Page 1
ESR SPECROSCOPY (Electron paramagnetic resonance (EPR) or electrons spin resonance (ESR))
R.Sukumar, PG Asst (Chemistry), Dindigul, Cell: 7904168869l Page 1
ESR spectroscopy is an absorption spectroscopy which involves the absorption of radiation in the
microwave region (104–106 MHz)
ESR is applicable to
a. Free radicals
b. Bi-radicals
c. Triplet states
d. Transition metal ions and complexes
e. Conduction of electron in metals
All these are characterized by the presence of one or more unpaired electrons
(Electron spins S > 0)
ESR is a Non-destructively technique and in any sample phase (gas, liquid or solid).
Energy The energy required for transition in ESR spectra is given by E=h= gH
= Bohr magneton
g= spectroscopic splitting factor (lande splitting factor)
The spin motion of this single unpaired electron gives to this molecule a g-factor that very
nearly equals that of a free electron; g = 2.0038 instead of 2.00232.
g = 1
+
S(S + 1) - L(L + 1) + J(J + 1)
2J(J + 1)
If want complete study material
Call: 7904168869
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Padasalai
Page 2
ESR SPECROSCOPY (Electron paramagnetic resonance (EPR) or electrons spin resonance (ESR))
R.Sukumar, PG Asst (Chemistry), Dindigul, Cell: 7904168869l Page 2
EPR spectra of the odd electron in two modes:
i. Absorption mode
ii. Derivative mode
ESR spectra are generally broad(not sharp) it is difficult to locate the peak in the absorption
mode
But it is quite easy to locate it in the derivative mode. Hence, the ESR spectra are generally
recorded in the derivative mode.
Reference sample
I. -Diphenyl picryl hydrazyl radical (DPPH)
II. It gives 5 sharp peaks (1:2:3:2:1)
Hyper fine structure (HFS) occurs as a result of the magnetic interaction between the electronic
spin(s) and the nuclear spin (I).
1. Selection rule for hyper fine transitions is ∆mS = ±1, ∆mI = 0
2. Example: The total number of fine and hyperfine lines from high spin Mn(II) complex,
I=5/2
3. Number of fine lines = no of unpaired electrons in Mn =5
4. Number of hyperfine lines = no of unpaired electrons x (2nI+1)
= 5 x (2x1x5/2+1) = 5x6=30
Zero-field splitting
When the system contains even number of electrons,
The degeneracy of the spin states of electrons in such atoms is resolved by the internal
magnetic field of the electrons even in the absence of external magnetic field.
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Page 3
ESR SPECROSCOPY (Electron paramagnetic resonance (EPR) or electrons spin resonance (ESR))
R.Sukumar, PG Asst (Chemistry), Dindigul, Cell: 7904168869l Page 3
This phenomenon of removal of degeneracy of spin states by the internal magnetic field of the
paramagnetic electrons is termed as zero-field splitting.
Zero-field splitting present in non-cubic crystal field (rhombic, trional, tetragonal, spin-orbit, spin-
spin interactions).
Kramaers degeneracy
When the system contains odd number of electrons,
The spin degeneracy energy level remains doubly degenerate. This is known as kramaers
degeneracy.
This can be removed only by the application of an external magnetic field.
The spin degeneracy may be resolved by crystal field.
Example: Cr(III)(d3),Mn(II)(d5) show kramers degeneracy.
Cr(III) has 3 unpaired electrons(Cr3+ =d3 s =3/2), therefore number of in tetragonally
distorted Cr(III) complex in zero field splitting is 2 i.e 3/2, 1/2.
Zero field splitting results in the following number of kramers doublets---2.
Application ESR in transition metal ions
Metal ion Configuration s Fine line
structure I
hyper Fine line
structure
Cr3+ d3 3/2 3 I=0(52Cr) 0
I=3/2(53Cr) 4
Mn2+ d5 5/2 5 I=5/2 (55Mn) 6
Fe3+ d5 5/2 5 No hyperfine structure seen
Co2+ d7 3/2 3 7/2 8
Cu2+ 3/2 4
Ti3+ d1 1/2 1 5/2(47Ti) 6
7/2(49Ti) 8
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Padasalai
Page 4
ESR SPECROSCOPY (Electron paramagnetic resonance (EPR) or electrons spin resonance (ESR))
R.Sukumar, PG Asst (Chemistry), Dindigul, Cell: 7904168869l Page 4
zero( most
abundant Ti) no hyperfine
VO2+ d1 1/2 1 I=7/2( 51V most
abundant) 8
Ni2+(d8) has no unpaired electron(square planar) in its orbit. Therefore, It does not exhibit
EPR signal at room temperature.
