Nuclear Spin-lattice Relaxation Research Articles

Range and sensitivity of 17O nuclear spin-lattice relaxation as a probe of aqueous electrolyte dynamics.

The study of electrolytic solutions is of relevance in many research fields, ranging from biophysics, materials, and colloid science to catalysis and electrochemistry. The dependence of solution dynamics on the nature of electrolytes and their concentrations has been the subject of many experimental and computational studies, yet it remains challenging to obtain a full understanding of the factors that govern solution behavior. Here, we provide additional insights into the behavior of aqueous solutions of alkali chlorides by combining 17O relaxation data with diffusion and viscosity data and contrast their behavior with 1H nuclear magnetic resonance relaxation data. The main findings are that 17O relaxation correlates well with viscosity data but not with diffusion data, while 1H relaxation correlates with neither. Certain ionic trends match known ion-specific series behavior, especially at high concentrations. Notably, we also examine the ranges of the interactions and conclude that the majority of the effects are tied to local water reorientation dynamics.

Controlled Sublimation Rate of Guest Drug from Polymorphic Forms of a Cyclodextrin-Based Polypseudorotaxane Complex and Its Correlation with Molecular Dynamics as Probed by Solid-State NMR.

Encapsulation of active pharmaceutical ingredients (APIs) in confined spaces has been extensively explored as it dramatically alters the molecular dynamics and physical properties of the API. Herein, we explored the effect of encapsulation on the molecular dynamics and physical stability of a guest drug, salicylic acid (SA), confined in the intermolecular spaces of γ-cyclodextrin (γ-CD) and poly(ethylene glycol) (PEG)-based polypseudorotaxane (PPRX) structure. The sublimation tendency of SA encapsulated in three polymorphic forms of the γ-CD/PEG-based PPRX complex, monoclinic columnar (MC), hexagonal columnar (HC), and tetragonal columnar (TC), was investigated. The SA sublimation rate was decreased by 3.0-6.6-fold and varied in the order of MC form > HC form > TC form complex. The 13C and 1H magic-angle spinning (MAS) solid-state nuclear magnetic resonance (NMR) spectra and 13C spin-lattice relaxation time (T1) indicated that the encapsulated SA molecules existed as the monomeric form, and its molecular mobility increased in the order of MC form > HC form > TC form complex. In the complexes, a rapid chemical exchange between two dynamic states of SA (free and bound) was suggested, with varying adsorption/desorption rates accounting for its distinct molecular mobility. This adsorption/desorption process was influenced by proton exchange at the interaction site and interaction strength of SA in the complexes, as evidenced by 1H MAS spectra and temperature dependency of the 13C carbonyl chemical shift. A positive correlation between the molecular mobility of SA and its sublimation rate was established. Moreover, the molecular mobility of γ-CD and PEG in the complexes coincided with that of SA, which can be explained by fast guest-driven dynamics. This is the first report on the stability improvement of an API through complexation in polymorphic supramolecular host structures. The relationship between the molecular dynamics and physical properties of encapsulated API will aid in the rational design of drug delivery systems.

Nuclear spin-lattice relaxation rate in disordered paramagnetic diluted magnetic semiconductors

Nuclear spin-lattice relaxation rate in disordered paramagnetic diluted magnetic semiconductors

Reduction of the Spin–Phonon Coupling of Quadrupole Nuclei in NaF Crystals under Magnetic Saturation

The rate of nuclear spin-lattice relaxation is determined by the efficiency of interaction between thermal phonons and nuclear spins. The results on reducing the efficiency of spin–phonon coupling by suppressing the contribution from paramagnetic centers to quadrupole nucleus relaxation are presented. The suppression has been performed by stationary magnetic effect at the Larmor frequency. It is shown that, as in the presence of an acoustic field, the rate of spin-lattice relaxation of 23Na nuclei in a sodium fluoride crystal at magnetic saturation of the NMR signal does not change in the region of a negative average spin temperature.In the region of positive spin temperature, the rate of relaxation of 23Na spins significantly decreases and nuclear magnetization recovery with time is described by the sum of two exponentials. The contribution from nuclear spins with a lower efficiency of spin–phonon coupling, corresponding to the exponential with a long relaxation time, increases with increasing saturating field intensity. It is demonstrated that the efficiency of spin–phonon coupling for 19F nuclei without quadrupole moment does not change under the saturation conditions.The results obtained can be used for analyzing the structure of real crystals.

