Friday, December 28, 2018

BDPA-Nitroxide Biradicals Tailored for Efficient Dynamic Nuclear Polarization Enhanced Solid-State NMR at Magnetic Fields up to 21.1 T #DNPNMR

Wisser, Dorothea, Ganesan Karthikeyan, Alicia Lund, Gilles Casano, Hakim Karoui, Maxim Yulikov, Georges Menzildjian, et al. “BDPA-Nitroxide Biradicals Tailored for Efficient Dynamic Nuclear Polarization Enhanced Solid-State NMR at Magnetic Fields up to 21.1 T.” Journal of the American Chemical Society 140, no. 41 (October 17, 2018): 13340–49.

Dynamic nuclear polarization (DNP) solid-state nuclear magnetic resonance (NMR) has developed into an invaluable tool for the investigation of a wide range of materials. However, the sensitivity gain achieved with many polarizing agents suffers from an unfavorable field and magic angle spinning (MAS) frequency dependence. We present a series of new hybrid biradicals, soluble in organic solvents, that consist of an isotropic narrow electron paramagnetic resonance line radical, α,γ-bisdiphenylene-β-phenylallyl (BDPA), tethered to a broad line nitroxide. By tuning the distance between the two electrons and the substituents at the nitroxide moiety, correlations between the electron–electron interactions and the electron spin relaxation times on one hand and the DNP enhancement factors on the other hand are established. The best radical in this series has a short methylene linker and bears bulky phenyl spirocyclohexyl ligands. In a 1.3 mm prototype DNP probe, it yields enhancements of up to 185 at 18.8 T (800 MHz 1H resonance frequency) and 40 kHz MAS. We show that this radical gives enhancement factors of over 60 in 3.2 mm sapphire rotors at both 18.8 and 21.1 T (900 MHz 1H resonance frequency), the highest magnetic field available today for DNP. The effect of the rotor size and of the microwave irradiation inside the MAS rotor is discussed. Finally, we demonstrate the potential of this new series of polarizing agents by recording high field 27Al and 29Si DNP surface enhanced NMR spectra of amorphous aluminosilicates and 17O NMR on silica nanoparticles.

Friday, December 21, 2018

Tuning nuclear depolarization under MAS by electron T1e #DNPNMR

Lund, Alicia, Asif Equbal, and Songi Han. “Tuning Nuclear Depolarization under MAS by Electron T1e.” Physical Chemistry Chemical Physics, 2018, 19. 

Cross-Effect (CE) Dynamic Nuclear Polarization (DNP) mechanism under Magic Angle Spinning (MAS) induces depletion or “depolarization” of the NMR signal, in the absence of microwave irradiation. In this study, the role of T1e on nuclear depolarization under MAS was tested experimentally by systematically varying the local and global electron spin concentration using mono-, bi- and tri-radicals. These spin systems show different depolarization effects that systematically tracked with their different T1e rates, consistent with theoretical predictions. In order to test whether the effect of T1e is directly or indirectly convoluted with other spin parameters, the tri-radical system was doped with different concentrations of GdCl3, only tuning the T1e rates, while keeping other parameters unchanged. Gratifyingly, the changes in the depolarization factor tracked the changes in the T1e rates. The experimental results are corroborated by quantum mechanics based numerical simulations which recapitulated the critical role of T1e. Simulations showed that the relative orientation of the two g-tensors and e-e dipolar interaction tensors of the CE fulfilling spin pair also plays a major role in determining the extent of depolarization, besides the enhancement. This is expected as orientations influence the efficiency of the various level anti-crossings or the “rotor events” under MAS. However, experimental evaluation of the empirical spectral diffusion parameter at static condition showed that the local vs. global e-e dipolar interaction network is not a significant variable in the commonly used nitroxide radical system studied here, leaving T1e rates as the major modulator of depolarization.

Wednesday, December 19, 2018

Sensitivity Considerations in Microwave Paramagnetic Resonance Absorption Techniques

I recently read again through Feher's article on EPR sensitivity. After 61 years this article is still very important when building instrumentation for EPR or DNP. It explains many EPR concepts and the relevant instrumentation using simple, easy to understand concepts.
I must read for everyone working in the field of magnetic resonance instrumentation.

Feher, G. “Sensitivity Considerations in Microwave Paramagnetic Resonance Absorption Techniques.” Bell System Technical Journal 36, no. 2 (March 1957): 449–84.

