Magnetic Resonance Line Shapes Research Articles

Nuclear induction line shape: Non-Markovian diffusion with boundaries.

The dynamics of viscoelastic fluids are governed by a memory function, essential yet challenging to compute, especially when diffusion faces boundary restrictions. We propose a computational method that captures memory effects by analyzing the time-correlation function of the pressure tensor, a viscosity indicator, through the Stokes-Einstein equation's analytic continuation into the Laplace domain. We integrate this equation with molecular dynamics simulations to derive necessary parameters. Our approach computes nuclear magnetic resonance (NMR) line shapes using a generalized diffusion coefficient, accounting for temperature and confinement geometry. This method directly links the memory function with thermal transport parameters, facilitating accurate NMR signal computation for non-Markovian fluids in confined geometries.

Manipulation of spin-1 solid-state targets

The spin-1 nuclear magnetic resonance (NMR) lineshape for polycrystalline materials can be manipulated with selective radio frequency (RF) to alter the overlapping absorption lines in a dynamically polarized target. This process can be performed in solid-state targets either in a frozen spin state or during continuous microwave pumping to manipulate the vector and tensor polarizations of the spin-1 target. In this paper, we present an analytical description of the spin energy levels responsible for the dynamics in the continuous wave NMR spectrum under RF, as well as a new simplified analysis of the lineshape itself that relies on three basic conditions. These conditions can be used to measure the vector and tensor polarization in an RF-manipulated signal in real-time or to simulate the spin-1 NMR lineshape’s response to locally applied RF irradiation. The analytical description of the RF-manipulated lineshape as well as the resulting simulations can be used to optimize the figure of merit in high-energy and nuclear scattering experiments using spin-1 solid-state targets.

NMR lineshape analysis using analytical solutions of multi-state chemical exchange with applications to kinetics of host–guest systems

Nuclear magnetic resonance (NMR) lineshape analysis is a powerful tool for the study of chemical kinetics. Here we provide techniques for analysis of the relationship between experimentally observed spin kinetics (transitions between different environments A,B,dots) and corresponding chemical kinetics (transitions between distinct chemical species; e.g., free host and complexed host molecule). The advantages of using analytical solutions for two-, three- or generally N-state exchange lineshapes (without J-coupling) over the widely used numerical calculation for NMR spectral fitting are presented. Several aspects of exchange kinetics including the generalization of coalescence conditions in two-state exchange, the possibility of multiple processes between two states, and differences between equilibrium and steady-state modes are discussed. ‘Reduced equivalent schemes’ are introduced for spin kinetics containing fast-exchanging states, effectively reducing the number of exchanging states. The theoretical results have been used to analyze a host–guest system containing an oxoporphyrinogen complexed with camphorsulfonic acid and several other literature examples, including isomerization, protein kinetics, or enzymatic reactions. The theoretical treatment and experimental examples present an expansion of the systematic approach to rigorous analyses of systems with rich chemical kinetics through NMR lineshape analysis.

Insulator:conductor interfacial regions — Li ion dynamics in the nanocrystalline dispersed ionic conductor LiF:TiO2

Lithium fluoride is known to be a very poor ionic conductor. Here, we used it as a model substance to investigate the influence of the insulator TiO2 on ion dynamics in nanocrystalline composites of LiF and TiO2. In such two-phase systems a percolation network of fast Li+ pathways at the conductor:insulator interfaces may lead to enhanced overall, through-going ionic (and electronic) transport. Indeed, we observed such an effect and characterized it by conductivity spectroscopy as well as by variable-temperature 7Li and 19F Nuclear Magnetic Resonance (NMR) line shape measurements and diffusion-induced NMR relaxometry. Compared to nanocrystalline LiF, a remarkable increase in total conductivity of almost four orders of magnitude, that is, from 6×10−11 S cm−1 up to 2×10−7 S cm−1 (100 °C) was observed for LiF:TiO2 containing 40 vol.-% of TiO2. Direct current polarization measurements revealed that principally ionic charge carriers are responsible for this enhancement. As both ions in LiF, Li+ and F−, might be mobile, NMR helped revealing that Li+ is the charge carrier being mainly responsible for the increase in ionic conductivity. Compared to SiO2 and Al2O3 [see, S. Breuer et al. J. Phys. Chem. C, 2019, 123, 5222], with TiO2 the largest increase in conductivity was achieved. Hence the introduction of heterogeneous conductor:insulator contacts turned out to be a highly suitable tool to effectively engineer the interfacial regions in such two-phase systems. In general, such a composite effect is important for ion transport in components for energy storage devices and in the solid electrolyte interphase region that generally passivates the anode material in lithium-ion batteries.

