Aug 5, 2020

Heteronuclear DNP of 1H and 19F nuclei using BDPA as a polarizing agent #DNPNMR

Gennaro, Antonio, Alexander Karabanov, Alexey Potapov, and Walter Köckenberger. “Heteronuclear DNP of 1H and 19F Nuclei Using BDPA as a Polarizing Agent.” Physical Chemistry Chemical Physics 22, no. 15 (2020): 7803–16.

This work explores the dynamic nuclear polarization (DNP) of 1H and 19F nuclei in a sample of 25/75 (% v/v) fluorobenzene/toluene containing the radical 1,3-bisphenylene-2-phenylallyl radical (BDPA) as a polarizing agent. Previously, heteronuclear effects in DNP were studied by analysing the shapes of DNP spectra, or by observing cross-relaxation between nuclei of different types. In this work, we report a rather specific DNP spectrum, where 1H and 19F nuclei obtain polarizations of opposite signs upon microwave (MW) irradiation. In order to explain this observation, we introduce a novel mechanism called heteronuclear thermal mixing (hn-TM). Within this mechanism the spectra of opposite signs can then be explained due to the presence of four-spin systems, involving a pair of dipolar coupled electron spins and hyperfine coupled nuclear spins of 1H and 19F, such that a condition relating their Larmor frequencies |o1e o2e| E oH oF is satisfied. Under this condition, a strong mixing of electron and nuclear states takes place, enabling simultaneous four-spin flip-flops. Irradiation of electron spin transitions with MW followed by such four-spin flip-flops produces non-equilibrium populations of |aHbFi and |bHaFi states, thus leading to the enhancements of opposite signs for 1H and 19F. Signal enhancements, build-up times and DNP-spectra as a function of MW power and polarizing agent concentration, all provide additional support for assigning the observed DNP mechanism as hn-TM and distinguishing it from other possible mechanisms. We also develop a quantum mechanical model of hn-TM based on averaging of spin Hamiltonians. Simulations based on this model show very good qualitative agreement with experimental data. In addition, the system exhibits cross-relaxation between 1H and 19F induced by the presence of BDPA, which was detected by measuring the 19F signal build-up upon saturation of 1H nuclei with a train of radio-frequency pulses. We demonstrate that such cross-relaxation most likely originates due to the same electron and nuclear states mixing in the four-spin systems.

Aug 3, 2020

Gadolinium Effect at High-Magnetic-Field DNP: 70% 13C Polarization of [U-13C] Glucose Using Trityl #DNPNMR

Capozzi, Andrea, Saket Patel, W. Thomas Wenckebach, Magnus Karlsson, Mathilde H. Lerche, and Jan Henrik Ardenkjær-Larsen. “Gadolinium Effect at High-Magnetic-Field DNP: 70% 13C Polarization of [U-13C] Glucose Using Trityl.” The Journal of Physical Chemistry Letters 10, no. 12 (June 20, 2019): 3420–25.

https://doi.org/10.1021/acs.jpclett.9b01306.

We show that the trityl electron spin resonance (ESR) features, crucial for an efficient dynamic nuclear polarization (DNP) process, are sample-composition-dependent. Working at 6.7 T and 1.1 K with a generally applicable DNP sample solvent mixture such as water/glycerol plus trityl, the addition of Gd3+ leads to a dramatic increase in [U-13C] glucose polarization from 37 ± 4% to 69 ± 3%. This is the highest value reported to date and is comparable to what can be achieved on pyruvic acid. Moreover, performing ESR measurements under actual DNP conditions, we provide experimental evidence that gadolinium doping not only shortens the trityl electron spin−lattice relaxation time but also modifies the radical g-tensor. The latter yielded a considerable narrowing of the ESR spectrum line width. Finally, in the frame of the spin temperature theory, we discuss how these two phenomena affect the DNP performance.

Jul 31, 2020

Spatially resolved NMR spectroscopy of heterogeneous gas phase hydrogenation of 1,3-butadiene with para-hydrogen

Svyatova, Alexandra, Elizaveta S. Kononenko, Kirill V. Kovtunov, Dmitry Lebedev, Evgeniy Yu. Gerasimov, Andrey V. Bukhtiyarov, Igor P. Prosvirin, et al. “Spatially Resolved NMR Spectroscopy of Heterogeneous Gas Phase Hydrogenation of 1,3-Butadiene with Para Hydrogen.” Catalysis Science & Technology 10, no. 1 (2020): 99–104


