Electron Paramagnetic Resonance Absorption Research Articles

A MnII MnIII -Peroxide Complex Capable of Aldehyde Deformylation.

Ribonucleotide reductases (RNRs) are essential enzymes required for DNA synthesis. In class Ib Mn2 RNRs superoxide (O2.- ) was postulated to react with the MnII2 core to yield a MnII MnIII -peroxide moiety. The reactivity of complex 1 ([MnII2 (O2 CCH3 )2 (BPMP)](ClO4 ), where HBPMP=2,6-bis{[(bis(2-pyridylmethyl)amino]methyl}-4-methylphenol) towards O2.- was investigated at -90 °C, generating a metastable species, 2. The electronic absorption spectrum of 2 displayed features (λmax =440, 590 nm) characteristic of a MnII MnIII -peroxide species, representing just the second example of such. Electron paramagnetic resonance and X-ray absorption spectroscopies, and mass spectrometry supported the formulation of 2 as a MnII MnIII -peroxide complex. Unlike all other previously reported Mn2 -peroxides, which were unreactive, 2 proved to be a capable oxidant in aldehyde deformylation. Our studies provide insight into the mechanism of O2 -activation in Class Ib Mn2 RNRs, and the highly reactive intermediates in their catalytic cycle.

Structural isomers of the S2 state in photosystem II: do they exist at room temperature and are they important for function?

In nature, an oxo-bridged Mn4 CaO5 cluster embedded in photosystem II (PSII), a membrane-bound multi-subunit pigment protein complex, catalyzes the water oxidation reaction that is driven by light-induced charge separations in the reaction center of PSII. The Mn4 CaO5 cluster accumulates four oxidizing equivalents to enable the four-electron four-proton catalysis of two water molecules to one dioxygen molecule and cycles through five intermediate S-states, S0 -S4 in the Kok cycle. One important question related to the catalytic mechanism of the oxygen-evolving complex (OEC) that remains is, whether structural isomers are present in some of the intermediate S-states and if such equilibria are essential for the mechanism of the O-O bond formation. Here we compare results from electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy (XAS) obtained at cryogenic temperatures for the S2 state of PSII with structural data collected of the S1 , S2 and S3 states by serial crystallography at neutral pH (∼6.5) using an X-ray free electron laser at room temperature. While the cryogenic data show the presence of at least two structural forms of the S2 state, the room temperature crystallography data can be well-described by just one S2 structure. We discuss the deviating results and outline experimental strategies for clarifying this mechanistically important question.

ESR and luminescent properties of anion-deficient α-Al2O3 single crystals after high-dose irradiation by a pulsed electron beam

Electron spin resonance (ESR) and photoluminescence (PL) spectra of anion-deficient alumina single crystals were measured. High-dose irradiation by a pulsed electron beam (130 keV) causes an absorption line with g┴ = 2.008 ± 0.002 in the ESR spectrum. In contrast to the irradiation by the beta-source, there is no overlapping of this line with another one in the obtained ESR spectrum, which allowed to investigate the thermal stability of this line more in details and to prove that it consists of several elementary components. A drop in PL intensity of F+-centers (3.8 eV) and F-centers (3.0 eV) is observed with increasing of ESR absorption line with g┴ = 2.008 ± 0.002. Parallel studies of ESR and PL spectra under stepped annealing in the temperature range of 523–973 K showed that the radiation-induced paramagnetic center was stable up to 873–973 K. A possible association of the disappearance of the ESR absorption line with the destruction of F-type aggregate defect and with changing charge state of the paramagnetic center caused by emptying of deep electron and hole traps at Т = 773–973 K is discussed in the paper.

Electric field modulation of magnetic exchange in molecular helices.

