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ChemPhysChem 10, (2021). doi:10.1002/cphc.202100602, Published by Wiley-VCH Verl., Weinheim

Molecules : a journal of synthetic chemistry and natural product chemistry 26(21), 6690 (2021). doi:10.3390/molecules26216690 special issue: "Special Issue "Advances in NMR and MRI of Materials" / Special Issue Editor: Prof. Dr. Igor Ser��a, Guest Editor", Published by MDPI, Basel

Electron paramagnetic resonance (EPR) spectroscopy was utilized in operando to analyze redox mechanisms in lithium cobalt oxide (LCO) cathode material during battery cycling. In the pristine battery, a considerable amount of paramagnetic Co2+ was detected that was formed during initial contact of the electrolyte with the LCO. The Co2+ got oxidized during the formation cycle. The absence of an EPR signal during subsequent battery cycles, irrespective of the state-of-charge, indicates that no EPR active Co4+ is formed. This suggests the presence of an anionic oxygen redox mechanism in the active material.

The intrinsic ionic nature of room temperature ionic liquids (RTILs) bears the potential to replace classical aqueous electrolytes in electrochemical applications, for example in metal–air batteries. For a systematic adjustment of RTIL properties in porous cathodes, the ionic arrangement under confinement is of prime importance. Using spectrally resolved pulsed gradient stimulated echo nuclear magnetic resonance (PGSTE-NMR) and spin–lattice NMR relaxation time (T1) distributions, the dynamics of 1-methyl-1-propylpyrrolidiniumbis(trifluoromethylsulfonyl)imide ([Pyr13][Tf2N]) confined to carbon black were investigated. A considerable dependence of the [PYR13] mobility on the loading fraction of the carbon black pore space was found. There is evidence for a preferential layering of the RTIL adjacent to the carbon surface and a dependence of the ionic configuration on the local structure of the carbon surface. The inversion efficiency of inversion-recovery T1 data indicates a quasi-stationary layer at the carbon surface with solid-like properties, where the bulk-like properties of the RTIL are adopted as the distance to the surface increases. From the NMR diffusion data an intermediate layer between the quasi-stationary and the bulk-like RTIL is evident. This layer shows a particularly strong pore space loading dependence. While it has an anisotropic, two-dimensional mobility with reduced diffusion perpendicular to the surface at any loading, when it interfaces a gas phase at low loading its mobility is higher than bulk diffusion by up to an order of magnitude and chemical exchange with other layers is low. This layer appears to be of particular importance for the ion exchange between RTIL environments with different spacing from the carbon surface and hence crucial for the overall dynamics of RTILs in the investigated porous environment.

We report a trinuclear copper(II) complex, [(DAM)Cu3(μ3-O)][Cl]4 (1, DAM = dodecaaza macrotetracycle), as a homogeneous electrocatalyst for water oxidation to dioxygen in phosphate-buffered solutions at pH 7.0, 8.1, and 11.5. Electrocatalytic water oxidation at pH 7 occurs at an overpotential of 550 mV with a turnover frequency of ∼19 s–1 at 1.5 V vs NHE. Controlled potential electrolysis (CPE) experiments at pH 11.5 over 3 h at 1.2 V and at pH 8.1 for 40 min at 1.37 V vs NHE confirm the evolution of dioxygen with Faradaic efficiencies of 81% and 45%, respectively. Rinse tests conducted after CPE studies provide evidence for the homogeneous nature of the catalysis. The linear dependence of the current density on the catalyst concentration indicates a likely first-order dependence on the Cu precatalyst 1, while kinetic isotope studies (H2O versus D2O) point to involvement of a proton in or preceding the rate-determining step. Rotating ring-disk electrode measurements at pH 8.1 and 11.2 show no evidence of H2O2 formation and support selectivity to form dioxygen. Freeze-quench electron paramagnetic resonance studies during electrolysis provide evidence for the formation of a molecular copper intermediate. Experimental and computational studies support a key role of the phosphate as an acceptor base. Moreover, density functional theory calculations highlight the importance of second-sphere interactions and the role of the nitrogen-based ligands to facilitate proton transfer processes.

