Dr. Kalpana G.

Patel

INFRARED SPECTROSCOPY

UNIT–II
IR spectroscopy
Introduction, fundamental modes of vibrations in polyatomic molecules,
sample handling, factors affecting vibrations
Instrumentation-Sources of radiation, wavelength selectors, detectors-
Golaycell, Bolometer, Thermocouple, Thermistor, Pyroelectric detector and
applications

Sr.no. Questions
1. Enlist sample handling methods in IR spectroscopy. Discuss any two
method in detail.
2. Explain: Fermi resonance, coupling vibration, overtones.

3. Write a note on detectors used in IR spectroscopy.

4. Explain theory of IR spectroscopy. Discuss the requirements of IR
absorption by molecule.
5. Explain types of stretching and bending vibration in IR spectroscopy.
Explain Fingerprint Region.
6. Write a note on Application of IR Spectroscopy.
7. Explain various factors affecting vibrational frequency in IR
spectroscopy
8. Enlist various factors affecting vibrational frequency and explain
Hydrogen bonding and vibrational coupling in detail.
9. Write a brief note on thermal sources.

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INFRARED SPECTROSCOPY

Electromagnetic spectrum consists of electromagnetic radiations differing from

one another in wavelength or frequency. In this spectrum, infrared radiation is
part between visible and microwave region. Any compound having covalent
bonds absorbs various frequencies in infrared region of the spectrum which
ranges between 2.5 µm – 25 µm. During this absorption process, those
frequencies of IR which matches the natural vibrational frequencies of the
molecule are absorbed and after absorption of energy, amplitude of the
vibrational motions of bonds in the molecule gets increased. Infrared radiations
in the range of about 10,000 – 100 cm-1 are absorbed and converted by an
organic molecule into energy of molecular vibrations.
Infrared spectroscopy is certainly one of the most important analytical
techniques available to today‘s scientists. One of the great advantages of
infrared spectroscopy is that virtually any sample in virtually any state may be
studied. Liquids, solutions, pastes, powders, films, fibers, gases and surfaces
can all be examined with a judicious choice of sampling technique. Infrared
spectroscopy is a technique based on the vibrations of the atoms of a molecule.
An infrared spectrum is commonly obtained by passing infrared radiation
through a sample and determining what fraction of the incident radiation is
absorbed at a particular energy. The energy at which any peak in an absorption
spectrum appears corresponds to the frequency of a vibration of a part of a
sample molecule. Fingerprint of a sample with absorption peaks corresponds to
the frequencies of vibrations between the bonds of the atoms making up the
material. Each different material is a unique combination of atoms, no two
compounds produce the exact same spectrum, and therefore IR can result in a
unique identification of every different kind of material.
Infrared Spectroscopy is simply the study of the interaction of Infrared light
with matter. Most powerful aspect is the ability to identify complete unknown
compounds and for compound comparison.

WAVELENGTH RANGE
Wavelength Energy content
Region Wavenumber range (cm-1)
range (µm) (Kcal/mol)

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Near 0.78-2.5 12800-4000 37-10

Middle 2.5-50 4000-200 10-1
Far 50-1000 200-10 1- 0.1
Mostly
used IR 2.5-15 4000-670
region

Near IR
 0.8-2.5μm OR <650 cm-1 : Some low energy electron transition as well as
changes in vibrational and rotational levels can occur in this region.
 This region is generally restricted to the study of compounds that contain
OH, NH,CH groups.
 Absorption bands are harmonic overtones or combinations of
fundamental stretching vibrational bands
 Valuable tool for analyzing mixtures of aromatic 1º,2º,3ºamines
 Extends from the upper wavelength end of the visible region
Middle IR
 2.5-50 μm OR 650-4000 cm-1: Changes in vibrational levels of most
molecules occur in this region, so is of most use for analysis.
 Divided into 3 regions-
Group frequency region:3600-1500cm-1
 helps in determining the functional groups such as: C=O,C=C,C-
H,C=H,C≡N,C ≡C,C=C each one has a distinct peak
 No complex mixing or overlapping occurs. Here we have small weak
peaks because of combination tones or overtones which should not
mixed with fundamental peaks
 This region is used for Quantitative analysis of drugs
900-600 cm-1
This region is only useful for only two purposes
 Deciding substituent in the aromatic ring
 Geometrical isomers can be easily detected in this region
Finger print region :1500-900 cm-1
Absorptions due to single bond stretching frequencies

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 It is known as finger print region because absorption bands in this region

are characteristic for a particular compound
 Two compounds having very similar structure can be easily differentiated
in this region
 This region is very complex because of vibrations
 Finger print of a test compound is compared with that of standard
compound and individual peak is not compared
 This region is not used for quantitative analysis
Far IR
50-500μm OR >4000 cm:
 Many purely rotational changes occur in this region.
 Region of frequencies lower than 650/cm
 Contains a few absorptions of C-X bond.
 Pure rotational absorption by gases are observed in far IR
UNIT USED
Wavenumbers are commonly used; larger values and easy to express than
wavelength
• The position of an absorption band can be specified in units frequency ,
v(s-1 , or Hz) or Wavelength - Measured in micron (μ)
• Wavenumber - Measured in reciprocal cm (cm-1)

Relationship between units:

• Wavelength of radiation is inversely proportional to energy, while
wavenumber is directly proportional to energy.

IR SPECTRUM
• IR spectrum is a graphical representation of % transmittance Vs either
increasing wavelength or decreasing frequency. Each dip in spectrum is
called as band/ peak, represents absorption of IR radiation at that
frequency by the sample.
• It is plot of %T vs. wavelength/wavenumber. Ideally it should be plot of
frequency vs. %T. but here, frequency is not used because its unit is very
large.

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• IR Spectra are conveniently plotted with the 100 % T or zero absorbance

at the top of the spectrum, hence IR absorption bands are inverted
compared to those in UV-Visible spectra.
• Below 100 cm-1 only rotation occur known as rotational spectra. At 100-
10000 cm -1 vibrational and rotational transition occurs.

THEORY AND PRINCIPLE

 Atoms are in continuous motion. They are in quantized vibrational states
and quantized rotational states.
 Absorption of IR radiation occurs in molecules that have small energy
differences between various vibrational and rotational states.
 IR radiations of frequencies less than 100 cm-1 is absorbed and converted
by an organic molecule into energy of molecular rotation. This absorption
is quantized, thus molecular rotation spectrum consists of discrete lines.
Absorption by gases is characterized by discrete, well defined lines, while
in liquids and solids, intramolecular collisions and interactions cause
broadening of the lines into a continuum.
 IR radiation in the range from 10000 – 100 cm-1 is absorbed and
converted by an organic molecule into energy of molecular vibration.
This absorption is also quantized, but vibrational spectra appears as bands
rather than as lines because a single vibrational energy change is
accompanied by a number of rotational energy changes. Commonly
vibrational and rotational bands occur between 4000 and 400 cm -1 .
 The frequency of the wavelength of absorption depends on the relative
masses of the atoms, the force constants of the bonds and geometry of the
atoms.

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Absorption of IR radiation is confined to molecular species that have small

energy difference between various vibrational and rotational states.
 Basically two criteria for the IR absorption are there which are as follow:
1. The frequency of radiations should exactly match with the natural
vibrational frequency of the molecule.
2. A molecule must undergo a net change in dipole moment

1. The frequency of radiations should exactly matches with the natural

vibrational frequency of the molecule.
 If the frequency of radiation exactly matches natural vibrational
frequency of the molecule, a net transfer of the energy takes place. This
results in a change in the amplitude of the molecular vibration and
absorption of the radiation is the consequence. The rotation of
asymmetric molecules around the centers of the mass results in a periodic
dipole fluctuation that can interact with the radiation.
 If frequency of electrical field associated with incident radiation exactly
matches to the frequency of vibrational and rotational spectra then IR
radiation is absorbed.
2. A molecule must undergo a net change in dipole moment
 In order for IR radiationto be absorbed by a molecule, it is necessary for
the molecule to undergo a change in a dipole moment during absorption.
 If in any molecule, change in dipole moment then absorption of IR
Radiation occurs and IR band is observed.
 Dipole moment is determined by magnitude of the charge difference and
the distance between the two centers of the charge.
 When IR fall on molecule, increase in distance of bond therefore change
in dipole so molecule is said to be IR active.
 But if dipole moment not changes and only rotation/vibration occur then
molecule is said to be IR inactive rotation
 Relative dipole moments:- C=O > C- Cl > C-N > C-C-OH > C-C-H
 For change in dipole molecule must be asymmetric.
 For example, in HCl charge distribution is not symmetric because
chlorine has higher electron density than the hydrogen. Thus, hydrogen
chloride has a significant dipole moment and said to be polar. As HCl
molecule vibrates, a regular fluctuation in dipole moment occurs, and a

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field is established that can interact with the electrical field associated
with radiation.
 Substances those are transparent to IR, are primarily mono-atomic species
and homo-nuclear diatomic gases such as He, Cl2, N2, and O2.(i. e. no
change in dipole moment).
 If no net change in the dipole moment in the molecule, it cannot absorb
IR radiation, so, IR spectra cannot be observed.

Origin of IR spectra
 As absorption of the IR radiation causes change in vibrational level which
results in change in rotational level. Hence, the resulting spectra are also
called as vibrational- rotational spectrum.
 The absorption of the IR radiation causes excitation of molecule to higher
vibrational levels and is quantized when transition occurs from lowest
level (v=0) to first level (v=1). The frequency of that radiation can be
given by hν = E1-E0 and is called as fundamental vibrational frequency.
When transition occurs from v=0 to v=2 or 3, it is called as overtone
frequency. (v=o to v=2 first overtone, v=o to v=3 second overtone)
THEORY OF MOLECULAR VIBRATIONS/TYPES OF MOLECULAR
VIBRATIONS
• A relative change in a distance between two atoms in a molecule is
known as vibration.
• Any change in shape of the molecule- stretching of bonds, bending of
bonds, or internal rotation around single bonds.
• The covalent bonds in molecules are constantly vibrating.
• A bond vibrates with both stretching and bending motions.
• Each stretching and bending vibration of a bond occurs with a
characteristic frequency.

