This document provides an overview of infrared (IR) spectroscopy. It discusses the principle behind IR spectroscopy, the different IR regions, molecular vibrations, factors affecting vibrational frequencies, instrumentation components, sampling techniques, and applications. The key points are: - IR spectroscopy involves absorption of IR radiation which causes vibrational transitions in molecules. It is used to identify functional groups and study molecular structure. - The different IR regions are the photographic, very near IR, near IR, and far IR regions. Molecular vibrations include stretching and bending modes. - Factors like coupled vibrations, hydrogen bonding, and electronic effects influence vibrational frequencies. Instrumentation components are the radiation source, monochromators, detectors,

1. PREPARED BY:
DEVIPRIYA P V
M PHARM
DEPT OF PHARMACEUTICAL ANALYSIS

2. IR SPECTROSCOPY
• Study of absorption of infrared radiation , which
causes vibrational transition in the molecule.
• Hence, IR spectroscopy also known as
vibrational spectroscopy.
• IR radiation refers to that region of
electromagnetic spectrum which lies between
visible and microwave region.
2

3. • The photographic region : visible to 1.2 μ.
• The very near infrared region :1.2-2.5μ.
• The near infrared region :2.5-25μ.
• The far infrared region : 25 to 300-400μ.
• Energy of the molecule = electronic energy
+vibrational energy + rotational energy
3

4. PRINCIPLE
• Molecules are made up of atoms linked by
chemical bonds .
• Analogous to springs.
• Because of continuous motion of molecule they
maintain some vibrations with some frequency
• Characteristic vibration are called natural
frequency of vibration.
4

5. • When energy in the form of IR radiation is
applied and when,
Applied IR frequency= natural frequency of
vibration
• Absorption of IR radiation takes place and a
peak is observed.
5

6. Criteria for a compound to absorb IR
radiation
1. Correct wavelength of radiation
2. Change in dipole moment
6

7. Correct wavelength of radiation
• A molecule to absorb IR radiation, the natural
frequency of vibrations of some part of a
molecule is same as the frequency of incident
radiation.
7

8. Change in dipole moment
• A molecule can absorb IR radiation when its
absorption cause a change in its electric dipole.
• A molecule is said to have electric dipole when
there is a single slight positive and negative
charge on its component of atom.
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9. MODES OF MOLECULAR VIBRATIONS
STRECHING VIBRATION BENDING VIBRATION
• Involves a continuous change
in the inter atomic distance
along the axis of the bond
between two atoms
• Requires more energy , so
appear at shorter wavelength
• Characterized by a change in
the angle between two bonds
• Requires less energy , so
appear at longer wavelength
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10. Symmetrical stretching
• Two bonds increase or decrease in length
symmetrically.
Asymmetrical stretching:
• One bond length is increased and other is
decreased
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11. 11

12. In plane bending
• Change in bond angle
• Bending of bonds within the same plane
1. scissoring
▫ Bond angle decreases
2. rocking
▫ Bond angle is maintained but both bonds move
within the plane
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13. Out plane bending
• Bending takes places outside the plane of
molecule
1. wagging
▫ 2 atoms move to one side of the plane
▫ Move up and down the plane
2. Twisting
▫ One atom moves above the plane and another
atom moves below the plane
13

14. 14

15. VIBRATIONS
STRETCHING
VIBRATIONS
SYMMETRIC
VIBRATION
ASSYMETRIC
VIBRATION
BENDING
VIBRATIONS
IN PLANE
BENDING
SCISSORING ROCKING
OUT OF
PLANE
BENDING
WAGGING
TWISTING
15

16. Factors affecting vibrational
frequencies
1. Coupled vibrations
2. Fermi resonance
3. Electronic effects
4. Hydrogen bonding
16

17. Coupled vibrations
• Interactions between vibrations can occur if the
vibrating bonds are joined to a single , central
atom.
• This is because there is mechanical coupling
interaction between the oscillators
17

