Pulsed Electron Paramagnetic Resonance Spectroscopy Research Articles

Relativistic Quantum Chemical Investigation of Actinide Covalency Measured by Electron Paramagnetic Resonance Spectroscopy.

We investigate actinide covalency effects in two [AnCptt3] (An = Th, U) complexes recently studied with pulsed electron paramagnetic resonance spectroscopy, using the Hyperion package to obtain relativistic hyperfine coupling constants from relativistic multiconfigurational wave functions. 1H and 13C HYSCORE simulations using the computed parameters show excellent agreement with the experimental data, highlighting the accuracy of modern relativistic ab initio methods. The extent of covalency indicated from the calculations on [ThCptt3] is in agreement with the original report based on traditional spectral fitting methods, while the covalency in [UCptt3] is found to be previously overestimated. The latter is due to the paramagnetic spin-orbit effect that arises naturally in a relativistic theory of hyperfine coupling and yet was not accounted for in the original study, thus highlighting the necessity of relativistic approaches for the interpretation of magnetic resonance data pertaining to actinides.

Conformational Heterogeneity of β-Barrel Membrane Proteins Observed In Situ Using Orthogonal Spin Labels and Pulsed ESR Spectroscopy.

Outer membrane proteins (OMPs) of Gram-negative bacteria are involved in many essential functions of the cell. They are tightly packed in the outer membrane, which is an asymmetric lipid bilayer. Electron spin resonance (ESR) spectroscopic techniques combined with site-directed spin labeling (SDSL) enable observation of structure and conformational dynamics of these proteins directly in their native environments. Here we depict a protocol for site-directed spin labeling of β-barrel membrane proteins in isolated outer membranes and intact E. coli using nitroxide, triarylmethyl (trityl), and Gd3+-based spin tags. Furthermore, subsequent continuous wave (CW) and orthogonal pulsed electron-electron double resonance (PELDOR) measurements are described along with experimental setup at Q-band (34GHz), the data analysis, and interpretation.

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.

Enhancing Coherence Times of Chromophore-Radical Molecular Qubits and Qudits by Rational Design.

An important criterion for quantum operations is long qubit coherence times. To elucidate the influence of molecular structure on the coherence times of molecular spin qubits and qudits, a series of molecules featuring perylenediimide (PDI) chromophores covalently linked to stable nitroxide radicals were synthesized and investigated by pulse electron paramagnetic resonance spectroscopy. Photoexcitation of PDI in these systems creates an excited quartet state (Q) followed by a spin-polarized doublet ground state (D0), which hold promise as spin qudits and qubits, respectively. By tailoring the molecular structure of these spin qudit/qubit candidates by selective deuteration and eliminating intramolecular motion, coherence times of Tm = 9.1 ± 0.3 and 4.2 ± 0.3 μs at 85 K for D0 and Q, respectively, are achieved. These coherence times represent a nearly 3-fold enhancement compared to those of the initial molecular design. This approach offers a rational structural design protocol for effectively extending coherence times in molecular spin qudits/qubits.

Pulse and CW EPR Oximetry Using Oxychip in Gemcitabine-Treated Murine Pancreatic Tumors.

The goal of this work was to compare pO2 measured using both continuous wave (CW) and pulse electron paramagnetic resonance (EPR) spectroscopy. The Oxychip particle spin probe enabled longitudinal monitoring of pO2 in murine pancreatic tumor treated with gemcitabine during the course of therapy. Pancreatic PanO2 tumors were growing in the syngeneic mice, in the leg. Five doses of saline in control animals or gemcitabine were administered every 3 days, and pO2 was measured after each dose at several time points. Oxygen partial pressure was determined from the linewidth of the CW EPR signal (Bruker E540L) or from the T2 measured using the electron spin echo sequence (Jiva-25™). The oxygen sensitivity was determined from a calibration curve as 6.1 mG/mm Hg in CW EPR and 68.5 ms-1/mm Hg in pulse EPR. A slight increase in pO2 of up to 20 mm Hg was observed after the third dose of gemcitabine compared to the control. The maximum delta pO2 during the therapy correlated with better survival. Both techniques offer fast and reliable oximetry in vivo, allowing to follow the effects of pharmaceutic intervention.

Molecular Structure Related to an S = 5/2 High-Spin S2 State Manganese Cluster of Photosystem II Investigated by Q-Band Pulse EPR Spectroscopy.