Glass tube reactors with Pd, Pt, Rh or Ir nanoparticles dispersed on a thin layer of TiO2, CeO2, SiO2 or Al2O3 provided mechanistic insight into the hydrogenation of 1,3-butadiene using para hydrogen. Magnetic resonance-based methods such as nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) are widely used to provide in situ/ operando information of chemical reactions. However, the low spin density and magnetic field inhomogeneities associated with heterogeneous catalytic systems containing gaseous reactants complicate such studies. Hyperpolarization techniques, in particular para-hydrogen-induced polarization (PHIP), increase significantly the NMR signal intensity. In this study, we test 16 glass tube reactors containing Pd, Pt, Rh or Ir nanoparticles dispersed on a thin layer of TiO2, CeO2, SiO2 or Al2O3 for the hydrogenation of 1,3-butadiene using para-hydrogen. The catalytic coatings of Ir and Rh gave hydrogenation products with the highest nuclear spin polarization while the coatings of Pd are the most selective ones for the semihydrogenation of 1,3-butadiene to 1- and 2-butenes. Spatially resolved NMR spectroscopy of the reagent and the product distribution along the reactor axis provided further mechanistic insight into the catalytic function of these reactive coatings under operando conditions.

Jul 29, 2020

Aqueous aging of a silica coated TiO2UV filter used in sunscreens: investigations at the molecular scale with dynamic nuclear polarization NMR #DNPNMR

Slomberg, Danielle L., Riccardo Catalano, Fabio Ziarelli, Stéphane Viel, Vincent Bartolomei, Jérôme Labille, and Armand Masion. “Aqueous Aging of a Silica Coated TiO 2 UV Filter Used in Sunscreens: Investigations at the Molecular Scale with Dynamic Nuclear Polarization NMR.” RSC Advances 10, no. 14 (2020): 8266–74


Short-term, aqueous aging of a commercial nanocomposite TiO2 UV filter with a protective SiO2 shell was examined in abiotic simulated fresh- and seawater. Under these conditions, the SiO2 layer was quantitatively removed (∼88–98%) within 96 hours, as determined using inductively coupled plasma-atomic emission spectroscopy (ICP-AES). While these bulk ICP-AES analyses suggested almost identical SiO2 shell degradation after aging in fresh- and seawater, surface sensitive 29Si dynamic nuclear polarization (DNP) solid-state nuclear magnetic resonance (SSNMR), with signal enhancements of 5–10× compared to standard SSNMR, was able to distinguish differences in the aged nanocomposites at the molecular level. DNP-SSNMR revealed that the attachment of the silica layer to the underlying TiO2 core rested on substantial Si–O–Ti bond formation, bonds which were preserved after freshwater aging, yet barely present after aging in seawater. The removal of the protective SiO2 layer is due to ionic strength accelerated dissolution, which could present significant consequences to aqueous environments when the photoactive TiO core becomes exposed. This work demonstrates the importance of characterizing aged nanocomposites not only on the bulk scale, but also on the molecular level by employing surface sensitive techniques, such as DNP-NMR. Molecular level details on surface transformation and elemental speciation will be crucial for improving the environmental safety of nanocomposites.

Jul 27, 2020

Dynamic nuclear polarization enhanced neutron crystallography: Amplifying hydrogen in biological crystals #DNPNMR

Pierce, Joshua, Matthew J. Cuneo, Anna Jennings, Le Li, Flora Meilleur, Jinkui Zhao, and Dean A. A. Myles. “Chapter Eight - Dynamic Nuclear Polarization Enhanced Neutron Crystallography: Amplifying Hydrogen in Biological Crystals.” In Methods in Enzymology, edited by Peter C. E. Moody, 634:153–75. Neutron Crystallography in Structural Biology. Academic Press, 2020.


Dynamic nuclear polarization (DNP) can provide a powerful means to amplify neutron diffraction from biological crystals by 10–100-fold, while simultaneously enhancing the visibility of hydrogen by an order of magnitude. Polarizing the neutron beam and aligning the proton spins in a polarized sample modulates the coherent and incoherent neutron scattering cross-sections of hydrogen, in ideal cases amplifying the coherent scattering by almost an order of magnitude and suppressing the incoherent background to zero. This chapter describes current efforts to develop and apply DNP techniques for spin polarized neutron protein crystallography, highlighting concepts, experimental design, labeling strategies and recent results, as well as considering new strategies for data collection and analysis that these techniques could enable.

ERC-funded postdoctoral position in MAS-DNP of organic solids #DNPNMR

Hello everyone,
there is a postdoc position opening at the Institut of Radical Chemistry in Marseille (France).