Nitrogen‐containing hydrothermal carbon (N‐HTC) materials of spherical particle morphology were prepared by means of hydrothermal synthesis with glucose and urotropine as precursors. The molar ratio of glucose to urotropine has been varied to achieve a continuous increase in nitrogen content. By raising the ratio of urotropine to glucose, a maximal nitrogen fraction of about 19 wt % could be obtained. Decomposition products of both glucose and urotropine react with each other; this opens up a variety of possible reaction pathways. The pH has a pronounced effect on the reaction pathway of the corresponding reaction steps. For the first time, a comprehensive analytical investigation, comprising a multitude of analytical tools and instruments, of a series of nitrogen‐containing HTC materials was applied. Functional groups and structural motifs identified were analyzed by means of FTIR spectroscopy, thermogravimetric MS, and solid‐state NMR spectroscopy. Information on reaction mechanisms and structural details were obtained by electronic structure calculations that were compared with vibrational spectra of polyfuran or polypyrrole‐like groups, which represent structural motifs occurring in the present samples.

The journal of physical chemistry / C 123(41), 25146-25156 (2019). doi:10.1021/acs.jpcc.9b05323, Published by American Chemical Society, Washington, DC

Journal of the Electrochemical Society 166(3), A5403-A5409 (2019). doi:10.1149/2.0641903jes special issue: "JES Focus Issue of Selected Papers from IMLB 2018", Published by Electrochemical Soc., Pennington, NJ

Physico-chemical processes in batteries are taking place on a broad time scale from fractions of seconds to the order of days [1]. Thus, for an accurate characterization of transport and mobility processes in batteries using electrochemical impedance spectroscopy (EIS), a large frequency range of up to ten decades must be covered. This implies measurement durations on the order of days for low frequency measurements, yielding the risk of distorting electrochemical instabilities in the battery and a considerable change of its state of charge (SOC) due to the probing ac current excitation. It is shown that the SOC change is frequency dependent and with 10-15% of the nominal battery capacity for the sub-millihertz range hardly a small perturbation. Nevertheless, these obstacles can be mitigated by the time domain measurement (TDM) technique [2-4]. TDM is limited to impedance measurements at low frequencies, with a small and frequency-independent SOC change. The combination of TDM and EIS, called time-domain supported electrochemical impedance spectroscopy (TD-EIS), opens up the possibility for a time-efficient implementation of impedance spectroscopy over a large frequency range down to microhertz frequencies [5]. In this work, TD-EIS at varying temperatures in combination with data fitting using an electrical equivalent circuit battery model, is used for the high-accuracy quantification of low-frequency mobility parameters in lithium-ion batteries. It is experimentally demonstrated, for the first time, that the phase of the impedance measurements converges in the sub-millihertz range, which permits a reliable quantification of diffusion kinetics. Moreover, it is shown that with TD-EIS time savings of up to 80% compared to the standard EIS measurement are feasible. From the electrical equivalent circuit battery model fit, an accurate estimate of charge transfer resistance and, in particular, also solid-state diffusion rate are obtained. Both processes follow an Arrhenius law, allowing the determination of activation energies with small variance. The results for the charge transfer process and for the solid-state diffusion process are within the range of literature values measured for similar systems. This work has been published in JECS [6]. References [1] A. Jossen, “Fundamentals of battery dynamics”, Journal of Power Sources 154 (2) (2006) 530-538 [2] S. C. Creason, J. W. Hayes, D. E. Smith, “Fourier transform faradaic admittance measurements - III. Comparison of measurement efficiency for various test signal waveforms”, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 47 (1) (1973) 9-46 [3] J.-S. Yoo, S.-M. Park, “An electrochemical impedance measurement technique employing Fourier transform”, Analytical Chemistry 72 (9) (2000) 2035-2041 [4] D. Klotz, M. Schönleber, J. Schmidt, E. Ivers-Tiffée, “New approach for the calculation of impedance spectra out of time domain data”, Electrochimica Acta 56 (24) (2011) 8763-8769 [5] J. Illig, J. Schmidt, M. Weiss, A. Weber, E. Ivers-Tiffée, “Understanding the impedance spectrum of 18650 LiFePO4-cells”, Journal of Power Sources 239 (2013) 670-679 [6] A. Mertens, I.C. Vinke, H. Tempel, H. Kungl, L.G.J. de Haart, R.-A. Eichel, J. Granwehr, "Quantitative Analysis of Time-Domain Supported Electrochemical Impedance Spectroscopy Data of Li-Ion Batteries: Reliable Activation Energy Determination at Low Frequencies", Journal of the Electrochemical Society 163 (7) (2016) H521-H527 Figure caption: Arrhenius plot of the solid-state diffusion rate obtained from battery model fits of impedance data from three batteries measured at different temperatures. The activation energies are determined from the gradients. Figure 1