Temperature Dependence of Nuclear Quadrupole Resonance and the Observation of Metal-Ligand Covalency in Actinide Complexes: 35Cl in Cs2UO2Cl4.

We report a study of the temperature dependence of 35Cl nuclear quadrupole resonance (NQR) transition energies and spin-lattice relaxation times (T1) for 235U-depleted dicesium uranyl tetrachloride (Cs2UO2Cl4) aimed at elucidating electronic interactions between the uranium center and atoms in the equatorial plane of the UO22+ ion. The transition frequency decreases slowly with temperature below 75 K and with a more rapid linear dependence above this temperature. The spin-lattice relaxation time becomes shorter with temperature, and as temperatures increase, the T1 decrease becomes nearly quadratic. The observed trends are reproduced by a model that assumes phonon-induced fluctuations of the electric field gradient tensor and partial electron delocalization from Cl to U. The fit of the theoretical model to experimental data allows a Debye temperature of 96 K to be estimated. The generalization of this approach to investigations of covalency in actinide-ligand bonding is examined.

Inherent surface superconducting phase in the Mo8Ga41 single-gap bulk superconductor as seen by nuclear resonance and tunneling spectroscopy

A comprehensive study of the intermetallic superconductor Mo8Ga41 was conducted by means of HRPXRD, ab-initio calculations, NMR and NQR spectroscopy on 69Ga and 71Ga nuclei, as well as tunneling spectroscopy. All experimental methods used demonstrated the presence of an inherent surface superconducting phase in addition to the bulk superconducting phase. For the latter, the s-wave type single-gap superconducting behavior was observed with a slightly increased 2Δ0/kBTc = 3.89 in agreement with the known enhanced electron-phonon coupling in this compound. From nuclear spin-lattice relaxation experiment, the estimated superconducting gap value of the surface phase was 2Δ*0 = 22.2 K, which corresponds to the T*c ≈ 6.3 K in the weak-coupling BCS limit. Meanwhile, the tunneling spectroscopy reveals similar values of T*c ∼ 6.0–7.3 K for this surface phase. Thus, the known evidence of the Mo8Ga41 multi-gap behavior can be explained by the influence of the inherent surface superconducting phase.

Anion and cation dynamics in a carbon-substituted cesium closo-hydroborate CsCB11H12: 1H and 133Cs NMR studies

Cesium monocarba-hydroborate CsCB11H12 belongs to the family of alkali-metal closo- hydroborate salts which are considered as promising solid electrolytes for battery applications. To study the dynamical properties of this compound at the microscopic level, we have measured the 1H and 133Cs nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates over the temperature range of 80–434 K. Our measurements have revealed a coexistence of two types of reorientational jump processes of the complex [CB11H12]− anions. For the faster of these processes characterized by the average activation energy of 310 meV, the anion reorientations are not “frozen” at the NMR frequency scale down to 120 K. The behavior of the 133Cs NMR line width in CsCB11H12 is consistent with the onset of long-range diffusive motion of Cs+ ions at the frequency scale of ∼104 s−1 above 340 K, and the distinct effects of the diffusive Cs+ jumps on both the 1H and 133Cs spin-lattice relaxation rates become observable above 380 K.

Gapless fermionic excitation in the antiferromagnetic state of ytterbium zigzag chain

The emergence of charge-neutral fermionic excitations in magnetic systems is one of the unresolved issues in recent condensed matter physics. This type of excitations has been observed in various systems, such as low-dimensional quantum spin liquids, Kondo insulators, and antiferromagnetic insulators. Here, we report the presence of a pronounced gapless spin excitation in the low-temperature antiferromagnetic state of YbCuS2 semiconductor, where trivalent ytterbium atoms form a zigzag chain structure. We confirm the presence of this gapless excitations by a combination of experimental probes, namely 63/65Cu-nuclear magnetic resonance and nuclear quadrupole resonance, as well as specific heat measurements, revealing a linear low-temperature behavior of both the nuclear spin-lattice relaxation rate 1/T1 and the specific heat. This system provides a platform to investigate the origin of gapless excitations in spin chains and the relationship between emergent fermionic excitations and frustration.