The high-spin S2 state of the photosynthetic oxygen-evolving cluster Mn4CaO5, corresponding to the g = 4.1 signal for X-band electron paramagnetic resonance (EPR), was investigated using Q-band pulsed EPR, which detected a main peak at g = 3.10 and satellite peaks at 5.25, 4.55, and 2.80. We evaluated the spin state as the zero-field splitting of D = 0.465 cm-1 and E/D = 0.245 with S = 5/2. The temperature dependence of the T1 relaxation time revealed that the excited-state energy was 28.7 cm-1 higher than that of the S = 5/2 ground state. By comparing present quantum mechanical (QM) calculation models, a closed-cubane structure with the protonation state of two oxygens, W1 (= OH-) and W2 (= H2O), was the most probable structure for the S = 5/2 state. The three-pulse electron spin-echo envelope modulation (ESEEM) detected the nuclear signal, which was assigned to nitrogen as His332 ligated to the Mn1 ion. The obtained hyperfine constant for the nitrogen signal was significantly reduced from that in the S = 1/2 low-spin state. These results indicate that the S = 5/2 spin state arises from the closed-cubane structure.

Pulse EPR spectroscopy and molecular modeling reveal the origins of the local heterogeneity of dietary fibers

Optimizing human diet by including dietary fibers would be more efficient when the fibers' chain interactions with other molecules are understood in depth. Thereby, it is important to develop methods for characterizing the fiber chain to be able to monitor its structural alterations upon intermolecular interactions. Here, we demonstrate the utility of the electron paramagnetic resonance (EPR) spectroscopy, complemented by simulations in probing the atomistic details of the chain conformations for spin-labeled fibers. Barley β-glucan, a native polysaccharide with linear chain, was utilized as a test fiber system to demonstrate the technique's capabilities. Pulse dipolar EPR data show good agreement with results of the fiber chain modeling, revealing sinuous chain conformations and providing polymer shape descriptors: the gyration tensor, spin-spin distance distribution function, and information about proton density near the spin probe. Results from EPR measurements point to the fiber aggregation in aqueous solution, which agrees with the results of the dynamic light scattering. We propose that the combination of pulse EPR measurements with modeling can be a perfect experimental tool for in-depth structural investigation of dietary fibers and their interaction under such conditions, and that the presented methodology can be extended to other weakly ordered or disordered macromolecules.

Inorganic Chemistry in Dalian University of Technology: Fundamental Science and Solution for Renewable Energy, Environment, Health, and Materials

Inorganic Chemistry in Dalian University of Technology: Fundamental Science and Solution for Renewable Energy, Environment, Health, and Materials

High-resolution spectroscopy of a single nitrogen-vacancy defect at zero magnetic field

We report a study of high-resolution microwave spectroscopy of nitrogen-vacancy (NV) centers in diamond crystals at and around zero magnetic field. We observe characteristic splitting and transition imbalance of the hyperfine transitions, which originate from level anti-crossings (LACs) in the presence of a transverse effective field. We use pulsed electron spin resonance spectroscopy to measure the zero-field spectral features of single NV centers for clearly resolving such LACs. To quantitatively analyze the magnetic resonance behavior of the hyperfine spin transitions in the presence of the effective field, we present a theoretical model, which describes the transition strengths under the action of an arbitrarily polarized microwave magnetic field. Our results are of importance for the optimization of the experimental conditions for the polarization-selective microwave excitation of spin-1 systems in zero or weak magnetic fields.

Recent Developments on Understanding Charge Transfer in Molecular Electron Donor-Acceptor Systems.

Charge transfer (CT) in molecular electron donor-acceptor systems is pivotal for artificial photosynthesis, photocatalysis, photovoltaics and fundamental photochemistry. We summarized the recent development in study of CT and discussed its application in thermally activated delayed fluorescence (TADF) emitters. The direct experimental proof of the spin multiplicity of the charge separated (CS) state with pulsed laser excited time-resolved electron paramagnetic resonance (TREPR) spectroscopy was discussed. Experimental determination of the electron exchange energy (J) of the CS state, with magnetic field effect on its yield or lifetime was introduced. The electron spin transfer accompanying the CT, studied with pulsed EPR spectra was briefly discussed. Tuning of the CT yield and kinetics with selective vibration excitation of the linker (the bridge) with IR pulse was presented. Above all, these studies show that there are more fun than simply monitoring the formation of the cations and anions and the kinetics or CS yields in this area.

