Electron Spin Resonance (ESR)- Principle, Instrumentation, Applications


Electron spin resonance (ESR) is a spectroscopic technique that can detect and measure the magnetic properties of molecules or compounds that have unpaired electrons. Unpaired electrons are often found in free radicals, transition metal complexes, organic radicals, and some defects in solids. ESR can provide information about the structure, dynamics, and interactions of these paramagnetic species.

The principle of ESR is based on the fact that an electron is a charged particle that spins around its axis and behaves like a tiny bar magnet. When a paramagnetic sample is placed in a strong external magnetic field, the unpaired electrons can align their spins either parallel or antiparallel to the direction of the field. These two orientations have different energies, and the difference between them depends on the strength of the magnetic field and the intrinsic magnetic moment of the electron. The intrinsic magnetic moment of the electron is characterized by a dimensionless constant called the g-factor, which is slightly different from 2 for a free electron.

An unpaired electron can change its spin orientation from parallel to antiparallel, or vice versa, by absorbing or emitting a photon of electromagnetic radiation that has the same energy as the difference between the two spin states. This process is called resonance, and it occurs when the frequency of the radiation matches the condition:

$$\Delta E = h\nu = g\beta B$$

where $\Delta E$ is the energy difference between the two spin states, $h$ is Planck`s constant, $\nu$ is the frequency of radiation, $\beta$ is the Bohr magneton (a unit of magnetic moment), and $B$ is the strength of the magnetic field. This equation is known as the fundamental equation of ESR spectroscopy.

ESR spectroscopy uses microwaves as the source of electromagnetic radiation, typically in the range of 9-10 GHz. The magnetic field is varied to scan through different values of resonance. When resonance occurs, there is a net absorption of microwave energy by the sample, which can be detected by a device called a crystal detector. The absorption signal is plotted as a function of magnetic field to obtain an ESR spectrum.

An ESR spectrum consists of one or more peaks or lines that correspond to different types of unpaired electrons in the sample. The position, intensity, shape, and splitting of these lines can reveal various aspects of the paramagnetic species, such as their number, identity, environment, mobility, and interactions. Some of the parameters that can be derived from an ESR spectrum are:

  • The g-factor: This parameter indicates how much the magnetic moment of an electron deviates from that of a free electron. It can provide information about the electronic structure and symmetry of the paramagnetic species.
  • The hyperfine coupling constant: This parameter reflects how much the unpaired electron interacts with nearby nuclei that have nonzero nuclear spins. It can provide information about the distance and orientation of these nuclei relative to the electron.
  • The line width: This parameter measures how broad or narrow an ESR line is. It can provide information about the relaxation processes and dynamics of the paramagnetic species.
  • The line shape: This parameter describes how symmetric or asymmetric an ESR line is. It can provide information about the distribution and heterogeneity of the paramagnetic species in the sample.

ESR spectroscopy has many applications in various fields of science and technology, such as chemistry, biology, physics, medicine, materials science, and archaeology. Some examples of applications are:

  • Studying free radicals and their reactions in chemical and biological systems
  • Characterizing transition metal complexes and metalloproteins
  • Investigating defects and impurities in solids and semiconductors
  • Measuring oxidative stress and antioxidant activity in living organisms
  • Dating ancient artifacts and fossils