The possibility to operate on magnetic materials through the application of electric rather than magnetic fields-promising faster, more compact and energy efficient circuits-continues to spur the investigation of magnetoelectric effects. Symmetry considerations, in particular the lack of an inversion centre, characterize the magnetoelectric effect. In addition, spin-orbit coupling is generally considered necessary to make a spin system sensitive to a charge distribution. However, a magnetoelectric effect not relying on spin-orbit coupling is appealing for spin-based quantum technologies. Here, we report the detection of a magnetoelectric effect that we attribute to an electric field modulation of the magnetic exchange interaction without atomic displacement. The effect is visible in electron paramagnetic resonance absorption of molecular helices under electric field modulation and confirmed by specific symmetry properties and spectral simulation.

Motional Narrowing of Electron Spin Resonance Absorption in the Plastic-Crystal Phase of [(CH3)4N]FeCl4

We report the temperature dependence of the electron spin resonance (ESR) absorption and magnetic susceptibility of plastic crystal [(CH3)4N]FeCl4. We demonstrate the extreme sensitivity of ESR to the static and dynamic structural transitions, characteristic of this material. We observed the narrowing of the ESR line in the high temperature plastic-crystal phase, which is due to fast rotation (τ ≈ 10–9 s) of the [FeCl4]− anion. This is a rare example of the effect of fast motional narrowing in crystalline solids, in which ions occupy fixed positions in the crystal lattice.

Enhanced photocatalytic hydrogen evolution activity of carbon and nitrogen self-doped TiO2 hollow sphere with the creation of oxygen vacancy and Ti3+

Introducing oxygen vacancies and Ti3+ into TiO2 is thought to be an effective method of increasing the utilization of sunlight and promoting the efficiency of electron transfer. In this work, novel carbon and nitrogen self-doped TiO2 hollow spheres (CNTH) with oxygen vacancies and Ti3+ were synthesized by a one-pot hydrothermal strategy in H2OC2H5OHHFH2O2 mixed solution with TiCN as a precursor of anatase TiO2 and self-doping source of N and C. The presence of oxygen vacancies and Ti3+ in the resulting CNTH photocatalyst was supported by electron paramagnetic resonance (EPR) and X-ray absorption near-edge structure spectroscopy (XANES). X-ray photoelectron spectroscopy (XPS) analysis showed that the C and N atoms were successfully self-doped into TiO2 lattice, which resulted in the CNTH materials exhibiting the enhanced absorption in the UV–visible region with a clear red shift at the absorption edges. The electronic structure of CNTH was examined by first-principle density functional theory (DFT) calculations, which further confirmed the optical properties of CNTH for the enhanced visible-light absorption properties and the red-shift phenomenon of the absorption edges. The resulting CNTH photocatalyst showed enhanced visible-light photocatalytic activity for H2 production. The enhanced activity of CNTH was due to the introduction of C and N self-doping and the existence of oxygen vacancies and Ti3+. The former can promote the photocatalytic reactions of the photogeneration electrons and holes on CNTH under visible-light irradiation, while the latter create more active sites and prolong the life time of electrons and holes by their donor states.

Electron paramagnetic resonance and optical absorption study of acceptors in CdSiP2 crystals

Cadmium silicon diphosphide (CdSiP2) is a nonlinear material often used in optical parametric oscillators (OPOs) to produce tunable laser output in the mid-infrared. Absorption bands associated with donors and acceptors may overlap the pump wavelength and adversely affect the performance of these OPOs. In the present investigation, electron paramagnetic resonance (EPR) is used to identify two unintentionally present acceptors in large CdSiP2 crystals. These are an intrinsic silicon-on-phosphorus antisite and a copper impurity substituting for cadmium. When exposed to 633 nm laser light at temperatures near or below 80 K, they convert to their neutral paramagnetic charge states (SiP0 and CuCd0) and can be monitored with EPR. The corresponding donor serving as the electron trap is the silicon-on-cadmium antisite (SiCd2+ before illumination and SiCd+ after illumination). Removing the 633 nm light and warming the crystal above 90 K quickly destroys the EPR signals from both acceptors and the associated donor. Broad optical absorption bands peaking near 0.8 and 1.4 μm are also produced at low temperature by the 633 nm light. These absorption bands are associated with the SiP0 and CuCd0 acceptors.

