Temperature Dependence Of EPR Spectra Research Articles

Dynamics of hindered CF3-rotor in 2,5-bis (trifl uoromethyl )-nitroben zene radical -anion in anhydro us DMF. A temperature - activation theory of reversible HFS transformations of EPR spectrum

The temperature dependence of EPR spectrum of 2,5-bis(trifluoromethyl)nitrobenzene radical anion in anhydrous DMF was investigated. Internal dynamics of hindered rotation of the CF3-group in ortho-position to NO2-group causes HFS modulation. The spectrum changes are reversible and temperature-dependent. An original temperature-activation representation of complex spectral density was proposed instead of the traditional spectral-kinetics representation to explain the observed transformation. The change resulted in a new convenient phenomenological reconstruction model, which allowed simulating the spectra in the whole temperature range. Splitting constants and contributions to the spectral widths of the spectral lines were found. The activation energy of the CF3-group hindered rotation is significant and amounts to 37 kJ/mol.

Magnetic anisotropy and exchange coupling in a family of isostructural FeIII2LnIII2 complexes

The reaction of [Fe3O(O2CPh)6(H2O)3](O2CPh) with lanthanide/rare earth nitrate salts in the presence of triethanolamine (H3tea) in acetonitrile/methanol solution yields a series of compounds with isostructural tetranuclear core motifs [Fe(III)2Ln(III)2(μ3-OH)2(teaH)2(O2CCPh)6]·3MeCN (Ln = Ce (1), Pr (2), Nd (3), Sm (4), Eu (5), Gd (6), Tb (7), Dy (8), Ho (9), Er (10), Tm (11), Yb (12), Y (13)). In all cases the core topology is a defect-dicubane planar or "butterfly" Fe2Ln2 motif. Compounds 1-13 were investigated using a combination of experimental techniques and theoretical studies. Magnetic susceptibility measurements were carried out on all compounds. The magnetic coupling between the two Fe(III) centres is antiferromagnetic, with J(FeFe) ca.-6.71(4) cm(-1), while the Fe-Ln couplings are much weaker, e.g. J(FeGd) = 0.18(1) cm(-1). Compounds 6, 7, 8 and 13 were selected for Mössbauer studies in order to investigate the influence of isotropic (Gd(III)), highly anisotropic non-Kramers and Kramers (Tb(III) and Dy(III)) and diamagnetic (Y(III)) rare earth ions on the local environment of the Fe(III) centres. Compounds 3, 6, 8 and 13 were also studied using X-Band EPR spectroscopy. For 13, with the diamagnetic Y(III) ion, this made it possible to obtain the D, E, J(FeFe) and g parameters for the iron centres. It is shown that the low-temperature spectra of compounds 3, 6 and 8 are determined by magnetic properties of rare-earth ions and the dipole-dipole interactions between the Ln(III) ions. The Fe-Ln interactions were confirmed as very weak and dipolar in nature by the temperature dependence of EPR spectra at T > 20 K.

Synthesis, structural characterization, magnetic and EPR studies of heterometallic Cu2Cr2 and Cu2Ga2 complexes

Two heterometallic complexes [{Cu(phen)2}2{Cr2(μ-OH)2(nta)2}][Cr2(μ-OH)2(nta)2]·8H2O (I) and [{Cu(phen)2}2{Ga2(μ-OH)2(nta)2}][Ga2(μ-OH)2(nta)2]·dmf·16H2O (III), where nta is the nitrilotriacetate ligand and dmf is dimethylfomamide, phen is ο,ο′-phenanthroline, have been prepared and structurally characterized by single-crystal X-ray crystallography. Both compounds display an ionic type structure, with close geometric parameters, containing {Cr2(μ-OH)2(nta)2} and {Ga2(μ-OH)2(nta)2} entyties for I and III, respectively. These dimeric di-μ-hydroxy units exhibit different coordination functions. In the tetranuclear Cu2Cr2 and Cu2Ga2 complex cations they are involved in a bidentate bridging form being linked to the cooper atoms through two symmetrically related oxygen atoms of the carboxylate groups. The anionic forms [Cr2(μ-OH)2(nta)2]2− and [Ga2(μ-OH)2(nta)2]2− are non-coordinating and only involved in a multiple hydrogen bonds architecture.Comparative analysis of the temperature dependence of EPR spectrum and magnetic susceptibility of compound I and III, showed that the magnetic properties of tetranuclear cluster [{Cu(phen)}2Cr2(OH)2(nta)2] are dominated by strong antiferromagnetic interaction between chromium ions (JisoCrCr=−13.4cm−1). The interaction between chromium(III) and copper(II) ions via syn–anti carboxylic groups has a small antiferromagnetic values (JisoCrCu=−0.23cm−1). The small anisotropy of the exchange interaction between chromium ions manifests itself only in the EPR spectra.

