Influence of carbonic nanostructure formation on EPR line parameters during carbonization of wheat bran

Carbonized wheat bran samples were studied by electron paramagnetic resonance (EPR) spectroscopy, elemental analysis and electron microscopy. The carbonization was performed in the temperature range from 250 to 800 °C in argon medium. The carbon content increased with temperature. At high temperatures a sharp decrease in the concentration of free radicals takes place. It is shown that at high temperatures the EPR line width reduces from 0.87 (initial) to 0.11 mT. This phenomenon is explained by the fact that at high temperatures there occurs the formation not only of graphitelike structures (classical interpretation) but also of different nanosize structures. The electron microscopy data verify the formation of different forms of nanostructures in the carbonized wheat bran samples. Presented at the 5th Asia-Pacific EPR/ESR Symposium, August 24–27, 2006, Novosibirsk, Russian Federation.     

Intramolecular Dynamics of the 1,2,4-Trifluorobenzene Radical Anion: An Optically Detected EPR and Quantum Chemistry Study

A previously unknown electron paramagnetic resonance (EPR) spectrum of the 1,2,4-trifluorobenzene radical anion in liquid solution in nonpolar solvent has been obtained by means of the optically detected EPR technique. On the basis of quantum chemical calculations, the observed fluorine hyperfine coupling values and their temperature dependence have been interpreted as a consequence of fast conformational exchange among different nonplanar structures.     

Observation of electric quadrupole spin resonance of Ho<Superscript>3+</Superscript> impurity ions in synthetic forsterite

The anomalous dependence of the electron paramagnetic resonance (EPR) line shape on the microwave power in the resonator has been found when studying the continuous-wave EPR spectra of impurity holmium ions in synthetic forsterite on an ELEXSYS E 580 EPR spectrometer. The power-threshold transition from the conventional lines being the derivatives of the spectral line contours to the spectral line contours themselves has been observed as the power increased. The properties of the anomalous EPR lines are qualitatively explained assuming that the resonance electric quadrupole transitions take place between the electron spin levels.     

An EPR Study on Nanographites

We present the results of an electron paramagnetic resonance study (EPR) in the range of 4–290 K on samples of nanographites obtained by ball milling graphite for different times. With a careful simulation of the spectral line shapes, we disentangled the EPR bands, providing the spectral profiles and intensities of the components on varying the temperature, their g tensors, and the homogeneous line widths of the contributing spin packets. We have been able to follow the effect of decreasing progressively the size of the flakes on the EPR bands due to mobile electrons and on Lorentzian lines due to nonbonding electrons on the zigzag edges of the crystallites. The temperature dependence of the EPR intensities shows a common trend for the signals attributed to edge electrons and to mobile electrons, showing that they belong to the same bath.     

Time-Resolved EPR Spectra of Photoexcited Copper Porphyrin

Photoinduced spin-polarized transient electron paramagnetic resonance (EPR) spectra of copper 5,10,15,20-tetrakis(3-pyridyl)porphyrin (3PyNCu) in the frozen solution have been observed in the X-band. The time evolution and the temperature dependence of the spectra have been studied. The effect of molecular oxygen in the frozen solution on the polarization pattern has also been examined. The magnetic resonance parameters of the ground state of 3PyNCu have been obtained by comparing the experimental continuous-wave and echo-detected EPR spectra with the numerical computations. The magnetic resonance parameters of the excited states and the photoinduced polarizations have been investigated by time-resolved EPR (TREPR) spectroscopy and numerical analysis. The experimental spectra have been considered as a sum of the polarized spectra of the ground and excited states. Our analysis confirmed that the TREPR spectra consisted of two main patterns: the enhanced signal from the ground state and the multiplet contribution belonging to the excited quartet state.     