Nafion is the most common material used as a proton exchange membrane in fuel cells. Yet, details of the transport pathways for protons and water in the inner membrane are still under debate. Overhauser Dynamic Nuclear Polarization (ODNP) has proven to be a useful tool for probing hydration dynamics and interactions within 5-8 A of protein and soft material surfaces. Recently it was suggested that ODNP can also be applied to analyze surface water dynamics along Nafion's inner membrane. Here we interrogate the viability of this method for Nafion by carrying out a series of measurements relying on 1H nuclear magnetic resonance (NMR) relaxometry and diffusometry experiments with and without ODNP hyperpolarization, accompanied by other complementary characterization methods including small angle X-ray scattering (SAXS), thermal gravimetric analysis (TGA) of hydration, and proton conductivity by AC impedance spectroscopy. Our comprehensive study shows that commonly used paramagnetic spin probes-here, stable nitroxide radicals-for ODNP, as well as their diamagnetic analogues, reduce the inner membrane surface hydrophilicity, depending on the location and concentration of the spin probe. This heavily reduces the hydration of Nafion, hence increases the tortuosity of the inner membrane morphology and/or increases the activiation barrier for water transport, and consequently impedes water diffusion, transport, and proton conductivity.

An ab initio simulation scheme is introduced as a theoretical prescreening approach to facilitate and enhance the research for pH-sensitive biomarkers. The proton 1H and carbon 13C nuclear magnetic resonance (NMR) chemical shifts of the recently published marker for extracellular pH, [1,5-13C2]zymonic acid (ZA), and the as yet unpublished ( Z)-4-methyl-2-oxopent-3-enedioic acid (OMPD) were calculated with ab initio methods as a function of the pH. The influence of the aqueous solvent was taken into account either by an implicit solvent model or by explicit water molecules, where the latter improved the accuracy of the calculated chemical shifts considerably. The theoretically predicted chemical shifts allowed for a reliable NMR peak assignment. The p Ka value of the first deprotonation of ZA and OMPD was simulated successfully whereas the parametrization of the implicit solvent model does not allow for an accurate description of the second p Ka. The theoretical models reproduce the pH-induced chemical shift changes and the first p Ka with sufficient accuracy to establish the ab initio prescreening approach as a valuable support to guide the experimental search for pH-sensitive biomarkers.

For studying electrode and electrolyte materials for lithium ion batteries, solid-state (SS) nuclear magnetic resonance (NMR) of lithium moves into focus of current research. Theoretical simulations of magnetic resonance parameters facilitate the analysis and interpretation of experimental Li SS-NMR spectra and provide unique insight into physical and chemical processes that are determining the spectral profile. In the present paper, the accuracy and reliability of the theoretical simulation methods of Li chemical shielding values is benchmarked by establishing a reference scale for Li SS-NMR of diamagnetic compounds. The impact of geometry, ionic mobility and relativity are discussed. Eventually, the simulation methods are applied to the more complex lithium titanate spinel (Li4Ti5O12, LTO), which is a widely discussed battery anode material. Simulation of the Li SS-NMR spectrum shows that the commonly adopted approach of assigning the resonances to individual crystallographic sites is not unambiguous.

Spinel-type Li4Ti5O12 is an attractive anode material for lithium-ion battery applications owing to its high specific energy and almost vanishing mechanical strain upon Li-(de)intercalation. The impact of oxygen non-stoichiometry and aliovalent Mg-doping on the defect chemistry of Li4Ti5O12 has been characterized by quantitative electron paramagnetic resonance (EPR) spectroscopy. The results obtained demonstrate that the Mg2+-ions are amphoterically incorporated at the here studied concentrations on both lithium (MgLi •) and titanium (Mg′′Ti) sites. Mg2+-doping thus simultaneously results in the formation of donor (MgLi •) and acceptor (Mg′′Ti) centers. Furthermore, upon both oxygen deficiency and Mg2+-doping, the concentration of `polaronic'-type Ti3+-states is increased, accounting for the enhanced electronic conductivity.

