Dipolar Coupling Constants Research Articles

Dynamics in the Intact fd Bacteriophage Revealed by Pseudo 3D REDOR-Based Magic Angle Spinning NMR.

The development of robust NMR methodologies to probe dynamics on the atomic scale is vital to elucidate the close relations between structure, motion, and function in biological systems. Here, we present an automated protocol to measure, using magic-angle spinning NMR, the effective 13C-15N dipolar coupling constants between multiple spin pairs simultaneously with high accuracy. We use the experimental dipolar coupling constants to quantify the order parameters of multiple C-N bonds in the thousands of identical copies of the coat protein in intact fd-Y21M filamentous bacteriophage virus and describe its overall dynamics on the submillisecond time scale. The method is based on combining three pseudo three-dimensional NMR experiments, where a rotational echo double resonance (REDOR) dephasing block, designed to measure internuclear distances, is combined with three complementary 13C-13C mixing schemes: dipolar-assisted rotational resonance, through-bond transfer-based double quantum/single quantum correlation, and radio frequency driven recoupling. These mixing schemes result in highly resolved carbon spectra with correlations that are created by different transfer mechanisms. We show that the helical part of the coat protein undergoes a uniform small (∼30°) amplitude motion, while the N-terminus is highly flexible. In addition, our results suggest that the reduced mobility of lysine sidechains at the C-terminus are a signature of binding to the single stranded DNA.

Dissipative dynamics of multiple-quantum NMR coherences in two-spin systems

Multiple-quantum (MQ) NMR experiments were performed at a special orientation of a hambergite (Be2BO3OH) single crystal, which consists of alternating zigzag proton chains. At the chosen orientation, one of the dipolar coupling constants in the chain becomes zero and the system becomes a set of well-isolated dipolar coupled spin pairs. The relaxation of the spin pairs in the MQ NMR experiment was studied on the basis of the Lindblad equation. Fermi’s golden rule was used to investigate the relaxation mechanism. The agreement of the calculated relaxation time with the experimental value (125 μs) suggests that the dipole–dipole interactions with protons surrounding the pair are responsible for the relaxation of MQ coherences.

A machine learning approach for dynamical modelling of Al distributions in zeolites via23Na/27Al solid-state NMR.

One of the main limitations in supporting experimental characterization of Al siting/pairing via modelling is the high computational cost of ab initio calculations. For this reason, most works rely on static or very short dynamical simulations, considering limited Al pairing/siting combinations. As a result, comparison with experiment suffers from a large degree of uncertainty. To alleviate this limitation we have developed neural network potentials (NNPs) which can dynamically sample across broad configurational and chemical spaces of sodium-form aluminosilicate zeolites, preserving the level of accuracy of the ab initio (dispersion-corrected metaGGA) training set. By exploring a wide range of Al/Na arrangements and a combination of experimentally relevant Si/Al ratios, we found that the 23Na NMR spectra of dehydrated high-silica CHA zeolite offer an opportunity to assess the distribution and pairing of Al atoms. We observed that the 23Na chemical shift is sensitive not only to the location of sodium in 6- and 8MRs, but also to the Al-Sin-Al sequence length. Furthermore, neglect of thermal and dynamical contributions was found to lead to errors of several ppm, and has a profound influence on the shape of the spectra and the dipolar coupling constants, thus necessitating the long-term dynamical simulations made feasible by NNPs. Finally, we obtained a predictive regression model for the 23Na chemical shift in CHA (Si/Al = 35, 17, 11) that circumvents the need for expensive NMR density functional calculations and can be easily extended to other zeolite frameworks. By combining NNPs and regression methods, we can expedite the simulations of NMR properties and capture the effect of dynamics on the spectra, which is often overlooked in computational studies despite its clear manifestation in experimental setups.

Pair correlations of the easy magnetisation axes of superparamagnetic nanoparticles in a ferrofluid/ferrocomposite.

The widespread use of magnetic nanoparticles in modern technologies and medical applications highlights the need for reliable theoretical models that can predict their physical properties. The pair correlation function of two randomly selected superparamagnetic nanoparticles in a ferrofluid/ferrocomposite is studied to depict the joint probability density of the easy magnetisation axes across the planes of parameters of major importance; these are the interaction of ferroparticles with an external magnetic field, the energy of magnetic anisotropy inside the superparamagnetic nanoparticle, and the interparticle magnetic dipole-dipole interaction. Assuming the rotational symmetry of the system, we come to the conclusion that the pair correlations of interest are dependent only on the polar angles, determining the inclinations of the ferroparticle easy axes from the direction of an external magnetic field. The dimer configuration, where two ferroparticles are in close contact along a magnetic field with their easy magnetisation axes aligned, is the most probable. This configuration becomes more pronounced with increasing anisotropy energy, dipolar coupling constant, and external magnetic field.