Within the past few years the field of paramagnetic resonance absorption has become an important tool in physical and chemical research. In many ways its usefulness is limited by the sensitivity of the experimental setup. A typical example is the study of semiconductors in which case one would like to investigate as small a number of impurities as possible. It is the purpose of this paper to analyze the sensitivity limits of several experimental set ups under different operating conditions. This was done in the hope that an understanding of these limitations would put one in a better position to design a high sensitivity resonance experiment. In the last section the performance of the different experimental arrangements is tested. The agreement obtained with the predicted performance proves the essential validity of the analysis. This paper is primarily for experimental physicists confronted withe the problem of setting up a high sensitivity spectrometer.

Monday, December 17, 2018

Electron-Spin Relaxation of Triarylmethyl Radicals in Glassy Trehalose #DNPNMR

Triarylmethyl radicals are commonly used dissolution-DNP experiments (dDNP). This article is a good reference source for the electronic relaxation times T1e and T2e in different solvents.

Kuzhelev, Andrey A., Olesya A. Krumkacheva, Ivan O. Timofeev, Victor M. Tormyshev, Matvey V. Fedin, and Elena G. Bagryanskaya. “Electron-Spin Relaxation of Triarylmethyl Radicals in Glassy Trehalose.” Applied Magnetic Resonance 49, no. 11 (November 2018): 1171–80.

Trehalose was recently proposed as a promising immobilizer of biomolecules for room-temperature electron paramagnetic resonance (EPR) structural studies. The most crucial parameter in these investigations is electron-spin relaxation (namely, phase memory time Tm). Recently, triarylmethyl (TAM) spin labels attached to DNA in trehalose were found to have the longest Tm at room temperature as compared to the existing spin labels and immobilizers. Therefore, in this work, we investigated TAM radicals in trehalose including Finland trityl (H36 form), perdeuterated Finland trityl (D36 form), and a deuterated version of OX063. The temperature dependence of electron-spin relaxation time of these radicals immobilized in trehalose was measured at X-band frequency, and possible mechanisms of relaxation were considered. OX063D in glassy trehalose has longer Tm up to 200 K as compared to Finland trityl, but at higher temperatures, OX063D is inferior in its relaxation properties, and the deuterated form of Finland trityl is preferable for pulse dipolar EPR spectroscopy experiments at 298 K. The influence of various deuterations (TAM or trehalose) on the observed relaxation times was studied, being controlled by the electron-spin-echo envelope modulation at room temperature.

Friday, December 14, 2018

Large-scale ab initio simulations of MAS DNP enhancements using a Monte Carlo optimization strategy #DNPNMR

Perras, Frédéric A., and Marek Pruski. “Large-Scale Ab Initio Simulations of MAS DNP Enhancements Using a Monte Carlo Optimization Strategy.” The Journal of Chemical Physics 149, no. 15 (October 21, 2018): 154202.

Magic-angle-spinning (MAS) dynamic nuclear polarization (DNP) has recently emerged as a powerful technology enabling otherwise unrealistic solid-state NMR experiments. The simulation of DNP processes which might, for example, aid in refining the experimental conditions or the design of better performing polarizing agents, is, however, plagued with significant challenges, often limiting the system size to only 3 spins. Here, we present the first approach to fully ab initio large-scale simulations of MAS DNP enhancements. The Landau-Zener equation is used to treat all interactions concerning electron spins, and the low-order correlations in the Liouville space method is used to accurately treat the spin diffusion, as well as its MAS speed dependence. As the propagator cannot be stored, a Monte Carlo optimization method is used to determine the steady-state enhancement factors. This new software is employed to investigate the MAS speed dependence of the enhancement factors in large spin systems where spin diffusion is of importance, as well as to investigate the impacts of solvent and polarizing agent deuteration on the performance of MAS DNP.

Wednesday, December 12, 2018

Conformation of Bis-nitroxide Polarizing Agents by Multi- frequency EPR Spectroscopy #DNPNMR

To optimize the DNP process it is crucial to understand the EPR properties of the polarizing agents. This article demonstrates the need of multi-frequency, high-field EPR spectroscopy to gain a deep understanding of all EPR parameters and how they influence the DNP process.

Soetbeer, Janne, Peter Gast, Joseph J Walish, Yanchuan Zhao, Christy George, Chen Yang, Timothy M Swager, Robert G Griffin, and Guinevere Mathies. “Conformation of Bis-Nitroxide Polarizing Agents by Multi- Frequency EPR Spectroscopy,”

The chemical structure of polarizing agents critically determines the efficiency of dynamic nuclear polarization (DNP). For cross-effect DNP, biradicals are the polarizing agents of choice and the interaction and relative orientation of the two unpaired electrons should be optimal. Both parameters are affected by the molecular structure of the biradical in the frozen glassy matrix that is typically used for DNP/MAS NMR and likely differs from the structure observed with X-ray crystallography. We have determined the conformations of six bis-nitroxide polarizing agents, including the highly efficient AMUPol, in their DNP matrix with EPR spectroscopy at 9.7 GHz, 140 GHz, and 275 GHz. The multi-frequency approach in combination with an advanced fitting routine allows us to reliably extract the interaction and relative orientation of the nitroxide moieties. We compare the structures of six bis-nitroxides to their DNP performance at 500 MHz/330 GHz.

