Field-cycling Nuclear Magnetic Resonance Research Articles

Lyotropic Aqueous 2-Picolinium Ionic Liquid Crystals and Their Shear-Induced Foams.

1-Dodecyl-2-methylpyridinium bromide ([C12-2-Pic][Br]) and 1-hexadecyl-2-methylpyridinium bromide ([C16-2-Pic][Br]) are two ionic liquid crystals presenting thermotropic smectic phases above 80 °C. Aiming to take advantage of the liquid crystalline properties at lower temperatures, lyotropic aqueous systems were prepared from these two organic salts. Both systems were characterized by polarized optical microscopy (POM), X-ray powder diffraction (XRD), and fast field cycling nuclear magnetic resonance (FFC-NMR) relaxometry to assess their texture, phase structure, and molecular dynamics, respectively. The mesomorphic behavior was induced at room temperature. Moreover, the lyotropic [C12-2-Pic][Br]aq revealed a smectic phase with higher separation between layers, different from the lamellar phases found in the thermotropic system (S1 and SA), which is thermally stable up to 50 °C. Furthermore, the surfactant nature of the ionic liquids diluted solutions investigated in this work allowed the formation of foams. It was found that the precursor solutions of the lyotropic dilutions with the longest alkyl chain ([C16-2-Pic][Br]aq) originated liquid foams with more stable structures than those of [C12-2-Pic][Br]aq.

Detecting soil hydrological connectivity in a badland area by fast field cycling nuclear magnetic resonance relaxometry

AbstractThe ‘hydrological connectivity inside the soil’ refers to both the spatial pattern inside the soil (structural component) and the physical–chemical process at a molecular level (functional component). Fast field cycling (FFC) nuclear magnetic resonance (NMR) relaxometry allows for measuring structural and functional connectivity by two suitable indexes named structural connectivity index (SCI) and functional connectivity index (FCI). In this study, FFC NMR relaxometry was applied to soils sampled in a very degraded environment (i.e., a badland area) to detect the capability of the measurement technique to distinguish the hydrological connectivity of these samples having different conditions (layer explored by roots, sparsely‐vegetated and bare soil). The relaxograms measured by the FFC NMR, using Proton Larmor frequencies in the range 0.01–10 MHz, were integrated and the resulting S‐shaped curves were analysed to obtain the connectivity indexes. Results showed that the ‘Sparsely vegetated’ sample is characterized by more small‐sized pores than the ‘Rooted’ one. The comparison between the ‘Sparsely vegetated’ and ‘Bare’ conditions pointed out that the presence of vegetation reduces the measured relaxation times and, as a consequence, the corresponding pore sizes and modifies the structural connectivity. The analysis also revealed that the three samples are characterized by similar values of SCI, which are independent of the proton Larmor frequency, while the FCI values of the ‘Bare’ soil are the lowest. Conversely, samples from soil with vegetation (‘Rooted’ and ‘Sparsely vegetated’) present comparable functional connectivity. Finally, the analysis of the frequency distribution of the ratio of each connectivity index and its mean value (SCI/m(SCI) and FCI/m(FCI)) allowed to establish its normal distribution. For the investigated samples, this result established that FCI and SCI can be represented by their mean value.

Exploring Crystal-Phase Molecular Dynamics of the Low-Viscous Mesogen 6CHBT: A Combined FFC and High-Field NMR Relaxometry Investigation.

The molecular dynamics study of thermotropic mesogens exhibiting the crystal phases is valuable in unraveling the complex global (collective) and local (noncollective) motions executed by liquid crystal molecules, which would further advance the existing knowledge on orientationally disordered crystalline (ODIC) phases. Toward the fulfillment of such a task, a combined nuclear magnetic resonance (NMR) relaxometry approach employing the fast field cycling (FFC) NMR (10 kHz-30 MHz) and high-field pulsed NMR (400 MHz) techniques is utilized to sample the broad frequency range offered by molecular motions in the crystal phase of 4-(trans-4'-n-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT). The validity of the observed relaxation data is tested and interpreted by the Bloembergen-Purcell-Pound (BPP) model involving the superposition of four mutually independent Lorentzian spectral densities, reflecting molecular dynamical processes on different time scales. The salient feature of the detailed analysis reveals that the lengthening of temporal dynamics in the crystal phase due to molecular rotations by jumps, which are of intermolecular origin, is evident and further supports the presence of collective-like local dynamics. The analysis does permit decoupling of the molecular reorientations about their short axes (∼100 ns) as well as long axes (∼50 ns) and methyl group rotations (∼0.5 ns) on distinct time scales. The activation energies for reorientations about the short axes and methyl group rotations are found to be 27.3 ± 2.7 and 15.8 ± 1.1 kJ/mol, respectively. The fast methyl rotations in the crystal phase of 6CHBT obtained from FFC NMR are further well complemented by high-field NMR, where 1H NMR line shapes are relatively narrow when compared to those of the nematic phase.