Performance of the cross-polarization experiment in conditions of radiofrequency field inhomogeneity and slow to ultrafast magic angle spinning (MAS).

In this paper, we provide an analytical description of the performance of the cross-polarization (CP) experiment, including linear ramps and adiabatic tangential sweeps, using effective Hamiltonians and simple rotations in 3Dspace. It is shown that radiofrequency field inhomogeneity induces a reduction in the transfer efficiency at increasing magic angle spinning (MAS)frequencies for both the ramp and the adiabatic CPexperiments. The effect depends on the ratio of the dipolar coupling constant and the sample rotation frequency. In particular, our simulations show that for small dipolar couplings (1 ) and ultrafast MAS (above 100 ) the transfer efficiency is below 40 % when extended contact times up to 20 are used and relaxation losses are ignored. New recoupling and magnetization transfer techniques that are designed explicitly to account for inhomogeneous radiofrequencyfields are needed.

Measurement of hyperfine constants and the isotope shift of rubidium 5P1/2 excited-state using saturated absorption spectroscopy

The Saturated Absorption Spectroscopy (SAS) was performed to measure the hyperfine constants of rubidium 5P1/2 excited state using an external-cavity diode laser (ECDL) operating at 795 nm. We report our measurements for the magnetic dipole coupling constants 120.79(29) and 407.75(57) for 85 Rb and 87 Rb respectively. Our measurement of the 85 Rb hyperfine constant shows a good agreement with previous results and our measurement of the 87 Rb hyperfine constant resolves the discrepancy of results that exists in the literature.

Experimental and computational studies on the formation of mixed amide-hydride solid solutions for CsNH2–CsH system

In this study, experimental determination and computational prediction are combined to investigate the formation of a mixed amide-hydride solid solution for the CsNH2–CsH system in a wide compositional range. The experimentally obtained results strongly indicate that a complete amide-hydride solid solution Cs(NH2)xH1-x with a stable cubic structure is achievable when the molar fraction of amide (x) is lower than 0.9. These results validate and confirm our data computationally via first-principles calculations, including the simulations of infrared (IR) and nuclear magnetic resonance (NMR) spectra for structures of various compositions as well as the determination of the dipolar coupling constants. Both the computed vibrational frequencies and 1H chemical shifts of CsNH2 and CsH moieties in the Cs(NH2)xH1-x (x = 0.2, 0.5, 0.8, 1) solid solution structures agree with the experimental IR and 1H MAS NMR data of the mixed xCsNH2+(1-x)CsH samples, confirming the formation of the solid solutions. The closest interproton distance in the homogeneous Cs(NH2)0·5H0.5 solid solution is computed to be 3.67 Å, which is larger than that of the known Rb(NH2)0·5H0.5 solid solution (3.29 Å). This work's combination of theoretical research and experimentation provides a suitable framework for the structural analysis and property estimation of other M-N-H solid solutions.

Effects of artificial weathering in NR/SBR elastomer blends

Degradation of polymer blends occurs by the constituent phases undergoing distinct chemical changes that depend on their unique chemical structures. This makes predicting and establishing a structure-property relationship for each phase necessary as well as challenging. In this work, the molecular and physical changes occurring in sulfur-cross-linked natural rubber (NR), styrene−butadiene rubber (SBR), and their 50/50 blend subjected to accelerated weathering are analyzed by 1H nuclear magnetic resonance (NMR) spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, atomic-force microscopy (AFM), and dynamic mechanical thermal analysis (DMTA). NMR transverse relaxation time (T2) studies suggest the formation of rigid components due to weathering. FTIR and AFM reveal that this is related to the formation of a stiff surface due to chemical modifications, which shows up as an additional thermal transition in the DMTA curves. Low-field double-quantum (DQ) NMR studies of the cross-link density, by the residual dipolar coupling constant (Dres), of SBR show a continuous increase in its cross-link density over the weathering duration (988 h). In contrast, NR exhibits dominant chain scission reactions resulting in defects, with both materials demonstrating the formation of different chain lengths. During the first 168 h, NR also undergoes modification of sulfur bond lengths, which is also observed in the blend. The blend largely follows an intermediate trend of cross-link densities compared to the two polymers but shows signs of lesser chain modifications than a weighted average of the two polymers. This is confirmed by phase-resolved DQ magic-angle spinning (MAS) NMR experiments whereby the peak-specific Dres of the blend was measured to be less modified than that of the individual vulcanizates, thus proving that the blend is more resistant to weathering than its constituent elastomers.

