Two-dimensional Exchange Spectroscopy Research Articles

Rigidified and Hydrophilic DOTA-like Lanthanoid Ligands: Design, Synthesis, and Dynamic Properties.

Limiting the dynamics of paramagnetic tags is crucial for the accuracy of the structural information derived from paramagnetic nuclear magnetic resonance (NMR) experiments. A hydrophilic rigid 2,2',2″,2‴-(1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetraacetic acid (DOTA)-like lanthanoid complex was designed and synthesized following a strategy that allows the incorporation of two sets of two adjacent substituents. This resulted in a C2 symmetric hydrophilic and rigid macrocyclic ring, featuring four chiral hydroxyl-methylene substituents. NMR spectroscopy was used to investigate the conformational dynamics of the novel macrocycle upon complexation with europium and compared to DOTA and its derivatives. The twisted square antiprismatic and square antiprismatic conformers coexist, but the former is favored, which is different from DOTA. Two-dimensional 1H exchange spectroscopy shows that ring flipping of the cyclen-ring is suppressed due to the presence of the four chiral equatorial hydroxyl-methylene substituents at proximate positions. The reorientation of the pendant arms causes conformational exchange between two conformers. The reorientation of the coordination arms is slower when the ring flipping is suppressed. This indicates that these complexes are suitable scaffolds to develop rigid probes for paramagnetic NMR of proteins. Due to their hydrophilic nature, it is anticipated that they are less likely to cause protein precipitation than their more hydrophobic counterparts.

Anion dynamics and motional decoupling in a glycerol-choline chloride deep eutectic solvent studied by one- and two-dimensional 35Cl NMR.

Chlorine-35 is among the few nuclides that provide an experimental handle on the anion dynamics in choline based deep eutectic solvents. By combining several nuclear magnetic resonance (NMR) techniques, the present work examines the Cl- motions within glyceline, a glycerol : choline chloride 2 : 1 solution, in a large temperature range down to the glass transition temperature Tg. The applied methods include spin relaxometry, second-order line shape analysis, as well as two-dimensional central-transition exchange and stimulated-echo spectroscopy. The finding of unstructured central-transition NMR spectra characterized by a relatively small average quadrupolar coupling attests to a highly disordered, essentially nondirectional anionic coordination in glyceline. For temperatures larger than about 1.3Tg the chlorine motions are well coupled to those of the glycerol and the choline moieties. At lower temperatures the local translational anion dynamics become Arrhenian and increasingly faster than the motion of glyceline's matrix molecules. Upon further cooling, the overall ionic conductivity continues to display a super-Arrhenius behavior, implying that the choline cations rather than the Cl anions dominate the long-range charge transport also near Tg.

Spectral, structural, and pharmacological studies of perillaldehyde and myrtenal based benzohydrazides

N-Acylhydrazones have been an interesting scaffold due to their diverse biological activities toward different therapeutic targets. We synthesized seven new N'‐[(E)‐[(4S)‐4‐(prop‐1‐en‐2‐yl)cyclohex‐1‐en‐1‐yl]methylidene]benzohydrazides as well as seven new N'-[(E)-{6,6-dimethylbicyclo[3.1.1]hept-2-en-2-yl}methylidene]benzohydrazides. The structure of synthesized hydrazides has been studied by using X-ray analysis, FT-IR, NMR, and HRMS spectra. Derivatives containing fluorine atoms in position 2 of the phenyl ring display versatile structures in solution and the solid state. The 1H NMR spectra of 3c and 5c measured in DMSO-d6 display at ambient temperature two unequally populated sets of signals. These facts confirm the existence of the two conformers of 3c and 5c of different stability which interconvert by rotation about their C―N bond slowly enough to lead to discrete sets of signals in 1H NMR spectra at room temperature. The kinetics of the conformational interconversion of derivatives 3c and 5c in DMSO-d6 were studied by two-dimensional exchange spectroscopy (2D EXSY). In 1H NMR spectra of 3c and 5c measured in CDCl3 and acetone-d6 was observed the strong splitting of the H-2 signal into a doublet because of H-bond formation between organic fluorine and NH donor. The FT–IR spectra of 3a–g and 5a–g confirmed the presence of the H-bonds N–H···O=C and N–H···N. Hydrazones 3f, 5a, and 5c crystallize in the P1 space group and the asymmetric units contain two pairs of pseudo-centrosymmetric molecules. Hydrazone 5b crystallizes in the P41 space group and the asymmetric units contain only one molecule. In the crystal, the molecules in 3f, 5a and 5b are linked through N―H···O hydrogen bonds, in 5c also N―H···N hydrogen bonds are present. In contrast to the NMR, the X-ray and IR spectra did not give unambiguous evidence of the F···H−N H-bond. The synthesized hydrazones 3c and 3g exhibited excellent antiproliferative effects in Jurkat cells with IC50 values of 3.8 and 4.2 μM, respectively. Our findings indicate that these compounds are promising antitumor agents.

