A variable temperature EPR study of Mn(2+)-doped NH(4)Cl(0.9)I(0.1) single crystal at 170 GHz: zero-field splitting parameter and its absolute sign

EPR measurements have been carried out on a single crystal of Mn(2+)-doped NH(4)Cl(0.9)I(0.1) at 170-GHz in the temperature range of 312-4.2K. The spectra have been analyzed (i) to estimate the spin-Hamiltonian parameters; (ii) to study the temperature variation of the zero-field splitting (ZFS) parameter; (iii) to confirm the negative absolute sign of the ZFS parameter unequivocally from the temperature-dependent relative intensities of hyperfine sextets at temperatures below 10K; and (iv) to detect the occurrence of a structural phase transition at 4.35K from the change in the structure of the EPR lines with temperature below 10K.     

A variable temperature EPR study of Mn-doped NH4Cl0.9I0.1 single crystal at 170 GHz: Zero-field splitting parameter and its absolute sign

EPR measurements have been carried out on a single crystal of Mn(2+)-doped NH(4)Cl(0.9)I(0.1) at 170-GHz in the temperature range of 312-4.2K. The spectra have been analyzed (i) to estimate the spin-Hamiltonian parameters; (ii) to study the temperature variation of the zero-field splitting (ZFS) parameter; (iii) to confirm the negative absolute sign of the ZFS parameter unequivocally from the temperature-dependent relative intensities of hyperfine sextets at temperatures below 10K; and (iv) to detect the occurrence of a structural phase transition at 4.35K from the change in the structure of the EPR lines with temperature below 10K.     

Multifrequency (X-band to W-band) CW EPR of the organic free radical in petroleum asphaltene

Petroleum of Arabian and Colombian origin was studied by electron paramagnetic resonance (EPR) spectroscopy at X- (9 GHz), Q- (34 GHz) and W-bands (94 GHz). The experiments were performed at room temperature (about 300 K) and at 77 K (W-band only). The asymmetry in the lines corresponding to free radicals was observed more intensely in the W-band spectra. The values of the line width ΔH in the spectra increased linearly with the microwave frequency utilized in the EPR experiments. A mathematical simulation of the free radical signal for the EPR spectra in three bands with a set of parameters corresponding to a single species was attempted, but this was not exactly coincident with the experimental signals, suggesting that the hyperfine interaction of the unpaired electron with its neighborhood corresponds to more than one species of radical in the molecular structure of the petroleum asphaltene.     

A rigorous evaluation of spin-Hamiltonian parameters and linewidth from a polycrystalline EPR spectrum.

Details are given of a procedure to evaluate the spin-Hamiltonian (SH) parameters and the linewidth from a polycrystalline EPR spectrum by using a least-squares fitting (LSF) technique in conjunction with numerical diagonalization of the SH matrix. The required resonance line positions are computed rather quickly using a homotopy technique, in which the position at an external magnetic field (B) orientation (theta, phi) is used as the initial value in a LSF procedure to estimate the position at an infinitesimally close B-orientation, (theta + deltatheta, phi + deltaphi). The resonance line positions are calculated successively in this procedure for all orientations of B over a grid of (theta, phi) values for the unit sphere. The eigenvectors of the SH matrix are used to calculate the intensities of the EPR lines exactly for each orientation of B. Details are given of how to compute rigorously the first and second derivatives of the chi(2)-function with respect to the SH parameters and the linewidth using the eigenvalues and eigenvectors of the spin-Hamiltonian matrix for a polycrystalline spectrum required in the LSF procedure. It is explained how this technique is generalized to include two or more magnetically inequivalent paramagnetic species, as well as how it is used for the simulation of other EPR-related spectra. The procedure is illustrated by evaluation of the Mn(2+) SH parameters and Lorentzian linewidth from the 249.9-GHz EPR spectrum of Mn(gamma-picoline)(4)I(2).     