EPR investigation of two types of Syrian's natural zeolites

Two different samples of natural zeolite have been investigated by X-band electron paramagnetic resonance (EPR) spectroscopy. The observed EPR spectra are typical to those observed for Fe³⁺ and Mn²⁺ ions. The lines, related to the iron, are observed, respectively at g≈4.3 and g≈2. The observed six lines, at g≈2, are the hyperfine structure due to the Mn²⁺ ions. The simulation of the experimental EPR spectra suggests that both of the manganese and the iron are present in more one site. The temperature dependence of the EPR spectra has been also investigated. The nature of the different sites involved in the EPR absorption is discussed.     

Magnetic and dynamic properties of Sm x Mn1 − x S solid solutions

The real and imaginary parts of the magnetic permeability at frequencies of 0.1, 1.0, and 10.0 kHz, as well as the electron paramagnetic resonance (EPR) line width and g-factor, have been measured in Sm _x Mn_{1 ? x} S (0.10 < x < 0.25) solid solutions in the temperature range 5–300 K. The logarithmic dependence of the maximum in the imaginary part of the magnetic permeability on the frequency and the power-law dependence of Imμ on the temperature have been determined. The mechanism of relaxation of the magnetic moment in the magnetically ordered and paramagnetic phases has been established. The experimental results have been explained in terms of the Heisenberg model with competing exchange interactions and the formation of the antiaspiromagnetic state at low temperatures.     

EPR evidence of the low temperature phase transition in C60

A standard EPR technique has been applied to perform temperature studies of polycrystalline samples of C₆₀ fullerene. A comparison of the three main parameters of EPR spectra obtained for a C₆₀ sample before and after (solvent free) purification has been made in temperatures from 4 to 300 K. When compared to the spectrum of the as-obtained (crude) sample the spectrum of the purified sample revealed considerable changes i.e. an increase in the EPR signal intensity and evidence of a phase transition at about 90 K. This transition is proved to be related to the two processes: freezing of the orientational motion of C₆₀ molecules and redistribution of the positive and negative charge observed as paramagnetic centres localized on C₆₀ molecules.     

F NMR and EPR study of fluorinated buckminsterfullerene C60F58

Solid state ¹⁹F NMR in the temperature range from 96 to 366 K and room temperature EPR studies of fluorinated buckminsterfullerene C₆₀F₅₈ have been carried out. The temperature dependence of the line width and the spin-lattice relaxation time show hindered molecular motion with the activation energy of ΔE_a=1.9 kcal/mol. Neither phase transition nor random rotation of C₆₀F₅₈ have been obtained. The spin-lattice relaxation rate is strongly affected by the presence of paramagnetic centers, namely, dangling C-C bonds yielding localized unpaired electrons. Such broken bonds are caused by C-C bond rupture in a cage-opened structure of hyperfluorinated species.     

Electron paramagnetic resonance study of gamma-irradiated potassium hydroquinone monosulfonate single crystal

Gamma-irradiated potassium hydroquinone monosulfonate (PHM) single crystals were investigated between 125 and 300?K using the electron paramagnetic resonance (EPR) technique. Between 125 and 300?K no changes in the spectra have taken place. The spectra were found to be temperature independent. The investigation of EPR spectra of irradiated single crystals of PHM showed the presence of two PHM anion radicals. The g values of the radiation damage centers observed in the PHM single crystal and the hyperfine structure constants of the free electron with nearby protons were obtained.     

Dynamical Jahn-Teller distortion in single crystals of Cu(II) doped magnesium potassium phosphate hexahydrate: a variable temperature EPR study

Single crystal electron paramagnetic resonance (EPR) studies on Cu(II)-doped magnesium potassium phosphate hexahydrate have been carried out at room temperature. The temperature dependence of g and A values has been obtained for the polycrystalline sample and the ground state is unambiguously identified. These results indicate the existence of a dynamic Jahn-Teller distortion for Cu(II) ion. The g and A tensor direction cosines are evaluated and compared with Mg-O directions, which confirms that Cu(II) enters substitutionally in the lattice.     