Microsized Pore Structure Determination in EPDM Rubbers Using High-Pressure 129Xe NMR Techniques.

Microsized pore parameters, such as pore size and distance between pores in a series of model EPDM rubbers, were determined in situ under the pressure of 500 psi using 129Xe nuclear magnetic resonance (NMR) techniques: spin-lattice (T1) and spin-spin (T2) relaxation measurements, pulsed-field gradient (PFG) NMR, and two-dimensional exchange spectroscopy (2D EXSY). The T1/T2 (≫1) ratio for the xenon confined in the pores is larger than that for nonconfined free xenon. This suggests that almost the entire pore surface interacts with xenon atoms like a closed pore. While these pores still connect each other through very narrow diffusion/exchange channels, it is possible to observe the echo decay in PFG-NMR and cross-peaks in 2D EXSY. The results show that both diffusion (Dpore ≈ 2.1 × 10-10 m2/s) and exchange (exchange rate, τexch = a few tens of milliseconds) of xenon between a pore within the material and outer surface are prolonged. The exchange distances (l), which correspond to the xenon gas penetration depth, were estimated to be 70-100 μm based on the measured diffusion coefficients and exchange rate (1/τexch). NMR diffraction analysis reveals that pore size (a) and pore distance (b) are on the order of magnitude of micrometers and tens of micrometers, while the diffusion coefficients of xenon gas in the diffusion channels (Deff) are about 10-8 m2/s. Overall, this study suggests that the pores with a few micrometers connected through very narrow flowing channels with the length of several tens of micrometers are developed 70 to 100 μm below the rubber surface. Furthermore, the overall steady-state diffusion of xenon is slower, approximately 2 orders of magnitudes, than the diffusion in the channel between the pores. The pore and exchange distances correlated with the composition of rubbers showed that the properties of EPDM rubber as a high-pressure gas barrier could be improved by reducing the size of cracks and the depth of gas penetration by the addition of both carbon black and silica fillers.

Direct observation of reversible bond homolysis by 2D EXSY NMR.

Bond homolysis is one of the most fundamental bond cleavage mechanisms. Thus, understanding of bond homolysis influences the development of a wide range of chemistry. Photolytic bond homolysis and its reverse process have been observed directly using time-resolved spectroscopy. However, direct observation of reversible bond homolysis remains elusive. Here, we report the direct observation of reversible Co–Co bond homolysis using two-dimensional nuclear magnetic resonance exchange spectroscopy (2D EXSY NMR). The characterization of species involved in this homolysis is firmly supported by diffusion ordered NMR spectroscopy (DOSY NMR). The unambiguous characterization of the Co–Co bond homolysis process enabled us to study ligand steric and electronic factors that influence the strength of the Co–Co bond. Understanding of these factors will contribute to rational design of multimetallic complexes with desired physical properties or catalytic activity.

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.

Insights into the microstructure and interconnectivity of porosity in porous starch by hyperpolarized 129Xe NMR

For the first time, hyperpolarized (HP) 129Xe NMR measurements are utilized to explore porous structures of porous starch (PS) successfully. Some micropores resided inside the mesopore walls of PS were detected by variable temperature (VT) HP 129Xe NMR, and the pore sizes of micropores were also estimated using the empirical relationship. Furthermore, the interconnectivity of pores was investigated in detail by two-dimensional (2D) exchange spectroscopy (EXSY). The exchange process of xenon from microporosity within pore walls to the free gas space was occurred at the mixing time of ≥12 ms at 173 K, which indicated the well interconnectivity between micropores and mesopores. This study not only exhibits a new approach for investigation of pores and hollows of PS, but also provides a better understanding of porous structures for rational design in adsorbing functional compounds.

Reorientational dynamics of trimethoxyboroxine: A molecular glass former studied by dielectric spectroscopy and 11B nuclear magnetic resonance.