Direct EPR detection of transient and continuous wave signals at 2.5 GHz

Direct detection of free induction decays and electron spin echoes, and the recording of echo-detected EPR spectra and electron spin echo envelope modulation patterns at a microwave frequency of 2.5 GHz is demonstrated. This corresponds to the measurement of the transverse magnetization in the laboratory frame, rather than in the rotating frame as usually done by down-converting the signal (homodyne detection). An oscilloscope with a 6-GHz analog bandwidth, a sampling rate of 20 GigaSamples per second, and a trigger frequency of 5 GHz for the edge trigger and 750 MHz for the advanced trigger, is used in these experiments. For signal averaging a 3-GHz microwave clock divider has been developed to synchronize the oscilloscope with the frequency of the EPR signal. Moreover, direct detection of continuous wave EPR signals at 2.5 GHz is described.     

On the mode of hexacoordinated NO-binding to myo- and hemoglobin: Variable-temperature EPR studies at multiple microwave frequencies

Electron paramagnetic resonance (EPR) spectroscopy is performed on NO^?-ligated hemo- and myoglobin at temperatures between 5 and 300 K. Apart from the standard X-band (9.5 GHz), data are taken at K-(24 GHz), Q-(34 GHz), W-(94 GHz) and Y-band (285 GHz) frequencies, respectively. The spectra are delineated into contributions of two EPR signatures differing ing-tensor symmetry, an axial and a rhombic one. The state contributions are found to differ with temperature and the states show temperature-dependentg-factor variations. In MbNO the axial species (denoted state II) is the only species observed at 300 K with X-band. Analysis of its development with decreasing temperature shows a partial transformation into a rhombic state (I). For this, at an intermediate temperature range (240–100 K), irregular line shapes are apparent, indicating disordered geometries of the Fe-NO-heme group. Regular rhombic lineshapes develop increasingly at temperatures below about 70 K. At 5–10 K, the rhombic state I dominates the EPR spectra but state II is still present. A similar development of spectra is observed in HbNO, for which, however, a rhombic contribution is already present at 300 K. Subunit-associated variations in state contributions and EPR appearance are not resolved at any frequency. Also, lineshape irregularities with decreasing temperature are not manifest in HbNO. These findings are used to present a structure-based picture of states I and II and their transformations and to discuss the origin of the line shape irregularities in MbNO.     

Simulation studies of high-field EPR spectra of spin-labeled lipids in membranes

The high-field (i.e., 94 GHz) membrane EPR spectra of lipids spin labeled in their fatty acid chains have been simulated by using two limiting motional models. The aim was to identify the dynamic origin of the residual (g(xx) - g(yy)) anisotropy observed in the nonaxial EPR spectra of cholesterol-containing membranes. It is concluded that the residual spectral anisotropy arises from in-plane ordering of the lipid chains by cholesterol. The partial averaging of the (g(xx) - g(yy)) anisotropy was best described by restricted axial rotation with a frequency in the region of tau(-1)(R||) approximately 0.5-1 x 10(9) s(-1). Simulations for slower axial rotation of unrestricted amplitude produced less satisfactory fits. In phospholipid membranes not containing cholesterol, the nonaxial anisotropy is completely averaged in the fluid phase and substantially reduced even in the gel phase. The unrestricted axial rotation in the gel phase is of comparable frequency to that of the limited axial rotation in the liquid-ordered phase of membranes containing cholesterol. These results on in-plane ordering by cholesterol in the liquid-ordered phase could be significant for current proposals regarding domain formation in cellular membranes.     

Frequency dependence of EPR signal intensity, 250 MHz to 9.1 GHz

Experimental EPR signal intensities at 250 MHz, 1.5 GHz, and 9.1 GHz agree within experimental error with predictions from first principles. When both the resonator size and the sample size are scaled with the inverse of RF/microwave frequency, omega, the EPR signal at constant B(1) scales as omega(-1/4). Comparisons were made for three different samples in two pairs of loop gap resonators. Each pair was geometrically scaled by a factor of 6. One pair of resonators was scaled from 250 MHz to 1.5 GHz, and the other pair was scaled from 1.5 GHz to 9 GHz. All terms in the comparison were measured directly, and their uncertainties estimated. The theory predicts that the signal at the lower frequency will be larger than the signal at the higher frequency by the ratio 1.57. For 250 MHz to 1.5 GHz, the experimental ratio was 1.52 and for the 1.5-GHz to 9-GHz comparison the ratio was 1.14.     

