This document provides an introduction to NMR spectroscopy and its principles. It discusses the two main types of NMR - proton (1H NMR) and carbon-13 (13C NMR) spectroscopy. It covers the interpretation of 1H NMR spectra, including number of peaks, intensity of peaks, chemical shift, spin-spin splitting/multiplicity, and coupling constants. Interpretation of 13C NMR spectra is also discussed, including chemical shifts and spin-spin splitting. Examples of spectra are provided to illustrate these concepts. The document concludes that NMR spectroscopy is an effective tool for determining molecular structure.

1. Presented By: M.P. Harshita (Y13MPH440),
I/II-M.Pharmacy,
Dept. Of Pharmaceutical Analysis
Chalapathi Institute Of Pharmaceutical Sciences, Lam,
Guntur.
1

2. Contents
 Introduction
 Theory behind NMR
 Types of NMR
 Interpretation of proton NMR
 Interpretation of carbon-13 NMR
 Conclusion
 References
2

3. Introduction to NMR Spectroscopy
 Nuclear magnetic
resonance
spectroscopy is a
powerful analytical
technique used to
characterize
organic molecules
by identifying
carbon-hydrogen
frameworks
within molecules.
changes occur at nuclear level
magnetic field is applied along
with EMR
when a correct combination of
magnetic field and EMR is applied
resonance or flipping of
nucleus(proton) occurs
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4. Principle behind NMR
 Any charged particle has a tendency to spin.
 A spinning charged particle develops a magnetic field around
it.(behaves like a tiny bar magnet)
 The spinning particle can exist in 2 different spin states of
equal energy but in the presence of external magnetic field
one spin state is of lower energy(G.S) and the other is of high
energy(E.S).
 When the radio waves of exactly this energy difference are
supplied, the proton flips (resonance) giving the NMR signal.
4

5. Contd...
5

6. Types of NMR Spectroscopy
 Two common types of NMR spectroscopy are used
to study the structure of an organic molecule:
 1H NMR - used to determine the type and number
of H atoms in a molecule.
 13C NMR - used to determine the carbon skeleton
of the molecule
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7. Interpretation of 1H NMR spectrum
• No of different types of protons
Number of
peaks
• Ratio of each type of proton
Intensity of
peaks
• Type of proton and its chemical
environment
Chemical shift
• No of protons on adjacent
carbon
Splitting
/multiplicity
• characteristic for each type of
proton
Coupling
constant
7

8. Number Of Peaks : No of different types of protons
8

9. 9
All the hydrogens of ethane are
equivalent hence single signal is
given

10. Intensity Of Peaks: ratio of number of each type of protons
10

11. Some More Examples:
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12. Benzyl Acetate : (55:22:33) = 5:2:3
12

13. Chemical Shift δ
 The shifts in the position of NMR signals
(compared with a reference standard- TMS)
resulting from the shielding and deshielding of
electrons is referred as chemical shift.
13
δ
ppm

14. The concept of Shielding and Deshielding
14

15. Depending on their chemical environment, protons
in a molecule are shielded by different amounts
15

16. Factors Effecting Chemical Shift (Shielding And
Deshielding)
Electronegativity Hybridization
Hydrogen
Bonding
Magnetic
Anisotropy
16

17. Electronegativity
 Electronegative substituents due to their electron
withdrawing effect reduces the density of electrons
around the proton attached to that carbon.
17

18. Ethyl Iodide: Effect Of Electronegativity
18
CH2I
CH3

19. •Greater the electronegativity of the substituent, the
more it deshields the proton.
 Multiple substituents have stronger effect than
single substituent.
19
compound CH3I CH3Br CH3Cl CH3F
Chemical
shift
2.16 2.65 3.10 4.26

20. 1,1,2-Trichloroethane: multiple substituent effect
20
Multiple substituent
single substituent

21. Primary And Secondary Hydrogens
21
10 10
20
10
20

22. TMS – Internal Standard
22

23. Hybridization
 S orbital holds the electrons more closer to the nucleus
than the p orbital, this results in lesser shielding of
proton in case of Sp2 hybridization. Thus vinyl
hydrogens have greater chemical shift than aliphatic
hydrogens.
 The anomalous chemical shift in alkynes is
because of magnetic anisotropy.
23
Hybridization Sp3 Sp2 sp
% s- character 25 33.3 50
Chemical Shift 0.1-4.0 4.5-7.0 2.0-3.0

24. Effect of hybridization : 2-Methyl-1-pentene
24
Sp2
Sp3
Sp3
Sp3
Sp3

25. Magnetic Anisotropy
 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.
 In case of benzene like aromatic systems it causes
strong deshielding effect and in sp hybridized
systems like acetylene it causes shielding effect.
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26. Anisotropy in benzene like systems
 Due to the
presence of
hydrogens in the
region where the
induced field
reinforces the
applied field,
there is an
appreciable
downfield shift.
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27. Anisotropy in sp hybridized systems like acetylene
 Due to the
presence of
hydrogens in
the region
where the
induced field
opposes the
applied field,
there is an
upfield shift
than in benzene
and other Sp2
hybridized
systems
27