In this work, trimethoxyboroxine (TMB) is identified as a small-molecule glass former. In its viscous liquid as well as glassy states, static and dynamic properties of TMB are explored using various techniques. It is found that, on average, the structure of the condensed TMB molecules deviates from threefold symmetry so that TMB's electric dipole moment is nonzero, thus rendering broadband dielectric spectroscopy applicable. This method reveals the super-Arrhenius dynamics that characterizes TMB above its glass transition, which occurs at about 204 K. To extend the temperature range in which the molecular dynamics can be studied, 11B nuclear magnetic resonance experiments are additionally carried out on rotating and stationary samples: Exploiting dynamic second-order shifts, spin-relaxation times, line shape effects, as well as stimulated-echo and two-dimensional exchange spectroscopy, a coherent picture regarding the dynamics of this glass former is gained.

On the Differential Roles of Mg2+, Zn2+, and Cu2+ in the Equilibrium of β-N-Methyl-Amino-L-Alanine (BMAA) and its Carbamates.

β-N-methyl-amino-L-alanine (BMAA) in the presence of bicarbonate (HCO3-) undergoes structural modifications generating two carbamate species, α-carbamate and β-carbamate forms of BMAA. The chemical structure of BMAA and BMAA-carbamate adducts strongly suggest they may interact with divalent metal ions. The ability of BMAA to cross the blood-brain barrier and possibly interact with divalent metal ions may augment the neurotoxicity of these molecules. To understand the effects of divalent metal ions (Mg2+, Zn2+, and Cu2+) on the overall dynamic equilibrium between BMAA and its carbamate adducts, a systematic study using nuclear magnetic resonance (NMR) is presented. The chemical equilibria between BMAA, its carbamate adducts, and each of the divalent ions were studied using two-dimensional chemical exchange spectroscopy (EXSY). The NMR results demonstrate that BMAA preferentially interacts with Zn2+ and Cu2+, causing an overall reduction in the production of carbamate species by altering the dynamic equilibria. The NMR-based spectral changes due to the BMAA interaction with Cu2+ is more drastic than with the Zn2+, under the same stoichiometric ratios of BMAA and the individual divalent ions. However, the presence of Mg2+ does not significantly alter the dynamic equilibria between BMAA and its carbamate adducts. The NMR-based results are further validated using circular dichroism (CD) spectroscopy, observing the n ➔ π interaction in the complex formation of BMAA and the divalent metal ions, with additional verification of the interaction with Cu2+ using UV-Vis spectroscopy. Our results demonstrate that BMAA differentially interacts with divalent metal ions (Mg2+ < Zn2+ < Cu2+), and thus alters the rate of formation of carbamate products. The equilibria between BMAA, the bicarbonate ions, and the divalent metal ions may alter the total population of a specific form of BMAA-ion complex at physiological conditions and, therefore, add a level of complexity of the mechanisms by which BMAA acts as a neurotoxin.

Anomerisation of Fluorinated Sugars by Mutarotase Studied Using 19F NMR Two-Dimensional Exchange Spectroscopy

Five 19F-substituted glucose analogues were used to probe the activity and mechanism of the enzyme mutarotase by using magnetisation-exchange NMR spectroscopy. The sugars (2-fluoro-2-deoxy-d-glucose, FDG2; 3-fluoro-3-deoxy-d-glucose, FDG3; 4-fluoro-4-deoxy-d-glucose, FDG4; 2,3-difluoro-2,3-dideoxy-d-glucose, FDG23; and 2,2,3,3-tetrafluoro-2,3-dideoxy-d-glucose (2,3-dideoxy-2,2,3,3-tetrafluoro-d-erythro-hexopyranose), FDG2233) showed separate 19F NMR spectroscopic resonances from their respective α- and β-anomers, thus allowing two-dimensional exchange spectroscopy measurements of the anomeric interconversion at equilibrium, on the time scale of a few seconds. Mutarotase catalysed the rapid exchange between the anomers of FDG4, but not the other four sugars. This finding, combined with previous work identifying the mechanism of the anomerisation by mutarotase, suggests that the rotation around the C1–C2 bond of the pyranose ring is the rate-limiting reaction step. In addition to d-glucose itself, it was shown that all other fluorinated sugars inhibited the FDG4 anomerisation, with the tetrafluorinated FDG2233 being the most potent inhibitor. Inhibition of mutarotase by F-sugars paves the way for the development of novel fluorinated compounds that are able to affect the activity of this enzyme invitro and invivo.