Pulsed EPR spin-probe study of intracellular glasses in seed and pollen

EPR spectra of 3-carboxy-proxyl (CP) in dry biological tissues exhibited a temperature-dependent change in the principal value A'(zz) of the hyperfine interaction tensor. The A'(zz) value changed sharply at a particular temperature that was dependent on water content. At elevated water contents, the break occurred at lower temperatures and appeared to be associated with the melting of the cytoplasmic glassy state. To investigate the reason for the change in A'(zz), we employed echo-detected EPR (ED EPR) spectroscopy. The shape of the ED EPR spectrum revealed the presence of librational motion of the spin probe, a motion typically present in glassy materials. The similarities in temperature dependency of A'(zz) and librational motion of CP in pea seed axes indicated that the change in A'(zz) arose from librational motion. ED EPR measurements of CP as a function of water content in Typha latifolia pollen showed that librational motion decreased with decreasing water contents until a plateau or minimum was reached. ED EPR spectroscopy is a valuable technique for characterizing the relation between molecular motion and storage kinetics of dry seed and pollen.     

High-field EPR studies on polymer film formation from colloidal dispersions

High-field EPR on nitroxide spin probes is applied to characterize the dynamics of small additive molecules and surfactants in polymer films obtained from colloidal dispersions. Due to the increased width of the spectra and the smaller influence of hyperfine-dependent relaxation on the spectral lineshape at W band (94 GHz) compared to X band (9.6 GHz), it is possible to measure subnanosecond rotational correlation times for the isotropic motion of the unpolar spin probe TEMPO in the free volume of poly(acrylate) films. Likewise, the anisotropies of the rotational diffusion tensors of a surfactant and a small ionic additive molecule in poly(fluoroacrylate) films can be determined with better confidence at W band. From these anisotropies it is concluded that the surfactant aggregates exhibit low molecular order, whereas the ionic additives are strongly attached to immobilized ionic clusters. High-field EPR lineshapes at W band are also found to be more sensitive to slow motions on a microsecond time scale than X-band EPR lineshapes. The design of a Fabry-Pérot resonator for measurements on polymer films is discussed and its sensitivity is demonstrated on a wet polymer film with a thickness of 160 μm.     

Revealing the Spin-Hamiltonian and Characteristic Parameters of Paramagnetic Centers in Cu2+-Doped Potassium Metabisulfite (K2S2O5) Single Crystal by Electron Paramagnetic Resonance Technique

The present study is mainly related to finding out spin-Hamiltonian parameters of Cu²⁺-doped potassium metabisulfite [K₂S₂O₅] single crystals using electron paramagnetic resonance (EPR) technique which has been applied in the temperature range from 297 to 113 K for single and powder crystals of the title compound. The existence of two complexes and two Cu²⁺ sites for each complex was concluded from the angular variation of the EPR spectra. Having constructed the g and the hyperfine tensors, the spin-Hamiltonian parameters have been obtained for these two complexes. Using these parameters, the covalency parameter (α ²), mixing coefficients (α and β) and Fermi contact term (K) have been calculated.     

Assignment of EPR transitions in a manganese-containing lipoxygenase and prediction of local structure

A new variant of lipoxygenases, one containing manganese instead of iron, is characterized by electron paramagnetic resonance (EPR) at two frequencies. In the manganous state (S(e) = 5/2), maganese lipoxygenase (MnLO) yields very broad X-band (9.2 GHz) EPR signals, extending over about 800 mT. In contrast, at W-band (94 GHz), the signal is much simplified, consisting of nested transitions centered near the free electron g-value. Computer simulation has been employed to derive estimates of the zero-field splittings for MnLO, with data from these two EPR frequencies. The general features of both X- and W-band spectra are fit, first, by simulations with S(e) = 5/2, but no nuclear hyperfine splitting. The simulations are then refined by inclusion of the hyperfine splitting. On the basis of the simulations, the ranges of zero-field splitting parameters are D = +0.07 to +0.10 cm(-1), and E/D = 0.13 to 0.23. Comparison of the value of D for MnLO with that of other manganese-containing proteins suggests that MnLO has three N-ligands to the metal center and O-ligands in the remainder of 6 coordination positions. The coordination environment of MnLO is similar to that in iron lipoxygenases.     

