Dipolar Coupling Interactions Research Articles

Quantitative analysis of the slow exchange process by 19F NMR in the presence of scalar and dipolar couplings: applications to the ribose 2'-19F probe in nucleic acids.

Solution NMR spectroscopy is a particularly powerful technique for characterizing the functional dynamics of biomolecules, which is typically achieved through the quantitative characterization of chemical exchange processes via the measurement of spin relaxation rates. In addition to the conventional nuclei such as 15N and 13C, which are abundant in biomolecules, fluorine-19 (19F) has recently garnered attention and is being widely used as a site-specific spin probe. While 19F offers the advantages of high sensitivity and low background, it can be susceptible to artifacts in quantitative relaxation analyses due to a multitude of dipolar and scalar coupling interactions with nearby 1H spins. In this study, we focused on the ribose 2'-19F spin probe in nucleic acids and investigated the effects of 1H-19F spin interactions on the quantitative characterization of slow exchange processes on the millisecond time scale. We demonstrated that the 1H-19F dipolar coupling can significantly affect the interpretation of 19F chemical exchange saturation transfer (CEST) experiments when 1H decoupling is applied, while the 1H-19F interactions have a lesser impact on Carr-Purcell-Meiboom-Gill relaxation dispersion applications. We also proposed a modified CEST scheme to alleviate these artifacts along with experimental verifications on self-complementary RNA systems. The theoretical framework presented in this study can be widely applied to various 19F spin systems where 1H-19F interactions are operative, further expanding the utility of 19F relaxation-based NMR experiments.

Tunable zero-field magnetoresistance responses in Si transistors: Origins and applications

The near-zero-field magnetoresistance (NZFMR) response has proven to be a useful tool for studying atomic-scale, paramagnetic defects that are relevant to the reliability of semiconductor devices. The measurement is simple to make and, in some cases, simple to interpret. In other cases, more sophisticated modeling based on the stochastic Liouville equation (SLE) is needed to access valuable information from NZFMR results. It has been shown that hyperfine and dipolar coupling interactions at atomic-scale defects affect the NZFMR line shape, but experimental parameters related to the detection method of NZFMR can also affect the nature of the response. Here, we demonstrate four distinct NZFMR detection methods in Si MOSFETs, which all access identical Si/SiO2 interface defects. In all four cases, we show that the line shape of the response is tunable based on experimental parameters alone. Using SLE-based modeling, we verify that time constants connected to physical carrier capture rates at the defect sites lead to these NZFMR line shape changes. The results demonstrate a method to extract some atomic-scale information for the purpose of defect identification. They also have broader applications to the continued development of ultra-sensitive magnetometers based on NZFMR in semiconductors. Additionally, the NZFMR effect in common Si-based devices may provide an inexpensive and accessible platform that mimics similar radical pair mechanisms that have become increasingly important in various biology fields.

Role of Protons in and around Strongly Coupled Nitroxide Biradicals for Cross-Effect Dynamic Nuclear Polarization.

In magic angle spinning dynamic nuclear polarization (DNP), biradicals such as bis-nitroxides are used to hyperpolarize protons under microwave irradiation through the cross-effect mechanism. This mechanism relies on electron-electron spin interactions (dipolar coupling and exchange interaction) and electron-nuclear spin interactions (hyperfine coupling) to hyperpolarize the protons surrounding the biradical. This hyperpolarization is then transferred to the bulk sample via nuclear spin diffusion. However, the involvement of the protons in the biradical in the cross-effect DNP process has been under debate. In this work, we address this question by exploring the hyperpolarization pathways in and around bis-nitroxides. We demonstrate that for biradicals with strong electron-electron interactions, as in the case of the AsymPols, the protons on the biradical may not be necessary to quickly generate hyperpolarization. Instead, such biradicals can efficiently, and directly, polarize the surrounding protons of the solvent. The findings should impact the design of the next generation of biradicals.

Control of excitation selectivity in pulse EPR on spin-correlated radical pairs with shaped pulses.

