This presentation gives you thorough knowledge about the IR Spectroscopy. This include basic principle, type of vibrations, factors influencing vibrational frequency, instrumentation and applications of IR Spectroscopy. This is the most widely used technique for identifying unknown functional group depending on the vibrational frequency.

1. TITLE : INFRARED
SPECTROSCOPY
Presented by:
RAVI KUMAR YADAV
05/MPH/DIPSAR/17
M.PHARM(CLINICAL
RESEARCH)

2. CONTENT
 Introduction
 Principle
 Type of vibrations
 Factors influencing vibrational frequency
 Instrumentation
 Application of IR spectroscopy
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3. INTRODUCTION
 Infrared spectroscopy or vibrational spectroscopy
is concerned with the study of absorption of
infrared radiation, which results in vibrational
transitions.
 Infrared radiations refers broadly to that part of
electromagnetic spectrum between visible and
microwave region.
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4. PRINCIPLE
 When the frequency of the IR radiation is equal
to the natural frequency of vibration, the
molecule absorb IR radiation and a peak is
absorbed.
 Every bond or portion of a molecule or
functional group requires different frequency for
absorption.
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5. THEORY
 IR radiation does not have enough energy to
induce electronic transitions as seen with UV.
 For a molecule to absorb IR, it must be
accompanied by a change in dipole moment.
 Regions of wavelength range
There are three regions:
1. Very near IR : Overtone region(2 - 2.5µ)
2. Near IR : Vibration region(2.5 – 25µ)
3. Far IR : Rotational region(25 – 400µ)
HOOKE'S LAW :
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6. TYPES OF VIBRATIONS
 There are different types of vibrations:
1.Stretching
i. Symmetric
ii. Asymmetric
2. Bending
i. In-plane bending
a. Scissoring
b. Rocking
ii. Out-of-plane bending
a. Wagging
b. Twisting
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8. FACTORS INFLUENCING
VIBRATIONAL FREQUENCY
1. Symmetry
Symmetric compounds do not possess dipole moment and are
IR inactive.
E.g. symmetric acetylene
2. Fermi resonance
Fermi resonance results in an unexpected shift in energy and
intensity of the bands.
E.g. the overtone of C-H deformation mode at 1400 cm‾¹ is
always in Fermi resonance with the stretch of the same band at
2800 cm‾¹.
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9. 3. Hydrogen bonding
Hydrogen bonding brings about remarkable downward
frequency shifts.
Intermolecular – broad bands
Intra-molecular – sharp bands
4. Electronic effect
Electronic effects such as inductive, mesomeric and field
effect may cause shift in absorption bands due to change in
absorption frequency.
E.g. Inductive – acetone(1715cm‾¹) and
chloroacetone(1725cm‾¹)
Mesomeric- acetophenone(1693cm‾¹) and p-amino
acetophenone(1677cm‾¹)
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10. 5.Bond angles
Difference in bond angles also lead to the changes in
absorption bands.
E.g.
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11. INSTRUMENTATION
 The main parts of IR spectrometer are as follows :
 1. IR radiation sources
 2. Monochromators
 3. Sample cells and sampling of substances
 4. Detectors
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12. 1.IR radiation sources
 The radiation source must emit IR radiation which must be
(i) intense enough for detection
(ii) steady
(iii) extend over the desired wavelengths
 The various popular sources of IR radiations are :
(i) Incandescent lamp
(ii) Nernst glower
(iii) Globar Source
(iv) Mercury Arc
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14. 2.Monochromators
A. Prism:-
 Used as dispersive element.
 Constructed of various metal halide salts.
 Sodium chloride is most commonly prism salt used.
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15. B. Grating
 Grating are nothing but rulings made on some materials
like glass, quartz or alkylhalides depending upon the
instrument. The mechanism is that diffraction produces
reinforcement. The rays which are incident upon the
gratings gets reinforced with the reflected rays.
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16. 3.Sample cell & Sampling of
substance
 Infrared spectra may be obtained for gases, liquids or
solids.
 Materials containing sample must be transparent to
the IR radiation. So, the salts like NaCl , KBr are only
used.
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17. Sample handling
 Samples of the same substance shows shift in
absorption bands as we pass from solid to gases
and hence the samples of different phases have to
be treated differently in IR spectroscopy.
