Observed Electron Paramagnetic Resonance Spectra Research Articles

Graphite Anodes for Li-Ion Batteries: An Electron Paramagnetic Resonance Investigation.

Graphite is the most commercially successful anode material for lithium (Li)-ion batteries: its low cost, low toxicity, and high abundance make it ideally suited for use in batteries for electronic devices, electrified transportation, and grid-based storage. The physical and electrochemical properties of graphite anodes have been thoroughly characterized. However, questions remain regarding their electronic structures and whether the electrons occupy localized states on Li, delocalized states on C, or an admixture of both. In this regard, electron paramagnetic resonance (EPR) spectroscopy is an invaluable tool for characterizing the electronic states generated during electrochemical cycling as it measures the properties of the unpaired electrons in lithiated graphites. In this work, ex situ variable-temperature (10-300 K), variable-frequency (9-441 GHz) EPR was carried out to extract the g tensors and line widths and understand the effect of metallicity on the observed EPR spectra of electrochemically lithiated graphites at four different states of lithiation. We show that the increased resolution offered by EPR at high frequencies (>300 GHz) enables up to three different electron environments of axial symmetry to be observed, revealing heterogeneity within the graphite particles and the presence of hyperfine coupling to Li nuclei. Importantly, our work demonstrates the power of EPR spectroscopy to investigate the local electronic structure of graphite at different lithiation stages, paving the way for this technique as a tool for screening and investigating novel materials for use in Li-ion batteries.

Structural characterization, continuous-wave optically stimulated luminescence, and correlation between thermoluminescence and EPR of Ce-doped Ca2Al2SiO7 phosphor synthesized by a solid-state reaction method

A calcium aluminum silicate (Ca2Al2SiO7) phosphor doped with different concentrations of cerium has been prepared by a solid-state reaction method. From X-ray diffraction (XRD) data and using the Rietveld refinement method, the phase, structure, and average crystalline size have been determined. Luminescence emissions have been investigated by optically stimulated luminescence (OSL) and thermoluminescence (TL) methods. TL studies reveal a broad peak centered at 220 °C with an emission band at 410 nm. The TL intensity of a sample doped with 0.1 mol% Ce proved to be 115 times stronger than that of an undoped sample. Electron paramagnetic resonance (EPR) has been utilized to detect the defect centers responsible for the TL process in Ce-doped Ca2Al2SiO7 phosphor. The observed EPR spectrum results from a superposition of three defect centers. Center I with a g-value of 2.0090 is assigned to an O− ion and exhibits hyperfine interaction with a nearby nucleus with spin 3/2. This center relates to the TL peak at 220 °C. Center II with an isotropic g-value of 2.0073 is also ascribed to an O− ion and correlates with the dominant 220 °C TL peak. Center III has a g-value of 2.0076 and is identified as an F+ center (singly-ionized oxygen vacancy).

A Theoretical Study of Local Electronic–Magnetic Properties for LiCoMnO 4 in the Charge and Discharge Process

Doping transition metal ions in LiMnCoO4 batteries is proved an efficient method to improve the electrochemical properties in experiment, which is related to the local environment of the cation. According to the observed electron paramagnetic resonance spectra for different signals, three different Mn4+ centers (C1, C2, and C3) are constructed and the local electronic–magnetic properties are theoretically studied based on the perturbation calculations of Spin-Hamilton Parameters (g factors). By comparing the results of conventional method (based on merely the crystal-field contribution) and the improved method (based on both the crystal-field and electron-transfer contributions), the proper theoretical model is constructed for the investigation of the [MnO6]8− clusters in LiMnCoO4. Furthermore, the electronic structure of the [MnO6]8− cluster is consistently discussed from the covalency and partial density of states in the charge and discharge processes, which may account for irreversible capacity of LiCoMnO4 batteries.

EPR characteristics of radiation-induced defects in Ge-rich α-quartz

Electron paramagnetic resonance (EPR) in a synthetic Si0.975Ge0.025O2 single crystal of α–quartz structure has been studied. After fast electron irradiation previously studied and new Ge-related paramagnetic centers (PCs) have been recognized using computer simulation of the observed EPR spectra. Increasing radiation doses allow to distinguish four paramagnetic [GeO4/M+]0 centers AGeLi, CGeLi, DGeLi, and AGeNa as well as four oxygen-vacancy electron centers Ge Eˊ1, Ge Eˊ2, Ge(IV) and a new Ge(V) center. The selective UV bleaching and distinct thermal stability of different centers allow us to determine the spin Hamiltonian parameters for the Ge(V) center.

