Powder EPR Spectra

While the effective g value is expressed in terms of three principal values directed along three axes or directions in a single crystal, only the principal values of g can be extracted from the powder spectrum rather than the principal directions of the tensor with respect to the molecular axes. (Therefore it is more correct to label the observed g values as gi, g2, g3 rather than g gyy, in a powder sample.) In the simplest case, an isotropic g tensor can be observed, such that all three principal axes of the paramagnetic center are identical (x = y = z and therefore gi= gi = g-i). In this case, only a single EPR line would be observed (in the absence of any hyperfine interaction). With the exception of certain point defects in oxides and the presence of signals from conduction electrons, such high symmetry cases are rarely encountered in studies of oxides and surfaces. 

More commonly the symmetry of the paramagnetic centers studied in metal oxides will be lower than isotropic, such as axial (g = gyy fe and AJ 

Figu re 1.9 (a) Absorption and (b) first derivative EPR lineshape for a randomly oriented S = 1 /2 spin system with axial symmetry. The angular dependence curve (0 vs field) is shown in (c). 

In the second example, consider the case of a paramagnetic species with rhombic symmetry x y z), characterized by three g values of gi, g2 and g. The variation in the g values now depends on the two polar angles of 0 and (Equation 1.29) and a typical example of the absorption and first derivative profiles for such a 

7 EPR (Electron Paramagnetic Resonance) Spectroscopy of Polycrystalline Oxide Systems 

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Spalek, T., Pietrzyk, P. and Sojka, Z. (2005) Application of the genetic algorithm joint with the Powell method to nonlinear least-squares fitting of powder EPR spectra,. /. Chem. Inf. Model., 45, 18. 

Biochemical EPR samples are almost always collections of randomly oriented molecules (frozen) aqueous solutions in which each paramagnetic molecule points in a different direction. In order to generate simulations of these powder EPR spectra we have to calculate the individual spectrum for many different orientations and then add these all up to obtain the powder pattern. Numerical procedures that generate sufficient spectra to approximate a powder pattern are collectively known as walking the unit sphere algorithms. Here is the basic procedure ... 

Recently, hf structure associated with the copper signal of cytochrome c oxidase has been reported by Frondsz et al.210 which used octave bandwidth S-band EPR spectroscopy (2-4 GHz). The observed structure has been attributed to copper hfs and to an additional magnetic interaction. Data obtained from powder simulation of the EPR spectra at 2.62 GHz and 3.78 GHz are collected in Table 12.2. In a subsequent paper Frondsz and Hyde211 have shown that in S-band EPR spectra of copper complexes in frozen solutions, improved spectral resolution can be achieved. This new technique, which allows a proper selection of the microwave frequency between 2 and 4 GHz, is therefore recommended for studying powder EPR spectra of these types of compounds. 

The various special ENDOR techniques summarized in Sect. 4 widen the field of applications considerably. They allow investigations either of complex, oriented spin systems, or of paramagnetic centers in randomly oriented large molecules. The ENDOR techniques are particularly useful to study biochemical systems, which are often characterized by very poorly resolved powder EPR spectra. 

Powder EPR spectra for Mnu-doped compounds (M(acac)2(bipy)] (M = Zn or Cd), [Zn(acac)2(phen)] and [Cd(acac)2(phen)H20)]7<47 indicate that distortions from octahedral symmetry were greater for the bipyridine adduct than for the phenanthroline adduct, and greater for Cd than for Zn. IR measurements confirm that all the compounds are tris-bidentate, except for [Cd(acac)2(phen)(H20)] which probably has coordinated water and a free carbonyl group. 

In the actual experiments, EPR spectra are also recorded at every orientation, from which one can make good estimates of anisotropic nitrogen hyperfine couplings which are not normally detected in the ENDOR experiments. In most cases complicated single crystal (and even powder) EPR spectra can be faithfully reproduced with the accurate proton couplings obtained from the ENDOR experiments and the nitrogen hyperfine couplings obtained from the EPR spectra. Examples of these combined results will be presented. 

Long range dipolar interactions between an unpaired electron and nuclear spins on adjacent atoms will not normally be resolved in conventional powder EPR spectra.The pulse technique of electron spin echo modulation (ESEM) is in favourable cases able to detect very weak hyperfine interactions not seen in CW EPR. The method measures modulation of the electron spin echo signal by dipolar hyperfine coupling in the time domain at fixed magnetic field. Until recently,... 

Copper(II) bis[(0-alkyl)-4-ethoxyphenyldithiophosphonato] and chromium(III) tris[(0-alkyl)-4-ethoxyphenyldithiophosphonato] complexes (alkyl = Me, Et, Pr1) were prepared and studied by magnetic measurements, and electronic, IR, and EPR spectroscopy. The valence vibrations of the PS2 group show that this group coordinates as isobidentate. The powder EPR spectra are typical for square-planar monomeric species and present hyperfine and superhyperfine structure. The EPR bands of the chromium(III) complexes may be attributed to metal ions in a pseudo-octahedral environment, coupled by dipole-dipole interaction.123... 

Fig. 4 Absorption (top) and derivative (bottom) powder EPR spectra of a nitroxide radical measured at X-band frequencies (9.5 GHz)...

Figure 3 Experimental and simulated X-band powder EPR spectra of H2(V(R,R-HIDPA)2] diluted in the corresponding zirconium compound, where HIDPA=hydroxyiminodipropionate. (A) Simulated spectrum with axial symmetry and gn = 1.9195, = 1.9839, A i = IT5mT and A, = 4.9mT (B) experimental spectrum and (C) simulated spectrum with rhombic symmetry = 1.9195, g = 1.9848, gyy= 1.9829, /W = 17.15 mT.

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