Spin-correlated radical pairs generated by photoinduced electron transfer are characterised by a distinctive spin polarisation and a unique behaviour in pulse electron paramagnetic resonance (EPR) spectroscopy. Under non-selective excitation, an out-of-phase echo signal modulated by the dipolar and exchange coupling interactions characterising the radical pair is observed and allows extraction of geometric information in the two-pulse out-of-phase electron spin echo envelope modulation (ESEEM) experiment. The investigation of the role of spin-correlated radical pairs in a variety of biological processes and in the fundamental mechanisms underlying device function in optoelectronics, as well as their potential use in quantum information science, relies on the ability to precisely address and manipulate the spins using microwave pulses. Here, we explore the use of shaped pulses for controlled narrowband selective and broadband non-selective excitation of spin-correlated radical pairs in two model donor-bridge-acceptor triads, characterised by different spectral widths, at X- and Q-band frequencies. We demonstrate selective excitation with close to rectangular excitation profiles using BURP (band-selective, uniform response, pure-phase) pulses and complete non-selective excitation of both spins of the radical pair using frequency-swept chirp pulses. The use of frequency-swept pulses in out-of-phase ESEEM experiments enables increased modulation depths and, combined with echo transient detection and Fourier transformation, correlation of the dipolar frequencies with the EPR spectrum and therefore the potential to extract additional information on the donor-acceptor pair geometry.

Examining ionicity and conductivity in poly(methyl methacrylate) containing imidazolium-based ionic liquids

Ionic liquid (IL) interactions with amorphous polymers or polyelectrolytes attract interests due to their propensity to form ionogels with enhanced viscosity and reduced ionic conductivity when compared to their neat IL. A good understanding on the interactions between polymer and IL is needed to design better IL-based hybrid electrolytes. This study examines the mixtures of IL-polymer electrolytes with relatively low polymer concentration to improve their conductivity. Ion transport properties of three types of imidazolium-based ionic liquids are investigated with the addition of linear poly(methyl methacrylate) (PMMA) homopolymer. Dielectric properties of PMMA/IL mixtures indicated the enhancement of apparent ionicity of ILs, which is attributed to the preferential interactions between PMMA and bis(trifluoromethylsulfonyl)imide (TFSI-) anions. The frequency-dependent conductivity data analyzed by the Jonscher universal power law infer that ion–dipole coupling interactions between PMMA and IL help in the solvation and enhance the ionicity by disrupting the cage structure and facilitating the fast ion motions of IL.

Theoretical description of pulse induced resonances in the homonuclear PIRATE experiment

Rotor-synchronous π pulses applied to protons (S) enhance homonuclear polarisation transfer between two spins (I) such as 13C or 15N as long as at least a single I–S heteronuclear dipolar-coupling interaction exists. The enhancement is maximum when the chemical-shift difference Δν between two spins equals an integer multiple, n, of the pulse-modulation frequency, which is half the rotor frequency νr. This condition, applied in the Pulse Induced Resonance with Angular dependent Total Enhancement (PIRATE) experiment, can be generalised for any spacing of the pulses k/νr such that Δν=nνr2k . We show, using average Hamiltonian theory (AHT) and Floquet theory, that the resonance conditions promote a second-order recoupling consisting of a cross-term between the homonuclear and heteronuclear dipolar interactions in a three-spin system. The minimum requirement is a coupling between the two I spins and a coupling of one of the I spins to the S spin. The effective Hamiltonian at the resonance conditions contains three-spin operators of the form 2I1±I2∓Sz with a non-zero effective dipolar coupling. Theoretical analysis shows that the effective strength of the resonance conditions decreases with increasing values of k and n. The theory is backed by numerical simulations, and experimental results on fully labelled 13C-glycine demonstrating the efficiency of the different resonance condition for k=1,2 at various spinning frequencies.

Intuitive Control of Low-Energy Magnetic Excitations via Directed Dipolar Interactions in a Series of Er(III)-Based Complexes.