Dynamic nuclear polarization weighted spectroscopy of multispin electronic-nuclear clusters

Nuclear spins and paramagnetic centers in a solid randomly group to form clusters featuring nearly degenerate, hybrid states whose dynamics are central to processes involving nuclear spin-lattice relaxation and diffusion. Their characterization, however, has proven notoriously difficult mostly due to their relative isolation and comparatively low concentration. Here, we combine magnetic field cycling experiments, optical spin pumping, and variable radiofrequency (RF) excitation to probe transitions between hybrid multispin states formed by strongly coupled electronic and nuclear spins in diamond. Leveraging bulk nuclei as a collective time-integrating sensor, we probe the response of these spin clusters as we simultaneously vary the applied magnetic field and RF excitation to reconstruct multidimensional spectra. We uncover complex nuclear polarization patterns of alternating sign that we qualitatively capture through analytical and numerical modeling. Our results unambiguously expose the impact that strongly hyperfine-coupled nuclei can have on the spin dynamics of the crystal, and inform future routes to spin cluster control and detection.

Magnetic resonance study of novel detonation nanodiamonds originated from non-conventional explosives

Detonation nanodiamonds (DND) are conventionally synthesized from a mixture of trinitrotoluene (TNT) and hexogen (RDX) explosives. It is generally believed that the internal structure of DNDs, which consists of a diamond core covered with a partially disordered shell, is very similar in all DNDs. Variations of the properties of DNDs are usually associated with the surface functionalization of the hydrogen-terminated surface with various chemicals to obtain a family of different nanodiamonds. In this paper, we report on the synthesis, electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) study of novel DNDs synthesized from a few non-conventional explosives, which have not previously been used for this purpose. We show that DNDs synthesized from different explosive precursors display different ratios of carbon atoms in the core and shell. The variation of this ratio correlates well with variations in the content of paramagnetic defects and 13C nuclear spin-lattice relaxation rates. The presented data advances new opportunities for controllable synthesis of DNDs with engineered properties.

Nuclear spin relaxation in aqueous paramagnetic ion solutions.

A Brownian shell model describing the random rotational motion of a spherical shell of uniform particle density is presented and validated by molecular dynamics simulations. The model is applied to proton spin rotation in aqueous paramagnetic ion complexes to yield an expression for the Larmor-frequency-dependent nuclear magnetic resonance spin-lattice relaxation rate T_{1}^{-1}(ω) describing the dipolar coupling of the nuclear spin of the proton with the electronic spin of the ion. The Brownian shell model provides a significant enhancement to existing particle-particle dipolar models without added complexity, allowing fits to experimental T_{1}^{-1}(ω) dispersion curves without arbitrary scaling parameters. The model is successfully applied to measurements of T_{1}^{-1}(ω) from aqueous manganese(II), iron(III), and copper(II) systems where the scalar coupling contribution is known to be small. Appropriate combinations of Brownian shell and translational diffusion models, representing the inner and outer sphere relaxation contributions, respectively, are shown to provide excellent fits. Quantitative fits are obtained to the full dispersion curve of each aquoion with just five fit parameters, with the distance and time parameters each taking a physically justifiable numerical value.

Calorimetric measurement of nuclear spin-lattice relaxation rate in metals

The quasiparticle density of states in correlated and quantum-critical metals directly probes the effect of electronic correlations on the Fermi surface. Measurements of the nuclear spin-lattice relaxation rate provide one such experimental probe of quasiparticle mass through the electronic density of states. By far the most common way of accessing the spin-lattice relaxation rate is via nuclear magnetic resonance and nuclear quadrupole resonance experiments, which require resonant excitation of nuclear spin transitions. Here we report non-resonant access to spin-lattice relaxation dynamics in AC-calorimetric measurements. The nuclear spin-lattice relaxation rate is inferred in our measurements from its effect on the frequency dispersion of the thermal response of the calorimeter-sample assembly. We use fast, lithographically-defined nanocalorimeters to access the nuclear spin-lattice relaxation times in metallic indium from 0.3~K to 7~K and in magnetic fields up to 35~T.

