An Introduction to EPR Theory
Electron Paramagnetic Resonance (EPR) spectroscopy is a magnetic resonance technique used to examine the structure, dynamics and spatial distribution of systems containing unpaired electrons. Such systems include free radicals, paramagnetic compounds, defects, etc., The technique provides an unparalleled glimpse into the electronic structure of the paramagnetic compound, since the magnetic parameters are related to the electronic wavefunction and configuration of surrounding nuclei with non-zero spins. EPR is sometimes referred to as Electron Spin Resonance (ESR) spectroscopy. Although the conventional spectrometers operating in continuous wave (CW) mode at X-band (9 GHZ) frequencies are most widely used, higher/lower frequency spectrometers and pulsed spectrometers are now vital in the EPR toolbox to unravel the inner secrets of any paramagnetic system. The following sections give a very simple overview of CW-EPR and the associated CW-ENDOR technique, with some suitable examples.
The electron spin of a paramagnetic centre can interact with ligand nuclear spins via dipolar and Fermi contact interactions, producing shifts in the NMR lines of the ligand nuclei. The dipolar interaction depends on the relative position of the nuclear spins with respect to the metal atom, so the NMR spectrum can yield information on nuclear co-ordinates. However, this is not always easy for a paramagnetic complex, because the presence of the unpaired electron will broaden the NMR lines considerably. In that case, EPR or ENDOR spectroscopy is required to study the system.
Although EPR is extremely important in characterising paramagnetic systems, it’s one major drawback however, is the low resolution which results from line broadening effects caused in some cases by unresolved electron spin – nuclear spin interactions. These electron-nuclear couplings, an important parameter providing information on the ligand co-ordinates, can be extracted by performing a double resonance experiment (i.e., by detecting the NMR resonances via intensity changes of a simultaneously irradiated EPR line). In this Electron Nuclear DOuble Resonance (ENDOR) experiment, the NMR quanta are detected in the microwave, rather than the RF range (known as quantum transformation) resulting in a sensitivity enhancement of several orders of magnitude over conventional NMR spectroscopy. Therefore, ENDOR can be regarded as NMR spectroscopy on an EPR spectrometer.