Determination of the 14N quadrupole coupling constant of nitroxide spin probes by W-band ELDOR-detected NMR

Nitroxide spin probe electron paramagnetic resonance (EPR) has proven to be a very successful method to probe local polarity and solvent hydrogen bonding properties at the molecular level. The g(xx) and the (14)N hyperfine A(zz) principal values are the EPR parameters of the nitroxide spin probe that are sensitive to these properties and are therefore monitored experimentally. Recently, the (14)N quadrupole interaction of nitroxides has been shown to be also highly sensitive to polarity and H-bonding (A. Savitsky et al., J. Phys. Chem. B 112 (2008) 9079). High-field electron spin echo envelope modulation (ESEEM) was used successfully to determine the P(xx) and P(yy) principal components of the (14)N quadrupole tensor. The P(zz) value was calculated from the traceless character of the quadrupole tensor. We introduce here high-field (W-band, 95 GHz, 3.5 T) electron-electron double resonance (ELDOR)-detected NMR as a method to obtain the (14)N P(zz) value directly, together with A(zz). This is complemented by W-band hyperfine sublevel correlation (HYSCORE) measurements carried out along the g(xx) direction to determine the principal P(xx) and P(yy) components. Through measurements of TEMPOL dissolved in solvents of different polarities, we show that A(zz) increases, while |P(zz)| decreases with polarity, as predicted by Savitsky et al.     

推广的等效自旋哈密顿

本文用羣论的方法提出一种推广的等效自旋哈密顿H_SG,用它来描述晶体中过渡族离子的顺磁共振性质。H_SG严格地考虑了空间和时间的对称性质。在一阶微扰计算的近似上,证明自旋空间的不可约张量组T_LM(L=0,1,…,2S)可以完全描述基态有2S+1个能级的顺磁离子的行为。本文给出了某些常见点羣对称度的不可约张量的形式。这样求出的H_SG和Koster-Statz矩阵完全等价。     

Gamma-radiolysis of deaerated cysteine solutions

The electron paramagnetic resonance (EPR) spectra of gamma-irradiated single crystals of phenidone (fenidon C₉H₁₀N₂O) have been studied for different orientations of crystals in a magnetic field. Phenidone single crystals have been irradiated with ⁶⁰Co-γ rays at room temperature. The EPR spectra have been investigated at temperatures between 125 and 450 K. The spectra have been found to be temperature independent. The spin-Hamiltonian parameters have been obtained from the single-crystal EPR analysis. The principal values of the hyperfine coupling tensor of the unpaired electron and the principal values of the g-tensor have been determined.     

A Low Temperature (10 K) High-Frequency (208 GHz) EPR Study of the Non-Kramers Ion Mn3+ in a MnMo6Se8 Single Crystal

A high-frequency (208 GHz) electron paramagnetic resonance (EPR) study on Mn³⁺ (3d⁴, S = 2) ions embedded in a MnMo₆Se₈ single crystal has been performed at 10 K. The experimental spectra reveal the presence of only one set of EPR lines from Mn³⁺ ions, whose magnetic axes are oriented along the crystal axes. The spin-Hamiltonian parameters are evaluated by the method of least-squares, fitting all the observed line positions simultaneously, for the three orthogonal orientations of the external magnetic field. The symmetry of the spin Hamiltonian at the site of the Mn³⁺ ions has been deduced from the EPR spectra.     

