Infrared spectroscopy measures the bond vibrations in molecules to determine their functional groups. There are two main types of instruments - dispersive and Fourier transform infrared spectroscopy. Dispersive instruments use gratings to separate infrared frequencies, while FT-IR uses interferometers and Fourier transforms. Samples can be analyzed as solids, liquids in cells, or gases in gas cells. The infrared region is divided into functional group and fingerprint regions that are used for structure elucidation and identification of compounds, drugs, polymers, and more. Molecular vibrations occur as stretching and bending modes. Factors like hydrogen bonding, conjugation, and inductivity affect vibrational frequencies.

1. IR SPECTROSCOPY
Infrared spectroscopy (IR) measures the bond vibration frequencies in a molecule and is used to determine the
functional groups.
The infrared region constitutes 3 parts-
a) The near IR (0.8 -2.5µm) (12,500-4000cm-1)
b) The middle IR (2.5 -15µm) (4000-667cm-1)
i) Group frequency Region (4000-1500cm-1)
i i) Finger print Region (1500-667cm-1)
c) The far IR (15-200µm) (667-50cm-1)
 Most of the analytical applicationsare confinedtothe middle IR region because absorption of organic molecules are
high in this region.
Wave numberismostlyusedmeasure inIRabsorptionbecause wave numbersare largervalues&easyto handle than
wave length which are measured in µm.
 E = hν = hc/λ = hcν¯
It gives sufficient information about the structure of a compound.
PRINCIPLE
When infrared light or radiation hits a molecule , the bonds in the molecule absorb the energy of the infrared and
respond by vibrating.
IR SPECTROSCOPY SAMPLE DETECTOR
MODES OF MOLECULAR VIBRATIONS
There are 2 types of vibrations.
1) Stretching vibrations 2) Bending vibrations
• 1)Stretching vibrations: in this bond length is altered. They are of 2 types
a) symmetrical stretching: 2 bonds increase or decrease in length.
b) Asymmetrical stretching: in this one bond length is increased and other is decreased.
2)Bending vibrations: These are also called as deformations. •In this bond angle is altered. •These are of 2 types
•a) in plane bending→ scissoring, rocking •b) out plane bending→ wagging, twisting
Scissoring: Thisisan in plane bending.Inthisbondanglesare decreased. 2 atoms approacheach other.
Rocking: In thismovementof atomstakesplace insame direction.
Wagging: It isan out of plane bending.Inthis2 atomsmove to one side of the plane.
Twisting: In this one atom moves above the plane and the other atom moves below the plane.
NUMBER OF VIBRATIONAL MODES
A molecule can vibrate in many ways, and each way is called a vibrational mode.
If a molecule contains ‘N’ atoms, total number of vibrational modes
For linear molecule it is (3N-5)
For non linear molecule it is (3N-6)
Eg: H2O, a non-
linear molecule, will have 3 × 3 – 6 = 3 degrees of vibrational freedom, or modes.
VIBRATIONAL MOTION :

2. It occurs when atoms in a molecule are in periodic motion while the molecule as a whole has constant
translational and rotational motion.
The frequency of the periodic motion is known as a vibration frequency.
The value of stretching vibrational frequency of a bond can be calculated by the application of hooke’s law.
ν/c = ν¯ = 1/2пc[k/m1m2/m1+m2]1/2
= 1/2пc√k/μ
Where, μ→ reduced mass
m1&m2 →masses of the atoms
k →force constant
c →velocity of radiation
Factors influencing vibrational Frequencies
Calculated value of frequency of absorption for a particular bond is never exactly equal to its experimental
value.
There are many factors which are responsible for vibrational shifts
1)Vibrational coupling:
•it is observed in compounds containing –CH2 &
-CH3.
EG. Carboxylic acid anhydrides , amides , aldehydes
2) Hydrogen bonding:
Hydrogenbonding brings about remarkable downward frequency shifts.
Strongerthe hydrogenbonding,greateris theabsorptionshift towards lower wave length than thenormal
value.
There is 2 types of hydrogen bonding
a) inter molecular→ broad bands
b) intra molecular → sharp bands
•hydrogen bonding in O-H and N-H compounds deserve special attention.
•Eg: alcohols & phenols , enols & chelates
3) Electronic effects:
In this the frequency shifts are due to electronic effects which include conjugation, mesomeric effect,
inductive effect.
a) conjugation: conjugation lowers the absorption frequency of C=O stretching whether the conjugation
is due to α,β- unsaturation or due to an aromatic ring.
b) mesomeric effect: a molecule can be represented by 2or more structures that differ only in the
arrangement of electrons.
c) inductive effect: depends upon the intrinsic tendency of a substituent to either release or withdraw
electrons.
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)
In dispersive type the infrared light is separated into individual frequencies by dispersion, using a grating
monochromator.
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.

