Electron spin Resonance Spectroscopy

Electron e-

J.J. Thomson's Experiments leading to the "Discovery of the Electron" in 1897:

The mass of an electron is 9.10938 x 10-31 kg.

Each electron carries one unit of negative charge (1.602 x 1019 coulomb)

The electron is a subatomic particle e− or β− a negative elementary charge

Spin

The electron spin is the electron’s electromagnetic field angular momentum

spin is like a vector quantity

All Electron carry a charge. In some electron this charge spins on

the electron axis and this circulation of e charge generates a

magnetic dipole along the axies.

these particle also have the properties to spin on their

own axis and each of them possesses angular momentum1/2(h/2π)

in accordance with the quantum theory. The net resultant of the

angular momentum of all nuclear particles is called electron spin.

For a electron having a spin quantum number I, these are(2I +1)

spin states.

SPIN QUANTUM OF VARIOUS NUCLEI

Number of

protons

Number of

neutrons

Spin

quantum

number. I

Example

Even Even 0 12C, 16O, 32S

Odd/

Even

Even/

Odd 1/2,3/2,5/2

1H, 19F, 31P, 11B, 79Br & 13C, 127I,

odd odd 1 2H, 14N

6

17

35Cl,

17

16O

ESR Spectroscopy

Electron Spin Resonance Spectroscopy

@lso called as

EPR Spectroscopy

Electron Paramagnetic Resonance Spectroscopy

PRINCIPLE

▪ ESR spectroscopy is based on the absorption of microwave radiation by an unpaired electron when it is exposed to a strong magnetic field.

▪ The electronic energy levels of the atom or molecule will split into different levels.

▪ The magnitude of the splitting is dependent on the strength of the applied magnetic field.

▪ The atom or molecule can be excited from one split level to another in the presence of an external radiation of frequency corresponding to the frequency obtained from the difference in energy between the split levels. Such an excitation is called a magnetic resonance absorption.

▪ The magnetic resonance frequency will hence be influenced by the local environment of the atom or molecule.

• Klystron tube acts as the source of radiation.

• The frequency of the monochromatic radiation is determined by the voltage applied to klystron.

• It is kept a fixed frequency by an automatic control circuit and provides a power output of about 300 milli watts.

The wave meter is put in between the oscillator and attenuator to know the frequency of microwaves produced by klystron oscillator.

The wave meter is usually calibrated in frequency unit (megahertz) instead of wavelength. Wave guide is a hollow, rectangular brass tube. It is used to convey the wave radiation to the sample and crystal.

The power propagated down the wave guide may be continuously decreased by inserting a piece of resistive material into the wave guide.

The piece is called variable attenuator and used in varying the power of the sample from the full power of klystron to one attenuated by a force 100 or more.

It is a non-reciprocal device which minimizes vibrations in the frequency of microwaves produced by klystron oscillator.

Isolators are used to prevent the reflection of microwave power back into the radiation source.

It is a strip of ferrite material which allows micro waves in one direction only.

It also is being stabilizing the frequency of the klystron.

The heart of the ESR spectrometer is the resonant sample cavity.

In most of the ESR spectrometers, dual sample cavities are generallyused.This is done for simultaneous observation of a sample and areference material.

Since magnetic field interacts with the sample to cause spinresonance the sample is placed where the intensity of magneticfield is greatest.

A measure of quality of the cavity is ‘Q factor’ which is defined as Thesensitivity of the spectrometer is directly proportional to this value ofQ.

The various components of the micro waveassembly to be coupled together by making use ofirises or slots of various sizes.

A Silicon crystal detectors, which converts the radiation in D.C., has widely been used as a detector of microwave radiation.

Microwave Bridge such as magic T and hybrid ring variety are most common.

The resonant cavity is placed between the poles pieces of an electromagnet.

An electro magnet capable of producing magnetic field of at least 5000 gauss is required for ESR.

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

The stability of field is achieved by energizing the magnet with a highly regulated power supply.

The ESR spectrum is recorded by slowly varying the magnetic field through the resonance condense by sweeping the current supplied to the magnet by the power supply.

This sweep is usually accomplished by with a variable speed motor drive.

Both the magnet as well as the power supply may require water cooling.

The modulation of the signal at a frequencyconsistent with good signal noise ratio in the crystaldetector is accomplished by a small alternatingvariation of the magnetic field.

The variation is produced by supplying an A.C.signal to modulation coil oriented with respect thesample in the same direction as the magnetic field.

If the modulation is of low frequency (400cycles/sec or less), the coils can be mountedoutside the cavity and even on the magnet polepieces.