Thermoluminescence and electron paramagnetic resonance correlation studies in lithium silicate phosphor

Lithium silicate phosphor, synthesized by the solid-state reaction method, displays three thermoluminescence (TL) peaks at 155 °C, 240 °C and 430 °C. Activation energy, frequency factor and kinetic order associated with the TL peaks have been determined using E-Tstop and glow curve deconvolution (GCD) methods. Electron Paramagnetic Resonance (EPR) studies have been carried out to identify the defect centers induced in the phosphor by gamma irradiation and also to find the centers responsible for the TL process in the system. The observed EPR spectrum arises from a superposition of three defect centers. One of the centers (center I) with a g-value 2.0097 is identified as the O− ion. Center II with an isotropic g-value 2.0002 is assigned to a F+-type center (singly ionized oxygen vacancy). The F+ center is likely to be associated with the TL peak at 156 °C. Center III characterized by a rhombic g-tensor with g1 = 1.9831, g2 = 1.9676, and g3 = 1.9208 is identified as a Ti3+ center. The Ti3+ center relates to the low temperature TL peak at 155 °C.

Quantitative EPR study of poly(vinylidene fluoride) activated by electron beam treatment

The electron paramagnetic resonance (EPR) spectra of poly (vinylidene fluoride) (PVDF) films were studied in order to identify type and intensity of trapped radicals after an electron treatment with 150 kGy in air at room temperature. The observed EPR spectra consist of superimposed spectra of eight different radicals. The individual spectra were simulated by first derivatives of normalized Gaussian functions including intensity, linewidth, g value, and hyperfine splitting constants of the unpaired electron with hydrogen or fluorine atoms in α or β positions. In addition, the number of trapped radicals was estimated using a standard sample of known spin concentrations. Although initially generated radicals tend to be converted to peroxy radicals in the presence of oxygen, the predominant radical is the CH-based mid-chain radical (–CF2–C*H-CF2-) with a relative intensity of 62.3%. This is attributed to CH-based radicals in the crystalline domains with low molecular mobility leading to an enhancement in its linewidth (ΔHpp ⁓ 3.6 mT). A fraction of 20.7% of all radicals are peroxy radicals with anisotropic g values, g∥ = 2.0210 and g⊥ = 2.0095. The relative intensity of allyl, dienyl and polyenyl radicals amounts to 4.9%.

High-spin Fe(III) Schiff based complexes with photoactive ligands. Synthesis, EPR study and magnetic properties

A series of three novel Fe(III) compounds of the formula [FeL2]X (where X = Cl−(1), PF6−(2), NO3– (3), and L is a photoactive ligand, (4)-4-(((2-(ethylamino)ethyl)imino)methyl)-3-hydroxyphenyl 4-bromobenzoate) was synthesized and studied by means of electron paramagnetic resonance (EPR) and pulsed laser irradiation. The Fe3+ ions in these compounds are in a high-spin state. A thorough analysis of the EPR data suggests that compounds 1 and 2 undergo an order–disorder ferroelectric phase transition, and below the phase transition temperature (Tc = 100 and 200 K for compounds 1 and 2, respectively) a nonzero average electric dipole moment appears. To get an insight into molecular structure of Fe3+ ions and their supramolecular organization in low-temperature (LT) and high-temperature (HT) phases of compounds 1 and 2, a series of density functional theory calculations was performed. On the basis of our findings, the LT- and HT-phase structures were proposed for these compounds. It was also shown that, whereas the chloride and hexafluorophosphate anions are able to form a network of hydrogen bonds between the [FeL2]X units (ionic pairs), which enable an electric dipole ordering in the sample, the nitrate anions, in contrast, tend to form hydrogen bonds inside the ionic pair. This conclusion is evidenced by the observed EPR spectra, which are different for compound 3 and are not indicative of the existence of an order–disorder ferroelectric phase transition. The EPR data obtained upon irradiation of compound 1 show that photoexcitation in the UV region at 5 K destroys hydrogen bonds and converts cationic complexes into ligand-to-metal charge transfer (LMCT) states, in which the iron is ferrous, and the unpaired electron is located on the salicylidene moieties. The LMCT states decay back to the ferric one, and ferric complexes further form the most stable (LT) phase structure.

