ELECTRON SPIN RESONANCE SPECTROSCOPY

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E- ELECTRON

S-SPINA spin s = 1/2 is an intrinsic property of electrons, characterized by quantum number 1/2.

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R-RESONANCE

An excited state of a stable particle causing a sharp maximum in the probability of absorption of electromagnetic radiation.

S-SPECTOSCOPY

ESR comes under magnetic spectroscopy where study is done using EMR under influence of magnetic field. 3

WHAT IS ESR???

It is a branch of absorption spectroscopy in which   radiation having frequency in microwave region is absorbed by paramagnetic substance to induce transition between magnetic energy level of electron with unpaired spin.

It is also called as electron paramagnetic resonance spectroscopy.

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ESR PHENOMENON IS SHOWN BY …………..

Atoms having an odd number of electrons.

Ions having partly filled inner electron shells.

Molecule that carry angular momentum of

electronic  origin.

Free radicals having unpaired electrons. 5

PRINCIPLE OF ESR

ESR spectra is given only by molecules having unpaired electrons.

The unpaired electrons are excited to a high energy state under the magnetic field by the absorption of microwave .

In absence of magnetic field, only one spin state of electrons.

Then spin angular momentum quantum number will have 2 values of m s +1/2 and -1/2. 6

When magnetic field is applied, leads to 2 energy states.

Lower energy state will have spin magnetic moment aligned with field corresponding to quantum no, m s = -1/2.

High energy state will have spin magnetic moment opposed with field corresponding to quantum no, m s = +1/2.

m s = +1/2

.

No field ΔE=hv=gβH m s = -1/2

,

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The equation describing the absorption of microwave energy between two spin states is

∆E = hv = gβH where: ∆E is the energy difference between the two spin

states .

h is Planck’s constant

v is the microwave frequency

g  is the proportionality factor which is a function of electron’s environment.

H is the applied magnetic field.

β is the electron Bohr magneton,8

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SCHMATIC DIAGRAM FOR INSTRUMENTATION OF

ESR

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INSTUMENTATION

Source

Magnetic bridge or circulator

Sample cavity

Magnet system

crystal detector

Auto amplifier

oscilloscope11

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SOURCE:

A)klystron:

Klystron oscillator operates in microwave band region of 3cm wave length.

It is a vacuum tube which can produce microwave oscillations.

It offers microwave of 9500 MHz.

B)Isolator: It minimises the vibrations in the frequency

of microwaves produced by klystron. It isolates the narrow range of microwaves

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C)wavemeter:

To measure the microwave radiation.

It is usually calibrated in frequency units(MHz) instead of wavelength.

D)Attenuator:

It adjusts the level of microwave power incident upon the sample.

It is similar to filter in spectrophotometer.14

MICROWAVE BRIDGE/CIRCULATOR:

It is made up of hollow rectangular copper or brass tubing having silver or gold plating inside to produce highly conducting flat surface.

It consists of 4arms.

The microwave radiations enter into arm1. Arm2 is connected to resonant cavity and

sample. Arm3 having terminating load which

absorbs any power reflected from detector. Arm4 is attached to the detector.

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3 terminating load

1 4 To detector

microwave power in 2 To resonant

cavity16

SAMPLE CAVITY:

In ESR spectrometer the resonant cavity contaning the sample.

Sample volume: 0.15 to 0.5 ml.

In case of high dielectric constant containing samples ,flat cells with thickness of 0.25mm and sample volume of 0.05 ml are used.

In most, dual sample cavities are used for the simultaneous observation of sample and reference.

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MAGNETIC SYSTEM:

It provides a homologous magnetic field can be varied from 0 to 500 gauss.

The field should be stable and uniform over the sample volume.

The stability can be achieved by energizing the magnet with highly regulated power supply.

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CRYSTAL DETECTORS:

Commonly used detector is silicon crystal which act as microwave rectifier.

It converts microwave power in to direct current output.

AUTO AMPLIFIER: By this signal undergoes narrow band

amplification.

OSCILLOSCOPE: Signal from detector and sweep unit is

recorded by oscilloscope.20

CHOICE OF SOLVENT

Water, alcohol and other high dielectric constant solvents are not used because they strongly absorb microwave power.

ESR spectra can be obtained for gases, solutions,powders,single crystals and frozen solutions.

The best frozen solutions results are when the solvent freezes to form a glass.

Eg: methyl cyclo hexane forms good glass where as cyclo hexane does not 21

DETERMINATION OF g-VALUE

In order to measure the g value, this is the best method to measure the field separation between the center of unknown spectrum and reference substance.

DPPH(2,2-diphenyl-1-picrylhydrazyl) is generally used as standard whose g-value is 2.0036.

The g-value for unknown is given by

g=g s(1-ΔH/H) Where H is resonance frequency ΔH is field separation between

standard and sample.22

HYPERFINE SPLITTING

This is occurred by the interactions between the spinning electrons and adjacent spinning nuclei.

When single electron interacts with one nucleus, the number of splittings will be equal to 2I +1.

If a single electron interacts magnetically with n equivalent nuclei, the electron signal is split up into 2nI+1 multiplet.

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IN CASE OF HYDROGEN………………

external field+ magnetic effect of proton external field +1/2 m s = +1/2

-1/2 no field -1/2 m s = -1/2 +1/2

splittings will be 2(1/2)+1=2 where I=1/2

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Spin level changes at nuclear level.

Radio frequency is applied.

Nuclear-nuclear Spin coupling occurs.

Nuclei with odd mass number only give NMR spectra.

Spin level changes at electronic level.

Microwave frequency is applied.

hyerrfine coupling occurs.

ESR spectra shown by paramagnetic substances and free radicals.

NMR ESR

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APPLICATIONS OF ESR

Study of free radicals

Investigation of molecules in the triple state

Study of inorganic compounds such as study of catalysts and determination of oxidation state of metal eg: copper.

The information of unpaired electrons is very useful in various aspects in applications of ESR. Like, Spin labels, Structural determination, and Reaction velocities. 27

Analytical applications:

Mn+2 ions can be measured and detected even when present in trace quantities.

ESR method is convenient method for determination of Vanadium in petroleum products.

In Biological systems:

ESR has studied the presence of free radicals in healthy and diseased tissues. 28

ESR in molecular biotechnology :

DNA: ESR is used to investigate the nucleotide-centered free radicals in DNA, either produced by irradiation, or indirectly by other free radicals.

Activity of enzymes:

ESR is used in the analysis of enzymatic activity of nitric oxide synthetase (NOS),

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