The effect of the zero-field splitting in light-induced pulsed dipolar electron paramagnetic resonance (EPR) spectroscopy.

Laser-induced magnetic dipole (LaserIMD) spectroscopy and light-induced double electron-electron resonance (LiDEER) spectroscopy are important techniques in the emerging field of light-induced pulsed dipolar electron paramagnetic resonance (EPR) spectroscopy (light-induced PDS). These techniques use the photoexcitation of a chromophore to the triplet state and measure its dipolar coupling to a neighboring electron spin, which allows the determination of distance restraints. To date, LaserIMD and LiDEER have been analyzed with software tools that were developed for a pair of two spins and that neglected the zero-field splitting (ZFS) of the excited triplet. Here, we explore the limits of this assumption and show that the ZFS can have a significant effect on the shape of the dipolar trace. For a detailed understanding of the effect of the ZFS, a theoretical description for LaserIMD and LiDEER is derived, taking into account the non-secular terms of the ZFS. Simulations based on this model show that the effect of the ZFS is not that pronounced in LiDEER for experimentally relevant conditions. However, the ZFS leads to an additional decay in the dipolar trace in LaserIMD. This decay is not that pronounced in Q-band but can be quite noticeable for lower magnetic field strengths in X-band. Experimentally recorded LiDEER and LaserIMD data confirm these findings. It is shown that ignoring the ZFS in the data analysis of LaserIMD traces can lead to errors in the obtained modulation depths and background decays. In X-band, it is additionally possible that the obtained distance distribution is plagued by long distance artifacts.

Dipolar-Coupled Entangled Molecular 4f Qubits.

We demonstrate by use of continuous wave- and pulse-electron paramagnetic resonance spectroscopy on oriented single crystals of magnetically dilute YbIII ions in Yb0.01Lu0.99(trensal) that molecular entangled two-qubit systems can be constructed by exploiting dipolar interactions between neighboring YbIII centers. Furthermore, we show that the phase memory time and Rabi frequencies of these dipolar-interaction-coupled entangled two-qubit systems are comparable to the ones of the corresponding single qubits.

Optical Initialization of Molecular Qubit Spin States Using Weak Exchange Coupling to Photogenerated Fullerene Triplet States.

The ability to initialize an electron spin qubit into a well-defined state is an important criterion for quantum information applications. To achieve this goal, a chromophore photoexcited to its triplet state is used to strongly spin polarize a nearby stable radical in a series of C60 fullerene derivatives containing a covalently linked α,γ-bisdiphenylene-β-phenylallyl (BDPA) radical. Selective photoexcitation of C60 results in up to 20-fold enhancement of the BDPA spin polarization observed by pulse electron paramagnetic resonance spectroscopy at room temperature. The sign of the spin polarization depends on the nature of the molecular spacer between C60 and BDPA. In addition, transient absorption spectroscopy and pulse-EPR measurements reveal that the BDPA spin polarization is derived from spin polarization transfer from the C60 triplet state by weak exchange coupling over a 1 nm distance.

Hyperfine interactions and coherent spin dynamics of isotopically purified 167Er3+ in polycrystalline Y2O3

167Er3+ doped solids are a promising platform for quantum technology due to erbium’s telecom C-band optical transition and its long hyperfine coherence times. We experimentally study the spin Hamiltonian and dynamics of 167Er3+ spins in Y2O3 using electron paramagnetic resonance (EPR) spectroscopy. The anisotropic electron Zeeman, hyperfine and nuclear quadrupole matrices are fitted using data obtained by X-band (9.5 GHz) EPR spectroscopy. We perform pulsed EPR spectroscopy to measure spin relaxation time T 1 and coherence time T 2 for the 3 principal axes of an anisotropic g tensor. Long electronic spin coherence time up to 24.4 μs is measured for lowest g transition at 4 K, exceeding previously reported values at much lower temperatures. Measurements of decoherence mechanism indicates T 2 limited by spectral diffusion and instantaneous diffusion. Long spin coherence times, along with a strong anisotropic hyperfine interaction makes 167Er3+:Y2O3 a rich system and an excellent candidate for spin-based quantum technologies.

