Unit II IR spectroscopy-

Presented by,
Mrs. T. A. Mandhare
Navsahyadri Institute of Pharmacy, Pune
1NIP, Pune.

 Introduction
 Fundamental modes of vibrations in poly atomic
molecules
 sample handling
 factors affecting vibrations
 Instrumentation
- Sources of radiation,
-wavelength selectors
-detectors-Golay cell, Bolometer,Thermocouple,
Thermister, Pyroelectric detector
 Applications
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 Spectroscopy
 “seeing the un seeable”
 Using electromagnetic radiation as a probe to
obtain information about atoms and molecules
that are too small to see.
 Electromagnetic radiation is propagated at the
speed of light through a vacuum as an
oscillating wave.
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 λ = wave length
 υ = frequency
 c = speed of light
 E = kinetic energy
 h = Planck’s constant
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 The electromagnetic spectrum contains many types of waves, one of which
is infrared waves. Other electromagnetic waves include gamma rays, x-
rays, ultraviolet, visible light, microwaves, and radio.
 Infrared Spectroscopy includes radiation with wavelengths between 2.5 µm
– 25 µm.
 Technically correct Unit is µm( micrometer) but often used unit is µ
(micron).
 But mostly referred unit is wave number( Reciprocal centimeter (cm )
 Infrared Spectroscopy generally refers to the analysis of the interaction of a
molecule with infrared light.
 An IR spectrum is essentially a graph plotted with the infrared light
absorbed on the Y-axis against. frequency or wavelength on the X-axis.
 The major use of infrared spectroscopy is to determine the functional
groups of molecules, relevant to both organic and inorganic chemistry.
 IR Spectroscopy detects frequencies of infrared light that are absorbed by a
molecule. Molecules tend to absorb these specific frequencies of light since
they correspond to the frequency of the vibration of bonds in the molecule.
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 The energy required to excite the bonds belonging to a
molecule, and to make them vibrate with more amplitude,
occurs in the Infrared region. A bond will only interact with
the electromagnetic infrared radiation, however, if it is polar.
 Regions of IR
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 In other words, IR spectra is nothing but a finger print region of a
molecule.
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 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
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 Vibration along the line of bond.
 Change in bond length.
 2 types:
 a) Symmetrical stretching
 b) Asymmetrical stretching
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 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.
 Asymmetrical stretching
vibrations occur at higher
frequency than Symmetrical stretching
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 Vibration not along the line of bond.
 In this, bond angle is altered.
 Occurs at lower frequencies than stretching vibrations.
 2 types:
 a)In plane bending:
 i. Scissoring
 ii. Rocking
 b)Out plane bending:
 i. Wagging
 ii. Twisting
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 a)In plane bending:
 i. scissoring :
 This is an in plane bending
 Bond angles are decrease
 ii. Rocking :
 Bond angle is maintained.
 Movement of atoms take place in the same
direction.
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 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
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➢Sample handling is considered as an important technique in the infrared
spectroscopy.
➢The samples used in IR spectroscopy can be either in the solid, liquid, or
gaseous state.
➢ Samples of different phases have to be handled differently.
➢Common point is that the material containing the sample must be
transparent to IR radiations.
a) Sampling of solids:
• Solid runs in solution- solid dissolved in non-aq. ---drop of solution is
placed on an alkali metal disk ---solvent allow to evaporate---thin film of
solute is formed(some time entire solution is placed in a liquid sample cell)
• Solid films- if solid is amorphous in nature—the sample is deposited on
the surface of KBr or NaCl by evaporation of solution of solid.

• Mull technique-nujol mull( mineral oil)
• Pressed pellet technique-
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Sample+ nujol Nujol
mull(thick paste) spread
between IR transmitting
window & the mounted in
path of infrared beam,
spectrum is run.
• Nujol mull is transparent
through IR region but shows
absorption maximas at
2915,1462,1376,719 cm-1
Sample + KBr mixture is passed
under very high pressure in
press(2500 Psi). Formed pellet is
transperant to IR radiations & is run
as such.
( blank KBr pellet-reference beam)
•Resolution of spectrum is superior.
•Stored for long time.