Tuesday, December 11, 2018

[NMR] Postdoctoral position in NMR of polymer electrolytes for energy storage at UCSB (Santa Barbara, USA) #DNPNMR

Postdoctoral position in NMR of polymer ionic liquid electrolytes for energy storage at UCSB (Santa Barbara, USA)

The postdoctoral research position is based at the Materials Department, University of California Santa Barbara (UCSB), in the group of Prof. Raphaële Clément. The position is available from January, 1st, 2019, and comes with an initial one-year contract. 

Project Description

The research program focuses on the study of polymer ionic liquids (PILs), used as electrolytes in Li-ion rechargeable batteries, with solid-state NMR techniques. The postdoc will be working in close collaboration with the group of Prof. Segalman at UCSB, in charge of the synthesis of the PILs. 

Li-ion batteries are the technology of choice for numerous applications, yet the energy density and safety of commercial devices is often limited by using organic liquid electrolytes with high flammability and poor stability of electrode/electrolyte interfaces during operation. Ionic liquids are a class of functional liquid salts that address both voltage and thermal stability concerns [1]. Incorporation of ionic liquid moieties onto a polymer to form PILs synergistically combines the functionality of ionic liquids with the mechanical robustness of the polymer backbone, an important criterion for lithium metal batteries. Unfortunately, polymer electrolytes generally exhibit low ionic conductivity due to a fundamental trade-off between improved mechanical properties and ion mobility [2]. Thus, an understanding of promising systems that would enable a decoupling of polymer mechanics from ion transport would be beneficial towards the targeted design of novel polymer electrolytes with good mechanical properties and high ionic conductivity.

The postdoc will investigate the Li+ transport properties of several PIL compositions using 7Li pulsed field gradient (PFG) NMR to determine the Li self-diffusion coefficient on the macroscopic (1-2 mm) lengthscale, as well as NMR relaxometry to characterize ionic conductivity at the microscopic scale. In addition, NMR will be used to investigate Li-Li site exchange [3] and Li-ligand binding kinetics [4]. NMR measurements will be correlated with conductivity measurements obtained via AC impedance techniques.

Relevant publications 

[1] B. Garcia, S. Lavalle, G. Perron, C. Michot, and M. Armand, Room temperature molten salts as lithium battery electrolyte, Electrochim. Acta 49 (2004) 4583–4588. DOI: 10.1016/j.electacta.2004.04.041

[2] J. R. Sangoro, C. Iacob, A. L. Agapov, Y. Wang, S. Berdzinski, H. Rexhausen, V. Strehmel, C. Friedrich, A. P. Sokolov, and F. Kremer, Decoupling of ionic conductivity from structural dynamics in polymerized ionic liquids, Soft Matter 10 (2014) 3536–3540.

DOI: 10.1039/C3SM53202J

[3] M. N. d’Eurydice, E. T. Montrazi, C. A. Fortulan and T. J. Bonagamba, T2-tiltered T2-T2 Exchange NMR. J. Chem. Phys. 144 (2016) 204201. DOI: 10.1063/1.4951712

[4] Tan-Vu Huynh. NMR study of lithium mobility in polymer electrolytes. Ph.D. thesis. Université d’Orléans, 2015. English.

Requirements and Preferred Experience

The requirements for the position are a Ph.D. in Chemistry, Materials Science, Physics or Engineering, and experience with PFG-NMR and standard 2D solid-state NMR techniques. In addition to performing research, the postdoc will be expected to provide assistance with the training of graduate and undergraduate students in the group.

The Magnetic Resonance Facilities at UCSB

The spectroscopy facility of the Materials Research Laboratory at UCSB (see comprises several solid-state NMR spectrometers, including a 300 MHz system equipped with a MRI/diffusion probe, a 400 MHz DNP-NMR spectrometer, and 500 MHz and 800 MHz systems.