Comparative EPR Study of Monomer and Polymerized Phases of C60

Temperature dependences of EPR spectra of the monomer, 1D (orthorhombic), and 2D (tetragonal and rhombohedral) polymerized phases of C60 were studied in the range of 11–295 K. It was shown that the concentration of paramagnetic centers depends radically on the type of polymerized phases. The temperature dependences allow one to separate temperature-dependent and temperature-independent components of the main doublet signals, which were attributed to C60 + radical and to conducting electrons, respectively. Additional triplet EPR signals assigned to dimer centers formed by a coupling of two C60 + radical were observed for the tetragonal and rhombohedral phases.

Magnetic and EPR studies of edge-localized spin paramagnetism in multi-shell nanographites derived from nanodiamonds

Prolonged (up to 2 h) heat treatment at 1600 °C of nanodiamond particles (5 nm) leads to their conversion to the mixture of quasi-spherical carbon onions and multi-shell polyhedral nanographites. Structural and magnetic properties of two (A and B) series of nanographite samples obtained at various annealing intervals were studied. XRD data show that both multi-shell nanographite samples have practically the same crystalline structures. HRTEM evidences that the most of particles obtained by short time (7 min) annealing have a spherical like shape whereas the long time (~ 2 h) annealing leads to the majority of particles having a polyhedral shape with a hollow inside. Electronic and magnetic properties of these nanocarbons were investigated by magnetic susceptibility and EPR. Annealing results in entire transformation of the EPR signal of nanodiamond to new EPR signals of various line shapes and widths. These signals are extremely sensitive to ambient oxygen. Concentrations for all EPR active spins vary from ~ 1 × 10 19 spins/g (7 min) to ~ 2 × 10 19 spins/g (2 h). Temperature dependences of EPR spectra' parameters were obtained for vacuum-processed samples within the range 4−600 K. Intensity vs. T plots may be well-fitted by the combination of Curie–Weiss and temperature-independent Pauli susceptibility contributions. The latter one increases on heat treatment. Significant extension of electron spin-lattice relaxation time on decreasing temperature was found.

Structural determinations, magnetic and EPR studies of complexes involving the Cr(OH) 2Cr unit