Investigating the Dynamic Behavior of Zinc Oxide Discharge Product in Rechargeable Zinc Electrodes

Rechargeable zinc-alkaline batteries are attractive alternatives to established battery technologies such as lithium-ion and lead-acid in certain applications because of their inherent low cost, safety, and environmental friendliness. Commercialization of zinc-alkaline batteries has been largely unsuccessful because of the limited cycle life of zinc electrodes at high depth of discharge. At high depths of discharge, dissolved zinc precipitates from the electrolyte as solid zinc oxide. Zinc oxide buildup causes passivation of zinc metal particles, which increases resistance within the battery and results in eventual cell death. It can also electrically isolate metal particles from the current collector, resulting in loss of active zinc mass. Zinc oxide in alkaline batteries has been studied for decades, but an understanding of the properties of the material and effects on battery performance is still not complete. A complete understanding of the discharge products of zinc electrodes is a necessary step toward wide-spread commercialization of zinc metal electrodes. We demonstrate in this work that zinc oxide produced during discharge undergoes chemical changes as a function of electrode potential. In operando optical microscopy shows that the oxide reversibly changes color from blue at more negative zinc electrode potentials to white at more positive zinc electrode potentials. In operando Raman spectroscopy shows differences in Raman scattering between the blue and the white oxide species, indicating that additional bonds are present in the blue material, which are not observed in the white material. Solid-state 1H magic angle spinning NMR spectroscopy on extracted oxide material suggests that changes to electrode potential result in simultaneous insertion or disinsertion of protons and electrons into the oxide material. More immobile species are present in the blue material compared to the white, indicating inserted protons may be more abundant in the blue oxide. Additionally, two-dimensional 1H-1H exchange spectroscopy shows more chemical exchange between sites in the white material compared to the blue, indicating protons may be more strongly bound in the blue oxide. The behavior observed in these studies is similar to that observed for electrochromic materials. In electrochromic materials, inserted electrons can increase the electrical conductivity of the oxide by orders of magnitude. At more positive zinc potentials, an increase in the electrical resistance of the oxide may result in conductivity losses between zinc metal particles. Additionally, when protons are disinserted from the oxide, they result in a local pH decrease in the electrolyte surrounding the oxide, which affects the ionic conductivity of the electrolyte. The work outlined in this study demonstrates the effects of this behavior on electrode performance and explores avenues for controlling the behavior through electrode additives and operation controls. Figure 1

Formation Constants and Conformational Analysis of Carbamates in Aqueous Solutions of 2-Methylpiperidine and CO2 from 283 to 313 K by NMR Spectroscopy.

Quantitative 13C nuclear magnetic resonance (NMR) spectroscopy was used to investigate the speciation in (2-methylpiperidine + H2O + CO2) systems at 283.2-313.2 K. The carbamate of 2-methylpiperidine(2-methylpiperidine- N-carboxylate) was shown for the first time to be a stable species in aqueous solutions. The spectroscopic results were used to obtain temperature-dependant formation constants for the carbamate using a simplified model for the activity coefficients from which the standard molar enthalpy of reaction was estimated. The results were incorporated into a self-consistent chemical equilibrium model, which includes vapor-liquid equilibria and all aqueous species, including the formation of carbamate. The predominant conformation of the sterically hindered carbamate, which was determined using two-dimensional exchange spectroscopy NMR, has the methyl group in the axial orientation and is in agreement with the density functional theory quantum chemical calculations.

An Electric Trap: Electron‐Rich Carbonyl Axis Ends Slow Threading/Dethreading Exchange Dynamics of Pillar[5]arene Ring along Axis

AbstractViologen axes with negative carbonyl ends such as carboxylic acid and carbonylethyl groups were synthesized. The threading/dethreading exchange dynamics of the pillar[5]arene ring and these axes were studied by variable‐temperature 1H NMR and two‐dimensional exchange spectroscopy NMR spectroscopies. The exchange rate increased on introduction of electron‐rich carbonyl ends into the axis: the exchange rate constants of the pillar[5]arene ring along the carboxylic acid‐ended and carbonylethyl‐ended viologen axes were ca. 2000 and 360 times smaller, respectively, than that along the alkyl‐ended viologen axis.

Zwitterionic Rhodium and Iridium Complexes Based on a Carboxylate Bridge-Functionalized Bis-N-heterocyclic Carbene Ligand: Synthesis, Structure, Dynamic Behavior, and Reactivity.