For a complete description of the project and to apply, please follow this link to the offer on the job portal of the CNRS:


For more details, contact Giulia Mollica at giulia.mollica@univ-amu.fr
Thank you

Giulia


Missions

The solid-state NMR group of the University of Aix Marseille is looking for motivated candidates for a postdoctoral research position in chemical sciences funded by the European Research Council (ERC). The project aims to develop and apply an emerging NMR hyperpolarization technique, MAS-DNP (nuclear dynamic polarization under magic-angle spinning). This approach is used to increase the sensitivity of solid-state nuclear magnetic resonance (SSNMR), allowing the extraction of unique structural information on solid materials at the atomic scale without requiring isotopic enrichment.
Activities

The aim of the project is to develop a new experimental approach to study the structure and the formation of organic solids. These materials, essentially composed of small organic molecules, can assume different crystallize forms, a phenomenon known as polymorphism, whose physical origins are not yet fully understood.

The postdoctoral researcher recruited as part of this project will:

  • develop new NMR experiments for the structural study of organic powders by hyperpolarized NMR (MAS DNP)
  • develop new tools to increase the temporal resolution of the analysis of crystallizing systems by DNP NMR.

Skills

Applicants must have a PhD in physical or physical-chemistry sciences and a previous experience in NMR.
Work Context

Host Institution - Research carried out under this project will be conducted at the Institut de Chimie Radicalaire (ICR, UMR7273). Located in the south of France in Marseille, the ICR is internationally recognized for its double expertise in i) the development of new DNP approaches for the characterization of organic solids and ii) the synthesis of radical species currently used as polarizing agents the most effective for the analysis of solids via MAS DNP. The laboratory has free access to high performance computing facilities through the Mesocentre of Aix-Marseille Université. With the Spectropole analytical facility on-site, ICR has access to a wide range of instruments for material analysis, and several NMR spectrometers for liquids and solids whose fields vary from 300 to 600 MHz. In particular, the laboratory is equipped with a new 400 MHz MAS-DNP spectrometer as well as with a 400 MHz wide bore NMR spectrometer with numerous MAS probes up to 60 kHz.
The postdoctoral position is funded by the CNRS initially for one year, renewable up to 2.5 years depending on the results. Salary depends on experience (2200 € - 3100 € net / month). Applications will be considered on a rolling basis.
The CNRS is labeled HR Excellence for Research (HRS4R) and promotes the transparency of the recruitment process and equal opportunities.

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Jul 24, 2020

Study of electron spectral diffusion process under DNP conditions by ELDOR spectroscopy focusing on the 14N Solid Effect #DNPNMR

Ramirez Cohen, Marie, Akiva Feintuch, Daniella Goldfarb, and Shimon Vega. “Study of Electron Spectral Diffusion Process under DNP Conditions by ELDOR Spectroscopy Focusing on the 14N Solid Effect.” Magnetic Resonance Discussions, February 24, 2020, 1–26.


Electron spectral diffusion (eSD) plays an important role in solid state, static DNP with polarizers having in-homogeneously broadened EPR spectra, such as nitroxide radicals. It affects the electron spin polarization gradient within the EPR spectrum during microwave irradiation and thereby determines the effectiveness of the DNP process via the so called indirect cross effect (iCE) mechanism. The electron depolarization profile can be measured by Electron-Electron Double Resonance (ELDOR) experiments and a theoretical framework for deriving eSD parameters from ELDOR spectra and employing them to calculate DNP profiles has been developed. The inclusion of electron depolarization arising from the <sup>14</sup>N Solid Effect (SE) has not yet been taken into account in this theoretical framework and is the subject of the present work. The <sup>14</sup>N SE depolarization was studied using W-band ELDOR of a 0.5&thinsp;mM TEMPOL solution, where eSD is negligible, taking into account the hyperfine interaction of both <sup>14</sup>N and <sup>1</sup>H nuclei, the long microwave irradiation applied under DNP conditions and electron and nuclear relaxation. The results of this analysis were then used in simulations of ELDOR spectra of 10 and 20&thinsp;mM TEMPOL solutions, where eSD is significant using the eSD model and the SE contributions were added ad-hoc employing the <sup>1</sup>H and <sup>14</sup>N frequencies and their combinations, as found from the analysis of the 0.5&thinsp;mM sample. This approach worked well for the 20&thinsp;mM solution where a good fit for all ELDOR spectra recorded along the EPR spectrum was obtained and the inclusion of the <sup>14</sup>N SE mechanism improved the agreement with the experimental spectra. For the 10&thinsp;mM solution, simulations of the ELDOR spectra recorded along the gz position gave a lower quality fit than for spectra recorded in the center of the EPR spectrum, suggesting that the simple approach used to the SE of the <sup>14</sup>N contribution, when its contribution is high, is lacking as the anisotropy of its magnetic interactions has not been considered explicitly.