EPR Linewidth Research Articles

Study of CsPbCl3between T = 47°C and the melting point by the133Cs NMR AND Gd3+epr measurements

Abstract An investigation of the approach to the melting point in CsPbCl3 by 133Cs NMR and Gd3+ EPR measurements is reported. A pretransitional phenomenon is observed with the two techniques. The evolution with temperature of the NMR and EPR linewidths gives some information on the dynamics around the solid-liquid transition.

96/04906 Determination of magnetic and nuclear quadrupole interactions in La@C82 from EPR linewidth data

96/04906 Determination of magnetic and nuclear quadrupole interactions in La@C82 from EPR linewidth data

Hydrogen-induced phase separation in La 1.89Sr 0.11CuO 4 as revealed by EPR of Fe spin probes

EPR measurements of Fe 3+ spin probes doped to La 1.89Sr 0.11CuO 4 superconductor were carried out to study the hydrogen absorption effect on this compound. In the high-temperature region the EPR linewidth was determined by the Korringa relaxation. By extracting the Korringa contribution, we were able to evaluate a dependence of the density of states at the Fermi surface N( E F ) on hydrogen concentration. It has been found that N( E F ) is unchanged during the hydrogenation process. An analysis of the EPR spectra shows that hydrogen doping induces phase separation of the compound, creating insulating, magnetically ordered regions in a metallic, superconducting background.

Determination of Magnetic and Nuclear Quadrupole Interactions in La@C82from EPR Linewidth Data

Abstract From the analysis of the temperature dependence of the EPR line width, an estimate of the anisotropic parts of the dipolar and quadrupolar hyperfine couplings of lanthanum was derived. Furthermore, the rotational correlation time of the nearly spherical molecule was found to deviate considerably from that of the solvent molecules.

Intramolecular and reorientation dynamics of bis(triphenylphosphine)-3,6-di-tert-butyl-4,5-dimethoxy-o-semiquinone complex of copper(I) in viscous media

Two simultaneous dynamical processes are studied in solutions of varying viscosity by using the EPR linewidth data of a Cu(I) o-semiquinone complex with two triphenylphosphine ligands (PPh 3), i.e. the overall reorientation (tumbling) of the complex and intramolecular exchange of the PPh 3 ligands in the coordination sphere of the complex. It is shown that for fast motion the line broadening of the hyperfine components is due to both modulation of the anisotropic magnetic parameters by rotational diffusion and to modulation of the isotropic part of the 31P hyperfine interaction by intramolecular exchange. The intramolecular dynamics of the coordination sphere appears to be dependent on the solvent dynamics.

Ionic Association and Electron Spin Relaxation Rates in Aquo Gadolinium(III) Complexes

The electron paramagnetic resonance linewidth of aquo gadolinium(III) ion changes with the counter-ion identity and concentration in aqueous solutions. The EPR linewidth of 2 mMgadolinium(III) chloride increases from 49.2 to 89.0 mT when carbonate ion is added and decreases to 17.3 mT when nitrite ion is added. These observations suggest association reactions between aquo gadolinium(III) ion and anions that change the electron spin relaxation rates of the aquo ion. The concentration dependence of the gadolinium(III) EPR linewidth is consistent with binding constants for nitrite and nitrate ion with the aquo gadolinium(III) ion of 37 ± 6 and 2.3 ± 0.3 L mol−1respectively. The decreases in the EPR linewidth factors with these association reactions are difficult to understand unless the anion reactions increase the symmetry of the metal center. Although first-coordination reactions may not be ruled out, the decrease in EPR linewidth is more consistent with an outer-sphere association reaction that also reduces the coordination number of the metal center from 9 to 8.

EPR Linewidth (T2) Method to Measure Oxygen Permeability of Phospholipid Bilayers and Its Use to Study the Effect of Low Ethanol Concentrations

It is well known that continuous-wave EPR spectra of nitroxide probes (labels) introduced into phospholipid bilayers are sensitive to molecular oxygen. However, accurate determination of oxygen broadening from these experiments is complicated by the complex shapes of EPR spectra, which are strongly influenced by anisotropic restricted motion of the probe molecules. An accurate method is presented to extract the oxygen broadening from the spectra measured with and without oxygen and at the same temperature. The method is based on a fast convolution algorithm with Levenberg–Marquardt optimization. This method was previously applied to EPR oximetry with nitroxides exhibiting rotational motion in the fast limit. It is shown that for several membrane spin probes, the oxygen broadening can be described as homogeneous; thus, a one-linewidth-parameter fitting model is appropriate. The method is applied to measure permeability profiles of model membranes composed from 1,2-dimyristoyl-sn-glycero-3-phosphocholine above and below the main phase transition. For both membrane phases, the broadening of doxyl- and sterol-type labels is found to be homogeneous, a finding consistent with the model of Heisenberg exchange between molecular oxygen and spin probes. As an example, the method is applied to study the ethanol effect on local oxygen permeability of a phospholipid bilayer. It is shown that ethanol concentrations as low as 1% (v/v) increase oxygen permeability of the bilayer. The effect is larger at the surface of the membrane than at its center, indicating that ethanol molecules interact primarily within the polar head region of the bilayer.