On the greenish-yellow color of natural Brazilian titanite

Natural greenish-yellow titanites from two localities in Brazil (Bananal/Capelinha, MG and Xambioa, TO) were characterized by electron paramagnetic resonance (EPR) and optical absorption. Along with vanadyl ions (VO2+) substituting Ti4+, two other paramagnetic centers were identified: the Fe3+ and the Mn2+. For the VO2+, the principal values and orientations of the g and A tensors were derived from a careful analysis of the angular dependence of the EPR spectra. The obtained results, although different to previously reported data, still support the model for the VO2+ bonds to the nearest oxygen (O1) in the TiO6-distorted octahedra. In addition, the titanite optical absorption spectra were analyzed, and the yellow color was ascribed to Fe3+ ↔ O2− ligand–metal charge transfer transitions (LMCT) in the near-ultraviolet, extending to the violet and blue spectral ranges. The VO2+ center seen by EPR was in low concentration and had no visible effect on the color. Finally, γ-irradiation up to 200 kGy and thermal treatments in oxidizing/reducing atmospheres up to 600–700 °C also had no big influence on the dominant yellow color.

Morphine, a potential inhibitor of myeloperoxidase activity

Morphine is an opioid alkaloid commonly used in clinical practice for its analgesic properties. The phenolic hydroxyl group of that phenanthrene derivative is pivotal for binding to opioid receptors but it may also be responsible for the antioxidant behavior of morphine reported in several in vitro experiments. In this study, we assessed the effect of morphine on myeloperoxidase (MPO), a hemic enzyme from azurophilic granules of polymorphonuclear neutrophils involved in the production of cytotoxic and microbicidal reactive oxidants during inflammatory response. Specific immunological extraction followed by enzyme detection (SIEFED) and molecular modeling (docking) were performed to study the potential anti-catalytic action of morphine on MPO in comparison with the inhibitory effects of reference antioxidant molecules quercetin, gallic acid and ascorbic acid. The reducing action of morphine on the MPO peroxidase cycle has been investigated using electron paramagnetic resonance (EPR) and UV-visible absorption spectroscopy. Morphine acted as a reducing substrate in the peroxidase cycle of MPO and therefore protected the enzyme against the suicide action of its natural substrate, hydrogen peroxide. The SIEFED experiments associated with the docking study, further demonstrated a lack of strong and sustained anti-catalytic activity of morphine. In summary, from the results of this study, it appears that morphine acts as a weak and reversible inhibitor of MPO that may nonetheless contribute to immunomodulatory and antioxidant effects of this opioid analgesic in vivo.

Landscape of Charge Carrier Transport in Doped Poly(3-hexylthiophene): Noncontact Approach Using Ternary Combined Dielectric, Paramagnetic, and Optical Spectroscopies.

We report on a comprehensive measurement system for mobility and energy states of charge carriers in matter under dynamic chemical doping. The temporal evolution of the iodine doping process of poly(3-hexylthiophene) (P3HT) was monitored directly through electron paramagnetic resonance (EPR) and optical absorption spectroscopy, as well as differential electrical conductivity by the microwave conductivity measurement. The increase in conductivity was observed after the EPR intensity reached a maximum and declined thereafter, and the conductivity finally reached ∼80 S cm-1. The carrier species changed from a paramagnetic polaron with an estimated mobility of μP+ ≈ 2 × 10-3 cm2 V-1 s-1 to an antiferromagnetic polaron pair with μPP+ ≈ 0.6 cm2 V-1 s-1. The technique presented here can be a ubiquitous method for rapid and direct observation of charge carrier mobility and energy states in p-type semiconducting materials as a completely noncontact, experimental, and quantitative technique.

Spectroscopic characterization of the Co-substituted C-terminal domain of rubredoxin-2.

