ELECTRON SPIN RESONANCE

it is basically about one of biochemical technique.

1. ELECTRON SPIN RESONANCE (ESR)

2. INTRODUCTION Electron spin resonance is also known as electron para magnetic resonance. It is a absorption spectroscopy similar to NMR, possible only with the molecules having unpaired electron. Instead of radio waves in NMR.Microwaves are used in ESR.

3. PRINCIPLE The phenomena of ESR is based on the fact that electron is a charged particle. It spin around its axis and this causes to act like a tiny magnet. When a molecule or compound with unpaired electron is placed in a strong electric field , the spin of the unpaired electron can align in two different ways creating to spin state Ms- +1/2 and -1/2. The alignment can either be along the direction to the magnetic field which corresponds to the lower energy state. Opposite i.e.. Antiparallel to the direction of applied magnetic field. The two different alignments have different energies and this difference in energy lifts the degeneracy of electron spin states. Whereas the energy difference is given by: Δ E= hv =g μ B B

4. Energy levels for an electron spin (M S  = ±1/2) in an applied magnetic field B.

5. INSTRUMENTATION

6. WHAT WE CAN LEARN FROM ESR? ESR measurements afford information about the existence of unpaired electrons, as well as quantities, type, nature, surrounding environment, and behavior. The ‘g’ value, which reflects the orbit level occupied by the electron. Line width, which is related to the transverse relaxation time. Saturation characteristics, which are related to the longitudinal relaxation time. Number of unpaired electrons. Hyperfine structure : hfs, which represents the interactions between electrons and nuclei. Fine structure: fs, which represents the interactions between electron and electron. Exchange interactions reflecting the exchanges between electrons.

7. HYPERFINE SPLITTING The ESR signal is due to transition of electrons from spin state +1/2 to -1/2. This spin state may interact with the magnetic moment of nuclei, with which the unpaired electron maybe partially or wholly associated. This interaction may lead to the splitting of the resonance signals in to several lines. This is hyperfine splitting.

8. APPLICATIONS Electron state, such as magnetic materials and semiconductors Electron state of semiconductor lattice defects and impurities (dopants) Structure of glass and amorphous materials Tracking of catalytic reactions, changes in charge state Photo-catalytic reactivity and photochemical reaction mechanisms Radicals of polymer polymerization processes (photo-polymerization, graft polymerization) Polymer resolution (photolysis, radiolysis, pyrolysis, chemical decomposition) Active oxygen radicals related to aging in disease in living organisms Oxidative degradation of lipids (food oils, petroleum, etc.) Detection of foodstuffs exposed to radiation Measurement of the age of fossils and geological features using lattice defects

9. COMPARISON BETWEEN ESR AND NMR EPR is fundamentally similar to the more widely familiar method of  NMR spectroscopy , with several important distinctions. While both spectroscopies deal with the interaction of electromagnetic radiation with magnetic moments of particles, there are many differences between the two spectroscopies : EPR focuses on the interactions between an external magnetic field and the unpaired electrons of whatever system it is localized to, as opposed to the nuclei of individual atoms . The electromagnetic radiation used in NMR typically is confined to the radio frequency range between 300 and 1000 MHz, whereas EPR is typically performed using microwaves in the 3 - 400 GHz range . In EPR, the frequency is typically held constant, while the magnetic field strength is varied. This is the reverse of how NMR experiments are typically performed, where the magnetic field is held constant while the radio frequency is varied . Due to the short relaxation times of electron spins in comparison to nuclei, EPR experiments must often be performed at very low temperatures, often below 10 K, and sometimes as low as 2 K. This typically requires the use of liquid helium as a coolant. EPR spectroscopy is inherently roughly 1,000 times more sensitive than NMR spectroscopy due to the higher frequency of electromagnetic radiation used in EPR in comparison to NMR. It should be noted that advanced pulsed EPR methods are used to directly investigate specific couplings between paramagnetic spin systems and specific magnetic nuclei. The most widely application is Electron Nuclear Double Resonance ( ENDOR ). In this method of EPR spectroscopy, both microwave and radio frequencies are used to perturb the spins of electrons and nuclei simultaneously in order to determine very specific couplings that are not attainable through traditional continuous wave methods.