18. Requirements for coupling
• The vibrations must be of the same symmetry species
• Strong coupling of stretching vibration occur when there is a
common atom between the two vibrating bonds
• Coupling of bending vibrations occur when there is a common
bond between vibrating groups
• Coupling is greatest when the coupled groups have
approximately equal energies
• No coupling is seen between two groups separated by two or
more groups
18

19. Fermi resonance
• The energy of an overtone level changes to coincide
with the fundamental mode of different vibrations.
• Molecule transfer its energy from fundamental to
overtone and back again.
• Resonance pushes the two levels apart and mix their
character .
• Give rise to pair of transitions with equal frequencies
19

20. Electronic effects
• Change in absorption frequencies
• The frequency shift are due to electronic effects
which include:
1. Inductive effect
2. Mesomeric effect.
3. Field effects
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21. Hydrogen bonding
• Downward frequency shift.
• Stronger the H-bonding , greater is the
absorption shift towards lower wave number.
• Intra molecular H –bonds=sharp bands.
• Inter molecular gives broad bands
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22. INSTRUMENTATION
• Radiation source
• Monochromators
• Sample cells and sampling of substances
• Detectors
• Recorders
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23. IR RADIATION SOURCE
• Require a source of radiant energy which emit
IR radiation which must be :
▫ Continuous
▫ Stable
▫ Sufficient intensity
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24. Incandescent lamp:
• used in near IR instruments
Nernst glower:
• composed of zirconia, yttria and thoria.
• Non conducting at room temperature
• Heated b/w 1000-1800°C.
• Provide radiation of about 7100 cm-1
Globar source:
• A rod of sintered silicon carbide.
• Emit radiation of 5200 cm-1
Mercury arc:
• In far IR region
24

25. MONOCHROMATORS
• To select desired frequencies from the radiation
source and reject the radiations of other
frequencies
▫ Prism monochromator
▫ Grating monochromator
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26. SAMPLE CELLS AND SAMPLING OF
SUBSTANCES
• Sampling of solids
▫ Solids run in solutions
▫ Solid films
▫ Mull technique
▫ Pressed pellet technique
• Sampling of liquids
• Sampling of gases
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27. Solids run in solutions
• Solid sample dissolved in a non aqueous solvent
• A drop of solution is placed on an alkali metal
disc and the solution is allowed to evaporate
leaving a thin film of the solute
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28. Solid films
• For amorphous solids
• Sample is deposited on the surface of a KBr or
NaCl cell evaporation of a solution of the solid.
• Useful for rapid qualitative analysis
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29. Mull technique
• Finely ground solid sample is mixed with Nujol
• Form a thick paste
• Spread between IR transmitting windows
• Mounted in a path of IR beam and spectrum is
run
• Good for qualitative analysis
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30. Pressed pellet technique
• Finely ground solid sample is
mixed with 100 times its weight
of KBr
• Passed under very high pressure
(25000 psig)
• Forms small pellet (1-2 mm
thick and 1 cm diameter)
• KBr pellets can be stored for
long periods
• Resolution of the spectrum in
KBr is high
• Concentration can be adjusted,
used for quantitative analysis
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31. 31

32. Sampling of liquids
• Placed in rectangular cells made of NaCl, KBr or
ThBr.
• Sample thickness should be so selected that the
transmittance lies b/w 15 & 20%.
• 0.01-0.05 thickness.
32

33. Sampling of gases
• Similar to sampling of
liquids.
• The gas must not react
with the cell windows or
the reflecting surfaces
• Not commonly used(lack
of sensitivity).
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34. DETECTORS
Thermal detector Non thermal detector
1. Bolometer
2. Thermocouple and
thermopile
3. Pyro electric detector
4. Golay cell
1. Photo voltaic detector
34

35. BOLOMETER
• Electrical resistance of a metal increases
approximately 0.4% for every degree Celsius
increase in temperature.
• When IR radiation falls on the metal conductor
its temperature changes.
• The degree of change in resistance is the
measure of radiation that falls.
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36. 36

