Nmr spectroscopy by dr. pramod r. padole

For B. Sc. & M.Sc. students

1. NMR Spectroscopy
By
Dr. P. R. Padole
Department of Chemistry
ShriShivajiScience College, Amravati.

2. BASIC:
Nuclear Magnetic
Resonance
spectroscopy.
For mapping
molecular structures
& learning how
molecules function &
relate to each other.
It is recognized as one
of the most powerful
techniques for
chemical analysis.

3. Contents
Introduction
1
Principles of NMR
2
NMR INSTRUMENTATION3
Chemical Equivalence
4
Splitting of Signal
5
Problems
6

4. Electromagnetic Spectrum:
4
Radio frequency
ʋ = 60 –500 MHz

5. iNTRODUCTION
Powerful analytical technique used to characterize organic
molecules by identifying carbon-hydrogen frameworks within
molecules.
 2 Types:
The source of energy in NMR is radio waves which have longer
(higher) wavelengths, and thus lower energy and frequency.
This waves can change the nuclear spins of some elements,
including
1
Hand
13
C.
Frequency in the range of 60MHz -500MHz.
13
C -NMR
1
H -NMR

6. Do you know?
NMR

7. The study of absorption of
radio frequency radiation
by nuclei
in presence of magnetic field
is called
Nuclear Magnetic Resonance.
Definationof NMR Spectroscopy:
The spectra produced due to;
(i) Absorption of radio frequency radiation by the nucleus &
(ii) Nuclear spin state transition with inversion of spin in presence of magnetic field,
is called as NMR spectra.

8. The study of absorption of radio
frequency radiation by atomic nuclei
in presence of magnetic field is called
as NMR spectroscopy.
Definition of NMR Spectroscopy
Nuclei of atoms rather than outer electrons are involved
in the absorption process, because of low energy and frequency.

9. Continuation of NMR History
Nobel prizes
1944 Physics Rabi (Columbia)
1991 Chemistry Ernst (ETH)
"for his resonance
method for
recording the
magnetic properties
of atomic nuclei"
1952 Physics Bloch
(Stanford), Purcell
(Harvard)
"for their development
of new methods for
nuclear magnetic
precision
measurements and
discoveries in
connection therewith"
"for his contributions
to the development of
the methodology of
high resolution nuclear
magnetic resonance
(NMR) spectroscopy"

10. Continuation of NMR History
2002 Chemistry Wüthrich (ETH)
2003 MedicineLauterbur (University of Illinois in
Urbana ), Mansfield (University of Nottingham)
"for his development of nuclear magnetic resonance
spectroscopy for determining the three-dimensional
structure of biological macromolecules in solution"
"for their discoveries
concerning magnetic
resonance imaging" (MRI)

12. 12
Q.1) Describe the principle or theory of nuclear magnetic resonance spectra.
Q.2) Give in short the principle of nuclear magnetic resonance.
(S-12, 4 Mark)
Q.3) What do you mean by: (W-12, 3 Mark)
(i) Magnetic nuclei, (ii) Flipping of nuclei &
(iii) Induced magnetic field
Q.4) Explain, why
13
C is NMR active while
12
C is not. (W-11, 1 Mark)
Q.5) What is the condition of the nucleus to be NMR active?
Q.6) What are magnetic and non-magnetic nuclei?
University Questions: Principle of NMR

13. Spinning of Proton:
ʋ α H
o
(i.e, Precessional frequency is directly proportional to Strength of external magnetic field)
Precessionalfrequency is defined as the number of revolutionsper second
made by the magnetic moment vector of the nucleus around the external field.
(H
oor B
o)