A series of water-soluble zwitterionic complexes featuring a carboxylate bridge-functionalized bis-N-heterocyclic carbene ligand of formula [Cp*MIIICl{(MeIm)2CHCOO}] and [MI(diene){(MeIm)2CHCOO}] (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl; M = Rh, Ir; MeIm = 3-methylimidazol-2-yliden-1-yl; diene = 1,5-cyclooctadiene (cod), norbornadiene (nbd)) were prepared from the salt [(MeImH)2CHCOO]Br and suitable metal precursor. The solid-state structure of both types of complexes shows a boat-shaped six-membered metallacycle derived of the κ2C,C' coordination mode of the bis-NHC ligand. The uncoordinated carboxylate fragment is found at the bowsprit position in the Cp*MIII complexes, whereas in the MI(diene) complexes it is at the flagpole position of the metallacycle. The complexes [RhI(diene){(MeIm)2CHCOO}] (diene = cod, nbd) exist as two conformational isomers in dichloromethane, bowsprit and flagpole, that interconvert through the boat-to-boat inversion of the metallacycle. An inversion barrier of ∼17 kcal·mol-1 was determined by two-dimensional exchange spectroscopy NMR measurements for [RhI(cod){(MeIm)2CHCOO}]. Reaction of zwitterionic Cp*MIII complexes with methyl triflate or tetrafluoroboric acid affords the cationic complexes [Cp*MIIICl{(MeIm)2CHCOOMe}]+ or [Cp*MIIICl{(MeIm)2CHCOOH}]+ (M = Rh, Ir) featuring carboxy and methoxycarbonyl functionalized methylene-bridged bis-NHC ligands, respectively. Similarly, complexes [MI(diene){(MeIm)2CHCOOMe}]+ (M = Rh, Ir) were prepared by alkylation of the corresponding zwitterionic MI(diene) complexes with methyl triflate. In contrast, reaction of [IrI(cod){(MeIm)2CHCOO}] with HBF4·Et2O (Et = ethyl), CH3OTf, CH3I, or I2 gives cationic iridium(III) octahedral complexes [IrIIIX(cod){(MeIm)2CHCOO}]+ (X = H, Me, or I) featuring a tripodal coordination mode of the carboxylate bridge-functionalized bis-NHC ligand. The switch from κ2C,C' to κ3C,C',O coordination of the bis-NHC ligand accompanying the oxidative addition prevents the coordination of the anions eventually formed in the process that remain as counterions.

Restricted amide rotation with steric hindrance induced multiple conformations

The CN bond character is dependent directly upon the resonance-contributor structure population driven by the delocalized nitrogen lone-pair of electrons. In the case of N, N-dibenzyl-ortho-toluamide (o-DBET), the molecule adopts subpopulations of conformers with distinct NMR spectral features, particularly at low temperatures. This conformational adaptation is unique to o-DBET, while the corresponding meta- and para- forms do not show such behavior. Variable-temperature (VT) NMR, two-dimensional exchange spectroscopy (EXSY), and qualitative molecular modeling studies are used to demonstrate how multiple competing interactions such as restricted amide rotation and steric hindrance effects can lead to versatile molecular adaptations in the solution state.

Molecular Dynamics Study of Nitrogen-Pyramidalized Bicyclic β-Proline Oligomers: Length-Dependent Convergence to Organized Structures.

In this study, the solution structures of the homooligomers of a conformationally constrained bicyclic proline-type β-amino acid were studied by means of molecular dynamics (MD) calculations in explicit methanol and water using the umbrella sampling method. The ratio of trans-amide and cis-amide was estimated by NMR and the rotational barrier of the amide of acetylated bicyclic amino acid monomer was estimated by two-dimensional (2D) exchange spectroscopy (EXSY) or line-shape analysis. A bias potential was introduced with respect to the amide torsion angle ω to enhance conformational exchange including isomerization of amide bonds by lowering the rotation energy barrier. After determination of reweighting parameters to best reproduce the experimental results of the monomer amide, the free energy profile around the amide torsion angle ω was obtained from the MD trajectory by reweighting of the biased probability density. The MD simulation results support the existence of invertomers of nitrogen-pyramidalized amide. Furthermore, extended structures with a high fraction of trans-amide conformation appear to be increasingly stabilized as the oligomer is elongated, both in methanol and in water. Our conformational analysis of natural and non-natural tertiary-amide-based peptide oligomers indicates that these oligomers preferentially adopt a limited number of conformations.

