NMR Spectroscopy

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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