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Types of vibration

A. Stretching B. Bending

1. 2. 1. In- 2. Out of
Symmetric Asymmetric plane plane

a. b. a. b.
Rocking Scissoring Wagging Twisting

A. Stretching:
• Definition:-A stretching vibration involves a continuous change in the
interatomic distance between two atoms without altering bond axis or
bond angle.
• It means distance between two atoms increases or decreases but atoms
remains in the same bond axis.
• Here energy absorbed is high.

1. Symmetrical stretching
Symmetrical stretching are vibrations where two atoms either
move towards or away from the central atom, thereby changing
interatomic distance but there is no change in bond angle.
2. Asymmetrical stretching
Asymmetrical stretching are vibrations where one atom approaches
the central atom while other moves away from the central atom.

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Bending (Deformation):
• Definition: It is characterized by continuous change in the bond angle and
axis with common atom or the movement of the group of atoms with
respect to the remainder of the molecule without movement of the atoms
in the group with respect to one another.
• Less energy is absorbed.
• 2 types:
(1) In plane: - Here bending occurs in same plane
(a) Rocking:It occurs when atoms or structural unit swings back
and forth in to the plane of the molecule.
(b) Scissoring (Symmetrical bending vibration): It occurs when
two atoms connected to central atom either move towards or away
from each other with certain deformation of valence angle.

(2) Out of plane: - Here back and forth rotation occurs but in other plane.
(a) Wagging: These types of vibrations occur when structural unit
swing back and forth out of the plane of the molecule.
(b) Twisting: These vibrations occur when structural unit rotates
about the bond which joins to the molecule.

* + sign indicates motion in same (towards) plane, - sign indicates

motion away from the plane.
Bending vibration generally requires less energy and takes place at longer
wavelength (shorter wavenumber)

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Types Absorption of bands (vibrations)

• Fundamental
• Non fundamental
• Fermi resonance

Absorption
bands

Fundamental Non fundamental Fermi

bands band resonance

The longest Overtones or Combinational Coupling

WL that harmonic bands bands Interaction
induces the which occur
change in between
vibrational When two Fundamental
motion of the bands of same band & Non
molecule gives symmetry are fundamental
rise to an closely located band
Sum or
absorption Multiples of within the
difference of
band fundamendal same molecule
two or more
band the interaction
fund.bands
take place
which is
known as
coupling

Fundamental vibrations/bands
 On the spectrum we have recording of bands depending on
absorption by vibration
 These vibrations are known as Normal or Fundamental vibrations
 The frequency of the radiation which causes transition of molecule
from v=0 to v=1 are known as fundamental vibrational
frequency.
 The absorption bands so obtained are known as Normal /
Fundamental vibrational absorption band
 Fundamental peak occurs if molecule vibrates from 0 ground level
to 1st vibrational level.
Non fundamental/Abnormal bands (vibrations)
 The bands which are not due to any explained vibration
Overtones

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 Molecule vibrates from 0 ground level to 2nd vibrational level.

 These are the vibrational bands having twice the frequency of
normal vibrational bands with a low intensity
 E.g. Carbonyl groups has normal absorption band at frequency
1715 cm-1 but overtones of carbonyl function appears at 3430 cm-1
Combination tones
 These are the weak absorption bands which occasionally appear at
a frequencies that are sum or differences of two or more
fundamental bonds
 Normally OH stretching occurs at 1250cm-1 and CO stretching
occurs at 1000 cm-1 while combination tones occurs at w 2250 cm-1
 Combination tones also occurs at 250 cm-1 but it out of the range of
IR Spectrum and so not seen
 Difference bands are similar to combination bands. The observed
frequency in this case results from the difference between the two
interacting bands
Coupling bands
 When two bands of same symmetry are closely located within the
same molecule the interaction take place which is known as
coupling.
 This result in a new strong absorption band which is different
from the normal absorption band which is itself a characteristic
absorption band
 Acetic anhydride contains two C=O bonds
 IR is usually concerned with C=O bond as it gives a distinct band
having constant position
 Any modification in nearby structure produces a predictable
change.
 Normal frequency of C=O function is 1760 cm-1 and coupled
absorption band appears at 1820 cm-1
 If a molecule shows a strong absorption band at 1820 cm-1 the
molecule may be acetic anhydride.
Fermi resonance bands
 It refers to coupling between fundamental and non fundamental
absorption bands.

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 Interaction may occur between fundamental vibrations and

overtones or combination tones.
 Because of fermi resonance we have change in absorption
frequency; we can have intensification of absorption or splitting of
absorption band.
 For example, absorption pattern of CO2. Four modes of absorption
pattern are possible. Symmetrical stretching band of CO2 appears
in the Raman spectrum near 1340 cm-1(Fundamental). Actually
two bands are observed i.e. one at 1286 cm-1 and one at 1388 cm-1.
The splitting results from coupling between the fundamental C=O
stretching vibration near 1340 cm-1 and the first overtone of the
bending vibration. The two fundamental bending vibrations are
equivalent and occurs near 667 cm-1. So, the first overtone of it
occurs near 1334 cm-1 (667 x 2 = 1334).
 Fermi resonance is a common phenomenon in IR and Raman
spectra. It requires that the vibrational levels be of the same
symmetry species and that is the interacting groups are located in
the molecule so that mechanical coupling is appreciable.
 Normal frequency of C-H bond is 810 cm -1 and overtones occur
at 1620 cm -1. Also normal frequency of C=C stretching is 1600
cm -1 . If the overtones of C-H bond and normal C=C couples then
absorption band splits.
 For example, fermi resonance is the doublet appearance of C=O
stretch of cyclopentanone. (1746 cm-1 and 1750 cm-1).
 In aldehyde, C-H stretching absorption usually appears as a
doublet (2820 cm-1 and 2720 cm-1), because of the interaction
between C-H stretching (fundamental) and overtone of C-H
bending.
VIBRATIONAL FREQUENCY:

Molecular vibration depends upon,

a. Masses of the atoms:
 Heavier atoms vibrate slowly than lighter ones.
 Frequency decreases with the increasing atomic weight.
b. Stiffness of the bonds:
 Stronger bonds usually absorb at higher frequency.

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 Frequency increases with increasing bond energy.

 The stiffness of the bond can be characterized by a proportionality
constant termed the force constant, k (derived from Hook’s law).
 Exposure of the molecule to IR radiation results in stretching and
bending vibrations. For example, assume that diatomic molecule is
held by a covalent bond with mass m1 and m2of the two atoms.
Each atom will consist of nuclei alongwith electrons.
 This diatomic molecule resembles spring with two atoms at either
end. The stiffness of bond is the force constant. By applying
Hook‘s law, two atoms and their connecting bond are treated as
harmonic oscillator composed of two masses joined by spring. The
following equation is derived from Hook‘s law.
 k/mreduced

m1

m2

mreduced m1m2
(μ) = m1 + m2
Where, k is the force constant of the spring and is related to the strength of the
bond. m1 and m2 are the masses of the atoms at the ends of the bond.
 Stronger the bond, larger will be k value and therefore  will be high.
 The equation relating the force constant, the reduced mass (μ) and the
frequency of oscillation is,

 This equation may be modified so that direct use of the wavenumber

values for bond vibrational frequencies can be made namely

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According to Hooke‘s law. The vibrational frequency of band stretching

depends upon bond strength (force constant) and Atomic mass.

ῡ= 1/2πc [ƒ/( m1m2/ m1 + m2)]1/2

ῡ = wavenumber; C = velocity of light in cm/s or 3 *1010 cm/s
f or k = force constant of bond in dynes/cm
Comparison of C-H group with F-H group,
1) Atomic mass
• F-H  Higher atomic masses
• C-H  lower atomic masses
• As vibrational frequency is inversely proportional to atomic mass.
• Therefore, stretching frequency of F-H should occur at lower frequency
than that for C-H bond.

2) Force constant
• As the first two rows of the periodic table, the value of force constant
increases as going from left to right.
• Therefore value of force constant for F-H is greater than C-H.
• As vibrational frequency is directly proportional to force constant value.
The stretching frequency of F-H should occur at higher value than that for
C-H bond.