Additional Information

Interested candidates should send a cover letter, a résumé (including a list of publications), and the names and email addresses of at least two references to
Raphaële Clément

Assistant Professor, Materials Department
Materials Research Laboratory, Room 3009
University of California, Santa Barbara, CA 93106-5121

Phone: 805-893-4294

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NMR web database:

Monday, December 10, 2018

DNP NMR Studies of Crystalline Polymer Domains by Copolymerization with Nitroxide Radical Monomers #DNPNMR

Verde-Sesto, Ester, Nicolas Goujon, Haritz Sardon, Pauline Ruiz, Tan Vu Huynh, Fermin Elizalde, David Mecerreyes, Maria Forsyth, and Luke A. O’Dell. “DNP NMR Studies of Crystalline Polymer Domains by Copolymerization with Nitroxide Radical Monomers.” Macromolecules 51, no. 20 (October 23, 2018): 8046–53.

Dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR) spectroscopy is increasingly recognized as a powerful and versatile tool for the characterization of polymers and polymer-based materials. DNP requires the presence of unpaired electrons, usually mono- or biradicals, and the method of incorporation of these groups and their distribution within the structure is crucial. Methods for covalently binding the radicals to the polymer and controlling their location (e.g., exclusively within a specific phase or at an interface) can allow the selective enhancement of a particular region or the measurement of domain sizes. We have prepared a series of polyurethanes by copolymerization of a nitroxide radical monomer with poly(ethylene glycol) (PEO) and diisocyanate linkers. The PEO is shown to form crystalline domains with the radical monomers in a separate phase, providing DNP enhancements of around 10 and allowing the domain size and morphology to be probed with the aid of X-ray scattering data. Additionally, electron paramagnetic resonance is used to estimate the inter-radical distances and density functional theory calculations are used to refine the PEO crystal structure.

Saturday, December 8, 2018

[NMR] Post-doctoral position available #DNPNMR

A postdoctoral position is available to join the Emsley group at EPFL, Lausanne

We are looking for highly motivated candidates to take up a Postdoctoral position developing new methods in NMR spectroscopy to address challenging problems in chemistry and materials science. In particular we will be working on dynamic nuclear polarization enhanced NMR methods for materials. Examples of our recent work and the application areas that we work on can be found on our website:

We are looking for highly motivated candidates with strong scientific background, independence, and who enjoy teamwork. You should hold a relevant qualification in chemistry, physics or related disciplines. Skills in developing experimental multi-dimensional nuclear magnetic resonance are a plus. 

Our laboratory at EPFL is part of one the world’s leading chemistry departments, and is located Lausanne on the north shore of Lake Geneva. The laboratory is equipped with unique state of the art NMR spectrometers (including gyrotron DNP accessories at 400 and 900 MHz, a dissolution-DNP machine, and 100 kHz magic angle spinning probes). 
Motivated candidates should contact Lyndon Emsley by email at

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Friday, December 7, 2018

Long-range heteronuclear J-coupling constants in esters: Implications for 13C metabolic MRI by side-arm parahydrogen-induced polarization

Stewart, Neil J., Hiroyuki Kumeta, Mitsushi Tomohiro, Takuya Hashimoto, Noriyuki Hatae, and Shingo Matsumoto. “Long-Range Heteronuclear J-Coupling Constants in Esters: Implications for 13C Metabolic MRI by Side-Arm Parahydrogen-Induced Polarization.” Journal of Magnetic Resonance 296 (November 2018): 85–92.

Side-arm parahydrogen induced polarization (PHIP-SAH) presents a cost-effective method for hyperpolarization of 13C metabolites (e.g. acetate, pyruvate) for metabolic MRI. The timing and efficiency of typical spin order transfer methods including magnetic field cycling and tailored RF pulse sequences crucially depends on the heteronuclear J coupling network between nascent parahydrogen protons and 13C, post-parahydrogenation of the target compound. In this work, heteronuclear nJHC (1<n≤5) couplings of acetate and pyruvate esters pertinent for PHIP-SAH were investigated experimentally using selective HSQMBC-based pulse sequences and numerically using DFT simulations. The CLIP-HSQMBC technique was used to quantify 2/3-bond JHC couplings, and 4/5-bond JHC ≲ 0.5 Hz were estimated by the sel-HSQMBC-TOCSY approach. Experimental and numerical (DFT-simulated) nJHC couplings were strongly correlated (P < 0.001). Implications for 13C hyperpolarization by magnetic field cycling, and PH-INEPT and ESOTHERIC type spin order transfer methods for PHIP-SAH were assessed, and the influence of direct nascent parahydrogen proton to 13C coupling when compared with indirect TOCSY-type transfer through intermediate (non-nascent parahydrogen) protons was studied by the density matrix approach.