For higher modulation frequencies, modulationcoils must be mounted inside the resonant cavityor cavities constructed of a non-metallicmaterial

e.g., Quartz with a tin silvered plating, becausemetallic penetration is not very effective in caseof higher modulation frequencies.

In order to adjust the spectrometer and to observethe signal, a cathode ray oscilloscope has beenemployed. A strip chart or X-Y recorder is used for

recording the signal.

EPR spectra are usually displayed in derivativeform to improve the signal-to-noise ratio.

SCHEMATIC DIAGRAM OF AN ESR SPECTROMETER

▪ The sample is placed in a resonant cavity which admits microwavesthrough an iris.

▪ The cavity is located in the middle of an electromagnet and helps toamplify the weak signals from the sample.

▪ Numerous types of solid-state diodes are sensitive to microwave energy

▪ Absorption lines are detected when the separation of the energy levelsis equal to the energy of the incident microwave.

What causes the energy levels?

Resulting energy levels of an electron in a magnetic field

Describing the energy levels

▪ Based upon the spin of an electron and its associated magnetic moment

▪ For a molecule with one unpaired electron– In the presence of a magnetic field, the two electron spin energy levels are

E = gmBB0MS

g = proportionality factor mB = Bohr magnetonMS = electron spin B0 = Magnetic field

quantum number(+½ or -½)

Hyperfine Interactions

▪ EPR signal is ‘split’ by neighboring nuclei–Called hyperfine interactions

▪ Can be used to provide information–Number and identity of nuclei–Distance from unpaired electron

▪ Interactions with neighboring nuclei

E = gmBB0MS + aMsmIa = hyperfine coupling constant

mI = nuclear spin quantum number

Hyperfine Interactions

Interaction with a single nucleus of spin ½

Hyperfine Interactions

▪ Coupling patterns same as in NMR

▪ More common to see coupling to nuclei with spins greater than ½

▪ The number of lines:

2NI + 1N = number of equivalent nuclei

I = spin

▪ Only determines the number of lines--not the intensities

Hyperfine Interactions

▪ Relative intensities determined by the number of interacting nuclei

▪ If only one nucleus interacting– All lines have equal intensity

▪ If multiple nuclei interacting– Distributions derived based upon spin

– For spin ½ (most common), intensities follow binomial distribution

Hyperfine Interactions

▪ Example:

– VO(acac)2– Interaction with vanadium nucleus

– For vanadium, I = 7/2– So,

2NI + 1 = 2(1)(7/2) + 1 = 8

– You would expect to see 8 lines of equal intensity

vanadyl acetylacetonate

Hyperfine Interactions

EPR spectrum of vanadyl acetylacetonate

Hyperfine Interactions

Pyrazine anion Electron delocalized over ring

Exhibits coupling to two equivalent N (I = 1)

2NI + 1 = 2(2)(1) + 1 = 5

Then couples to four equivalent

H (I = ½)

2NI + 1 = 2(4)(1/2) + 1 = 5

So spectrum should be a quintet with intensities 1:2:3:2:1 and each of those lines should be split into quintets with intensities 1:4:6:4:1

Hyperfine Interactions

EPR spectrum of pyrazine radical anion

• Electron Spin Resonance is a powerful

non-destructive & non-intrusive analytical method.

• ESR yields meaningful structural information even

from ongoing chemical or physical processes,

without influencing the process itself.

• It is the ideal technique to complement other

analytical methods in a wide range of application

areas.

Free Radicals

Odd-electron Molecules

Transition Metal Complexes

Molecular Motion

Rare Earth Ions

Crystal / Ligand Fields

Electron Transport

Reaction Kinetics etc.. Can be detected by ESR.

1. Clearly mention state the model of ESR Spectormeter for your sample analysis.

2. Analysis charges would vary depending on model selection. Relevent charges are given under each model

3. Clearly state the type of analysis required. Powder/Solution and also RT/77K For solution sample suggest name of solvent.

4. Experiments can be carried out at ambient temperature, liquid nitrogen temperature and most of the temperatures between liquid nitrogen temperature and 200o C.

5. Experiments can be carried out with samples in liquid, solid or gaseous form, which includes single crystals, metal alloys, liquid crystals, organic radicals, polymers, glasses, powders, triplets, polyradicals, conduction electrons, drugs, rare-earths, enzymes, proteins and gases.

6. Provide minimum 25-50 mg of solid sample in powder form.

References

http://web.mit.edu/speclab/www/PDF/DCIF-EPR-training-n03.pdf

http://www.rsic.iitb.ac.in/esr.html

http://www.pharmatutor.org/articles/instrumentation-electron-spin-resonance-spectroscopy

THANK YOUBy

ARJUN KUMAR B

M.Sc. Nano science & technology

I year