Infrared spectroscopy analyzes the absorption of infrared radiation by molecules to determine their structure. It works by exciting the vibrational modes of molecules, which correspond to characteristic absorption frequencies. The fingerprint region between 1500-500 cm-1 is especially useful for identifying functional groups and establishing molecular identity. Infrared spectrometers contain an infrared source, sample holder, detector, and recorder. Applications include identification of functional groups, structural elucidation of drugs and polymers, and quantitative analysis.

1. INFRARED
SPECTROSCOPY
by
P. viji
M.Pharm I year
(Pharmaceutics)

2. INTRODUCTION
Infrared spectroscopy or vibrational spectroscopy is
concerned with the study of absorption of infrared
radiation, which results in vibrational transitions.
It is also known as vibrational spectroscopy.
Infrared (IR) radiation lies in the part of the
electromagnetic spectrum i.e. between the visible and
microwave regions.
Near IR region: 0.8μm to 2.5μm
Middle IR region: 2.5μm to 50μm
Far IR region : 50μm to 1000μm

3. PRINCIPLE
 Molecules are made up of atoms linked by bonds.
 The movement of atoms and the chemical bonds like
spring and balls (vibration)
 This characteristic vibration are called Natural
vibration.

4. When IR is applied then it causes the vibration between
the atoms of the molecules when,
Applied infrared frequency = Natural frequency of
vibration
Then, Absorption of IR radiation takes place and a peak
is observed.
Example : Infrared vibrations of ethanol.

5. Every bond or portion of a molecule or functional group
requires different frequency of absorption. Hence
characteristic peak is observed for every functional
group or part of the molecule.
In other words, IR spectra is nothing but a finger print
region of a molecule.

6. Fingerprint Region
Absorption band in the region 1500-500 cm‾¹.
Useful for establishing the identity of a compound.
It consists of :
 Region 1500-1350 cm‾¹ :Appearance of doublet
near 1380 cm‾¹ and 1365cm‾¹ shows the presence
of 3º butyl group.
 Region 1350-1000 cm‾¹ : All classes of compound
viz. alcohol, esters, lactones shows absorptions in
the region due to C-O stretching.
 Region below 1000 cm‾¹ : This region distinguishes
between cis and trans alkene.

7. CRITERIA FOR A COMPOUND TO
ABSORB IR RADIATION
 Dipole Moment:
The bonds in a molecule can absorb IR
radiation only when there is a change in dipole moment
due to electric field of IR radiation.
 Applied IR frequency should be equal to the natural
frequency of radiation, otherwise compounds do not
give IR peaks

8. TYPES OF VIBRATIONS

9. 1. Stretching vibrations:
Vibration along the line of bond.
Change in bond length.
2 types:
a) Symmetrical stretching
b) Asymmetrical stretching

10. a) Symmetrical stretching
 Both bonds increase or decrease in length
simultaneously.
b) Asymmetrical stretching
 In this, one bond length is increased and other is
decreased.

11. 2. Bending vibrations
 Vibration not along the line of bond.
 In this, bond angle is altered.
2 types:
a)In plane bending:
i. scissoring
ii.rocking
b)Out plane bending:
i. wagging
ii.twisting

12. a)In plane bending:
i. scissoring :
 This is an in plane blending
 Bond angles are decrease
ii. Rocking :
 Bond angle is maintained.
 Movement of atoms take place in the same
direction.

13. b) OUT PLANE BENDING
vibrations takes place outside the plane of molecule.
i. Wagging:
 both atoms move to
one side of the plane.
ii. Twisting:
 One atom moves above the plane and another atom
moves below the plane

14. 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
5. recorders

15. 1.IR radiation sources
Incandescent lamp
Nernst glower
Globar Source

16. a) Incandescent Wire Source

17. b) The Nernst Glower

19. c) The Globar source

20. 2.Monochromators
A.Prism:-
 Used as dispersive element.
 Constructed of various metal halide salts.
 Sodium chloride is most commonly prism salt used.

21. 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.

22. 3.SAMPLE CELL
 Made up of alkali halides like NaCl or KBr .
 Aqueous solvents cannot be used - they
dissolve alkali halides.
 Only organic solvents like chloroform is used.