Vegan and sugar-substituted chocolates: assessing physicochemical characteristics by NMR relaxometry, rheology, and DSC

The main physicochemical characteristics of novel artisanal chocolates (both dark and milky) intended for vegan consumers or for those requiring assumption of fewer simple sugars, were analysed. Replacement of milk (with coconut copra, almonds, and soy protein isolates), and sucrose (with coconut sugars, stevia and erythritol, respectively) in dark chocolate, were accounted for by means of texture analysis, rheology, water activity, fatty acid composition, differential scanning calorimetry (DSC) and fast field cycling (FFC) nuclear magnetic resonance (NMR) relaxometry. The vegan sample (i.e., the milk-less one) showed lower values of hardness and adhesiveness as well as a larger peak in the melting behavior at the calorimetric evaluation (DSC). Moreover, the absence of milk resulted in the halving of the yield stress and a decrease in both the apparent and Casson’s viscosity. In the sample of chocolate with less sucrose, the peak temperatures measured at the DSC indicate crystallization of cocoa butter in its best form (Vβ2), unlike in dark chocolate, due to the different sugar composition. Similarly, the Casson yield stress (τ0), increased significantly (almost 70%), with the substitution of sugar. Finally, the results of NMR FFC relaxometry made it possible to identify aggregates of different sizes, laying the basis for its use as a rapid, non-destructive method for chocolate analysis.

NMR spectroscopy of a 18O-labeled rhodium paddlewheel complex: Isotope shifts, 103Rh-103Rh spin-spin coupling, and 103Rh singlet NMR.

Despite the importance of rhodium complexes in catalysis, and the favorable 100% natural abundance of the spin-1/2 103Rh nucleus, there are few reports of 103Rh nuclear magnetic resonance (NMR) parameters in the literature. In part, this is the consequence of the very low gyromagnetic ratio of 103Rh and its dismal NMR sensitivity. In a previous paper [Harbor-Collins et al., J. Chem. Phys. 159, 104 307 (2023)], we demonstrated an NMR methodology for 1H-enhanced 103Rh NMR and demonstrated an application to the 103Rh NMR of the dirhodium formate paddlewheel complex. In this paper, we employ selective 18O labeling to break the magnetic equivalence of the 103Rh spin pair of dirhodium formate. This allows the estimation of the 103Rh-103Rh spin-spin coupling and provides access to the 103Rh singlet state. We present the first measurement of a 18O-induced 103Rh secondary isotope shift as well as the first instance of singlet order generated in a 103Rh spin pair. The field-dependence of 103Rh singlet relaxation is measured by field-cycling NMR experiments.

Water Dynamics in Dextran-Based Hydrogel Micro/Nanoparticles Studied by NMR Diffusometry and Relaxometry.

Water dynamics in mesoporous dextran hydrogel micro/nanoparticles was investigated by means of nuclear magnetic resonance (NMR) techniques. High-resolution 1H NMR spectra and pulsed field gradient (PFG) NMR diffusometry measurements obtained on swollen state dextran micro/nanogel revealed the existence of different fractions of water molecules based on their interaction with the gel matrix. In addition to the translational diffusion of bulk water, two more diffusion processes characterized with self-diffusion coefficients 1 and 2 orders of magnitude smaller than that of bulk water were identified. 1H spin-lattice relaxation dispersion profiles obtained for a broad range of Larmor frequencies using fast field cycling (FFC) and conventional NMR relaxometry techniques allowed us to further clarify the mechanisms of molecular motion. According to the water proton pool fractions and associated self-diffusion coefficients, it is shown that the relaxation contribution associated with reorientation-mediated translational motions (RMTDs) dominates the relaxation dispersion observed at intermediate frequencies. At very low frequencies, the spin-lattice relaxation rate is dominated by the slow solid-gel dynamics probed by the water molecules interacting with the pores' surface hydroxyl groups due to the rapid chemical exchange between surface hydroxyl groups and free water. The correlation time for the thumbling-like motion of the dextran gel was found to be in the submillisecond range. The values of the self-diffusion and coherence lengths associated with motion of water molecules interacting with the solid-gel particles are consistent with the particle size and pore size distributions obtained for the studied dextran gels.