NMR investigation on the honeycomb iridate Ag3LiIr2O6

We investigate the structural and magnetic properties of a Kitaev spin liquid candidate material Ag$_3$LiIr$_2$O$_6$ based on $^7$Li nuclear magnetic resonance line shape, Knight shift and spin-lattice relaxation rate $1/T_1$. The first sample A shows signatures of magnetically ordered spins, and exhibits one sharp $^7$Li peak with FWHM increasing significantly below 14~K. $1/T_1^{stretch}$ of this sample displays a broad local maximum at 40~K, followed by a very sharp peak at $T_N = 9\pm1$~K due to critical slowing down of Ir spin fluctuations, a typical signature of magnetic long range order. In order to shed light on the position-by-position variation of $1/T_1$ throughout the sample, we use Inverse Laplace Transform $T_1$ analysis based on Tikhonov regularization to deduce the density distribution function $P(1/T_1)$. We demonstrate that $\sim 60\%$ of Ir spins are statically ordered at the NMR measurement timescale but the rest of the sample volume remains paramagnetic even at 4.2~K, presumably because of structural disorder induced primarily by stacking faults. In order to further investigate the influence of structural disorder, we compare these NMR results with those of a second sample B, which has been shown by transmission electron microscope to have domains with unwanted Ag inclusion at Li and Ir sites within the Ir honeycomb planes. The sample B displays an additional NMR peak with relative intensity of $\sim 17\%$. The small Knight shift and $1/T_1$ of these defect-induced $^7$Li sites and the enhancement of bulk susceptibility at low temperatures suggest that these defects generate domains of only weakly magnetic Ir spins accompanied by free spins, leading to a lack of clear signatures of long-range order. The apparent lack of long-range order could be easily misinterpreted as evidence for the realization of a spin liquid ground state in highly disordered Kitaev lattice.

2H NMR study on temperature-dependent water dynamics in amino-acid functionalized silica nanopores.

We prepare various amino-acid functionalized silica pores with diameters of ∼6 nm and study the temperature-dependent reorientation dynamics of water in these confinements. Specifically, we link basic Lys, neutral Ala, and acidic Glu to the inner surfaces and combine 2H nuclear magnetic resonance spin-lattice relaxation and line shape analyses to disentangle the rotational motions of the surfaces groups and the crystalline and liquid water fractions coexisting below partial freezing. Unlike the crystalline phase, the liquid phase shows reorientation dynamics, which strongly depends on the chemistry of the inner surfaces. The water reorientation is slowest for the Lys functionalization, followed by Ala and Glu and, finally, the native silica pores. In total, the rotational correlation times of water at the different surfaces vary by about two orders of magnitude, where this span is largely independent of the temperature in the range ∼200-250 K.

Solid-State NMR Spectroscopy Study of Molecular Dynamics of Pyridine 1-Oxide Encapsulated in p-tert-Butylcalix[4]arene.