b) Sampling of liquids:
Liquid samples are generally kept between two salt
plates of salts and measured since the plates are
transparent to IR light. Salt plates can be made up
of sodium chloride, calcium fluoride, or even
potassium bromide.
c) Sampling of Gases:
 Since the concentration can be in parts per million,
the sample cell must have a relatively long
pathlength, i.e. light must travel for a relatively long
distance in the sample cell.
samples of multiple physical states can be used in
Infrared Spectroscopy.
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 Fundamental Vibrations- Fundamental vibrational frequencies of a molecule
corresponds to transition from v=0 to v=1.
 For a non-linear molecule there will by 3N-6 (where N is the number of atoms)
number vibrations. The same holds true for linear molecules, however the
equations 3N-5 is used, because a linear molecule has one less rotational degrees
of freedom.
 Overtone- Overtones occur when a vibrational mode is excited from v=0 to v=2,
which is called the first overtone, or v=0 to v=3, the second overtone.
 Combination band- combination bands are observed when more than two or more
fundamental vibrations are excited simultaneously. One reason a combination
band might occur is if a fundamental vibration does not occur because of
symmetry.
 Difference band-Difference bands are similar to combination bands. The observed
frequency in this case results from the difference between the two interacting
bands.
 Usually spectrum is complicated because of presence of weak overtone,
Combination band and Difference band.
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 The value of stretching vibrational frequency of a bond
can be calculated by using HOOKE’S LAW.
 Value of vibrational frequency or wave number
depends upon:
 Bond strength
 Reduced mass
 Ex: C=C has higher vibrational frequency than C-C
stretching.
 O-H has higher vibrational frequency than C-C
bonding.
 F-H has higher vibrational frequency than O-H
stretching.
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 Coupled Vibrations
 Fermi Resonance
 Electronic Effects
 Hydrogen Bonding
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 Vibrations which occurs at different frequencies than that
required for an isolated vibrations (stretching) are called
coupled vibrations.
 Eg. Consider methylene group(-CH2-) two absorptions occurs
which corresponds to symmetric and asymmetric stretching
vibrations.
 Eg. Consider methyl group(-CH3-) two absorptions occurs
which corresponds to symmetric and asymmetric stretching
vibrations.
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 A Fermi resonance is the shifting of the energies and
intensities of absorption bands in an infrared or Raman
spectrum.
 In IR spectrum, absorption bands are spread over a wide
range of frequencies. Then the energy of a overtone level
chances to coincide with the fundamental mode of different
vibrations. This type of resonance is called Fermi Resonance.
 This phenomenon was first observed by Enrico Fermi in case
of carbon dioxide.
 Ex: Carbon dioxide (triatomic) linear and four fundamental
vibrations are expected. In this two symmetric stretching
vibration is IR inactive(no change in dipole moment).
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 The frequency shifts from the normal position of
absorption because of electronic effects.
 The force constant or the bond strength
changes and it’s absorption frequency shifts
from normal value.
 The frequency shifts includes:
 Inductive Effect
 Mesomeric Effect
 Field Effect
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 It is the electronic effect occurs due to polarization
of sigma bonds within the molecules or ion.
 Introduction of alkyl groups causes + I effect.
 It results in lengthening or weakening of the bond
and hence the force constant is lowered and wave
number of absorption decreases.
 Eg. wave number for c=o for following compounds
 Formaldehyde 1750 cm-1
 Acetaldehyde 1745 cm-1
 Acetone 1715 cm-1
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 The introduction of electronegative atom
causes – ve I effect.
 It results in the increase of bond order.
 Thus the force constant increases and hence
the wave number of the bonds also increases.
 Eg.
 Acetone 1715 cm-1
 Chloroacetone 1725 cm-1
 Dichloroacetone 1740 cm-1
 Tetachloroacetone 1750 cm-1
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 Conjugation occurs between two pi bonds or
a pi bond and lone pair of electrons present
on an adjacent atom.(polarity is produced)
 In some cases –I effect is dominated by
mesomeric effect and the absorption
frequency falls.
 Ex: Absorption frequency of amides and
esters
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 Lone pair of electrons present on the atoms
influence each other through space
interactions and changes the vibrational
frequencies of both the groups.
 This effect is called as Field effect.
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Ortho halo acetophenon

