Mr. Sanket P. Shinde provides an overview of infrared (IR) spectroscopy, detailing its importance in identifying functional groups in various compounds through their vibrational characteristics. The document explains the different regions of the infrared spectrum, the relationship between dipole moments and IR activity, and the significance of vibrational modes, emphasizing the use of Hook's law in understanding absorption frequencies. Detailed tables present typical infrared absorption frequencies for various functional groups, illustrating the diversity in vibrational behavior and energy levels.

1. Mr. Sanket P. Shinde
Assistant Professor
Pune-Maharashtra.

2. Mr. S. P. Shinde
❑ Infrared spectroscopy is one of the most important analytical technique
used for determining the functional group present in both inorganic &
organic compounds.
❑ Liquids, solutions, paste, powders, films, fibres, gases, and surfaces can
all be examined with a choice of sampling technique.
❑ IR spectroscopy is a technique based on the vibrations of the atom of a
molecule.
❑ IR spectroscopy measures the vibrations of atoms, through which it is
possible to determine the functional groups.
❑ An infrared spectrum is commonly obtained by passing infrared radiation
through a sample and determining the fraction of the incident radiation
which is absorbed at a particular energy.
❑ The term infra red covers the range of the electromagnetic spectrum
between 0.78 and 1000μm.
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3. Mr. S. P. Shinde
Infra red region has been divided into three sections
1. Near infra red region
2. Middle infra red region
3. Far infra red region
The wavelength ranges for the above regions are given below.
Region Wavelength range (μm) Wave number range (cm-1)
Near 0.78-2.5 12800-4000
Middle 2.5-50 4000-200
Far 50-1000 200-10
Continue…
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4. Mr. S. P. Shinde
❑ Atoms or atomic groups in a molecules are in continuous motion with
respect to one another.
❑ IR spectra originate from the difference modes of vibration and rotation of
a molecule, whereas the UV-visible absorption bands are primarily due to
electronic transition.
❑ In order to absorb IR radiation, a molecule must undergo a net change in
dipole moment as a consequence of its vibrational or rotational motion.
❑ The dipole moment is determined by the magnitude of the charge
difference and the distance between the two centers of charge.
❑ The change in bond length or angle due to vibrational or rotational motion
must cause a net change in the dipole moment of the molecule.
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5. Mr. S. P. Shinde
❑ No net change in dipole moment occurs during the vibration or rotation
of homonuclear species such as O2, N2, or Cl2; consequently, such
compounds cannot absorb in the IR.
❑ Vibrational modes which do not involve a change in dipole moment are
said to be IR-inactive.
❑ With exception of a few compounds of this type, all molecular species
exhibit IR-active.
❑ The total energy of a molecule at any given moment is defined as the sum
of the contributing energy terms:
Etotal = EElectronic + Evibrational + Erotational + Etranslation
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6. Mr. S. P. Shinde
❑ Among these energies the vibrational energy component is a higher
energy term and absorption of energy by a molecule as the component
atoms vibrate about the mean centre of their chemical bonds.
❑ The fundamental vibrational frequency of molecule can be expressed by
Hooks Law.
❑ Where m1 and m2 are the component masses for the chemical bond under
consideration.
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7. Mr. S. P. Shinde
❑ Consider a bond and the connected atoms to be a spring with two masses
attached.
❑ Using the force constant k and the two masses m1 and m2 then the
equation indicates how the frequency of the absorption should change
as the properties of the system change.
❑ The hooks law provides link between the strength of the covalent bond,
masses of atoms and vibrational frequency.
❑ The greater the masses of attached atoms, the lower the IR frequency at
which bond will absorb.

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Continue…

8. Mr. S. P. Shinde
1. For a stronger bond (larger k value), μ increases :
As examples of this, in order of increasing bond strength compare:
• CC bonds: C−C (1000 cm-1), C=C (1600 cm-1) and C≡C (2200 cm-1)
• CH bonds: C−C−H (2900 cm-1), C=C−H (3100 cm-1) and C≡C−H (3300 cm-1)
2. For heavier atoms attached (larger m value), μ decreases :
As examples of this, in order of increasing reduced mass compare:
• C−H (3000 cm-1)
• C−C (1000 cm-1)
• C−Cl (800 cm-1)
• C−Br (550 cm-1)
• C−I (about 500 cm-1)
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9. Mr. S. P. Shinde
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Thus Hook’s Law states :
• The vibrational frequency is proportional to the strength of the spring;
the stronger the spring; the higher the frequency.
• The vibrational frequency inversely proportional to the masses at the
ends of the spring; the lighter the weights, the higher the frequency.
As per the Hook’s Law :
• Stronger bonds absorb at higher frequencies.
• Weaker bonds absorb at lower frequencies.
• Bonds between lighter atoms absorb at higher frequencies.
• Bonds between heavier atoms absorb at lower frequencies.
Continue…