14. Condition of the nucleus to be NMR active:
•When such spinning nucleus (proton) is placed in external magnetic field (H
oor
B
o), the number of possible orientations calculated by (2 I + 1).
•Thus, spin quantum number Iof proton (Hydrogen) is ½
(or spin number, I > O) are called as magnetic nucleiand hence it spins and
possible orientation is (2 x ½ +1=2) two,
i.e., + ½ & -½ and that are NMR active.
I = 1/2 (
1
H,
13
C,
19
F,
31
P ), I = 3/2 (
11
B,
33
S ) & I = 5/2 (
17
O ).
I = 1 (
2
H,
14
N )
•Whereas value of Iis zero ( I = 0) are called as non magnetic nuclei and hence
these nuclei do not spin. Such nuclei are NMR inactive. (i.e., the spins of all
the particles are paired, there will be no net spin and the nuclear spin quantum
number ( I) will be zero. This type of nucleus is said to have zero spin.)
I = 0 (
12
C,
16
O and
32
S)

15. Spin quantum number of some common nuclei:
•Spin quantum number (I) is related to the
atomic and mass number of the nucleus.
I = 1/2 (
1
H,
13
C,
19
F,
31
P ),
I = 3/2 (
11
B,
33
S ) &
I = 5/2 (
17
O ).
I = 1 (
2
H,
14
N )
I = 0 (
12
C,
16
O and
32
S)

16. Principle of NMR:
The precessingproton will only absorb energy from RF source,
if the precessionalfrequency is the same as the frequency of RF beam;
when this occurs, the nucleus and the RF beam are said to be in resonance,
hence the term Nuclear Magnetic Resonance.

17. Opposed orientation
(High energy)
Less stable
Aligned orientation
(Low energy)
More stable
The nuclear spin states
are separated from each
other by energy
difference ∆E= E
2 -E
1

18. Opposed orientation
(High energy)
Less stable
Aligned orientation
(Low energy)
More stable
The nuclear spin states
are separated from each
other by energy
difference ∆E= E
2 -E
1

19. Opposed orientation
(High energy)
Less stable
Aligned orientation
(Low energy)
More stable
The nuclear spin states
are separated from each
other by energy
difference ∆E= E
2 -E
1
The nuclear magnetic resonance occurs when nuclei aligned (↑) with an
applied magnetic field are induced to absorb energy (Rf) and
change their spin (↓) with respect to the applied magnetic field.

20. Opposed orientation
(High energy)
Less stable
Aligned orientation
(Low energy)
More stable
The nuclear spin states
are separated from each
other by energy
difference ∆E= E
2 -E
1
It can lose this extra energy and relax back into the aligned orientation (↑) and
same will be occurs continuously by the absorption of radio frequency radiation
(RF energy) ) in presence of external magnetic field (of strength H
oor B
o).

21. 21

22. 22
The precessingproton will only absorb energy from RF source, if the
precessionalfrequency is the same as the frequency of RF beam;
when this occurs, the nucleus and the Rfbeam are said to be in
resonance, hence the term Nuclear Magnetic Resonance.

23. 23

24. Instrumentation or Experimental Method:
(Field Sweep Method):
To produce the equal amount of magnetic field pass through the sample
provides a very strong homogeneous magnetic field at 60-100 MHz
Radio frequency source (ʋ = 60 MHz)
is made to fall on the sample tube detects emitted radio frequencies
as nuclei relax to a lower energy level

25. 25
Instrumentation or Experimental Method:
(Field Sweep Method):
For PMR, the solvent should not contain H-atoms.
Solvents: CDCl
3is an ideal solvent.

26. Choice of Solvent used in NMR
Spectroscopy:
The conditions for the solvent used in NMR spectroscopy
are given below:
It should be transferent to radio-frequency region, i.e.,
It should not absorbs with in radio -frequency region.
It should be non viscous.
It should dissolve the solute.
It should be pure.
For PMR, the solvent should not contain H-atoms.
Solvents: CDCl
3is an ideal solvent.
Other solvents like CS
2, CCl
4, pyridine, trifluroacetic acid,
DMSO, C
6D
6may also be used.
Dimethyl sulfoxide(DMSO) is an organosulfur compound with the formula (CH
3)
2SO.
Sample: 10-50 mg of the sample is dissolved in proper
solvent (0.5 ml)
26
Q.1) Give the choice of solvent in NMR spectroscopy?

