Electron paramagnetic resonance description

The description theoretical study of defects frequently refers to some computation of defect electronic structure i.e., a solution of the Schrodin-ger equation (Pantelides, 1978 Bachelet, 1986). The goal of such calculations is normally to complement or guide the corresponding experimental study so that the defect is either properly identified or otherwise better understood. Frequently, the experimental study suffices to identify the basic structure of the defect this is particularly true when the system is EPR (electron paramagnetic resonance) active. However, if the computational method properly simulates the defect, we are provided with a wealth of additional information that can be used to reveal some of the more basic and general features of many-electron defect systems and defect reactions. 

Electron Paramagnetic Resonance Spectra. Only two of these complexes exhibit well-resolved EPR spectra. A narrow, isotropic signal observed at g = 2.005 for the trinuclear complex 12 at low temperatures is consistent with an S = 1/2 ground state169), but a detailed description of the electronic properties of the complex remains to be developed. The [Fe(MoS4)2]3 ion shows a rhombic S = 3/2 EPR spectrum that is very solvent dependent and, under certain conditions, is somewhat similar in apperance to that of FeMo-com). For example, in frozen aqueous solution, the apparent g values are 5.3,2.6, and 1.7181). If complex 14 also proves to have an S = 3/2 ground state, a somewhat similar EPR spectrum at low temperature would be expected as well. 

As yet, no X-ray crystal structures are available for any of the molybdenum enzymes in Table I. Therefore, present descriptions of the coordination environment of the molybdenum centers of the enzymes rest primarily upon comparisons of the spectra of the enzymes with the spectra of well-characterized molybdenum complexes. The two most powerful techniques for directly probing the molybdenum centers of enzymes are electron paramagnetic resonance (EPR) spectroscopy and X-ray absorption spectroscopy (XAS), especially the extended X-ray absorption fine structure (EXAFS) from experiments at the Mo K-absorption edge. Brief summaries of techniques are presented in this section, followed by specific results for sulfite oxidase (Section III.B), xanthine oxidase (Section III.C), and model compounds (Section IV). 

Chemically-induced spin polarization was one of the last truly new physical phenomena in chemistry to be discovered and explained during this century. So unusual were the observations and so ground-breaking the theoretical descriptions that, over a very short time period, the chemist s way of thinking about free radical reactions and how to study them was fundamentally changed. After the earliest experimental reports of unusual phases of electron paramagnetic resonance (EPR) (1963) [1] and nuclear magnetic resonance (NMR)... 

An overview of die free radicals and reactions thereof is presented. Free radicals are atoms or groups having an unpaired electron and hence are paramagnetic. Electron paramagnetic resonance spectroscopy (EPR) and trapping methods are used to analyze radicals. In lipids, radical reactions lead to autoxidation and hence flavor reversion. Reactive oxygen species are key components involved in such reactions. Finally, descriptions for phenolic, sequesterant and enzymatic antioxidants and their mode of action are provided. 

Chang, T., Kahn, A.H. NBS Special Publication 260-59 Standard Reference Materials Electron Paramagnetic Resonance Intensity Standard SR M-2601 Description and Use. Washington, D.C. NBS, 1978. 

The spin polarization effect was first identified in free radicals, and in particular in conjugated hydrocarbons [48, 49]. In such radicals (the simplest one being the planar CH3 molecule), where the unpaired electron is supposed to occupy a n type MO, the EPR (Electron Paramagnetic Resonance) experiment evidenced the existence of spin density on the nuclei of the hydrogen atoms. This seems in contradiction with the nuUity of the SOMO in the plane of the molecule and thus on the H atom nuclei. This observation could interpreted only if on leaves the closed-shell description of the core . 

Chang T, Kahn AH. Standard reference materials electron paramagnetic resonance intensity standards SRM 2601, description and use. NIST s special publication 1978 260-59,38 5. 

It is a great pleasure to introduce Volume 24 of our SPR series in Electron Paramagnetic Resonance. As in previous volumes, we have tried to embrace the enormous and diverse areas of science where EPR has made such an important impact, by carefully selecting a number of Chapters that will appeal to all practitioners of the technique. From innovative and translational technologies to the core science disciplines, EPR continues to provide an unrivalled description of systems containing unpaired electrons, and the following Chapters will serve to illustrate and exemplify these elegant contributions. 

The electron spin resonance (ESR) technique has been extensively used to study paramagnetic species that exist on various solid surfaces. These species may be supported metal ions, surface defects, or adsorbed molecules, ions, etc. Of course, each surface entity must have one or more unpaired electrons. In addition, other factors such as spin-spin interactions, the crystal field interaction, and the relaxation time will have a significant effect upon the spectrum. The extent of information obtainable from ESR data varies from a simple confirmation that an unknown paramagnetic species is present to a detailed description of the bonding and orientation of the surface complex. Of particular importance to the catalytic chemist... 

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