23. Sample handling
a) Sampling of solids
 Solids run in solution
 Mull technique
 Pressed pellet technique
 Solids films

24. i.) 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
carbon tetrachloride etc.

25. ii. 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.

26. iii. Mull technique:
 In this technique a small quantity of sample is
thoroughly ground in a clean mortar until the powder is
very fine.
 After grinding, the mulling agent (mineral oil or Nujol) is
introduced in small quantities just sufficient to take up
the powder (mixture approximates the consistency of
a toothpaste).
 The mixture is then transferred to the mull plates & the
plates are squeezed together to adjust the thickness
of the sample between IR transmitting windows.
 This is then mounted in a path of IR beam and the
spectrum is run.

27. iv. Pressed pellet technique:
 In this technique a small amount of finely ground solid
sample is intimately mixed with about 100 times its
weight of powdered Potassium bromide, in a vibrating
ball mill.
 This finely ground mixture is then pressed under very
high pressure (25000 p sig) in evacuable die or
minipress to form a small pellet (about 1-2 mm thick
and 1cm in diameter).
 The resulting pellet is transparent to IR radiation and is
run as such.

29. b) Sampling of liquids
 Liquids sample can be sandwiched between two alkali
halide salt plates ( NaCl , KBr , CaF2 ) .
 The sample cell thickness is 0.01-0.05mm.

30. c) Sampling of gases
 Here the spectrum of gas can be obtained by gases
sample or low boiling i.e. volatile liquid is introduced
into a glass evacuated cell made up of NaCl.
 Very few organic compounds can be examined as
gases.
 E.g. : 1 , 4-dioxane

31. 4) DETECTOR
TYPES OF DETECTOR:
 Bolometers detector
 Thermocouple and
thermopile detector
 Pyro electric detector
 Golay cell

32. a) Thermocouple detector

35. b) Bolometer

37. c) GOLAY CELL:
 Golay cell consists of a small metal cylindrical closed
by a rigid blackened metal plate.
 Pneumatic chamber is filled with xenon gas.
 At one end of cylinder a flexible silvered diaphragm
and at the other end Infra red transmitting window is
present.

38.  When infra red radiation is passed through infrared
transmitting window the blackened plate absorbs the
heat. By this heat the xenon gas causes expand
 The resulting pressure of gas will cause deformation
of diaphragm. This motion of the diaphragm detects
how much IR radiation falls on metal plate.
 Light is made to fall on diaphragm which reflects light
on photocell

39. d) Pyroelectric Detectors
 Pyroelectric detector contain certain crystal Such as
lithium tantalate, barium titanate and triglycine sulfate
i.e. they produce temperature sensitive dipole
moments.
 To construct a pyroelectric detector , a pyroelectric
substance is placed between two electrodes .
 Two electrodes are connected to each other via a
voltmeter.
 one of which has IR transparent window .

40.  When the IR radiation falls upon a pyroelectric
substance , it absorbs energy from the radiation .

41. 5) Recorder:
 The radiant energy received by detector is converted
into measurable electrical signal and is amplified by
amplifier.
 The amplified signals are recorded and plotted.

42. TYPES OF INSTRUMENTATION
 There are 2 types of infrared spectrophotometer
characterized by the manner in which the IR
frequencies are handled.
1) dispersive type (IR)
2) Interferometric type(FTIR)

43. 1) dispersive type (IR)
 In dispersive type the infrared light is separated into
individual frequencies by dispersion, using a grating
monochromator.

44. 2) Interferometric type(FTIR)
 In interferometric type the ir frequencies are allowed to
interact to produce an interference pattern and this
pattern is then analyzed, to determine individual
frequencies and their intensities.

45. APPLICATIONS
 Identification of functional group and structural
elucidation.
 Identification of drug substances.
 Identifying impurities in drug sample.
 Study of hydrogen bonding.
 Study of polymers.
 Identify ratio of cis-trans isomers in a mixture of
compounds.
 Quantitative analysis.
 To find out difference between hydrogen bonding.