The mixture of guests 25% pyridine 1-oxide (PNO) and 75% nitrobenzene (NB) encapsulated in a host cavity of p-tert-butylcalix[4]arene (p-tBC) is a suitable system for the study of intermolecular weak interactions, and the preferred orientation of the guest molecules within the host was derived. Variable temperature deuterium nuclear magnetic resonance line shape and spin-lattice relaxation studies were performed on three samples of 25% PNO as a guest (selectively and entirely deuterated) with 75% NB encapsulated in p-tBC as a host system. It is found that the PNO molecular motion supports the two-site jump model and is highly mobile throughout the temperature range from -130 to 20 °C. PNO reorients about its C2 molecular symmetry axis followed by reorientation around the compound's C4 axis of symmetry of a host. Calculation showed that besides precise molecular jumping, the guest also experiences the vibrational motion with a variation angle dispute about 15°. The correlation times for the molecular guest motion and two-dimensional exchange spectroscopy confirmed a fast exchange limit inside the cavity. These molecular dynamics follow an Arrhenius behavior motion from which the small activation energy is delivered that strongly suggests a relatively weak intermolecular interaction between the host and guest species. PNO has definite dipole orientational motion in the cavity where its aromatic covalent bond structure minimizes contact with the host p-tert-butyl groups. Experimentally found, PNO-d5 deuterons in the single crystal collapse at 55 and 125° at ambient temperature. The first splitting of 124 kHz belongs to D3(D5), the second splitting of 100 kHz belongs to D2(D6), and the last splitting of 52 kHz belongs to D4 of the ring of PNO inside the p-tBC.

A Compact Laser Pumped 4He Magnetometer with Laser-Frequency Stabilization by Inhomogeneous Light Shifts

We propose a compact 4He magnetometer realizing magnetic field measurement and laser-frequency stabilization simultaneously in a single 4He atomic cell. The frequency stabilization scheme is based on the asymmetric line shape of magnetic resonance which is induced by spatially inhomogeneous light shifts. We investigate the asymmetric line shape of the magnetic resonance signal theoretically and experimentally in laser pumped 4He magnetometer with the magneto-optical double-resonance configuration. Notice that, due to the asymmetric line shape, the in-phase component of the magnetic resonance signal is shown to have a linear dependence with respect to the laser frequency detuning and is used to actively lock the laser frequency to the resonant point. The method reduces the complexity of the system and improves the stability of the magnetometer, making the laser-pumped 4He magnetometer more compact and portable.

Lineshape of Magnetic Resonance and its Effects on Free Induction Decay and Steady-State Free Precession Signal Formation

Magnetic resonance imaging based on steady-state free precision (SSFP) sequences is a fast method to acquire T1, T2, and T2∗-weighted images. In inhomogeneous tissues such as lung tissue or blood vessel networks, however, microscopic field inhomogeneities cause a nonexponential free induction decay and a non-Lorentzian lineshape. In this work, the SSFP signal is analyzed for different prominent tissue models. Neglecting the effect of non-Lorentzian lineshapes can easily result in large errors of the determined relaxation times. Moreover, sequence parameters of SSFP measurements can be optimized for the nonexponential signal decay in many tissue structures.

Reproducibility of the field homogeneity of a metal-clad no-insulation all-REBCO magnet with a multi-layer ferromagnetic shim

We report that the field homogeneity of a large-scale metal-clad no-insulation all-ReBa2Cu3O7−x magnet is reproducible after occasional operating conditions, in this case the current ramp down/up and magnet warm-up/cool-down conditions. First, we shimmed the magnet by cylindrically arraying ferromagnetic shims on the wall of the room-temperature bore of the magnet to improve the spatial field homogeneity. As a result, the field homogeneity of the magnet was improved from 557.7 to 14.1 ppm in a 10 mm diameter spherical volume. The resultant acquisition of field homogeneity tremendously enhanced from an inhomogeneous state greatly degraded by screening currents qualified us to study the temporal stability of the field homogeneity. We used a magnet-energizing protocol with the field drift minimized to shrink a nuclear magnetic resonance (NMR) lineshape into a much narrower super-imposable lineshape. We demonstrate the technique by NMR experiments, showing excellent reproducibility in every case, presenting strong evidence of recovery to certain time-unvarying field gradients and to a state of homogeneity. These findings suggest a new route for high-resolution high-temperature superconducting NMR and magnetic-resonance-imaging magnet technology.

