Residual Quadrupolar Coupling Research Articles

Cadmium-rich intermetallic phases ARh2Cd20 - structure, magnetic behavior, 151Eu Mössbauer and 113Cd solid-state NMR spectroscopy.

The cadmium-rich intermetallic compounds ARh2Cd20 (A = Ca, Sr, Y, La-Nd, Sm-Lu) were synthesized from the elements in sealed tantalum tubes. The elements were reacted in an induction furnace and the samples were post-annealed to increase phase purity and crystallinity. The ARh2Cd20 phases crystallize with the cubic CeCr2Al20 type structure, space group Fd3̄m. The polycrystalline samples were characterized by X-ray powder diffraction. The structures of SrRh2Cd20, LaRh2Cd20, CeRh2Cd20, TbRh2Cd20 and DyRh2Cd20 were refined from X-ray single crystal diffractometer data. Temperature dependent magnetic susceptibility data show diamagnetism for CaRh2Cd20, SrRh2Cd20, YRh2Cd20, LaRh2Cd20 and YbRh2Cd20, thus substantiating stable divalent ytterbium in the latter phase. The remaining phases are Curie Weiss paramagnets. EuRh2Cd20 contains stable divalent europium and orders ferromagnetically at TC = 13.8 K. Antiferromagnetic ordering was detected for SmRh2Cd20 (TN = 4.3 K), GdRh2Cd20 (TN = 9.3K), TbRh2Cd20 (TN = 6.6 K) and DyRh2Cd20 (TN = 3.9 K). TbRh2Cd20 and DyRh2Cd20 exhibit metamagnetic transitions at critical fields of 19 respectively 10 kOe. The divalent ground state in EuRh2Cd20 was also confirmed by a 151Eu Mössbauer spectrum which shows an isomer shift of δ = -10.86(1) mm s-1. Solid-state NMR spectroscopy was performed for the samples YRh2Cd20, LaRh2Cd20, and YbRh2Cd20. The 113Cd NMR spectra include three distinct Cd sites for each sample in accordance with the crystallographic data. All samples further show negative Knight shifts for 103Rh, suggesting high s-d exchange interaction at the Rh site. In the case of 139La, a residual quadrupolar coupling was observed despite cubic site symmetry, confirming the existence of local defect sites. 89Y and 171Yb NMR spectra were recorded, the latter confirming the divalent nature of Yb in YbRh2Cd20.

Multiple quantum filtered nuclear magnetic resonance of 23Na+ in uniformly stretched and compressed hydrogels.

Stretching or compressing hydrogels creates anisotropic environments that lead to motionally averaged alignment of embedded guest quadrupolar nuclear spins such as 23Na+. These distorted hydrogels can elicit a residual quadrupolar coupling that gives an oscillation in the trajectories of single quantum coherences (SQCs) as a function of the evolution time during a spin-echo experiment. We present solutions to equations of motion derived with a Liouvillian superoperator approach, which encompass the coherent quadrupolar interaction in conjunction with relaxation, to give a full analytical description of the evolution trajectories of rank-1 (T^1±1), rank-2 (T^2±1), and rank-3 (T^3±1) SQCs. We performed simultaneous numerical fitting of the experimental 23Na nuclear magnetic resonance (NMR) spectra and rank-2 (T^2±1) and rank-3 (T^3±1) SQC evolution trajectories measured in double and triple quantum filtered experiments, respectively. We estimated values of the quadrupolar coupling constant CQ, rotational correlation time τC, and 3 × 3 Saupe order matrix. We performed simultaneous fitting of the analytical expressions to the experimental data to estimate values of the quadrupolar coupling frequency ωQ/2π, residual quadrupolar coupling ωQ/2π, and corresponding spherical order parameter S0*, which showed a linear dependence on the extent of uniform hydrogel stretching and compression. The analytical expressions were completely concordant with the numerical approach. The insights gained here can be extended to more complicated (biological) systems such as 23Na+ bound to proteins or located inside and outside living cells in high-field NMR experiments and, by extension, to the anisotropic environments found in vivo with 23Na magnetic resonance imaging.