Aprotic lithium–oxygen (Li-O2) batteries have attracted considerable attention in recent years owing to their outstanding theoretical energy density.[1] Parasitic side reactions are the central problem for the development of a reversible Li-O2 cell chemistry. They mainly occur upon charge, causing the degradation of both the carbon electrode and the electrolyte upon battery cycling, leading to cell death within a few cycles. The exact electrochemical or chemical nature of the side reactions during charge remains largely unclear, but some form of “nascent” oxygen produced during charging has been suggested to contribute to the degradation of several cell components.[2] Singlet oxygen (term symbol 1Δg, thereafter 1O2) could potentially be this reactive nascent oxygen, as it is a strong oxidizing agent and known to form upon chemical oxidation of Li2O2, Na2O2 and a series of organic peroxides.[3,4] We present the first experimental investigation of 1O2 formation during the charge of an aprotic Li-O2 battery. The identification of 1O2 is based on its reactivity towards a specific spin trap (4-Oxo-TEMP), which is added to the electrolyte, forming a stable radical (4-Oxo-TEMPO) that can be detected by in-operando electron paramagnetic resonance (EPR) spectroscopy. The EPR experiments were carried out in a purpose-built spectro-electrochemical cell[5] , modified by adding a reference electrode and a gas purging unit, which can be cycled directly inside the cavity of the EPR spectrometer. Firstly, the electrochemistry in the presence of the spin trap is investigated. By a combination of UV/VIS experiments and On-line electrochemical mass spectrometry (OEMS) it is shown that the presence of the spin trap does not affect the charging processes up to 3.9 V. In the next step, a constant current charging experiment with the 4-Oxo-TEMP spin trap as electrolyte additive (100 mM) is carried out. Figure 1 shows the charging potential and the concentration of the 4-Oxo-TEMPO radical as determined by in-operando EPR spectroscopy. The charging processes can be separated into four distinct phases (s. Fig. 1): During phase I and II, a normal charging process with Li2O2 oxidation as the main electrochemical reaction is taking place, as also indicated by the oxygen evolution determined by OEMS. The 4-Oxo-TEMPO amount starts to increase once the electrode potential exceeds ∼3.55 V, which is close to the thermodynamic threshold for 1O2 formation (3.45 to 3.55 V), suggesting that this increase is caused by the reaction of the spin trap with 1O2. In phases III and IV electrochemical side reactions set in that are not related to the 1O2 cell chemistry. The presence of these four phases shows that a time and voltage resolved in-operando EPR experiment, as presented in this work, is critical for unraveling the different mechanisms for forming and decomposing the 4-Oxo-TEMPO radical. To provide further evidence that the increase of 4-Oxo-TEMPO during phase II is caused by 1O2, potential controlled experiments were carried out, which fully confirm the results of the constant current experiments. Lastly, based on a calibration procedure and a simple mechanistic consideration, the lower limit of the 1O2 fraction formed during cell charge was estimated to be 0.5%. The real fraction of 1O2 is most likely higher, as a significant part of the evolved 1O2 will be quenched by the solvent or other side reactions before being trapped by 4-Oxo-TEMP. However, this simple estimate already shows that 1O2 is formed in substantial quantities. We believe that the occurrence of the highly reactive singlet oxygen might be the long overlooked missing link in the understanding of the electrolyte degradation and carbon corrosion reactions that occur during the charge of Li-O2cells. References [1] J. Christensen, P. Albertus, R. S. Sanchez-Carrera, T. Lohmann, B. Kozinsky, R. Liedtke, J. Ahmed, A. Kojic, J. Electrochem. Soc. 2012, 159, R1–R30. [2] H. Beyer, S. Meini, N. Tsiouvaras, M. Piana, H. A. Gasteiger, Phys. Chem. Chem. Phys. 2013, 15, 11025–37. [3] W. Adam, D. V Kazakov, V. P. Kazakov, Chem. Rev. 2005, 105, 3371–87. [4] Q. Li, F. Chen, W. Zhao, M. Xu, B. Fang, Y. Zhang, L. Duo, Y. Jin, F. Sang, Bull. Korean Chem. Soc. 2007, 28, 1656–1660. [5] J. Wandt, C. Marino, P. Jakes, R. Eichel, H. A. Gasteiger, J. Granwehr, Energy Environ. Sci. 2015, 8, 1358–1367. Acknowledgement TUM gratefully acknowledges financial support by the Bavarian Ministry of Economic Affairs and Media, Energy and Technology under the auspices of the EEBatt project. Forschungszentrum Jülich gratefully acknowledges financial support by the German Ministry of Education and Research (BMBF) in the framework of the MEET-HiEnD project. Figure 1