27. Protonsindifferentenvironmentsabsorbatslightlydifferentfrequencies,
sotheyaredistinguishablebyNMR.

28. 28
NMR spectrum of CH
3
_
CH
2-Brat Low Resolution:
NMR spectrum of CH
3
_
CH
2-Brat High Resolution:

29. Information from
1
H-NMR spectra:
Position of
signals
(chemical
shift):
What types
of
hydrogens?
Number
of
signals:
How many
different
types of
hydrogens
in the
molecule?
1 2 3 4
Relative
areas under
signals
(integration):
How many
hydrogens
of each
type?
Splitting
pattern:
How many
neighboring
hydrogens?

30. 30

31. 31
Equivalent and Non-Equivalent Protons:
Q.1) Explain the terms: Equivalent and non-equivalent
protons. (S-12 & W-18, 2 Mark)
Q.2) Define the terms in NMR spectroscopy with example:
Equivalent and non-equivalent protons.(W-15, 4 Mark)
Q.3) In a molecule protons with same environment
absorbing at same magnetic field strength are called
equivalent protons. (S-14, ½ Mark)
Q.4) Define and explain:
(i) Equivalent proton & (ii) Non-equivalent proton.
Q.5) Explain equivalent and non-equivalent protons with
suitable examples. (S-16, W-17, & W-19, 4 Mark)
Number of Signals:

32. 32
Equivalent and Non-Equivalent Protons:
(i) Equivalent Protons:
Defination:
Theprotonshavingsamechemicalenvironment
(identicalelectronicenvironmentinamolecule)arecalled
asequivalentprotons.
•The set of equivalent proton gives one absorption signal
(peak) at low resolution.
•Because they absorbs at same frequency and at same
field strength.
•They have same chemical shift.

33. 33
Examples of Equivalent Protons:H
3C CH
3 1Set(i)
aa
H
3C CH
2 2Set(ii)
ba
CH
3
a
H
CH
3
HH
H
3C
H
(iii)
a a
b
b
b
b
2Set
CH
3CH
2OCH
2CH
3
a b b a
(iv)
2Set

34. 34
Non-Equivalent Protons:
Defination:
Theprotonshavingdifferentchemicalenvironment
(differentelectronicenvironmentinamolecule)arecalled
asnon-equivalentprotons.
•Thesetofnon-equivalentprotongivesdifferent
absorptionsignals(peaks)atlowresolution.
•Theyabsorbsatdifferentfrequencyordifferentfield
strength.
•Non-equivalentprotonundergospin-spinsplitting
athighresolution.
•Theyhavedifferentchemicalshift.

35. 35
Common explanation of equivalent and non-equivalent protons:
Example: Consider ethyl chloride molecule ,
Ethyl chloride contains two sets (kinds) of equivalent proton:
Set -(a): It contains three equivalent protons of methyl (CH
3
-) group.
Set -(b): It contains two equivalent protons of methylene(-CH
2
-) group.
The proton of set (a) & set (b) are non-equivalent to each other.
So, at low resolution three methyl (CH
3
-) proton will give one
absorption signal at particular frequency or field strength.
Similarly, two methylene(-CH
2
-) proton will give one absorption
signal at another frequency or field strength.

36. 36
To determine if protons are equivalent,
one may carry out a simple test:
Such pairs of protons are called enantitopic protons.
The environment of these protons is mirror images of each other and
these are said to be equivalent proton and give one NMR signal.
CH
3CH
2Br
CH
3CH
2ClCl
CH
aH
b
CH
3
Enantionmers
Cl
CZH
b
CH
3
Cl
CH
aZ
CH
3
;
Mirror image
-H / + Z

37. Enantiomersor Enantiomorphs or Enantiomerism:
(Mirror images)
(Enantiomers Greek word: Enantio = Opposite; Meros= Parts)
(Enantiomorphs Greek word: Enantio = Opposite; Morphs= forms)