Near-zero-field magnetoresistance measurements: A simple method to track atomic-scale defects involved in metal-oxide-semiconductor device reliability.

We demonstrate the ability of a relatively new analytical technique, near-zero-field magnetoresistance (NZFMR), to track atomic-scale phenomena involved in the high-field stressing damage of fully processed Si metal-oxide-semiconductor field-effect transistors. We show that the technique is sensitive to both the Pb0 and Pb1 dangling bond centers and that the presence of both centers can be inferred through NZFMR via hyperfine interactions with the central 29Si atoms of the dangling bonds. The NZFMR results also provide evidence for the redistribution of mobile hydrogen atoms at the Si/SiO2 interface and also a potential change in the average dipolar coupling constant between electrons in neighboring defects. This work shows that NZFMR offers significant analytical power for studying technologically relevant semiconductor device reliability problems and has advantages in experimental simplicity over comparable techniques.

NMR‐Based Cross‐Link Densities in EPDM and EPDM/ULDPE Blend Materials and Correlation with Mechanical Properties

AbstractThe role of cross‐linking in dictating the microstructural and mechanical properties in ethylene‐propylene‐diene‐monomer rubber (EPDM) and EPDM/ULDPE blends cross‐linked by different sulfur amounts is investigated by solid‐state1H time‐domain NMR spectroscopy and tensile‐tests. Analyses of spin‐spin relaxation time (T2), by combining free‐induction decay (FID), magic‐sandwich echo‐FID, and Hahn‐echo experiments demonstrate a reduction in crystal‐amorphous interface regions of pure ultralow‐density polyethylene (ULDPE) upon curative addition. The blends demonstrate a complete loss of these fractions due to curative‐induced plasticization and solvation by polyethylene segments of EPDM. Cross‐link densities, quantified by the magnitude of residual dipolar coupling constant (Dres), arising from topological restrictions to segmental motions, are measured by multiple‐quantum experiments. The entanglement‐dominated EPDMs demonstrate a significant reduction in ultimate tensile properties with increasingDres. The analogous blends yield similarDresvalues up to 0.36 phr of free sulfur. Thereafter, a deviation from the cross‐linking trend of the EPDMs is observed with the blends approaching a cross‐linking limit, thus emphasizing the migration of additives to the amorphous phase of the ULDPE. From the additional contributions of solvation and complex entanglement scenarios in the blends, restoration and even significant enhancement in ultimate tensile strength are achieved. Limitations in applying the popular Mooney–Rivlin analysis are also briefly discussed.

Double echo symmetry-based REDOR and RESPDOR pulse sequences for proton detected measurements of heteronuclear dipolar coupling constants

1H{X} symmetry-based rotational echo double resonance pulse sequences (S-REDOR) and symmetry-based rotational echo saturation pulse double resonance (S-RESPDOR) solid-state NMR experiments have found widespread application for 1H detected measurements of difference NMR spectra, dipolar coupling constants, and internuclear distances under conditions of fast magic angle spinning (MAS). In these experiments the supercycled R412 (SR412) symmetry-based recoupling pulse sequence is typically applied to the 1H spins to reintroduce heteronuclear dipolar couplings. However, the timing of SR412 and other symmetry-based pulse sequences must be precisely synchronized with the rotation of the sample, otherwise, the evolution of 1H CSA and other interactions will not be properly refocused. For this reason, significant distortions are often observed in experimental dipolar dephasing difference curves obtained with S-REDOR or S-RESPDOR pulse sequences. Here we introduce a family of double echo (DE) S-REDOR/S-RESPDOR pulse sequences that function in an analogous manner to the recently introduced t1-noise eliminated (TONE) family of dipolar heteronuclear multiple quantum coherence (D-HMQC) pulse sequences. Through numerical simulations and experiments the DE S-REDOR/S-RESPDOR sequences are shown to provide dephasing difference curves similar to those obtained with S-REDOR/S-RESPDOR. However, the DE sequences are more robust to the deviations of the MAS frequency from the ideal value that occurs during typical solid-state NMR experiments. The DE sequences are shown to provide more reliable 1H detected dipolar dephasing difference curves for nuclei such as 15N (with isotopic labelling), 183W and 35Cl. The double echo sequences are therefore recommended to be used in place of conventional S-REDOR/S-RESPDOR sequences for measurement of weak dipolar coupling constants and long-range distances.