Due to low fluorine background signal in vivo, 19F is a good marker to study the fate of exogenous molecules by magnetic resonance imaging (MRI) using equilibrium nuclear spin polarization schemes. Since 19F MRI applications require high sensitivity, it can be important to assess experimental feasibility during the design stage already by estimating the minimum detectable fluorine concentration. Here we propose a simple method for the calibration of MRI hardware, providing sensitivity estimates for a given scanner and coil configuration. An experimental "calibration factor" to account for variations in coil configuration and hardware set-up is specified. Once it has been determined in a calibration experiment, the sensitivity of an experiment or, alternatively, the minimum number of required spins or the minimum marker concentration can be estimated without the need for a pilot experiment. The definition of this calibration factor is derived based on standard equations for the sensitivity in magnetic resonance, yet the method is not restricted by the limited validity of these equations, since additional instrument-dependent factors are implicitly included during calibration. The method is demonstrated using MR spectroscopy and imaging experiments with different 19F samples, both paramagnetically and susceptibility broadened, to approximate a range of realistic environments.

In this contribution, negative charges and electronic traps related to the Si/Al2O3 interface were measured and related to paramagnetic point defects and molecular vibrations. To this end, contactless capacitance voltage measurements, X-band electron paramagnetic resonance (EPR), and infrared spectroscopy were carried out, and their results were compared. A change in the negative charge density and electron trap density at the Si/Al2O3 interface was achieved by adding a thermally grown SiO2 layer with varying thicknesses and conducting an additional temperature treatment. Using EPR, five paramagnetic moments were detected in Si/(SiO2)/Al2O3 samples with g values of g1 = 2.0081 ± 0.0002 , g2 = 2.0054 ± 0.0002 , g3 = 2.0003 ± 0.0002 , g4 = 2.0026 ± 0.0002, and g5 = 2.0029 ± 0.0002. Variation of the Al2O3 layer thickness shows that paramagnetic species associated with g1, g2, and g3 are located at the Si/Al2O3 interface, and those with g4 and g5 are located within the bulk Al2O3. Furthermore, g1, g2, and g3 were shown to originate from oxygen plasma exposure during Al2O3 deposition. Comparing the g values and their location within the Si/Al2O3 system, g1 and g3 can be attributed to Pb0 centers, g3 to Si dangling bonds (Si-dbs), and g4 and g5 to rotating methyl radicals. All paramagnetic moments observed in this contribution disappear after a 5-min temperature treatment at 450 ° C. The deposition of an additional thermal SiO2 layer between the Si and the Al2O3 decreases the negative fixed charge density and defect density by about one order of magnitude. In this contribution, these changes can be correlated with a decrease in amplitude of the Si-db signal. Pb0 and the methyl radical signals were less affected by this additional SiO2 layer. Based on these observations, microscopic models for the negative fixed charge density (Qtot) and the interface trap density (DQit) and the connection between these values are proposed.

Several different processes and potentially even multiple different mechanisms are involved in the dynamic nuclear polarization (DNP) of bulk nuclei in a solid matrix doped with paramagnetic centers. This has to date prevented the quantification of DNP on the basis of a mechanistic understanding for paramagnetic agents with an electron paramagnetic resonance (EPR) spectrum broader than the Larmor frequency of the nuclei that are polarized. To compare theoretical models with experiments, it is necessary to gather experimental data to quantify all the involved processes. On the basis of an EPR setup using the same unstabilized microwave source as is used for DNP, we present multidimensional correlation EPR experiments to study the electron spin dynamics under DNP conditions. Low-power pulse EPR methods are used to measure the transient saturation and saturation–recovery on a timescale of tens to hundreds of milliseconds. Furthermore, the absence of a microwave resonator or cavity enables us to perform pump–probe experiments with switching of the microwave frequency. The correlation patterns obtained with these electron–electron double-resonance experiments can be used to analyze spectral diffusion in the EPR spectrum of the investigated radicals. In addition to studying the dynamical properties of the electron spins, it is shown for TEMPO in a glassy matrix that these experiments can be used to directly measure DNP of strongly hyperfine-coupled nuclei, in this case the nitroxyl-14N.