Cover Feature: Structure Elucidation of Lithium Compounds Using 7Li Residual Quadrupolar Couplings (Chem. Eur. J. 23/2023)

Shedding light on the structure and solvation state of lithium compounds in aligned solution: The residual quadrupolar coupling of 7Li in a range of lithium complexes due to the interaction of the electric field gradient (shown in its graphical tensor form) with the alignment in a stretched polymer gel has been investigated using NMR spectroscopy. The preferred orientation of the molecules in the anisotropic environment is illustrated in the background. More information can be found in the Research Article by M. John and co-workers (DOI: 10.1002/chem.202203995).

Magnetically aligned nanodiscs enable direct measurement of 17O residual quadrupolar coupling for small molecules

The use of 17O in NMR spectroscopy for structural studies has been limited due to its low natural abundance, low gyromagnetic ratio, and quadrupolar relaxation. Previous solution 17O work has primarily focused on studies of liquids where the 17O quadrupolar coupling is averaged to zero by isotropic molecular tumbling, and therefore has ignored the structural information contained in this parameter. Here, we use magnetically aligned polymer nanodiscs as an alignment medium to measure residual quadrupolar couplings (RQCs) for 17O-labelled benzoic acid in the aqueous phase. We show that increasing the magnetic field strength improves spectral sensitivity and resolution and that each satellite peak of the expected pentet pattern resolves clearly at 18.8 T. We observed no significant dependence of the RQC magnitudes on the magnetic field strength. However, changing the orientation of the alignment medium alters the RQC by a consistent factor, suggesting that 17O RQCs measured in this way can provide reliable orientational information for elucidations of molecular structures.

Molecular enantiodiscrimination by NMR spectroscopy in chiral oriented systems: Concept, tools, and applications.

The study of enantiodiscriminations in relation to various facets of enantiomorphism (chirality/prochirality) and/or molecular symmetry is an exciting area of modern organic chemistry and an ongoing challenge for nuclear magnetic resonance (NMR) spectroscopists who have developed many useful analytical approaches to solve stereochemical problems. Among them, the anisotropic NMR using chiral aligning solvents has provided a set of new and original tools by making accessible all intramolecular, order-dependent NMR interactions (anisotropic interactions), such as residual chemical shift anisotropy (RCSA), residual dipolar coupling (RDC), and residual quadrupolar coupling (RQC) for spin I>1/2, while preserving high spectral resolution. The force of NMR in enantiopure, oriented solvents lies on its ability to orient differently in average on the NMR timescale enantiomers of chiral molecules and enantiotopic elements of prochiral ones, leading distinct NMR spectra or signals to be detected. In this compendium mainly written for all chemists playing with (pro)chirality, we overview various key aspects of NMR in weakly aligning chiral solvents as the lyotropic liquid crystals (LLCs), in particular those developed in France to study (pro)chiral compounds in relation with chemists needs: study of enantiopurity of mixture, stereochemistry, natural isotopic fractionation, as well as molecular conformation and configuration. Key representative examples covering the diversity of enantiomorphism concept, and the main and most recent applications illustrating the analytical potential of this NMR in polypeptide-based chiral liquid crystals (CLCs) are examined. The latest analytical strategy developed to determine in-solution conformational distribution of flexibles solutes using NMR in polypeptide-based aligned solvents is also proposed.

New experimental observations of the behavior of sodium ions in saturated rock samples.