EPR and impedance spectroscopic investigations on lithium bismuth borate glasses containing nickel and vanadium ions

Glasses having composition 7NiO∙23Li2O∙20Bi2O3∙50B2O3, 7V2O5∙23Li2O∙20Bi2O3∙50B2O3 and x(2NiO∙V2O5)∙(30−x)Li2O∙50B2O3∙20Bi2O3 (with x=0, 2, 5, 7 & 10mol%) prepared through melt-quench route are explored by analyzing density, impedance spectroscopy and electron paramagnetic resonance (EPR). It is found that both density and molar volume increase with an increase in substitution of 2NiO∙V2O5 in the base glass matrix. Different dielectric parameters viz. dielectric loss (ε), electrical modulus (M), loss tangent (tanδ) etc. are evaluated and their variations with frequency and temperature are analyzed which reveals that these glasses exhibit a non-Debye relaxation behavior. A phenomenal description of the capacitive behavior is obtained by considering the circuitry as a parallel combination of bulk resistance (Rb) and constant phase element (CPE). The conduction mechanism is found to follow Quantum Mechanical Tunneling (QMT) model. Spin Hamiltonian Parameters (SHPs) and covalency rates are calculated from the EPR spectra of vanadyl ion. The observed EPR spectra confirmed that V4+ ion exists as vanadyl ion in the octahedral coordination with tetragonal compression.

Features of Structural Changes in Travertine Like Barite Influenced by X-Ray Irradiation and Thermal Exposure

The results of our research on travertine-like barite by electron paramagnetic resonance (EPR) are presented. The growth of these crystals was held in continuous motion of gas-water solutions at a constant radiation exposure of the nuclei 226Ra, 232Th and 40K. The objective of this work was to identify the crystal-structure features of travertine-like barite associated with growth conditions under the influence of radiation exposure. Radiographic studies of barite samples have shown that the aggregates are virtually mono-mineral formations. It has a compressed unit cell with lower volume and higher density compared to the parameters of sedimentary barites. Only weak lines of the EPR spectrum of the ion-radical SO2− were fixed in the natural sample of barite. Therefore, laboratory experiments were carried out with X-ray ionization of samples, not only of the original rock, but also of samples exposed to thermal annealing. The EPR spectra of the following ion radicals: О− (gxx =2.0186; gyy =2.0121; gzz =2.0093), SO2− (gxx =2.0033; gyy =2.0126; gzz =2.0105) and two type - SO3− (I) (gxx =2.0034; gyy =2.0024; gzz =1.9995), SO3-(II) (gxx =2.0076; gyy =2.0022; gzz =1.9979) were detected after X-ray irradiation. The intensities of the lines of the observed EPR spectra of ion radicals change with annealing temperature. Paramagnetic centres form an integrated system with the structure of the crystal. Within the system, the transfer of electrons and holes is observed.

A DFT, X- and W-band EPR and ENDOR Study of Nitrogen-Centered Species in (Nano)Hydroxyapatite

Incorporation of the nitrogen-containing impurities in hydroxyapatite (HAp) powders with the sizes of the crystallites of (20–50) nm was studied using first-principles modeling combined with the multi-frequency (9 and 94 GHz) electron paramagnetic resonance (EPR) methods. It is shown that the observed EPR spectra are undoubtedly due to the presence of the bulk radiation-induced NO3 2− radicals. This conclusion is based on spin-polarized density functional theory calculations of spectroscopic parameters within gauge-including projector augmented wave framework followed by the exact comparison of the simulated EPR and electron–nuclear double resonance spectra with the experimental findings. In addition, a comprehensive analysis of the simulated properties allows us to suggest that the paramagnetic centers preferably occupy PO4 3− sites in the HAp structure.