37. THERMOCOUPLE
• Electrical current will flow when two dissimilar
metal wires are connected together at both ends
and a temperature differential exists between the
two ends.
37

38. PYROELETCRIC DETECTOR
• Pyro-electric materials exhibit electrical
polarization.
• When the temperature is altered, polarization
changes and is observed as an electrical signal.
38

39. GOLAY CELL
Metal cylinder and flexible
diaphragm
Temperature increases
Gas is expanded and
diaphragm deforms
Detect as a signal
39

40. PHOTOVOLTAIC DETECTOR
IR radiation
Photovoltaic detector
Generates a small voltage
Detected as a signal
40

41. Single beam IR spectrophotometer
41

42. Double beam IR instrument
42

43. FTIR
• FT-IR stands for Fourier Transform Infra Red,
the most preferred method of infrared
spectroscopy.
• In infra red spectroscopy ,IR radiation is passed
through a sample, some of the radiation is
absorbed and some is transmitted.
• The resulting spectrum represents the molecular
absorption and transmission , creating a
molecular finger print of the sample
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44. INSRUMENTATION
44

45. APPLICATIONS
 Qualitative analysis
 Identification of substances
 Determination of molecular structure
 Studying progress of reaction
 Detection of impurities
 Isomerism in organic chemistry.
 Shape of symmetry of a molecule
 Identification of functional groups
45

46. Identification of substances
• To compare spectrums.
• No two compounds have identical IR spectrum.
• Criteria : sample and reference must be tested in
identical conditions.
• The finger print region (1200 -700 cm-1).
• Small difference in structure and constitution of
molecule result in significant changes in the
peak in this region.
46

47. 47

48. 48

49. Determination of molecular structure
• Identification is done based on position of
absorption bands in the spectrum.
• Eg: c=o at 1717 cm-1.
• Absence of band of a particular group indicates
absence of that group in the compound.
49

50. Studying progress of reaction
• Observing rate of disappearance of characteristic
absorption bands in reaction.
• Rate of increasing absorption bands in products
of a particular product.
• Eg: O-H = 3600-3650cm-1
• C=O =1680-1760cm-1
50

51. Functional group isomerism
• Isomerism shown by compounds having same
molecular formula but different functional
groups.
• Eg: CH3-O-CH3 and CH3-CH2-OH.
• OH=3500-3100cm-1
51

52. Applications in inorganic complexes
• Geometrical isomerism
• Determination of purity
• Shape of symmetry of a molecule.
• Presence of water in a sample.
• Measurement of paints and varnishes.
• Examination of oils and paintings and artifacts.
52

53. Quantitative analysis
• Determination of concentration of one of the
functional groups of the compounds being
estimated.
• Eg: concn of hexanol in hexane -hexanol
mixture.
• A=abc
• 2 methods to determine A and C
1. Cell-in cell-out method
2. Baseline method
53

54. IR SPECTRUM
 The functional group region:
• Identifies the functional group with the
consequence of changing stretching vibrations.
• Ranges from 4000-1600cm-1
 The fingerprint region:
• Identifies the exact molecule with the
consequence of changing bending vibrations.
• Ranges from 1600-625 cm-1
54

55. INTERPRETATION OF IR SPECTRA
• Structural information about compounds is
mainly derived from the presence or absence of
characteristics absorption bands of various
functional groups in the IR spectrum of the
compounds.
55

56. INTERPRETATION
56

57. HOOKE’S LAW
• The stretching frequency is related to the masses
of the atom and the force constant (a measure of
resistance of a bond to stretching) of a bond.
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58. 58

59. IR SPECTRUM OF ALKANES
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60. IR SPECTRUM OF ALKENES
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61. 61

62. IR SPECTRUM OF ALCOHOLS
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63. IR SPECTRUM OF ALDEHYDES AND
KETONES
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64. IR SPECTRUM OF CARBOXYLIC ACID
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65. IR SPECTRUM OF AMINES
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