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Electron Spin Resonance, BY IMRAN AZIZ

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Electron Spin Resonance, BY IMRAN AZIZ Powered By Docstoc
					  ELECTRON SPIN
   RESONANCE

 MOHAMMAD IMRAN AZIZ
    Assistant Professor
  PHYSICS DEPARTMENT
SHIBLI NATIONAL COLLEGE,
    AZAMGARH (India).
  ELECTRON SPIN RESONANCE SPECTROSCOPY

1. INTRODUCTION

   •The spin of an electron and its associated magnetic moment are
     the basis of ESR spectroscopy

   •The technique can only be applied to species having one or more
     electrons [e.g., free radicals, biradicals and other triplet states,
     and many transition metal compounds]

   •The technique is particularly valuable for the study of unstable
     paramagnetic species generated in situ by electrochemical
     oxidation/reduction.
2. BASIC PRINCIPLE

In the presence of a magnetic field B, a molecule or ion having one e-
has two electron-spin energy levels given by,
                             Ems = geBBms,
where ms = ½,
B is the Bohr magneton and
g is a proportionality factor,
equal to 2.00232 for a free
electron                                      E = E - E = geBB
 [for radicals g ~ ge while for
transition metal compounds
g = 2 to 4].

When the resonance
condition [h = geBB] is
satisfied, strong absorption
of the radiation (microwave
frequency) occurs.
•Just as in NMR, the spin magnetic moment interacts with the local
    magnetic field and so, the resonance condition is given by the
    modified equation,

           h = geBBlocal = geB(1-)B = gBB

   where g = (1-) ge = g-factor of the radical or complex.

•Most commonly, fields of 0.34 and 1.24T (1T = 104 Hz) are used,
   with corresponding frequencies of 9.5 and 35 GHz, which are in
   the microwave region.
Numerical illustration-1


The center of an ESR spectrum of methyl radical occurred at
329.4 mT in a spectrometer operating at 9.233 GHz. What is
the g-value?

    [h = 6.626093 X 10-34 Joules/sec;
     Bohr Magneton (B) = 9.274026 X 10-24 Joules/Tesla;
     Thus, h/B = 71.4448 mT/GHz]

 Use the equation, h = gB
       g = h/B = (71.44X 9.233)/329.4 = 2.0024
Numerical illustration-2


 Calculate the magnetic field at which a methyl radical (g =
 2.0024) comes into resonance in a spectrometer operating
 at 9.468 GHz.

          [h = 6.626093 X 10-34 Joules/sec; Bohr Magneton
            (B) = 9.274026 X 10-24 Joules/Tesla; Thus,
            h/B = 71.4448 mT/GHz]

    Use the equation, h = gB
     = (h/B)(/g) = (71.44X 9.468)/2.0024 = 337.8 mT
3. RELAXATION PROCESSES AND SIGNAL INTENSITIES

   Relaxation processes:

   (a) Spin-lattice relaxation: The absorbed microwave energy is
        transferred from the spin system to its surroundings and

   (b) Spin-spin relaxation: The absorbed microwave energy is
        transferred from the spin system to the adjacent spin.

         If the relaxation time is long, the population of the upper state
   will increase during observation and the signal intensity will
   saturate or decrease in intensity.

          If the relaxation time is short, then by the uncertainty principle,
   the resonance lines must be wide. This is the case with transition
   metal complexes, where the spectral lines are observed at liq. N2 or
   liq. He temperatures only.
4. LAY OUT AND PRESENTATION OF THE SPECTRUM

 A typical ESR spectrometer consists of the following layout:

   Radiation Source: This consists of a Klystron source which
   generates microwave frequency

   Cavity or Sample Chamber: The cavity size is so chosen that a
   standing wave is set up and the location of the cavity coincides with
   a region of uniform magnetic field.

   Detection and Recording System: The detection system utilizes a
   small-amplitude magnetic field modulation and a phase-sensitive
   detector as a means of reducing noise.

   Presentation of Spectrum: As in NMR, the ESR spectrum can be
   represented by plotting intensity against the strength of the applied
   field, but ESR spectra are generally presented as derivative curves
   (i.e., the slope of the absorption is plotted against the magnetic field
   strength). Much greater sensitivity can be achieved by this
   detection method if the line shape is broad.
5. THE g VALUE

     The g value is the proportionality constant in the basic equation,
               h = g B.
       [For  in kG, and  in MHz, g = 0.71446 X /
       where  is the fixed frequency of the microwave radiation
     and  is the magnitude of the static field at resonance.]

   •If the electron spin is the only source of magnetism, then
                               ge = 2.0023.
       Fortunately, g can be measured with great accuracy (usually > 
       0.001) and hence small deviations from 2.0023 will help to
       characterize the species.
   •The g value is the unique property of the molecule as a whole and is
     independent of any electron-nuclear hyperfine interactions that
     may be present.
   •When the unpaired electron is in an orbital that is far removed from
     other levels, g will be close to the spin-only value of 2.0023.

				
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posted:2/23/2010
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Description: concept of esr