Definition of electron spin resonance (esr)

Also known as electron paramagnetic resonance (EPR), this method is
used to determine whether radicals are formed in solution.  Although
it is very useful at this purpose, the sensitivity toward determination
of structure is poor.  It is the study of molecules containing
unpaired electrons by observing the magnetic fields at which they
come into resonance with monochromatic radiation.  The radiation
is in the microwave region, and the magnetic fields are about .3 Tesla. 
It can be used to study radicals generated either through chemical
reactions or radiation, as well as d-metal complexes and molecules
in triplet states.