Dipolar coupling is rarely invoked as a driving force for slow relaxation dynamics in lanthanide-based single-molecule magnets, though it is often the strongest mechanism available for mediating inter-ion magnetic interactions in such species. Indeed, for multinuclear lanthanide complexes, the magnitude and anisotropy of the dipolar interaction can be considerable given their ability to form highly directional, high-moment ground states. Herein, we present a mono-, di-, and trinuclear erbium-based single-molecule magnet sequence, ([Er-TiPS2COT]+)n (n = 1-3), wherein a drastic reduction in the allowedness of magnetic relaxation pathways is rationalized within the framework of the dipole-dipole interactions between angular momentum quanta. The resulting design principles for multinuclear molecular magnetism arising from intramolecular dipolar coupling interactions between highly anisotropic magnetic states present a nuanced justification of the relaxation dynamics in complex manifolds of individual quantized transitions. Experimental evidence for the validity of this model is provided by coupling the relaxation dynamics to an AC magnetic field across an unprecedented frequency range for molecular magnetism (103-10-5 Hz). The combination of slow dynamics and multiple, low-energy transitions leads to a number of noteworthy phenomena, including a lanthanide single-molecule magnet with three well-defined relaxation processes observable at a single temperature.

Transverse optical binding for a dual dipolar dielectric nanoparticle dimer

The physical origins of the transverse optical binding force and torque beyond the Rayleigh approximation have not been clearly expressed to date. Here, we present analytical expressions of the force and torque for a dual dipolar dielectric dimer illuminated by a plane wave propagating perpendicularly to the dimer axis. Using this analytical model, we explore the roles of the hybridized electric dipolar, magnetic dipolar, and electric-magnetic dipolar coupling interactions in the total force and torque on the particles. We find significant departures from the predictions of the Rayleigh approximation, particularly for high-refractive-index particles, where the force is dominated by the magnetic interaction. This results in an enhancement of the dimer stability by one to four orders of magnitude compared to the predictions of the Rayleigh approximation. For the case of torque, this is dominated by the coupling interaction and increases by an order of magnitude. Our results will help to guide future experimental work in optical binding of high-refractive-index dielectric particles.

Fabrication of Co/Al2O3 Composite Nanofiber via Electrospinning with Tunable Magnetic Properties

In this paper, Co/Al2O3 composite nanofibers with different Co content are fabricated by electrospinning combined with selective reduction. By controlling the Co content in the composite nanofiber, not only the fiber’s morphology, but also the magnetic properties can be well adjusted to meet different applications’ requirements. The magnetic hysteresis loops of composite nanofiber show two different types of shape, typical ferromagnetism and wasp-waisted ferromagnetism. Interestingly, the saturation magnetization, which has a close relationship with the content of magnetic component, is not simply shows an increasing tendency with the increases of Co content in the composite fiber. For the Co content below 50 wt%, the Ms value increases with the Co content. After that, the saturation magnetization decreases to a valley value at near 60 wt% Co content, then increases again. The coercivity also shows the same tendency. The results indicate that the dipolar coupling interaction plays a key role in the magnetic property adjustment of Co/Al2O3 composite nanofiber.

Spectroscopic properties of the V-doped borate glasses

Electron paramagnetic resonance (EPR) and optical absorption spectra in borate glasses of Li2B4O7:V, LiKB4O7:V, CaB4O7:V, and LiCaBO3:V compositions with added 0.5 and 1.0 mol.% V2O5, were investigated. The EPR spectroscopy clearly shows that the V impurity is incorporated into the network of studied glasses, mainly, as isolated vanadyl (VO2+) molecular complex centres. Spin Hamiltonian parameters of axial symmetry (g||, g⊥, A||, A⊥), dipolar hyperfine coupling (P) and Fermi contact interaction (K) constants of the VO2+ centres in octahedral sites with a tetragonal compression (C4v local symmetry) for all investigated glasses were determined from their experimental EPR spectra. Optical absorption bands, observed in the investigated glasses have been identified and interpreted. Crystal field parameters (Dq, Ds, Dt) and molecular orbitals (bonding) coefficients (β⁎2, επ⁎2) for VO2+ centres in the studied glasses were calculated and compared with referenced data for other V-doped borate glasses with similar compositions.