Influence of the Mn2+Ion Cluster Formation on the EPR Spectrum of Zn1−xMnxTe

The second and fourth central moments of the EPR line of the Mn2+ion clusters are calculated for the case when isotropic and anisotropic superexchange interactions exist between paramagnetic ions. The infinite-temperature linewidth for each type of the cluster is found taking Lorentzian lineshape function. Using cluster-method calculations of central moments, infinite-temperature EPR linewidth in Zn1−xMnxTe is evaluated forx≤ 0.20, and the results are compared with the theory of the EPR line broadening developed for randomly diluted magnetic semiconductors. The cluster-method approach to the EPR linewidth offers an explanation of the narrow resonance observed in Zn1−xMnxTe forx> 0.20.

Inferring unresolved hyperfine structure in liquid solution EPR:14N hyperfine structure in the 1∶1 copper-calcimycin complex

Theoretical expressions for EPR line widths in liquids suggest the following two strategies for inferring unresolved hyperfine structure in near square planar copper(II) complexes and thereby identify the coordinating ligand atoms: (i) Increasing the resolution by reducing the line widths, by forming a mixed ligand complex of lower magnetic anisotropy. (ii) By demanding that the magnetic anisotropy determined from line widths, assuming unresolved hyperfine structure, should agree with that determined from frozen solution EPR spectrum.14N hyperfine structure from a coordinating cal (A N ∼ 12 G) in 1∶1 copper-calcimycin complex (Cu−cal) has been observed / inferred by the application of these strategies in 1∶1 chloroform-ethanol mixtures. It has been suggested that Cu−cal prepared in the presence of the bidentate ligand diethyldithiocarbamate (dtc−) is the mixed ligand complex Cu(dtc)(cal) in which cal is a bidentate ligand. The Cu−cal prepared in the presence of a monodentate anion AN (AN=NO 3 − , Br−, Cl− etc.) is Cu(AN)(cal) in which cal is a tridentate ligand. These complexes have near planar coordination geometry.

The structure and EPR properties of (CH2OHCH2NH3)2CuCl4

The title compound is a member of the (RNH3)2CuCl4 layer perovskite family, space group C2/c, witha=21.624(5),b=7.511(1),c=7.431(2)A, β=89.56(2)° andV=1206.9(4)A3 withZ=4. In the antiferrodistortive structure, the Cu(II) ion assumes an elongated octahedral coordination (unique Cu−Cl distances=2.274(2), 2.288(2), and 3.010(2)A). The octahedra share corners with four neighbors to form a two-dimensional network. The −NH3 moieties hydrogen bond to the layers, so that the −C2H5OH moieties form sheathes about each metal halide layer. A twofold disorder of the −C2H5OH groups is observed. The EPR spectrum is investigated to help understand the magnetic properties of the systems. Theg-values are consistent with strong intralayer exchange coupling. The EPR linewidths show evidence of spin diffusion effects at liquid nitrogen temperature. However, at room temperature, the spin anisotropies lead to broadening of the EPR lines through a phonon modulation mechanism.

Electronic Properties of Beta-Vanadium Bronzes Li xNa yV 2O 5 (0.23 ≤ x + y ≤ 0.37) Obtained by the Sol-Gel Process

Vanadium bronzes β-Li x Na y V 2O 5 (0.23 ≤ x + y ≤ 0.37) have been synthesized by the sol-gel process. These compounds exhibit the monoclinic structure of compounds of the same composition prepared by solid state reaction, but they are obtained as thin layers with a highly preferred orientation. Studies of these compounds by magnetic susceptibility, electron paramagnetic resonance, and electrical conductivity measurements are presented. It is demonstrated from magnetic susceptibility and EPR linewidth that the bipolaron model describes accurately the electronic properties of the lithium and mixed lithium/sodium bronzes synthesized by the sol-gel process. Dc-conductivity measurements between 15 and 300 K show that the bronze conductivities are anisotropic and depend on the nature of the exchanged ion. More complete study on Li 0.23V 2O 5, including ac-conductivity and impedance measurements from 10 Hz to 10 GHz, gives a good correlation between all the experimental techniques described in this paper.