1H NMR relaxometry of saturated rock samples has become a useful tool for the characterization of porosity and transport phenomena of enclosed fluids. The pore size can be measured using the difference between inverse relaxation values of protons absorbed by the saturated rock and that present in the bulk fluids. These experiments are usually performed at low magnetic fields to reduce the influence of the diffusion on the relaxation values in the presence of Internal Gradient Fields. Recently, sodium ions have become objects of investigation. The main advantage of sodium ions over protons as measured spins in the Petrophysic NMR experiments is their presence in water and not other phases like oil and gas. However unlike protons, sodium ions can have slow motion properties like appearance of the bi-exponential relaxation and residual quadrupolar distribution, which can lead to complex behavior of spins inside the pores. Here, we describe eight 23Na NMR experiments at 9.39 T external magnetic field, in which we have investigated the behavior of sodium ions in 4 saturated rock samples: Berea 500, Fontainebleau 1, Bentheimer sandstones, and Austin chalk. We show that the reduction of spin-spin relaxation is caused by anisotropic motion and not diffusion in the presence of Internal Gradient Fields. There can be a link between free diffusional and motional averaging regimes regardless of the size of environment in which the measured spin ions are. Using different NMR sequences, we reveal and quantitatively describe bi-exponentially relaxing spins and spins with residual quadrupolar coupling. This work demonstrates unique model for the behavior of ions inside porous media, which is different than known models (Brownstein-Tarr model and "agarose gel model").

Investigating Water Interactions with Collagen Using2H Multiple Quantum Filtered NMR Spectroscopy To Provide Insights into the Source of Double Quantum Filtered Signal in Tissue

In an effort to provide insight into the molecular origins of the (2)H double quantum filtered (DQF) NMR signal observed in connective tissue, specifically spinal disc tissue, (2)H multiple quantum filtered (MQF) NMR spectroscopy is used to study the structure and dynamics of D2O in collagen as a function of hydration. Residual quadrupolar coupling constants are measured and decrease from 3500 to 20 Hz while T2 relaxation times increase from 0.65 to 20 ms as hydration increases. Analysis of the data indicates that the quadrupolar coupling and T2 relaxation arises when water molecules spend time in restricted environments. The residual quadrupolar coupling is influenced almost exclusively by the most restricted water sites, the clefts of the triple helices not exposed on the surface of the fibrils, while the T2 relaxation has secondary contributions from less restricted water environments. The magnitudes of the measured values are consistent with results from DQF NMR studies of spinal disc tissue, supporting the assertion that water binding to collagen is a major contributor to the DQF NMR signal observed in spinal disc tissue.

Investigating the Water in Hydrated sPEEK Membranes Using Multiple Quantum Filtered 2H NMR Spectroscopy

Double and zero quantum filtered (2)H NMR spectroscopy is used to study the structure and dynamics of D(2)O in sulfonated poly(ether ether ketone) membranes as a function of membrane hydration. Both residual quadrupolar coupling constants and T(2) relaxation values are obtained as a function of hydration. The residual couplings vary from 160 Hz at low hydration to 30 Hz at high hydration. The T(2) relaxation times range from 3 to 14 ms, with the high hydration values having longer T(2). Results from this study are compared to results obtained for Nafion membranes, revealing similarities and differences in the water environments of the two membranes that result from the structure of the polymers and can be related to properties such as water diffusion.