The implementation of electron paramagnetic resonance (EPR) detection in a low-temperature dissolution dynamic nuclear polarization (DNP) setup is presented. Using a coil oriented parallel to the static magnetic field, the change of the longitudinal magnetization of free radicals is measured upon resonant irradiation of an amplitude or frequency modulated microwave (mw) field. The absorption EPR spectrum is measured if the amplitude of the mw field is modulated, whilst the first derivative of the spectrum is obtained with frequency modulation. Using a burst of pulses, it is also possible to perform pump–probe experiments such as saturation-recovery or electron–electron double resonance experiments. Furthermore, the magnetization could be monitored in a time-resolved manner during amplitude modulation, thus making it possible to record its transient as it is approaching an equilibrium value. Experimental examples are shown with frozen solutions of trityl radical and TEMPO, two commonly used radicals for dissolution DNP experiments.

Remote detection of NMR and MRI is an alternative detection modality where the information about a sample of interest is stored as longitudinal magnetization of a fluid. This fluid is then extracted and transported to a detector optimized for sensitivity. Hyperpolarized 129Xe gas is ideally suited for this purpose due to its long spin–lattice relaxation time constant, its chemical inertness and the low background signal in the absence of targeted addition of prepolarized gas. The method is described in some detail, with a summary of potential sensitivity advantages and implications of non-ideal fluid transport. The time-of-flight remote detection technique is described to quantify fluid flow and dispersion in a laboratory frame of reference. Different applications are reviewed, including gas flow in rocks, in wood, in microporous materials and in microfluidic devices.

The formation of mossy lithium and lithium dendrites so far prevents the use of lithium metal anodes in lithium ion batteries. To develop solutions for this problem (e.g., electrolyte additives), operando measurement techniques are required to monitor mossy lithium and dendrite formation during electrochemical cycling. Here we present a novel battery cell design that enables operando electron paramagnetic resonance (EPR) spectroscopy. It is shown that time-resolved operando EPR spectroscopy during electrochemical cycling of a lithium-metal/LiFePO4 (LFP) cell provides unique insights into the lithium plating/dissolution mechanisms, which are consistent with ex situ scanning electron microscopy (SEM) analysis. To demonstrate the viability of the operando EPR method, two cells using different electrolytes were studied. When using an electrolyte containing fluoroethylene carbonate (FEC) additive, a higher reversibility of the lithium anode and reduced formation of micro-structured (mossy/dendritic) lithium were observed.

Pore structure and connectivity determine how microstructured materials perform in applications such as catalysis, fluid storage and transport, filtering or as reactors. We report a model study on silica aerogel using a time-of-flight magnetic resonance imaging technique to characterize the flow field and explain the effects of heterogeneities in the pore structure on gas flow and dispersion with 129Xe as the gas-phase sensor. The observed chemical shift allows the separate visualization of unrestricted xenon and xenon confined in the pores of the aerogel. The asymmetrical nature of the dispersion pattern alludes to the existence of a stationary and a flow regime in the aerogel. An exchange time constant is determined to characterize the gas transfer between them. As a general methodology, this technique provides insights into the dynamics of flow in porous media where several phases or chemical species may be present.

We have used nuclear magnetic resonance (NMR) to obtain spatially and temporally resolved profiles of gas flow in microfluidic devices. Remote detection of the NMR signal both overcomes the sensitivity limitation of NMR and enables time-of-flight measurement in addition to spatially resolved imaging. Thus, detailed insight is gained into the effects of flow, diffusion, and mixing in specific geometries. The ability for noninvasive measurement of microfluidic flow, without the introduction of foreign tracer particles, is unique to this approach and is important for the design and operation of microfluidic devices. Although here we demonstrate an application to gas flow, extension to liquids, which have higher density, is implicit.

Dolomite from the Jhamarkotra phosphate mine (India) contains 680 ppm Mn 2 + as indicated by electron-spin resonance spectroscopy (EPR). Dipolar broadening of the EPR signal prevents a quantitative assignment of Mn 2 + to the Ca and Mg sites. Upon heating to 700 °C, over 99% of the dolomite is decomposed and all the Mn 2 + is released from the dolomite structure. Approximately 95% of the original Mn 2 + is oxidized and forms Mn-oxide. The remaining Mn 2 + preferentially migrates into CaO. This behavior is interpreted by the Goldschmidt rule stating that smaller ions are taken up at the sites of larger ions. During subsequent hydration of CaO into Ca(OH) 2 , the Mn 2 + remains stable. The protection of Mn 2 + against oxidation is explained by the topotaxical alteration of the host minerals. Carbonation of the Ca(OH) 2 leads to Mn 2 + oxidation caused by the dissolution of the hydroxide prior to the formation of CaCO 3 . Divalent Mn monitored by EPR can be used to unravel the behavior of trace elements on the molecular level during mineral alteration.