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• As per the atomic mass F-H should occur at lower frequency than that for
C-H bond.
• But practically F-H group absorbs at higher frequency (4138 cm-1) than
the C-H group ( 3040 cm-1)
• Thus, force constant has greater effect on stretching frequency than
atomic mass increases.
• The force constants for bonds are:
– single bond = 5 x 105 dyne/cm
– double bond = 10 x 105 dyne/cm
– triple bond = 15 x 105 dyne/cm
• The regions of an IR spectrum where bond stretching vibrations are seen
depends primarily on whether the bonds are single, double, or triple or
bonds to hydrogen.
• The following table shows where absorption by single, double, and triple
bonds are observed in an IR spectrum.
Bond absorption region, cm–1
C–C, C–O, C–N 800–1300
C=C, C=O, C=N, N=O 1500–1900
C≡C, C≡N 2000–2300
C–H, N–H, O–H 2700–3800

VIBRATIONAL/FUNDAMENTAL MODES/PEAKS:
 The numbers of vibrations in a polyatomic molecules can be calculated.
In polyatomic molecules the atoms and bonds are not rigidly linked,
hence vibrate from their resulting position to higher vibrational level
giving fundamental and overtone bands.
 The numbers of vibrations (fundamental bands) is related to the degree of
freedom in a molecule. A molecule has as many degrees of freedom as
the total degrees of freedom of its individual atoms. The number of
degree of freedom is equal to the sum of the coordinates necessary to
locate all atoms of a molecule in space.
 Each atom has three degrees of freedom corresponding to 3 coordinates x,
y, z necessary to describe its position relative to other atoms in the
molecule. A molecule of n atoms therefore has 3 N degrees of freedom.
 In defining motion of a molecule, we have to consider,
o Translational motion (motion of the entire molecule through space)

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o Rotational motion of the entire molecule around its centre of

gravity
o Vibrational motions of each atom relative to the other.
 When a molecule absorbs IR radiation, its chemical bonds vibrate. The
bonds can stretch, contract, and bend. Vibrational motion can be broken
down into a number of constituent vibrations called normal modes.
 Hence, 3 degrees of freedom from 3N describe translational motion and
another 3 degrees from 3 N describe rotational motion, as a whole.
Hence, a non linear molecule containing 3N degrees of freedom has 3N-6
degrees of freedom which describe vibrational motion. Hence, 3N-6
degrees of freedom represent the number of possible vibrations within a
molecule which is also called as normal or fundamental modes.
 So, for any nonlinear molecule, no. of peaks for vibrational transition
=3 n-3(for translational motion)-3(for rotational motion)
=3 n-6
• For a linear molecule, rotation about bond axis is not possible, hence only
2 degrees of freedom are required for representing rotational motion in
linear molecule.
• For, linear molecule degree of freedom for rotational transition is only 2.
=3n-3(for translational motion) -2(for rotational motion)
=3 n-5
Type of Molecule Normal Mode Formula # of Modes in a 3-Atom
Molecule
Linear 3N-5 4
Non- Linear 3N-6 3

Where N= the number of atoms in a molecule

Linear molecule with n atoms usually has 3n -5 fundamental vibrational

modes.
Linear molecules: CO2 ,HCl.
Eg: CO2 3(3)-5=4

Nonlinear molecule with n atoms usually has 3n - 6 fundamental vibrational

modes.
Eg: H2O 3(3) – 6 = 3
Nonlinear molecules : benzene, hexane, methane, H2O
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• A single absorption peak should appear for each vibration in which there
is a change in dipole.
• All the vibrations in the molecule occur at the same time and IR
spectrophotometer is linked to spectroscope which enables recording of
particular frequencies of vibrations.
• Increase in the theoretical numbers of fundamental modes of vibrations
(absorption frequencies) will be observed because of overtones
(multiplies of a given frequency) and combination tones (sum of two
other vibrations) increase the number of bands, whereas various other
phenomenon decrease the number of fundamental bands which are as
follows,
1. The symmetry of the molecule is such that no change in the
dipole occurs from a particular vibration.
2. The energies of absorption of two or more vibrations are
identical; hence there is occurrence of a degenerate band from
several other absorptions of the same frequency in highly
symmetrical molecules.
3. Fundamental vibrations are sometimes so close that they coalesce
and give only one peak.
4. Fundamental bands that are too weak are not observed because
absorption intensity is so low that it is not detectable by ordinary
means.
5. The vibrational energy is in a wavelength region beyond the
range of the instrument i.e. fundamental frequencies that fall
outside of the 4000-400 cm-1 region.

• Sometimes numbers of peaks observed are more than theoretical number

of peaks, which is because of overtone peaks which occur at 2 or 3 times
the frequency of fundamental peaks. Combination bands or peak is
observed when a photon excites two vibrational modes simultaneously.
The frequency of combination band is approximately the sum or
difference of the two fundamental frequencies and this phenomenon
occurs when a quantum of energy is absorbed by two bonds rather than
one.

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Reasons for increase in no. of peaks

• If single photon exciting 2 vibrator simultaneously than results in
combination peak. Frequency of this combination peak in addition or
differentiation of 2 fundamental peaks.
• Due to same vibrator that jumps from 2nd to 3rd or so on.
• Appearance of overtone peak frequency of which is twice or trice than
fundamental peak.

Reasons for decrease in no. of peaks

• If the molecule absorbs very less radiation which is difficult to detect by
detector.
• Fundamental frequency that fall outside of the 2.5-15 μm region.
• Fundamental bands that are too weak to be absorbed.
• If 2 fundamental peaks are close together coalescence of peaks.
• If molecule is symmetric.
• Where energy of absorption of 2 vibrator are same then only single peak
appears in spectra.
• This kind of peak is known as degenerate peak.

FACTORS INFLUENCING THE VIBRATIONAL FREQUENCIES

1. Vibrational coupling
2. Bond strength
3. Hydrogen bonding
4. Electronic effect
5. Bond angle/strain
6. Field effect
7. Interaction between solute and solvent
8. Concentration of analytes
9. Nature of solvent
10.Temperature effect

1. VIBRATIONAL COUPLING/ COUPLED INTERACTIONS

 The energy of the vibration and thus, the wavelength of its absorption
peak may be influenced by or coupled with the other vibrators in the
molecule.

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 Here, interaction of 2 vibrational levels which vibrate at same frequency

and nearby in molecule is known as vibrational coupling.
 It occurs only when bonds are located closely to each other.
 Here, 2 more peaks appear in spectra
1)Symmetric
2)Anti symmetric
Requirements:
 2 Stretching vibrational coupling occur when it is separated through
common atom.
 2 bending vibrational coupling occur when it is separated by common
bond.
 Interaction is greatest when the coupled groups absorb individually, near
the same frequency.
 Coupling can occur between stretching and bending if bond involved in
stretching is involved in bending also.
 Both vibrators should vibrate with same frequency.
 Coupling is negligible when groups are separated by one or more carbon
atoms (i.e. two or more bonds) and the vibrations are mutually
perpendicular.
 Hence, when two bonds oscillators share a common atom, they cannot
behave as individual oscillators unless the oscillation frequencies are
widely different. This is because there is mechanical coupling interaction
between the oscillators.
E.g;-(1) Isolated -C-H bond has only one stretching frequency but the
stretching vibration of C-H bond in CH2 group combine together to produce
two couple vibrations of different frequency i.e. symmetric (2900 cm-1) and
antisymmetric (3000 cm-1). C-H bands in CH3 group give rise to
symmetric(3000 cm-1) and antisymmetric (3100 cm-1) vibration but they are
of different frequency from those of CH2 group.
(2)CO2
 CO2 molecule consist of two C=O bonds with a common carbon atom,
hence coupling can occur. If no coupling occurred between the two C=O
bonds, an absorption peak would be expected at the same wave number
as the peak for the C=O stretching vibration in an aliphatic ketone at 1700
cm-1.
 It is linear molecule so no. of peaks should be 3n-5 = 3*3-5= 4 (n= No. of
atoms). Two stretching vibrations are possible i.e. symmetric and
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antisymmetric. Interaction between two vibrations can occur because

bonds involved are associated with common carbon atom. The symmetric
vibration causes no change in the dipole because two oxygen atoms
simultaneously move away from or towards the central carbon atom.
Thus symmetric vibration is IR inactive.
 In the asymmetric vibration, one oxygen atom moves away from the C
atom as the C atom moves towards the Oxygen atom. Hence, as a result,
net change in charge distance occurs periodically, producing the change
in dipole moment, so absorption occurs at 2350 cm-1, which is at a higher
frequency (shorter wavelength) than observed for a carbonyl group in
aliphatic ketone (1700 cm-1).
 The remaining two vibrational modes of CO2 involves scissoring as
shown below.

 Anti symmetric peak appear at 2350 cm-1

 Here in symmetric peak doesn‘t appear because of there is no change in
dipole moment➙IR inactive
 Instead of 2 bending peaks only one bending peak appear because energy
of absorption is same
 The bending vibrations are the resolved components at 900 to one another
of the bending motion in all possible planes around the bond axis.
 The two vibrations are identical in energy and produce peak at 667 cm-1 is
called as degenerate band or peak.

(3) H2O
 Another example is nonlinear triatomic molecules such as H2O, SO2 or
NO2 . These molecules have 3 x 3 – 6 = 3 vibrational modes which are
as follows:

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 Dr. Kalpana G. Patel

 Because the central atom is not in line with the other two, a symmetric
stretching vibration will produce a change in dipole and will thus be
responsible for IR absorption. For example, stretching peaks appear at
3650 (symmetric) and 3760 (antisymmetric) cm-1. There is only one
component to the scissoring vibration for their nonlinear molecule
because motion in the plane of the molecule constitutes a rotational
degree of freedom. For water, the bending vibration causes absorption at
1595 cm-1.
 The difference in behavior of linear and nonlinear molecule with 2 and 3
absorption peaks respectively shows how IR spectroscopy can sometimes
be used to deduce the molecular shapes.

(4) Acetaldehyde (CH3=CHO)

 Here n=7 and molecule is non linear so no. of vibrational peaks should
be 15 according to 3n-6
 But here only 5 peaks appear in spectra
(5) Anhydride R-C-O-C-R
O O
 These give rise to 2-C=O stretching absorption, 1 antisymmetric and 1
symmetric peak approximately at 1800-1900 cm-1.
 Here, coupling occurs between 2 carbonyl groups which are indirectly
linked through oxygen atom. Interaction is due to slight = bond
character in carbonyl oxygen bond due to resonance forms.
(6) Amide R-C-N-H
O H

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 Shows 2 absorption bands around 1600-1700cm-1 i.e. due to –C=O

stretching and –N-H bending but due to coupling original character is
changed.
 Here coupling between –C-N stretching and –N-H bending vibrational
level takes place
 Here amide I peak is observed due to –C=O stretching
 Amide II peak is observed due to coupling.
 It refers to coupling between fundamental and non fundamental
absorption bands.
 Interaction may occur between fundamental vibrations and overtones or
combination tones.
7. Coupling of vibrations is a common phenomenon, hence the position of
the absorption peak corresponding to the given organic functional group
cannot always be specified exactly. For example, C-O stretching
frequency in methanol is 1034 cm-1, in ethanol is 1053 cm-1, in 2- butanol
is 1105 cm-1. These variations results from a coupling of the C-O
stretching with adjacent C-C stretching or C-H vibrations.

