Infrared spectroscopy is a technique used to identify chemical functional groups in molecules by detecting the vibrational transitions of bonds between atoms. It works by measuring how infrared radiation is absorbed by a sample based on the vibrational frequencies of the chemical bonds present. The main components of an IR spectrometer are an infrared radiation source, a sample holder, a detector, and a recorder. Factors like electronic effects, hydrogen bonding, and bond angles can affect the vibrational frequencies observed in IR spectra. Infrared spectroscopy has many applications including structure elucidation and identification of organic compounds.

1. INFRARED SPECTROSCOPY
Amjad Hussain
M.Pharm

2. CONTENTS
• Introduction
• Principles of IR Spectroscopy
• Types of Vibrations in IR
• Instrumentation
• Factors Affecting Vibrational Frequencies
• Applications of IR
• References

3. Introduction
• Infrared waves have wavelength longer than visible but shorter than
microwaves.
• Infrared spectroscopy is also called as vibrational spectroscopy.
• The study of absorption of infrared radiation, which results in vibrational
transitions of I.R spectra.
• It is mainly used for structural elucidation to determine the functional groups.
• The ranges of infrared radiations are 0.8-1000 µm.
a) Near IR radiation: 0.8-2.5 µm
b) Mid IR radiation: 2.5-15 µm
c) Far IR radiation: 15-200 µm

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

5. Principle
• In any molecule it is known that atoms or group of atoms are
connected by bonds.
• These bonds are analogous to spring.
• When IR radiation absorbs it undergoes vibrations.
• Criteria for absorbing IR radiation-
i. Change in dipole moment
ii. Applied IR frequency = Natural frequency of vibrations
• Bond intensity in IR spectra may be expressed as
transmittance (T) and absorbance (A).
A=log10 (1/T)

6. Types of Vibrations
There are two types of vibrations
1. Stretching vibrations -
a) Symmetrical
b) Asymmetrical
2. Bending vibrations:
a) In-plane bending
i. Scissoring
ii. Rocking
b) Out-plane bending
i. Wagging
ii. Twisting

7. Instrumentation
• The main parts of IR spectrometer are :
– IR radiation sources
– Monochromator
– Sample Cell & Sampling of Substances
– Detectors
– Recorder

8. Radiation Source
1. Nerst Glower
– It operates at 1500 0C
– It is composed of zirconium, yttrium and thorium.
– Used in near IR region.
– Used for detection of carbohydrates and proteins.
– Disadvantage is its frequent mechanical failure.
2. Globar Source
– It operates at 1300 0C
– It is composed of silicon carbide.
– Used in middle IR region.
– Used to detect simple functional group.

9. • Incandescent Lamp
– It is made up of nichrome wire.
– Used in near IR region.
– It is heated up to 1100 K.
– Used to detect complex organic molecules.
• Mercury Arc
– Quartz jacketed tube containing mercury vapor at a
pressure grater than 1 atm.
– Used in far IR region.
– Intensity of radiation is greater.
– Used to detect inorganic complexes.

10. • Tungsten Lamp
– Used in mid IR region.
– Intensity of radiation is mild.
– Used to detection of organic functional groups.

11. Monochromators
• Monochromators converted polychromatic light into
monochromatic light.
• The radiation source emits radiation of various frequencies.
• The sample in IR spectroscopy absorbs only at certain frequencies,
it is therefore becomes necessary to select desired frequencies
from the radiation source and reject the radiation of the
frequencies.
• This selection has been achieved by means of monochromators
which are mainly of two types-
– Prism
– Grating

12. Prism
• Prism are made up of various metal halides salts.
• NaCl is probably the most common prism salt.
• While glass & quartz were utilized in visible and UV they
absorb & unsatisfactory in the IR.
Grating
• Commonly used because of high resolving power.
• Dispersion of grating follows law of diffraction.

13. Sample Cell
• Sample cells are made up of alkali halides like NaCl or KBr are
used.
• Infrared spectra may be obtained for gases, liquids or solids.

14. Sampling Methods
• IR spectroscopy has been used for the characterization of
solid, liquid or gas samples.
1. Solids
2. Liquids
3. Gases

15. Solids
• Generally 3 techniques are employed for preparing solid
samples:
1. Solid Run in Solution.
2. Mull Technique
3. Pressed Pellet Technique

16. • Dissolve the organic compound in a solvent (CCl4).
• A drop of solution is placed on plates and solvent is allowed to
evaporate leaving a thin film.
Solid Run in Solution

17. Pressed Pellet Technique
• Finely ground solid sample is mixed with about 100x its
weight of powdered KBr and pressing the matrix under high
pressure.
• This results KBr pellet that can be inserted into the holder of
spectrometer.

18. Mull Technique
• Finely grounded solid sample is mixed with mulling agent
(Nujol) to make a thick paste.
• This paste is placed between salt plates.

19. Liquids
i. The sample that are liquid at room temp. are usually
put frequently with no preparation, into rectangular cell
made of NaCl, KBr, and their IR spectra are obtained
directly.
ii. A drop of liquid organic compound is placed between a pair
of polished NaCl or KBr plates, referred as Salt plates.
– When the plates are squeezed gently, a thin liquid film
forms between them.
– A spectrum determined by this method is referred to as
neat spectrum since no solvent is used.
– The pair of plates is inserted into the holder of
spectrometer.

