ELECTRON SPIN RESONANCE (ESR) Principle, Theory, Application and MCQ for GPAT, GATE, NET JRF Exam

Electron spin resonance (ESR) spectroscopy is also known as Electron paramagnetic resonance (EPR)
It is absorption spectroscopy similar to NMR. possible only with molecules having unpaired electrons Instead of Radiowaves in NMR, Microwaves is used in ESR

PRINCIPLE  :-

The phenomenon of electron spin(ESR) is based on the fact that an electron is a charged particle it spins around its axis and this causes it to act like a tiny bar magnet

1. When a molecule or compound with an unpaired electron is placed in a strong magnetic field
   •The spin of the unpaired electron can align in two different ways creating two spin states ms = ± ½
2. The alignment can either be along the direction (parellel) to the magnetic filed which corresponds to the lower energy state m s =- ½• Opposite (antiparallel) to the direction of the applied magnetic field
   mrs=+1/2
3. The two allignments have different energies and this difference in energy lifts the
   degeneracy of the electron spin states
   • The energy difference is given by:

E=E+-E-=hv=gmBB
• h = planck’s constant (6.626 x 10^-34)
• v = the frequency of radiation
• B = Bohr magneton (9.274 x 10^-24)
• B = strength ot the magnetic field in Tesla
• g = the g-factor (more explanation on g
The g-factor is a unit less measurement of the intrinsic magnetic moment of the electron, and its value for a free electron is 2.0023.

Hyperfine splitting:-

•The ESR signal is due to transition of electrons from the spin state ms =-l/2 to the spin state ms = +1/2 . — spin flip
• This spin state may interact with magnetic moment of nuclei, with which the unpaired electron may be partially or wholly associated

‘This interaction may lead to splitting of resonance signals in to several lines. This is Hyperfine splitting

INSTRUMENTATION :-

1.SOURCE :-

A source of microwave radiation with constant frequency and variable amplitude is used. It contains

• Klyston :- Which offers microwave of 9500 MHz (If 35000 MHz is used it gives 20 time more resolution.)
• Isolator :- To isolate a narrow range of microwave.
• Wavemeter :- To measure the radiation.
• Attenuator :- Similar to a filter in a spectrophotometer.

2.Microwave bridge (circular -T or Magic-T)

3.Magnetic field :- A magnetic field sweep from 0 to 500 gauss or 0 to 3400 gauss is used.

4.Sample cavity :- Sample in the form of gas , liquid, or solid can be used. solvent of low dielectric constant are preferred. A solution conc. of 10^-6M-10^-9 is used in sample cells which are flat and can be hold 0.05ml to 0.5 ml.

5.Detector :- Usually a silicon tungsten rectifier detector is used.

6. Amplifier & Recored :- To record the derivative spectra it is used.

REFERENCE STANDARD (INTERNAL REFERENCE)

Any one of the following is used as reference standard in ESR.

1. DPPH :- 1,1-Di phenyl-2-picryl Hydrazyl  has g value is 2.0036.
2. Cr in a tiny chip of ruby crystal with a g value of 1.4

APPLICATION

1. Study of free radical including reaction velocity and mechanism.
2. Structure elucidation of organic and inorganic molecules.
3. Study of biological system using spin labelling technique.
4. Quantitative analysis :- It is done in instrument having dual sample cavity.

MCQ 

1. Which of the following is used as detector crystal in ESR spectrometer?
a) Silicon rectifier
b) Silicon tungsten rectifier
c) Silicon boron rectifier
d) Silicon quartz rectifier

2. Which of the following is the oscilloscope field width of the ESR spectrometer?
a) 0.2 to 40 G
b) 0.1 to 20 G
c) 0 to 10 G
d) 0.01 to 20 G

3.Klyston is produce frequency

a.95000 MHz

b.900 MHz

c.9500 MHz

d.500 MHz

4. How many ml of sample is hold by sample holder in ESR ?

a. 0.05ml to 10 ml

b. 0.05ml to 5 ml

c. 0.05ml to 0.5 ml

d. 50ml to 0.5 ml

5.Which is not application of ESR ?

a.Structure elucidation of organic and inorganic molecules

b.Study of biological system using spin labelling technique

c.Study of free radical including reaction velocity and mechanism.

d. assay of drug

6.Which is reference standared in ESR ?

a.KBr

b.NaCl

c.DPPH

d.Cu

ANSWER KEY 

1.b

2.a

3.c

4.c

5.d

6.c

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REFERENCE :-

TEXT BOOK OF PHARMACEUTICAL ANALYSIS THIRD EDITION BY DR.S.RAVI SANKAR (PG.NO. 7.1-7.5)