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• The positions of atoms in molecules are not fixed; they are subject to a
number of different vibrations.
• There are two simplest modes of molecular vibrations in a molecule
which are infrared active and give rise to absorptions as given below.
1. Stretching vibrations :
Stretching vibrations can be explained as the change in inter-atomic distance
along bond axis. As shown in the diagram below, the stretching involves
vibrations of the atoms towards the centre atom by contraction and expansion
movement thus the distance between the atoms changes along the bond axis.
Symmetric Asymmetric

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There are two types of stretching vibrations :
• Symmetric stretching :
Symmetric stretching involves the change in inter atomic distance in
equal manner whereas asymmetric stretching involves unequal change.
• Asymmetric stretching :
Generally asymmetric stretching vibrations occur at higher frequencies
than symmetric stretching vibrations.
2. Bending vibrations :
• Bending vibrations are explained as change in angle between two bonds.
• The atoms vibrate by changing the bond angle between two atoms.
• They are also called as deformation vibrations.
• Bending vibrations occur at lower frequencies than stretching vibrations.
There are four types of bending vibrations which are as follows.

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Rocking: Rocking involves in-plane bending. The atoms are vibrated in the plane
of molecule towards one direction. The angle of the bonds changes towards the
same direction.
Scissoring: Scissoring involves in-plane bending. There will be vibrations of
atoms in the plane of molecule towards each other like movement of a scissor.
Wagging: Wagging involves out-of -plane bending in which the atoms vibrate
near to the attached atom at either side away from the plane.
Twisting: Twisting involves out-of-plane bending in which the atoms vibrate in
opposite directions away from the plane in twisted manner.

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• Transition from the ground state to the first excited state absorbs light
strongly in IR region and give rise to intense bands called the fundamental
bands.
• These bands in IR spectrum resulted due to the various stretching and
bending vibrations.
• For example, in a group that contains three or more atoms and at least two
of which are identical like, CH3, CH2, NO2, NH, give rise to two modes of
stretching vibrations that is symmetric and asymmetric.
• The CH3 group gives symmetric stretching vibration at about 2872 cm-1
and an asymmetric stretching vibration at about 2962 cm-1, These types of
bands are called as fundamental bands.
• These absorptions arise from excitation from the ground state to the lowest
energy excited state.

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• Vibrational frequency is defined as the rate at which the atoms of a
molecule vibrate.
• The vibrational frequencies for each functional groups are unique.
• Various functional groups vibrate in different frequencies.
• This unique property is used to identify the various functional groups
using IR spectrum.
• The following table gives a detailed account on vibrational frequencies
of various functional groups.

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Typical infrared Absorption Frequencies
Functional
Class
Stretching Vibrations Bending Vibrations
Range (cm-1) Intensity Assignment Range (cm-1) Intensity Assignment
Alkanes 2850-3000 Strong
CH3, CH2 & CH
2 or 3 bands
1350-1470
1370-1390
720-725
medium
medium
weak
CH2 &
CH3 deformation
CH3 deformation
CH2 rocking
Alkenes
3020-3100
1630-1680
1900-2000
medium
variation
strong
=C-H &
=CH2 (usually
sharp)
C=C (symmetry
reduces intensity)
C=C asymmetric
stretch
880-995
780-850
675-730
strong
medium
medium
=C-H & =CH2
(out-of-plane bending)
cis-RCH=CHR
Alkynes
3300
2100-2250
strong
variation
C-H (usually sharp)
C≡C (symmetry
reduces intensity)
600-700 strong C-H deformation

