Angular Dependence Of EPR Spectra Research Articles

Spin Probe Determination of Molecular Orientation Distribution and Rotational Mobility in Liquid Crystals: Model-Free Approach.

A model-free approach for simulation of EPR spectra of nitroxide spin probes in liquid-crystalline materials was suggested and used to obtain parameters of molecular orientation and rotational mobility. The developed method is based on experimental recording and numerical simulation of the angular dependence of EPR spectra, which is shown to be much more informative in comparison with a single EPR spectrum. Quantitative spectral simulations considering both local orientational ordering and distribution of local directors in the sample were used for discrimination of models of rotational mobility and orientational alignment. The method was applied for detailed quantitative characterization of axial, orthorhombic, and low-symmetry non-orthorhombic molecular orientation distributions. It is shown that the ordinarily used model of rotational diffusion in a mean-field potential is suitable for the description of molecular mobility and orientational ordering only for relatively low sample temperatures and low-mobility probe molecules with large sizes. In cases of high molecular mobility, the more realistic jump mechanism of molecular moves can be approximately described as quasi-librations. For ordered liquid crystals it was found that mostly the order parameters up to rank 12-14 are essential and easily determined. When well-aligned materials are described, the order parameters up to 18th rank or even higher become meaningful. Both molecular and sample biaxiality is analyzed and quantitatively characterized. The local molecular ordering and sample orientational alignment are quantitatively characterized separately.

Identification of environment symmetry for iron centers in aluminosilicates by EPR

We study an angular dependence of EPR spectra of the own defects in topaz (Al2[SiO4][F,OH]2). The topaz crystal structure is built of silicon-oxygen tetrahedral (SiO4) and octahedral aluminum surrounded by four oxygen ions and fluorine ions or a hydroxyl group. There are two types of substitution defects in topaz, such as octahedral and tetrahedral, depending on the location of the impurity ion. The point defects with octahedral symmetry arise on local substitution of aluminum (Fe3+ → Al3+), while tetrahedral coordinated centers are formed by substitution of silicon (Fe3+→ Si4+) in the silicon-oxygen tetrahedral (SiO4). To determine the symmetry of the impurity centers, the angular dependence of the EPR spectrum is calculated. The excited energy states for defects are determined by the magnitude of the g-factor shift. These states equal 1.6 eV and 3.2 eV, respectively. A super-hyperthin structure (SHTS) of the EPR lines of iron has been observed. This SHTS is determined by the magnetic moments of the fluorine nuclei F19 located in the first coordination sphere of the paramagnetic iron center. The models of the three new paramagnetic centers in topazes are suggested, one of them being of orthorhombic symmetry for aluminum substitution and the two others being of tetragonal symmetry for silicon substitution with oxygen vacancies.

High temperature phase transition and multiphase state formation in K3H(SO4)2 superprotonic conductor

EPR spectra of K3H(SO4)2 (KHS) doped with VO2+ or Mn2+ recorded at the temperature higher than about 440K are time-dependent and indicate on anomalously slow kinetics of the paraelastic/ferroelastic phase transition which takes place far below the temperature proposed by Baranov et al. (TII=463K). The formation of a multidomain state upon heating below TII is manifested as irreversible changes in EPR spectra after sample cooling to room temperature. The angular dependence of EPR spectra indicates that K3H(SO4)2 undergoes a ferroelastic phase transition between the point groups 2/m and 3¯m. The EPR spectra of KHS doped with VO2+ or Mn2+ are rapidly changed into the pseudo-powder ones after annealing above TI=480K. This points to a decomposition of the KHS crystal and a multiphase state formation at TI=480K, rather than at superprotonic phase transition, which is typical for the majority of compounds of the M3H(XO4)2 type.