Conventional and high-field pulsed EPR experimental studies on Bazhenov oil formation under the influence of 50 Hz electromagnetic field

Heavy and extra-heavy oil are generating considerable interest in terms of replacing light oil role in the next few years. Nowadays, various enhanced oil recovery (EOR) methods are under development for an effective exploration and oil refining of unconventional oil sources. Electrical methods are among the most commonly discussed types of EOR techniques. Despite this interest, no one as far as we know has studied the effectiveness and mechanisms of electromagnetic (EM) heating influence on heavy oil conversion. It is common knowledge that heavy oils often contain stable paramagnetic centers, which potentially can serve as at least signaling species to follow the EOR upgrading degree. For the first time, to the best of our knowledge, pulsed and high-frequency (high-field, with 3.4 T detection magnetic field) electron paramagnetic resonance (EPR) techniques were exploited to investigate the EM heating influence on Bazhenov formation heavy oil by subjecting it to 50 Hz (0.75 T) electromagnetic field in laboratory conditions. The obtained samples were analyzed by a set of physico-chemical methods including SARA analysis (S: Saturates, A: Aromatics, R: Resins, A: Asphaltenes), gas chromatography–mass spectrometry (GCMS) in addition to pulsed and conventional (cw) EPR spectroscopy. SARA analysis has shown an increase in the content of asphaltenes from 1.2 ± 0.3 wt% in the initial oil to 1.4 ± 0.4, 1.4 ± 0.3, 1.3 ± 0.2, 1.3 ± 0.5 wt% in the samples obtained at 10, 20, 30 and 40 min respectively. Interestingly, this content has drastically dropped to 0.2 ± 0.05 and 0.3 ± 0.02 wt% in the samples obtained at 50 and 60 min of EM heating. Analogically for resins content change which has firstly increased from 1.5 ± 0.5 wt% in the initial oil to 1.9 ± 0.5, 2.6 ± 0.6, 3.0 ± 0.2, 3.0 ± 0.6 wt% in the samples obtained at 10, 20, 30 and 40 min respectively. Then this content has decreased to 0.5 ± 0.05 and 1.0 ± 0.04 wt% in the samples obtained at 50 and 60 min of EM heating. The content of saturated and aromatic compounds has been found to reach 99.3 ± 0.6 wt% in the sample obtained at 50 min comparing to 97.3 ± 1.1 wt% in the initial oil. Moreover, the GCMS data have showed a significant increase in the content of normal alkanes, which resulted from the processes of oil high molecular compounds fractions conversion as a consequence of alkyl substituents elimination. Our data points towards the idea that the investigated oil sample EPR signal intensity and their spectroscopic characteristics do not change with the EM heating either in the X- or W-band frequencies. g|| and g⊥ were found equal to 2.0024 and 2.0015 respectively at 3357 and 3358 mT for the initial sample and the obtained sample after 60 min of EM heating. Similar results (g||=2.0004 and g⊥=1.9991) were obtained for these samples at 3360 and 3362 mT. The findings of this study can help for better understanding of the mechanisms ruling the impact of electromagnetic heating on enhancing oil recovery.

Continuous wave and pulsed EPR study of Cd1-xMnxTe crystals with different Mn content

The cadmium manganese telluride (Cd1–xMnxTe) crystals (x < 0.001 and x = 0.02, 0.04, 0.1) grown using the Bridgman method were studied by applying continuous wave and pulsed electron paramagnetic resonance (EPR) spectroscopy in the wide temperature range. The Cd1–xMnxTe crystals with x < 0.001 revealed the EPR spectrum from isolated Mn2+ with g⊥ = g∥ = 2.0074(3), |A⊥| = |A∥| = 56.97 ⋅ 10–4 cm–1, |a| = 30.02⋅10–4 cm–1, while Cd1–xMnxTe crystals with x = 0.02…0.04 are characterized by two single broad isotropic EPR lines of Lorentzian shape (g ~ 2.009 and g ~ 1.99) due to Mn clusters of different sizes. The EPR spectrum of Cd1–xMnxTe crystals with x = 0.01 consists of the single broad line at g ~ 2.01 due to higher level of homogeneity inherent to these crystals. The temperature dependence of spin relaxation times for the isolated Mn2+ center in the Cd1–xMnxTe crystals with x < 0.001 has been described using the conceptions of Orbach process for TM–1 and two-phonon Raman process for T1–1.

