Structural characterization electron spin resonance

Electron paramagnetic resonance spectroscopy (HER), also called electron spin resonance spectroscopy (ESR), may be used for direct detection and conformational and structural characterization of paramagnetic species. Good introductions to F.PR have been provided by Fischer8 and I.effler9 and most books on radical chemistry have a section on EPR. EPR detection limits arc dependent on radical structure and the signal complexity. However, with modern instrumentation, radical concentrations > 1 O 9 M can be detected and concentrations > I0"7 M can be reliably quantified. 

Let us take 1978 as the starting point. Massoth [51] then published an extensive review of what was known about the structure of HDS catalysts. Characterization was essentially based on techniques such as X-ray diffraction, electron microscopy, photoelectron spectroscopy, electron spin resonance and magnetic methods. Massoth was rather unhappy with the state of affairs in 1978. He was struck by the ...diversity and apparent contradictions of results and interpretations... It almost seems as though everyone is working with a different catalyst . 

The format of this section is similar to that of the review by Knowles <1996CHEC-II(7)489>, where only the main structure elucidation techniques are reviewed. None of the heterocyclic systems included in this chapter exists as radicals thus, electron spin resonance (ESR) spectroscopy is not included. Mass spectrometry is also omitted from discussion, as this technique is always used in conjunction with other analytical techniques to ensure full characterization of compounds. Nevertheless, mass spectra for most of the compounds in this chapter have been reported, although assignments of fragmentation patterns are rarely given. 

It is true that in some cases, the spectroscopic data on a reactive intermediate are so persuasive that the connection between structure and spectroscopic features is firm. However, in general this will not be the case, and additional spectroscopic or preparative criteria will have to be provided. So we are faced with the question How can we connect the information obtained, for example, from observations in matrices or in solution-phase fast kinetic studies, to molecular structure How do we know that the results of these experiments, using what we hopefully call direct methods, really pertain to the species we are trying to characterize I attempt to deal with this issue in what follows. Since the methods used vary from one class of non-Kekule species to another, specific classes are individually discussed, and special techniques are introduced as needed. Electron spin resonance spectroscopy has played such a pervasive role that it will be useful to give first a brief outline of that method. 

In order to characterize the active site structure of Ca ATPase from sarcoplasmic reticulum, we have employed Gd + as a paramagnetic probe of this system in a series of NMR and EPR investigations. Gadolinium and several other lanthanide ions have been used in recent years to characterize Ca + (and in some cases Mg2+) binding sites on proteins and enzymes using a variety of techniques, including water proton nuclear relaxation rate measurements (35,36,37), fluorescence (38) and electron spin resonance (39). In particular Dwek and Richards (35) as well as Cottam and his coworkers (36,37) have employed a series of nuclear relaxation measurements of both metal-bound water protons and substrate nuclei to characterize the interaction of Gd + with several enzyme systems. 

M(VI) and M(IV) oxidation states. The M(V) state is generated by a one-electron reduction of the M(VI) state, or the one-electron oxidation of the M(IV) state, and occurs during the catalytic cycle—en route to the regeneration of the catalytically active state. Spectroscopic studies of the Mo—MPT enzymes, notably electron spin resonance (EPR) investigations of the Mo(V) state, have clearly demonstrated that the substrate interacts directly with the metal center (37). The first structural characterization of a substrate-bound complex was achieved for the DMSOR from Rhodobacter capsulatus DMS was added to the as-isolated enzyme to generate a complex with DMSO that was O-bound to the molybdenum (43). 

In addition to the structure in the dehydrated state, the structure of supported vanadia catalysts under redox reaction conditions is directly related to the catalytic performance. Vanadia catalysts are usually reduced to some extent during a redox reaction, and the reduced vanadia species have been proposed as the active sites [4, 19-24]. Therefore, information on the valence state and molecular structure of the reduced vanadia catalysts is of great interest. A number of techniques have been applied to investigate the reduction of supported vanadia catalysts, such as temperature programmed reduction (TPR) [25-27], X-ray photoelectron spectroscopy (XPS) [21], electron spin resonance (ESR) [22], UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) [18, 28-32], X-ray absorption fine structure spectroscopy (XAFS) [11] and Raman spectroscopy [5, 26, 33-41]. Most of these techniques give information only on the oxidation state of vanadium species. Although Raman spectroscopy is a powerful tool for characterization of the molecular structure of supported vanadia [4, 29, 42], it has been very difficult to detect reduced supported... 

A considerable number of different techniques has been employed in the past to characterize the porosity and surface chemistry of porous carbon materials. These include gas adsorption (mostly N2 and CO2) [9-14], immersion calorimetry [9], small-angle X-ray [11,15] and neutron [14] scattering, inverse gas chromatography [12,13], differential thermal analysis [12], Fourier transform infrared [12], Raman [16] and X-ray photoelectron [17] spectroscopies and electron spin resonance [16]. It is worth mentioning that the information about the porous structure of the material provided by this array of techniques is only indirect... 

Electron spin resonance (ESR) is extensively used in the study of fulleride ions, as the magnetic characterization of these molecular ions yields fundamental information on the electronic structure. An ESR signal can in principle appear in any system containing fulleride ions, as the configuration can involve unpaired spins even in systems with an even number of electrons. In solids, the Pauli susceptibility indicates a metallic state. 

Due to the quite similar structure of HA and other GAGs, e.g. ChS, clear distinction between both species can hardly be established by H NMR. This is, however, possible by using NMR that is characterized by higher resolution than H NMR [249]. The considerable role of HO radicals in the synovial fluids from patients with RA was recently proven also by electron spin resonance spectroscopy (ESR) using the spin trap 5,5-dimethyl-l-pyrroline-A-oxide to convert the highly reactive HO radical into a more stable compound [250]. 

Electron spin resonance spectroscopy (ESR) has been used by several research groups to characterize the local structure of CTB-Cu (iO, 11, 13). For the neat Ionomer, both Isolated and dimeric copper complexes, as shown In Fig. 9, have been reported. Fig. 10 compares the ESR spectra for CTB-Cu and Blend 1, PSVP/ CTB-Cu (1 1). The strong signal near 3160 G was due to Isolated copper Ions with a square planar structure as In Fig. 9a. The measured g-Lande factor and hyperflne Interaction parameters were gy" 2.320, g = 2.059, A = 145 6, and 30 + 5 G, which agreed with those reported for Isolated Cu(II) Ions In model compounds (14). 

We ve tried to include all substantial developments and advances in this new edition. Significant developments in biomedical applications, microelectromechani-cal systems, and electronic textiles have been included, as has synthesis of nano-structured CEPs. New methods for characterizing CEPs, such as electrochemical Raman and electron spin resonance spectroscopy, have also been described. Significant progress is also detailed in techniques for processing CEPs and the fabrication of devices. 

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