Five heterometallic compounds with formulae [Ba(H 2O) 4Cr 2(μ-OH) 2( nta) 2] · 3H 2O ( I), [M( bpy) 2(H 2O) 2] [Cr 2(OH) 2( nta) 2] · 7H 2O, where M 2+ = Zn, ( II); Ni, ( III); Co, ( IV) and [Mn(H 2O) 3( bpy)Cr 2(OH) 2( nta) 2] · ( bpy) · 5H 2O ( V); bpy = 2,2′-bipyridine, ( nta = nitrilotriacetate ion) have been prepared by reaction of I with the corresponding M II-sulfates in the presence of 2,2′-bipyridine. Substances I– V have been characterized by magnetic susceptibility measurements, EPR and X-ray determinations. I represents a 2D coordination polymer formed by coordination of centrosymmetrical dimeric chromium(III) units and Barium cations. The 10-coordinate Ba polyhedron is completed by four water molecules. Compounds II– IV are isostructural and consist of non-centrosymmetric dimeric anions [Cr 2(μ-OH) 2( nta) 2] 2−, complex cations [M II( bpy) 2(H 2O) 2] 2+ and solvate water molecules. The octahedral coordination of chromium atoms implies four donor atoms of the nta 3− ligands and two bridging OH groups. Multiple hydrogen bonds of coordinated and solvate water molecules link anions and cations in a 3D network. A similar [Cr 2(μ-OH) 2( nta) 2] 2− unit is found in V. The bridging function is performed by a carboxylate oxygen atom of the nta ligand that leads to the formation of a trinuclear complex [Mn( bpy)(H 2O) 2Cr 2(μ-OH) 2( nta) 2]. Experimental and calculated frequency and temperature dependences of EPR spectra of these compounds are presented. The fine structure appearing on the EPR spectra of compound V is analyzed in detail at different temperatures. It is established that the main part of the EPR signals is due to the transitions in the spin states of a spin multiplet with S = 2. Analyses of experimental and calculated spectra confirm the absence of interaction between metal ions (M II) and Cr-dimers in complexes III and IV and the presence of weak Mn–Cr interactions in V. The temperature dependence of magnetic susceptibilities for I– V was fitted on the basis of the expression derived from isotropic Hamiltonian including a bi-quadratic exchange term.

Temperature dependence of EPR spectrum of 2-trifluoromethylnitrobenzene radical anion in acetonitrile

Temperature dependence of the EPR spectrum of 2-trifluoromethylnitrobenzene radical anion in anhydrous acetonitrile in the temperature range 217 K ≤ T ≤ 296 K was studied and simulated. Temperature-dependent dynamic modulation of the fluorine isotropic hyperfine structure is caused by slow hindered rotation of CF3 group with an activation energy of E F* = 36.5±0.5 kJ mol−1, which is the highest value for motions in π-type free radicals studied to date.

Localization of conduction-band electrons in β ″-(BEDT-TTF)4NH4[ Cr (C2O4)3] . DMF single crystals

Temperature dependence of EPR spectrum of β-(BEDT-TTF) 4 NH 4 [Cr (C 2 O 4 ) 3 ].DMF single crystals, which consists of two separate lines characterizing magnetic features of BEDT-TTF and Cr 3+ ions, was studied within the range 1.5-300 K. It was found that reconstruction of the EPR spectrum occurs under decrease of the temperature down to T 20 K. The reconstruction consists in change of Lorentzian shape of the BEDT-TTF line to Gaussian one and in decrease of the effective magnetic moment of Cr 3+ ions. Analysis of temperature and orientation dependencies of parameters of the EPR spectra allows one to suppose that localization of conduction electrons within regions with dimensions close to the size of individual BEDT-TTF molecules occurs at T 20 K. Exchange interactions between Cr 3+ ions change towards antiferromagnetic spin correlations as well.

EPR spectra of Gd 3+ ions and localized paramagnetic centers in La 0.5Gd 0.5Ba 3Cu 2O 6+χ

Oxygen deficient La 0.5Gd 0.5Ba 2Cu 3O 6+χ has been studied by EPR spectroscopy. The temperature dependence of EPR spectrum reveals the presence of sustained magnetic ordering of the Gd 3+ sublattice and localized paramagnetic centers corresponding to copper-oxygen clusters.

EPR study of K2ZnCl4 in incommensurate phase

Temperature dependence of EPR spectra of Mn2+ is studied in paraelectric and incommensurate phases of K2ZnCl4 between 390 and 580 K. The splitting and broadening of the EPR lines are analysed on the basis of phenomenological model of the incommensurate modulation of the fine structure tensor. The temperature dependence of the splitting follows the power law (T-T1)2β with 2β=0.70. The temperature variation of the soliton density is also determined.