Rx, Ry and Rz Rotation Operators of Spin 4 Systems in Quantum Information Theory

Quantum computing requires use of various physical techniques together with quantum theory. One of the promising systems is spin systems as applied and seen in pulsed nuclear magnetic resonance (NMR) and pulsed electron paramagnetic resonance (EPR) spectroscopies and hence spin-based quantum information technology. Construction of higher spin systems and related rotation operators is important for the theoretical infrastructure that can be used in quantum information theory. It is expected that as the value of spin increases, it will give way to longer time in the computation with bigger data. Spin operators up to spin-4 have been published in previous studies. In this work, explicit symbolic expressions of x, y and z components of rotation operators for spin-4 were worked out via exponential operator for each element of related spin operator matrices and simple linear curve fitting process. The procedures gave out exact expressions of each element of the rotation operators. It can be predicted that quantum rotation operators for higher spins, like spin-4, will theoretically and practically contribute to spin-based quantum information technology.

Design Principles for the Development of Gd(III) Polarizing Agents for Magic Angle Spinning Dynamic Nuclear Polarization

Nuclear magnetic resonance suffers from an intrinsically low sensitivity, which can be overcome by dynamic nuclear polarization (DNP). Gd(III) complexes are attractive exogenous polarizing agents for magic angle spinning (MAS) DNP due to their high chemical stability in contrast to nitroxide-based radicals. However, even the state-of-the-art Gd(III) complexes have so far provided relatively low DNP signal enhancements of ca. 36 in comparison to standard DNP biradicals, which show enhancements of over 200. Here, we report a series of new Gd(III) complexes for DNP and show that the observed DNP enhancements of the new and existing Gd(III) complexes are inversely proportional to the square of the zero-field splitting (ZFS) parameter D, which is in turn determined by the ligand-type and the local coordination environment. The experimental DNP enhancements at 9.4 T and the ZFS parameters measured with pulsed electron paramagnetic resonance (EPR) spectroscopy agree with the above model, paving the way for the development of more efficient Gd(III) polarizing agents.

Pulsed Multifrequency Electron Paramagnetic Resonance Spectroscopy Reveals Key Branch Points for One- vs Two-Electron Reactivity in Mn/Fe Proteins.

Traditionally, the ferritin-like superfamily of proteins was thought to exclusively use a diiron active site in catalyzing a diverse array of oxygen-dependent reactions. In recent years, novel redox-active cofactors featuring heterobimetallic Mn/Fe active sites have been discovered in both the radical-generating R2 subunit of class Ic (R2c) ribonucleotide reductases (RNRs) and the related R2-like ligand-binding oxidases (R2lox). However, the protein-specific factors that differentiate the radical reactivity of R2c from the C-H activation reactions of R2lox remain unknown. In this work, multifrequency pulsed electron paramagnetic resonance (EPR) spectroscopy and ligand hyperfine techniques in conjunction with broken-symmetry density functional theory calculations are used to characterize the molecular and electronic structures of two EPR-active intermediates trapped during aerobic assembly of the R2lox Mn/Fe cofactor. A MnIII(μ-O)(μ-OH)FeIII species is identified as the first EPR-active species and represents a common state between the two classes of redox-active Mn/Fe proteins. The species downstream from the MnIII(μ-O)(μ-OH)FeIII state exhibits unique EPR properties, including unprecedented spectral breadth and isotope-dependent g-tensors, which are attributed to a weakly coupled, hydrogen-bonded MnIII(μ-OH)FeIII species. This final intermediate precedes formation of the MnIII/FeIII resting state and is suggested to be relevant to understanding the endogenous reactivity of R2lox.

Librational Dynamics of Spin-Labeled Membranes at Cryogenic Temperatures From Echo-Detected ED-EPR Spectra.

Methods of electron spin echo of pulse electron paramagnetic resonance (EPR) spectroscopy are increasingly employed to investigate biophysical properties of nitroxide-labeled biosystems at cryogenic temperatures. Two-pulse echo-detected ED-spectra have proven to be valuable tools to describe the librational dynamics in the low-temperature phases of both lipids and proteins in membranes. The motional parameter, , given by the product of the mean-square angular amplitude, , and the rotational correlation time, , of the motion, is readily determined from the nitroxide ED-spectra as well as from the W-relaxation rate curves. An independent evaluation of is obtained from the motionally averaged 14N-hyperfine splitting separation in the continuous wave cw-EPR spectra. Finally, the rotational correlation time can be estimated by combining ED- and cw-EPR data. In this mini-review, results on the librational dynamics in model and natural membranes are illustrated.