Infra red spectroscopy

INFRA RED
SPECTROSCOPY
 Siddu K M
 M Pharm 1st year
 Department of pharmaceutics
 Al Ameen College of Pharmacy
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Topics to be covered…
Introduction
Theory
Modes of molecular vibration
Sample handling
Instrumentation
Dispersive IR spectrophotometer
Fourier-Transform IR spectrophotometer
How to find out functional groups
Factors affecting vibrational frequencies
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Introduction :
• Infrared Spectroscopy is the analysis of infrared light interacting with a
molecule. This can be analysed in three ways by measuring absorption,
emission and reflection.
• IR spectroscopy or vibrational spectroscopy is concerned with the study of
absorption of infrared radiation, which results in vibrational transitions.
• IR Spectroscopy measures the vibrations of atoms, and based on this it is
possible to determine the functional groups
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Theory :
• In IR spectroscopy when a compound is exposed to IR radiations, it
selectively absorbs the radiation resulting in vibration of molecule of the
compound.
• This results in closely packed absorption band which are characteristic to the
functional groups and bonds present in the compound.
Region Wavelength range (mm) Wavenumber range (cm-1)
Near 0.78 - 2.5 12800 - 4000
Middle 2.5 - 50 4000 - 200
Far 50 -1000 200 - 10
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Modes of molecular vibrations:
a) Stretching vibration
a) Symmetrical stretching
b) Asymmetrical stretching
b) Bending vibrations
a) In-plane scissoring
b) In-plane rocking
c) Out-plane wagging
d) Out-plane twisting
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Stretching vibrations:
vibrations in which bond length is altered at regular intervals
without change in bond angle.
• Symmetrical : Two bonds increase or decrease in length symmetrically.
• Asymmetrical : bond length of one increases and another decreases.
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Bending vibrations:
vibrations in which change in bond angle or bond axis takes place
without change in bond length.
• In-plane rocking: Bond angle is maintained but both the bonds moves
within the plane.
• In-plane scissoring: Bond angle decreases but takes place within the same
plane.
• Out-plane wagging: Two atoms move up or down the plane with respect
to central atom.
• Out-plane twisting: One of the atom moves up the plane and another
down the plane with respect to central atom.
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Sample handling :
• Different states of sample are
a) Solid samples
b) Liquid samples
c) Gas samples
• The sample of same substance shows difference in frequency of absorption as it
passes from solid to gas state.
• In some cases additional bands are also observed with the change in sample state.
• Sample should be placed in sample holder or cuvettes.
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Solid samples:
1. Solid solutions
• In this technique solid sample is dissolved in inert solvent/non aqueous solvent, and
the solution is analysed in liquid sample cells.
• Solvent doesn’t react with solute and as well as does not absorb in the studied range.
• Solvents must be dry and transparent in the region of absorption.
• Ex: CCl4 , chloroform.
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• In this technique the amorphous solid sample is dissolved into soluble volatile
solvent.
• A drop of this solution is placed on the cell made up of NaCl or KBr.
• The solvent is evaporated by gently heating, which leaves a thin film on the surface
of cell.
• The resultant film is used for qualitative analysis.
2. Sample films
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3. Mull technique
• This method is used for crystalline compounds.
• A thick paste of finely powdered sample and mineral oil is sandwiched between two
plates either of NaCl or KBr.
• The most widely used mineral oil is Nujol oil.
• Nujol is transparent in most region of IR spectra but it has characteristic absorption
of C-C & C-H vibrations of hydrocarbons at 2915, 1462, 1376 &719 cm-1.
• When hydrocarbon bands interfere with spectrum, Fluorolube or
Hexachlorobutadiene may be used.
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4. Pressed pellet technique
• In this technique finely powdered solid sample is mixed with about 100 times of its
weight of powdered KBr.
• This mixture is pressed under a mini press with a pressure of about 25000 psi to
form a pellet.
• The sample disc so formed is then kept in the path of beam of IR
spectrophotometer
• This method eliminates a problem of bands which appear in the IR spectrum due
to the mulling agent.
• This method is not suitable for some polymers which are difficult to grind with
KBr.
15

