Electron paramagnetic resonance spectroscopy transition metal complexes

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]. 

Electron paramagnetic resonance (EPR) or electron spin resonance (ESR) spectroscopy is suitable for the study of systems containing unpaired electrons, i.e., free radicals, transition metal ions, lanthanide metal ions, transuranium ions and complexes. Systems containing transition metal ions or lanthanide metal ions are particularly important in the study of solution structure. 

Poppl A, Kevan L. 1996. A practical strategy for determination of proton hyperfine interaction parameters in paramagnetic transition metal ion complexes by two-dimensional HYSCORE electron spin resonance spectroscopy in disordered systems. J Phys Chem 100 3387-3394. 

A large number of paramagnetic transition metal nitrosyl complexes have been studied using electron spin resonance (ESR) spectroscopy. Information on the electronic ground state can be derived from the g-value and the hyperfine coupling constants, and many [MLslNO)]" (see Table IV) and nitrosyl porphyrin complexes (99) have been studied in this way with a view to understanding their electronic structures. 

The third principal application of the electron spin resonance technique is to the study of paramagnetic transition metal ions in biochemical systems. Most examples are complexes of copper, iron, manganese, chromium, cobalt and molybdenum. Other metals such as titanium, vanadium and nickel are sometimes employed as structural probes. Only four of these ions, Cu ", Mn, Gd " and VO ", are seen in ESR spectroscopy at room temperature under virtually all conditions. Therefore, they are of special importance. 

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