Category Archives: Solid-Effect

Elucidation of Oxygen Chemisorption Sites on Activated Carbons by 1 H DNP for Insight into Oxygen Reduction Reactions #DNPNMR

Liu, Xiaoyang, Juan Gu, James Wightman, and Harry C. Dorn. “Elucidation of Oxygen Chemisorption Sites on Activated Carbons by 1 H DNP for Insight into Oxygen Reduction Reactions.” ACS Applied Nano Materials, November 25, 2019, acsanm.9b01308.

Activated carbons (ACs) are widely used in many industrial and medical adsorbent applications because of their distinct ability to adsorb numerous gaseous and/or liquid analytes. More recently, ACs have been actively explored as an inexpensive alternative to metal catalysts (Pt) for numerous oxygen reduction reactions including microbial fuel cells (MFCs) for wastewater treatment. Although it is well established that O2 is chemisorbed on ACs, the actual chemical site has not been elucidated. In this study, we characterize adsorption of benzene on the surface of ACs in the presence and absence of O2. The AC samples have been heat treated and cover the range of 350−600 °C. The flowing benzene is monitored by solid/liquid intermolecular transfer (SLIT) 1H dynamic nuclear polarization (DNP). We find that the introduction of benzene (N2 atmosphere) flowing over an AC interface leads to a scalar (positive) 1H Overhauser effect in high temperature activated carbons (550−600 °C), whereas this nanoscale close-in Fermi interaction is completely suppressed upon introduction of oxygen (air) to the flowing benzene/ activated carbon interface. We propose these results are consistent with a benzene/delocalized singlet−triplet radical carbene or diradical interaction at the zigzag sites edges of disordered graphene motifs. These unique radical sites chemically react with O2 to form quenched diamagnetic sites. In contrast, a solid-state 1H DNP effect is observed at lower heat treatment temperatures representing different radical sites (e.g., aromatic heteroatom radical sites) in ACs.

Native Vanadyl Complexes in Crude Oil as Polarizing Agents for In Situ Proton Dynamic Nuclear Polarization #DNPNMR

This is really exciting work and the first time a vanadyl complex is used as the polarizing agent for DNP. Typically, nitroxides are used as polarizing agents, and this work clearly demonstrates the potential of metal complexes for DNP.

Gizatullin, Bulat, Marat Gafurov, Alexey Vakhin, Alexander Rodionov, Georgy Mamin, Sergei Orlinskii, Carlos Mattea, and Siegfried Stapf. “Native Vanadyl Complexes in Crude Oil as Polarizing Agents for In Situ Proton Dynamic Nuclear Polarization.” Energy & Fuels 33, no. 11 (November 21, 2019): 10923–32.

The presence of paramagnetic species such as vanadyl complexes (VO2+) and free carbon radicals in petroleum disperse systems (PDSs) such as crude oil, bitumen, or kerogen causes significant interest of studying the structure of PDS, high-molecular weight components, and their effects on the physical and chemical properties of PDS products by magnetic resonance techniques. However, the lack of detailed studies keeps the exact structure, aggregation mechanism, and interaction with complex composites of the PDS still disputable. In this contribution, detailed electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) investigations, including advanced fast field cycling dynamic nuclear polarization, of heavy crude oil focused on vanadyl complexes are presented. A perceptible room-temperature 1H dynamic nuclear polarization (DNP) solid effect at the X-band (magnetic field of 300−400 mT corresponding to the EPR frequency of 9.5 GHz and NMR frequency of 14.6 MHz), with enhancement ±5, is observed at moderate microwave irradiation power in crude oil with a high concentration of VO2+, while no Overhauser DNP contribution is found. Using NMR T2-encoding, DNP spectra and molecular dynamics, two components are distinguished, from which the one with slower dynamics exhibits higher DNP enhancement via VO2+ complexes. The observed difference is discussed in terms of electron−nuclear interaction and relative parts of hyperpolarized nuclear spins using an advanced model for DNP data simulation.

Three-spin solid effect and the spin diffusion barrier in amorphous solids #DNPMR

Tan, Kong Ooi, Michael Mardini, Chen Yang, Jan Henrik Ardenkjær-Larsen, and Robert G. Griffin. “Three-Spin Solid Effect and the Spin Diffusion Barrier in Amorphous Solids.” Science Advances 5, no. 7 (July 2019): eaax2743.

Dynamic nuclear polarization (DNP) has evolved as the method of choice to enhance NMR signal intensities and to address a variety of otherwise inaccessible chemical, biological and physical questions. Despite its success, there is no detailed understanding of how the large electron polarization is transferred to the surrounding nuclei or where these nuclei are located relative to the polarizing agent. To address these questions we perform an analysis of the three-spin solid effect, and show that it is exquisitely sensitive to the electron-nuclear distances. We exploit this feature and determine that the size of the spin diffusion barrier surrounding the trityl radical in a glassy glycerol–water matrix is <6 Å, and that the protons involved in the initial transfer step are on the trityl molecule. 1H ENDOR experiments indicate that polarization is then transferred in a second step to glycerol molecules in intimate contact with the trityl.