Chromatographic NMR spectroscopy: the effect of hollow silica microspheres on magnetic field inhomogeneities and resonance lineshapes.

Micrometric hollow silica spheres can effectively reduce magnetic field inhomogeneities when employed as a stationary phase in the context of NMR chromatography. We here provide a description of the NMR line broadening phenomenon for physically representative collections of hollow spheres with different geometries and filling factors. Our results highlight how, within the explored conditions, a proper modelling of the line broadening phenomenon should consider the enhanced relaxation of the spins during their diffusion across the spherical shells, and possibly other slow motional effects.

Complexation Kinetics of Cyclodextrins with Bile Salt Anions: Energy Barriers for the Threading of Ionic Groups.

Binding constants for thousands of cyclodextrin complexes have been reported in the literature, but much less is known about the kinetics of these host-guest complexes. In the present study, inclusion complexes of bile salts with β-cyclodextrin, γ-cyclodextrin, and a methylated β-cyclodextrin were studied by nuclear magnetic resonance (NMR) lineshape analysis to explore the structural factors that govern the complexation kinetics. For complexes with β-cyclodextrin, the association rate constants ranged from 2 × 106 to 2 × 107 M-1 s-1 while the dissociation rate constants ranged from 12 to 6000 s-1 at 25 °C. The kinetics were thus significantly slower than for any other β-cyclodextrin complex reported in the literature, due to the large energy barrier for threading the ionic sidechains of the bile salt anions. Bile salts with taurine and glycine sidechains had identical binding affinities, but the kinetics differed by a factor of 10. Introduction of a single hydroxyl group at the binding site of the bile salts reduced the lifetimes and binding constants of the complexes by more than 50 times. The strong temperature dependence of the rate constants revealed that the large activation energies were mainly enthalpic with a small contribution from entropy. The larger γ-cyclodextrin was threaded by the nonionic end of the bile salts, and the kinetics were too fast to be accurately determined. The study demonstrates that ionic groups on guest molecules constitute significant energy barriers for the threading and dethreading of β-cyclodextrin hosts.

Revealing weak spin-orbit coupling effects on charge carriers in a π -conjugated polymer

We measure electrically detected magnetic resonance (EDMR) on organic light-emitting diodes (OLEDs) made of the polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) at room temperature and high magnetic fields, where spectral broadening of the resonance due to spin-orbit coupling (SOC) exceeds that due to the local hyperfine fields. Density-functional-theory calculations on an open-shell model of the material reveal g-tensors of charge-carrier spins in the lowest unoccupied (electron) and highest occupied (hole) molecular orbitals. These tensors are used for simulations of magnetic resonance line-shapes. Besides providing the first quantification and direct observation of SOC effects on charge-carrier states in these weakly SO-coupled hydrocarbons, this procedure demonstrates that spin-related phenomena in these materials are fundamentally monomolecular in nature.

Resistively detected NMR line shapes in a quasi-one-dimensional electron system

We observe variation in the resistively-detected nuclear magnetic resonance (RDNMR) lineshapes in quantum Hall breakdown. The breakdown is locally occurred in a gate-defined quantum point contact (QPC) region. Of particular interest is the observation of a dispersive lineshape occured when the bulk 2D electron gas (2DEG) is set to $\nu_{\rm{b}} = 2$ and the QPC filling factor to the vicinity of $\nu_{\rm{QPC}} = 1$, strikingly resemble the dispersive lineshape observed on a 2D quantum Hall state. This previously unobserved lineshape in a QPC points to simultaneous occurrence of two hyperfine-mediated spin flip-flop processes within the QPC. Those events give rise to two different sets of nuclei polarized in the opposite direction and positioned at a separate region with different degree of electronic polarizations.