Controllable Interlayer Charge and Energy Transfer in Perovskite Quantum Dots/ Transition Metal Dichalcogenide Heterostructures

AbstractEfficient interlayer charge and energy transfer are explored in CsPbBr2I quantum dots/MoS2 heterostructure. When perovskite QDs directly contact MoS2, charge transfer from QDs to MoS2 dominates the nonradiative exciton relaxation in QDs/MoS2 heterostructure. With the layer number of MoS2 reducing to a monolayer, nonradiative exciton relaxation rate increases due to reduced dielectric effect and changed energy band structure of MoS2. Then an hexagonal boron nitride (h‐BN) spacer is inserted between QDs and a monolayer MoS2, in which nonradiative exciton relaxation rate is dominated by excitonic energy transfer from QDs to MoS2. With the spacer thickness increasing, nonradiative exciton relaxation rate decays as 1/d3.03 that is slower than the result 1/d4 of Förster theory. This is attributed to strong near‐field dipole–dipole coupling interaction in QDs/h‐BN/MoS2 heterostructure. Controllable interlayer charge and energy transfer potentially enable new avenues for designing optoelectronic devices, as well as for studying fundamental issue of tunable light–matter interaction at nanoscale vision.

Biradical rotamer states tune electron J coupling and MAS dynamic nuclear polarization enhancement

Cross Effect (CE) Dynamic Nuclear Polarization (DNP) relies on the dipolar (D) and exchange (J) coupling interaction between two electron spins. Until recently only the electron spin D coupling was explicitly included in quantifying the DNP mechanism. Recent literature discusses the potential role of J coupling in DNP, but does not provide an account of the distribution and source of electron spin J coupling of commonly used biradicals in DNP. In this study, we quantified the distribution of electron spin J coupling in AMUPol and TOTAPol biradicals using a combination of continuous wave (CW) X-band electron paramagnetic resonance (EPR) lineshape analysis in a series of solvents and at variable temperatures in solution – a state to be vitrified for DNP. We found that both radicals show a temperature dependent distribution of J couplings, and the source of this distribution to be conformational dynamics. To qualify this conformational dependence of J coupling in both molecules we carry out Broken Symmetry DFT calculations which show that the biradical rotamer distribution can account for a large distribution of J couplings, with the magnitude of J coupling directly depending on the relative orientation of the electron spin pair. We demonstrate that the electron spin J couplings in both AMUPol and TOTAPol span a much wider distribution than suggested in the literature. We affirm the importance of electron spin J coupling for DNP with density matrix simulations of DNP in Liouville space and under magic angle spinning, showcasing that a rotamer with high J coupling and optimum relative g-tensor orientation can significantly boost the DNP performance compared to random orientations of the electron spin pair. We conclude that moderate electron spin J coupling above a threshold value can facilitate DNP enhancements.

Characteristics of signal propagation in multiferroic majority logic gates subjected to thermal noise

The error rates of multiferroic majority logic gate (MLG) in the presence of thermal noise fluctuation are evaluated via solving the Landau–Lifshitz–Gilbert equations. It is found that in the pre-stress condition, magnetisation orientations of nanomagnets in the MLG denote an initial 4°–10° departure from the easy axis direction due to random thermal noise field. This phenomenon may be the main origin of switching errors of multiferroic logic gates at high temperature. Specifically, in the post-stress condition, it has been seen that larger spacing between nanomagnets leads to a high error rate, up to 50%, which arises from weak dipolar coupling interactions and strong thermal fluctuations. However, opposed to their expected, small spacing between nanomagnets leads to a higher error rate, up to 90%, which mainly arises from spontaneous magnetisation reversal induced by weak stress anisotropy energy. Therefore, moderate spacing is very important to MLG switching at room temperature, for example the optimal spacing is 250–310 nm for MLG constructed with nanomagnets having shape anisotropy energy of 623 kT. These findings can provide guides on multiferroic logic circuit design.

Nitrogen-Doped Silver-Nanoparticle-Decorated Transition-Metal Dichalcogenides as Surface-Enhanced Raman Scattering Substrates for Sensing Polycyclic Aromatic Hydrocarbons