38. S limonene (lemons) R limonene (oranges)CH3
HCCH
2
CH
3 CH3
H CCH
2
H
3C

39. LOGO
www.themegallery.com
(Not mirror images)
(Diastereoisomers Greek word: Diastereo = different; Meros= Parts )
(Diastereoisomorphs Greek word: Diastereo = different; Morphs= forms )
Diastereoisomers
or
Diastereoisomorphs
or
Diastereoisomerism:
[email protected]

40. LOGO
Diastereoisomers:
For example:
Tartaric acid having two asymmetric carbon atoms,

41. 41
To determine if protons are non-equivalent,
one may carry out a simple test:
Suchpairsofprotonsarecalleddiastereomers(geometricisomers)protons.
Theenvironmentoftheseprotonsisnotmirrorimagesofeachotherandthesearesaid
tobenon-equivalentproton.
SoitgivedifferentNMRsignals.H
3C
Br H
Ha
b
H
3C
Br H
Z
H
3C
Br Z
H
I
II
Diastereoisomers
Ex
No mirror image
CH
3CH(Cl)CH
2CH
3
CH
3C(Br)=CH
2

42. 42CH
3-CH
2-CH
2-Cl
CH
3-CH-CH
2-Cl
Cl

43. 43
Important Example:
In 2-bromopropene we observe that the replacement of either of the vinylic protons gives
one of a pair of diastereomers (geometric isomers)
Similarly in 1,2-dichloropropane, the two protons on C-1are
nonequivalentand hence will give separate NMR signals.H
3C
Br H
Ha
b
H
3C
Br H
Z
H
3C
Br Z
H
I
II
Diastereoisomers
Ex
No mirror image HCH
C
Cl
ClH
CH
3
b a
-Ha
-Hb
+Z
+Z
HCZ
C
Cl
ClH
CH
3
ZCH
C
Cl
ClH
CH
3
Diastereosisomers
1
2
3
No mirror image

44. 44

45. 45C C 4Set
v)
b
a
H
3C
H
H
H
c
d
CH
3 CH CH
2Cl
Cl
Ex.
H C H
C
Cl
ClH
CH
3
: protons on C-1 carbon atom are different.
4 - Set of non-equvalent protons
a
b
c d
1
2
Br CH
2CH
2Cli) ii)
a b c
a b
3Set
2Set
iii)
iv)
OH
Cl
H
bH
b
H
aH
a
c
3Set
In 1,2-dichloropropane, we get 4 NMR signals indicating four different types ofprotons.
the environment of two protons on C-1 is not same and hence they are non-equivalent
and will show separate signals, these protons are also called as diastereotropic
protons.
1

46. 46C
CH
3H
3C
C
H
3CCH
3
CH
3
CH
3
a a
a
b
a
a H
3CC
CH
3
Br
CH
3
CH
3CH
2-Br
CH
3CH
2CH
2-Br
CH
3CHCH
3
Cl
a a
a
a b
a b c a b a CH
3
CH
2Cl
CH
3CHCH
2CH
3
Br
Cl-CH
2CH
2CH
2-Cl
b d c a
b a b
a
b
c
d
e
f

47. Do you know?
Shielding Effect:

48. 48

49. Shielding & Deshieldingof proton:
49
Explanation or Causes of Chemical Shift:
Shielding & Deshielding of proton by electron:
Q.1) What do you mean by shielding and deshielding
of a nucleus? (W-11, 2 Mark)
Q.2) Explain in brief shielding and deshielding effect
in NMR spectroscopy. (S-15 & W-19, 4 Mark)
Q.3) Explain the term: Shielding effect. (S-18, 2 Mark)

50. Meet Office Boss:
50

51. 51
Explanation or Causes of Chemical Shift:
Shielding & Deshieldingof proton by electron:
Shielding Effect:

52. 52
Shielding Effect:
Thegeneratedsecondarymagneticfieldalignedinsuchaway
thatitopposes(decreases)themainapplied(external)magneticfield
(H
oorB
o)thattheproton“feels”.
Thus,thesecondarymagneticfield(Inducedmagneticfield)
experiencedbytheprotonisless(weaker).Suchproton(nucleus)is
calledasshieldedprotonorshielding.