Materials Technology for Heterogeneous Polymer Systems : Part II. Investigation of Motional Heterogeneity in Rubber Vulcanizates

For rubber vulcanizates or crosslinked rubbers, inherent characteristics of rubber elasticity are well known to be mainly controlled by the change in conformational entropy under deformation, causing constrained molecular mobility of rubbery network chains. In this part II of special lecture, 1H-pulsed NMR method is applied to observe the motional heterogeneity in the crosslinked rubbers, related to effects of crosslink density and incorporation of reinforcing carbon black fillers on the tensile failure behaviors under various conditions. Furthermore, as for analysis of thermo-oxidative aging behaviors for the crosslinked rubbers, the relationships between the breaking stretch ratio in tensile measurements, λb, and NMR relaxation properties such as spin-spin relaxation time, T 2, and residual dipolar coupling constant , D res, were discussed to predict the failure properties of small pieces sampled from engineering rubber parts, for which tensile tests could not be easily performed.

Extraction of dipolar coupling constants from low-frequency electrically detected magnetic resonance and near-zero field magnetoresistance spectra via least squares fitting to models developed from the stochastic quantum Liouville equation

We report low-frequency electrically detected magnetic resonance (EDMR) and near-zero field magnetoresistance (NZFMR) measurements observed through spin-dependent trap-assisted tunneling on unpassivated 28Si/28SiO2 metal–insulator–semiconductor (MIS) capacitors. This study both experimentally and theoretically explores the effects of the low-frequency EDMR response and the mechanisms responsible for spin-mixing, which leads to the NZFMR response in the absence of electron–nuclear hyperfine interactions. Previous reports that utilized high-frequency EDMR and NZFMR on these devices indicated that the observed trap-assisted tunneling spectra are dominated by silicon dangling bonds back bonded to silicon at the Si/SiO2 interface, Pb0 and Pb1 centers. These previous results also suggest that the rate limiting step in trap-assisted tunneling is the interface to an oxide trapping event. In this work, we extend the theory to show the explicit connection of the defects observed between the NZFMR response and the EDMR, which has not yet been demonstrated. We also extend a theoretical approach to the analysis of both the EDMR and NZFMR spectra and match the theory to experimental observations made in 28Si/28SiO2 MIS capacitors. The method utilizes a least squares fitting algorithm of models developed from the stochastic quantum Liouville equation. We find that we can extract a dipolar coupling constant by fitting both the NZFMR and EDMR spectra. Our experimental results and resulting fitted spectra from our quantitative model suggest the mechanism responsible for spin-mixing, which leads to the NZFMR response in the absence of electron–nuclear hyperfine interactions, is predominately magnetic dipolar interactions between Pb centers at the interface.

Double Echo Symmetry-Based REDOR and RESPDOR Pulse Sequences for the Improved Proton Detected Measurement of Heteronuclear Dipolar Coupling Constants

Double Echo Symmetry-Based REDOR and RESPDOR Pulse Sequences for the Improved Proton Detected Measurement of Heteronuclear Dipolar Coupling Constants

Calculation of DEER spectrum by the use of doubly rotating frames: Three-pulse and four-pulse nitroxide biradical DEER signals

A novel doubly rotating frame (DRF) technique has been exploited to calculate three- and four-pulse DEER signals for a system of two dipolar-coupled nitroxides. The three-pulse DRF technique is illustrated for the case, wherein the relative intensity of the peak, appearing at ±d/2, is compared with that of the peak at ±d, where d=23D (D = dipolar-coupling constant, as a function of B1, the amplitude of the pulses. It is found for three-pulse DEER signals that for the commonly used B1 fields the three-pulse DEER sequence is capable of measuring distances between the two nitroxides of the biradical corresponding to d≤8 MHz, or distances r≥ 18.7 Å. As for the four-pulse DEER signal, using the distribution of the distance between the nitroxide radicals, determined experimentally, the simulated DEER signal shows excellent agreement with the experiment.

Quantum Fisher information and skew information correlations in dipolar spin system

Quantum Fisher information (QFI) and skew information (SI) plays a key role in the quantum resource theory. Understanding these measures in the physical system has practical significance in the state parameter estimation and quantum metrology. In this article, we consider a pair of spin-1/2 particles coupled with dipolar and Dzyaloshinsky-Moriya (DM) interactions, serving as the physical carrier of quantum information. We examine the bipartite nonlocal correlations of pair of spin-1/2 particle system for the thermal equilibrium states, characterized by local quantum uncertainty (LQU) and local quantum Fisher information (lQFI). The effects of dipolar coupling constants on quantum correlation quantifiers are studied. The DM interaction greatly enhances the quantum correlation in the system whereas the temperature tends to annihilate the amount of quantum correlations.