MRC special issue on NMR of small molecules in anisotropic media

It was a great pleasure to edit and even a greater one to present this special issue to the readers of Magnetic Resonance in Chemistry. The relatively recent developments of weak alignment media compatible with organic solvents have opened new avenues on the structural analysis of complex natural and synthetic small organic molecules by NMR spectroscopy.1 The first NMR experiment of a small molecule in anisotropic media was performed by Saupe and Englert in 1963, in which benzene was partially aligned in the nematic phase of p-azoxyanisole.2 This experiment led to the later development of the theory of alignment3 and also permitted the empirical determination of the chemical shielding anisotropy of the benzene ring. The strong degree of alignment induced by nematic liquid crystal prevented their practical application to the structural analysis of complex small organic molecules. However, NMR spectroscopy of liquid crystals developed itself into a strong field of research and applications. This topic is outside the scope of this special issue. Weak alignment was first introduced by Tjandra and Bax in 1997 as a new and powerful tool for the study of the structure and dynamics of proteins.4 Its application quickly extended to the analysis of nucleic acids, and several alignment media compatible with water were developed since then. However, small organic molecules had to wait nearly 40 years to be analyzed by NMR in weak alignment media compatible with organic solvents. The deuterium quadrupolar coupling and the dipolar coupling are anisotropic NMR parameters not observable in conventional liquid state NMR experiments, but save for relaxation effects, they can be revealed when molecules are exposed to orienting media. Residual dipolar couplings (RDCs) have proven to be very powerful for the constitutional, configurational, and conformational analysis of small molecules,1 as a complementary tool to existing NMR parameters used for the same purpose, e.g. NOE and the J-based analysis. For certain compounds, the use of only these two existing NMR parameters leads to more than one solution structure, and this ambiguity can be lifted if RDCs are added to the analysis. The residual deuterium quadrupolar coupling has been extensively used as a tool for enantio-discrimination when small molecules are aligned in chiral non-racemic lyotropic liquid crystalline phases, e.g. different types of helical polymers dissolved in organic solvents.5 On the other side, a third anisotropic NMR parameter, the residual chemical shift anisotropy, has been explored as a complementary tool to the application of RDCs to the structural analysis of small molecules.6 This special issue is focused on new NMR methodologies for the analysis of small organic molecules oriented in weak alignment media. This is an emerging field with plenty of room for new discoveries and development of necessary tools such as software and methodologies for data analysis, pulse sequences, and alignment media. However, its application is not yet widespread in the small molecules community. In this special issue, the readers will have the opportunity to enjoy a diverse set of research articles contributed by the experts in the field. I hope that the scientific community finds this special issue particularly inspiring to bring these new methodologies into their everyday work. I want to thank the Editor In Chief, Prof. James Keeler, for the invitation to edit this special issue. I also want to thank the members of the Editorial Board, Prof. Helmut H. Duddeck, Prof. Bozhana P. Mikhova, and Prof. Angela M. Gronenborn for their encouragement and support. Finally, my thanks to all my colleagues who gave me the honor of editing their contributions.

2H double- and zero-quantum filtered NMR spectroscopy for probing the environments of water in Nafion

Multiple quantum 2H NMR spectroscopy is used to study the structure and dynamics of D2O in Nafion membranes as a function of membrane hydration. By employing both double- and zero-quantum filtered experiments, residual quadrupolar coupling constants and T2 relaxation values are obtained. The residual couplings vary from 240 to 20 Hz and the T2 values range from 20 to 180 ms, with the high hydration values having smaller couplings and longer T2 values. Analysis of the data using a water-exchange model suggests that the changes in parameters arise from a change in the fraction of time water spends in the anisotropic environments and not from changes in the order parameters that characterize the anisotropic sites. It has been found that a two-site model is needed to accurately fit the spectra above a hydration level of 10 D2O per sulfonate, with the second site having a negligible residual quadrupolar coupling. The data supports a model with two different hydration layers at high hydration and can be understood in terms of the recently proposed parallel-channel model for Nafion hydration.

Probing the Local Structure of Pure Ionic Liquid Salts with Solid‐ and Liquid‐State NMR

Room-temperature ionic liquids (RTILs) are gaining increasing interest and are considered part of the green chemistry paradigm due to their negligible vapour pressure and ease of recycling. Evidence of liquid-state order, observed by IR and Raman spectroscopy, diffraction studies, and simulated by ab initio methods, has been reported in the literature. Here, quadrupolar nuclei are used as NMR probes to extract information about the solid and possible residual order in the liquid state of RTILs. To this end, the anisotropic nature and field dependence of quadrupolar and chemical shift interactions are exploited. Relaxation time measurements and a search for residual second-order quadrupolar coupling were employed to investigate the molecular motions present in the liquid state and infer what kind of order is present. The results obtained indicate that on a timescale of approximately 10(-8) sec or longer, RTILs behave as isotropic liquids without residual order.