Eigenstate versus Zeeman-based approaches to the solid effect #DNPNMR

Rodríguez‐Arias, Inés, Alberto Rosso, and Andrea De Luca. “Eigenstate versus Zeeman-Based Approaches to the Solid Effect.” Magnetic Resonance in Chemistry 0, no. 0 (2017).

The solid effect is one of the simplest and most effective mechanisms for dynamic nuclear polarization. It involves the exchange of polarization between one electron and one nuclear spin coupled via the hyperfine interaction. Even for such a small spin system, the theoretical understanding is complicated by the contact with the lattice and the microwave irradiation. Both being weak, they can be treated within perturbation theory. In this work, we analyze the two most popular perturbation schemes: the Zeeman and the eigenstate-based approaches, which differ in the way the hyperfine interaction is treated. For both schemes, we derive from first principles an effective Liouville equation that describes the density matrix of the spin system; we then study numerically the behavior of the nuclear polarization for several values of the hyperfine coupling. In general, we obtain that the Zeeman-based approach underestimates the value of the nuclear polarization. By performing a projection onto the diagonal part of the spin-system density matrix, we are able to understand the origin of the discrepancy, which is due to the presence of parasite leakage transitions appearing whenever the Zeeman basis is employed.

Dynamic nuclear polarisation by thermal mixing: quantum theory and macroscopic simulations #DNPNMR

Karabanov, A., et al., Dynamic nuclear polarisation by thermal mixing: quantum theory and macroscopic simulations. Phys. Chem. Chem. Phys., 2016. 18(43): p. 30093-30104.

A theory of dynamic nuclear polarisation (DNP) by thermal mixing is suggested based on purely quantum considerations. A minimal 6-level microscopic model is developed to test the theory and link it to the well-known thermodynamic model. Optimal conditions for the nuclear polarization enhancement and effects of inhomogeneous broadening of the electron resonance are discussed. Macroscopic simulations of nuclear polarization spectra displaying good agreement with experiments, involving BDPA and trityl free radicals, are presented.

Theory of solid effect and cross effect dynamic nuclear polarization with half-integer high-spin metal polarizing agents in rotating solids #DNPNMR

Corzilius, B., Theory of solid effect and cross effect dynamic nuclear polarization with half-integer high-spin metal polarizing agents in rotating solids. Phys. Chem. Chem. Phys., 2016. 18(39): p. 27190-27204.

Dynamic nuclear polarization (DNP) is a powerful method to enhance sensitivity especially of solid-state magic-angle spinning (MAS) NMR by up to several orders of magnitude. The increased interest both from a practical as well as theoretical viewpoint has spawned several fields of active research such as the development of new polarizing agents with improved or unique properties and description of the underlying DNP mechanisms such as solid effect (SE) and cross effect (CE). Even though a novel class of unique polarizing agents based on high-spin metal ions such as Gd(iii) and Mn(ii) has already been utilized for MAS DNP a theoretical description of the involved DNP mechanism is still incomplete. Here, we review several aspects of DNP-relevant electron-paramagnetic resonance (EPR) properties of the general class of these half-integer high-spin metal ions with isotropic Zeeman interaction but significant zero-field splitting (ZFS). While the SE can be relatively easily described similar to that of a S = 1/2 system and is assumed to be effective only for polarizing agents featuring a narrow central EPR transitions (i.e., mS = -1/2 [rightward arrow] +1/2) with respect to the nuclear Larmor frequency, the CE between two high-spin ions requires a more detailed theoretical investigation due to a multitude of possible transitions and matching conditions. This is especially interesting in light of recent understanding of CE being induced by MAS-driven level anti-crossings (LACs) between dipolar-coupled electron spins. We discuss the requirements of such CE-enabling LACs to occur due to anisotropy of ZFS, the expected adiabaticity, and the resulting possibilities of high-spin metal ion pairs to act as polarizing agents for DNP. This theoretical description serves as a framework for a detailed experimental study published directly following this work.

Gd(iii) and Mn(ii) complexes for dynamic nuclear polarization: small molecular chelate polarizing agents and applications with site-directed spin labeling of proteins #DNPNMR

Kaushik, M., et al., Gd(iii) and Mn(ii) complexes for dynamic nuclear polarization: small molecular chelate polarizing agents and applications with site-directed spin labeling of proteins. Phys Chem Chem Phys, 2016. 18(39): p. 27205-27218.