Fermi resonance:
 Because of fermi resonance we have change in absorption frequency; we
can have intensification of absorption or splitting of absorption band.
 For example, absorption pattern of CO2. Four modes of absorption
pattern are possible. Symmetrical stretching band of CO2 appears in the
Raman spectrum near 1340 cm-1 (Fundamental). Actually two bands are
observed i.e. one at 1286 cm-1 and one at 1388 cm-1. The splitting results
from coupling between the fundamental C=O stretching vibration near
1340 cm-1 and the first overtone of the bending vibration. The two
fundamental bending vibrations are equivalent and occurs near 667 cm-1.
So, the first overtone of it occurs near 1334 cm-1 (667 x 2 = 1334).
 Fermi resonance is a common phenomenon in IR and Raman spectra. It
requires that the vibrational levels be of the same symmetry species and
that is the interacting groups are located in the molecule so that
mechanical coupling is appreciable.
 Normal frequency of C-H bond is 810 cm -1 and overtones occur at 1620
cm -1. Also normal frequency of C=C stretching is 1600 cm -1 . If the
overtones of C-H bond and normal C=C couples then absorption band
splits.

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 For example, fermi resonance is the doublet appearance of C=O stretch of

cyclopentanone. (1746 cm-1 and 1750 cm-1).
 In aldehyde, C-H stretching absorption usually appears as a doublet (2820
cm-1 and 2720 cm-1), because of the interaction between C-H stretching
(fundamental) and overtone of C-H bending.
2. Bond strength
 According to Hooke‘s law. The vibrational frequency depends upon (1)
bond strength (force constant) and (2) Atomic mass.
 The stretching vibration occurs in the order of bond strengths:
For example, C-N (1050 cm-1) < C=N (1650 cm-1) <C≡N (2250 cm-1)
 The variation of vibrational frequency is due to decreasing force constant
from triple bond to the single bond, when the atomic masses are constant.
For example, C≡C (2200 cm-1) > C=C (1650 cm-1) > C-C (1200 cm-1)
 C-H bond dissociation energy is 104 Kcal / mole which is greater than C-
C bond dissociation energy, 88 Kcal / mole. Vibrational frequency for C-
H bond is 3000 cm-1 and C-C bond is 1200 cm-1.
 The stronger the bond, the greater the amount of energy required to
stretch it. Moreover, the mass of C-H bond is much less than that of C-C
bond and this factor should be also taken into consideration.
 Hence, we can conclude that,
o Vibrational frequency of a bond increases as strength of bond
increases and reduced mass decreases. Hence, C=C, and C=O
stretching will have higher frequency than C-C and C-O
stretching respectively.
o C-H and O-H stretching occur at higher frequency than C-C and
C-O stretching.
o O-H stretching occurs at higher frequency than O-D stretching.
The value of O-H stretching is a measure of bond strength. In
general, stronger the H bonding, 1) longer the O-H bond, 2)
lower the vibrational frequency, 3) more intense band observed.
3. Hydrogen bonding
 Hydrogen bonding is possible only if any system contains 1 proton donor
(X-H) group and 1 proton acceptor groups in system. Here,‖s‖ orbital of
protondonor overlaps ‖p‖ or ―‖orbital of proton acceptor group.

Example of proton donors:--COOH

-OH

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-NH2
-CONH2
Example of proton acceptor:-any system containing ―=―. Common proton
acceptor atoms are O, N and halogens. Unsaturated groups like C=C
linkage can also act as proton acceptors.
 H bonding is denoted by ―-----‖
2 types of H-bonding
(1) Inter molecular
(2) Intra molecular
Whenever H-bond is present in system, lengthening of the bond occur. As
bond length is increased it decreases the bond strength. So, there is
decrease in wavenumber this leads to decreased in vibrational frequency
change in shape and position of IR peak.
 The strength of hydrogen bond is maximum when proton donor group
and axis of lone pair orbital are linear. Strength of the bond decreases as
the distance between proton donor and proton acceptor increases.
 Hydrogen bonding alters the force constant of both the groups and thus
the frequencies of both stretching and bending vibrations are altered. The
X-H stretching bands move to lower frequencies usually with increased
intensity and band widening. The stretching frequency of the acceptor
group for example, C=O is also decrease but to a lesser degree than the
proton donor group. The H-X bending vibrations usually shifts to a
shorter wavelength when bonding occurs. This shift is less
pronounced/weaker than that of the stretching frequency.
 Intermolecular hydrogen bonding involves association of two or more
molecules of the same or different compounds. It results in dimer
compound or in polymeric molecular chains which exist in neat samples
or concentrated solution of monohydroxy alcohols. Intermolecular
hydrogen bonds are broken on dilution (at lower concentration); hence
there is decrease in bonded O-H absorption and increase in appearance of
free O-H absorption.
 Intramolecular bonds are formed when proton donor and acceptor are
present in single molecule under spatial conditions that allows the
required overlap of the orbital, for example, the formation of 5 or 6 –
membered ring.
 Intramolecular hydrogen bonding is independent of concentration. For
example, at low ethanol concentration stretching vibrations of free -

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OH group gives narrow peak at 3640 cm-1. Because intermolecular H-

bonding is minimum, but at higher ethanol concentration broad and wide
band appears 3350 cm-1 which is due to vibration of OH groups involved
in hydrogen bonding. With increase on ethanol concentration, band
intensity at 3640 cm-1 decreases (disappears) and at 3350 cm-1 increases.
 The extent of both inter and intra molecular H-bonding is temperature
dependent. The effect of concentration on intermolecular and
intramolecular H-bonding is markedly different. The bands that result
from intermolecular bonding generally disappear at low concentration
(less than about 0.01 M in non polar solvents). Intramolecular H bonding
is an internal effect and persists at very low concentration. For example,
salicylic acid.
 In general, intermolecular H bonds give rise to broad bands while
intramolecular gives sharp and well defined bands.
 The change in frequency between free OH absorption and bonded OH
absorption is a measure of the strength of the H bond. Molecular
geometry, nature of protondonor/acceptor, ring strain existing in molecule
relative acidity and basicity of the proton donor and acceptor groups
affect the strength of Hydrogen bonding.
 Intramolecular bonding involving the same bonding groups is stronger
when a six me mbered ring is formed than when a smaller ring results
from bonding. Hydrogen bonding is strongest when a bonded structure is
stabilized by resonance.
 The effect of H bonding on the stretching frequency from its value due to
H bonding.
 An important aspect involves interaction between functional groups of
solute and solvent. If the solute is polar then it is important to note the
solvent used and the solute concentration.
 Examples in which intermolecular H-bonding occur are alcohol, phenol,
carboxylic acid, etc.
 Examples in which intramolecular H-bonding occur are, o-chloro and o-
alkoxy phenols, beta-hydroxy amino/nitro compounds.

(1) Salicylic acid (o-hydroxy benzoic acid –OHBA) and p-hydroxy

benzoic acid PHBA):-here, both shows different peaks because OHBA
shows intramolecular H-bond. In OHBA benzoic acid, C=O stretching
vibration occurs at 1665 cm-1, in whereas PHBA absorption occurs at

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1680 cm-1. This is because intramolecular hydrogen bonding decreases

frequency of C=O stretching to a greater extent than intermolecular
hydrogen bonding. In salicylic acid due to closeness of OH and COOH
groups (ortho position), intramolecular hydrogen bonding takes place.
Hence, salicylic acid (OHBA) absorbs at lower frequency than p-hydroxy
benzoic acid (PHBA).
(2)phenol shows inter/intramolecular H-bonding
(3)enols/chelates:-here, H-bonding is so strong that even though diluting
solution it can‘t break
C=O and –OH:-here, H-bond with oxygen of –C=O, So, decreased
double bond characteristic, which depends on basicity of –C=O group. If
more basicity of group, stronger will be H bond.
-COOH also shows H-bonding
(4) Aromatic compounds possess pie-system. So they possess
conjugation. These compounds act as Lewis base. So, decreased
‗=‗characteristic and thereby vibrational frequency will be decreased.
(5)-NH2:-shows 2 peaks, at 3000cm-1 and 3600cm-1. If free amino group
is present it shows peak at 3600cm-1. If H-bonded amino group it shows
peak at 3000cm-1. H-bond in –N-H is weaker than that of in –O-H. In
amines N is less electronegative than oxygen. Hydrogen bonds in amines
are weaker than in alcohols and also shift in vibrational frequency is less
as compared to alcohols.
4. Electronic effects-
 It depends on presence of substituent.
(a)Conjugation:- It causes delocalization of electron, so, decreases ―=―
characteristics, which increases bond length. So decreases bond strength
and hence, decrease in vibrational frequency.