The modification of transition-metal dichalcogenides (TMDs), incorporating nitrogen (N) doping and silver nanoparticles (AgNPs) decoration on the skeleton of exfoliated MoS2 and WS2, was accomplished. The preparation of N-doped and AgNPs-decorated TMDs involved a one-pot treatment procedure in a vacuum-sputtering chamber under N plasma conditions and in the presence of a silver (Ag) cathode as the source. Two different deposition times, 5 and 10 s, respectively, were applied to obtain N-doped with AgNPs-decorated MoS2 and WS2 hybrids, abbreviated as N5-MoS2/AgNPs, N10-MoS2/AgNPs, N5-WS2/AgNPs, and N10-WS2/AgNPs, respectively, for each functionalization time. The successful incorporation of N as the dopant within the lattice of exfoliated MoS2 and WS2 as well as the deposition of AgNPs on their surface, yielding N-MoS2/AgNPs and N-WS2/AgNPs, was manifested through extensive X-ray photoelectron spectroscopy measurements. The observation of peaks at ∼398 eV derived from covalently bonded N and the evolution of a doublet of peaks at ∼370 eV guaranteed the presence of AgNPs in the modified TMDs. Also, the morphologies of N-MoS2/AgNPs and N-WS2/AgNPs were examined by transmission electron microscopy, which proved that Ag deposition resulted in nanoparticle growth rather than the creation of a continuous metal film on the TMD sheets. Next, the newly developed hybrid materials were proven to be efficient surface-enhanced Raman scattering (SERS) platforms by achieving the detection of Rhodamine B (RhB). Markedly, N10-MoS2/AgNPs showed the highest sensitivity for detecting RhB at concentrations as low as 10–9 M. Charge-transfer interactions between RhB and the modified TMDs, together with the polarized character of the system causing dipole–dipole coupling interactions, were determined as the main mechanisms to induce the Raman scattering enhancement. Finally, polycyclic aromatic hydrocarbons such as pyrene, anthracene, and 2,3-dihydroxynaphthalene, coordinated via π–S interactions with N-MoS2/AgNPs, were screened with high sensitivity and reproducibility. These findings highlight the excellent functionality of the newly developed N-MoS2/AgNPs and N-WS2/AgNPs hybrid materials as SERS substrates for sensing widespread organic and environmental pollutants as well as carcinogen and mutagen species.

Magic-Angle Spinning Frequencies beyond 300 kHz Are Necessary To Yield Maximum Sensitivity in Selectively Methyl Protonated Protein Samples in Solid-State NMR

In the last decade, proton detection in magic-angle spinning (MAS) solid-state NMR became a popular strategy for biomolecular structure determination. In particular, probe technology has experienced tremendous progress with smaller and smaller diameter rotors achieving ever higher MAS frequencies. MAS rotation frequencies beyond 100 kHz allow to observe and assign protons in fully protonated samples. In these experiments, resolution is however compromised as homogeneous proton–proton dipolar coupling interactions are not completely averaged out. Using a combination of experiments and simulations, we analyze the MAS frequency-dependent intensities of the 1H,13C methyl correlation peaks of a selectively methyl protonated (CH3) microcrystalline sample of the chicken α-spectrin SH3 domain (α-SH3). Extensive simulations involving nine spins employing the program SIMPSON allow to predict the MAS frequency dependence of the proton intensities. The experimental results are used to validate the simulations. As qua...

Nuclear magnetic resonance surface relaxation mechanisms of kerogen

Nuclear magnetic resonance (NMR) relaxometry is an excellent tool for probing the interactions between solid pore surface and pore fluids in porous media. Surface relaxation is a key component of NMR relaxation. It is well-known that in conventional rocks, paramagnetic centers contribute most to the surface relaxation phenomenon. However, the interactions between organic pore surfaces and pore fluids, and the mechanism of surface relaxation in organic shale pores, are not well-understood. We tackle the issue using deuterated compounds to adjust the proton density in the liquid phase and monitoring the transverse relaxation rate changes of kerogen-fluid mixtures. With the Barnett and Eagle Ford kerogen isolates, we found that for alkanes, it is intramolecular dipolar coupling that dominates among the magnetic interactions. As a result, the transverse relaxation rate of alkane proton spins is more likely to be dependent on the concentration of active adsorption sites on the kerogen surface, rather than the kerogen proton density. For water inside organic pores, surface relaxation most likely originates from hydrogen bonding and intermolecular dipolar coupling. We also examined the temperature effect on kerogen surface relaxation and found temperature-dependent behavior that is consistent with surface relaxation by hydrogen bonding and homonuclear dipolar coupling interactions.