 Hydrogen bonding gives rise to downward
frequency shifts.
 Stronger is the hydrogen bonding, greater is
the absorption shift towards lower wave
numbers.
 Hydrogen bonding is of two types
 Intermolecular hydrogen bonding
 Intramolecular hydrogen bonding
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 INTERMOLECULAR HYDROGEN BONDING
 Gives rise to broad bands
 These are concentration dependent.
 INTRAMOLECULAR HYDROGEN BONDING
 Gives rise to sharp and well defined bands.
 These are concentration independent.
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 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
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 Incandescent lamp
 Nernst glower
 Globar Source
 Mercury Arc
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 In far IR region (<200cm-1) the described
sources lose their effectiveness at that time
mercury arc lamps are used.
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 A. Prism:-
 Used as dispersive element.
 Constructed of various metal halide salts.
 Sodium chloride is most commonly prism salt
used.
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 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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 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.
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 a) Sampling of solids
 Solids run in solution
 Mull technique
 Pressed pellet technique
 Solids films
 b) Sampling of liquids
 c) Sampling of gases
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 Detectors are used to measure the intensity of
unabsorbed infrared radiation.
 The detectors can be classified into three categories:
 Thermal detectors
 Pyroelectric detectors
 Photoconducting detectors
DETECTORs:
 Bolometers detector
 Thermocouple
 Thermistors
 Golay cell
 Pyro electric detector
 Photocoductivity detector
 Semiconductor detectors
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Thermal detectors can be used over a wide range of
wavelengths and they operate at room temperature.
Their main disadvantages are slow response time and
lower sensivity relative to other types of detectors.
 Bolometers detector
 Thermocouple
 Thermistors
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 Made up of fused mixture of metal oxides
 As the temperature of mixture increases
 It’s electrical resistance decreases
 (opposite to bolometer)
 Response time is slow
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 Golay cell consists of a small metal cylinder
closed by a rigid blackened metal plate at one
side and by a flexible metal diaphragm at other
end.
 Pneumatic chamber is filled with xenon gas and
sealed.
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 When infra red radiation is passed through infrared
transmitting window the blackened plate absorbs the
heat.
 By this heat the xenon gas expands, 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
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 Pyroelectric detector contain certain crystal Such
as lithium tantalate, barium titanate and
triglycine sulfate
 They produce temperature sensitive dipole
moments.
 To construct a pyroelectric detector, pyroelectric
substance is placed between two electrodes .
 Two electrodes are connected to each other via a
voltmeter.
 one of which has IR transparent window.
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 When the IR radiation falls upon a pyroelectric
substance, it absorbs energy from the
radiation.
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 This is non-thermal detector of great
sensitivity.
 Consists of thin layer of lead sulphide or lead
telluride supported on glass and enclosed into
an evacuated glass envelope.
 When IR radiation is focused on lead sulphide
or lead telluride, its conductance increases and
causes more current to flow.
 Response time is 0.5msec.
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 Exposure to radiation causes very rapid
change in their electrical resistance and
therefore very rapid response to IR signal.
 The basic concept behind this system is that
IR photon displaces an electron in the
detector, changing it’s conductivity.
 The response time for this detector is the
time required to change the semiconductor
from insulator to conductor.
 It is very short -1 nsec.
 Eg. Lead sulphide detector
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 Recorders are used to record the IR spectrum
 The radiant energy received by detector is
converted into measurable electrical signal
and is amplified by amplifier.
 The amplified signals are recorded and
plotted.
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 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)
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 In dispersive type the infrared light is separated
into individual frequencies by dispersion, using a
grating monochromator.
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Single beam IR spectrophotometer

 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.
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 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.
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 With IR spectroscopy it is not possible to
know molecular weight of substance.
 It is frequently non-adherence to Beer’s law
of complexity spectra.
 The narrowness of spectra and effect of stray
radiations make the measurements of
absorbance upon slit width and wavelength
setting.
 Generally, IR spectroscopy does not provide
information of the relative positions of
different functional groups on a molecule.
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Q.1.Which molecule is responsible for following IR Spectrum ?

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Q.2.Which molecule is responsible for following IR Spectrum ?

 Elementary organic spectroscopy, principles
andchemical applications, Y. R. Sharma.
 Instrumental methods of pharmaceutical
analysis, Dr. Gurdeep Chatwal, Dr. Anand.
 Introduction to Spectroscopy by Pavia,
Lampman & Kriz.
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Unit II IR spectroscopy-

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Unit II IR spectroscopy-