A new setup for longitudinal detection (LOD) of EPR signals based on a commercial pulse EPR spectrometer equipped with an ENDOR probehead is presented. The design is suited for pulse LOD EPR and amplitude-modulated cw LOD EPR experiments. The sensitivity is substantially increased compared with earlier designs. Two new pulse schemes that take full advantage of the special properties of the setup are invented. In transient-nutation longitudinally detected EPR (TN-LOD EPR), the nutation of magnetization during a microwave pulse is used to measure the EPR signal. In pulse-train excited longitudinally detected EPR (PT-LOD EPR), a train of microwave pulses that periodically inverts the magnetization is applied. First experimental results on radicals and metal complexes at room temperature are presented.

A two-dimensional electron spin echo envelope modulation (ESEEM) experiment, called nuclear-Zeeman-resolved ESEEM (NZ-ESEEM), that correlates nuclear transition frequencies with nuclear Zeeman frequencies is introduced. NZ-ESEEM is basically a three-pulse ESEEM experiment complemented by a magnetic-field pulse applied during part of the free evolution period between the second and third microwave pulse. The inner working of the new approach is explained and the instrumentation is discussed. The capacity of the method is illustrated by two examples of applications.

The defect structure of CuO-doped PbTiO3 has been analyzed using the Hyperfine Sublevel Correlation Experiment to identify hydrogen interstitials. The formation of (CuTi″−(OH)O•)′ defect complexes has been observed, which exist in addition to the (CuTi″−VO••)× complexes. On this basis, modified reorientation characteristics are proposed due to a change in hopping mechanism from an oxygen-vacancy mediated migration mechanism to a proton hopping process. Furthermore, mobile hydrogen interstitials are generated that increase conductivity in terms of a “Grotthuss”-type charge-transport mechanism.

Nuclear magnetic resonance (NMR) in combination with pulsed magnetic field gradients has proven very successful for measuring molecular diffusion, where the correlation time of the motion is much shorter than the timescale of the experiment. In this article, it is demonstrated that a single-scan NMR technique to measure molecular diffusion can be employed to also study incoherent random motions over macroscopic length scales that show correlation times similar to the timescale of the experiment. Such motions are observed, for example, after the mixing of two components or after transferring a sample from one container into another. To measure the fluid settling, a series of magnetization helices were encoded onto a sample. Stimulated gradient echo trains were then generated after different mixing times, which enabled the determination of an effective dispersion coefficient for the fluid. This technique was used to optimize the timing of NMR experiments combined with dissolution dynamic nuclear polarization, where a sample was shuttled between two magnets. In addition to the decay of fluid turbulences, the presence of microbubbles in the sample tube at the end of the shuttling step was identified as another contribution to the NMR linewidth. Microbubbles could be indirectly observed through the line broadening effect on the NMR signal due to their different susceptibility compared to the solvent, which induced field gradients near the interfaces. Using these data, the signal attenuation caused by sample motion in single-scan two-dimensional correlation spectroscopy NMR experiments could be predicted with reasonable accuracy.

Using low temperature dynamic nuclear polarisation (DNP) in conjunction with dissolution makes it possible to generate highly polarised nuclear spin systems for liquid state applications of nuclear magnetic resonance spectroscopy. However, in its current implementation, which requires the transfer of the solute between two different magnets, the hyperpolarisation strategy is limited to spin systems with relatively long longitudinal relaxation time constants. Here we describe the design and construction of a dedicated spectrometer for DNP applications that is based on a magnet with two isocentres. DNP enhancement is carried out in the upper compartment of this magnet in a low temperature environment at 3.35 T, while a 9.4 T isocentre in the lower compartment is used for high resolution NMR spectroscopy. The close proximity (85 cm) of the two isocentres makes it possible to transfer the sample in the solid state with very little loss of spin polarisation. In first performance tests this novel experimental set-up proved to be superior to the strategy involving two separate magnets.

