IR spectroscopy is a powerful analytical technique used to identify chemical substances based on their absorption of infrared radiation. It provides information about molecular structure without decomposition. The IR region extends from 0.8-2.5 microns. IR spectroscopy works by detecting the vibrational and rotational frequencies of molecules when IR radiation is passed through a sample. The resulting absorption spectrum is unique to a compound's molecular structure and functional groups. Modern IR instruments contain an IR source, monochromator, sample cells, detectors, and recorders to produce IR spectra.

1. Under the guidance of- Presented by-
DR.P.NAGARAJU, M.pharm, Ph.D. Bhavana.G.(Y15MPh223)
Professor, I/II M.pharm
Dept. Pharm. Analysis

2. INTRODUCTION
IR Spectroscopy is one of the most powerful analytical technique which is used
for chemical identification of substances.
One of the most important advantage of IR spectroscopy over the other methods
are it provides useful information about the structure of the molecule without
evaluation.
IR is useful for determination of organic and inorganic structure. IR extends
between 0.8-2.5 microns/ 12800-4000 cm-¹.
IR REGIONS
1. Near IR (Over tone Region)
2. Mid IR (Vibrational Region)
3. Far IR (Rotational Region)

4. PRINCIPLE
In this technique chemical substances show absorption in the IR region.
After absorption vibration takes place and forms closed packed absorption bands,
called IR absorption spectrum .
Various bands are present in this spectrum which correspond to various
functional groups. Thus IR spectrum of a chemical substance is a finger print
for its identification.
Band absorption is identified by –
V=C/λ
ῡ=1/λ

5. Band intensities in IR spectrum is expressed by
transmittance(T).
The spectrum is measured based on wave number and % intensity of
transmittance.

6. THEORY
For a molecule to absorb IR radiation, it has to full fill
Certain requirements :
1. Correct wave length: A molecule absorb radiation only when natural
frequency is same as the frequency of incident radiation
2. Electric dipole: A molecule absorb radiation only when its absorption
causes a change in its electric dipole.
The closer the atoms in a molecule , the greater will be the strength of
the dipole, faster will be the rate of change of dipole, the higher will be the
frequency of the vibration, and the more intense will be the absorption of
radiation.

7. MOLECULARVIBRATIONS

9. Mol. vibration divided into 2 main types:
FUNDAMENTAL
VIBRATIONS
• Vibrations which appear as band in the
spectra.
NON-
FUNDAMENTAL
VIBRATIONS
• Vibrations which appears as a result
of fundamental vibration.

10. Fundamental vibration is also divided into types:
These are vibrations
in which the bond
length is altered i.e.
increased/ decreased.
STRETCHING
VIB.
1.Bending vibrations
are characterized by a
change in the bond
angle b/w two bonds.
2.It requires less
energy so appear at
longer wavelength.
BENDING
VIB.

11. Now, stretching vibration is further divided into :
Symmetric
• In this the 2 bonds increase/ decrease
in length, symmetrically.
Asymmetric
• In this one bond length is increased
and the other one decreases.

12. Bending vibration is divided into:
IN-PLANE
BENDING
• In this change in bond angle
is observed. Bending of
bonds takes place within
same plane
SCISSORING
• Here bond angle
decreases/2 atoms move
towards centre of atom
ROCKING
• Here bond angle is maintained,
but bonds move within plane in
1 direction.

13. OUT OF PLANE
BENDING • Out the plane of molecule.
WAGGING
• In this both atoms
move to one side of
plane.
TWISTING
• In this one atom is above
the plane and the other is
below the plane.

14. NON-FUNDAMENTAL VIBRATIONS
NON-
FUNDAMENTAL
OVER TONES:
These are observed at
twice the frequency
of strong band.
Ex: carbonyl group.
COMBINATION
TONES:
Weak bands that
appear occasionally
at frequencies that
are sum/difference
of 2 or more
fundamental bands.
FERMI RESONANCE:
Interaction b/w
fundamental vibration
& overtones or
combination tones.
Ex:CO2

15. INSTRUMENTATION

16. INSTRUMENTATION
The usual optical materials, glass or quartz absorb strongly in the IR region,
the apparatus for measuring of IR spectra is different from that of UV-
visible regions.
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

17. Schematic diagram of IR spectrophotometer is:
SOURCE
The instrument requires a source of radiant energy which provides isolating
narrow frequency bands. The radiation source must emit IR radiation which must
be
1. Steady
2. Intense enough for detection and extend over desired wavelength.
Source Sample Monochromator
Recorder Detector

18. The various popular sources of IR radiation are:
1. Incandescent lamp:
This is used for near IR instruments, this fails in far IR because it is glass
enclosed and has low spectral emissivity.