We investigate complexes of two paramagnetic metal ions Gd3+ and Mn2+ to serve as polarizing agents for solid-state dynamic nuclear polarization (DNP) of 1H, 13C, and 15N at magnetic fields of 5, 9.4, and 14.1 T. Both ions are half-integer high-spin systems with a zero-field splitting and therefore exhibit a broadening of the mS = -1/2 <–> +1/2 central transition which scales inversely with the external field strength. We investigate experimentally the influence of the chelator molecule, strong hyperfine coupling to the metal nucleus, and deuteration of the bulk matrix on DNP properties. At small Gd-DOTA concentrations the narrow central transition allows us to polarize nuclei with small gyromagnetic ratio such as 13C and even 15N via the solid effect. We demonstrate that enhancements observed are limited by the available microwave power and that large enhancement factors of >100 (for 1H) and on the order of 1000 (for 13C) can be achieved in the saturation limit even at 80 K. At larger Gd(iii) concentrations (>/=10 mM) where dipolar couplings between two neighboring Gd3+ complexes become substantial a transition towards cross effect as dominating DNP mechanism is observed. Furthermore, the slow spin-diffusion between 13C and 15N, respectively, allows for temporally resolved observation of enhanced polarization spreading from nuclei close to the paramagnetic ion towards nuclei further removed. Subsequently, we present preliminary DNP experiments on ubiquitin by site-directed spin-labeling with Gd3+ chelator tags. The results hold promise towards applications of such paramagnetically labeled proteins for DNP applications in biophysical chemistry and/or structural biology.

Solid Effect DNP in a Rapid-melt setup #DNPNMR

van Bentum, P.J.M., et al., Solid Effect DNP in a Rapid-melt setup. J. Magn. Reson., 2016. 263: p. 126-135.

Dynamic Nuclear Polarization (DNP) has become a key element in nuclear magnetic resonance (NMR). Recently, we developed a novel approach to DNP enhanced liquid-state NMR based on rapid melting of a solid hyperpolarized sample followed by ‘in situ’ liquid-state NMR detection. This method allows 1 H detection with fast cycling options for signal averaging. In nonpolar solvents, doped with BDPA radicals, proton enhancement factors were achieved of up to 400. A short recycling delay of about 5 s allows for a fast determination of the hyper-polarization dynamics as function of the microwave frequency and power. Here, we use the rapid melt dnp method to study the mechanisms for DNP in the solid phase in more detail. Solid Effect, Cross Effect, Solid Overhauser and Liquid-state (supercritical) Overhauser DNP enhancement can be observed in the same setup. In this paper, we concentrate on Solid Effect DNP observed with both homogeneous narrow line radicals such as BDPA and with wide line anisotropic nitroxide radicals such as TEMPOL. We find indications that BDPA protons play an important role in Solid Effect DNP with this radical. A simplified spin diffusion model for BDPA can give a semi-quantitative description of the enhancements as function of the microwave power and as function of the proton concentration in the solid solution. For aqueous frozen samples we observe a similar Solid Effect DNP enhancement, which is analyzed within the simplified spin diffusion model.

Solid effect DNP polarization dynamics in a system of many spins

Wisniewski, D., et al., Solid effect DNP polarization dynamics in a system of many spins. J Magn Reson, 2016. 264: p. 30-8.

We discuss the polarization dynamics during solid effect dynamic nuclear polarization (DNP) in a central spin model that consists of an electron surrounded by many nuclei. To this end we use a recently developed formalism and validate first its performance by comparing its predictions to results obtained by solving the Liouville von Neumann master equation. The use of a Monte Carlo method in our formalism makes it possible to significantly increase the number of spins considered in the model system. We then analyse the dependence of the nuclear bulk polarization on the presence of nuclei in the vicinity of the electron and demonstrate that increasing the minimal distance between nuclei and electrons leads to a rise of the nuclear bulk polarization. These observations have implications for the design of radicals that can lead to improved values of nuclear spin polarization. Furthermore, we discuss the potential to extend our formalism to more complex spin systems such as cross effect DNP.

Solid-State Dynamic Nuclear Polarization at 9.4 and 18.8 T from 100 K to Room Temperature

This is an incredible article. It shows the temperature dependence of the DNP enhancement over a wide temperature regime. Most importantly it shows that at room temperature still an enhancement of 15-20 can be achieved.
Just a few years ago the common believe was that solid-state MAS-DNP experiments have to be performed at 90 K or below. This article clearly demonstrates that there is still so much room for improvements of DNP. I think the most exciting moments in DNP are still ahead of us and the method has the potential to become an integral part of the DNP toolbox.

Lelli, M., et al., Solid-State Dynamic Nuclear Polarization at 9.4 and 18.8 T from 100 K to Room Temperature. J Am Chem Soc, 2015. 137(46): p. 14558-61.

Efficient dynamic nuclear polarization (DNP) in solids, which enables very high sensitivity NMR experiments, is currently limited to temperatures of around 100 K and below. Here we show how by choosing an adequate solvent, (1)H cross effect DNP enhancements of over 80 can be obtained at 240 K. To achieve this we use the biradical TEKPol dissolved in a glassy phase of ortho-terphenyl (OTP). We study the solvent DNP enhancement of both TEKPol and BDPA in OTP in the range from 100 to 300 K at 9.4 and 18.8 T. Surprisingly, we find that the DNP enhancement decreases only relatively slowly for temperatures below the glass transition of OTP (Tg = 243 K), and (1)H enhancements around 15-20 at ambient temperature can be observed. We use this to monitor molecular dynamic transitions in the pharmaceutically relevant solids Ambroxol and Ibuprofen.

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