Liquid samples :
• Liquid samples are usually handled pure without mixing them with any solvent
because all solvents have their own characteristics absorption spectra.
• For liquid samples, highly polished salt (NaCl, KBr or AgCl) plates are used.
• A drop of liquid sample is placed on the face of first plate and the second plate is
placed on the top to form a uniform film of a sample.
• Wipe off the excess liquid spilled from the edge of the plate then place the plates in
the sample compartment of spectrophotometer and run the spectrum.
• Aqueous liquid and volatile liquid can’t be examined with this technique.
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Gas samples :
• A special sample cell made up NaCl or KBr is used which is internally gold-
plated or stainless steel is used.
• These cells will cause multiple reflections of IR radiations which will help in
getting better absorption spectra.
• The vapours of gas are placed into the cell and directly place in the path of
Infrared radiation.
• The gas sample is introduced into the cell through a stop cock under a
pressure of 5-50 mmHg
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INSTRUMENTATION
• The main parts of IR spectrophotometer are
1. Radiation source
2. Monochromators
3. Sample cells and sample
4. Detectors
5. Recorder
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1. Radiation source:
a. Incandescent lamp: It consists of a closely wounded nichrome coil enclosed in a
glass covering which upon heating, a black oxide film is formed on nichrome coil which
emits IR radiation.
b. Nerst glower: The Nernst glower is constructed of rare earth oxides in the form of a
hollow cylinder. Platinum leads at the ends of the cylinder permit the passage of electricity.
c. Globar source: It consists of silicon carbide rod with a diameter of 6-8 mm & length
50mm and enclosed in a brass tube which is provided with provision for emission of
radiation when heated electrically.
d. High pressure Hg arc
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2. Monochromators:
A. Prism monochromators:
• Prism monochromators are used for radiations of shorter wavelength i.e, ≤ 350 nm.
• There are two types.
i. Mono pass prism monochromator: Radiation will pass once through the prism.
ii. Double pass prism monochromator: Radiation will pass twice through the prism.
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B. Grating monochromators:
• Used to get uniform dispersion and can be used at wide range of wavelength.
• A grating is a set of equally spaced, narrow, parallel sources. A grating disperses light
of different wavelengths to give, for any wavelength, a narrow fringe.
• There are two types.
i. Diffraction grating.
ii. Transmittance grating.
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4. Detectors
• The detectors used in IR spectroscopy are thermal detectors.
• When IR radiations fall on a thermal detectors, its temperature increases due
to heating.
• The increase in temperature produces potential difference which is
proportional to the amount of radiation falling on the detector.
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 Thermocouple detector:
• Works on the principle that when two different semi conductor metal wires are connected
and kept at different temperatures, a potential difference is developed between them which
causes the flow of current.

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 Bolometer :
• A bolometer works on the principle of Wheatstone bridge
arrangement.
• Wheatstone bridge has 4 arms, one arm of the bridge consists of metal of unknown
resistance and the other three are having known resistance.

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Golay cell:
• Golay cell consists of small metal cylinder closed by a rigid
blackened metal plate. Pneumatic chamber is filled with Xenon gas.
• At one end of the cylinder a flexible silvered diaphragm and at the
other end IR transmitting window is present.
• When IR radiation is passed through the window, the blackened
plate absorbs the heat and causes the Xenon gas to expand and
creates pressure.
• The resulting pressure of gas will deform the diaphragm. The
motion of diaphragm shows the amount of IR radiations that fall on
the metal plate.