3. DISPERSIVE INSTRUMENTS
These are often double-beam recording instruments, employing diffraction gratings for dispersion of
radiation.
These 2 beams are reflected to a chopper which consists of rotating mirror.
It sends individual frequencies to the detector thermopile.
Detector will receive alternately an intense beam & a weak beam
This alternate current flows from detector to amplifier.
GRATING DETECTOR
SLIT
SAMPLE
LIGHT SOURCE
In order to measure an IR spectrum, the dispersion Spectrometer takes several minutes. Also the detector receives
only a few % of the energy of original light source.
FOURIER TRANSFORM IR SPECTROMETER
In the FT-IR instrument, the sample is placed between the output of the interferometer and the detector.
The sample absorbs radiation of particular wavelengths.
An interferogram of a reference is needed to obtain the spectrum of the sample.
After an interferogram has been collected, a computer performs a Fast Fourier Transform, which results in a
frequency domain trace (i.e intensity vs wavenumber).
The detector used in an FT-IR instrument must respond quickly because intensity changes are rapid .
Pyroelectric detectors or liquid nitrogen cooled photon detectors must be used. Thermal detectors are too
slow.
To achieve a good signal to noise ratio, many interferograms are obtained and then averaged. This can be
done in less time than it would take a dispersive instrument to record one scan.
In order to measure an IR spectrum, FTIR takes only a few seconds. Moreover, the detector receives up to 50% of
the energy of original light source. (much larger than the dispersion
spectrometer.)
SAMPLE HANDLING:
1. SAMPLING OF SOLID: Solids run in solution
 Solids dissolved in a aqueous solvent

4.  Placed over the alkali metal disk
 Solvent is allowed to evaporate
 Thin film of solute formed
 Entire solution is run in one of the cells for liquids
MULL TECHNIQUE
 Finely ground solid sample is used.
 Mixed with Nujol (mineral oil)
 Thick paste is made.
 Spread between I R transmitting windows
 Mounted in path of I R beam & The spectrum is run.
2. SAMPLING OF LIQUID:
•Liquid samples taken.
•Put it into rectangular cells of KBr, NaCl etc.
•I R spectra obtained.
•Sample thickness … such that transmittance lies between 15 – 20 % i.e., 0.015 – 0.05 mm in thickness.
•For double beam, matched cells are generally employed
•One cell contains sample while other has solvent used in sample.
•Matched cells should be of same thickness, protect from moisture.
3. SAMPLING OF GASES:
i. Gas samples are examined in the lR spectrometer after removal of water vapour.
ii. The simplest gas cell consists of metal/gas cylinder of 10cm long and closed with a appropriate window.
partial pressure of 5 to 15mmHG gives a reasonable level of absorption in most cases .
iii. The end wall of gas cell is made of NaCl and for low concentrated gases long path legths are required.
iv. Multi reflection can be used to make effective path length as long as 40cm ,so that constituent of gas can
be determind.
APPLICATION OF IR SPECTROSCOPY
A. Identification of functional group and Structure elucidation:
The entire IR region is devided into
Group frequency region- 4000 – 1500 cm-1
Finger print region – 1500 – 400 cm-1
In the group frequency region, the peaks corresponding to different functional groups can be
observed.
B. Identification drug substance:
IR spectra of sample and standard can be compared to identify a substance.
C. 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 pure drug. By comparing these, we can identify the
presence of impurity.
D. Study of polymers
E. Pharmaceutical research
F. Forensic investigations
G. Lubricant formulation and fuel additives
H. Quality assurance and control
I. Environmental and water quality analysis methods
J. Biochemical and biomedical research
I R Radiation Source –

5. Incandescent lamp
Nernst Glower
Globar Source
Mercury Arc
DETECTORS:-
-Bolometers
– Thermocouple
– Thermistors
– GolayCells
– Photoconductivitycell
– Semiconductor
– Pyroelectricdetectors