Infrared spectroscopy analyzes the absorption of infrared radiation by molecules. When the frequency of infrared radiation matches the natural vibrational frequency of bonds in a molecule, absorption occurs. Different functional groups absorb characteristic frequencies allowing infrared spectroscopy to determine a molecule's structure. Molecular vibrations include stretching and bending motions that change the dipole moment. Factors like mass, bond strength, and geometry affect vibrational frequencies.

1. INFRARED SPECTROSCOPY
THEORY
PRESENTED BY: ANUPAMA RAMACHANDRAN
1st Year M Pharm Pharmacology
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2. CONTENTS
• INTRODUCTION
• IR SPECTROSCOPY
• PRINCIPLE
• MOLECULAR VIBRATIONS
• COUPLED INTERACTIONS
• DEGREE OF FREEDOM
• SUMMARY
• REFERENCE
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3. INTRODUCTION
• Spectroscopy is the measurement and interpretation of EMR absorbed or emitted
when the molecules or atoms or ions of a sample moves from one energy state to
another energy state.
• Spectroscopy is an instrumentally aided study of the interactions between matter
(sample being analyzed) and energy (any portion of the electromagnetic
spectrum)
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4. SPECTROSCOPY
ABSORPTION SPECTROSCOPY EMISSION SPECTROSCOPY
*UV SPECTROSCOPY * FLUORIMETRY
*IR SPECTROSCOPY * FLAME PHOTOMETRY
*NMR SPECTROSCOPY
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5. IR SPECTROSCOPY
• Energy of molecule = Electronic energy+ Vibrational energy + Rotational
energy
• IR spectroscopy is concerned with the study of absorption of infrared
radiation, which causes vibrational transition in the molecule. Hence, IR
spectroscopy also known as Vibrational spectroscopy.
• IR spectra mainly used in structure elucidation to determine the functional
groups.
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6. 6

7. IR REGION: 0.8 µm (800nm) to 1000 µm (1mm)
subdivided into
1.Near IR: 0.8-2 µm
2.Middle IR: 2-15 µm
3.Far IR: 15-1000 µm
Most of the analytical applications are confined to the middle IR region
because absorption of organic molecules are high in this region.
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8. • GROUP FREQUENCY REGION:( 4000 to 1500 cm-1)
Contain relatively few peaks.
Associated with stretching vibrations.
Almost all the stretching vibrations are above 1450 cm-1.
IR stretching frequencies
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9. FINGER PRINT REGION: (1500 – 400 cm-1 )
• Usually consist of bending vibrations within the molecule.
• pattern of peaks are more complicated & is much difficult pickout
individual bonds in this region
• Each compound produce its own unique pattern of peaks like a finger
print in this region .
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10. • In pharmaceutical analysis, infrared radiation (mid IR) of wavelength
25µ to 2.5µ or wave numbers from 400cm-1 to 4000 cm-1.
• we use wave no. instead of wave length for mentioning characteristic
peak ,because wave numbers are larger values & easy to handle than
wave lengths.
• Wave no: no. of waves present per cm
•
1
𝑤𝑎𝑣𝑒𝑙𝑒𝑛𝑔𝑡ℎ 𝑖𝑛 𝜇
× 104
= wave no. per cm
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11. • Molecules are made up of atoms linked by chemical bonds. The
movement of atoms and the chemical bonds is like spring and balls
(vibration).
• This characteristic vibration are called Natural frequency of vibration.
PRINCIPLE OF IR SPECTROSCOPY
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12. • When energy in the form of infrared radiation is applied ,then it
causes vibration between the atoms of the molecules and when,
Applied infrared frequency = Natural frequency of vibration
Then, Absorption of IR radiation takes place and a peak is observed.
Different functional groups absorb characteristic frequencies
of IR radiation. Hence gives the characteristic peak value. Therefore, IR
spectrum of a chemical substance is a finger print of a molecule for its
identification.
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13. CRITERIA FOR A COMPOUND TO ABSORB IR
RADIATION
• 1. Correct wavelength of radiation
• 2. Change in dipole moment
1. Correct wavelength of radiation:
A molecule to absorb IR radiation, the natural frequency of vibrations
of some part of a molecule is the same as the frequency of incident
radiation.