53. 53

54. 54
So, shielded proton requires a more (greater or higher)
external applied magnetic field strength (H
oor B
o) to
bring the proton in resonance and to observed the
absorption of radio-frequency (RF) signal (spectrum) on
the up-field (High-field) to the right in an NMR spectrum.

55. Do you know?
DeshieldingEffect:

56. 56

57. Deshielding Effect:
(Circulation of π-electrons):
Circulationofπ-electronsaroundtheproton
(nucleus)itselfgeneratesasecondarymagneticfield
(Inducedmagneticfield)thatcaneitheropposeor
reinforce(stronger)theappliedmagneticfieldatthe
proton,dependingontheprotonslocation.
•If the induced magnetic field opposes (decreases) the applied magnetic
field (H
oor B
o), proton is said to be shielded.
•If the induced field reinforces(stronger) the
applied magnetic field, then the field felt by the
proton is more and such a proton is said to be
deshielded,

58. De-shielding Effect:
(Circulation of π-electrons):
Paramagnetic
Region
Circulation
of
Π-
electrons
Diamagnetic Region

59. De-shielding Effect:
(Circulation of π-electrons):
Paramagnetic
Region
Circulation
of
Π-
electrons
Diamagnetic Region

60. De-shielding Effect:
(Circulation of π-electrons):
Paramagnetic
Region
Circulation
of
Π-
electrons
Diamagnetic Region

61. Example of De-shielding: Benzene
61
Where induced magnetic field reinforces (to make strong)
the applied magnetic field (H
oor B
o), in the surrounding
protons & protons are called as deshielded.

62. 62
So, de-shielded proton requires lower external applied magnetic
field strength (H
oor B
o) to bring the proton in resonance and to
observed the absorption of radio-frequency (Rf) signal (spectrum)
on the down field (Low field) region.

63. 63
Whereas induced magnetic field opposes(decreases) the applied
magnetic field (H
o
or B
o
), then the protons are called as shielded:
Acetyleneprotons present in shielded zone and hence have low δ value.
Because acetylenic proton lies along the axis where the induced magnetic
field generates, which opposed (decreases) the external magnetic field.

64. 64

65. 65

66. Do you know?
Why is TMSused as an internal standard
in NMR spectral determination?

67. 67
Common explanation of equivalent and non-equivalent protons:
Example: Consider ethyl chloride molecule ,
Ethyl chloride contains two sets (kinds) of equivalent proton:
Set -(a): It contains three equivalent protons of methyl (CH
3
-) group.
Set -(b): It contains two equivalent protons of methylene(-CH
2
-) group.
The proton of set (a) & set (b) are non-equivalent to each other.
So, at low resolution three methyl (CH
3
-) proton will give one
absorption signal at particular frequency or field strength.
Similarly, two methylene(-CH
2
-) proton will give one absorption
signal at another frequency or field strength.

68. 68

69. Choice of Solvent used in NMR
Spectroscopy:
The conditions for the solvent used in NMR spectroscopy
are given below:
It should be transferent to radio-frequency region, i.e.,
It should not absorbs with in radio -frequency region.
It should be non viscous.
It should dissolve the solute.
It should be pure.
For PMR, the solvent should not contain H-atoms.
Solvents: CDCl
3is an ideal solvent.
Other solvents like CS
2, CCl
4, pyridine, trifluroacetic acid,
DMSO, C
6D
6may also be used.
Dimethyl sulfoxide(DMSO) is an organosulfur compound with the formula (CH
3)
2SO.
Sample: 10-50 mg of the sample is dissolved in proper
solvent (0.5 ml)
69
Q.1) Give the choice of solvent in NMR spectroscopy?