 Sampling of solids
1. Solids run in solution
2. Mull technique
3. Pressed pellet technique
4. Solids films
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18. 1. Solids run in solution
Dissolve solid sample in non -aqueous solvent and place
a drop of this solution in alkali metal disc and allow to
evaporate, leaving a thin film which is then mounted
on a spectrometer.
E.g. of solvents – acetone, cyclohexane, chloroform etc.
2. Mull technique
Finely powdered sample + mulling agent (Nujol) and
make a thick paste (mull). Transfer the mull to the mull
plates and the plates are squeezed together to adjust the
thickness it is then mounted in spectrometer.
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19. 3. Pressed pellet technique
Finely powdered sample is mixed with about 100 times
its weight of KBr in a vibrating ball mill and the mixture
is then pressed under very high pressure in a die to
form a small pellet ( 1-2mm thick and 1cm in diameter).
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20. 4. Solid films
Here amorphous solid is dissolved in volatile solvents
and this solution is poured on a rock salt plate (NaCl
or KBr), then the solvent is evaporated by gentle
heating.
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21. Sampling of liquids
Liquids sample can be sandwiched between two alkali
halide plates ( NaCl , KBr , CaF2 ) .
The sample cell thickness is 0.01-0.05mm.
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22. Sampling of gases
Here gases sample is introduced into a glass cell made
up of NaCl.
Very few organic compounds can be examined as gases.
E.g. : 1 , 4-dioxane
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23. 4.Detectors
 The detectors can be classified into three categories:
1. Thermal detectors:- Their responses depend
the heating effect of radiation.
2. Pyroelectric detectors:- Pyroelectric effect
depends on the rate of change of the detector
temperature rather than on the temperature itself.
3. Photoconducting detectors:- Most sensitive.
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25. Dispersive IR instrument
 Dispersive IR instruments are introduced in 1940’s.
 Double-beam instruments are mostly used than
Single beam instrument.
 In dispersive IR sequential scanning of wave numbers
of light takes place.
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26. Fourier Transform IR Instrument
 FTIR collects all wavelengths simultaneously and at once.
 FTIR works based on Michelson Interferometer which
having
(i) Beam splitter
(ii) Fixed mirror
(iii)Movable mirror
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28. APPLICATIONS OF IR
SPECTROSCOPY
1. Identification of an organic compound
 To measure spectrums.
 No two samples will have identical IR spectrum.
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29. Fingerprint Region
 Absorption band in the region 1500-500 cm‾¹.
 Useful for establishing the identity of a compound.
It consists of :
1. Region 1500-1350 cm‾¹ : Appearance of doublet near
1380 cm‾¹ and 1365cm‾¹ shows the presence of 3º
butyl group.
2. Region 1350-1000 cm‾¹ : All classes of compound viz.
alcohol, esters, lactones shows absorptions in the region
due to C-O stretching.
3. Region below 1000 cm‾¹ : This region distinguishes
between cis and trans alkene.
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30. 2.Qualitative determination of
functional groups
 The presence or absence of absorption bands help in
predicting the presence of certain functional group
in the compound.
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31. 3.Quantitative analysis
 It can be done by measuring the intensity of the
absorption bands.
 This is done by baseline technique and is thus used to
determine the quantity of a substance.
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32. 4.Identifying the impurities in a
drug sample
 Impurities have different chemical nature when compared
to the pure drug.
 Hence these impurities give rise to additional peaks than
that of the pure drug.
 By comparing these we can identify the presence of
impurities.
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33. LIMITATION OF INFRARED
SPECTROSCOPY
 By IR spectroscopy , it is not possible to know the
molecular weight of a substance.
 It does not provide information of the relative positions
of different functional groups on a molecule.
 From a single IR spectrum of an unknown substance,
it is not possible to know whether it is a pure compound
or a mixture of compounds.
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34. REFERENCES
 Pavia, Introduction to Spectroscopy, fourth edition,
Cengage Learning, 2009.
 Chatwal G.R. , Instrumental methods of Chemical Analysis,
fifth edition, 2010.
 R.M. Silverstein and G.C. Bassler, Spectroscopic
identification of organic compounds, Wiley, New
York (1964).
 https://www.omicsonline.org/scholarly/infrared-
spectroscopy-journals-articles-ppts-list.php
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