A sensitivity analysis is presented of the remote detection NMR technique, which facilitates the spatial separation of encoding and detection of spin magnetization. Three different cases are considered: remote detection of a transient signal that must be encoded point-by-point like a free induction decay, remote detection of an experiment where the transient dimension is reduced to one data point like phase encoding in an imaging experiment, and time-of-flight (TOF) flow visualization. For all cases, the sensitivity enhancement is proportional to the relative sensitivity between the remote detector and the circuit that is used for encoding. It is shown for the case of an encoded transient signal that the sensitivity does not scale unfavorably with the number of encoded points compared to direct detection. Remote enhancement scales as the square root of the ratio of corresponding relaxation times in the two detection environments. Thus, remote detection especially increases the sensitivity of imaging experiments of porous materials with large susceptibility gradients, which cause a rapid dephasing of transverse spin magnetization. Finally, TOF remote detection, in which the detection volume is smaller than the encoded fluid volume, allows partial images corresponding to different time intervals between encoding and detection to be recorded. These partial images, which contain information about the fluid displacement, can be recorded, in an ideal case, with the same sensitivity as the full image detected in a single step with a larger coil.

Hyperpolarized (129)Xe can be used as a sensor to indirectly detect NMR spectra of heteronuclei that are neither covalently bound nor necessarily in direct contact with the Xe atoms, but coupled through long-range intermolecular dipole-dipole interactions. To reintroduce long-range dipolar couplings the sample symmetry has to be broken. This can be done either by using an asymmetric sample arrangement, or by breaking the symmetry of the spin magnetization with field gradient pulses. Experiments are performed where only a small fraction of the available (129)Xe magnetization is used for each point, so that a single batch of xenon suffices for the point-by-point acquisition of a heteronuclear NMR spectrum. Examples with (1)H as the analyte nucleus show that these methods have the potential to obtain spectra with a resolution that is high enough to determine homonuclear J couplings. The applicability of this technique with remote detection is discussed.

We report the use of an atomic magnetometer based on nonlinear magneto-optical rotation with frequency modulated light (FM NMOR) to detect nuclear magnetization of xenon gas. The magnetization of a spin-exchange-polarized xenon sample ($1.7 $cm$^3$ at a pressure of $5 $bar, natural isotopic abundance, polarization 1%), prepared remotely to the detection apparatus, is measured with an atomic sensor (which is insensitive to the leading field of 0.45 G applied to the sample; an independent bias field at the sensor is $140 \mu$G). An average magnetic field of $\sim 10 $nG induced by the xenon sample on the 10-cm diameter atomic sensor is detected with signal-to-noise ratio $\sim 10$, limited by residual noise in the magnetic environment. The possibility of using modern atomic magnetometers as detectors of nuclear magnetic resonance and in magnetic resonance imaging is discussed. Atomic magnetometers appear to be ideally suited for emerging low-field and remote-detection magnetic resonance applications., Comment: 4 pages, 4 figures

Two schemes for the measurement of orientation-dependent spin–lattice relaxation times are introduced, which combine the inversion-recovery experiment with electron-Zeeman-resolved or right-angle wiggling EPR. The principles of the experiments are outlined and their performance is illustrated by examples of application. With the electron Zeeman-resolved approach the relaxation times of two metal complexes with different g values are unraveled, whereas with right-angle wiggling the orientation-dependent relaxation behavior of a metal complex with large hyperfine anisotropy is analyzed.

Short acquisition time and single scan capability of gradient-assisted ultrafast multidimensional spectroscopy makes it possible to record 2D spectra of highly polarised spin systems in the liquid state using dynamic nuclear polarization (DNP) in conjunction with fast dissolution. We present a slice selective experiment, suitable for back-to-back acquisition of two independent single-scan 2D experiments from different sample volumes. This scheme maximizes the amount of information obtainable from a sample that is prepolarised with a non-repeatable DNP technique. It is particularly suitable for samples with the short longitudinal relaxation times common to proton NMR spectroscopy. This technique is demonstrated by applying two filtered proton 2D COSY experiments on a DNP-polarised mixture of glutamine and glutamate to selectively amplify the correlation pattern of the protons connected to the beta and gamma carbons of either one of the two amino acids. Particular emphasis was put on the reproducibility of the experiments, especially the polarisation enhancement. Data for the liquid-state proton enhancement from amino acids and small proteins was assembled in a map that allowed the prediction of signal levels in liquid-state NMR experiments employing dissolution DNP.