Broadband NMR Relaxometry as a Powerful Technique to Study Molecular Dynamics of Ionic Liquids.

Fast field cycling nuclear magnetic resonance (FFC NMR) relaxometry technique has been demonstrated to be a useful analytical tool to investigate molecular dynamics in very diverse systems during the last decades. Of particular importance has been its application in studying ionic liquids, upon which this review article is based. Some of the research carried out on ionic liquids during the last ten years using this technique is highlighted in this article with the aim of promoting the favorable features of FFC NMR applied toward understanding dynamics of complex systems.

Advanced insight on the water dynamics of anisotropic hydrogels by field-cycling nuclear magnetic resonance: Application of 3-Tau model

Fast field cycling (FFC) nuclear magnetic resonance (NMR) relaxometry is used to investigate an anisotropic polygalacturonate hydrogel formed by the diffusion of calcium ions from an external reservoir (external gelation). Such a hydrogel has a gradient of polymer density accompanied by a gradient of the mesh size of its 3D network. The NMR relaxation process is dominated by the interaction of proton spins between water molecules located at polymer interfaces and in nanoporous spaces. The FFC NMR experiment provides the spin-lattice relaxation rate R1ω as a function of Larmor frequency ω producing dispersion (NMRD) curves that are highly sensitive to the dynamics of the protons at the surfaces. The hydrogel is sliced into three parts and the NMR profile for each hydrogel slice is measured. The NMRD data for each slice is interpreted using the 3-Tau Model with the aid of user-friendly fitting software called 3TM. The key fit parameters include three nano-dynamical time constants and the average “mesh size” which collectively determine the bulk water and water surface layer contribution to the total relaxation rate. The results are consistent with independent studies where comparison is possible.

An automatic [formula omitted]-based regularization method for the analysis of FFC dispersion profiles with quadrupolar peaks

Fast Field-Cycling Nuclear Magnetic Resonance relaxometry is a non-destructive technique to investigate molecular dynamics and structure of systems having a wide range of applications such as environment, biology, and food. Besides a considerable amount of literature about modeling and application of such technique in specific areas, an algorithmic approach to the related parameter identification problem is still lacking. We believe that a robust algorithmic approach will allow a unified treatment of different samples in several application areas. In this paper, we model the parameters identification problem as a constrained L1-regularized non-linear least squares problem. Following the approach proposed in [Analytical Chemistry2021 93 (24)], the non-linear least squares term imposes data consistency by decomposing the acquired relaxation profiles into relaxation contributions associated with 1H−1H and 1H−14N dipole-dipole interactions. The data fitting and the L1-based regularization terms are balanced by the so-called regularization parameter. For the parameters identification, we propose an algorithm that computes, at each iteration, both the regularization parameter and the model parameters. In particular, the regularization parameter value is updated according to a Balancing Principle and the model parameters values are obtained by solving the corresponding L1-regularized non-linear least squares problem by means of the non-linear Gauss-Seidel method. We analyse the convergence properties of the proposed algorithm and run extensive testing on synthetic and real data. A Matlab software, implementing the presented algorithm, is available upon request to the authors.

A Relation between the Formation of a Hydrogen-Bond Network and a Time-Scale Separation of Translation and Rotation in Molecular Liquids.