Optimal excitation of N23a nuclear spins in the presence of residual quadrupolar coupling and quadrupolar relaxation

Optimal control theory is applied for designing pulse sequences to optimally excite a spin-3/2 system with residual quadrupolar coupling in the presence of quadrupolar relaxation. A homogeneous form of the master equation is constructed to simulate the dynamics of the spin system, and a general optimization procedure with a homogeneous form of the equation of motion is described. The optimized pulses are tested with (23)Na NMR, and their performance is compared with that of pulses optimized in the absence of relaxation.

Long-Time Scale Ionic Dynamics in Dense Clay Sediments Measured by the Frequency Variation of the7Li Multiple-Quantum NMR Relaxation Rates in Relation with a Multiscale Modeling

7Li NMR relaxation measurements under spin-locking conditions are used to probe the dynamical properties of the lithium counterions within dense dispersions of charged anisotropic nanoplatelets. By simultaneously measuring the T1ρ and T2ρ relaxation times in addition to triple-quantum filtered relaxation times under the same spin-locking conditions, it is possible to separately quantify the contributions from the quadrupolar and the heterogeneous dipolar relaxation mechanisms. Thanks to the contribution from the residual quadrupolar coupling felt by the condensed lithium counterions, that procedure allows a broad dynamical range to be probed by performing spin-locking relaxation measurements using a limited number of irradiation powers. As illustrated by a multiscale modeling of the lithium diffusion and relaxation within such heterogeneous system, the frequency variation of the spectral densities characterizing the decorrelation of the quadrupolar coupling is a sensitive probe of the ionic mobility and the structure of the colloidal dispersion.

Optimal nuclear magnetic resonance excitation schemes for the central transition of a spin 3/2 in the presence of residual quadrupolar coupling

Optimal control theory is applied for enhancing the intensity of the central peak of a spin 3/2 signal in the presence of a residual quadrupolar coupling. While a maximum enhancement is always possible in the regime omega(rf) << omega(Q) via the use of modulated and shaped pulses, the intermediate rf-power regime omega(rf)-omega(Q) does not admit simple solutions based on intuition. In this work we present optimized shaped pulses that have been derived using an optimization algorithm based on optimal control and test these with (23)Na NMR in this regime. In addition to enhancing the intensity of the central transition signal, the satellite peaks can be effectively suppressed, which is a useful feature for the implementation in (23)Na imaging sequences.

The interaction of water molecules with purple membrane suspension using 2H double-quantum filter, 1H and 2H diffusion nuclear magnetic resonance.

Bacteriorhodopsin is a membrane protein of the purple membrane (PM) of Halobacterium salinarum, which is isolated as sheets of highly organized two-dimensional hexagonal microcrystals and for which water molecules play a crucial role that affects its function as a proton pump. In this paper we used single- and double-quantum (2)H NMR as well as (1)H and (2)H diffusion NMR to characterize the interaction of water molecules with the PM in D(2)O suspensions. We found that, under the influence of a strong magnetic field on a concentrated PM sample (0.61 mM), the PM sheets affect the entire water population and a residual quadrupolar splitting (upsilon(q) approximately 5.5 Hz, 298 K, at 11.7 T) is observed for the D(2)O molecules. We found that the residual quadrupolar coupling, the creation time in which a maximal DQF signal was obtained (tau(max)), and the relative intensity of the (2)H DQF spectrum of the water molecules in the PM samples (referred to herein as NMR order parameters) are very sensitive to temperature, dilution, and chemical modifications of the PM. In concentrated PM samples in D(2)O, these NMR parameters seem to reflect the relative organization of the PM. Interestingly, we have observed that some of these parameters are sensitive to the efficiency of the trimer packing, as concluded from the apo-membrane behavior. The data for dionized blue membrane, partially delipidated sample, and detergent-treated PM show that these D(2)O NMR order parameters, which are magnetic field dependent, are sensitive to the structural integrity of the PM. In addition, we revealed that heating the PM sample inside or outside the NMR magnet has, after cooling, a different effect on the NMR characteristics of the water molecules in the concentrated PM suspensions. The difference in the D(2)O NMR order parameters for the PM samples, which were heated and cooled in the presence and in the absence of a strong magnetic field, corroborates the conclusions that the above D(2)O order parameters are indirect reflections of both microscopic and macroscopic order of the PM samples. In addition, (1)H NMR diffusion measurements showed that at least three distinct water populations could be identified, based on their diffusion coefficients. These water populations seem to correlate with different water populations previously reported for the PM system.

