Electron Spin Resonance Spectroscopy:

Electron Paramagnetic Resonance also known as Electron Spin Resonance is a spectroscopic technique for studying the materials with unpaired electrons. It is a versatile and non-destructive technique, and the basic concepts are analogous to those of NMR, but it is the electron spins that are excited instead of the spins of atomic nuclei.

This technique is quite useful for a variety of applications such as oxidation and reduction processes, biradicals and triplet state molecules, reaction kinetics, as well as numerous additional applications in biology, medicine and physics.

When a molecule/compound with unpaired electrons is placed in a strong magnetic field, the spin of the unpaired electron can align in two different ways and thus creating two spin states: m_s= ± ½; (a) The alignment is along the direction to the magnetic filed (parallel, m_s= - ½) corresponding to lower energy state or opposite (antiparallel, m_s=+½) to the direction of the applied magnetic field. These two alignments have different energies and the difference in energy lifts the degeneracy of the electron spin states.

The energy difference is given by:

where B is the strength of the magnetic field in Tesla, h is the Planck's constant, v is the frequency of radiation, μ_B is the Bohr magneton, g is a g-factor which is 2.0023 for free electrons.