Elecrton Spin Resonance By Imran Aziz
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ELECTRON SPIN RESONANCE MOHAMMAD IMRAN AZIZ Assistant Professor PHYSICS DEPARTMENT SHIBLI NATIONAL COLLEGE, AZAMGARH (India). pdfMachine Is a pdf writer that produces quality PDF files with ease! Produce quality PDF files in seconds and preserve the integrity of your original documents. Compatible across nearly all Windows platforms, if you can print from a windows application you can use pdfMachine. Get yours now!
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Transcript of Elecrton Spin Resonance By Imran Aziz
ELECTRON SPIN RESONANCE
MOHAMMAD IMRAN AZIZAssistant Professor
PHYSICS DEPARTMENTSHIBLI NATIONAL COLLEGE,
AZAMGARH (India).
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ELECTRON SPIN RESONANCE SPECTROSCOPY
1. INTRODUCTION
�The spin of an electron and its associated magnetic moment arethe basis of ESR spectroscopy
�The technique can only be applied to species having one or moreelectrons [e.g., free radicals, biradicals and other triplet states,and many transition metal compounds]
�The technique is particularly valuable for the study of unstableparamagnetic species generated in situ by electrochemicaloxidation/reduction.
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E = E - E = geBB
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 = geBBms,
where ms = ½, B is the Bohr magneton and g is a proportionality factor, equal to 2.00232 for a free electron[for radicals g ~ ge while for transition metal compounds g = 2 to 4].
When the resonance condition [h = geBB] is satisfied, strong absorption of the radiation (microwave frequency) occurs.
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�Just as in NMR, the spin magnetic moment interacts with the localmagnetic field and so, the resonance condition is given by themodified equation,
h = geBBlocal = geB(1-)B = gBB
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 inthe microwave region.
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Numerical illustration-1
The center of an ESR spectrum of methyl radical occurred at329.4 mT in a spectrometer operating at 9.233 GHz. What isthe 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 = gB
g = h/B = (71.44X 9.233)/329.4 = 2.0024
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Numerical illustration-2
Calculate the magnetic field at which a methyl radical (g =2.0024) comes into resonance in a spectrometer operatingat 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 = gB
= (h/B)(/g) = (71.44X 9.468)/2.0024 = 337.8 mT
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3. RELAXATION PROCESSES AND SIGNAL INTENSITIES
Relaxation processes:
(a) Spin-lattice relaxation: The absorbed microwave energy istransferred from the spin system to its surroundings and
(b) Spin-spin relaxation: The absorbed microwave energy istransferred from the spin system to the adjacent spin.
If the relaxation time is long, the population of the upper statewill increase during observation and the signal intensity willsaturate 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 transitionmetal complexes, where the spectral lines are observed at liq. N2 orliq. He temperatures only.
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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 whichgenerates microwave frequency
Cavity or Sample Chamber: The cavity size is so chosen that astanding wave is set up and the location of the cavity coincides witha region of uniform magnetic field.
Detection and Recording System: The detection system utilizes asmall-amplitude magnetic field modulation and a phase-sensitivedetector as a means of reducing noise.
Presentation of Spectrum: As in NMR, the ESR spectrum can berepresented by plotting intensity against the strength of the appliedfield, but ESR spectra are generally presented as derivative curves(i.e., the slope of the absorption is plotted against the magnetic fieldstrength). Much greater sensitivity can be achieved by thisdetection method if the line shape is broad.
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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, thenge = 2.0023.
Fortunately, g can be measured with great accuracy (usually >
0.001) and hence small deviations from 2.0023 will help tocharacterize the species.
�The g value is the unique property of the molecule as a whole and isindependent of any electron-nuclear hyperfine interactions thatmay be present.
�When the unpaired electron is in an orbital that is far removed fromother levels, g will be close to the spin-only value of 2.0023.
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