Motional smearing of electrically recovered couplings measured from multipulse transients

AbstractThe measurement of residual dipolar and quadrupolar coupling constants in the liquid phase by using an electric field to destroy the isotropic nature of molecular tumbling is complicated by charge‐induced turbulent motion. In many cases this motion is due to charge injection at electrode surfaces, an effect that leads to an apparent removal of electrically recovered anisotropic spectral splittings when measured from a spin‐echo envelope modulation produced by a train of radio frequency (rf) pulses. To understand this averaging, the effect of quadrupolar couplings and enhanced molecular diffusion on free‐induction, spin‐echo, and Carr–Purcell signals is analytically determined in the special case of homogeneous rf pulses. Additional signal damping due to rf inhomogeneity and coupling constant heterogeneity is determined by numerically extending the kernel formalism introduced by Herzog and Hahn to understand spin diffusion in solids. Finally, the merit of the numerical approach is tested by comparison with analytical results for homogeneous rf pulses and experimental results for perdeuterated nitrobenzene involving inhomogeneous rf pulses and coupling heterogeneity. © 2001 John Wiley &amp; Sons, Inc. Concepts Magn Reson 13: 171–189, 2001

Fast Diffusion of Br- Ions on a Micellar Surface

An accurate determination of the field-dependent spin relaxation rates of 81Br by NMR provides, through the motional correlation time, the lateral surface diffusion coefficient of the Br- ions that are electrostatically confined to the vicinity of the oppositely charged surface of C16TABr micelles. We find that the Br- counterions diffuse about 1 order of magnitude faster than the C16TA+ molecules. The obtained diffusion coefficient is about 10-9 m2/s, roughly half the bulk diffusion coefficient of Br-. The residual quadrupole coupling constant is small, around 1 MHz, indicating a high symmetry for the Br- environment on the ≥100 ps time scale.

Establishing a degree of order: obtaining high-resolution NMR structures from molecular alignment

I would like to thank Eva de Alba for providing structural data as well as Ad Bax and James Baber for their critical comments.

NMR oxygen-17 studies of the state of water in a saturated sucrose solution

Abstract The frequency dependence of the water oxygen-17 transverse and longitudinal relaxation times is used to investigate the dynamic state of water in a saturated sucrose solution at 298K. Assuming that the observed frequency dependence originates from the slow modulation of the residual quadrupole coupling of the water-sucrose interaction, an anisotropic rotation model is used to extract water rotational correlation times and the order parameter characterizing the water-sucrose dynamic complex. Although the theory succeeds in providing a quantitative fit to the longitudinal data there is an unexpected anomalous dephasing enhancement in the transverse relaxation, and the paper concludes with a discussion of the possible origins of this enhancement and of alternative models for the dynamic state of the water.

Simultaneous Acquisition of Quadrupolar Order and Double-Quantum23Na Signals

Na+with both residual quadrupolar coupling and biexponential relaxation contributes to the signal acquired from DQF-4, while only Na+with residual quadrupolar coupling contributes to the signal acquired with the Jeener–Broekaert sequence. Since RF phases and flip angles for DQF-4 and Jeener-Broekaert sequences are identical, these different types of signals can be generated simultaneously. A phase-cycling scheme is developed to differentiate the signals corresponding to residual quadrupolar coupling and biexponential relaxation after the signals are acquired by use of the same RF sequences. This technique can maximize the attainable information from Na+in biological tissues in a given acquisition time.