Pseudomonas putida rubredoxin-2 (Rxn2) is an essential member of the alkane hydroxylation pathway and transfers electrons from a reductase to the membrane-bound hydroxylase. The regioselective hydroxylation of linear alkanes is a challenging chemical transformation of great interest for the chemical industry. Herein, we report the preparation and spectroscopic characterization of cobalt-substituted P. putida Rxn2 and a truncated version of the protein consisting of the C-terminal domain of the protein. Our spectroscopic data on the Co-substituted C-terminal domain supports a high-spin Co(II) with a distorted tetrahedral coordination environment. Investigation of the two-domain protein Rxn2 provides insights into the metal-binding properties of the N-terminal domain, the role of which is not well understood so far. Circular dichroism, electron paramagnetic resonance and X-ray absorption spectroscopies support an alternative Co-binding site within the N-terminal domain, which appears to not be relevant in nature. We have shown that chemical reconstitution in the presence of Co leads to incorporation of Co(II) into the active site of the C-terminal domain, but not the N-terminal domain of Rxn2 indicating distinct roles for the two rubredoxin domains.

Experimental and theoretical investigations on the EPR parameters and molecular orbital bonding coefficients of VO2+ ions in BTTB glasses

The effect of the spin probe VO2+ in 15BaO–15TeO2–10TiO2-(60-x) B2O3–xV2O5 (x = 0.2, 0.4, 0.6, 0.8 mol %) glasses has been studied by employing Electron Paramagnetic Resonance (EPR) and optical absorption spectroscopic techniques. The observed EPR spectra of VO2+ ions were attributed on the basis of well-known spin-Hamiltonian of C4V symmetry. The simulated EPR spectra for VO2+ ions in the present glass system were drawn using Easy spin software. Both the experimental and simulated spectra were found to be in good agreement with each other. The optical absorption spectra exhibited three d–d transition bands due to crystal and tetragonal fields of VO2+ ions. These bands were assigned to 2B2g→ 2Eg, 2B2g→ 2B1g and 2B2g→ 2A1g transitions. The crystal field parameters Dq, Ds and Dt values are calculated. From the EPR and optical data, the molecular bonding coefficients were evaluated. Employing the higher order perturbation formulae of the g factors for 3d1 ion under tetragonally compressed octahedral fields, theoretical studies were carried out. The spin-Hamiltonian parameters and obtained from both the experimental and theoretical methods were in good agreement with each other.

Magnetoelectric properties of Ca2CoSi2O7 studied by high-field electron spin resonance

We have performed electron spin resonance (ESR) experiments of multiferroic Ca2CoSi2O7 single crystals in high magnetic fields perpendicular to the easy plane. We observed many ESR absorption signals in the magnetization plateau phase below the saturation field which form branches with a linear magnetic field dependence. At zero magnetic field, large energy gaps with Δ = 1.7 ∼ 1.8 THz are identified by linear extrapolations of the resonance branches. We speculate that the high-energy ESR modes would be caused by electromagnons. With increasing magnetic field, some of these ESR modes decrease linearly with field even within the magnetization plateau phase and intersect the zero-frequency line at the magnetic field where the magnetization plateau terminates, suggesting a relationship between these energy gaps and the magnetization plateau.

Defects induced in cerium dioxide single crystals by electron irradiation

Micro-Raman spectroscopy, X-band electron paramagnetic resonance (EPR) spectroscopy, and UV-visible optical absorption spectroscopy were used to study the damage production in cerium dioxide (CeO2) single crystals by electron irradiation for three energies (1.0, 1.4, and 2.5 MeV). The Raman-active T2g peak was left unchanged after 2.5-MeV electron irradiation at a high fluence. This shows that no structural modifications occurred for the cubic fluorite structure. UV-visible optical absorption spectra exhibited a characteristic sub band-gap tail for 1.4-MeV and 2.5-MeV energies, but not for 1.0 MeV. Narrow EPR lines were recorded near liquid-helium temperature after 2.5-MeV electron irradiation; whereas no such signal was found for the virgin un-irradiated crystal or after 1.0-MeV irradiation for the same fluence. The angular variation of these lines in the {111} plane revealed a weak g-factor anisotropy assigned to Ce3+ ions (with the 4f1 configuration) in a high-symmetry local environment. It is concluded that Ce3+ ions may be produced by a reduction resulting from the displacement damage process. However, no evidence of F+ or F0 center or hole center formation due to irradiation was found from the present EPR and optical absorption spectra.

