Nmr spectroscopy by dr. pramod r. padole

NMR Spectroscopy
By
Dr. P. R. Padole
Department of Chemistry
Shri Shivaji Science College, Amravati.

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.

Contents
Introduction1
Principles of NMR2
NMR INSTRUMENTATION3
Chemical Equivalence4
Splitting of Signal5
Problems6

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

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 1H and 13C.
 Frequency in the range of 60MHz - 500MHz.
13C -NMR1H -NMR

The study of absorption of
radio frequency radiation
by nuclei
in presence of magnetic field
is called
Nuclear Magnetic Resonance.
Defination of 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.

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.

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"

Continuation of NMR History
2002 Chemistry Wüthrich (ETH)
2003 Medicine Lauterbur (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
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 13C is NMR active while 12C 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

Spinning of Proton:
ʋ α Ho (i.e, Precessional frequency is directly proportional to Strength of external magnetic field)
Precessional frequency is defined as the number of revolutions per second
made by the magnetic moment vector of the nucleus around the external field.
(Ho or Bo)

Condition of the nucleus to be NMR active:
• When such spinning nucleus (proton) is placed in external magnetic field (Ho or
Bo), the number of possible orientations calculated by (2 I + 1).
• Thus, spin quantum number I of proton (Hydrogen) is ½
(or spin number, I > O) are called as magnetic nuclei and 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 ( 1H, 13C, 19F, 31P ), I = 3/2 ( 11B, 33S ) & I = 5/2 ( 17O ).
I = 1 ( 2H, 14N )
• Whereas value of I is 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 (12C, 16O and 32S)

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 ( 1H, 13C, 19F, 31P ),
I = 3/2 ( 11B, 33S ) &
I = 5/2 ( 17O ).
I = 1 ( 2H, 14N )
I = 0 (12C, 16O and 32S)

Principle of NMR:
The precessing proton will only absorb energy from RF source,
if the precessional frequency 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.

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= E2 - E1

Opposed orientation
(High energy)
Less stable
Aligned orientation
(Low energy)
More stable
other by energy
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.

Opposed orientation
(High energy)
Less stable
Aligned orientation
(Low energy)
More stable
other by energy
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 Ho or Bo).

22
The precessing proton will only absorb energy from RF source, if the
precessional frequency 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.

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
Instrumentation or Experimental Method:
(Field Sweep Method):
For PMR, the solvent should not contain H-atoms.
Solvents: CDCl3 is an ideal solvent.

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.
 Solvents: CDCl3 is an ideal solvent.
Other solvents like CS2, CCl4, pyridine, trifluroacetic acid,
DMSO, C6D6 may also be used.
Dimethyl sulfoxide (DMSO) is an organosulfur compound with the formula (CH3)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?

Protons in different environments absorb at slightly different frequencies,
so they are distinguishable by NMR.

28
NMR spectrum of CH3
_CH2-Br at Low Resolution:
NMR spectrum of CH3
_CH2-Br at High Resolution:

Information from 1H-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?

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
Equivalent and Non-Equivalent Protons:
(i) Equivalent Protons:
Defination:
The protons having same chemical environment
(identical electronic environment in a molecule) are called
as equivalent protons.
• 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
Examples of Equivalent Protons:
H3C CH3 1 Set(i)
aa
H3C CH2 2 Set(ii)
ba
CH3
a
H
CH3
HH
H3C
H
(iii)
a a
b
b
b
b
2 Set
CH3 CH2 O CH2 CH3
a b b a
(iv)
2 Set

34
Non-Equivalent Protons:
Defination:
The protons having different chemical environment
(different electronic environment in a molecule) are called
as non-equivalent protons.
• The set of non-equivalent proton gives different
absorption signals (peaks) at low resolution.
• They absorbs at different frequency or different field
strength.
• Non-equivalent proton undergo spin-spin splitting
at high resolution.
•They have different chemical shift.

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 (CH3-) group.
Set - (b): It contains two equivalent protons of methylene (-CH2-) group.
The proton of set (a) & set (b) are non-equivalent to each other.
So, at low resolution three methyl (CH3-) proton will give one
absorption signal at particular frequency or field strength.
Similarly, two methylene (-CH2-) proton will give one absorption
signal at another frequency or field strength.