Highly improved mechanical strength of aramid paper composite via a bridge of cellulose nanofiber

A novel aramid paper composite based on pretreated meta-aramid fiber via the addition of cellulose nanofiber (CNF) was fabricated, and the mechanical strength and interfacial strength of the aramid paper composite were investigated. The results indicated that modified fibers showed higher roughness and more available hydrophilic groups. Besides, compared with the pristine aramid paper, it turned out that the tensile index, tear index and interlayer bonding strength of the paper composites with CNF increased by 2.04 times, 2.36 times and 3 times, respectively. In addition, tensile energy absorption (TEA) was also improved by an increment of 99.7% with 20 wt% CNF. These apparent evidences can be accounted for the following mechanisms. On the one hand, enhanced mechanical properties of aramid paper composite were derived from the strong hydrogen bonding or dipole–dipole coupling interaction between aramid fiber and CNF. On the other hand, significant reinforcement of interlayer bonding strength can be attributed to the pivotal bonding bridge and filling agent between aramid chopped fibers (ACFs) and fibrid, which could improve interfacial adhesion of paper sheet. The thin film structure like “spider web” or “silk” from SEM images indicated the CNF was used as a bridge actually.

Dynamics of atoms in strong laser fields I: A quasi analytical model in momentum space based on a Sturmian expansion of the interacting nonlocal Coulomb potential

In this study we present a model that we have formulated in the momentum space to describe atoms interacting with intense laser fields. As a further step, it follows our recent theoretical approach in which the kernel of the reciprocal-space time-dependent Schrödinger equation (TDSE) is replaced by a finite sum of separable potentials, each of them supporting one bound state of atomic hydrogen (Tetchou Nganso et al. 2013). The key point of the model is that the nonlocal interacting Coulomb potential is expanded in a Coulomb Sturmian basis set derived itself from a Sturmian representation of Bessel functions of the first kind in the position space. As a result, this decomposition allows a simple spectral treatment of the TDSE in the momentum space. In order to illustrate the credibility of the model, we have considered the test case of atomic hydrogen driven by a linearly polarized laser pulse, and have evaluated analytically matrix elements of the atomic Hamiltonian and dipole coupling interaction. For various regimes of the laser parameters used in computations our results are in very good agreement with data obtained from other time-dependent calculations.

Improving spectral resolution in biological solid-state NMR using phase-alternated rCW heteronuclear decoupling

The successful application of solid-state NMR spectroscopy for structural study of biological macromolecules requires high spectral resolution. In presence of abundant H1 spins, the resolution of the prevailing C13 or N15 chemical shift encoding experiments critically depends on the availability of efficient and robust heteronuclear decoupling methods in addition to the use of high-field instrumentation and fast sample spinning. Robustness of the decoupling method towards alterations in amplitude/offset of radio frequency fields due to varying sample states is important to ensure recording of spectra with high resolution over long sampling periods for insensitive samples. Here, we present a phase-alternated refocused continuous-wave decoupling method offering better resolution, easier setup, and higher robustness than previous methods. Improved decoupling is in part ascribed to more efficient cancellation of the residual heteronuclear, H1–C13, dipolar coupling interactions which are induced by homonuclear, H1–H1, dipolar coupling interactions.

Preparation of Two‐Dimensional Au Nanoparticle Arrays with Controllable Gap Distance: Interparticle Coupling Effect on Surface Plasmon Resonance and SERS#

In this paper, we demonstrate the fabrication of highly ordered, close‐packed, two‐dimensional Au nanoparticle (2D Au NP) arrays by surface modification of Au NPs with self‐assembly chemistry of (3‐mercaptopropyl)‐trimethoxysilane (MPTMS) at a water/hexane interface. Aside from functioning as a packing molecule, MPTMS provided active sites for direct substrate polymerization of polydimethylsiloxane (PDMS). The optical properties of the fabricated 2D Au NP array were then analyzed by UV–vis‐NIR and Raman spectroscopy. The 2D Au NP array showed unique surface plasmon resonance (SPR) modes that came from the bare dipole–dipole coupling interaction of neighboring nanoparticles, which was due to the condition of sub‐1‐nm distance between the NPs. We were able to deduce these SPR modes by investigating their characteristics by controlling the gap between the Au NPs through mechanical force expansion. Second, Raman spectroscopy indicated that the close‐packed Au nanoparticle array exhibits strong surface‐enhanced Raman scattering response; and, as expected, an inverse relationship was observed between Raman intensity and gap length. In addition, the Au nanorod array showed higher sensitivity compared to the Au nanosphere array as building block for the 2D Au NP array.