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Typical infrared Absorption Frequencies
Functional
Class
Stretching Vibrations Bending Vibrations
Range (cm-1) Intensity Assignment Range (cm-1) Intensity Assignment
Arenes
3030
1600 & 1500
variation
med-weak
C-H (may be
several bands)
C=C (in ring) (2
bands)
(3 if conjugated)
690-900 strong-med
C-H bending &
ring puckering
Alcohols &
Phenols
3580-3650
3200-3550
970-1250
variation
strong
strong
O-H (free), usually
sharp
O-H (H-bonded),
usually broad
C-O
1330-1430
650-770
medium
var-weak
O-H bending (in-
plane)
O-H bend (out-of-
plane)
Amines
3400-3500 (dil.
soln.)
3300-3400 (dil.
soln.)
1000-1250
weak
weak
medium
N-H (1°-amines), 2
bands
N-H (2°-amines)
C-N
1550-1650
660-900
med-strong
variation
NH2 scissoring (1°-
amines)
NH2 & N-H wagging
(shifts on H-bonding)

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Typical infrared Absorption Frequencies
Functional
Class
Stretching Vibrations Bending Vibrations
Range (cm-1) Intensity Assignment Range (cm-1) Intensity Assignment
Aldehydes &
Ketones
2690-2840(2 bands)
1720-1740
1710-1720
1690
1675
1745
1780
medium
strong
strong
strong
strong
strong
strong
C-H (aldehyde C-H)
C=O (saturated
aldehyde)
C=O (saturated ketone)
aryl ketone
α, β-unsaturation
cyclopentanone
cyclobutanone
1350-1360
1400-1450
1100
strong
strong
medium
α-CH3 bending
α-CH2 bending
C-C-C bending
Carboxylic
Acids &
Derivatives
2500-3300 (acids)
overlap C-H
1705-1720 (acids)
1210-1320 (acids)
1785-1815 ( acyl
halides)1750 &
1820 (anhydrides)
1040-1100
1735-1750 (esters)
1000-1300
1630-1695(amides)
strong
strong
med-strong
strong
strong
strong
strong
strong
strong
O-H (very broad)
C=O (H-bonded)
O-C (sometimes 2-peaks)
C=O
C=O (2-bands)
O-C
C=O
O-C (2-bands)
C=O (amide I band)
1395-1440
1590-1650
1500-1560
medium
medium
medium
C-O-H bending
N-H (1¡-amide) II band
N-H (2¡-amide) II band
Nitriles
Isocyanates,
Isothiocyanates,
Diimides, Azides
& Ketenes
2240-2260
2100-2270
medium
medium
C≡N (sharp)
-N=C=O, -N=C=S
-N=C=N-, -N3, C=C=O

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Functional Class Characteristic Absorptions
Sulfur Functions
S-H thiols 2550-2600 cm-1 (wk & shp)
S-OR esters 700-900 (str)
S-S disulfide 500-540 (wk)
C=S thiocarbonyl
1050-1200 (str)
S=O sulfoxide
Sulfone
sulfonic acid
sulfonyl chloride
Sulfate
1030-1060 (str)
1325± 25 (as) & 1140± 20 (s) (both str)
1345 (str)
1365± 5 (as) & 1180± 10 (s) (both str)
1350-1450 (str)

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Functional Class Characteristic Absorptions
Phosphorous Functions
P-H phosphine 2280-2440 cm-1 (med & shp)
950-1250 (wk) P-H bending
(O=)PO-H phosphonic acid 2550-2700 (med)
P-OR esters 900-1050 (str)
P=O phosphine oxide
Phosphonate
Phosphate
phosphoramide
1100-1200 (str)
1230-1260 (str)
1100-1200 (str)
1200-1275 (str)

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Functional Class Characteristic Absorptions
Silicon Functions
Si-H silane 2100-2360 cm-1 (str)
Si-OR 1000-11000 (str & brd)
Si-CH3 1250± 10 (str & shp)
Oxidized Nitrogen Functions
=NOH oxime
O-H (stretch)
C=N
N-O
3550-3600 cm-1 (str)
1665± 15
945± 15
N-O amine oxide
Aliphatic
aromatic
960± 20
1250± 50
N=O nitroso
nitro
1550± 50 (str)
1530± 20 (as) & 1350± 30 (s)

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• When a fundamental vibration couples with an overtone or combination
band it results in different vibration, this coupled vibration is called Fermi
resonance.
• Fermi resonance results in the splitting of two vibrational bands.
• The wave functions for the two resonant vibrations mix according to the
harmonic oscillator and the result is a shift in frequency and a change in
intensity in the spectrum.
• As a result, two strong bands are observed in the spectrum, instead of the
expected strong and weak bands.
• Example In the given figure, the bands shown in the top represent two
fundamental vibrations without Fermi resonance whereas the bands shown
in the bottom represent change in bands as a result of Fermi resonance.
• The Fermi resonance resulted in increase in energy of first band and
decrease in energy of second bang thus resulted in "Fermi doublet".