Syntheses, structures and theoretical calculations of stable triarylarsine radical cations.

The first stable triarylarsine radical cation salts 1˙+[BArF4]- and 2˙+[BArF4]- were afforded by one-electron oxidation of triarylarsines Trip3As (1, Trip = 2,4,6-iPr3C6H2) and Dipp3As (2, Dipp = 2,6-iPr2C6H3) with AgSbF6 and NaBArF4 (ArF = 3,5-(CF3)2C6H3), respectively. Their molecular and electronic structures were investigated by single-crystal X-ray diffraction, electron paramagnetic resonance (EPR) and UV-vis absorption spectroscopy, in conjunction with density functional theory (DFT) calculations. The EPR and computational results demonstrate that the spin density of 1˙+ and 2˙+ is mainly distributed on the As nuclei.

EPR and Optical absorption studies of Na2O-Al2O3- B2O3-CuO glasses

Alkali borate glasses are promising materials having applications in solid state batteries, fuel cells etc. These glasses are little hygroscopic due to the presence of alkali content. The addition of Al2O3 makes these glasses resistant to alkali oxides. Al2O3 depending upon its concentration in the glass matrix acts as network modifier or glass former. Hence an attempt is made to prepare the glasses of the composition (30-x) Na2O-xAl2O3-69B2O3-1CuO by varying Al2O3 by 0, 5, 10 & 15 mole % replacing equal amounts of Na2O. Glasses are prepared by conventional melt quenching method. X-ray diffractograms are recorded at room temperature. Peak-free X-ray diffractograms confirmed the amorphous nature of these glass samples. Electron Paramagnetic Resonance (EPR) and Optical absorption studies were carried out on Na2O-Al2O3-B2O3 glasses using Cu2+ as spin probe. From the EPR Spectra spin-Hamiltonian parameters were evaluated. It was observed that ((∼2.33)>>>ge(=2.0023) and >>.. From the above results it was concluded that the ground state of Cu2+ orbital is (2_(B_1g)) state and the site symmetry around the Cu2+ is tetragonally distorted Octahedral. The optical absorption spectra revealed a broad absorption band which is assigned to 2B1g→2B2gtransition. By correlating EPR and Optical data bonding coefficients α2,β2,wereevaluated.were evaluated.

Binding of Oxovanadium(IV) Complexes to Blood Serum Albumins

In this work the binding of VIVO2+ and VIVO-complexes to serum albumins {human serum albumin (HSA), bovine serum albumin (BSA) and porcine serum albumin (PSA)} are studied using circular dichroism (CD), electron paramagnetic resonance (EPR) and visible absorption spectroscopy. The results confirm previous findings that VIVO2+ occupies at least two types of binding sites on albumin: ‘the strong vanadium binding site’ (designated by VBS1) and ‘the weak vanadium binding sites’ (designated by VBS2). VBS1 binds 1 mol equivalent of VIVO2+. On the other hand VBS2 correspond to binding of several mol equivalents of VIVO, and studies done with PSA in the presence of excess ZnII ions indicate that VSB2 corresponds to two distinct types of sites. The hyperfine coupling constant Az for VIVO2+ binding at VBS2 on HSA and BSA are all very similar (~168 × 10-4 cm-1) but differ slightly on PSA (~166 × 10-4 cm-1) due to differences in the binding sets. When (VIVO)-HSA systems are titrated with maltol ternary species of (maltol)m(VIVO)mHSA and (maltol)2m(VIVO)mHSA stoichiometry form which are clearly distinguishable from the binary (VIVO)-HSA system by the type and intensity of the CD spectra recorded. Changes are also observable in the intensity of the X-band EPR spectra, but not much in the hyperfine coupling constants Az, which are all in the range 166-167 × 10-4 cm-1. The results further demonstrate that the presence of maltol may enhance the binding of VIVO to albumin.