70. Tetramethylsilane(TMS) as an Internal Standard:
70
Q.1) Why is TMS used as an internal standard in NMR
spectral determination? (W-13 & S-14, 3 Mark)
Q.2) Explain why TMS is used as internal reference
standard in NMR spectra?
Q.3) Give the advantages of TMS as internal reference
standard in NMR spectra.
Q.4) Which substance is taken as a standard for
recording chemical shift? (S-14, ½ Mark)
(a) Dimethylsilane (b) Methylsilane
(c) Trimethylsilane (d) Tetramethylsilane
Q.5) Why is TMS selected as an internal standard
reference in NMR spectroscopy? (S-18, 4 Mark)
Q.6) Tetramethylsilane(TMS)is the internal standard
used in NMR spectroscopy. (W-19, ½ Mark)

71. Tetramethyl silane (TMS) as
an Internal Standard:
1
It contains 12
equivalent protons.
It is symmetrical
gives a single strong
and sharp
absorption peak for
all 12 equivalent
protons.
2
The silicon atom has very
low electronegativity.
So, the protons are highly
shielded by the induced
magnetic field of
circulating electron.
So TMS absorbs at unique
highest magnetic field
strength, where other
protons do not absorbs.
3
It minimizes
the effect of
solvent on
chemical
shift.
Tetramethyl silane (TMS) is used as reference (standard) compound in
measurement of position of other absorption peaks in NMR spectra.
Because

72. Tetramethyl silane (TMS) as
an Internal Standard:
4
It is
chemically
inert and
does not
interact with
the sample
5
It is volatile
(boiling point
= 27
o
C).
Hence the
recovery of
sample (organic
compound) is
possible.
6
It is soluble
in most of
the organic
solvent.
Tetramethyl silane (TMS) is used as reference (standard) compound in measurement of
position of other absorption peaks in NMR spectra. Because
Cyclohexane, dioxanecan also be used as internal reference compound.

73. Do you know?
Chemical Shift

74. 74

75. Chemical Shift:
75
Position of Signals -Shielding and Deshielding:
Chemical Shift:
Q.1) Explain the term: Chemical shift with suitable
examples. (S-10, S-13, W-16, S-17 & S-19, 1-3 Mark)
Q.2) What is chemical shift? What is its unit? Write the
approximate chemical shift values for
(i) aromatic protons &
(ii) aldehydic proton. (W-14, 4 Mark)
Q.3) What is Chemical shift? Explain the cause of
Chemical shift? How it is calculated?
Q.4) Define and explain: Chemical shift. Name various
scale for measurement of Chemical shift.
Q.5) Define the term Chemical shift.(S-16, 1 Mark)
Q.6) What is chemical shift? (W-17, 1 Mark)

76. 76
Position of signals (chemical shift):
What types of hydrogens?
Position of Signals –
Shielding and Deshielding:
Chemical Shift:
Defination:
TherelativepositionofNMRabsorptionsignalofa
setofequivalentprotonsfromreferencecompoundlike
TMS(tetramethylsilane)iscalledasChemicalshift.

77. 77
Position of signals (chemical shift):
What types of hydrogens?
Defination:
Thepositionofthesignalintermsofδ-valuefrom
TMSiscalledasChemicalshift.
OR
Thedifferencebetweenthepositionoftheabsorption
band(signal)foragiventypeofhydrogenatomandthe
positionofthepeakforTMSiscalledchemicalshiftfor
thathydrogen.
TheequivalentprotonshavesameChemicalshift.
Thenon-equivalentprotonshavedifferentChemicalshift.
Theunitinwhichchemicalshiftismostconvenientlyexpressedispartspermillion(ppm)

78. 78
•ThechemicalshiftofthexaxisgivesthepositionofanNMR
signal,measuredinppm,accordingtothefollowingequation:
1
HNMR—TheSpectrum-ChemicalShift