19. 2.Nernest Glower:
It consists of hollow rod which is about 2mm in diameter and 30 mm in
length.
The glower is made of rare earth oxides like Zirconia, Yttria and thoria. It is
heated to a temperature between 1000-1800c.
Dis-advantage:
1. Energy is concentrated in near IR and Visible regions of spectrum.

20. 3.Globar Source:
It is a rod of sintered silicon carbide which is about 50mm in length and
4mm in diameter.
When heated at a temperature between 1300 and 1700c, it emits radiation in
IR region.
Disadvantage:
Less intense than Nernst glower.

21. 4. Mercury Arc:
It is effective in far IR region(wave number <200cm-1)
At shorter wavelengths, heated glass envelope emits radiation whereas at
longer wavelengths the mercury plasma provides radiation through quartz.

22. MONOCHROMATOR
The radiation source emits radiations of various frequencies.
Desired frequencies absorb radiations at desired wavelengths.
TYPES OF MONOCHROMATORS
(a) Prism Monochromator-

23. Single pass Monochromator: PROCEDURE:
* Sample is kept near the focus of the beam before the entrance slit.
* The radiation from the source after passing through sample, strikes the
entrance slit which renders the radiation parallel and sends it to the prism.
* The dispersed radiation after reflecting from plane enters the 2nd slit and
focuses on the sample cell, through which it finally passes into the detector
section.

24. Double pass Monochromator:
The double pass produces more resolution than monochromator in
the radiation, before it passes on to the detector.

25. Grating Monochromator:
The grating monochromator has a series of parallel straight lines cut into
plane surface. Dispersion by grating follows law of diffraction.
n=d(sin I  sin )

26. SAMPLE CELLS
IR spectroscopy is used for characterization of solid liquid or gas.
Sampling of substances
1) Sampling of solids:
 SOLIDS RUN IN SOLUTION.
Solids may also be dissolved in non-aqueous solvents provided there is no chemical
interaction with the solvent.

27. It involves:
(A) Solid films: If solid is amorphous then sample is deposited on surface of
KBr/ NaCl by evaporation of solid.
This tech. is used in qualitative analysis.
(B) Mull technique:
Grinding sample Mix with nujol

29. • (C) Pressed pellet technique:
Here finely ground solid sample is mixed with KBr.
The mixture is then passed under high pressure in a press to form a small pellet.
IR radiation is passed through it.

30. 2)Sampling of liquids:
Samples that are liquids at room temperature are directly put into the cells
which are made up of NaCl, KBr and their IR spectra are obtained directly.
3)Sampling of gases:
The gas sample is introduced in such a way that it should to react with reflecting
surfaces.

31. DETECTORS
These are used to detect signals and responses.
The various types of detectors used in IR spectroscopy are:
 BOLOMETERS: In this electrical resistance of metal increases with increase in
temperature. When IR radiation falls on conductor change in resistance occurs.

32.  THERMOCOUPLE: Here electric current flows between 2 metal wires
which are connected at both ends.
The end which is exposed to IR radiation and is called as “Hot junction”.
The other end is called as “Cold junction” which is away from light. A
thermocouple is made by welding the 2 wires at ends. In IR 1 end is kept at
constant temperature and is not exposed to radiation. The temperature
difference generates P.D .

33.  THERMISTORS:
It is made of mixture of metal oxides. As temp. increases electrical
resistance decreases.
 GOLAY CELL:

34. APPLICATIONS OF IR
1) Determination of purity of compounds.
2) Shape and symmetry of the compound.
3) In industry for detection of impurities, and to produce satisfactory
products.
REFERENCES
 Instrumental methods of chemical analysis by
 Gurdeep R. Chatwal
 Sham K. Anand
 Instrumental analysis by Skoog
 https://en.wikipedia.org/wiki/Infrared_spectroscopy