Reactive Nitrogen Species Reactivities with Nitrones: Theoretical and Experimental Studies

Reactive nitrogen species (RNS) such as nitrogen dioxide ((•)NO(2)), peroxynitrite (ONOO(-)), and nitrosoperoxycarbonate (ONOOCO(2)(-)) are among the most damaging species present in biological systems due to their ability to cause modification of key biomolecular systems through oxidation, nitrosylation, and nitration. Nitrone spin traps are known to react with free radicals and nonradicals via electrophilic and nucleophilic addition reactions and have been employed as reagents to detect radicals using electron paramagnetic resonance (EPR) spectroscopy and as pharmacological agents against oxidative stress-mediated injury. This study examines the reactivity of cyclic nitrones such as 5,5-dimethylpyrroline N-oxide (DMPO) with (•)NO(2), ONOO(-), ONOOCO(2)(-), SNAP, and SIN-1 using EPR. The thermochemistries of nitrone reactivity with RNS and isotropic hfsc's of the addition products were also calculated at the PCM(water)/B3LYP/6-31+G**//B3LYP/6-31G* level of theory with and without explicit water molecules to rationalize the nature of the observed EPR spectra. Spin trapping of other RNS such as azide ((•)N(3)), nitrogen trioxide ((•)NO(3)), amino ((•)NH(2)) radicals and nitroxyl (HNO) were also theoretically and experimentally investigated by EPR spin trapping and mass spectrometry. This study also shows that other spin traps such as 5-carbamoyl-5-methyl-pyrroline N-oxide, 5-ethoxycarbonyl-5-methyl-pyrroline N-oxide, and 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline N-oxide can react with radical and nonradical RNS, thus making spin traps suitable probes as well as antioxidants against RNS-mediated oxidative damage.

Deconvolution of the EPR spectra of vanadium oxide nanotubes

In this work we report results of continuous wave (CW) electron paramagnetic resonance (EPR) spectroscopy of vanadium oxide nanotubes. The observed EPR spectra are composed of a weak well-resolved spectrum of isolated V4+ ions on top of an intense and broad structure-less line shape, attributed to spin–spin exchanged V4+ clusters. With the purpose to deconvolute the structured weak spectrum from the composed broad line, a new approach based on the Krylov basis diagonalization method (KBDM) is introduced. It is based on the discrimination between broad and sharp components with respect to a selectable threshold and can be executed with few adjustable parameters, without the need of a priori information on the shape and structure of the lines. This makes the method advantageous with respect to other procedures and suitable for fast and routine spectral analysis, which, in conjunction with simulation techniques based on the spin Hamiltonian parameters, can provide a full characterization of the EPR spectrum. Results demonstrate and characterize the coexistence of two V4+ species in the nanotubes and show good progress toward the goal of obtaining high fidelity deconvoluted spectra from complex signals with overlapping broader line shapes.

Lifetime of the unpaired electron species in calf thymus DNA thin films induced by nitrogen and oxygen K-shell photoabsorption

Purpose: To study the lifetime of the short-lived unpaired electron species induced in a calf thymus DNA film by soft X-rays in the energy regions around nitrogen and oxygen K-edge.Materials and methods: Dry calf thymus DNA films were measured by electron paramagnetic resonance (EPR) spectrometer during the monochromatic soft X-ray irradiation provided from a synchrotron accelerator (SPring-8, Super Photon ring-8 GeV). Two operation modes of SPring-8 with different electron bunch structures were used to explore dependence of EPR intensity on pulse interval of soft X-rays.Results: Observed EPR spectra did not show significant difference, in the spectral shape, intensity, g-factor and photon energy dependence, between the two bunch modes.Conclusions: The total yield of the unpaired electron species were estimated by assuming the exponential relaxation of the unpaired electrons with two different bunch modes of the synchrotron. The lifetime of the unpaired electron species is estimated to be in the range of a few tens to several hundreds of μs.

Nitroxide sensing of a DNA microenvironment: mechanistic insights from EPR spectroscopy and molecular dynamics simulations.

