Electron paramagnetic resonance organic radicals

The reaction of bis(benzene)vanadium [12129-72-5] with TCNE affords an insoluble amorphous black soHd that exhibits field-dependent magnetization and hysteresis at room temperature, an organic-based magnet (12). The anion radical is quite stable in the soHd state. It is paramagnetic, and its intense electron paramagnetic resonance (epr) spectmm has nine principal lines with the intensity ratios expected for four equivalent N nuclei (13) and may be used as an internal reference in epr work (see Magnetic spin resonance). 

In 1960 Rex (22) first reported the use of electron paramagnetic resonance spectrometry as a method for demonstrating the presence of stable organic free radicals in humic acid. We felt that this technique might provide useful information about the structure of humic acid which was not readily available by other physical methods, particularly if relations between EPR spectra and chemically modified humic acids could be demonstrated. Our preliminary studies (26) confirmed this presumption. 

Tn 1963 we began to investigate lignin preparations by electron paramagnetic resonance (EPR) spectrometry. This technique can detect paramagnetic species, particularly organic free radicals. The types of information available from EPR measurements can be summarized as follows ... 

As noted at the beginning of this chapter, radicals are a unique class of organic intermediates because they can be studied using magnetic resonance techniques such as electron paramagnetic resonance spectroscopy (EPR). The EPR technique has proven valuable for ascertaining the local structure about a radical in a supercritical fluid solvent. 

A purely organic chiral nitroxide which shows liquid crystalline behaviour as well as intriguing magnetic properties and a dependence on the enantiomeric nature has been reported [180]. The reason for studying the compounds was to increase the sensitivity of mesophases to magnetic and electric fields. The racemic modification of the radical, which displays a nematic phase, proved to be more sensitive to alignment than the cholesteric phase with the enantiomers present. It was proposed that the compounds may also be used to study the dynamic nature of mesophases by electron paramagnetic resonance spectroscopy. 

Electron paramagnetic resonance spectroscopy has proved a valuable tool in the study of AdoCbl-dependent enzymes. AdoCbl itself is EPR-silent, but upon homolysis to form Cbl(II), two spins are formed, one on the cobalt (which now has low-spin d configuration) and one on the organic radical. Typically, the two unpaired electrons remain close enough in the enzymeis active site that they interact with one another to give complex, but informative, EPR spectra. 

Binet et al (2002) have undertaken an initial electron paramagnetic resonance study to examine the distribution of free radicals in Murchison and Orgueil macromolecular material. They suggest that there are radical-rich regions, which could represent regions of pristine interstellar organic matter preserved within the macromolecular material. 

The electron-donor centers on metal oxides can be measured by adsorbing certain organic molecules on the surface of the oxide. The transfer of an electron from the donor site of the oxide to the adsorbed molecule creates a paramagnetic ion detectable using electron paramagnetic resonance (EPR) spectroscopy. Che et al. (1972) studied the adsorption of tetracya-noethylene (TCNE) on MgO that had been pretreated between 100 and 800°C. Using EPR methods they identified the presence of adsorbed TCNE radical anion. As the pretreatment temperature increased from 100 to 800°C, the concentration of the radical anion passed through two maxima, one at 200°C and the other at 700°C. The electron-donor centers were found to be associated with OH and O ions with a low coordination number. 

Electron paramagnetic resonance (EPR) spectroscopy [1-3] is the most selective, best resolved, and a highly sensitive spectroscopy for the characterization of species that contain unpaired electrons. After the first experiments by Zavoisky in 1944 [4] mainly continuous-wave (CW) techniques in the X-band frequency range (9-10 GHz) were developed and applied to organic free radicals, transition metal complexes, and rare earth ions. Many of these applications were related to reaction mechanisms and catalysis, as species with unpaired electrons are inherently unstable and thus reactive. This period culminated in the 1970s, when CW EPR had become a routine technique in these fields. The best resolution for the hyperfine couplings between the unaired electron and nuclei in the vicinity was obtained with CW electron nuclear double resonance (ENDOR) techniques [5]. 

---