Probing 29Si-17O connectivities and proximities by solid-state NMR

The measurement of dipolar and J- couplings between 29Si and 17O isotopes is challenging owing to (i) the low abundance of both isotopes and (ii) their close Larmor frequencies, which only differ by 19%. These issues are circumvented here by the use of isotopic enrichment and dedicated triple-resonance magic-angle spinning NMR probe. The surface of 29Si-enriched silica was labelled with 17O isotope and heated at 80 and 200 °C. 29Si-17O connectivities and proximities were probed using two-dimensional (2D) through-bond and through-space heteronuclear multiple-quantum coherences (J- and D-HMQC) experiments between 17O and 29Si nuclei. The simulation of the build-up of the J- and D-HMQC signals allowed the first experimental measurement of J- and dipolar coupling constants between 17O and 29Si nuclei. These HMQC experiments allow distinguishing two distinct siloxane (SiOSi) oxygen sites: (i) those covalently bonded to Q3 and Q4 groups, having a hydroxyl group as a second neighbour and (ii) those covalently bonded to two Q4 groups. The measured J- and dipolar coupling constants of siloxane 17O nucleus with Q4 29Si nuclei differ from those with Q3 29Si nuclei. These results indicate that the 29Si-17O one-bond J-coupling and Si-O bond length depend on the second neighbours of the Si atoms.

How chains and rings affect the dynamic magnetic susceptibility of a highly clustered ferrofluid.

The dynamic magnetic susceptibility, χ(ω), of a model ferrofluid at a very low concentration (volume fraction, approximately 0.05%), and with a range of dipolar coupling constants (1≤λ≤8), is examined using Brownian dynamics simulations. With increasing λ, the structural motifs in the system change from unclustered particles, through chains, to rings. This gives rise to a nonmonotonic dependence of the static susceptibility χ(0) on λ and qualitative changes to the frequency spectrum. The behavior of χ(0) is already understood, and the simulation results are compared to an existing theory. The single-particle rotational dynamics are characterized by the Brownian time, τ_{B}, which depends on the particle size, carrier-liquid viscosity, and temperature. With λ≤5.5, the imaginary part of the spectrum, χ^{''}(ω), shows a single peak near ω∼τ_{B}^{-1}, characteristic of single particles. With λ≥5.75, the spectrum is dominated by the low-frequency response of chains. With λ≥7, new features appear at high frequency, which correspond to intracluster motions of dipoles within chains and rings. The peak frequency corresponding to these intracluster motions can be computed accurately using a simple theory.

Free Induction Decay in Zigzag Spin Chains in a Multi-pulse NMR Experiment

The free induction decay (FID) is investigated for zigzag (alternating) spin chains in the approximation of the nearest neighbor interactions. The dependence of the FID on the ratio of dipolar coupling constants (the parameter of dimerization) is also studied. Possible comparison of the obtained results with the experimental data for proton zigzag chains in a single crystal of hambergite is discussed.

Polymer Networks Synthesized from Poly(Sorbitol Adipate) and Functionalized Poly(Ethylene Glycol).

Polymer networks were prepared by Steglich esterification using poly(sorbitol adipate) (PSA) and poly(sorbitol adipate)-graft-poly(ethylene glycol) mono methyl ether (PSA-g-mPEG12) copolymer. Utilizing multi-hydroxyl functionalities of PSA, poly(ethylene glycol) (PEG) was first grafted onto a PSA backbone. Then the cross-linking of PSA or PSA-g-mPEG12 was carried out with disuccinyl PEG of different molar masses (Suc-PEGn-Suc). Polymers were characterized through nuclear magnetic resonance (NMR) spectroscopy, gel permeation chromatography (GPC), and differential scanning calorimetry (DSC). The degree of swelling of networks was investigated through water (D2O) uptake studies, while for detailed examination of their structural dynamics, networks were studied using 13C magic angle spinning NMR (13C MAS NMR) spectroscopy, 1H double quantum NMR (1H DQ NMR) spectroscopy, and 1H pulsed field gradient NMR (1H PFG NMR) spectroscopy. These solid state NMR results revealed that the networks were composed of a two component structure, having different dipolar coupling constants. The diffusion of solvent molecules depended on the degree of swelling that was imparted to the network by the varying chain length of the PEG based cross-linking agent.