The behavior of the nitroxide spin labels 1-oxyl-4-bromo-2,2,5,5-tetramethylpyrroline (R5a) and 1-oxyl-2,2,5,5-tetramethylpyrroline (R5) attached at a phosphorothioate-substituted site in a DNA duplex is modulated by the DNA in a site- and stereospecific manner. A better understanding of the mechanisms of R5a/R5 sensing of the DNA microenvironment will enhance our capability to relate information from nitroxide spectra to sequence-dependent properties of DNA. Toward this goal, electron paramagnetic resonance (EPR) spectroscopy and molecular dynamics (MD) simulations were used to investigate R5 and R5a attached as R(p) and S(p) diastereomers at phosphorothioate (pS)C(7) of d(CTACTG(pS)C(7)Y(8)TTAG). d(CTAAAGCAGTAG) (Y = T or U). X-band continuous-wave EPR spectra revealed that the dT(8) to dU(8) change alters nanosecond rotational motions of R(p)-R5a but produces no detectable differences for S(p)-R5a, R(p)-R5, and S(p)-R5. MD simulations were able to qualitatively account for these spectral variations and provide a plausible physical basis for the R5/R5a behavior. The simulations also revealed a correlation between DNA backbone B(I)/B(II) conformations and R5/R5a rotational diffusion, thus suggesting a direct connection between DNA local backbone dynamics and EPR-detectable R5/R5a motion. These results advance our understanding of how a DNA microenvironment influences nitroxide motion and the observed EPR spectra. This may enable use of R5/R5a for a quantitative description of the sequence-dependent properties of large biologically relevant DNA molecules.

Electron Paramagnetic Resonance of Mn-Doped Sn1−x Mn x O2 Powders

Electron paramagnetic resonance (EPR) data on manganese-substituted tin dioxide (Mn:SnO2) samples are presented. The investigations show that the observed EPR spectra arise from three different contributions of Mn ions viz., (1) interstitial Mn2+ ions situated on the particle surfaces (2) isolated Mn2+ ions located substitutionally at Sn sites and (3) the Mn4+ ions which substitute for Sn4+ ions in SnO2 lattice. The effect of the dopant concentration on EPR spectra is also presented and discussed.

Local Structure at Mn<SUP>2+</SUP> Ions in Vacuum Annealed Small Cubic ZnS Nanocrystals Self-Assembled Into a Mesoporous Structure

A mesoporous structure of self-assembled nanocrystals of cubic ZnS doped with Mn2+ ions with a homogeneous distribution of pores of similar size was synthesized at room temperature by a surfactant-assisted liquid-liquid reaction. The component nanocrystals exhibit a high crystallinity and a tight size distribution centered at 2 nm, as well as the narrowest Electron Paramagnetic Resonance (EPR) spectra linewidth and the best resolution reported so-far, effects attributed to self-assembling. The observed EPR spectra consist of lines from the substitutional Mn2+(I) and surface Mn2+(II) and Mn2+(III) centers. Here we show that, in contrast with previous reports, our EPR spectra are highly sensitive to structural changes during pulse annealing in vacuum up to 500 degrees C. The changes are related to the transformation of the surface Mn2+ centers in new Mn2+ centers, attributed to an oxidation process in which the thermal decomposition of the Tween 20 additive, also observed by EPR, seems to be involved. We have also been able to observe, for the first time by EPR spectroscopy, the formation of the ZnO phase and the nanocrystals size increase, which occur during annealing up to 500 degrees C, structural changes confirmed by XRD and TEM observations on the samples previously investigated by EPR.

A combined EPR and DFT study of the overcrowded aromatic radical cations from Friedel–Crafts alkylation reactions

► The polycyclic aromatic radical cations were investigated by EPR and ENDOR. ► The aromatic rings in both PAHs radical cations twist out of co-planarity. ► The theoretical hfcc support the experimental assignment of the observed radicals. Electron paramagnetic resonance and electron–nuclear double resonance methods were used to study the polycyclic aromatic radical cations produced in a Friedel–Crafts alkylating system, and the following radical cations were indentified: 3,6,11,14-tetramethyl dibenzo (a, c) triphylene and 2,6-dimethyl-9,10-di(p-methylbenzyl) anthracene radical cations. The results indicate that the observed electron paramagnetic resonance spectra are due to overcrowded polycyclic aromatic radical cations formed from the parent hydrocarbons. It is suggested that benzyl halides produced in the Friedel–Crafts alkylation reactions undergo Scholl condensation to give polycyclic aromatic hydrocarbons, which are converted into corresponding polycyclic aromatic radical cations in the presence of AlCl 3 . We carried out the theoretical calculation of the isotropic 1 H hyperfine coupling constants for studied both PAHs radical cations. The results indicate that the IEFPCM–DFT calculation at B3LYP level with 6-31++G(d,p), EPRII and EPRIII basis sets could well support the experimental hfcc assignment of the observed radicals. Optimized geometry indicates that the aromatic rings in both PAHs radical cations twisted significantly out of co-planarity.

