EPR (electron paramagnetic resonance), often also referred to as ESR (electron spin resonance), is a spectroscopic method that allows one to obtain information on the structure and dynamics of systems with unpaired electrons (paramagnetic systems). In general the concept of EPR is similar to that of NMR (nuclear magnetic resonance) just that the magnetic moments observed at are electron spins rather than nuclear spins.
Example of systems amenable to EPR investigations
Intrinsic paramagnetic centers
- Stable Radicals in chemistry and biology
- Short lived species, intermediates in chemical reactions (min life time 100 ns)
- Radical pairs and triplet states in photosynthesis and related biomimetic systems
- Radicals generated by irradiation damage (DNA, polymers)
- Paramagnetic transition metals ions Such as Cu(II), Mn(II), V(IV), Fe(III), Cr(III), Cr(V), Co(II), Rh(II), Ni(I), Mo(V),Ti(I), Ti(III) in catalysis and metalloproteins (ET) and metalloenzymes
- Defects in semiconductors
- Endohedral fullerens
Diamagnetic systems with artificially introduced spin
Because EPR spectroscopy is an excellent method for probing structure and dynamics in liquids and solids it is also being applied to diamagnetic systems through the introduction of spin labels, in analogy to the use of fluorescence probes.
Mn(II) as a probe for Mg(II) in biological systems.
Nitroxide spin-probes for:
- Protein structure and dynamics obtained through site directed spin labeling
- Ordering and dynamics in membranes
- Polymers – Structure, dynamics and degradation of polymers
- Ordering and dynamics in liquid crystals
- pH sensors
- Self-assembly processes
- Adsorption on surfaces
Reactive radical species like singlet oxygen and OH radicals are probed by spin traps that upon the reaction with the radicals turn into stable radicals
A very simple explanation of the EPR experiment
In the EPR experiment the sample is placed in a magnetic field, which removes the degeneracy of the various spin states of the paramagnetic center. Transitions between the different spin states can then be induced by irradiation at the appropriate microwave frequency. The registration of the absorption of the microwave by the sample produces the EPR spectrum. This spectrum is highly sensitive to the physical and chemical environment of the unpaired electrons and therefore it is very useful for the characterization of paramagnetic centers. There are different ways for carrying out EPR measurements, such as continuous irradiation of the microwaves, at a fixed frequency, while changing the magnetic field or by application of series of microwave pulses at a fixed magnetic field. Like NMR, EPR is a very rich spectroscopy, including many different experimental techniques, relying on well established theoretical foundations based on both quantum and statistical mechanics. Nonetheless, the challenge for devising new experimental techniques is still there to improve resolution and sensitivity.