ESr Spectrum of the complex ion [ Mo(CN)8]3- in solution consist of one line.
If the sample is enriched with 13C, the spectrum consists of nine lines.
But Mo(v) in [ Mo(CN)8]3- has d1 configuration and single line expected.
Beacuse13C enriched sample (13CI=3/2), there will be 2nI+1 or 9 lines because 8C atoms
in the CN groups surround the Mo atom.
Vanadyl acetyl acetone shows 8 lines in the hyperfine structure of ESR spectrum. It means
the spin of V51 nucleus (I) value is 7/2 as 2I+1=8 or I=7/2.
ESR spectrum of [ Fe(CN)5(NO)]3- ion consists of 3 lines with the same intensity because of
the electron interaction with the nitrosyl nitrogen nucleus(I=1)
Application ESR in ORGANIC RADICAL ZND IONS
Radical I Peaks (2I+1) Hyperfine
spliting
Hydrogen atom 1/2 2(1/2)+1=2 1:1 Doublet
Deuterium 1 2(1)+1=3 1:2:1
Methyl radical(CH3) 3/2 2(3/2)+1=4 1:3:3:1 quartet
(SO3)2NO- 1 2(1)+1=3 1:2:1
1,4-benzosemiquinone
radical anion 2(4/2=2) 2(2)+1=5 1:4:6:4:1 quintet
Benzene radical 3(6/2=3) 2(3)+1=7 1:6:15:20:1
5:6:1 Septet
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Page 5
ESR SPECROSCOPY (Electron paramagnetic resonance (EPR) or electrons spin resonance (ESR))
R.Sukumar, PG Asst (Chemistry), Dindigul, Cell: 7904168869l Page 5
Naphthalene negative ion
two sets of proton
2I=4/2=2
2I=4/2=2
[2(2)+1]
[2(2)+1]=25
Anthracene negative ion
I=4/2=2(4)
I=4/2=2(4)
I=2/2=1(2 )
[2(2)+1]
[2(2)+1]
[2(1)+1]=75
Triphenyl methyl
radical(C6H5)3C
I=6/2=3(6ortho)
I=6/2=3(6meta)
I=3/2=1.5(3 para)
[2(3)+1]
[2(3)+1]
[2(1.5)+1]
7x7x4= 196
CRYSTAL DETECTORS AND HOLDERS: Klystron tube acts as the source of radiation.
Silicon crystal detector, which converts the radiation in D.C., has widely been used as a detector
of microwave radiation.
Difference between NMR and ESR
Parameter NMR ESR
Spinning possible for Nuclei (I > 0) Electron (S > 0)
Observed region Radio frequencies Microwave-frequency radiation
Frequency range 300-1000MHz 3-400GHz
Constant parameter Magnetic field strenth Microwave-frequency
Varied parameter Radio frequencies Magnetic field strenth
Sensitivity 1000 times less than ESR 1000 times more than NMR
Relaxation time More for nucleus Less for electron
Parameter measured Coupling constant(J) Splitting constant(g)
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Page 6
ESR SPECROSCOPY (Electron paramagnetic resonance (EPR) or electrons spin resonance (ESR))
R.Sukumar, PG Asst (Chemistry), Dindigul, Cell: 7904168869l Page 6
ANSWER THE FOLLOWING QUESTIONS
1. The ESR spectrum of anthracene radical anion consists of
(a) 25 lines (b) 50 lines (c) 75 lines (d) 100 lines
2. The ESR spectrum ofCD3 Free radical (the nuclear spin of 2D=1) consists of
(a) Four lines (b) 1:2:3:2:1 quinet (c) 1:6:15:20:15:6:1 septet (d) 1:3:6:7:6:3:1
sepet
3. ESR spectra are observed in ----------- region.
(a) Microwave (b) radiofrequency (c) X-ray (d) UV/VIZ
4. The ESR spectrum of p-benzo semiquinone radical anion consists of a
(a) 1:2:3:2:1 quinet (b) 1:3:3:1 quartet (c) 1:4:6:4:1 quinet (d) equally intense
quintet
5. The ESR spectrum of naphthalene radical anion consists of
(a) 15 lines (b) 25 lines (c) 30 lines (d) 40 lines
6. The compound DPPH, used for calibrating ESR spectra, shows ESR spectrum consisting of a
(a) 1:2:3:2:1 quinet (b) 1:3:3:1 quartet (c) 1:4:6:4:1 quinet (d) equally intense quintet
7. Among the following diatomic molecules, which one will show EPR signal?
(a) Li2 (b) B2 (c) C2 (d) N2
8. The g value for the benzene radical anion is 2.0025 at what field would you search for resonance in
a spectrometer operating at 9.302GHz?