Ex Situ 23Na Solid-State NMR Reveals the Local Na-Ion Distribution in Carbon-Coated Na2FePO4F during Electrochemical Cycling

The potential Na-ion cathode material Na2FePO4F is investigated here by ex situ 23Na solid-state nuclear magnetic resonance (ssNMR) in order to characterize the structure and ion mobility as a function of electrochemical cycling. The use of fast magic angle spinning (MAS) speeds of 65 kHz allows for the collection of high-resolution 23Na NMR spectra that reveal two unique peaks at +450 and −175 ppm, corresponding to the two crystallographically unique Na sites in the material of interest. Two-dimensional NMR exchange spectroscopy results reveal that chemical exchange between the Na ions residing in distinct environments has a maximum hopping rate of ∼200 Hz. The collection of one-dimensional NMR spectra as a function of electrochemical cycling reveals the reproducible formation of a new peak at +320 ppm in the 23Na NMR spectrum at all intermediate states of charge. The appearance of this resonance at +320 ppm is attributed to the fully oxidized (NaFePO4F) phase that is present even upon initial electroche...

Interchannel Hopping in Single Crystalline Lithium Triborate Probed by 7Li NMR: Spin Relaxation, Line Shape Analysis, Selective-Inversion Spin Alignment, and Two-Dimensional Exchange Spectra

The ion dynamics of single crystalline lithium triborate, LiB3O5, an important material in nonlinear optics, is studied using various 7Li and 11B nuclear magnetic resonance (NMR) techniques at temperatures from about 480 to 780 K in order to elucidate the apparent discrepancies underlying previous interpretations of NMR line shape analyses and results from dielectric spectroscopy. Rotating frame spin–lattice relaxation as well as line shape measurements are carried out and are combined with selective-inversion spin alignment as well as two-dimensional chemical exchange spectroscopy to track the temperature-dependent Li ion motion. From symmetry considerations the latter is clearly identified as interchannel hopping. By combining the present results with those from the published, yet so far not fully analyzed, dielectric loss spectra, it is shown how seeming differences in energy barriers hindering the ion motion and in the evolution of the distribution of correlation times can be reconciled.

Dynamic UltraFast 2D EXchange SpectroscopY (UF-EXSY) of hyperpolarized substrates

In this work, we present a new ultrafast method for acquiring dynamic 2D EXchange SpectroscopY (EXSY) within a single acquisition. This technique reconstructs two-dimensional EXSY spectra from one-dimensional spectra based on the phase accrual during echo times. The Ultrafast-EXSY acquisition overcomes long acquisition times typically needed to acquire 2D NMR data by utilizing sparsity and phase dependence to dramatically undersample in the indirect time dimension. This allows for the acquisition of the 2D spectrum within a single shot. We have validated this method in simulations and hyperpolarized enzyme assay experiments separating the dehydration of pyruvate and lactate-to-pyruvate conversion. In a renal cell carcinoma cell (RCC) line, bidirectional exchange was observed. This new technique revealed decreased conversion of lactate-to-pyruvate with high expression of monocarboxylate transporter 4 (MCT4), known to correlate with aggressive cancer phenotypes. We also showed feasibility of this technique in vivo in a RCC model where bidirectional exchange was observed for pyruvate–lactate, pyruvate–alanine, and pyruvate–hydrate and were resolved in time. Broadly, the technique is well suited to investigate the dynamics of multiple exchange pathways and applicable to hyperpolarized substrates where chemical exchange has shown great promise across a range of disciplines.

NMR detection of dynamical processes in antiferroelectric nanoclusters during the order-disorder transition inNH4H2AsO4

We study the dynamics of inorganic antiferroelectric nanoclusters formed during an order-disorder transition and demonstrate the coexistence of the two phases in a region of 2--3 K around the transition temperature ${T}_{N}\ensuremath{\sim}215$ K. Single crystals of ${\mathrm{NH}}_{4}{\mathrm{H}}_{2}{\mathrm{AsO}}_{4}$, a model hydrogen-bonded compound, show an antiferroelectric-paraelectric transition studied by means of highly sensitive magic angle spinning ${}^{15}\mathrm{N}$ NMR at 21.1 T. The phase coexistence is demonstrated by a double-peak structure of the chemical shift. Two-dimensional chemical exchange spectroscopy and spin-lattice relaxation time $({T}_{1})$ measurements show that the clusters are dynamic with sizes $\ensuremath{\sim}50$ nm and lifetimes approaching seconds as $T\ensuremath{\rightarrow}{T}_{N}$. Their occupancy increases rapidly to fill the crystal volume below ${T}_{N}$. This study provides evidence for the commonality of the phase transitions in systems with electric properties and provides an improved spectroscopic method for such studies.