ELECTRON SPIN RESONANCE

1
ELECTRON SPIN RESONANCE
2
Definition

• It is also called electron paramagnetic
  resonance (EPR) or electron magnetic resonance
  (EMR).
• Is a branch of absorption spectroscopy in which
  radiation of microwave frequency is absorbed by
  paramagnetic substances.
• It was invented by Zavoisky in 1944.

3

• ESR is concerned with magnetic behaviour of
  spinning electrons.
• ESR spectrum is obtained by transition from one
  spin state to other state of an electron,
  transition is induced by radiation of microwave
  frequency.
• ESR uses reagents called electron spin reagents,
  which are stable containing odd electrons have
  the capacity to react with aminoacids.
• ESR of compounds give information about
  structure, viscosity, polarity, phase
  transformation and chemical reactivity.

4

• ESR

• NMR

• Chemical shift is absent.
• The transition b/w electrons occurs upon the
  absorption of a quantum of radiation in microwave
  region .
• The frequency required is 8000MCS-1 .
• Limited application due to systems containing
  unpaired electrons.

• Chemical shift is present .
• The transition occurs upon the application of
  radiofrequency.
• The frequency required is 40MCS-1

5
THEORY

• The energy levels are produced by the
  interaction of the magnetic moment of an unpaired
  electron in a molecule ion with an applied
  magnetic field.
• ESR spectrum is due to transition b/w these
  energy levels by absorbing radiations of
  microwave frequency.

6

• Like a proton, an electron has a spin, which
  gives it a magnetic property known as a magnetic
  moment.
• When an external magnetic field is supplied, the
  paramagnetic electrons can either orient in a
  direction parallel or antiparallel to the
  direction of the magnetic field
• This creates two distinct energy levels for the
  unpaired electrons.

7
(No Transcript)
8
(No Transcript)
9

• For an electron of spin s1/2, the spin angular
  momentum have values of
• ms 1/2 or -1/2
• In the absence of magnetic field, the two values
  give rise to doubly degenerate energy levels(same
  energy).

10

• If a magnetic field is applied, this leads to two
  non-degenerate energy levels.
• The low energy state will have the spin magnetic
  moment alligned with the field (ms -1/2), and
• high energy state - opposite to field (ms
  1/2).

11
(No Transcript)
12
(No Transcript)
13

• These two states possess energies split up from
  original state with no applied magnetic field by
  µe H (low energy state)
• -µeH (high energy state)
• Where, H magnetic field
• µe magnetic moment
• In ESR, transition b/w different energy levels
  take place by absorbing quantum of radiaton of
  ferquency in microwave region

14
?EhvgßB
h Plancks constant 6.626196 x 10-27
erg.sec n frequency (GHz or MHz) g g-factor
(approximately 2.0) ß Bohr magneton (9.2741 x
10-21erg.Gauss-1) B magnetic field (Gauss or mT)
15
Thus ESR spectrum of free electron consist of
single peak corresponding to transition b/w two
levels. When absorption occurs 2 µeH
hv Where , v - frequency of absorbed radiation
(cycles/sec) The energy of transition is given
by ?E hv gßH Where, ß Bohrs
magneton ( a factor for converting angular
momentum into magnetic moment) g
proportionality factor(a function of electron
environment). Also called spectroscopic
splitting factor OR Landes splitting factor.
The value is not constant
16

• g is proportionality factor, function of
  electrons enviornment,also called as
  spectroscopic spliting factor or Landes spliting
  factor
• g is not constant.
• For a free electron , value is 2.0023.In free
  radicals, value is same(electron character is
  same).

17

• In some crystals, g vary from 0.2 to 0.8.The
  reason is that unpaired electron is localised
  in a paricular orbital about the atom and the
  orbital angular momentum couples with spin
  angular momentum give rise to a low value of g
  in ionic crystals.
• g value depend upon the orientation of the
  molecule having the unpaired electron w.r.t
  applied magnetic field.
• In a crystal the value of g along x,y,z axes
  denoted as gx,gy,gz.

18
INSTRUMENTATION
19
Most EPR spectrometers are similar to the
original instrumentation used for NMR before the
development of pulsed Fourier Transform
spectroscopy. Most also use electromagnets
rather than permanent magnets or cryomagnets.
20
(No Transcript)
21
(No Transcript)
22

• SOURCE
• Klystron
• most common source operating in MW band region of
  3 cm ?
• Vacuum pump, can produce MW oscillations centered
  on small range of frequency
• Frequency on monochromatic radiation determined
  by applied voltage to Klystron
• Operated at 9500 Mc/sec.

