NMR spectroscopy is a technique used to determine the structure of compounds. It is based on the absorption of radiofrequency radiation by atomic nuclei placed in a strong magnetic field. The document discusses the theory, principles, instrumentation, and applications of NMR spectroscopy. It explains key concepts such as chemical shifts, spin-spin splitting, coupling constants, and how they provide information about molecular structure. In summary, the document provides an introduction to NMR spectroscopy including the basic principles, parameters, and applications of this analytical technique.

1. Presented by
Rahul Chakraborty
Student of Sem-(IV) , Chemistry
honours
Department of Chemistry
Raiganj Surendranath
Mahavidyalaya
1
NMR SPECTROSCOPY

2.  Introduction
 Theory&principleofNMR
 Chemicalshift
 Sheilding&desheildingofprotons
 Effectof Resonanceinchemical
shift
 Effectof Hydrogenbondingin
chemicalshift
 Spin-spin splitting
 Couplingconstant
 KarplusEquation& KarplusCurve
 ApplicationsofNMR
2
Content

3.  Nuclear magnetic resonance (NMR) is a spectroscopic
technique which is based on the absorption of
electromagnetic radiation in the radio frequency region
4 to 900 MHz by nuclei of the atoms.
 NMR Spectroscopy is used to determine the structure
of a compound.
3
Introduction

4.  Spin quantum number “I” is related to the atomic
number and atomic mass of the nucleus.
Atomic
number
Atomic mass
NMR
(Active/
Inactive)
Spin quantum
number (I)
Examples
Odd/Even Odd Active I= ½,3/2,5/2,... 1H1,
13C6,
19F9 etc.
Even Even Inactive I=0 12C6,16O8 etc.
Odd Even Active I=1,2,3,4,… 2H1,14N7 etc.
4
Theory of NMR

5.  If external magnetic field is applied, then total
number of possible orientation calculated by (2I+1).
 Hydrogen has spin quantum number I=½ and
possible orientation is (2×½+1=2) two +½ & -½.
5

6.  Magnetic nuclei have two type of of motions; spin and
precession. The Spining nucleus generates a magnetic
field.
6
Principles of NMR

7.  The theory behind NMR comes from the spin of a nucleus
and it generates magnetic field.
 If an external magnetic field is applied, an energy transfer
(∆E) is possible between ground state to excited state.
7

8.  When the spin returns to its ground state level, the
absorbed radiofrequency energy is emitted at the same
frequency level.
 The emitted radiofrequency signal that give the NMR
spectrum of the concerned nucleus.
8

9.  The emitted radio frequency 𝛎 is directly proportional
to the strength of the applied field.
ν=
 B0=External magnetic field experienced by proton.
 𝛾 = Gyromagnetic ratio (The ratio between the nuclear
magnetic moment and angular moment).


9

10. NMR Instrumentation
10

11.  The following solvents are normally used in which
hydrogen replaced by deuterium.
 CCl4 – Carbon tetrachloride.
 CS2 – Carbon disulfide.
 CDCl3 – Deuteriochloroform.
 D2O - Deuterium oxide.
11
Solvents used in NMr

12.  A chemical shift is defined as the difference in parts per
million (ppm) between the resonance frequency of the
observed proton and tetramethylsilane (TMS) hydrogen.
 δx = the chemical shift of proton Hx, in ppm
 ν X = the frequency of signal for proton x in Hz
 ν TMS= the frequency of signal for TMS in Hz and
 ν0= the operating frequency of the instrument in MHz

x S
0
Chemical shift
12

13. TMS is the most common reference compound in NMR, it is set
at δ=0 ppm.
13
Internal standard
Si
CH3
H3C
CH3
CH3
Tetramethylsilane
1. TMS having 12 magnetically equivalent protons
gives a sharp peak even at low concentration,
2. Inert towards most of reagents ,
3. Soluble in most of the organic solvents.

14.  High electron density around a nucleus shields the
nucleus from the external magnetic magnetic field and
the signals are upfield in the NMR spectrum.
 Lower electron density around a nucleus deshields the
nucleus from the external magnetic field and the signals
are downfield in the NMR spectrum.
14
Sheiding & Desheiding of protons

15. Effect of resonance on chemical shift
15
In case of nitrobenzene, due
to –R effect of –NO2 group
both the o-positions and p-
position become electron
deficient i.e, desheilded. The
protons at o-positions(Hc)
and p-position (Ha) resonate
higher δ values compared to
m-Hb .