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.
CH3CH2Br
CH3CH2Cl Cl
C HaHb
CH3
Enantionmers
Cl
C ZHb
CH3
Cl
C HaZ
CH3
;
Mirror image
-H / + Z

Enantiomers or Enantiomorphs or Enantiomerism:
(Mirror images)
(Enantiomers Greek word: Enantio = Opposite; Meros= Parts)
Enantiomorphs Greek word: Enantio = Opposite; Morphs= form

S limonene (lemons) R limonene (oranges)
CH3
HCCH2
CH3
CH3
H C CH2
H3C

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(Not mirror images)
(Diastereoisomers Greek word: Diastereo = different; Meros= Parts)
(Diastereoisomorphs Greek word: Diastereo = different; Morphs= forms)
Diastereoisomers
or
Diastereoisomorphs
or
Diastereoisomerism:
pramodpadole@gmail.com

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

41
To determine if protons are non-equivalent,
one may carry out a simple test:
Such pairs of protons are called diastereomers (geometric isomers) protons.
The environment of these protons is not mirror images of each other and these are said
to be non-equivalent proton.
So it give different NMR signals.
H3C
Br H
H a
b
H3C
Br H
Z
H3C
Br Z
H
I
II
Diastereoisomers
Ex
No mirror image
CH3CH(Cl)CH2CH3
CH3C(Br)=CH2

42
CH3-CH2-CH2-Cl
CH3-CH-CH2-Cl
Cl

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-1 are
nonequivalent and hence will give separate NMR signals.
H3C
Br H
H a
b
H3C
Br H
Z
H3C
Br Z
H
I
II
Diastereoisomers
Ex
No mirror image
H C H
C
Cl
ClH
CH3
b a
-Ha
-Hb
+Z
+Z
H C Z
C
Cl
ClH
CH3
Z C H
C
Cl
ClH
CH3
Diastereosisomers
1
2
3
No mirror image

45
C C 4 Set
v)
b
a
H3C
H
H
H
c
d
CH3 CH CH2 Cl
Cl
Ex.
H C H
C
Cl
ClH
CH3
: protons on C-1 carbon atom are different.
4 - Set of non-equvalent protons
a
b
c d1
2
Br CH2 CH2 Cli) ii)
a b c
a b
3 Set 2 Set
iii) iv)
OH
Cl
HbHb
HaHa
c
3 Set
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
C
CH3H3C
C
H3C CH3
CH3
CH3
a a
a
b
a
a
H3C C
CH3
Br
CH3
CH3CH2-Br
CH3CH2CH2-Br CH3CHCH3
Cl
a a
a a b
a b c a b a
CH3
CH2Cl
CH3CHCH2CH3
Br
Cl-CH2CH2CH2-Cl
b d c a b a b
a
b
c
d
e
f

Do you know?
Shielding Effect:

Shielding & Deshielding of 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)

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

52
Shielding Effect:
The generated secondary magnetic field aligned in such a way
that it opposes (decreases) the main applied (external) magnetic field
(Ho or Bo) that the proton “feels”.
Thus, the secondary magnetic field (Induced magnetic field)
experienced by the proton is less (weaker). Such proton (nucleus) is
called as shielded proton or shielding.

54
So, shielded proton requires a more (greater or higher)
external applied magnetic field strength (Ho or Bo) 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.

Do you know?
Deshielding Effect:

Deshielding Effect:
(Circulation of π-electrons):
Circulation of π-electrons around the proton
(nucleus) itself generates a secondary magnetic field
(Induced magnetic field) that can either oppose or
reinforce (stronger) the applied magnetic field at the
proton, depending on the protons location.
• If the induced magnetic field opposes (decreases) the applied magnetic
field (Ho or Bo), 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,

De-shielding Effect:
Paramagnetic
Region
Circulation
of
Π-
electrons
Diamagnetic Region

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

62
So, de-shielded proton requires lower external applied magnetic
field strength (Ho or Bo) 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
Whereas induced magnetic field opposes (decreases) the applied
magnetic field (Ho or Bo), then the protons are called as shielded:
Acetylene protons 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.

Do you know?
Why is TMS used as an internal standard
in NMR spectral determination?

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 (CH3-) group.
Set - (b): It contains two equivalent protons of methylene (-CH2-) group.
The proton of set (a) & set (b) are non-equivalent to each other.
So, at low resolution three methyl (CH3-) proton will give one
absorption signal at particular frequency or field strength.
Similarly, two methylene (-CH2-) proton will give one absorption
signal at another frequency or field strength.

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.
 Solvents: CDCl3 is an ideal solvent.
Other solvents like CS2, CCl4, pyridine, trifluroacetic acid,
DMSO, C6D6 may also be used.
Dimethyl sulfoxide (DMSO) is an organosulfur compound with the formula (CH3)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?

Tetramethyl silane (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)

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

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
= 27oC).
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, dioxane can also be used as internal reference compound.

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
Position of signals (chemical shift):
What types of hydrogens?
Position of Signals –
Shielding and Deshielding:
Chemical Shift:
Defination:
The relative position of NMR absorption signal of a
set of equivalent protons from reference compound like
TMS (tetra methyl silane) is called as Chemical shift.