EPR study of the mobility of paramagnetic species on the surface and in the bulk of solids

The temperature dependence of EPR spectra provides information on the mobility of paramagnetic species at the gas (liquid)/solid interface and in the bulk of solids. Changes in the environment of molecules on solid surfaces caused by their motion occurring upon thermal treatment at various temperatures are observed. Superoxide radical can migrate from Co(III) to Mg(II) surface sites of the CoO–MgO solid solutions. In aqueous solutions transition metal ions coordinate water molecules, forming aquacomplexes which are usually free to tumble within the liquid medium. Their mobility is, however, strongly modified in the vicinity of the solid surface or inside the narrow pores. In solids the migration of paramagnetic species from the surface into the bulk is controlled by the temperature of thermal treatment. In the case of V 2O 5–ZrO 2 catalyst this process is strongly influenced by the phase transitions occurring in the solid matrix and by the presence of alkali metals.

The temperature dependence of EPR spectra of Cu(II)-doped hexakis(imidazole)cadmium(II) perchlorate

EPR spectra of powder samples of Cu(II)-doped Cd(Im) 6(ClO 4) 2 are studied at X-band over a wide temperature range, 7–300 K. The observed g and copper hyperfine tensors are found to be strongly temperature dependent and are interpreted in terms of Jahn–Teller effect involving a d x 2− y 2 ground state. Single crystal spectra indicate the presence of two misaligned elongated tetragonal octahedral CuN 6 chromophores. The g tensor of Cu(II)-doped Cd(Im) 6(ClO 4) 2 is axially symmetric, implying that the orthorhombic strain component is insignificant. Influence of the structure of host crystals on the observed behaviour is discussed by comparing the EPR results of Cu(Im) 6 2+ in different lattices.

Electron spin relaxation studies of La@C82 in the solid state by EPR

The temperature dependence of EPR spectrum of La@C82 in the powder of empty C60 and C70 mixed crystals was studied by EPR spectroscopy employing X- and Q-band microwave frequencies. The rigid limit spectra (at 4.2 K for the X-band and at 132 K for the Q-band) could be analyzed by static spectral simulation which yielded the EPR parameters,g ‖=2.0021,g ⊥=1.9970,La A ⊥=7.8 MHz,La A ‖∼0 MHz and an isotropic13C coupling value of about 3 MHz. For higher temperatures an appreciable motional averaging effect was observed and the spectra were analyzed by using dynamic spectral simulation based on the stochastic Liouville equation, where we assumed an isotropic rotational motion with the Brownian diffusion. The calculated spectra reproduced the dominant feature of the temperature dependence of the spectra almost satisfactorily for both the X-and Q-band frequencies with the appropriate rotational correlation times. The Arrhenius plots of the correlation time gave two activation energies of 0.9 kcal/mol and 2.9–3.8 kcal/mol for the temperatures below and above 200 K, respectively.

EPR observation of phase transitions in calcium cadmium acetate hexahydrate: Using Mn2+ and Cu2+ ions as probe

Results of temperature dependence of EPR spectra of Mn2+ and Cu2+ ions doped calcium cadmium acetate hexahydrate (CaCd(CH3COO)4·6H2O) have been reported. The investigation has been carried out in the temperature range between room temperature (∼ 300 K) and liquid nitrogen temperature. A I-order phase transition at 146 ± 0.5 K has been confirmed. In addition a new II-order phase transition at 128 ± 1 K has been detected for the first time. There is evidence of large amplitude hindered rotations of CH3 groups which become frozen at ∼ 128 K. The incorporation of Cu2+ and Mn2+ probes at Ca2+ and Cd2+ sites respectively provide evidence that the phase transitions are caused by the molecular rearrangements of the common coordinating acetate groups between Ca2+ and Cd2+ sites. In contradiction to the previous reports of a change of symmetry from tetragonal to orthorhombic below 140 K, the symmetry of the host is concluded to remain tetragonal in all the three observed phases between room temperature and liquid nitrogen temperature.