We study the relation between the translational and rotational motions of liquids, which is anticipated in the framework of the Stokes-Einstein-Debye (SED) treatment. For this purpose, we exploit the fact that 1H field-cycling nuclear magnetic resonance relaxometry and molecular dynamics simulations provide access to both modes of motion. The experimental and computational findings are fully consistent and show that the time-scale separation between translation and rotation increases from the van der Waals liquid o-terphenyl over ethylene glycol to the hydrogen-bonded liquid glycerol, indicating an increasing degree of breakdown of the SED relation. Furthermore, the simulation results for two ethylene glycol models with different molecular conformations indicate that the translation is more retarded than the rotation when the density of intermolecular hydrogen bonds increases. We conclude that an increasing connectivity of a hydrogen-bond network leads to an increasing time-scale separation and, thus, to a stronger SED violation.

Changes in Physicochemical Properties of Biochar after Addition to Soil

It is recognized that biochar undergoes changes when it is applied to soils. However, the mechanisms of biochar alterations are not fully understood yet. To this purpose, the present study is designed to investigate the transformations in the soil of two different biochars obtained from pyrolysis of fir-wood pellets. The production of the biochars differed for the dry and wet quenching procedures used to terminate the pyrolysis. Both biochars were applied to clay soil (26% sand, 6% silt, and 68% clay) placed into lysimeters. After water saturation and 15 days of equilibration, seeds of watercress (Lepidium sativum) were cultivated. After a further 7 weeks, the biochars were manually separated from the systems. A total of four samples were collected. They were analyzed for chemical–physical characteristics by using an innovative technique referred to as fast field cycling nuclear magnetic resonance relaxometry. The results showed that the dry−quenching produced a material that was mainly chemically altered after application to soil compared to the biochar obtained by the wet−quenching. Indeed, the latter was both chemically and physically modified. In particular, results showed that water was better retained in the soil treated with the dry−quenched material. Consequently, we may suggest that crop productivity and environmental remediation may be modulated by applying either the dry−quenched or the wet−quenched biochar.

Broadband NMR relaxometry of electrolytes for energy storage

An increasing use of the fast field cycling nuclear magnetic resonance relaxometry technique to investigate dynamics in electrolytes for energy storage has been evidenced during the last decade. Therefore, this review article describes some of the research studies carried out on electrolytes during the last ten years using this technique. These studies include various types of liquid electrolytes, such as ionic liquids and deep eutectic solvents, semi-solid-state electrolytes, in particular, ionogels and polymer gels, and solid electrolytes such as glasses, glass ceramics, and polymers. An extended description of the different models used to explain the relaxation rate profiles is presented throughout this article.

Field-cycling 31P and 1H NMR relaxometry studying the reorientational dynamics of glass forming organophosphates.

We apply field-cycling (FC) 31P nuclear magnetic resonance (NMR) to access the reorientational susceptibility of two glass formers, m-tricresyl phosphate (m-TCP) and tri-butyl phosphate (TBP). Although FC 31P studies are still instrumentally demanding, together with FC 1H data, they provide site-resolved information. A crossover from dipolar relaxation at low frequencies to relaxation determined by chemical shift anisotropy at high frequencies and probed by conventional NMR is identified. A comparison is made between dielectric (DS) and depolarized light scattering (DLS) relaxation spectra demonstrating similar behavior close to Tg, including an excess wing contribution for m-TCP. The time constants of 31P NMR and DLS, probing the molecular core, agree. The 1H data monitoring the dynamics of the phenyl groups yield slightly shorter correlation times. At high temperatures, the DS relaxation spectra show a bimodal character: a fast component in agreement with 1H data, and a slow component much slower than 31P NMR and DLS suggest. We discuss the possible origins of the slow component. All time constants tend to merge toward Tg. Hence, we propose that site-specific dynamics disappear and a common α-relaxation establishes near Tg. In addition, we compare the diffusion coefficient D(T) determined by FC and static field gradient 1H NMR. Concerning TBP, we present FC 31P data of both α- and β-processes. Regarding the latter, we compare the DS and NMR susceptibility on absolute scale, yielding a significantly stronger β-relaxation in the 31P NMR spectra.

Nuclear Spin Relaxation in Viscous Liquids: Relaxation Stretching of Single-Particle Probes.