77
Position of signals (chemical shift):
What types of hydrogens?
Defination:
The position of the signal interms of δ-value from
TMS is called as Chemical shift.
OR
The difference between the position of the absorption
band (signal) for a given type of hydrogen atom and the
position of the peak for TMS is called chemical shift for
that hydrogen.
The equivalent protons have same Chemical shift.
The non-equivalent protons have different Chemical shift.
The unit in which chemical shift is most conveniently expressed is parts per million (ppm)

78
• The chemical shift of the x axis gives the position of an NMR
signal, measured in ppm, according to the following equation:
1H NMR—The Spectrum- Chemical Shift

79
The chemical shift in terms of δ value is given by,
X 106 ; ppm
Where, ʋsample is frequency of absorption of set of equivalent proton in the sample.
ʋTMS is frequency of absorption of TMS.
ʋinstrument is input frequency of spectrophotometer.
On δ scale, the δ value for TMS is taken as 0 (Zero) ppm.
At constant
Applied field strength

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

1) Electronegative group or electron attracting group:
 Electronegative groups attached to the C-H system
decrease the electron density around the protons, and
there is less shielding (i.e., more deshielding) and
chemical shift increases.
81

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

Deshielding Effect:
Paramagnetic Region
Circulation
of
Π-electrons
Diamagnetic Region

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

Other examples of Deshielding:
87

88
So, deshielded proton requires lower external applied magnetic
field strength (Ho or Bo) to bring the proton in resonance and to
observed the absorption of radio-frequency (Rf) signal (spectrum)
on the down field (Low field) region.

90
Whereas induced magnetic field opposes (decreases) the applied
magnetic field (Ho or Bo), then the protons are called as shielded:
Acetylene protons 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.

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

Do you know?
Splitting of a Signal or Splitting pattern

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)

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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.

Difference between chemical shift & spin-spin splitting
/coupling constant:
S.No. Chemical Shift Spin-spin splitting / Coupling constant(J)
1.
The relative positions of
NMR absorption peak of set of
equivalent protons of sample
from TMS are called as chemical
shift.
The splitting of the absorption peak
(signal) into multiplets due to spin-spin
coupling is called as spin-spin splitting.
The distance between two adjacent peaks
in a multipets is called as coupling
constant.
2.
It occurs due to shielding or
deshielding of protons by
circulating electrons from
magnetic field.
It occurs due to spin-spin coupling
(spin interaction between adjacent non-
equivalent protons)
3. It depends upon magnetic field. It is independent of magnetic field.
4. It is primary environmental effect.
It is secondary environmental effect of
neighboring non-equivalent protons.
98

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

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Area Ratios: Pascal’s Triangle:
100

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Splitting of a Signal:
 Spin spin splitting for one adjacent non equivalent proton:
 Spin-spin splitting for two adjacent (-CH2-) non equivalent proton:
 Spin-spin splitting for three adjacent non equivalent protons:
101
CH3
Signal from
uncoupled
proton
2 spin states
of CH protons
H0
Coupling with one H, split
the signal into dooublet (1:1)
Signal of
uncoupled
proton
3 spin states
of CH2 protons
H0
Coupling with two H, split
the signal into triplet (1:2:1)
,
Splitting of absorption
signal of CH3 proton
into triplet
Splitting of absorption signals of CH3 proton
into triplet due to spin coupling of-CH2- proton
Signal of
uncoupled
proton
4 spin states
of CH3 protons
H0
Coupling with three H, split
the signal into quartet (1:3:3:1)
, ,
, ,
Splitting of
absorption
signal of CH2
proton
into quartet
Splitting of absorption signals of CH2 proton
into quartet due to spin coupling of CH3- proton

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

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Spin-Spin Splitting:
103
Explanation with examples:

LOGO
Rules of spin-spin splitting:
Rule No.-2): (na +1).(nc +1) rule:
104
Q.1) How many peaks observed in high resolution NMR for 1-bromopropane
(CH3-CH2-CH2-Br)? (S-16, 4 Mark)

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.

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Values of Chemical shifts:
106

LOGO
Signals appears at different δ – Values:
107

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

LOGO
109
How many hydrogens of each type.
peak height 1 mm = 1H

LOGO
110
How many hydrogens of each type.

Do you know?
Coupling constant or Splitting constant (J):

LOGO
Define & explain: Coupling constant.
Coupling constant, J (J value):
 The distance (in Hz) between adjacent peaks in
a multiplet is called as Coupling constant or
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.

Nmr spectroscopy by dr. pramod r. padole

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