A new type of p-doping for polyacetylene

We report here a new type of p-doping for polyacetylene which seems to be very promising. The new dopant is the (CF 3SO 2) 2N− ion. It is possible to form salts with various counter-ions and to perform electrochemical or chemical doping. With electrochemical doping we reached a maximum doping level close to 8% and the conductivity increased up to 18 S/cm. We performed Raman measurements with various wavelengths and observed the characteristic peaks of cis and trans polyacetylene simultaneously. This behaviour can be interpreted as the result of an inhomogenous and weak doping of the sample. We observed also photoluminescence in these doped samples. We present also the dependence of the EPR linewidth as a function of the chemical doping time.

Dynamic Nuclear Polarization with Nitroxides Dissolved in Biological Fluids

The most widely used free radicals for dynamic nuclear polarization (DNP) experiments or related Overhauser imaging are nitroxides. The DNP parameters in biological fluids were measured in order to provide guidelines for the design of new nitroxides, adapted to the biological applications of DNP. Eighteen nitroxides were studied at a concentration of 1 m M. Extrapolation at complete electron paramagnetic resonance saturation and proton longitudinal-relaxation-time measurements enable calculation of the coupling factor between nitroxide free electrons and water protons. In deoxygenated phosphate-buffered solutions, the NMR signal enhancement by DNP ranged from −36.3 to −6.7, and the coupling factor ranged from 0.31 to 0.03, Nitroxides with a long side chain yield poor enhancement, although their relaxivity is far greater than that of nitroxides with small chains. In a 1 m M albumin solution, the loss in enhancement factor is mainly caused by the fact that proton relaxation occurs via interactions, not only with the dissolved free radicals but also with the albumin macromolecules. In serum, the enhancement factor is lower than that in an albumin solution, because of the higher protein concentration in serum. In red-blood-cell suspensions, the enhancement factor was further decreased, Two effects contribute to this decrease: first, the increased viscosity due to the presence of red blood cells, and second, the susceptibility effects of the paramagnetism of deoxyhemoglobin. The high sensitivity to oxygen of DNP in phosphate-buffered solution is also greatly reduced when nitroxides are dissolved in blood, In red-blood-cell solutions there is competition between the removal of O 2, which favors proton relaxation by the free radical, and the increase of deoxyhemoglobin, which provides the water protons with another sink of relaxation, On the basis of this study, the properties of nitroxides for use in biological application must fulfill four criteria; i.e., they must have an EPR linewidth narrower than 100 mG, a relaxivity greater than 2.5 (m M s) −1, a good stability with a half-life time in biological fluids exceeding 1 hour, and low toxicity,

EPR and magnetization of Gd2BaNiO5

We report EPR and magnetization for Gd2BaNiO5 single crystals. The high-temperature susceptibility gives μeff = 7.9μB/mol Gd and ΘGd = 11 K (JGdGd/kB = 0.6 K). The coupling of 1-D Ni chains through the Gd-Ni interaction induces 3-D antiferromagnetic order with TN ⋍ 55(3) K, indicated by a divergence of the EPR linewidth. A mean field analysis gives JGd−Ni/kB = 3 K.

Alternative synthesis of (Z),(Z)-4,5;4′,5′-bis(1,4-dioxanediyl-2,3-dithio)tetrathiafulvalene (cis,cis-BDDT–TTF), and the preparation, EPR properties and structure of the radical cation salt (cis,cis-BDDT–TTF)2I3

An alternative synthesis of the title compound (a derivative of BEDT–TTF or ET)via the cis-thione 1, which was unexpectedly obtained while attempting to prepare the trans isomer in the reaction between (E)-2,3-dichloro-1,4-dioxane and bis(4,5-dimercapto-1,3-dithiole-2-thione)zincate, is presented. The radical cation salt (cis,cis-BDDT–TTF)2I3 was prepared by electrochemical methods, and its structure, as determined by X-ray diffraction, consists of a columnar packing of the donor molecules with the triiodide anions residing between the columns. This structure is distinctly different from the layered structure, donor layers alternating with anion layers, of organic conductors and superconductors based on ET and related electron-donor molecules. The salt was found to exhibit a large EPR linewidth (130 G, g= 2.0045), which is much larger than those found in other organic conductors and superconductors. The large EPR linewidth may be a reflection of the three-dimensional (3D) electron delocalization, a consequence of the columnar structure of the salt.