(a) 0.432T (b) 0.332T (c) 0.872T (d) 0.243T
Solution : E=h=6.626x10-34Js X 9.3x109s-1 = 6.6164X10-24J
B=E/gB = (6.6164X10-24J) / (2.0025)(9.2741X10-24JT-1) =0.332T
9. In an ESR spectrometer operating at 9.302GHz, the ESR spectrum of hydrogen atom gave two lines,
one at 357.3mT and the other at 3o6.6mT.the hyperfine coupling constant of hydrogen atom is
(a) 2.3mT (b) 2.3mT (c) 1.15mT (d) none of these
10. Calculate the frequency at which the transition between the two spin states of a free electron may
be observed at field strength of 0.3000T.
(a) 8.4 x 109s-1 (b) 5.39 x 108s-1 (c) 4.82 x 106s-1 (d) 1.93 x 1010s-1
Solution: E=h=geB N
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Page 7
ESR SPECROSCOPY (Electron paramagnetic resonance (EPR) or electrons spin resonance (ESR))
R.Sukumar, PG Asst (Chemistry), Dindigul, Cell: 7904168869l Page 7
=geB N/h = (2.002 )( 9.274 x 10-24JT-1) ( 0.3000T) / (6.626x10-34J-s) = 8.4 x 109s-1
11. The non-magnetic isotopes of Mo(Mo94,Mo(6) with I =0 exhibit.
(a) 6 ESR lines (b) 1ESR lines ( c ) 5ESR lines (d ) 2ESR lines
12. Which of the metal ion is EPR ACTIVE?
(a) Ni2+ (b) Fe2+ ( c ) Cr2+ (d ) Co2+
12. Which of the following species is ESR active?
(a) VOSO4 (b) K2CrO7 ( c ) KMnO4 (d ) [Co(NH3)6 ]Cl3
Solution:
In VOSO4 ,V4+ is 3d1 so it is ESR active.Cr6+,Mn7+ are 3d0.in [Co(NH3)6 ]Cl3, because of strong NH3 ligand
d-electrons are paired up.
13. EPR spectroscopy is often used to investigate Copper (II) complexes but not copper (I) complexes.
This is because-
(a) Cu2+ has and s1d8 configuration but Cu+ is d10 (b) Cu2+ has and s2d7 configuration but Cu+ is d10
(c) Cu2+ has and d9 configuration but Cu+ is d10 (d) Cu2+ has and d9 configuration but Cu+ is s1d9
14. The number of lines exhibited by a high resolution EPR spectrum of the species,[Cu(en)2]2+ is
[nuclear spin(I) of Cu=3/2 and that of N=1]-----------------
(a) 12 (b) 15 (c) 20 (d) 36
15. The number of EPR signals observed for octahedral Ni(II) complexes is ------------
(a) 1 (b) 2 (c) 3 (d) 0
16. In the EPR spectrum of tetragonal Cu(II) complex, when g ll g ge the unpaired electron
Resides in the orbital-----------------
(a) dxy (b) dx2
-y2 (c) dz
2 (d) dxz
17. For a tetraonally distorted Cr(III) complex. Zero field splitting results in the following number of
kramers doublets---
(a) 1 (b) 2 (c) 3 (d) 0
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Page 8
ESR SPECROSCOPY (Electron paramagnetic resonance (EPR) or electrons spin resonance (ESR))
R.Sukumar, PG Asst (Chemistry), Dindigul, Cell: 7904168869l Page 8
18. The total numbers of fine and hyperfine EPR Lines expected for octahedral high-spin Mn(II)
complexes are respectively(I=5/2 for Mn)---------
(a) 3 and 30 (b) 5 and 33 (c) 5 and 30 (d) 4 and 24
19. An ESR spectrum of hydrogen atoms shows two lines. This is due to
(a) spin-spin coupling (b) hyper fine coupling(c) quadrupole coupling (d) spin-spin decoupling