Temperature dependence of EPR spectrum of Mn 2+ doped in ammonium iodide single crystals

EPR of Mn 2+ doped in ammonium iodide single crystal has been studied at X-band in the temperature range 573–577 K. The observed temperature dependence of line widths and spin Hamiltonian parameter b 2 0 below room temperature is related to the structural transformations in the crystal. The coexistence of high temperature phase (NaCl) and low temperature phase (CsCl) is attributed to the large thermal hysteresis in line widths and b 2 0. The dissociation of ion vacancy pairs occurs near 500 K and is reflected in the reversible change of an anisotropic EPR spectrum in an isotropic sextet near this temperature. The ion vacancy pair models for NaCl and CsCl phases are discussed along with the effects of thermal processing of the samples. Heating the crystals above 500 K leads to expulsion of Mn 2+ impurity from the crystal.

EPR and optical absorption study of VO 2+ ion doped in (NH 4) 6Sb 4(SO 4) 3F 12 single crystal

The results of an EPR and optical absorption study of vanadyl ion (VO 2+) doped in (NH 4) 6Sb 4(SO 4) 3F 12 single crystal are reported. The EPR study has been undertaken in the temperature range, from RT to LNT. In the upper hexagonal phase of (NH 4) 6Sb 4(SO 4) 3F 12 the EPR spectrum is a single isotropic octet and indicates the rapid tumbling motions of the vanadyl complex. Below 256 K a different type of orientation independent spectrum comprising two sets of octets is observed and indicates that the tumbling motions are completely quenched below 256K. On the basis of temperature dependence of EPR spectrum a phase transition in (NH 4) 6 Sb 4(SO 4) 3F 12 is confirmed. The optical and EPR data are correlated to suggest symmetry and coordination of the vanadyl complex in the lost crystal.

Temperature Dependence of EPR Spectrum from VO2+Ions in (NH4)2SO4

Temperature dependence of EPR spectra from polar VO 2+ probes doped in ferroelectric (NH 4 ) 2 SO 4 crystal was studied in the range between 110 K and 300 K. Impurity ion, VO 2+ , takes substitutionally the position of two NH 4 + ion vacancies so as to satisfy the local charge neutrality. Three kinds of VO 2+ sites in the crystal are identified. One is VO 2+ substituted for II-II pair of NH 4 + ions, another is I-II and the other is I-I. When the temperature is lowered, VO 2+ molecular axes are slightly displaced in a different manner, respectively. From this fact it is concluded that the local electric field is reflected in the VO 2+ displacement. The analyses of the polar angles to the crystal c -axis for VO 2+ in II-II and I-I sites show quite similar profile with anomalous temperature dependence of polarization observed by Unruh. The two-sublattice model for (NH 4 ) 2 SO 4 is supported by the present study.

Exchange integral for a variety of tetranuclear ferredoxins

The temperature dependence of EPR spectra of oxidized [4Fe-4S∗] (−1, −2) ferredoxins (previously designated HiPIP) and a reduced [4Fe-4S∗] (−2,−3) ferredoxin have been analyzed so as to determine the energy of a low-lying excited electronic state. The values obtained were: Center S-3 from beef heart, 44 cm −1; Center S-3 from mung bean, 53 cm −1; the [4Fe-4S∗] (−1,−2) ferredoxin from Thermus thermophilus, 78 cm −1; Center N-2 of NADH ubiquinone reductase, 83 cm −1. Increasing axial distortion in the EPR spectra of the [4Fe-4S∗] (−1,−2) ferredoxins was associated with higher energy differences. Center N-2, a [4Fe-4S∗] (−2,−3) iron-sulfur cluster does not fit this relationship.

Temperature dependence of EPR spectra of light-induced Cr 3+ centers in silver chloride crystals

In chromium-doped AgCl crystals, the conversion from Cr 2+ ions to Cr 3+ ions by illumination with blue light below 178 K has been detected by means of EPR. Three kinds of spectra could be identified. The angular dependence of the EPR signals and the temperature dependence of signal intensities indicate two [100]-axial centers of the point groups C 4v and D 4h, and an orthorhombic center of the point group D 2h. The centers are interpreted as Cr 3+ ions associated with one or two Ag + vacancies. At temperatures above 178 K, the recharging to Cr 2+ takes place. The interaction between Cr 3+ and vacancies is estimated. The mechanism of charge transfer between Cr 2+ and Cr 3+ is discussed.