Transition from static to dynamic Jahn-Teller distortion in (P(C 6H 5) 4) 2C 60I

Air stable bulk single crystals of (P(C 6H 5) 4) 2C 60I can be grown by electrocrystallisation. We will prove that this salt is an excellent model system for the ferromagnetic [TDAE]C 60 and furthermore a good tool, to study the dynamics of C 60 mono anions in the solid state. We could directly observe the temperature dependence of the Jahn-Teller distortion of the C 60- and the transition from a static to a dynamic Jahn-Teller distortion at about 140 K. We will present detailed EPR linewidth and g-factor measurements at 9.7 and 34 GHz between 3.5 and 300 K.

Density of states and magnetic correlations in the La 2- xSr xCuO 4 system studied by EPR of Fe spin probes

EPR of Fe 3+ ions doped to La 2- x Sr x CuO 4 samples with various Sr content ( x=0-0.25) has been carried out. In the high-temperature region the EPR linewidth was determined by the Korringa relaxation. By extracting the Korringa contribution, we were able to evaluate a dependence of the density of states N F on x. It has been found that N F continuously grows with Sr doping from under- to overdoped regimes. The obtained N F( x) dependence indicates that the insulator-metal transition near x=0.06 is driven by hole delocalization. In the low-temperature region the relaxation rate was determined by spin-spin relaxation. The low-temperature upturn observed for the temperature dependence of the EPR linewidth can be understood by a slowing down of spin fluctuations due to spin freezing and/or carrier localization. We suggest that carrier localization enhancement plays an important role in the occurrence of the well known anomaly of T c near x=0.125 (so-called 1 8 problem).

Identification of Coordinating Ligand Atoms in Cu(calcimycin) 2 Complex from EPR Linewidths in Chloroform Solutions

The X-band EPR spectra of Cu(cal) 2 complex in CHCl 3 solutions (cal = calcimycin) do not show resolved 14N hyperfine structure (J. S. Puskin and T. E. Gunter, Biochemistry 14, 187, 1975) even though they are expected from the nitrogen coordination to divalent metal ions inferred by NMR acid by model building (C. M. Deber and D. R. Pfeiffer, Biochemistry 15, 132, 1976). In the present work, unresolved hyperfine structure from two equivalent 14N has been inferred in the EPR spectra of Cu(cal) 2 from an analysis of linewidths and lineshapes of m I = 3 2 and 1 2 transitions at 298 K and m I = 3 2 transition at 253 K, in CHCl 3 solutions ( m I = nuclear magnetic quantum number associated with the Cu hyperfine transition); g(parallel to); g || − g ⊥(=0.24), | A || − A ⊥| (=131 G), and 14N hyperfine constant A N (∼11.3 G) determined from liquid solution linewidth studies in this work favor a solution structure of the complex in which two nitrogens and two oxygens are involved in the square-planar coordination at the metal ion similar to that suggested on the basis of the model for Ca(cal) 2 complex.

Ni 3+ −Ni 2+ segregation in Li xNi 2− xO 2 solid solutions (0.6 ⩽ x < 1)

EPR of low-spin Ni 3+ ions is used to refine Ni 3+ −Ni 2+ segregation in Li x Ni 2- x O 2 oxides with 0 0.6 ⩽ x< 1 where a transformation from cubic to layered structure occurs. It is shown that, above the temperature of magnetic ordering, the spin-lattice relaxation comprising phonon modulation of antiferromagnetic interplane Ni 3+ONi 2+ and ferromagnetic intraplane Ni 3+ONi 3+ interactions governs the linear temperature dependence of the EPR line width (Δ H pp). The dΔ H pp/d T coefficient can be used as a parameter sensitive to the Ni 2+Ni 3+ segregation in neighbouring cubic (111) planes. For Li x Ni 2- x O 2 (0.6 < x< 0.7), where Li +, Ni 3+ and Ni 2+ are approximately in the same amounts, clustering of the Li +, Ni 3+ and Ni 2+ ions takes place in the alternative (111) planes. For x>0.7, Li + and Ni 3+ segregation evolve two discrete LiO 2- and NiO 2-layers, but a small part of Li + and Ni 3+ fall into the “alien” layer. In addition, Ni 2+ segregate into both layers, their amount being almost equal when x>0.7.

Coordinating iigand atoms in Cu2+-calcimycin complex: Conclusions from EPR linewidths in chloroform solutions

Coordinating iigand atoms in Cu2+-calcimycin complex: Conclusions from EPR linewidths in chloroform solutions