16. N
O
O
N
O
O
N
O
O
etc.
canonical forms showing position charge at o- and p-positions
i.e, these positions are deshielded
16

17. 17
Similarly,the +R effect of –OCH3
group in anisole increases the
electron density at both the ortho
and para positions conseqently
these positions become more
sheilded.Thus the ortho-H and
para-H resonate at lower ẟ values
(upfield) compared to Hs of
benzene (ẟ =7.37).
OCH3
H
H
H
H
H





18. 18
canonical forms showing negative charge at o- and p-positions
i.e, these positions are shielded
OCH3 OCH3
etc.
OCH3

19. H-bonding effect on chemical shift
 The chemical shift depends on how much hydrogen
bonding is taking place (observed in high conc).
 Hydrogen bonding lengthens the O-H and reduces
the valance electron density around proton.
 It is deshielded and shifted downfield in the NMR
spectrum.
 Alcohols vary in chemical shift
from 0.5 ppm (free OH) to about
5.0 ppm (lots of H-bonding).
O H
R
O
R
H O R
H
19

20.  The interaction between the spins of neighboring
nuclei in a molecule may cause the splitting of NMR
spectrum.This is known as spin-spin coupling or
splitting.
 The number of peaks in a PMR signals for a
particular set of protons depends on the number of
equivalent protons (n) in the neighbouring C-atoms.
20
Spin-spin splitting

21.  The splitting of a signal can be predicted by (n+1) rule.
 Zero H atom as neighbour n+1=0+1=1(singlet)
 One H atom as neighbour n+1=1+1=2(doublet)
 Two H atom as neighbour n+1=2+1=3(triplet)
21

22. 22
C C
H
C
no of peaks=(n+1)=(0+1)
=1(singlet)

23. 23
C C
H
H
no of peaks=(n+1)=(1+1)
=2(doublet)

24. 24
C C
H
H
H
no of peaks=(n+1)=(2+1)
=3(triplet)

25. 25

26. 26
C
H
H
C
H
C
H
H
A A chemically equivalent H's
No of lines = (nA+1)
= 4+1= 5 (quintet)

27. 27
C C C
H
H C
R
C
H
H
H
R
H
A B C
Chemically non equivalent H's
no of peaks=(nA+1)(nB+1)(nC+1)
=(2+1)(1+1)(2+1)
=18 (multiplet)

28. 28
Example Of Spin-spin splitting
CH3
C
H2
H3C
O
2.09
2.49
1.06
0
1
2
PPM
Singlet
Quartet
Triplet
TMS
2-Butanone

29. 29

30. N+
O
O-
4.41
1.95
0.96
0
1
2
3
4
PPM
H3C
H2
C
H2
C NO2
a
b
c
Ewg(-NO2) is present
deshelding of proton Ha
value is more.
a
b
c
Triplet
Multiplet
Triplet
1-Nitropropane
TMS
30

31.  The distance between the peaks of a doublet or triplet
or quartet is called the Coupling constant (J).
 The coupling constant denoted by J .
 Coupling constant are measure of the effectiveness of
spin-spin coupling and very useful in 1H NMR of a
complex structures.
31
Coupling constant

32. 32

33. 33
Vicinal Coupling ConstantS: Karplus equation
 The values of vicinal coupling constant (3JH-C-C-H)
varies from 0-16 Hz depending upon the dihedral
angle (ϕ). The relationship between dihedral angle
(ϕ) & vicinal coupling constants (3JH-C-C-H) is given by
the Karplus equation.
 The coupling constants gives information about the
molecular structure/geometry.

34. 3JHA-HB =8.5 cos²ϕ – 0.28 for 0°≦ϕ≦90°
3JHA-HB=9.5 cos²ϕ – 0.28 for 90°≦ϕ≦180°
34
3JHA-HB =J0 cos²ϕ – 0.28
3JHA-HB =J180 cos²ϕ – 0.28
Karplus curve
3
J
HA-HB
(Hz)→
Dihedral angle (ϕ)→

35. 35
cis coupling & trans coupling
H3C
CH3
CH3
H3C
cis-2-butene
trans-2-butene
H
H
H H
Doublet
Doublet
Trans Coupling Cis Coupling
J=12-18 Hz
J=5-12 Hz



36. H
H
Ortho coupling
=7-10 Hz
Ortho, meta & coupling
H
H
H
H ortho coupling
=7-10 Hz
meta coupling
=2-2.5 Hz
H
H
para coupling
< 0.5 Hz
36

37. 37
Applications of nmr
Detection of hydrogen bonding: intermolecular hydrogen bonding shifts
the absorption for a concerned proton downfield.
• Splitting pattern : how many neighbouring hydrogens.
Major application in chemistry is to determine the structure of
molecules.
• Medical practitioners employ magnetic resonance imaging (MRI),a
multidimensional NMR imaging technique , for diagnostic purposes.

38. 38
references
Pavia,
Introduction
to
Spectroscopy ,
(fifth edition)
Organic
Spectroscopy
by William
Kemp, (third
edition)
Organic
Molecular
Spectroscopy-
Ajay Kumar
Manna
Elementary
Organic
Spectroscopy ,
Y.R.Sharma , S
Chand

39. AKNOWLEDGEMENT
39
I would to like to express my gratitude and appreciatiation to all who
gave me this opportunity to complete this seminar.
Also, i would like to express my deep sense of gratitude to our chemistry
teacher Dr. Sujit Ghosh under whose valuable guidance, this, seminar
has been carried out.
I would like to extend my special thanks to our HOD Dr. Kamala
Bhattyacharya, Sumi Saha, Mini Ghosh, Subrata Basak & my classmates,
without their support and coordination we would not have been able to
complete this seminar.

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