Spin-lattice relaxation rates R1(ω,T), probed via high-field and field-cycling nuclear magnetic resonance (NMR), are used to test the validity of frequency-temperature superposition (FTS) for the reorientation dynamics in viscous liquids. For several liquids, FTS is found to apply so that master curves can be generated. The susceptibility spectra are highly similar to those obtained from depolarized light scattering (DLS) and reveal an excess wing. Where FTS works, two approaches are suggested to access the susceptibility: (i) a plot of deuteron R1(T) vs the spin-spin relaxation rate R2(T) and (ii) a plot of R1(T) vs an independently measured reference time τref(T). Using single-frequency scans, (i) allows one to extract the relaxation stretching as well as the NMR coupling constant. Surveying 26 data sets, we find Kohlrausch functions with exponents 0.39 < βK ≤ 0.67. Plots of the spin-spin relaxation rate R2─rescaled by the NMR coupling constant─as a function of temperature allow one to test how well site-specific NMR relaxations couple to a given reference process. Upon cooling of flexible molecule liquids, the site-specific dynamics is found to merge, suggesting that near Tg the molecules reorient essentially as a rigid entity. This presents a possible resolution for the much lower stretching parameters reported here at high temperatures that contrast with the ones that were reported to be universal in a recent DLS study close to Tg. Our analysis underlines that deuteron relaxation is a uniquely powerful tool to probe single-particle reorientation.

Water Dynamics in Composite Aqueous Suspensions of Cellulose Nanocrystals and a Clay Mineral Studied through Magnetic Resonance Relaxometry.

1H spin-lattice relaxation time (T1) measurements were performed to probe the dynamic behavior of water in aqueous suspensions of cellulose nanocrystals (CNCs) and a layered smectite clay mineral with different degrees of concentration. 1H-T1 experiments were carried out over a wide frequency domain, ranging from a few kilohertz to 500 MHz, with the aid of conventional and fast field cycling nuclear magnetic resonance (NMR) techniques. The experimental relaxometry data illustrate differences between the dynamic behavior of bulk water and that confined in the vicinity of CNC-clay surfaces. Clay alone in moderate concentration was found to enforce almost no effect on the water dynamics, whereas introducing CNCs to the system presented a significantly enhanced relaxivity. The modeling of the relaxation dispersions allowed the determination of dynamical processes and variables explaining the dynamic behavior of water in CNC-clay suspensions. It turned out that reorientations mediated by translational displacements are a leading NMR relaxation mechanism for water interacting with the surfaces of CNC-clay particles in the low-frequency domain. In the high-frequency regime, however, the inner-sphere paramagnetic relaxation mechanism dominates, which is caused by the interaction of water protons with dissolved Fe ions.

Measuring hydrological connectivity inside soils with different texture by fast field cycling nuclear magnetic resonance relaxometry

The locution “hydrological connectivity inside the soil” is generally used to disclose how the spatial patterns inside the soil affect the physical–chemical processes at a molecular level to influence water transfer into the soil, the surface runoff and related sediment transport. Fast Field Cycling (FFC) Nuclear Magnetic Resonance (NMR) relaxometry has been used to measure both structural and functional connectivity by two indexes indicated as structural (SCI) and functional (FCI) connectivity index. Here, FFC-NMR relaxometry has been applied to analyze three samples: two non-degraded soils, having different grain-size distribution, and a degraded soil sampled in a badland area. Proton Larmor frequencies (νL) ranging in the 0.015–35 MHz interval were used on water suspended samples. The relaxograms (i.e., the distributions of the longitudinal relaxation times, T1) obtained by the FFC-NMR investigations were integrated. The resulting S-shaped curves were handled to obtain the aforementioned FCI and SCI parameters. Results showed that the empirical frequency distribution of the SCI index is related to soil texture and structure, while soils having similar SCI revealed different functional connectivity values. This approach allowed (i) to distinguish SCI values of degraded soils from those of non-degraded ones and (ii) to verify whether the structural connectivity index is related to soil texture and structure. The analysis also showed that the variation coefficient of SCI and FCI, achieved from the measurements done at different νL values, is minimum in the measurement range 0.015–1 MHz. The frequency of 1 MHz can be considered the optimal value to have accurate measured connectivity indexes. For the investigated samples, the obtained relationship between FCI and SCI allowed to conclude that high structural connectivity values correspond to a better functional connectivity. Finally, the hydrological connectivity inside a soil was defined as the sum of the two components representing structural and functional perspective.

Differentiation among dairy products by combination of fast field cycling NMR relaxometry data and chemometrics.