Sequence-Specific Stereomeric Environment in a DNA Duplex Revealed by a Nucleotide-Independent Nitroxide Probe

In site-directed spin labeling, structural and dynamic information on a parent macromolecule is obtained by monitoring a covalently linked nitroxide radical using electron paramagnetic resonance (EPR) spectroscopy. Our group have developed a method of attaching nitroxide species, such as 1-oxyl-4-bromo-2,2,5,5-tetramethylpyrroline (R5a), to a specific nucleotide position within a target DNA or RNA sequence. The method relies on site-specific introduction of a phosphorothiate during the solid phase chemical synthesis of nucleic acids, and at each given labeling site the nitroxide is attached to one of two phosphorothioate diastereomers (Rp or Sp) in an approximately 50/50 ratio. We have recently reported that variations in DNA structural and dynamic features at the level of an individual nucleotide can be detected using R5a attached to mixed phosphorothioate diastereomers, in which an observed EPR spectrum is presumably a sum of those obtained from either diastereomer (Popova et al., Biochemistry, 2009, 48, 8540-50). In this work, we report X-band EPR spectra of R5a attached to purified Rp and Sp diastereomers at different sites within a B-form DNA duplex. Results are compared to those obtained with mixed nitroxide diastereomers, and advantages and limitations are discussed regarding the necessity of diastereomer separation when probing DNA local environment. Our work is a further step forward in developing a SDSL methodology that may provide a mean for studying structure and dynamics in large DNA molecules.

A 236 GHz Fe3+ EPR Study of Nanoparticles of the Ferromagnetic Room-Temperature Semiconductor Sn1−x Fe x O2 (x = 0.005)

High-frequency (236 GHz) electron paramagnetic resonance (EPR) studies of Fe3+ ions at 255 K are reported in a Sn1−x Fe x O2 powder with x = 0.005, which is a ferromagnetic semiconductor at room temperature. The observed EPR spectrum can be simulated reasonably well as the overlap of spectra due to four magnetically inequivalent high-spin (HS) Fe3+ ions (S = 5/2). The spectrum intensity is calculated, using the overlap I(BL) + (I(HS1) + I(HS2) + I(HS3) + I(HS4)) × exp(−0.00001B), where B is the magnetic field intensity in Gauss, I represents the intensity of an EPR line (HS1, HS2, HS3, HS4), and BL stands for the baseline (the exponential factor, as found by fitting to the experimental spectrum, is related to the Boltzmann population distribution of energy levels at 255 K, which is the temperature of the sample in the spectrometer). These high-frequency EPR results are significantly different from those at X-band. The large values of the zero-field splitting parameter (D) observed here for the four centers at the high frequency of 236 GHz are beyond the capability of X-band, which can only record spectra of ions with much smaller D values than those reported here.

EPR and DFT Study of the Polycyclic Aromatic Radical Cations from Friedel-Crafts Alkylation Reactions

Electron paramagnetic resonance and electron-nuclear double resonance methods were used to study the polycyclic aromatic radical cations produced in a Friedel-Crafts alkylating system, with m-xylene, or p-xylene and alkyl chloride. The results indicate that the observed electron paramagnetic resonance spectra are due to polycyclic aromatic radicals formed from the parent hydrocarbons. It is suggested that benzyl halides produced in the Friedel-Crafts alkylation reactions undergo Scholl self-condensation to give polycyclic aromatic hydrocarbons, which are converted into corresponding polycyclic aromatic radical cations in the presence of AlCl3. The identification of observed two radicals 2,6-dimethylanthracene and 1,4,5,8-tetramethylanthracene were supported by density functional theory calculations using the B3LYP/631G(d,p)//B3LYP/631G(d) approach. The theoretical coupling constants support the experimental assignment of the observed radicals.