Nuclear magnetic resonance line shapes of Wigner crystals in C13 -enriched graphene

Assuming that the nuclear magnetic resonance (NMR) signal from a $^{13}$C isotope enriched layer of graphene can be made sufficiently intense to be measured, we compute the NMR\ lineshape of the different crystals ground states that are expected to occur in graphene in a strong magnetic field. We first show that in nonuniform states, there is, in addition to the frequency shift due to the spin hyperfine interaction, a second contribution of equal importance from the coupling between the orbital motion of the electrons and the nuclei. We then show that, if the linewidth of the bare signal can be made sufficiently small, the Wigner and bubble crystals have line shapes that differ qualitatively from that of the uniform state at the same density while crystal states that have spin or valley pseudospin textures do not. Finally, we find that a relatively small value of the bare linewidth is sufficient to wash out the distinctive signature of the crystal states in the NMR line shape.

Multiparametric quantification of thermal heterogeneity within aqueous materials by water 1H NMR spectroscopy: Paradigms and algorithms.

Processes involving heat generation and dissipation play an important role in the performance of numerous materials. The behavior of (semi-)aqueous materials such as hydrogels during production and application, but also properties of biological tissue in disease and therapy (e.g., hyperthermia) critically depend on heat regulation. However, currently available thermometry methods do not provide quantitative parameters characterizing the overall temperature distribution within a volume of soft matter. To this end, we present here a new paradigm enabling accurate, contactless quantification of thermal heterogeneity based on the line shape of a water proton nuclear magnetic resonance (1H NMR) spectrum. First, the 1H NMR resonance from water serving as a "temperature probe" is transformed into a temperature curve. Then, the digital points of this temperature profile are used to construct a histogram by way of specifically developed algorithms. We demonstrate that from this histogram, at least eight quantitative parameters describing the underlying statistical temperature distribution can be computed: weighted median, weighted mean, standard deviation, range, mode(s), kurtosis, skewness, and entropy. All mathematical transformations and calculations are performed using specifically programmed EXCEL spreadsheets. Our new paradigm is helpful in detailed investigations of thermal heterogeneity, including dynamic characteristics of heat exchange at sub-second temporal resolution.

Nonclassical dynamics of the methyl group in 1,1,1-triphenylethane. Evidence from powder 1H NMR spectra.

According to the damped quantum rotation (DQR) theory, hindered rotation of methyl groups, evidenced in nuclear magnetic resonance (NMR) line shapes, is a nonclassical process. It comprises a number of quantum-rate processes measured by two different quantum-rate constants. The classical jump model employing only one rate constant is reproduced if these quantum constants happen to be equal. The values of their ratio, or the nonclassicallity coefficient, determined hitherto from NMR spectra of single crystals and solutions range from about 1.20 to 1.30 in the latter case to above 5.0 in the former, with the value of 1 corresponding to the jump model. Presently, first systematic investigations of the DQR effects in wide-line NMR spectra of a powder sample are reported. For 1,1,1-triphenylethane deuterated in the aromatic positions, the relevant line-shape effects were monitored in the range 99-121 K. The values of the nonclassicality coefficient dropping from 2.7 to 1.7 were evaluated in line shape fits to the experimental powder spectra from the range 99-108 K. At these temperatures, the fits with the conventional line-shape model are visibly inferior to the DQR fits. Using a theoretical model reported earlier, a semiquantitative interpretation of the DQR parameters evaluated from the spectra is given. It is shown that the DQR effects as such can be detected in wide-line NMR spectra of powdered samples, which are relatively facile to measure. However, a fully quantitative picture of these effects can only be obtained from the much more demanding experiments on single crystals.

Isomerase-catalyzed binding of interleukin-1 receptor-associated kinase 1 to the EVH1 domain of vasodilator-stimulated phosphoprotein.