20. DPPH solid gives line in EPR whereas in solution the sample gives
(a) 4 ESR lines (b) 5ESR lines ( c ) 1ESR lines (d ) 2ESR lines
21. The correct nuclear magnetic resonance condition is
(a) hg=H (b) h =gH ( c ) h = gH (d ) hg=H
22. The ESR specturum of p-benzo semiquinone radical anion contains a
(a) Sextet (b) quartet (c) quinet (d) triplet
23. Which of the following molecules shows ESR speecturm
(a) N2 (b) O2 (c) 12C (d) Cu+
24. How many signals are expected in the ESR spectrum of ethyl radical?
(a) 6 (b) 12 (c) 8 (d) 10
25. ESR can be estimated for species with
(a) Lone pair (b) free radical (c) bonded pair (d) diradical
26. ESR spectroscopy is used tostudy
(a) Na+ (b) K+ (c) Ca2+ (d) Cu2+
27. The presence of odd electron can be identified by--------------- spectroscopy.
(a) IR (b) NMR (c) ESR (d) MASS
28. Hyperfine structure in ESR spectra arises due to interaction between ---------
(a) Spins of electron and positron (b) spins of electron and electron
(c) Spins of electron and nuclear spin (d) spins of 2 nuclear spin
29. Which of the following statements is not correct?
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Page 9
ESR SPECROSCOPY (Electron paramagnetic resonance (EPR) or electrons spin resonance (ESR))
R.Sukumar, PG Asst (Chemistry), Dindigul, Cell: 7904168869l Page 9
(a) Microwaves are generated by the klystron tube
(b) Typical frequencies/energies ESR transitions are much higher than those for
NMR transitions
(c) Magnitude of hyperfine coupling constant aH proportional to electron density at nucleus
(d) The value of g is 2.0023 for DPPH
30. The ESR spectrum of (SO3)2NO- will have
(a) A triple with intensity ratio 1:2:1 (b) A triple with intensity ratio 1:1:1
(c) A triple with intensity ratio 1:3:3:1 (d) A triple with intensity ratio 1:1:1:1
31. The paramagnetic nature of a sample can be detected by___________________
(a) NMR spectroscopy (b) IR spectroscopy (c) ESR spectroscopy (d) Mossbauer spectroscopy
32. The reference compound used in ESR spectroscopy is
(a) diphenylpicrylhydrazyl radical (b) diphenylpicryl hydrazine
(c) diphenylpicryl hydrazine dehydrate (d) diphenylpicrylhydraziniumiodine
33. A metal ion with S=1/2 andI=7/2 has isotropic ESR. How many hyperfine lines are expected to be
observed in the ESR spectrum?
(a) 7 (b) 8 (c) 14 (d) 15
34. The esr spectrum of a radical with a single magnetic nucleus is split into 6 lines. What is the spin of
the nucleus?
(a) 5 (b) 5/2 (c) 3/2 (d) 3
35. The No. of hyperfine lines in the ESR spectrum of .CD2CH3 radical is
(a) 12 (b) 20 (c) 15 (d) 25
36. Which of the following species show ESR spectrum?
(i) CO2 (ii) N2+ (iii) NO (iv)H2O (v) O2
(A) (II),(III)&(V) ONLY (B) (i),(ii)&(iv) 0nly
(c)(i), (ii),(iii)&(iV) only (d) all of these
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Page 10
ESR SPECROSCOPY (Electron paramagnetic resonance (EPR) or electrons spin resonance (ESR))
R.Sukumar, PG Asst (Chemistry), Dindigul, Cell: 7904168869l Page 10
37. The kramers degeneracy is
(a) due to the presence of paired electrons
(b) Developed due to the application of magnetic field
( c) due to the presence of an odd number of unpaired electrons.
(d) due to the application of electric field
38. How many hyper fine lines in ESR spectrum are shown by Mn2+?
(a) 3 (b) 5 (C) 2 (d) 6
39 .which of the following will not show ESR spectra?
(a)O2 (b) C2H5, (c) N2 (d)Cu2+
40. Predict the number of lines in the ESR spectra of the following systems respectively
(1)CH3CH2 (2) CH3
(3) C6H6-
(a) 4,7,6 (b)2,1,6 , (c) 5,3,6 (d)12,4,7
41. What will be the Esr frequency in a magnetic field of 25000 gauss of g =2 and =9.273x10-24JT-1?
(a) 6000MHz (b) 7000MHz (c) 25000MHz (d) 600000MHz
42. The electron spin resonance is an example of -------
(a) Stark effect (b) Russell-saunders state (c) Paschen-back effect (d) Vibration effect
43. How many Esr lines are observed if an unpaired electron, when closed to two non- equivalent
protons is placed in magnetic field?
(a) 2 (b) 8 (C) 16 (d)4
44. If the two equivalent protons are considered in the above question, predict the number of lines-----
(a) 2 (b) 8 (C) 4 (d) 3
45. Among the following complex species the one which is EPR active is-------
(a) Ni(CO)4, (b)[ Co(NH3)5Cl]2+ (c) [Cu(C2O4)2]2- (d) Mo(CO)6
If want complete study material
Call: 7904168869
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