulk of proton-free solids. This is achieved by generating the polarization in a wetting phase, transferring it to nuclei near the surface and relaying it towards the bulk through homonuclear spin diffusion between weakly magnetic nuclei. Pulse cooling is a strategy to achieve this that uses a multiple contact cross-polarization sequence for bulk hyperpolarization. Here, we show how to maximize sensitivity using the pulse cooling method by experimentally optimizing pulse parameters and delays on a sample of powdered SnO2. To maximize sensitivity we introduce an approach where the magic angle spinning rate is modulated during the experiment: the CP contacts are carried out at a slow spin rate to benefit from faster spin diffusion, and the spin rate is then accelerated before detection to improve line narrowing. This method can improve the sensitivity of pulse cooling for 119Sn spectra of SnO2 by an additional factor of 3.5.

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