Electron paramagnetic resonance (EPR)
Electron paramagnetic resonance (EPR) spectroscopy is the study of materials or molecules with unpaired electrons. The technique makes use of the intrinsic angular momentum of an electron – its ‘spin’ – by placing the paramagnetic material into a magnetic field and applying microwave radiation.
For a given molecule, such as the fictive molecule in the figure below, an EPR field sweep (traditionally performed by continuous wave EPR) provides us with information on the type of chemical centre the unpaired electron resides on. However, spectral and time resolution is limited and exploiting the full potential of EPR spectroscopy requires pulsed methods. These allow the separation of different types of interactions.
Fig. 1: Illustration of the use of EPR for structural analysis. The position of the unpaired electron is indicated by the full black circle in the fictive molecule. The coloured rings and corresponding schematic spectra indicate which parts of the molecule can approximately be studied with a particular EPR technique. Figure adapted from C. Calle et al., Chimia,2001, 55, 763.
Hyperfine techniques such as ENDOR (electron nuclear double resonance), ESEEM (electron spin-echo envelop modulation) and HYSCORE (hyperfine sublevel correlation) spectroscopy yield information on interactions with more distant nuclei and spectral resolution can be improved by orders of magnitude. We can thus learn about distances between electron and nuclear spins, the spin density distribution (the type of bonding), and electric field gradients.
Double electron-electron resonance (DEER), or synonymously pulse electron-electron double resonance (PELDOR) spectroscopy, has been very widely applied to many different types of systems, especially in biology, to determine the distance between two unpaired electrons.
Whilst the majority of EPR experiments are carried out at the X-band frequency, going to higher field and frequencies (e.g. Q- and/or W-band) can be extremely useful in detangling complicated spectra, by separating field dependent from field-independent components and thereby increasing spectral resolution.