Variable-frequency EPR study of Mn(2+)-doped NH(4)Cl(0.9)I(0.1) single crystal at 9.6, 36, and 249.9 GHz: structural phase transition

Multifrequency electron paramagnetic resonance studies on the Mn(2+) impurity ion in a mixed single crystal NH(4)Cl(0.9)I(0.1) were carried out at 9.62 (X-band) in the range 120-295 K, at 35.87 (Q-band) at 77 and 295 K, and at 249.9 GHz (far-infrared band) at 253 K. The high-field EPR spectra at 249.9 GHz are well into the high-field limit leading to a considerable simplification of the spectra and their interpretation. Three magnetically inequivalent, but physically equivalent, Mn(2+) ions with their respective magnetic Z-axes oriented along the crystallographic [100], [010], [001] axes were observed. Simultaneous fitting of EPR line positions observed at X-, Q-, and far infra-red bands was performed using a least-squares procedure and matrix diagonalization to estimate accurately the Mn(2+) spin-Hamiltonian parameters. The temperature variation of the linewidth and peak-to-peak intensities of the EPR lines indicate the presence of lambda-transitions in the mixed NH(4)Cl(0.9)I(0.1) crystal at 242 and 228 K consistent with those observed in the pure NH(4)Cl and NH(4)I crystals, respectively. A superposition-model analysis of the spin-Hamiltonian parameters reveals that the local environment of the Mn(2+) ion is considerably reorganized to produce axially symmetric crystal fields about the respective Z-axes of the three magnetically inequivalent ions as a consequence of the vacancy created due to charge-compensation when the divalent Mn(2+) ion substitutes for a monovalent NH(4)(+) ion in the NH(4)Cl(0.9)I(0.1) crystal. This reorganization is almost the same as that observed in NH(4)Cl and NH(4)I single crystals, although the latter two are characterized by different, simple cubic and face-centered cubic, structures.     

Quasioptical study of antiferromagnetic resonance in YFeO3 at submillimeter wavelength under high pulsed magnetic fields

Transmission spectra, T(H), of linearly polarized electromagnetic waves through YFeO(3), weak ferromagnet, measured at frequencies nu=96-1000 GHz in long-pulsed magnetic fields (H||k||c-axis, Faraday geometry) exhibit strong rotation of the polarization plane near the quasiferromagnetic AFMR as well as low frequency impurity modes. New ascending impurity branch including five lines was observed at high magnetic field (10-30 T) at 96 GHz and 140 GHz in addition to the known low-field descending impurity branch. Behavior of all the impurity modes assigned to transitions in (6)S(5/2) multiplet of Fe(3+) "impurity" ions in c-sites was described self-consistently by one spin-Hamiltonian. A theoretical calculation of dynamical magnetic susceptibility at AFMR and impurity modes and further simulation of transmission spectra made it possible to describe the main features of the observed spectra T(H). It was found that the T(H) behavior is determined at resonances not only by non-diagonal components of the magnetic susceptibility but also by the anisotropy of the dielectric permittivity (epsilon(xx)(') not equal epsilon(yy)(')), i.e. birefringence.     

Single-crystal EPR studies of transition-metal ions in inorganic crystals at very high frequency

Electron paramagnetic resonance (EPR) single-crystal rotation studies at very high frequency (249.9 GHz) of transition metal ions with electron spins greater than one-half are reported. At 249.9 GHz, the spectra are in the high-field limit despite large zero-field splittings. This leads to a considerable simplification of the spectra, and aids in their interpretation. Single-crystal 249.9 GHz EPR spectra of Ni²⁺ in Ni₂CdCl₆· 12H₂O, Mn²⁺ (0.2%) in ZnV₂O₇, and Fe³⁺ (2%) in CaYA10₄ were recorded at 253 K in an external magnetic field of up to 9.2 T, along with those at X-band and Q-band frequencies at 295 K and lower temperatures. The goniometer used at 249.9 GHz for single-crystal rotation is based on a quasi-optical design and is an integral part of a special Fabry-Pérot resonator. The values of the spin-Hamiltonian parameters were estimated from a simultaneous fitting of all of the observed line positions at several microwave frequencies recorded at various orientations of each crystal with respect to the external magnetic field with least-squares fitting in conjunction with matrix diagonalization. Estimates of zero-field splitting parameterD at room temperature are: for Ni²⁺, about ?31 GHz (site I) and about ?7 GHz (site II); for Mn²⁺, about 6 GHz; and for Fe³⁺, about 29 GHz.