A set of commercial milk and Sicilian cheeses was analysed by a combination of fast field cycling (FFC) nuclear magnetic resonance (NMR) relaxometry and chemometrics. The NMR dispersion (NMRD) curves were successfully analysed with a mathematical model applied on Parmigiano-Reggiano (PR) cheese. Regression parameters were led back to the molecular components of cheeses (water trapped in casein micelles, proteins and fats) and milk samples (water belonging to hydration shells around dispersed colloidal particles of different sizes and bulk water). The application of chemometric analysis on relaxometric data enabled differentiating milk from cheeses and revealing differences within the two sample groups of either cheeses or milk samples. Marked differences among cheeses were evidenced by statistical analysis of the sole quadrupolar peaks parameters, suggesting that these contain information on the nature of the milk used during cheese production. Hence, combination of FFC NMR and chemometrics represents a powerful tool to investigate alterations in dairy products.

Unravelling Main- and Side-Chain Motions in Polymers with NMR Spectroscopy and Relaxometry: The Case of Polyvinyl Butyral.

Polyvinyl butyral (PVB) is an amorphous polymer employed in many technological applications. In order to highlight the relationships between macroscopic properties and dynamics at a microscopic level, motions of the main-chain and of the propyl side-chains were investigated between Tg − 288 °C and Tg + 55 °C, with Tg indicating the glass transition temperature. To this aim, a combination of solid state Nuclear Magnetic Resonance (NMR) methods was applied to two purposely synthesized PVB isotopomers: one fully protonated and the other perdeuterated on the side-chains. 1H time domain NMR and 1H field cycling NMR relaxometry experiments, performed across and above Tg, revealed that the dynamics of the main-chain corresponds to the α-relaxation associated to the glass transition, which was previously characterized by dielectric spectroscopy. A faster secondary relaxation was observed for the first time and ascribed to side-chains. The geometry and rate of motions of the different groups in the side-chains were characterized below Tg by 2H NMR spectroscopy.

The Field-Frequency Lock for Fast Field Cycling Magnetic Resonance: From NMR to MRI

Magnetic field stability plays a fundamental role in Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI) experiments, guaranteeing accuracy and reproducibility of results. While high levels of stabilization can be achieved for standard NMR techniques, this task becomes particularly challenging for Fast Field Cycling (FFC) NMR and MRI, where the main magnetic field is switched to higher or lower levels during the pulse sequence, and field stabilization must be guaranteed within a very short time after switching. Recent works have addressed the problem with rigorous tools from control system theory, proposing a model based approach for the synthesis of magnetic field controllers for FFC-NMR. While an experimental proof of concept has underlined the correctness of the approach for a complete FFC-NMR setup, the application of the novel, model based Field-Frequency Lock (FFL) system to a FFC-MRI scanner requires proper handling of field encoding gradients. Furthermore, the proof of concept work has also stressed how further advances in the hardware and firmware could improve the overall performances of the magnetic field control loop. The main aim of this perspective paper is then discussing the key challenges that arise in the development of the FFL system suitable for a complete MRI scanner, as well as defining possible research directions by means of preliminary, simulated experiments, with the final goal of favoring the development of a novel, model based FFL system for FFC-MRI.

Detection of Authenticity and Quality of the Turkish Delights (Lokum) by Means of Conventional and Fast Field Cycling Nuclear Magnetic Resonance Relaxometry.

Turkish delights (lokum) are traditional confectionery products that contain mainly sucrose as the sugar source and starch as the gelling agent. However, manufacturers sometimes might prefer to use corn syrup instead of sucrose to decrease the cost. This jeopardizes the originality of Turkish delights and leads to production of adulterated samples. In this study, Turkish delights were formulated using sucrose (original sample) and different types of corn syrups (SBF10, SCG40, and SCG60). Results clearly indicated that corn-syrup-containing samples had improved textural properties and were less prone to crystallization. However, this case affected authenticity of the products negatively. Both time domain nuclear magnetic resonance (TD NMR) and fast field cycling nuclear magnetic resonance (FFC NMR) techniques were found to be effective to discriminate the original samples from the corn-syrup-containing samples. In addition, quantitative analysis of FFC NMR showed that, apart from the rotational motions, molecules in Turkish delights (mainly water and also sugar molecules) undergo two types of translational dynamics.