Electron Spin Resonance (ESR) commonly referred to by the name Electron Magnetic Resonance (EMR) or Electron Paramagnetic Resonance (EPR) is an absorption spectroscopy where radiation with frequencies within the microwave range (0.04 to 25 cm) is absorption by paramagnetic materials to cause changes in the magnetic levels of electrons having unpaired spins.
ESR is built on the idea that molecules, ions, atoms or molecular fragments with an odd amount of electrons have distinctive magnetic properties. The electron is spin-dependent and as a result of spin , there is a magnetic moment. In 1944 when it was discovered by E.K. Zavoisky, EPR spectroscopy has been utilized as a highly precise and useful technique to study different types of paramagnetic species found in liquid and solid states.
Principle of Electron Spin Resonance (ESR)
The concept that is known as electron spin resonance (ESR) is due to the fact that electrons are charged particle. It is able to spin around its axis, which creates the impression of the tiny bar magnet. When a molecule with an unpaired electron placed in a magnetic field, the electron’s spin can be aligned in two different ways, resulting in two different spin states. ms = +/-1/2 .
The alignment could be in either direction (parellel) of the magnetic field that represents the state of lower energy, ms = 1/2 opposite (antiparallel) in the direction to which is magnetic field, ms = + 1/2
The two alignments have distinct energies. This difference in energy can alter the degeneracy of state of electrons. The energy difference can be determined by:
∆ E = E+ – E- = hv = gmßB
Where,
h = Planck’s constant (6.626 x 10-34 J s-1)
v = the frequency of radiation
ß = Bohr magneton (9.274 x 10-24 J T-1) B = strength of the magnetic field in Tesla
The g = g-factor is a measurement in units less of the intrinsic magnetic force of the electron. the value for a free electron would be 2.0023.
An unpaired electron may be able to move between two levels of energy by emitting or absorbing the energy-displaystyle hnu photon at hv, so that the resonance requirement hv = E is observed. This results in the basic equation for EPR spectroscopy.
Working of Electron Spin Resonance (ESR)
- The equation can allow the use of a wide range of frequencies and magnetic field measurements The majority of EPR measurements are done using microwaves that are in between 9000 and 10000 Mhz (9-10 GHz) range (9-10 GHz).
- EPR spectra are generated generally by keeping the frequency of the photon constant while altering the magnetic field that hits the specimen.
- A group of paramagnetic centres that include free radicals, are exposed to microwaves with a set frequency.
- When you increase the magnetic field the distance between the energy states increases until it reaches the power of microwaves.
- The electrons that are not paired can shift between two states of spin. Because there are typically higher numbers of electrons within the lower state because of the Maxwell-Boltzmann spectrum There is an energy absorption net.
- This absorption is tracked and converted into an spectral.
Instrumentation of Electron Spin Resonance (ESR)
1. Klystrons
Klystron tube functions as the radiation source. It is stabilized against temperature fluctuations through the immersion of an oil bath, or the forced cooling of air. The frequency of monochromatic radiation is controlled by the voltage applied to the klystron. It is maintained at a certain frequency through an automated control circuit, and has an output power of approximately 300 milliwatts.
2. Wave Guide Or Wavemeter
The wave meter is placed between the oscillator and attenuator. To determine the frequency of the microwaves produced by the klystron oscillator. The wavemeter is generally calibrated using frequency units (megahertz) rather than the wavelength. The wave guide is an hollow rectangle brass tube. It’s used to transmit waves of radiation to the crystal as well as the sample.
3. Attenuators
The power that is transmitted through the wave guide could be decreased continuously by inserting a piece resistive material within the guide. This is known as a variable attenuator. It’s used to vary the strength of the sample. It can be used to vary the full strength of klystron, to one that is reduced by a force of 100 or more.
4. Isolators
It’s a device that reduces the fluctuations that occur in microwave frequencies created by the oscillator klystron. Isolators can be used to block radiation of microwave energy back to the source of radiation. It is made of ferrite that allows microwave waves to travel only in one direction. It also regulates frequencies of Klystron.
5. Sample Cavities
The central component of an ESR instrument is its resonant cavity that holds the sample. The cylindrical cavity TE120 and the rectangular TE011 cavities are widely utilized. In the majority of ESR spectrometers, dual-sample cavities are typically used. this is to enable simultaneous examination of a sample as well as an underlying material. Because magnetic fields interact with the sample, causing spin resonance, the sample will be located where the strength of magnetic field is the highest.
6. Couplers And Matching Screws
The different components of the microwave assembly are able to be connected by the use of slots or irises with various dimensions.
7. Crystal Detectors
Silicon crystal detectors, which convert the radiation to D.C is widely employed as detectors of microwave radiation.
8. Magnet System
The resonant cavity is located between the poles in an electric magnet. The field must be uniform and stable across the entire sample. The field’s stability is made possible by activating the magnet using a controlled power supply. It is the ESR spectrum can be recorded slowly altering the magnetic field via the resonance condense through sweeping the current that is supplied via the power source to the magnetic field.
9. Modulation Coil
A modulation to an appropriate frequency for a good signal noise ratios of the crystal detection device is done through a tiny fluctuation in that magnetic field. The effect is achieved through the supply of the A.C. signal to modulation coils that are oriented to an object in the direction of magnetic fields. If the modulation has a lower frequency (400 cycle/sec and less) The coils may be placed outside of the cavity, and on the magnet poles. For higher frequencies of modulation the modulation coils should be placed within the resonant cavity, or cavities made from non-metallic materials e.g. Quartz coated with a tin silvered plated.
10. Display Devices
To monitor the signal to which a system is connected, various devices can be utilized.
Applications of Electron Spin Resonance (ESR)
- ESR spectrometry is among the primary methods for studying the properties of transition metals that contain the metalloproteins.
- To determine the catalytic rate
- To find out about the current geometries of the site
- Study the process of protein folding and denaturation.
- In studies that study the interaction between ligands and enzymes
- In Biological Systems
- Research into Free Radicals
- Spin Labels
- The study of Inorganic Compounds
- Reaction Velocity & Mechanisms
- Research into naturally occurring substances , such as minerals that contain Transition elements and minerals that have imperfections (e.g quartz), Hemoglobin (Fe) Coal, Petroleum, Rubber , etc.
- Conducting Electrons
