Basics of ESR
Electron spin resonance (ESR) spectroscopy is closely related to the probably better known nuclear magnetic resonance (NMR). It uses magnetic fields corresponding to a microwave frequency range between 1 GHz and 500 GHz to study paramagnetic materials and molecules. That is, molecules with unpaired electrons, like radicals, transition metal complexes, etc.
ESR can be used to obtain structural information of molecules together with details about their electron density distributions. In solutions and solids, the dynamics of molecules can be determined and the kinetics of chemical reactions can be studied. Quantitatively, analytical applications such as dosimetry can be mentioned, as well as characterization of the redox-active centers in proteins. All of these properties can be observed at room temperature, but often lower temperatures are used.
ESR was discoveed in 1946 by the Russian scientist Zavoisky, and itís use has increased steadily since then. The main categories of ESR are continuous-wave (cw) and pulse ESR. In cw ESR, molecules are irradiated continuously by low-power microwaves, whereas in pulse ESR, microwave is applied in very short and high-powered pulses. Additionally, radio frequencies (in a range of up to 150 MHz) can be applied, in which case one speaks of ENDOR (electron nuclear double resonance) spectroscopy. Both cw and pulse ENDOR experiments are possible.
ESR is sometimes also refered to as EPR (electron
paramagnetic resonance) or EMR (electron magnetic resonance).
In our group we mainly study the properties of organic radicals in solution through cw ESR. These studies mainly concern simple reactions like self-exchange and dimerization where we seek to explain reaction kinetics and through that obtain useful physical constants associated with the given compounds. Additionally we use methods like ENDOR and CIDEP (Chemically Induced Dynamic Electron Polarisation) on radicals in solution, as well as solid state ESR on powder samples.