23

• b. Isolator
• A non-reciprocal device, a strip of ferrite
  material
• Minimises vibrations in frequency of MW produced
  by Klystron oscillator
• Variation in frequency due to backward reflection
  in region b/w Klystron circulator
• Wavemeter
• b/w isolator attenuator to know frequency of MW
  produced by Klystron oscillator
• Calibrated in frequency units
• Attenuator
• b/w wavemeter circulator, adjusts the level of
  MV power incident upon sample
• Possesses an absorption element

24

• CIRCULATOR / MAGIC T
• MW radiations enter circulator through a wave
  guide by a loop of wire that couples with
  oscillating magnetic field sets up a
  corresponding field in wave guide
• Made up of hollow rectangular copper or brass
  tubing with silver or gold plating inside to
  produce a highly conducting flat surface

25

• SAMPLE CAVITY
• Cavity system is constructed in such a way as to
  maximise applied magnetic field along the sample
  dimension
• 0.15 0.5 ml sample can be used (less DEC)
• Flat cells (0.25 mm thickness) used for samples
  (high DEC) 0.05 ml sample
• Most ESR spectrometers use dual sample cavities
  for sample and reference

26

• 4. MAGNET SYSTEM
• A static magnetic field is provided by an
  electromagnet with a current- regulated power
  supply.
• A homogeneous and stable field is required for
  best results.
• The resonant cavity is placed b/w pole pieces of
  electromagnet
• A Hall probe, driven from a stable
  constant-current power system, with a digital
  multimeter (DMM) reading the Hall voltage, is
  used to measure the value of the magnetic field
  between the poles of the magnet

27

• CRYSTAL DETECTORS
• Silicon crystal is most commonly used detector
• Act as a microwave rectifier
• Convert MW power into direct current output
• AMPLIFIER AND PHASE SENSITIVE DETECTOR
• The signal from crystal detector undergo
  narrow-band amplification by auto amplifier
• Noise present in the amplified signal is removed
  by phase sensitive detector
• OSCILLOSCOPE AND PEN RECORDER

28

• WORKING
• Sample is kept in resonant cavity
• The cavity is a long path-length cell in which
  wavas are reflected to fro thousand of times
• Klystron oscillator produce MW that pass through
  isolator, wave meter and attenuator to reach
  circulator through arm 1
• MW power divide b/w arms 2 3 (sample cavity)
• Arm 2 have balancing resistance
• When total resistance on 2 3 are same, MW power
  is absorbed completely
• When R on arm 3 changes due to some ESR resonance
  absorption by the sample, then some MW power
  enter into detector through arm 4

29

• detector convert the MW power into DC
• If the magnetic field around the sample is
  changed to the value required for resonance,
  absorption takes place by the sample
• The current is recorded by the recorder and show
  an absorbance peak
• If main magnetic field is swept slowly for
  several minutes, recorder shows the derivative of
  the absorption spectrum
• generally absorption spectra are recorded as
  first derivative spectra

30
(No Transcript)
31

• PRESENTATION OF ESR SPECTRUM
• ESR spectrum is obtained by plotting intensity
  against the strength of magnetic field
• ESR spectrum is better represented by derivative
  curve
• First derivative (slope) of absorption curve is
  plotted against strength of magnetic field
• Each negative slope in derv. curve represents a
  peak or shoulder in absorption spectrum
• Every crossing of the derv. axis with a negative
  slope indicates a true maximum and crossing with
  positive slope indicates minimum
• Thus no. of peaks/shoulders in absorption curve
  can be determined from no. of minima or maxima in
  the derivative curve.

32

• Area covered by either the absorption or
  derivative curve is proportional to no of
  unpaired electrons in the sample
• To calculate no of electrons in the sample,
  comparison is made with standard sample having
  known no of unpaired electrons and possessing the
  same line shape
• Most widely used std is 1,1-diphenyl-picrylhydrazy
  l (DPPH) free radical

33
DPPH is chemically stable Splitting
factor g 2.0036 1,53 1021unpaired electrons
per gm