<Superscript>29</Superscript>Si nuclear spin relaxation in microcrystals of plastically deformed Si: B samples

Single crystals and microcrystals Si: B enriched with ²⁹Si isotopes have been studied using nuclear magnetic resonance and electron paramagnetic resonance (EPR) in the temperature range from 300 to 800 K. It has been found that an increase in the temperature from 300 to 500 K leads to a change in the kinetics of the relaxation of the saturated nuclear spin system. At 300 K, the relaxation kinetics corresponds to direct electron–nuclear interaction with inhomogeneously distributed paramagnetic centers introduced by the plastic deformation of the crystals. At 500 K, the spin relaxation occurs through the nuclear spin diffusion and electron–nuclear interaction with an acceptor impurity. It has been revealed that the plastic deformation affects the EPR spectra at 9 K.     

Synthesis and properties of polyvinylidene fluoride high-temperature treatment products

Simultaneous thermal, gravimetric and mass spectrometric analyses made it possible to reveal considerable differences in fluorine desorption and changes in weight of filmy samples of original and partly chemically dehydrofluorinated polyvinylidene fluoride (PVDF) under an increase in temperature up to 800°C in argon flow. To determine the thermal treatment effect on the final material’s properties, the PVDF film was heated in a backing vacuum (0.5 Torr) up to 600°C with 40-minute isothermal exposure at a maximum temperature. Raman scattering spectroscopy was applied to compare molecular structures of the final products obtained by the high-temperature treatment of all samples and to prove their similarity. The spectrum of the chemically dehydrofluorinated sample, not subjected to a thermal effect, was characterized by a weak band in the region of 2000–2200 cm^–1, characteristic of the carbyne-like structures dominated by a polyyne type.

     

EPR evidence of antiferromagnetic ordering in single‐layer graphene

We report electron paramagnetic resonance (EPR) evidence of the antiferromagnetic ordering in pristine single‐layer graphene. Temperature dependences of the parameters of EPR spectra obtained for vacuum‐processed samples were studied within the temperature range of 4.2–300 K. Our experiment has confirmed recent theoretical predictions that in single‐layer graphene the carrier‐mediated exchange interaction leads to antiferromagnetic coupling. We note some quantitative discrepancies between the theory and experimental findings and discuss their origins. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

EPR, X-ray, and magnetization studies of metastable alloys Ti2Fe, Al40Cu10Mn25Ge25 and Al65Cu20Fe15 as prepared by mechanical alloying

Mechanical alloying (MA) technique has been used to synthesize metastable alloys with nominal compositions Ti₂Fe (MA for 1, 5, and 26 h), Al₆₅Cu₂₀Fe₁₅ (MA for 26 and 34 h) and Al₄₀Cu₁₀Mn₂₅Ge₂₅ (MA for 42 h). These have been studied by EPR, X-ray diffraction, differential thermal analysis (Ti₂Fe only), and magnetization (Ti₂Fe only) techniques. X-ray diffraction provided information on transformation to metastable phases, while the EPR spectra gave insight into inequivalent paramagnetic transition metal sites in the alloys and the temperature variation of magnetic ordering of the samples. Magnetization data on Ti₂Fe system has been interpreted in terms of phase separations within the amorphous phase. It is concluded that MA process significantly influences the magnetic properties of the samples, wherein the duration of MA process plays an important role. The EPR and magnetization data indicate that the disorder of paramagnetic ions within the samples increases with increasing temperature.     

Effect of Zn2+ and Cu2+ on free radical properties of melanin fromCladosporium cladosporioides

Effect of metal ions on free radical properties of natural melanin produced by soil fungiCladosporium cladosporioides was studied. The electron paramagnetic resonance (EPR) spectrum of the studied melanin consists mainly of a single line of eumelanin, and only a very weak signal of pheomelanin was observed. o-Semiquinone free radicals form paramagnetic centers in melanin. Diamagnetic Zn²⁺ ions produce an increase in the free radical concentration in melanin. Quenching of melanin EPR lines was obtained for melanin and paramagnetic Cu²⁺ ion complexes. Slow spin-lattice relaxation processes are characteristic for the free radicals in melanin samples and fast spin-lattice relaxation was observed for Cu²⁺ ions. The EPR lines of copper ions saturate at higher microwave powers than the EPR lines of melanin.     