79. 79
The chemical shift in terms of δ value is given by,
X 10
6
; ppm
Where, ʋ
sampleis frequency of absorption of set of equivalent proton in the sample.
ʋ
TMSis frequency of absorption of TMS.
ʋ
instrumentis input frequency of spectrophotometer.
On δ scale, the δ value for TMS is taken as 0 (Zero) ppm.
At constant
Applied field strength

80. Factors affecting Chemical Shift:
Electronegative group
Or Electron attracting group:1
Anisotropic Effect2
Electron Donating Group3

81. 1) Electronegative group or electron attracting group:
ElectronegativegroupsattachedtotheC-Hsystem
decreasetheelectrondensityaroundtheprotons,and
thereislessshielding(i.e.,moredeshielding)and
chemicalshiftincreases.
81

82. EN increases:
82

83. 2) Anisotropic Effect:
The word “anisotropic” means “non-uniform”.
So, magnetic anisotropy means that there is a
“non-uniform magnetic field”.
Electrons in π-systems (e.g. Aromatics, alkenes,
alkynes, carbonyls, etc.) interact with the applied
field which induces a magnetic field that causes
the anisotropy.
It causes both shielding and deshielding of
protons.
83

84. DeshieldingEffect:
(Circulation of π-electrons):
Circulationofπ-electronsaroundtheproton
(nucleus)itselfgeneratesasecondarymagneticfield
(Inducedmagneticfield)thatcaneitheropposeor
reinforce(stronger)theappliedmagneticfieldatthe
proton,dependingontheprotonslocation.
•If the induced magnetic field opposes (decreases) the applied magnetic
field (H
oor B
o), proton is said to be shielded.
•If the induced field reinforces (stronger) the
applied magnetic field, then the field felt by the
proton is more and such a proton is said to be
deshielded,

85. DeshieldingEffect:
(Circulation of π-electrons):
Paramagnetic Region
Circulation
of
Π-electrons
Diamagnetic Region

86. Example of Deshielding: Benzene
86
Where induced magnetic field reinforces (to make strong)
the applied magnetic field (H
oor B
o), in the surrounding
protons & protons are called as deshielded.

87. Other examples of Deshielding:
87

88. 88
So, deshieldedproton requires lower external applied magnetic
field strength (H
oor B
o) to bring the proton in resonance and to
observed the absorption of radio-frequency (Rf) signal (spectrum)
on the down field (Low field) region.

89. 89

90. 90
Whereasinducedmagneticfieldopposes(decreases)theapplied
magneticfield(H
o
orB
o
),thentheprotonsarecalledasshielded:
Acetyleneprotonspresentinshieldedzoneandhencehavelowδvalue.
Becauseacetylenicprotonliesalongtheaxiswheretheinducedmagnetic
fieldgenerates,whichopposed(decreases)theexternalmagneticfield.

91. 91

92. 92

93. 93
3) Electron Donating Group
Effect of Alkyl Group Substituents:

94. Do you know?
Splitting of a Signal or Splitting pattern

95. 95

96. Splitting of a Signal or Splitting pattern:
96
Q.1) Define and explain:
(i) Spin-spin coupling &
(ii) Spin-spin splitting.
Q.2) Explain with suitable examples:
Spin-Spin splitting. (W-12 & W-16, 3 Mark)
Q.3) Explain the term Spin-spin coupling with suitable
example. (S-17 & S-19, 2 Mark)
Q.4) Give the ideal relative intensities ratio for: (S-18, 4 Mark)
(i) a triplet (ii) a quartet (iii) a quintet (iv) a doublet
Q.5) Explain the term Spin-spin splitting. (W-18, 2 Mark)

97. LOGO
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Splitting of a Signal:
Spin-Spin Coupling
The small magnetic
interaction occurring
between
spin states of
neighboring
non-equivalent protons
at high resolution is
called as
spin-spin coupling
Splitting
of a Signal
How many neighboring hydrogens.
Spin-Spin Splitting
The splitting of broad NMR
signal of a set of equivalent
protons at low resolution
into multiplets
due to
spin-spin coupling
with the neighboring
non-equivalent proton
at high resolution
is called as
spin-spin splitting.
The source of signal splitting is a phenomenon called spin-spin coupling, a term that describes
the magnetic interactions between neighboring, non-equivalent NMR-active nuclei.