20. Gas
• The sample cell is made up of NaCl, KBr etc. and it is similar to
the liquid sample cell. A sample cell with a long path length (5
– 10 cm) is needed because the gases show relatively weak
absorbance.
• The dried gas sample introduced into gas cell which is made
up of glass or metal cylinder of about 10 cm long.
• Cell is equipped with mirrors and to bring multiple reflection
to increase effective path length.

21. Detectors
The various types of detectors used in IR spectroscopy are-
a) Bolometer
b) Thermocouple
c) Thermistor
d) Golay cell
e) Pyroelectric detectors

22. Bolometer
• Bolometer are constructed from metals.
• Radiation fall on bolometer the bridge become unbalanced
due to change in electric resistance.
• Amount of current flowing through galvanometer is measure
of intensity of radiation
• Germanium bolometer excellent for Far IR.
• Response time is few milliseconds.

23. Thermocouple
• Most commonly used, constructed based upon Wheatstone
bridge
• It contain two fine wire of metal which has different thermo
electrical properties are welded with blackened gold coil.
• Cold junction is kept constant temp.
• Hot junction is expose to radiation.
• Two junction are at different temp. it cause a potential
difference depending on amount of radiation fall on hot
Junction.
• To enhance sensitivity several thermocouple are joined in
series to give thermopile.
• Response time is about milliseconds.

24. Thermistor
• Semiconductors device called thermistor.
• Similar to bolometer.
• Resistance made by fusing several metallic oxide.
• They shows the negative thermal co-efficient of electrical
resistance.

25. Golay Cell
• It consists of small metal cylinder, one end of which closed by
blackened metal plate
• Other with metalized diaphragm
• A cylinder is filled with non absorbing gas like xenon
• Radiation fall on cell, metal plate is heated which causes the
expansion of gas which in turn affects the diaphragm
• This cause the change in output of cell.

26. Pyroelectric
• Pyroelectric detectors are ferroelectric devices, electrical
conductors, or semiconductors which change electric
polarization as a function of temperature .
• The degree of polarization decreases with increase in
temperature. A signal is produced at electrodes which are
placed across the surface of the detector material as small
polarization changes occur.
• Other materials such as strontium barium niobate and
especially lithium tantalite which is used commercially in fire
detectors also function as suitable detectors for IR
instruments.

27. Factors Affecting Vibrational
Frequencies
There are 4 factors:-
1. Electronic Effects
2. Hydrogen Bonding
3. Bond Angle

28. Electronic Effect
• Changes in the absorption frequencies for a particular group
take place when the substituents in the neighborhood of that
particular group are changed.
• The frequency shifts are due to the electronic effects which
include:-
i. Inductive effect
ii. Mesomeric effect
iii. Field effect

29. Inductive Effect
• The introduction of alkyl group cause +I effect which results in
the lengthening or the weakening of the bond and hence the
force constant is lowered and the wave number of absorption
decreases.
• The introduction of group causes –I effect or an
electronegative atom which results in the bond order to
increase. Thus, the force constant increases and hence, the
wave number of absorption increases.

30. Mesomeric Effect
• They cause lengthening or the weakening of a bond leading in
the lowering of the absorption frequency. It is found in
conjugated systems.
• More will be the conjugation, less will be the bond strength
and lower will be the wave number.

31. Field Effect
• In ortho substituted compounds, the lone pair of electrons on
two atoms influence each other through space interactions
and change the vibrational frequencies of both the groups.
This effect is called field effect.
• It is generated due to steric effect.

32. Hydrogen Bonding
 It gives rise to downward frequency shifts.
 Stronger the hydrogen bonding grater the absorption shift
towards lower wave number from the normal value.
 There are two types of hydrogen bonding:-
i. Intermolecular Hydrogen Bonding
ii. Intra-molecular Hydrogen Bonding
 The H- bonding which is between two different molecules is
called intermolecular H-bonding.
 The H-bonding which is within the same molecules is called
intra-molecular H-bonding.
 Intermolecular H-bonding gives rise to broad bands, while
intra-molecular H-bonds give sharp and well defined bands.

33. Bond Angle
• Bond angle will decrease, bond strength will increase,
vibrational frequency will increase and wave number will
increase.

34. FTIR
• FTIR is a powerful tool for identifying the type of chemical bond
present in a molecule.
• FTIR have greatly extended the capabilities of IR and applied to
many areas that are difficult to analyze by dispersive instrument.
• In dispersive IR , IR light is separated into it’s individual frequency
by dispersion using a grating monochromatic.
• All frequencies and intensities can be simultaneously determined.
• The instrument used for FTIR is Scanning Michelson interferometer.
• FTIR frequencies are allowed to intersect to produce an
interference pattern and this pattern is analyzed mathematically
using “FOURIER TRANSFORM”, to determine the individual
frequencies and their intensities.

35. Applications
For organic compounds:
 Identification of substance like (hydrocarbons hydroxy
compounds, aldehydes and ketones, carboxylic groups, amines
etc)
 Determination of molecular structure
 Studying the progress of the reaction
 Detection of impurities
For in organic compounds:
 High modes of vibrations
 Lower symmetry of complexes
 Formation of chelates.
 Geometrical isomerism

36. Reference
• Instrumental Method of Chemical Analysis, Gurdeep R
Chatwal, Sham K Anand.