Identification of paramagnetic nitrogen centers (P1) in diamond crystallites synthesized via the sintering of detonation nanodiamonds at high pressure and temperature

Diamond single crystals synthesized from powder detonation nanodiamonds (DNDs) by means of treatment at high pressures (P ~ 7 GPa) and temperatures (T > 1300°C) have been studied by electron paramagnetic resonance (EPR). A key feature of treatment (high-pressure high–temperature (HPHT) sintering) is the use of low molecular weight alcohols in the process. The appearance of a hyperfine EPR signal structure due to “paramagnetic nitrogen” (P1 centers) is explained by the growth of submicron and micron diamond single crystals from DND nanocrystals by the oriented attachment and coalescence mechanism. Such growth and coarsening of crystals appreciably decreases the concentration of paramagnetic centers, the presence of which hinders the detection of a hyperfine structure in the EPR signal from P1 centers, in the near-surface areas of coalesced and grown together DND particles. It has been shown that the concentration of paramagnetic defects of all types decreases to ~3.1 × 1018 g^–1 (~60 ppm) during HPHT treatment at T = 1650°C. This causes the successful identification of P1 centers, whose fraction is no less than ~40% of the total amount of paramagnetic centers in microcrystals synthesized by HPHT sintering.     

Applications of automatic fittings to powder EPR spectra of free radicals,<Emphasis Type="Italic">S</Emphasis>>1/2, and coupled systems

An automatic fitting procedure has been employed to analyze experimentally studied paramagnetic complexes in powder form by electron paramagnetic resonance (EPR). A least-squares fitting procedure utilizing analytical derivatives of the theoretically calculated spectrum with respect to theg-, zero-field, nuclear quadrupole, and hyperfine tensors was used to refine those parameters. An anisotropic line width could also be fitted. The theoretically calculated spectra were obtained by matrix diagonalization of a general spin Hamiltonian allowing also magnetically coupled systems to be analyzed. A VO²⁺ S=1/2 complex showingg and hyperfine anisotropic interactions and free radical systems featuring Δm ₁≠0 transitions due to the direct field effect or the presence of quadrupolar nuclei have been analyzed as well as NO _x species on surfaces and radiation defects employed for EPR dosimetry. Analysis of systems withS>1/2, and magnetically coupled species has also been attempted.     

EPR Study of Iron Ion Complexes in Human Blood

Electronic states of iron ion complexes in human blood from patients with melanoma have been investigated by electron paramagnetic resonance (EPR). The measurements were performed at liquid nitrogen temperature (77 K) on an X-band EPR spectrometer. Numerous types of iron paramagnetic centers have been identified. In several kinds of protein complexes exemplified by methemoglobin, transferrin or ferritin, various forms of trivalent iron have been found. Three groups of patients with typical EPR spectra have been individualized. These groups differed in types and concentration of paramagnetic centers in peripheral blood. A good correlation has been found between the EPR results, the total iron ion complexes concentration and transferrin saturation.     

Spin relaxation studies on conducting poly(tetrathiafulvalene)

Magnetic parameters and the relaxation behavior of paramagnetic centers in an iodine-doped poly(tetrathiafulvalene) semiconductor with a d.c. conductivity of 10^?5 S·cm^?1 have been studied using mainly the 2 mm waveband EPR technique in the temperature range of 110–270 K. The EPR line shape analysis confirms the existence of immobile radicals pinne on short polymer chains and mobile polarons with different relaxation parameters in slightly doped poly(tetrathiafulvalene). The temperature dependences of electron spin-lattice and spin-spin relaxation times of paramagnetic centers of both types have been determined independently using the saturation method at the operation frequency ν _e = 140 GHz. An anisotropic slow libration of immobile polarons with an activation energy of 0.02 eV have been registered for the first time using the saturation transfer EPR method. The temperature dependences of intrachain diffusion and interchain hopping rates in poly(tetrathiafulvalene) are determined from theT ₁ andT ₂ EPR data. The interchain spin dynamics is shown to correlate with libration of polarons trapped on polymer chains and is in good agreement with a hopping charge transport mechanism.