DISPERSIVE IR
SPECTROPHOTOMETER
• Dispersive IR instruments are introduced in 1940’s.
• Double-beam instruments are mostly used than Single beam instrument.
• In dispersive IR sequential scanning of wave numbers of light takes place.
• Scanning instrument uses a frequency separation device (grating) to resolve the IR radiation into individual frequencies.
• An exit slit isolates a specific frequency for passage to the detector.
• The IR spectrum is obtained by moving (scanning) the grating over a given wavenumber region after passing through the
sample.
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Disadvantages:
• It containing all movable parts which causes mechanical slippage
• Less resolution, accuracy and sensitivity
• Slow scan speed
• Only narrow frequency range can be studied
• Involvement of stray light
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FOURIER-TRANSFORM IR
SPECTROPHOTOMETER
• A FT-IR spectrometer simultaneously collects high-spectral-resolution data over a wide
spectral range.
• Fourier Transform Infrared (FT-IR) spectrometry was developed in order to overcome
the limitations encountered with dispersive instruments mainly the slow scanning
process.
• A solution was developed which employed a very simple optical device called an
interferometer. The interferometer produces a unique type of signal which has all of
the infrared frequencies “encoded” into it. The signal can be measured very quickly,
usually on the order of one second or so.
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Advantages:
• Fast & sensitive
• All frequencies can be modulated at once
• Simple mechanical design with only one moving part
• No stray light is involved
• When using He-Ne laser as internal standard, no need of external calibration
• Availability of easy sampling accessories
• Air pollutants like CO, ethylene oxide etc. can be analysed
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HOW TO FIND FUNCTIONAL
GROUPS IN THE IR SPECTRUM
• To generate the IR spectrum, different frequencies of infrared light are
passed through a sample, and the transmittance of light at each frequency is
measured.
• The transmittance is then plotted versus the frequency of the light.
• Different functional groups produce bond absorptions at different locations
and intensities on the IR spectrum.
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Example of HEXANE
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• The IR spectrum of hexane (C6H14) is shown in the next figure. Because
hexane has only C-H and C-C bonds (and no functional groups)

FACTORS AFFECTING VIBRATIONAL
FREQUENCIES
• Factors responsible for shifting the vibrational frequencies from their normal
values are
1. Coupled vibrations
2. Fermi resonance
3. Hydrogen bonding
4. Electronic effects
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1. Coupled vibrations
• An isolated C-H bond has only one stretching vibrational frequency where
as methylene group ( C-H2) shows two stretching vibrations, symmetrical and
asymmetrical because of mechanical coupling or interaction between C-H
stretching vibrations in the CH2 group.
• Asymmetric vibrations occur at higher frequencies or wave numbers than
symmetric stretching vibrations.
• These are known as coupled vibrations because these vibrations occur at
different frequencies than that required for an isolated C-H stretching.
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2. Fermi resonance
• Coupling of two fundamental vibration modes produces two new modes of
vibration ,with frequencies higher and lower than that observed in absence
of interaction. Interaction can also take place between fundamental
vibrations and overtones such interactions are known as Fermi Resonance.
• Example: symmetrical stretching vibration of CO2 in IR spectrum shows
band at 1337 cm-1 and the two bending vibrations are equivalent and absorb
at same frequency of 667.3 cm-1 i.e. (667.3×2= 1334.6 cm-1)
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3. Hydrogen bonding
• It occurs in any system containing a proton donor group and a proton
acceptor i.e. (X-H)
• The N-H stretching frequencies of amines are also affected by hydrogen
bonding as that of the hydroxyl group but frequency shifts for amines are
lesser than that for hydroxyl compounds.
• Because nitrogen is less electronegative than oxygen so the hydrogen
bonding in amines is weaker than that in hydroxyl compounds.
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4. Electronic effect
• Changes in the absorption frequencies for a particular group take place when
the substituents in the neighbourhood of that particular group are changed.
• It includes :
1. Inductive effect
2. Mesomeric effect
3. Field effect
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 INDUCTIVE EFFECT:
• The introduction of alkyl group causes +I effect which results in the lengthening or the
weakening of the bond.
• Hence wave number of absorption decreases.
Formaldehyde (HCHO) 1750 cm-1
Acetaldehyde (CH3CHO) 1745 cm-1
Acetone (CH3COCH3) 1715 cm-1

MESOMERIC EFFECT:
• It causes lengthening or the weakening of a bond leading in the lowering of absorption
frequency.
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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.
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Infra red spectroscopy

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