(b)Resonance effect(mesomeric effect):-It means single molecule can be

represented in 2 or more than 2 forms. Stretching which differ in the
arrangementof electrons is called resonance effect. If electron releasing
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 Dr. Kalpana G. Patel

group present it increases in delocalization which decreases in ―=―

characteristic. This increases the bond length and decreases bond
strength. So, decreases vibrational frequency.
 Absorption frequencies of C=O stretching in amides and esters

O=C-O-CH3
H2N-C=O

Benzamide
C=O 1663 cm-1 Phenyl acetate
C=O 1730cm-1

As the N-atom is less electro negative than O-atom,

the electron pair on N-atom in amide is more liable to conjugation
andC=O absorption frequency is much lesser in amides than esters.

(c)Inductive effect:-
Inductive effect depends on tendency of substituent to release or
withdraw electrons.
 +I effect (electron donating groups): weakening of the bond-absorption at
lower wavenumber. Absorption frequency shifts from the normal value
because of electronic effects.
E.g.: HCHO- 1750cm-1, CH3CHO-1745 cm-1
Introduction of electronegative group: -I effect- increases wave no. of
absorption.
E.g.: CH3COCH3 1715cm-1
ClCH2 COCH3 1725cm-1
 Inductive and resonance both type of effects are existing in molecule.
Finally which type of effect is shown by molecule depends on which
effect is predominant.
(1) E.g.:-amides (1)R-C-NH2:-
O

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NH2 is electron releasing group, so, it will have more resonance effect.
So, it will lead to decrease in vibrational frequency.

(2)R-C-Cl:
O
Inductive effect of chloride is more dominating than mesomeric effect.
C=O is electron withdrawing group. So, it decreases delocalization of
electrons. This will increase = bond characteristics. So, increases bond
strength and thereby increases in vibrational frequency.

(3)Esters:-R-C-O-R1:
O
If alkyl esters are present then resonance effect is predominant
If R=benzene then inductive effect is predominant.
 The introduction of Halogen atom, an electronegative atom causes -I
(Inductive effect), which result in shortening as well as strengthening of
the bond. So value of force constant for C=O increases and that‘s way
frequency of absorption will also increase.
 When an alkyl group is attached in α- position of C=O group, it exerts +I
effect and causes the wavenumber of absorption to decrease.
 Eg: Formaldehyde HCHO:C=O will show absorption at 1735 cm-1.
o Acetaldehyde CH3CHO: C=O will show absorption at 1730 cm-1
o In acetone, two –CH3groups are present so two times +I effect will
be observed. This lead to C=O bond stretching weak absorption at
1715 cm-1
5. Field effect:-
 Vibrational frequencies of two groups often influence each other through
space interaction. If 2 functional group present and 1 affect the vibration
of other, it is known as field effect.
 Space interaction may be electrostatic or steric in nature. Eg: alpha chloro
keto steroids.
 C=O stretching frequencies is higher when chlorine in equatorial than
when it is axial. It may be due to repulsive force of non bonding electron
of oxygen and chlorine in the Molecule. This changes the hybridization
state of Oxygen. Therefore C=O stretching frequency shift to higher
frequency.

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6. Bond angle:-
 Normal bond angle is of 1200.
 If decrease there is decrease in angle strain, so bond length also decreases
and ―=―characteristic is increased. This leads to increased vibrational
frequency.
 If ring expands then increased length of bond, so ―=―characteristic
decreases and bond length increases. Therefore, vibrational frequency
will get decreases.
 In ketones, highest C=O frequencies observed in strained rings, which
can be explained in terms of bond angular strain. The C-CO-C bond angle
when reduced below 1200, results in increased s orbital character in C=O
bond which shortens C=O bond and therefore strengthen the bond and
finally frequency increases. As ring size decreases, an increase in rigidity
in C-CO-C bond system occurs, hence C=O stretching couples more
effectively with C-C stretching, leading to higher frequency.
7. Interaction between solute and solvent:-
 The solvent can affect peaks in the spectrum of an analyte if it can
associated with or in some other way react with analyte. this change in
position and shape of IR peak.
 So it is generally preferable to choose a solvent for which is relatively
inert and non-polar
 CCl4and CS2 are mostly recommended as solvents.

8. Concentration of analyte :-
 Increased concentration of analyte leads to more interaction between
molecules. So, this leads to band broadening
9. Nature of solvent:-
 Should be transparent to IR
 Should be free from water
 Should be inert. If solvent molecule reacts with the solute molecule then
it will lead to band broadening.
10.Temperature effect:-

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 If temperature increases, that leads to increase in interaction between

solute molecules, so there band broadening may be observed. There may
be change in position and shape of IR spectra.

INSTRUMENTATION
1. Source
2. Monochromator
3. Sample compartment
4. Detector
5. Recorder

1. Source (thermal source):-

 Single inert solid rod is heated to high temp between 1500-2200 °K.
 Heating is carried out by passing current.
Requirements:-
Continuous emission of IR Radiation
All sources used in IR region are thermal sources
Should emit high intensity radiation.

(1)Nernst glower source:-

 It is a cylindrical rod or hollow tube which is prepared by fusing mixture
of oxide of 4 metal i.e. yttrium, cerium, thorium and zirconium.
 Rod is 2 cm long and have 1 mm diameter.
 Platinum leads are sealed to the end of the cylindrical rod to permit
electrical connection.
 Rod is heated to high temperature between 1200-2200 °K by passing
electrical current.
Advantages:-
1. Emits radiation in range of 1000-10000cm-1.
2. Energy output of Nernst glower is between 1-10 µm. Radiation
intensity is approximately thrice of nichrome and glober source which
remains steady and constant over a long period of time.
3. It can be used in air as it is not oxidized.

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Disadvantages:-
1. Large negative temperature co-efficient of electrical resistance. It is
much conductive and so not easily heated. Hence, preheating is required
which is carried out by auxillary heater externally
2. High heat is generated, so ventilation is require to remove surplus heat
and evaporated oxides and binder.
3. Heated upto certain temperature range only.
4. Frequent mechanical failure.
(2) Glober source:-
 It is single silicon carbide rod usually about 50 mm long and 5 mm in
diameter and electrically heated to 1300 -1500 °K by passing electrical
current.
 Maximum radiation at 5200 cm-1
 Spectral energies of the Glober and the Nernst glower are comparable
except in the region below 5 µm, where the Glober provides a
significantly a greater output.
Advantage over Nernst:-
 Possess large positive temperature co-efficient. So no need of auxillary
heater.
 Not even preheating required.
Disadvantage:-radiation intensity is less than nerst glower.

(3) Nichrome wire source (Incandescent wire source):-

 It is heated to 1100 °K which emits IR radiation.
 It is simple wire of nichrome which is spirally wounded.
Advantage:-
 Simple, rugged and has longer life time and requires less care compared
to other sources.
Disadvantage:-
 It emits lower intensity radiation compared to other source.
(4) Tungsten filament lamp:-
 It is used for near IR only.
 Also used for visible IR
 It is the convenient source for the near IR region of 4000-12,800 cm-1
(0.78 - 2.5µm).
(5) Mercury arc lamp:-

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 It is used for far IR region. None of the thermal sources provide sufficient
radiation power for detection. Hence high pressure Hg arc is used.
 Hg vapour at high pressure > 1 atm filled in quartz envelop.
 Passage of the electrical current through the vapour forms an internal
plasma source which provides continuum radiation in far IR region.
(6) CO2 laser source:-
 It emits radiation in range of 900-1100 cm-1(11 to 9 µm) range, which
consist of about 100 closely spaced discrete lines. Any one of the lines
can be chosen by tuning the laser.
 Used for quantitative determination of C6H6, NH3, ethanol, NO2, trichloro
ethylene.
 Also used for monitoring/determination of concentration of atmospheric
pollutant.
Advantage:-
 The radiant power available in each line is several orders of magnitude
greater than that of black body sources (thermal detectors).
(7)Rhodium wire:-
 Incandescent wire source
 Heated at temp.at 1100ºk

SUMMARY
S.N Characte Nernst Globar Incandesc Mercur Tungst Co2
o r glower ent y arc en laser
lamp
1. Composit Rare earth Silicon Nichrome High Tungst Tunab
ion oxides carbide wire (Hg) en – le Co2
pressure Haloge laser.
n
2. Operating 1200 — 1300- 1100°K 1000°K 3500°K -------
temp. 2200°K 1500
°K
3. Radiation 12,800- 5200 10,800-- < 665 10,100 1100-
-1
s 4000 cm cm-1 8000 cm-1 cm-1 —4000 900
produced cm-1 cm-1
O.P

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4. IR region Near / Near Far Middle Middl

used visible Middle e /near
5. Intensity More As Less but Greater More
of intense equal to longer Mild effecti
radiation Nernst life. ve
6. Output >2µm >5µm 2-4µm 10µm 2- 5 µm
significan 4µm
t (λ)
7. Used for Carbohyd Simple complex In- Most NH3
rate , Functio organic organic organic C6H6,
protein nal molecules. complex functio C2H5
groups es. nal OH
groups

2. Monochromator:
Convert polychromatic light to monochromatic light
(1) Prism:-
 Quartz prism can be used between 0.8-3 µm and cannot be used above 4
µm because above 4 µm, it starts absorbing radiation strongly.
 NaCl prism is widely used in range of 5-15 µm and cannot be used above
15 µm because it absorbs radiation strongly.
 CsBr-KBr prism is used for far IR (15-40 µm)
 LiF2 prism is used for near IR(1-5 µm).
Disadvantages of prism:-
 Resolution is not good as that of grating monochromator.
 They absorb moisture and so fogging occurs.
 So it require polishing which can be done by polishing agent i.e. slurry of
aloxite and bansite which is prepared in ethanol-ethylene glycol water.
 Slurry is spreaded on silk cloth stretched on flat surface.
 Prism is polished by rubbing on flat surface and wipped by dry cloth.
2 types of monochromator:-
(1) single pass
(2) double pass/ double monochromator:-
 It has high resolution because here radiation passes 4 times
 Here 2 plain mirrors are to be used.
(2) Grating:-

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 Usage of reflection grating is more common than prism.