98. Difference between chemical shift & spin -spin splitting
/coupling constant:
S.No.ChemicalShift Spin-spinsplitting/Couplingconstant(J)
1.
The relative positions of
NMR absorption peakof set of
equivalent protons of sample
fromTMSare called as chemical
shift.
Thesplittingoftheabsorptionpeak
(signal)intomultipletsduetospin-spin
couplingiscalledasspin-spinsplitting.
Thedistancebetweentwoadjacentpeaks
inamultipetsiscalledascoupling
constant.
2.
It occurs due to shieldingor
deshieldingof protons by
circulating electrons from
magnetic field.
Itoccursduetospin-spincoupling
(spininteractionbetweenadjacentnon-
equivalentprotons)
3.Itdependsuponmagneticfield.Itisindependentofmagneticfield.
4.It is primaryenvironmental effect.
Itissecondaryenvironmentaleffectof
neighboringnon-equivalentprotons.
98

99. LOGO
Rules of spin-spin splitting:
Rule No.-1)

100. LOGO
Area Ratios: Pascal’s Triangle:
100

101. LOGO
Splitting of a Signal:
Spin spin splitting for one adjacent non equivalent proton:
Spin-spin splitting for two adjacent (-CH
2-) non equivalent proton:
Spin-spin splitting for three adjacent non equivalent protons:
101CH
3
Signal from
uncoupled
proton
2 spin states
of CH protons
H
0
Coupling with one H, split
the signal into dooublet (1:1) Signal of
uncoupled
proton
3 spin states
of CH
2 protons
H
0
Coupling with two H, split
the signal into triplet (1:2:1)
,
Splitting of absorption
signal of CH
3 proton
into triplet
Splitting of absorption signals of CH
3 proton
into triplet due to spin coupling of-CH
2- proton Signal of
uncoupled
proton
4 spin states
of CH
3 protons
H
0
Coupling with three H, split
the signal into quartet (1:3:3:1)
,,
,,
Splitting of
absorption
signal of CH
2
proton
into quartet
Splitting of absorption signals of CH
2 proton
into quartet due to spin coupling of CH
3- proton

102. LOGO
Some examples:
Signals at Low & High Resolution:
102

103. LOGO
Spin-Spin Splitting:
103
Explanation with examples:

104. LOGO
Rules of spin-spin splitting:
Rule No.-2): (n
a +1).(n
c +1) rule:
104
Q.1) How many peaks observed in high resolution NMR for 1-bromopropane
(CH
3-CH
2-CH
2-Br)? (S-16, 4 Mark)

105. Explanation of PMR spectrum of Ethanol:
105
In presence of small traces of acid or alkali, the proton of –OH is fast exchanging and
hence single peak is observed even in high resolution spectra.

106. LOGO
Values of Chemical shifts:
106

107. LOGO
Signals appears at different δ –Values:
107

108. Do you know?
Relative areas under signals (integration):

109. LOGO
Relative areas under signals (integration):
109
How many hydrogens of each type.
peak height 1 mm = 1H

110. LOGO
Relative areas under signals (integration):
110
How many hydrogens of each type.

111. Do you know?
Coupling constant orSplitting constant (J):

112. LOGO
Define & explain: Coupling constant.
Coupling constant, J(J value):
The distance (in Hz) between adjacent peaks in
a multiplet is called as Coupling constantor
splitting constant.
OR
The spacing between the two adjacent peaks of
a multiplet is called as coupling constant
between the two protons
112
Q.1) Explain the term: Coupling constant. (S-10 & W-18, 1-2 Mark)
Q.2) What is Coupling constant? (S-18, 1 Mark)
Q.3) Define& explain: Coupling constant.

113. LOGO
THEEND!