Wednesday, December 5, 2018

Electron decoupling with cross polarization and dynamic nuclear polarization below 6 K #DNPNMR

Sesti, Erika L., Edward P. Saliba, Nicholas Alaniva, and Alexander B. Barnes. “Electron Decoupling with Cross Polarization and Dynamic Nuclear Polarization below 6 K.” Journal of Magnetic Resonance 295 (October 2018): 1–5.

Dynamic nuclear polarization (DNP) can improve nuclear magnetic resonance (NMR) sensitivity by orders of magnitude. Polarizing agents containing unpaired electrons required for DNP can broaden nuclear resonances in the presence of appreciable hyperfine couplings. Here we present the first cross polarization experiments implemented with electron decoupling, which attenuates detrimental hyperfine couplings. We also demonstrate magic angle spinning (MAS) DNP experiments below 6 K, producing unprecedented nuclear spin polarization in rotating solids. 13C correlation spectra were collected with MAS DNP below 6 K for the first time. Longitudinal magnetization recovery times with MAS DNP (T1DNP, 1H) of urea in a frozen glassy matrix below 6 K were measured for both the solid effect and the cross effect. Trityl radicals exhibit a T1DNP (1H) of 18.7 s and the T1DNP (1H) of samples doped with 20 mM AMUPol is only 1.3 s. MAS below 6 K with DNP and electron decoupling is an effective strategy to increase NMR signal-to-noise ratios per transient while retaining short recovery periods.

Monday, December 3, 2018

Multi-resonant photonic band-gap/saddle coil DNP probehead for static solid state NMR of microliter volume samples #DNPNMR

Nevzorov, Alexander A., Sergey Milikisiyants, Antonin N. Marek, and Alex I. Smirnov. “Multi-Resonant Photonic Band-Gap/Saddle Coil DNP Probehead for Static Solid State NMR of Microliter Volume Samples.” Journal of Magnetic Resonance 297 (December 2018): 113–23. 

The most critical condition for performing Dynamic Nuclear Polarization (DNP) is achieving sufficiently high electronic B1e fields over the sample at the matched EPR frequencies, which for modern high-resolution NMR instruments fall into the millimeter wave (mmW) range. Typically, mmWs are generated by powerful gyrotrons and/or extended interaction klystrons (EIKs) sources and then focused onto the sample by dielectric lenses. However, further development of DNP methods including new DNP pulse sequences may require B1e fields higher than one could achieve with the current mmW technology. In order to address the challenge of significantly enhancing the mmW field at the sample, we have constructed and tested one-dimensional photonic band-gap (PBG) mmW resonator that was incorporated inside a double-tuned radiofrequency (rf) NMR saddle coil. The photonic crystal is formed by stacking ceramic discs with alternating high and low dielectric constants. The thicknesses of the discs are chosen to be λ/4 or 3λ/4, where λ is the wavelength of the incident mmW field in the corresponding dielectric material. When the mmW frequency is within the band gap of the photonic crystal, a defect created in the middle of the crystal confines the mmW energy, thus forming a resonant structure. An aluminum mirror in the middle of the defect has been used to split the structure in order to reduce its size and simplify the resonator tuning. The latter is achieved by adjusting the width of the defect by moving the aluminum mirror with respect to the dielectric stack using a gear mechanism. The 1D PBG resonator was the key element for constructing a multi-resonant integrated DNP/NMR probehead operating at 198 GHz EPR / 300 MHz 1H / 75.5 MHz 13C NMR frequencies. Initial tests of the multi-resonant DNP/NMR probehead were carried out using a quasioptical 200 GHz bridge and a Bruker Biospin Avance II spectrometer equipped with a standard Bruker 7 T wide-bore 89 mm magnet parked at 300.13 MHz 1H NMR frequency. The mmW bridge built with all solid-state active components allows for the frequency tuning between ca. 190 to ca. 198 GHz with the output power up to 27 dBm (0.5 W) at 192 GHz and up to 23 dBm (0.2 W) at 197.5 GHz. Room temperature DNP experiments with a synthetic single crystal high-pressure high-temperature (HPHT) diamond (0.3Å~0.3Å~3.0 mm3) demonstrated dramatic 1,500–fold enhancement of 13C natural abundance NMR signal at full incident mmW power. Significant 13C DNP enhancement (of about 90) have been obtained at incident mmW powers of as low as <100 μW. Further tests of the resonator performance have been carried out with a thin (ca. 100 μm thickness) composite polystyrene-microdiamond film by controlling the average mmW power at the optimal DNP conditions via a gated mode of operation. From these experiments, the PBG resonator with loaded Q≃250 and finesse provides up to 12-fold/11 db gain in the average mmW power vs. the non-resonant probehead configuration employing only a reflective mirror.