Electron paramagnetic resonance EPR spectroscopy

In the EPR experiment, transitions are observed between the Zeeman split Ms components of a spin S in an applied field. Typical commercial 

0 = 45° as a function of H. (b) Level crossing field as a function of orientation (0) of crystal unique axis with respect to H, measured at 0.4 K. Reprinted with permission from van Slageren et al., 2002 [20]. Copyright (2002) Wiley-VCH 

By measuring EPR on a single crystal as a function of its orientation, it is possible to determine the orientation of the principal axes of the ZFS [Dxx,yy,zz) with respect to the crystal and, therefore, the molecular geometry. For powder samples the weighted sum of all orientations is measured. 

However, measurement of the first derivative spectrum means that, in the high field limit, only features corresponding to x, y and z orientations will be observed, thus greatly simplifying matters. Unfortunately, measuring in the high field limit is not always possible. For very anisotropic samples it may be necessary to immobilise polycrystalhne samples [e.g. in wax or as a pellet) in order to prevent torquing of the crystallites (see above). 

A comprehensive text on EPR of exchange coupled materials has been published by Bencini and Gatteschi and an excellent tutorial on the benefits of high-frequency/field EPR as applied to high spin and anisotropic molecules by Barra et 

FIGURE 6.1 (a) EPR spectrum of horse spleen apoferritin incubated with haemin (8 haemin molecules/molecule of apoferritin) at pH 8 (b) 

the fine structure in the EPR spectrum reveals that delocalisation is limited to the PNSNP segment of the ring. 

For a more advanced treatment of the theory and applications of EPR spectroscopy, the reader is referred to the text by Weil et 

72 Electron paramagnetic resonance (EPR) spectroscopy. This is also known as electron spin resonance (ESR) spectroscopy and is the electron analogue of NMR. In the case of EPR, however, the magnetic moment is derived from unpaired electrons in free radical species and transition metal ions. The paramagnetism of many transition metal oxidation states has already been mentioned as a drawback to the observation of their NMR spectra, but it is the raison d etre behind EPR the technique is thus limited, in the case of metals, to those which are paramagnetic or which have free radicals as ligands. 

For isolated electron spins the magnetic moment, p, is equal to —gHoS, where Po is the Bohr magneton, the electron spin quantum number, S, is equal to %, and g is a constant (known as the g-value). The y-value is characteristic of the 

Ion d electrons ground term ground term ground term  

An additional interaction that is frequently seen in EPR spectra involves coupling of the electron and nuclear magnetic moments in molecules where there are nuclei with non-zero spins. These are the same isotopes as those used to generate NMR spectra and are summarised in Table 3.3. Spectra are split into 27+1 components, and the resulting peak separations are called hypefine splittings (which are 

A review of all of the different types of EPR spectrum that can be obtained from the various paramagnetic transition metal ions is beyond the scope of this chapter. However, a few examples will be presented below to illustrate the type of information that can be obtained. 

Electron paramagnetic resonance (EPR) [or electron spin resonance (ESR)] spectroscopy is useful for the characterization of spedes having unpaired electrons. Since most molecular dusters are diamagnetic, the application of this technique is limited. However, paramagnetic spedes may be formed under certain conditions when metal carbonyl clusters are formed in zeolite cages. Due to the 

Abdo and Howe [84] observed the EPR signal of a radical containing two equivalent molybdenum nuclei formed from activated samples prepared from [Mo(CO)6] in HY zeolite and suggested that dinuclear anionic carbonyl species analogous to the known complex [Mo2(CO)io] may have formed. Similarly, EPR spectroscopy has been used to investigate the conversion of [Fe2(CO)9] to [HFejCCO),]- in HNaY zeolite. [50] 

This technique can be used to measure the production of free radicals because the unpaired electron in a free radical has magnetic resonance. However, because the radicals are unstable, owing to their high chemical reactivity, the technique of spin-trapping is used. In this technique, the generated radicals react with a suitable probe, and the EPR spectra arising from the reaction of the probe with different radical species can then be identified. 

There is some evidence, from EPR spectroscopy and analysis of spin-trapped adducts, to suggest that OH may indeed be formed by activated neutrophils. However, caution must be exercised in interpreting such data because 02 -generated adducts may decay to form adducts that resemble those generated directly from -OH. For example, 5,5-dimethyl-l-pyrroline-l-oxide (DMPO) can react with C 2 to form DMPO-OOH, and with OH to form DMPO-OH formation of the latter adduct in phagocytosing neutrophils is taken as evidence for OH formation. However, two facts must be considered  

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Electron paramagnetic resonance (EPR) spectroscopy proves the formation of peroxyl radicals in oxidized hydrocarbons [12—15]. 

Reorienting an electron in a magnetic field ( electron spin) as seen in electron paramagnetic resonance (EPR) spectroscopy... 

Electron paramagnetic resonance (EPR) spectroscopy is yet another diagnostic tool for the detection of isomorphous substitution of Ti. Its sensitivity is very high, and investigations can be performed with samples even with very low contents of paramagnetic species. The spectra and g parameters are sensitive to the local structure and associated molecular distortions. Hence, it is an ideal tool to characterize Ti in titanosilicates. Ti in the + 4 oxidation state in titanosilicates is diamagnetic and hence EPR-silent. Upon contacting with CO or H2 at elevated... 

Analysis of antioxidant properties relative to the DPPH" radical involves observation of colour disappearance in the radical solution in the presence of the solution under analysis which contains antioxidants. A solution of extract under analysis is introduced to the environment containing the DPPH radical at a specific concentration. A methanol solution of the DPPH radical is purple, while a reaction with antioxidants turns its colour into yellow. Colorimetric comparison of the absorbance of the radical solution and a solution containing an analysed sample enables one to make calculations and to express activity as the percent of inhibition (IP) or the number of moles of a radical that can be neutralised by a specific amount of the analysed substance (mmol/g). In another approach, a range of assays are conducted with different concentrations of the analysed substance to determine its amount which inactivates half of the radical in the test solution (ECso). The duration of such a test depends on the reaction rate and observations are carried out until the absorbance of the test solution does not change [4]. If the solution contains substances whose absorbance disturbs the measurement, the concentration of DPPH radical is measured directly with the use of electron paramagnetic resonance (EPR) spectroscopy. 

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