 Either made up of glass or plastic coated with aluminum.
Advantages:-
 Covered with Aluminium so strong enough so life-time is high.
 Produce linear dispersion
 Not affected by moisture
 High resolution
3. Sample compartment and sample Preparation:-
 Sample handling is most difficult and most time consuming part of IR
analysis. So it is most important.
 Sample should be placed in sample holder or cell or cuvettes.
 Solvents used to prepare sample solutions has tendency to absorb the IR.
The sample cell windows should be transparent in the spectral region of
interest and are usually much narrower (0.01 – 1 mm) than those used in
UV-Visible regions
 There is no rugged window material for cuvettes that is transparent and
also inert over this region.
 The alkali halides are widely used, particularly NaCl, which are
transparent at wavelengths as long as 625cm-1.
 The sample cells are demountable and teflon spacers are used along with
the sample cell to adjust the path length. Teflon has only C-C and C-F
absorption bands. For frequencies less than 600cm-1, a polyethylene cell
is useful.
 Fixed path length cells are also available which can be filled or emptied
with hypodermic syringe.
 Sodium chloride windows are most commonly used but their surface
become fogged due to absorption of moisture and thus need polishing
with buffing powder which return them to their original condition.
 AgCl3 is often used for moist samples or aqueous solutions. But it is soft,
is easily deformed, and darkens on exposure to visible light.
 Sampling of the substance depends on state of the sample i.e., Solid,
Liquid or Gas. The intermolecular forces of attraction are most operative
in solid phase and least in case of gases. The sample of same substance
shows shift in frequencies of absorption as it passes from solid to the
gaseous state. In some cases additional bands are also observed with the
change in state of the sample.

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 Important to mention the state of the sample and solvent for scanning in
IR region for correct interpretation of spectra. The samples whose spectra
are to be recorded must be pure and free from water.
(A)GAS:-
 The gas sample cell is similar to cell for liquid samples and made of KBr,
NaCl and so on.
 The spectrum of low boiling liquid or gas can be obtained by permitting
the sample to expand into an evacuated cylindrical cell equipped with
suitable windows.
 Gas molecules are examined after removing moisture or water vapors
 A variety of cylindrical cells are available with path lengths ranging from
10cm up to 10m long or more.
 The longer pathlength are obtained in compact cells by providing
reflecting internal surfaces containing gold surfaced mirror, so that so that
beam makes numerous passes through the sample before exiting from the
sample cell.
 Multiple reflections can be used to make effective path length as long as
40m
 Choice depends on
o Nature of gas
o Concentration
 Path length should be long
o If highly absorbing gas, then 10cm length is sufficient
 If less absorbing gas, then more than 10cm length is required.
(1) single path cells:-
 It posses halide window.
 It possess two pots -entrance pot and exit pot
 Both are fitted with valve which is open and connected to tubes which
allows entry of gas from entrance valve till it recover all space and exit
from exit pot.
 Gas is filled at high pressure, so more amt of gas come out of exit pot.
(2) Multipath cell:-
 It is used for gas which required longer path length in cell.
 Internal surface is reflective so gas beam can possess no. of paths.
 Internal surface coated with Ag/Au.
 Low boiling point liquid sample allow to expand.

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 Gas must not react with the cell windows or the reflecting surfaces.
 Gas analysis is performed with IR but the method is not commonly used
because of its lack of sensitivity.
 Moisture should be avoided.
 Its strong absorption bands at 3710cm-1 and 1625cm-1 may interfere in the
analysis. In addition, the windows and other instrument components
which are constructed of soluble salts may be damaged.

(B) LIQUID:-
 Liquids at room temperature
 Rectangular cells made of NaCl, KBr or ThBr
 Sample thickness should be 0.01-0.05mm to give transmittance between
15% and 70%.
 If a cell possesses good quality windows, flat and parallel, its thickness, t,
in cm can be calculated from the following equation:
 2t=N/w1-w2
 Where, N is the number of fringes between wavenumber w1
and w2
 For double beam work, ―matched cells‖ are generally employed. One cell
will contain the sample while the other will have solvent used in the
sample. Matched cells must have same thickness.
 All cells should be protected from moisture because they dissolve in
water.
 For similar reasons organic liquid samples must be dried before pouring
into cells.
 For low viscous liquid  filled in cuvette.
 For highly viscous liquidsandwitched between 2 halide plats.
o For low viscous liquid:
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 Liquid sample is filled in cuvettes.

 Sample is taken by use of syringe.
 Dimension of cuvettes is in between 0.01-1 mm.
 Solvent should be transparent to IR
 But no solvent is available which is transparent in IR region.
o Desirable characteristic of solvent:-
• Transparent in wavelength range
• Not interact with solute, i.e. it should be inert.
• Adequate solubility property (sample should be soluble in solvent.)
• Refractory index of solvent should match to salts which are used to
prepare cell.
• We can‘t use polar solvents, as they absorb moisture.
• For that add 2,2-dimethoxypropane,so if any moisture is present that will
react with it.
• Examples of solvents used:-
(A) Combination of n-heptane and tetrachloro ethylene
n-heptane --- transparent in IR region of 250-1000 cm-1
tetrachloro ethylene--- transparent in IR region of 1000-4000 cm-1.
Range of mixture in IR region---250-4000 cm-1.

(B)CCl4 and CS2 combination

CCl4 transparent in 1350-4000cm-1
CS2 transparent in 625-1350 cm-1
So range of mixture is 625-4000 cm-1
• CS2 not used in sample containing primary/secondary amine or alcohol.
• Dioxane,CHCl3, and DMF(dimethyl formamide)-polar solvents are used.
• Now a days, ultra microcavity as a sample holder is available. For that
microlitre sample is required
For highly viscous liquid:-
• Two flat plates of halide salt is used and in between sample solution is
placed.
• Plates are made up of CsBr/CsI.
• Thickness should be very less which should be in range of 0.01-0.1 mm.
• Plats are held together by capillary action.
• For, volatile liquid AgCl plates are used.
(C) SOLID:-

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The solid whose IR spectra are to be recorded can be sampled in various

ways:
1. Nujol Mull Technique
2. Pressed Pellet Technique
3. Solid films
4. Solids run in solution
1. Nujol Mull Technique

 Mull is a thick paste formed by grinding an insoluble solid with an inert

liquid and used for studying spectra of the solid

Preparation of paste by addition of mulling oil (mineral oil)

• 2 to 5 mg drug is taken and grinded in a smooth agate mortar.
• Grinding is continued after the addition of 1 to 2 drops of mulling oil to
form a paste.
• Grinding should be such that particle size should be <2 µm to avoid
excessive scattering of radiation.
• This paste is then sandwitched between 2 halide plates and then used for
spectral measurement.
• Mulling oil is nujol (trade name for high boiling point petroleum oil,
hydrocarbon) which is widely used because characteristic of it in IR
region is known.
• 4 different peak for nujol oil-2915,1462,1376,719 cm-1
• If any sample absorbing at this range, we can‘t use nujol.
• For that other halogenated polymer like flourolube is used.
• Other mulling oil is Hexachlorobutadiene
• The use of both Nujol and flurolube mulls make possible IR scan sample
in range 250-4000cm-1 region.
• This technique is good for qualitative analysis but not for quantitative
analysis.

Disadvantage:-
• Although Nujol is transparent in most part of the IR spectra but it has
characteristic absorption of C-C and C-H vibrations of hydrocarbons at
2915, 1462, 1376 and 719cm-1.
• This is the major drawback in using Nujol for certain compounds which
may have absorption in the region similar to Nujol.

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(2) Pressed Pellet Technique (OR Pellet disc method)

• Finely ground solid sample is mixed with about 100 times its weight of
powdered KBr.
• It is prepared by mixing 1-2 mg of drug with 100 mg of KBr and grinding
and mixing is done in mortar or small vibrating ball-mill used until
particle size is < 2 µm which avoid scattering.
• The mixture is then pressed with special dies under high pressure 10000-
15000 psi into a transparent disk. Best results are obtained if the disk is
formed in vacuum to remove occluded air.
• Otherwise 2 additional peaks appear at 1640-3450cm-1
• Special dyes and punch are used to press the powder in specific shape.
• By this method, sample which is as small as 1 µg can be examined.
• The powder (KBr + sample) is introduced as shown, and then the upper
screw ‗A‘ is tightened until the powder is compressed into a thin disc.
• After compressing the sample, one removes the bolts (A and A‘) and
places the steel cylinder with the sample disc inside it in the path of the
beam of IR spectrometer and a blank KBr pellet of identical thickness is
kept in the path of reference beam.

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Advantages over Nujol Mull Technique:-

1) KBr eliminates the problem of bands which appear in the IR
spectrum due to the mulling agent. In this case, no such bands
appear.
2) KBr pellets can be stored for long periods of time.
3) As the concentration of sample can be suitably adjusted in the
pellets, it can be used for quantitative analysis.
4) The resolution of the spectrum in the KBr is superior to that
obtained with mulls.
Disadvantages:-
1) It always has a band at 3450cm-1, from the OH group of moisture
present (always) in the sample. Thus, care must be observed in
investigations concerning the region of the OH band in the sample.
2) The high pressure involved during the formation of pellets may
bring about polymorphic changes in crystallinity in the samples,
(especially inorganic complexes) which may cause complications
in IR spectrum.
3) This method is not successful for some polymers which are
difficult to grind with KBr.
(3)Deposited films (solid films):-.
• Useful only when the material can be deposited from solution or cooled
from a melt as microcrystals or as a glassy film.
• Useful for Polymers, resins and amorphous solids
• Dissolve them in reasonable volatile solvent and solution is formed.
• Pour solution on rock-salt plate

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• Evaporate solvent by gentle heating.

