Category Archives: 7Li

Endogenous Dynamic Nuclear Polarization for Sensitivity Enhancement in Solid-State NMR of Electrode Materials #DNPNMR

Harchol, Adi, Guy Reuveni, Vitalii Ri, Brijith Thomas, Raanan Carmieli, Rolfe H. Herber, Chunjoong Kim, and Michal Leskes. “Endogenous Dynamic Nuclear Polarization for Sensitivity Enhancement in Solid-State NMR of Electrode Materials.” The Journal of Physical Chemistry C 124, no. 13 (April 2, 2020): 7082–90.

Rational design of materials for energy storage systems relies on our ability to probe these materials at various length scales. Solid state NMR spectroscopy is a powerful approach for gaining chemical and structural insight at the atomic/molecular level, but its low detection sensitivity often limits applicability. This limitation can be overcome by transferring the high polarization of electron spins to the sample of interest in a process called dynamic nuclear polarization (DNP). Here we employ for the first time, metal ion-based DNP to probe pristine and cycled composite battery electrodes. A new and efficient DNP agent, Fe(III), is introduced, yielding lithium signal enhancement up to 180 when substituted in the anode material Li4Ti5O12. In addition to being DNP active, Fe(III) improves the anode performance. Reduction of Fe(III) to Fe(II) upon cycling can be monitored in the loss of DNP activity. We show that the dopant can be reactivated (return to Fe(III)) for DNP by increasing the cycling potential window. Furthermore, we demonstrate that the deleterious effect of carbon additives on the DNP process can be eliminated by using carbon free electrodes, doped with Fe(III) and Mn(II), which provide good electrochemical performance as well as sensitivity in DNP-NMR. We expect the approach presented here will expand the applicability of DNP for studying materials for frontier challenges in materials chemistry associated with energy and sustainability.

Paramagnetic NMR in solution and the solid state

Pell, Andrew J., Guido Pintacuda, and Clare P. Grey. “Paramagnetic NMR in Solution and the Solid State.” Progress in Nuclear Magnetic Resonance Spectroscopy 111 (April 2019): 1–271.

The field of paramagnetic NMR has expanded considerably in recent years, both in solution and the solid state. This review addresses both the theoretical description of paramagnetic NMR, and the way in which it is currently practised. We provide a review of the theory of the NMR parameters of systems in both solution and the solid state. Here we unify the different languages used by the NMR, EPR, quantum chemistry/DFT, and magnetism communities to provide a comprehensive and coherent theoretical description. We cover the theory of the paramagnetic shift and shift anisotropy in solution both in the traditional formalism in terms of the magnetic susceptibility tensor, and using a more modern formalism employing the relevant EPR parameters, such as are used in first-principles calculations. In addition we examine the theory first in the simple non-relativistic picture, and then in the presence of spin-orbit coupling. These ideas are then extended to a description of the paramagnetic shift in periodic solids, where it is necessary to include the bulk magnetic properties, such as magnetic ordering at low temperatures. The description of the paramagnetic shift is completed by describing the current understanding of such shifts due to lanthanide and actinide ions. We then examine the paramagnetic relaxation enhancement, using a simple model employing a phenomenological picture of the electronic relaxation, and again using a more complex state-of-the-art theory which incorporates electronic relaxation explicitly. An additional important consideration in the solid state is the impact of bulk magnetic susceptibility effects on the form of the spectrum, where we include some ideas from the field of classical electrodynamics. We then continue by describing in detail the solution and solid-state NMR methods that have been deployed in the study of paramagnetic systems in chemistry, biology, and the materials sciences. Finally we describe a number of case studies in paramagnetic NMR that have been specifically chosen to highlight how the theory in part one, and the methods in part two, can be used in practice. The systems chosen include small organometallic complexes in solution, solid battery electrode materials, metalloproteins in both solution and the solid state, systems containing lanthanide ions, and multi-component materials used in pharmaceutical controlled-release formulations that have been doped with paramagnetic species to measure the component domain sizes.