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14. 2. Change in dipole moment
• A molecule can only absorb IR radiation when its absorption cause a
change in its electric dipole
• A molecule is said to have an electric dipole when there is a slight
positive and a slight negative charge on its component of atoms.
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15. MOLECULAR VIBRATIONS
• Any change in shape of the
molecule.
2 types,
1) fundamental vibrations
2) non fundamental vibrations
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16. FUNDAMENTAL VIBRATIONS
There are 2 types of vibrations:
1.Stretching vibrations
2.Bending vibrations
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17. 1.STRETCHING VIBRATIONS:
 Vibration or oscillation along the line of bond
Change in bond length
 Occurs at higher energy: 4000-1250 cm-1
2 types:
a)Symmetrical stretching
b)Asymmetrical stretching
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18. a) Symmetrical stretching:
• 2 bonds increase or decrease in length simultaneously.
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19. Symmetrical stretching
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20. b) Asymmetrical stretching:
• in this, one bond length is increased and other is decreased
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21. Asymmetrical stretching
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22. 2. BENDING VIBRATIONS
 Vibration or oscillation not along the line of bond
 These are also called as deformations
 In this, bond angle is altered
 Occurs at low energy: 1400-666 cm-1
 2 types:
a) In plane bending: scissoring, rocking
b) Out plane bending: wagging, twisting
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23. a) In plane bending
i. Scissoring:
• This is an in plane bending
• 2 atoms approach each other
• Bond angles are decreased
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24. Scissoring
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25. ii. Rocking:
• Movement of atoms take place in the same direction.
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26. Rocking
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27. b) Out plane bending
i. Wagging:
• 2 atoms move to one side of the plane.
• They move up and down the plane.
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28. Wagging
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29. ii. Twisting:
• One atom moves above the plane and another atom moves below
the plane.
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30. Twisting
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31. COUPLED INTERACTIONS
• Interactions between vibrations can occur (Coupling) if the vibrating
bonds are joined to a single, central atom.
• This is because there is mechanical coupling interaction between the
oscillators.
Example: C=O (both symmetric and asymmetric stretching vibrations)
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32. REQUIREMENTS FOR COUPLING
• The vibrations must be of the same symmetry species if interaction is
to occur.
• Strong coupling of stretching vibrations occurs 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.
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33. • Coupling is greatest when the coupled groups have approximately
equal energies.
• Coupling between a stretching vibration and a bending vibration
occurs if the stretching bond is one side of an angle varied by
bending vibration.
• No coupling is seen between groups separated by two or more bonds
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35. FACTORS AFFECTING FREQUENCY OF IR
ABSORPTION
• Relative mass of the atom.
• Force constant of the bonds.
• Geometry of the atom.
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36. DEGREE OF FREEDOM
• Fundamental vibration of molecule depend on degree of freedom.
• Each atom has 3 degree of freedom depend on x, y, z.
• For a molecule containing n no. of atoms has 3n degree of freedom.
• For non – linear molecule, 3 degree of freedom represent rotational &
transational motion.
• For non-linear, ( 3n -6) degree of freedom represent fudamental vibrations.
• For linear (3n-5) degree of freedom represent fundamental vibrations.
• All vibrational changes don’t appear as band.
• Only those vibrational changes that result in change in dipole moment
appear as band.
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37. TYPICAL INFRARED SPECTRUM
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39. SUMMARY
• Absorption spectroscopy.
• Vibrational transitions.
• Applied infrared frequency = Natural frequency of vibration.
• Structure elucidation to determine functional groups.
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40. REFERENCES
1. Spectrometric identification of organic compounds, sixth edition,
Robert M. Silverstein & Francis X. Webster. Page no: 71 – 76
2. Principles of instrumental analysis, 6th edition, Douglas A. Skoog , E
James Holler, Stanley R Crouch . Page no: 430 – 455
3. Introduction to spectroscopy, Donald L Pavia, Gary M. Lampman,
James R. Vyvyan. Page no: 15 - 31
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41. Thank you…
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