• Care must be taken to free the sample of solvent.
• Films of crystalline solid generally causes excessive light scattering.
• Specific crystal orientation may lead to spectra differing from those
observed for randomly oriented particles such as exist in a mull or a
halide disk.
• This technique is useful for rapid qualitative analysis but is not suitable
for quantitative analysis.
• Used for obtaining IR spectra of plastic and resin.
• Disadvantages:- cause excessive scattering
(4) Solids Run in Solution:-
• Solids may be dissolved in a non-aqueous solvent.
• No chemical interaction between solute and solvent and also solvent does
not absorb in the studied range.
• A drop of the solution is placed on an alkali metal disk and the solvent
allowed to evaporate, leaving a thin film of the solute, or the entire
solution is placed in a liquid sample cell.
• If the solution can be prepared in a suitable solvent then the solution is
run in one of the cells for liquids.
• But this method is not for all solids as suitable solvents are limited and no
single solvent is transparent throughout the IR region.
Solvents
• Solvent must be dry and transparent in the region of interest.
• A common pair of solvents is CCl4 and CS2.
• CCl4 is relatively free of absorption at frequencies above 1333cm-1,
whereas CS2 shows little absorption below 1333cm-1.
• Solvent and solute combinations that react must be avoided.
• For example, CS2cannot be used as a solvent for primary or secondary
amines. Amino alcohols react slowly with CS2 and CCl4.
• To obtain the spectra of polar materials that are insoluble in CCl4 and
CS2, chloroform, methylene chloride, acetonitrile and acetone are
useful solvents.

4. DETECTORS:-
(1)Thermal detector
a. Thermocouple
b. Bolometer

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c. Thermister
d. Golay cell
(2)Pyroelectric Detectors
(3)Photon detectors

(1) Thermal detectors:-

• Important and used in simple IR
• Used over wide range of wavelength
• It measures radiant energy by means of its heating effect.
• With these devices, radiation is absorbed by a small black body material and
resultant temperature rise is measured.
• Temperature rise due to IR which is converted to potential difference and
that is measured. This potential difference depends on the amount of
radiation.

Disadvantages:-
• Low sensitivity.
• Thermal noise i.e. due to surrounding of detector. which is reduced by
<1> enclosing the detector in evacuated housing with KBr/CsI window
and operated in presence of vacuum.
<2> By placing chopper
• Here, radiation come out of analyte will pass through chopper which is
operated at low frequency because sensitivity of detector is less.
• Extraneous (Unwanted) radiation will be differentiated by analyte signal
which has frequency of chopper.

Types of thermal detectors:-

(1) Thermocouples
(2) Bolometers
(3) Thermisters
(4) Golay cell
They are less sensitive than pyroelectric and photon detectors.
(1) Thermocouple:-
• Most widely used due to simple construction.
• It works on principle of Peltier effect i.e. temperature difference between
2 junction rises potential difference which depends upon the amount of
radiation falling upon the hot junction.

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• Constructed by 2 junction which is prepared by fusing 2 similar metal to

dissimilar metal
• Metals areBismuth-Antimony
• Junctions are coated with metallic oxide film.
• Placed in evacuated steel housing having KBr / CsI window.
• One junction is cold junction and other is hot junction.
• Potential difference between 2 junction is 6-8μV
• Cold junction is kept at constant temperature and hot junction is exposed
to IR radiation So temperature of hot junction increases.
• here, temperature difference between Cold junction and hot junction is
calculated and converted to potential difference
• Potential difference of thermocouple is directly proportional to intensity
of IR radiation.
• Sensitivity is low and has slow response time.
• The thermocouple is usually connected to a preamplifier.
• A well designed thermocouple can detect temperature difference of 10-6
K, which corresponds to a potential difference of about 6 to 8 μV.
• The response time of a thermocouple is about 60 mili second.
• Prepared by making series of thermocouple by fusing and this is called as
thermopile for increasing sensitivity.
Advantage:-Independence of the response with changes in wavelength

(2) Bolometers (resistance thermometer):-

• Consist of single metallic strip like Pt or Ni or small strip of
semiconductor material.

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• When IR is absorbed strip is heated decrease in electrical resistance.

• Change in electrical resistance is converted to current.
• And measure amount of IR radiation.

• Constructed in form of Wheatstone bridge. On one arm of it, bolometer is

placed and on other arm, same type of strip is placed which is not
exposed to IR. Two arms are joined by Galvanometer.
• The bridge remains balanced, when no IR radiation fall on the bolometer,
no current flow.
• But as radiation fall, the bridge becomes unbalanced due to change in
electrical resistance and thus current through Galvanometer (G). The
amount of current flowing through galvanometer is a measure of intensity
of radiation falling on detector.
• The response time for bolometer is 4 mili second.
• Placed in evacuated steel housing with KBr/CsI window to minimize
thermal noise. Most widely used is germanium bolometer which is
operated at 1.5 K temperature and it is used in range 5 to 400 cm-1
(3) Thermistors:-
• Similar to bolometer but instead of metal, metallic oxide is used.
• Prepared by fusing mixture of metallic oxide such as cobalt, manganese
or nickel. Change in electrical resistance is converted to electrical
current.
(4) Golay cell or Golay Pneumatic detector

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Principle:-It responds to change in volume of non absorbing gas as a

function of temperature rise. As a result of expansion of the gas, the
pressure increases which is then converted to an electrical signal.
• Consist of small metallic cylinder in which inert gas like xenon is filled.
One side consists of black metallic plate and opposite side consists of
flexible metallic diaphragm.
• When no IR, Diaphragm is in contracted position.
• When contracted then the radiation is reflected on opaque surface.
• IR fall on metallic plate and so heated up. So temperature transfers to gas.
Diaphragm is pushed in outward direction. As soon as Diaphragm pushes
then radiation is reflected and passed through metallic grid and passes
from lens and detected by photocell due to change in angle of reflection.
The gas expansion is converted to electrical current.
• In other type of pneumatic detector-no need of visible optical system
• Here 2 capacitor plates are used.
• Here, capacitance between 2 capacitor plates is measured as electrical
current.
Advantages
1) Response is linear over the entire range from UV, Visible upto IR region
2) It require less response time about 10-2 sec, hence much faster than other
thermal detectors

Disadvantage
More expensive and bulky

(2)Pyroelectric Detector:-
• Constructed from pyroelectric material (dielectric material)

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• Here, polarised material is used  polarization depends on temperature

and it is continuous after removal of electrical field.
• Here,2 electrode are used and in between them the plate of pyroelectric
material is placed. And when IR fall change in charge it is registered
as electrical current.
Constructed from pyroelectric material (dielectric material) which have
both properties i.e. thermal and electrical.
• Example of pyroelectric material:-
• Triglycine sulfate
• Barium titanate
• Lithium niobate
• Lithium tantalate
• Here, polarised material is used  polarization depends on temperature
and it is continuous after removal of electrical field.
• Here, 2 electrode are used and in between them the plate of pyroelectric
material is placed. When IR fall change in charge it is registered as
electrical current.
• They should not heated above temperature which is known as curie point.
• Above curie point, polarisation capacity is lost.
e.g.:-diglycerine sulfate lose its polarization capacity above 47 °C
• Advantage:-responding time is less i.e. upto 1milisec or <1milisec
• Current depends on surface area of crystal

(3)Photon detector:-
• Principle:-as temperature increases decreases in electrical resistance.
• Constructed from thin film of both intrinsic and extrinsic semiconductor
material
e.g.:-lead sulphide, indium antimonide, indium arsenyl, lead selenile,
Hg/Cd teluride

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• Semiconductor material placed on glass surface which is non-conducting

at lower energy state. This is sealed into an evacuated envelop to protect
the semiconductor from atmosphere.
• When IR radiation fall on these material, the non conducting valence
electron are raised to higher energy state which can conduct and produces
a signal by decreasing the electrical resistance of the semiconductor
• Here, electrical current is measured.
• Change in electrical resistance is converted to electrical current.
• The voltage drop across the resistor is proportional to the amount of
radiation
Advantages:
o 10-100 times more sensitive than thermal detector.
o Response is more rapid than that of thermal detectors.
Disadvantage: Useful over a narrow wavelength range (1 to 6 micrometer)

S.N Parameters Thermocou Thermister Pyroelectric Golay

o ple
or or
Or
Bolometer Pneumatic
Thermopile

1 Principle Pelletier Whetsto Electric Expansi

. effect ne bridge polarizatio on of
n gases

2 Materials Bismuth Sintered TGS,

. used and oxides of DTGS, generall
Antimon Mn, Co, LiTGO3, y CO2
y, coated Ni LiTubO3
by metal
oxides

3 Material Thermall Thermall Non-center Inert

. should be y active y symmetric nature
sensitive crystal
resistors

4 Descriptio Half - ----------- ------------ Metal

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. n junction- --- cylinder

hot closed in
b/t metal
Alternate
plate and
-junction
Ag
-cold

5 Conversio Radiant Change Thermal Expancti

. n unit to in alteration on of gas
Electric resistanc to to
signal --- e-Q E.polarizat pressure
measured ion to
e.signal

6 Used Photocua Diffusive FTIR Non –

. stic reflectan dispersiv
spectrosc ce e IR
opy

7 Response 30 sec 4 sec multiple 0.01sec

. time scanning

Instrumentation
Types:
1. Single Channel Scanning Instruments:
Single piece of data transmitted through single channel.
Dispersive Spectrometers: With a monochromator to be used in the mid-IR
region for spectral scanning and quantitative analysis.
1) Single beam spectrometer
2) Double beam spectrometer
• Non dispersive spectrometer: Use filters for wavelength selection or an
infrared-absorbing gas in the detection system for the analysis of gas at
specific wavelength.
1) Positive non dispersive
2) Negative non dispersive
2. Single Channel/Multiplex Instruments: In which multiple piece of data are
simultaneously transmitted through single channel

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 FT-IR Spectrometers: Widely applied and quite popular in

the far-IR and mid-IR spectrometry
 Hadamard Spectrometers
3. Multichannel/Multiplex Instruments
 Multichannel detector/no mirror scanning interferometer
 Focal Array Detector/Step-Scan FT-IR

Dispersive Infrared Spectrophotometer

• Scanning instrument uses a frequency separation device (grating) to resolve
the IR radiation into individual spectral resolution elements.
• An exit slit isolates a specific spectral resolution element 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.
• Radiation from the source is flicked between the reference and sample paths.
Often, an optical null system is used.
• This is when the detector only responds if the intensity of the two beams is
unequal. If the intensities are unequal, a light attenuator restores equality by
moving in or out of the reference beam. The recording pen is attached to this
attenuator.