Orientation distribution function and order parameters of oriented spin probe as determined by EPR spectroscopy

Two methods for determination of molecular orientation distribution from angular dependence of EPR spectra are compared. One of these methods is based on direct expansion of the orientation distribution function in a series of generalized spherical harmonics, in another method the mean field potential is used. The limitations of these approaches as well as their capabilities and advantages are considered

Photostimulated electron transfer and its action on paramagnetism of Sp3Cr(C2O4)3 single crystals

The effect of irradiation by ultraviolet light on the effective magnetic moment of a paramagnetic single crystal based on photochrome spiropyran (Sp) and chromium oxalates Sp3Cr(C2O4)3 molecules is detected. It is shown that the deviation of the temperature dependence of the magnetic moment from the Curie law is caused not by the exchange interaction, but by electron redistribution between Cr3+ and Cr4+ ions and spiropyran molecules Sp0 and Sp+. Analysis of the angular dependence of EPR spectra makes it possible to determine the contribution of Cr3+ ions to the magnetic properties of the crystals and to determine the crystal field parameters D = 0.619 cm−1 and E = 0.024 cm−1. Irradiation of hydrated samples by ultraviolet light leads to intensity redistribution of EPR lines attributed to Cr3+ and Sp0. Thermally stimulated paramagnetism of triplet states of spiropyran ions Sp+ and the SpI salt is observed.

Evolution of ordered films of copper phthalocyanine according to EPR data

The procedure for calculating the orientation distribution of molecules using the angular dependence of EPR spectra was employed to study copper(II) phthalocyanine (CuPc) films varying in thickness and obtained by depositing the molecular complex on flat quartz plates. At the first stage of deposition, a layer of the α-CuPc phase with preferable orientation of molecular stacks along the plate surface is formed. At the second stage, a layer with an orthogonal arrangement of molecular stacks is condensed over the first layer. The interaction with NO2 forms CuPc binuclear associates. Analysis of the EPR spectra made it possible to determine the symmetry of the structure and the distance between the paramagnetic Cu2+ ions; the structure of the associates has been proposed. The orientation distribution of CuPc dimers in the film depends both on the initial ordering in the film and on processing conditions. Strong disordering of molecular stacks in ordered films during the α-CuPc to α-CuPc phase transition has been found.

Determination of molecular orientation distribution of stable paramagnetic probes in stretched polymers

The possibility of determination of the orientation distribution functions of paramagnetic probes in stretched polymers by analyzing the angular dependence of EPR spectra was demonstrated taking two stable radicals, 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl and 2-septadecyl-2,3,4,5,5-pentamethylimidazolidine, as examples. The method employed permits establishment of the preferred orientation of the probe molecules relative to the polymer macromolecules. The technique is responsive to changes in the orientation distribution of impurity particles on annealing of the matrix.

Microenvironments of tyrosine D + and tyrosine Z + in Photosystem II studied by proton matrix ENDOR

Proton matrix ENDOR spectra of tyrosines Z +(Y + Z) and D +(Y + D) were studied in the frozen Tris-treated PS II. ENDOR spectra of the tyrosines in oriented membranes observed at 10 K have shown similar angular dependence with similar line pair separations, suggesting that the local arrangement of protons surrounding both radical species is similar. The angular dependence of EPR spectra showed the C-O bonds in Y + Z and Y + D lie parallel to the membrane normal. With increasing temperature, the intensity of the line pair with 1.2 MHz separation decreased above 25 K for Y + Z, and above 110 K for Y + D, respectively. Assuming a motion of the proton responsible to the line pair, the activation energies for the motional broadening were estimated to be 86 ± 20 J/mol for Y + Z, and 810 ± 200 J/mol for Y + D, respectively. The broadening may be ascribed to small amplitude vibrations of protons surrounding tyrosine radicals. The intensities for the peaks with about 7 MHz separation assigned to the OH proton of the tyrosine decreased, which was ascribed to a similar motional broadening. The difference in temperature dependence of line broadening of the two radicals suggests that the functional difference between them may be ascribable to differences in flexibility in their microenvironment.

A Method to Obtain Directly the Angular Dependence of EPR Spectra in Single Crystal Studies

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