28. Effect Of Anisotropy: Benzene And Pentyne
28

29. Hydrogen Bonding
 Protons that are
involved in hydrogen
bonding typically
change the chemical
shift values.
 more the hydrogen
bonding, more the
proton is deshielded
and chemical shift
value is higher or
down field.
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30. Simplified correlation chart for proton chemical
shift values
30

31. Spin-Spin Splitting/Multiplicity/Coupling (n+1)
rule
 Each type of proton “senses” the number of protons on
the adjacent carbon.
 If there were “n” protons on the adjacent carbon, the
signal of this proton is split into “n+1” signals.
 Through this splitting phenomenon number of protons
on the adjacent carbon can be estimated.
31

32. 32
Examples

33. 33
Examples

34. 34

35. Splitting Phenomenon:
35
A
B
A
B
1,1,2-Trichloroethane

36. Ethyl Iodide
36
A
A
B
B

37. 2-Nitropropane
37
A
A
A
B
B

38. Theory Behind Spin- Spin Splitting
38
Though the applied magnetic
field is constant, due to the
different local magnetic fields
caused by the neighboring
protons, the magnetic field is
felt differently by the proton
leading to splitting of signal.

39. Pascal’s Triangle
 Gives the number
of peaks and the
ratio of each peak
in a multiplet.
 The difference in
the peak areas in a
multiplet is due to
the difference in
the probability of
spins of adjacent
protons.
39

40. 40
Coupling Constant (J)
•The distance between the center of the two adjacent peaks
in a multiplet is usually constant and is called the coupling
constant.
•It is independent of the external field and specific for each
type of proton attached to different carbons and functional
groups.
•It is measured in Hertz(Hz) or in cps (cycles per second).
δ δ

41. Interpretation of 13C NMR spectrum
 Carbon NMR provides information about the carbon
skeleton of the molecule. The basic principles involved in
the interpretation of proton NMR are applicable to carbon
NMR also.
 12C the most abundant isotope of carbon is NMR
inactive.(spin zero) but 13C has odd mass number and
hence spin.
 The resonance signals of 13C are difficult to observe than
1H, their signal is 6000 times weaker than 1H, because of its
low natural abundance(only 1.08% of all the carbons is in
the form of 13C) Majority of the molecules doesn’t contain
13C.
 Even if a molecule contains 13C, it is unlikely that the same
molecule contains another 13C. No single molecule can
provide the complete spectrum, it is a combination of a
greater number of individual scans.
41

42. 13C Chemical shifts
42

43. Some Important 13C Chemical shifts
43

44. Chart For Carbonyl And Nitrile Functional groups
44

45. Reason for greater chemical shift in 13C NMR
 Electronegativity, hybridization, anisotropy… all effect 13C
Chemical shifts in the similar fashion as they effect 1H
chemical shifts.
 The shift is greater for a 13C atom than for proton since the
electronegative atom is directly attached to carbon and the
effect occurs through single bond C-X.(more deshielding)
 With protons the electronegative atom is attached to carbon
not hydrogen. The effect occurs through 2 bonds H-C-X
rather than one(less deshielding) so the chemical shift is
smaller than that of 1H.
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46. Spin-Spin Splitting of Carbon-13 signals
46
(Hetero nuclear splitting of signals in 13C NMR)

47. Proton Coupled And Decoupled 13C NMR Spectra:
47
Ethyl phenylacetate

48. Some Sample Spectra
48
Proton decoupled 13C NMR spectrum of 2,2-Dimethylbutane:

49. Proton decoupled 13C NMR spectrum of Cyclohexanol:
49

50. Proton decoupled 13C NMR spectrum of Cyclohexene:
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51. Proton decoupled 13C NMR spectrum of Cyclohexanone:
51

52. Off- Resonance Decoupling:
 The decoupling technique has an advantage that all
peaks become singlet, but has a disadvantage of
loosing the valuable information regarding the
protons.
 Off-resonance decoupling can often provide multiplet
information while keeping the spectrum relatively
simple in appearance.
 In an Off-resonance-decoupled 13C spectrum, the
effect of protons directly attached to carbon is
retained, but the couplings between carbon and more
remote hydrogens is eliminated.
52

53. Off- Resonance Decoupled 13C NMR spectrum of 1-Propanol:
53

54. Conclusion
NMR spectroscopy
is an effective tool
in determining the
information
regarding the
carbon skeleton
and proton
environment of a
molecule.
Using it together
with Infrared
spectroscopy, the
complete structure
of the molecule can
often be
established.
It has wide range of
applications in
pharmaceutical
and health areas.
54

55. References:
 Elementary organic spectroscopy by Y.R SHARMA
page no 182-206.
 Spectroscopy by PAVIA,LAMPMAN,KRIZ
Page no .102-304
 Instrumental methods of chemical analysis by
GURDEEP R.CHATWAL page. No 619-640.
 Organic spectroscopy by WILLIAM KEMP page no
102-140
 Spectroscopy Of Organic Compounds , P.S. KALSI,
6th Edition,pg.no:193-196.
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