Interleukin-1 receptor-associated kinase 1 (IRAK1) is a crucial signaling kinase in the immune system, involved in Toll-like receptor signaling. Vasodilator-stimulated phosphoprotein (VASP) is a central player in cell migration that regulates actin polymerization and connects signaling events to cytoskeletal remodeling. A VASP–IRAK1 interaction is thought to be important in controlling macrophage migration in response to protein kinase C-ε activation. We show that the monomeric VASP EVH1 domain directly binds to the 168WPPPP172 motif in the IRAK1 undefined domain (IRAK1-UD) with moderate affinity (KDApp = 203 ± 3 μM). We further show that this motif adopts distinct cis and trans isomers for the Trp168–Pro169 peptide bond with nearly equal populations, and that binding to the VASP EVH1 domain is specific for the trans isomer, coupling binding to isomerization. Nuclear magnetic resonance line shape analysis and tryptophan fluorescence experiments reveal the complete kinetics and thermodynamics of the binding reaction, showing diffusion-limited binding to the trans isomer followed by slow, isomerization-dependent binding. We further demonstrate that the peptidyl-prolyl isomerase cyclophilin A (CypA) catalyzes isomerization of the Trp168–Pro169 peptide bond and accelerates binding of the IRAK1-UD to the VASP EVH1 domain. We propose that binding of IRAK1 to tetrameric VASP is regulated by avidity through the assembly of IRAK1 onto receptor-anchored signaling complexes and that an isomerase such as CypA may modulate IRAK1 signaling in vivo. These studies demonstrate a direct interaction between IRAK1 and VASP and suggest a potential mechanism for how this interaction might be regulated by both assembly of IRAK1 onto an activated signaling complex and PPIase enzymes.

Unconventional magnetism in the spin-orbit-driven Mott insulatorsBa3MIr2O9(M=Sc,Y)

We have carried out detailed bulk and local probe studies on the hexagonal oxides Ba3MIr2O9 (M=Sc,Y) where Ir is expected to have a fractional oxidation state of +4.5. In the structure, Ir-Ir dimers are arranged in an edge shared triangular network parallel to the ab plane. Whereas only weak anomalies are evident in the susceptibility data, clearer anomalies are present in the heat capacity data. Our 45Sc nuclear magnetic resonance (NMR) lineshape (first order quadrupole split) is symmetric at room temperature but becomes progressively asymmetric with decreasing temperatures. This is suggestive of distortions in the structure which could arise from progressive tilt/rotation of the IrO6 octahedra with a decrease in temperature T. The 45Sc NMR spectral weight shifts near the reference frequency with decreasing T indicating the development of magnetic singlet regions. Around 10K, a significant change in the spectrum takes place with a large intensity appearing near the reference frequency but with the spectrum remaining multi-peak. It appears from our 45Sc NMR data that in Ba3ScIr2O9 significant disorder is still present below 10K. In the case of Ba3YIr2O9, the 89Y NMR spectral lines are asymmetric at high temperatures but become nearly symmetric (single magnetic environment) below T~70K. Our 89Y spectra and T1 measurements confirm the onset of long range ordering (LRO) from a bulk of the sample at 4K in this compound. Our results suggest that Ba3YIr2O9 might be structurally distorted at room temperature (via, for example, tilt/rotations of the IrO6 octahedra) but becomes progressively a regular triangular lattice with decreasing T. The effective magnetic moments and magnetic entropy changes are strongly reduced in Ba3YIr2O9 as compared to those expected for a S=1/2 system. Similar effects have been found in other iridates which naturally have strong spin-orbit coupling (SOC).

B-field dependence of the electron spin resonance line shape in a quantum dot

I investigate the line shape of the electron magnetic resonance broadened by an unpolarized bath of nuclear spins using an exact diagonalization method for the central spin model. While the shape is invariably Gaussian above the maximum Overhauser field of fully polarized nuclei, I find that the shape becomes non-trivial and magnetic field dependent in an intermediate range of external fields above a typical Overhauser field at high nuclear spin temperature and below the maximum Overhauser field. This effect can be observed in semiconductor quantum dots using electric transport or quantum optics techniques.