Identification of Superoxide O2− during Thermal Decomposition of Molten KNO3-NaNO2-NaNO3 Salt by Electron Paramagnetic Resonance and UV-Vis Absorption Spectroscopy

On account of excellent thermal physical properties, molten nitrates/nitrites salt has been widely employed in heat transfer and thermal storage industry, especially in concentrated solar power system. The thermal stability study of molten nitrate/nitrite salt is of great importance for this system, and the decomposition mechanism is the most complicated part of it. The oxide species O22− and O2− were considered as intermediates in molten KNO3-NaNO3 while hard to been detected in high temperature molten salt due to their trace concentration and low stability. In this work, the homemade in situ high temperature UV-Vis instrument and a commercial electron paramagnetic resonance were utilized to supply evidence for the formation of superoxide during a slow decomposition process of heat transfer salt (HTS, 53 wt% KNO3/40 wt% NaNO2/7 wt% NaNO3). It is found that the superoxide is more easily generated from molten NaNO2 compared to NaNO3, and it has an absorption band at 420–440 nm in HTS which red shifts as temperature increases. The band is assigned to charge-transfer transition in NaO2 or KO2, responsible for the yellow color of the molten nitrate/nitrite salt. Furthermore, the UV absorption bands of molten NaNO2 and NaNO3 are also obtained and compared with that of HTS.

Study of CuO content on physical and structural properties of Li2O-B2O3-CuO glasses using electron paramagnetic resonance

Spectroscopic studies based on optical absorption, electron paramagnetic resonance (EPR), and Fourier transform infrared (FTIR) spectroscopy were performed to understand the effects of modifier oxides and CuO dopants in glass matrices. Glass with a composition of xLi2O-B2O3-yCuO was prepared with x fixed at 0.1 and y varied from 0.005 to 0.05. The optical absorption spectra were used to determine the absorption band, which was attributed to the 2B1g→2B2g characteristic transition of Cu2+ ions in the distorted octahedral sites. The values of the spin-Hamiltonian parameters determined by the EPR studies indicated that the site symmetry around the Cu2+ ions was a tetragonally distorted octahedral. The glasses for which y was greater than 0.02 exhibited bands caused by the Cu+ ions in the UV region. The EPR and optical absorption data were used to calculate the molecular orbital coefficients α2 and β12, and the FTIR studies showed that these glasses consisted of BO3 and BO4 units.

Broadband ESR Spectroscopy With a Tunable Interferometer

This paper presents a broadband and quantitative electron spin resonance (ESR) spectroscopy technique that is tunable over a wide frequency range. Different from the detection approaches in conventional ESR instrumentation, an interferometer-based method is used to remove radio frequency probing signals at the vector network analyzer (VNA) detector. As a result, high-resolution tuning enables highly sensitive ESR detection. A planar microwave resonator is designed at ~7.6 GHz and incorporated into the interferometer to measure $0.2~\mu \text{g}$ ( $2.8 \times 10^{14}$ spins) 2,2-diphenyl-1-picrylhydrazyl (DPPH) sample at a signal-to-noise ratio (SNR) of ~121 at room temperature. The broadband ESR operation capability is investigated with a broadband meander microstrip line (MML) by measuring $6~\mu \text{g}$ ( $8.4 \times 10^{15}$ ) DPPH between 8 and 13 GHz at a ~194 SNR with 10 kHz VNA intermediate frequency. The obtained sensitivity is significantly higher than that of current broadband ESR methods. In both MR and MML cases, dispersion and absorption ESR signals at room temperature are simultaneously obtained. With MML structures, the quantification of permeability $\mu (\omega )=\mu ' (\omega ) -j\mu ''(\omega )$ is possible. Thus, the ESR techniques are also a highly sensitive ferromagnetic resonance method. More work is needed to further improve ESR sensitivity and quantification accuracy.