1. Electron spin resonance spectroscopy-ESR
Introduction: 1. 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.
2. Magnetic energy splitting is done by applying a static magnetic field. ESR Phenomenon Is Shown By
* Atoms having an odd number of electrons. * Ions of partly filled inner electron shells. * Molecule of angular momentum (electronic). * Free radicals having
unpaired electrons. * Molecules of paired electrons & 0magnetic field
Principle
Excitation Process in ESR
Instrumentation
1. The unpaired electrons are
excited to a high energy state under
the magnetic field by the absorption
of microwave radiation (frequency).
2. The excited electron changes its
direction of spin and relaxes into the
ground state by emitting phonons.
Differences between ESR and NMR spectroscopy
S.No
1
2
3
4
5
6
7
8
Parameter
Spinning possible for
Observed region
Frequency range
Constant parameter
Varied parameter
Sensitivity
Relaxation time
Parameter measured
NMR spectroscopy
Nuclei
Radiofrequency
300 - 1000MHz
Magnetic field strength
Radio frequency
1000 times less than ESR
More for Nuclei
Coupling constant J
ESR spectroscopy
Electron
Microwave
3 - 400 GHz
Microwave frequency
Magnetic field strength
1000 times more than NMR
Less for Electrons
Splitting constant g
Reference standard:
Most commonly used reference standard materials for ESR are
1. DPPH: 1, 1-Di Phenyl Hydrazyl free radical which has g value 2.0036
2. Cr+3 in a tiny chip of crystal with a g value 1.4.
Determination of ‘g’ value: spectroscopic Splitting Factor / Lande’s Splitting Factor
g value gives an idea about electronic environment can be calculated by
g = gstd [1- ∆H/H], where H= resonance frequency used
∆H= field separation between standard and sample
gstd = g value of internal standard
Applications
All application of ESR is based on the following aspects,
1. Study of free radicals, including reaction mechanism, and velocity.
2. Investigation of molecules in the triple state, and
3. Study of inorganic and organic compounds
4. Study of biological systems, using spin labeling techniques.
5. Quantitative analysis is done in instrument having dual sample cavity.
(1) KLYSTRONS
Klystron tube acts as the source of radiation. The frequency of the
monochromatic radiation is determined by the voltage applied to
klystron.
(2) WAVE GUIDE OR WAVEMETER
The wave meter is put in between the oscillator and attenuator to know
the frequency of microwaves produced by klystron oscillator. Wave
guide is a hollow, rectangular brass tube. It is used to convey the wave
radiation to the sample and crystal.
(3) ATTENUATORS
(4) ISOLATORS
Reference standard In ESR
It is a non-reciprocal device which minimizes vibrations in the
frequency of microwaves produced by klystron oscillator. Isolators are
used to prevent, reflection of microwave power back into the source.
(5) SAMPLE CAVITIES
The heart of the ESR spectrometer is the resonant cavity containing the
sample. Rectangular TE120 cavity and cylindrical TE011 cavity have
widely been used.
(6) COUPLERS AND MATCHING SCREWS
The various components of the micro wave assembly to be coupled
together by making use of irises or slots of various sizes.
7) CRYSTAL DETECTORS AND HOLDERS
Silicon crystal detector, which converts the radiation in D.C., has widely
been used as a detector of microwave radiation.
(8) MAGNET SYSTEM
The resonant cavity is placed between the poles pieces of an
electromagnet. A magnetic field of 5000 gauss is required for ESR.
Dr N.Srinath and A.Anilkumar, Department of Pharmaceutical Analysis, Sri Siddhartha Pharmacy College, Nuzvid.
Contact email. - srinath.n2k@gmail.com, anilkumar.pharmacy@gmail.com