Bayesian Approach to T1 Analysis in NMR Spectroscopy with Applications to Solid State Physics

We applied the Bayesian framework to the analysis of nuclear spin–lattice relaxation curves. In the simplest case, nuclear spin–lattice relaxation is described by a single exponential function. However, relaxations to which the exponential function does not fit well are often observed in real NMR experiments. One of the reasons for those relaxations is the sample inhomogeneity. For such cases, it is common practice to analyze the curve by fitting a stretched exponential function exp(−(t/T1)β) to the data, where T1 is the nuclear spin–lattice relaxation time. Another possibility is the coexistence of different relaxation mechanisms. In this situation, the relaxation curves should be fitted by the summation of several relaxation functions. For scientists to interpret relaxation curves, a data-driven method is required to determine which of the two possibilities is more plausible. In this paper, we propose a Bayesian method to select the appropriate relaxation function and even the number of relaxation components. We demonstrate that our method effectively determines the relaxation model on the basis of numerical experiments and the experimental data of nuclear spin–lattice relaxation of the nonmagnetic semiconductor SmS.

Low-Temperature Magnetic Fluctuations Investigated by 125Te-NMR on the Uranium-Based Superconductor UTe2

To investigate the static and dynamic magnetic properties on the uranium-based superconductor UTe2, we measured the NMR Knight shift K and the nuclear spin–lattice relaxation rate 1/T1 in H || a by 125Te-NMR on a 125Te-enriched single-crystal sample. 1/T1T in H || a is much smaller than 1/T1T in H || b and c, and magnetic fluctuations along each axis are derived from the 1/T1T measured in H parallel to all three crystalline axes. The magnetic fluctuations are almost identical at two Te sites and isotropic at high temperatures, but become anisotropic below 40 K, where heavy-fermion state is formed. The character of magnetic fluctuations in UTe2 is discussed with the comparison to its static susceptibility and the results on other U-based superconductors. It is considered that the magnetic fluctuations probed with the NMR measurements are determined by the magnetic properties inside the two-leg ladder formed by U atoms, which are dominated by the qa = 0 ferromagnetic fluctuations.

Dynamical properties of the magnesium borohydride – ethylenediamine compound Mg(en)1.2(BH4)2: A nuclear magnetic resonance study

The magnesium borohydride – ethylenediamine (en) compound Mg(en)1.2(BH4)2 exhibits high Mg-ion conductivity above room temperature. To study the dynamical properties of this compound and its partially deuterium-substituted counterpart, we have measured the 1H, 2D, and 11B nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates over a wide temperature range (6–324 K). Our measurements have revealed a coexistence of four types of BH4 reorientational jump processes with different activation energies. For the fastest of these processes characterized by the activation energy of 47(3) meV, the unusually high reorientational mobility is retained down to low temperatures, so that the corresponding H jump rate is of the order of 108 s−1 near 70 K. The presence of fast reorientational motion of [BH4]− anions in Mg(en)1.2(BH4)2 supports the idea that anion reorientations may facilitate the cation diffusion. In the studied temperature range, we have not found any distinct signs of localized motion of the ethylenediamine molecules or their fragments at the NMR frequency scale.

Long-Lived, Transportable Reservoir of Nuclear Polarization Used to Strongly Enhance Solution-State NMR Signals

There is a fundamental issue with the use of dynamic nuclear polarization (DNP) to enhance nuclear spin polarization: the same polarizing agent (PA) needed for DNP is also responsible for shortening the lifetime of the hyperpolarization. As a result, long-term storage and transport of hyperpolarized samples is severely restricted and the apparatus for DNP is necessarily located near or integrated with the apparatus using the hyperpolarized spins. In this paper, we demonstrate that naphthalene single crystals can serve as a long-lived reservoir of proton polarization that can be exploited to enhance signals in benchtop and high-field NMR of target molecules in solution at a site 300 km away by a factor of several thousand. The naphthalene protons are polarized using short-lived optically excited triplet states of pentacene instead of stable radicals. In the absence of optical excitation, the electron spins remain in a singlet ground state, eliminating the major pathway of nuclear spin-lattice relaxation. The polarization decays with a time constant of about 50 h at 80 K and 0.5 T or above 800 h at 5 K and 20 mT. A module based on a Halbach array yielding a field of 0.75 T and a conventional cryogenic dry shipper, operating at liquid nitrogen temperature, allows storage and long distance transport of the polarization to a remote laboratory, where the polarization of the crystal is transferred after dissolution to a target molecule of choice by intermolecular cross-relaxation. The procedure has been executed repeatedly and has proven to be reliable and robust.

