Electron spin resonance spectroscopy limitations

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. 

The techniques available to achieve molecular structure determinations are limited. They include structural analysis with diffraction techniques—such as electron, neutron, and x-ray diffraction—and various absorption and emission techniques of electromagnetic radiation—such as microwave spectroscopy and nuclear magnetic resonance (NMR). For molecules with unpaired spins a companion technique of electron spin resonance spectroscopy (ESR) is highly informative. 

Electron Spin Resonance Spectroscopy. Practically all the ESR studies on dioxygen complexes have been concerned with 77 cobalt dioxygen complexes, and the results have been summarised in the review by Basolo et al. We limit our discussion here to the essential points relating to the electronic structure, and the reader is invited to consult the references given for fuller details. 

The applicability of spectroscopic methods (other than NMR) for determining functionality in humic substances is reviewed. Spectroscopic methods, like all other investigational techniques, are severely limited when applied to humic substances. This is because humic substances are comprised of complicated, ill-defined mixtures of polyelectrolytic molecules, and their spectra represent the summation of the responses of many different species. In some cases only a small fraction of the total number of molecules contributes to the measured spectrum, further complicating the interpretation of spectra. The applicability and limitations of infrared spectroscopy, Raman spectroscopy, UV-visible spectroscopy, spectrofiuorimetry, and electron spin resonance spectroscopy to the study of humic substances are considered in this chapter. Infrared spectroscopy, while still very limited when applied to humic substances, is by far the most useful of the methods listed above for determining functionality in these materials. Very little information on the functionality of humic substances has been obtained by any of the other spectroscopic methods. 

Since its discovery by Zavoisky in 1944, electron spin resonance spectroscopy (ESR) (also called electron paramagnetic resonance spectroscopy [EPR]) has become an essential tool for the study of the structure and dynamics of molecular systems containing one or more unpaired electrons. Such paramagnetic systems can frequently be examined using magnetic susceptibility techniques as well, but these do not provide the detailed information that ESR spectroscopy does. ESR spectroscopy and magnetic susceptibility methods each have their strengths and limitations and often provide complementary information. 

Applications of esr spectroscopy for monitoring the degree of functionalization of a polymer are limited, primarily because esr-active groups are mostly used as probes rather than as reactive functionalities. Electron spin resonance spectroscopy has, however, been used to estimate the proximity of titanium groups in a titanocene polymer (Grubbs, et al., 1973 Bonds et al., 1975). It has also been used to demonstrate the presence of copper prophyrins in polymer-bound metalloporphyrins (Rollmann, 1975). 

Electron spin resonance spectroscopy offers a unique technique to study the role of radical species as intermediates in both polymerization and polymer degradation processes. The technique has been developed significantly since its introduction to chemical applications in the 1950s [1], with major advances in the stability of the magnetic field, in the sensitivity to low radical concentrations— and hence the limit of detection and measurement—and in data collection and manipulation. ESR spectrometry enables both the identification of radicals and the measurement of their concentration. It is a non-destructive technique and spectra can be recorded both during polymerization, and, in suitable circumstances, during degradation of polymers [2]. 

In this chapter we have limited ourselves to the most common techniques in catalyst characterization. Of course, there are several other methods available, such as nuclear magnetic resonance (NMR), which is very useful in the study of zeolites, electron spin resonance (ESR) and Raman spectroscopy, which may be of interest for certain oxide catalysts. Also, all of the more generic tools from analytical chemistry, such as elemental analysis, UV-vis spectroscopy, atomic absorption, calorimetry, thermogravimetry, etc. are often used on a routine basis. 

The title Spectroscopy in Catalysis is attractively compact but not quite precise. The book also introduces microscopy, diffraction and temperature programmed reaction methods, as these are important tools in the characterization of catalysts. As to applications, I have limited myself to supported metals, oxides, sulfides and metal single crystals. Zeolites, as well as techniques such as nuclear magnetic resonance and electron spin resonance have been left out, mainly because the author has little personal experience with these subjects. Catalysis in the year 2000 would not be what it is without surface science. Hence, techniques that are applicable to study the surfaces of single crystals or metal foils used to model catalytic surfaces, have been included. 

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. 

The application of analytical methods to speciation measurements in complicated systems has remained rather limited, despite the considerable technological progress during the past 25 years. The characterisation methods (e.g. spectroscopy, nuclear magnetic resonance) are often limited to the study of isolated compounds at relatively high concentrations. They, therefore, necessitate the prior employment of sophisticated separation and pre-concentration methods which introduce severe risks of perturbation. The trace analysis methods are often insensitive to the chemical form of the elements measured (e.g. atomic absorption, neutron activation). Those which possess sufficient element specificity (e.g. electron spin resonance, fluorescence, voltammetry) still require significant development before their full potential can be realised. 

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