Endogenous Dynamic Nuclear Polarization for Natural Abundance 17O and Lithium NMR in the Bulk of Inorganic Solids #DNPNMR

Wolf, Tamar, Sandeep Kumar, Harishchandra Singh, Tanmoy Chakrabarty, Fabien Aussenac, Anatoly I Frenkel, Dan Thomas Major, and Michal Leskes. “Endogenous Dynamic Nuclear Polarization for Natural Abundance 17O and Lithium NMR in the Bulk of Inorganic Solids.” Journal of the American Chemical Society, n.d., 23.

In recent years magic angle spinning – dynamic nuclear polarization (MAS-DNP) developed as an excellent approach for boosting the sensitivity of solid state NMR (ssNMR) spectroscopy, thereby enabling the characterization of challenging systems in biology and chemistry. Most commonly, MAS-DNP is based on the use of nitroxide biradicals as polarizing agents. In materials science, since the use of nitroxides often limits the signal enhancement to the materials’ surface and subsurface layers, there is need for hyperpolarization approaches which will provide sensitivity in the bulk of micron sized particles. Recently, an alternative in the form of paramagnetic metal ions has emerged. 

Here we demonstrate the remarkable efficacy of Mn(II) dopants, used as endogenous polarization agents for MAS-DNP, in enabling the detection of 17O at a natural abundance of only 0.037%. Distinct oxygen sites are identified in the bulk of micron-sized crystals, including battery anode materials Li4Ti5O12 (LTO) and Li2ZnTi3O8, as well as the phosphor materials NaCaPO4 and MgAl2O4, all doped with Mn(II) ions. Density functional theory calculations are used to assign the resonances to specific oxygen environments in these phases. Depending on the Mn(II) dopant concentration, we obtain significant signal enhancement factors, 142 and 24, for 6Li and 7Li nuclei in LTO, respectively. We furthermore follow the changes in the 6,7Li LTO resonances and determine their enhancement factors as a function of Mn(II) concentration. The results presented show that MAS-DNP from paramagnetic metal ion dopants provides an efficient approach for probing informative nuclei such as 17O, despite their low gyromagnetic ratio and negligible abundance, without isotope enrichment.

Reversal of Paramagnetic Effects by Electron Spin Saturation #DNPNMR

Jain, Sheetal K., Ting A. Siaw, Asif Equbal, Christopher B. Wilson, Ilia Kaminker, and Songi Han. “Reversal of Paramagnetic Effects by Electron Spin Saturation.” The Journal of Physical Chemistry C 122, no. 10 (March 15, 2018): 5578–89.

We present a study in which both significant dynamic nuclear polarization (DNP) enhancement of 7Li NMR and reversal of the paramagnetic effects (PEs) are achieved by microwave (μw) irradiation-induced electron spin saturation of nitroxide radicals at liquid-helium temperatures. The reversal of the PE was manifested in significant narrowing of the 7Li NMR line and reversal of the paramagnetic chemical shift under DNP conditions. The extent of the PE was found to decrease with increased saturation of the electron paramagnetic resonance line, modulated as a function of microwave (μw) power, frequency, duration of irradiation, and gating time between μw irradiation and NMR detection. The defining observation was the shortening of the electron phase memory time, Tm, of the excited observer spins with increasing μw irradiation and concurrent electron spin saturation of the electron spin bath. This and a series of corroborating studies reveal the origin of the NMR line narrowing to be the reversal of paramagnetic relaxation enhancement (PRE), leading us to debut the term REversal of PRE by electron Spin SaturatION (REPRESSION). The shortening of electron Tm of any paramagnetic system as a function of electron spin saturation has not been reported to date, making REPRESSION a discovery of this study. The reversal of the paramagnetic dipolar shift is due to the decrease in electron spin order, also facilitated by electron spin saturation. This study offers new fundamental insights into PE under DNP conditions and a method to detect and identify NMR signal proximal to paramagnetic sites with reduced or minimal line broadening.

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