Radicals on the silica surface: probes for studying dynamics by means of fast field cycling relaxometry and dynamic nuclear polarization

Determining the dynamics of adsorbed liquids on nanoporous materials is crucial for a detailed understanding of interactions and processes on the solid-liquid interface in many materials and porous systems. Knowledge of the influence of the presence of paramagnetic species on the surface or within the porous matrices is essential for fundamental studies and industrial processes such as catalysts. Magnetic resonance methods, such as electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR) and dynamic nuclear polarization (DNP), are powerful tools to address these questions and to quantify dynamics, electron-nuclear interaction features and their relation to the physical-chemical parameters of the system. This paper presents an NMR study of the dynamics of polar and nonpolar adsorbed liquids, represented by water, n-decane, deuterated water and nonane-d20, on the native silica surface as well as silica modified with vanadyl porphyrins. The analysis of the frequency dependence of the nuclear spin-lattice relaxation time is carried out by separating the intra- and intermolecular contributions, which were analyzed using reorientations mediated by translational displacements (RMTD) and force-free-hard-sphere (FFHS) models, respectively.

Observation of multigap and coherence peak in the noncentrosymmetric superconductor CaPtAs: As75 nuclear quadrupole resonance measurements

We present synthesis and $^{75}$As-nuclear quadrupole resonance (NQR) measurements for the noncentrosymmetric superconductor CaPtAs with a superconducting transition temperature $T_c$ of $\sim 1.5$ K. We discovered two different forms of CaPtAs during synthesis; one is a high-temperature tetragonal form that was previously reported, and the other is a low-temperature form consistent with the orthorhombic structure of CaPtP. According to the $^{75}$As-NQR measurement for superconducting tetragonal CaPtAs, the nuclear spin-lattice relaxation rate $1/T_1$ has an obvious coherence peak below $T_c$ and does not follow a simple exponential variation at low temperatures. These findings indicate that CaPtAs is a multigap superconductor and a large $s$-wave component.

Rate constants for saturation-recovery EPR and ELDOR of 14N-Spin labels

Saturation-recovery (SR)-EPR can determine electron spin–lattice relaxation rates in liquids over a wide range of effective viscosity, making it especially useful for biophysical and biomedical applications. Here, I develop exact solutions for the SR-EPR and SR-ELDOR rate constants of 14N-nitroxyl spin labels as a function of rotational correlation time and spectrometer operating frequency. Explicit mechanisms for electron spin–lattice relaxation are: rotational modulation of the N-hyperfine and electron-Zeeman anisotropies (specifically including cross terms), spin-rotation interaction, and residual frequency-independent vibrational contributions from Raman processes and local modes. Cross relaxation from mutual electron and nuclear spin flips, and direct nitrogen nuclear spin–lattice relaxation, also must be included. Both the latter are further contributions from rotational modulation of the electron-nuclear dipolar interaction (END). All the conventional liquid-state mechanisms are defined fully by the spin-Hamiltonian parameters; only the vibrational contributions contain fitting parameters. This analysis gives a firm basis for interpreting SR (and inversion recovery) results in terms of additional, less standard mechanisms.

Giant Density of States Enhancement Driven by a Zero-Mode Landau Level in Semimetallic Black Phosphorus under Pressure.

Dirac fermion systems form a unique Landau level at the Fermi level-the so-called zero mode-whose observation itself will provide strong evidence of the presence of Dirac dispersions. Here, we report the study of semimetallic black phosphorus under pressure by ^{31}P-nuclear magnetic resonance measurements in a wide range of magnetic field up to 24.0T. We have found a field-induced giant enhancement of 1/T_{1}T, where 1/T_{1} is the nuclear spin lattice relaxation rate: 1/T_{1}T at 24.0T reaches more than 20 times larger than that at 2.0T. The increase in 1/T_{1}T above 6.5T is approximately proportional to the squared field, implying a linear relationship between the density of states and the field. We also found that, while 1/T_{1}T at a constant field is independent of temperature in the low-temperature region, it steeply increases with temperature above 100K. All these phenomena are well explained by considering the effect of Landau quantization on three-dimensional Dirac fermions. The present study demonstrates that 1/T_{1} is an excellent quantity to probe the zero-mode Landau level and to identify the dimensionality of the Dirac fermion system.