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• Light from Radiation source split into two beams with mirrors, half passing
from sample cell and other half from reference cell.
• The reference beam then passes through attenuator or optical wedge and into
chopper.
• The chopper consists of motor driven disk that alternatively reflects the
reference or transmits the sample beam to the monochromator.
• After dispersion by prism or grating, the alternating beams fall on detector
and converted into electrical signal.
• The signal is amplified and passed to synchronous rectifier that is connected
to chopper. Synchronous rectifier is device which mechanically or
electrically coupled to the chopper to cause the rectifier switch and the beam
leaving the chopper to change simultaneously.
• If two beams are identical in power than constant DC signal is produced. If
they differ in power than fluctuating or AC signal is produced, the phase of
which is determined by which beam is more intense.
• The current from the rectifier is filtered and further amplified to drive a
synchronous motor in one direction or the other depending upon the phase of
the input current.
• The synchronous motor which is attached to both attenuator and pen drive of
recorder and causes both to move until a null point is achieved.
• Low frequency chopper (5 to 13 cycles per minute) that permits the detector
to discriminate between signals from source and signals from extraneous
radiation.
• In null type, the power of the reference beam is reduced or attenuated to
match that of the beam passed through sample
Monochromator is placed after sample compartment in IR:
• Because the IR radiation compared to UV/visible radiation is not
sufficiently energetic to cause photo decomposition of the sample.
• The most scattered radiation generated within cell compartment which
is effectively removed by monochromator and do not reach transducer.
Limitation:
1. Response of attenuator system always lags behind the transmittance
changes, particularly in scanning region where the signal is rapidly
changing.
2. Momentum associated with both attenuator and recorder may result in
pen drive overshooting.

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3. In region where transmittance reaches 0 almost no radiation reach

transducer and exact null position can‘t be established.
Disadvantages
 Slow Scanning process
◦ "step-wise" nature of spectral acquisition
 Limited energy throughput.
◦ optical dispersion process throws energy away
◦ the exit and entrance slits allow throughput of only a small
fraction of the total IR energy (> 98%)
 Difficult to increase the S/N by multiple scanning
◦ wavelength reproducibility not sufficient due to mechanical
backlash
 Digitizing difficult due to mechanical backlash
◦ computer interfacing and processing difficult

Non dispersive Infrared spectrophotometer

Advantages:
1. Less complex
2. More rugged
3. Easier to maintain
4. Less expensive
5. No wavelength selection device is used.

a) Positive filter non dispersive IRS

• System contains source, 2 mirrors, 2 detectors, sample and reference

compartments

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• When there is no sample in the sample cell, the detector A and B

absorb the IR radiation and their temperature is increase.
• The temperature difference between two detectors is measured and it
is minimum when there is no sample in light path.
• When sample material is introduced in sample cell, it absorbs
radiation.
• The light falling on detector A is therefore decreased in intensity and
temperature of detector decreases and temperature difference T 2-T1
increases between two detectors.
• As concentration of sample increases,T2-T1 increases
• So the relationship between T2-T1 and sample concentration is positive
in slope –hence the term positive filter.
• The reference cell containing N2 gas does not absorb in IR
b) Negative filter non dispersive IRS

• Detector A contains non absorbing gas such as N2 and detector B

contains compound to be determined.
• When no sample in sample cell, temperature of detector B is a
maximum and detector A is at room temperature,T2-T1 is maximum
• As the concentration of sample increased, light is absorbed by sample
• So less light is fall on detector B, Temperature decreases, T2-T1
decreases
• The relationship between T2-T1 and sample concentration is negative
in slope, hence named negative filter.

Multiplex IR Instruments
Multiplexing: The simultaneous transmission of multiple resolution
elements along the same channel

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◦ All of the energy falls on the detector simultaneously measure

all the time.
Types of Multiplex instruments
◦ Transform multiplex devices- Fourier and Hadamard transform
spectrometers.
◦ Frequency-division multiplexing instruments.
◦ The instruments use a single detection channel to record the optical
information that has been encoded.
◦ Each individual measurement in a set of encoded data contains
information about all points in the spectrum.

APPLICATIONS OF IR SPECTROPHOTOMETRY:
Quantitative IR absorption methods differ somewhat from UV/Visible due to
complexity of spectra, narrowness of absorption bands and instrumental
limitations of IR instruments. It is particularly useful in pharma industry in
identification of drugs and detection of impurities.
A. Qualitative Analysis:
(1)Identification of substance:
• Each substance gives a characteristic IR spectrum.
• Thus, identification of substance done by comparing the IR spectra of
sample with IR spectra of authentic standard substance
If two samples afford identical spectra under similar conditions of
measurements they must be samples of same substance
• For organic compounds, large number of absorption bands found in IR
spectra, so probability that any two compounds produce identical spectra
is almost zero.
• Enantiomer produces exactly identical spectra so IR fails to distinguish
between enantiomer.
• Various pharmacopoeias like IP, BP, USP have included ―IR spectra‖ as
one of the test for identification of many drugs and substances.
(2)Determination of molecular structure:
• From the position of absorption bands in the spectrum, it is possible to
establish nature of groups in molecule
e.g. If a spectrum contains a strong band at 1717cm-1, the compound must
contain a carbonyl group.
(3) Studying the progress of reactions:

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Progress of a chemical reaction can be readily followed by examining spectra of

small portions of reaction mixture withdrawn from time to time.

e.g. Oxidation of an alcohol to a carbonyl compound

• There is gradual disappearance of hydroxyl stretching frequency at
3600-3650 cm-1 and appearance of carbonyl stretching frequency at
1680-1780 cm-1
(4)Detection of impurities:
• Possible to determine whether given sample of compound is pure or not
• By comparing the spectra of sample with reference spectra, the
spectrum of crude sample is blurred and reduces sharpness of
individual bands and contains many extra bands.
(5) Isomerism in organic chemistry
Detection of geometrical isomerism:
• Trans isomer give simpler spectra than cis isomer because vibration of
trans isomer give rise to little or no change in dipole moment
• Trans isomer shows band at lower frequency
e.g. cis alkenes-970 cm-1 and trans-730-650 cm-1
Detection of tautomerism:
• They have different chemical bonds
• e.g. Thiocrboxylic acids – two tautomers
C-O-H and C=O

S S-H
These two tautomer can be identify by presence or absence of bands due to O-
H, S-H, C=O, C=S and thus two groups can be differentiated
(6)Determination of Shape or symmetry of compound:
e.g. NO2, if it is liner only two bands are obtain like in CO2
• If three bands are obtain than bent structure (non linear) like in H2O
• NO2 gives three peaks at 750, 1323, 1616cm-1
• It shows NO2 is a bent structure, not a linear
(7)Presence of water in sample:
Presence of water in sample can be detected readily. Because it absorbs at 3600-
3200 cm-1, 1650-1620 cm-1 and 600-450 cm-1
(8)Measurement of paints and varnishes:
(9)Estimation of old paintings and artifacts:
(10) Used in forensic science

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(11)In industry:
• To determine impurities in raw materials
• For quality control, by continues checking of composition and the percent
present of the required product
• For identification of materials made in industrial research laboratories, or
materials made by competitors
• Beeswax, carnuba wax, and other waxes used for coating floors and
furniture can be identify
• To identify odor and taste components of food
• To distinguish one polymer from another
• To identify atmospheric pollutants in atmosphere
B. Quantitative Analysis:
• IR has traditionally been used for qualitative analysis. Quantitative
data obtained are significantly inferior in quality compared with
UV/Visible data.
• All quantitative analysis by IR are governed by Beer-Lambert‘s law.
But in IR deviations from Beer‘s law are more due to
1. Weak intensity of light source 2. Weak detection/sensitivity
by detectors and 3. Use of wide slit widths (due to low
intensity of source and low sensitivity of detectors).

Difficult to use quantitatively due to chemical or instrumental effects.

 Hence, it is necessary to check the plot of concentration vs %T before
proceeding for IR spectra recording at a selected wavelength calibration
curves are that required for quantitative work.
 The quantitative determination of various compounds by infrared
spectroscopy is based on the determination of the concentration of one of
functional groups of compound being estimated. For example, if there is a
mixture of hexane and hexanol, the hexanol can be determined by
measuring how much absorption is taking place by OH bond.
 Convenient for measuring concentrations of substance by using Beer-
Lambert‘s law.
A= -log I1/ I0 =abc
• Large sloping background often interferes with normal spectrum.
• The baseline method corrects involves selection of absorption band of
the substance under analysis which is sufficiently separated from other
matrix peaks and corrected.

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