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Overtones
• Overtones occur when a vibrational mode is excited from v=0 to v=2,
which is called the first overtone, or v=0 to v=3, the second overtone.
• Overtones are generally not detected in larger molecules.
Combination Bands
• Combination bands are observed when more than two or more
fundamental vibrations are excited simultaneously.
• One reason a combination band might occur is if a fundamental
vibration does not occur because of symmetry.
• Combination implies addition of two frequencies, but it also possible
to have a difference band where the frequencies are subtracted.

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Vibrational coupling
• In addition to the vibrations mentioned above, interaction between
vibrations can occur (coupling) if the vibrating bonds are joined to a
single, central atom. This is called as vibrational coupling.
Vibrational coupling is influenced by a number of factors.
1. When a common atom is present between two bonds and during their
vibration coupling of stretching vibrations occurs. When a bond is
common between two vibrating groups then coupling of bending
vibrations occurs.
2. Coupling between a stretching vibration and a bending vibration occurs If
the stretching bond is one side of an angle varied by bending vibration
3. Coupling is greatest when the coupled groups have approximately equal
energies.
4. No coupling is seen between groups separated by two or more bonds.

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• A typical infrared spectrum can be visually divided into two regions.
• The left half, above 1500 cm-1 called as Functional group region.
• The right half from 1500-500 cm-1 called as Fingerprint region.
1. Functional group region :
• The first region above 2000 cm-1 usually contains relatively few peaks, but
some very diagnostic information can be found in this region. This region
is called as functional group region.
• First, alkane C- H stretching absorptions just below 3000 cm-1 demonstrate
the presence of saturated carbons, and signals just above 3000 cm-1
demonstrate unsaturation.
• A very broad peak in the region between 3100 and 3600 cm-1 indicates the
presence of exchangeable protons, typically from alcohol, amine, amide or
carboxylic acid groups.
• The frequencies from 2800 to 2000 cm-1 are normally void of other
absorptions, so the presence of alkyne or nitrile groups can be easily seen
here. C = O group show absorption at 1715 cm-1

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2. Fingerprint region :
• The region is between 1500 to 500 cm-1 usually contains very
complex bands of absorptions.
• These absorption bands are due to the mainly all manner of bending
vibrations within the molecule. This is called the fingerprint region.
• During interpretations it is more difficult to characterize individual
bonds in this region as compared to the functional group region at
higher wave numbers.
• The importance of the fingerprint region is that each different
compound produces a different pattern of troughs thus useful in
interpretation.

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• Fourier Transform Infrared (FT-IR) spectrometry was developed to
overcome the limitations encountered with dispersive instruments.
• The main difficulty was the slow scanning process.
• A method for measuring all of the infrared frequencies simultaneously,
rather than individually as with dispersive instruments
• A very simple optical device called an interferometer was developed.
• 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.
• Thus, the time element per sample is reduced to a matter of a few
seconds rather than several minutes.
Introduction

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FTIR spectrometers have several prominent advantages:
1. The signal-to-noise ratio of spectrum is significantly higher than the
previous generation infrared spectrometers.
2. The accuracy of wave number is high. The error is within the range of
±0.01 cm-1
3. The scan time of all frequencies is short (approximately 1 s).
4. The resolution is extremely high (0.1 -0.005 cm-1).
5. The scan range is wide (1000 - 10 cm-1).
6. The interference from stray light is reduced.
Due to these advantages, FTIR Spectrometers have replaced dispersive IR
spectrometers.

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Fourier transform infrared spectroscopy is preferred over dispersive or
filter methods of infrared spectral analysis for several reasons:
• It is a non-destructive technique.
• It provides a precise measurement method which requires no
external calibration.
• It can increase speed, collecting a scan every second.
• It can increase sensitivity.
• It has greater optical throughput.
• It is mechanically simple with only one moving part.

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• FTIR measures all frequencies simultaneously rather individually using an
interferometer.
• An interferometer is the exceptional component in FTIR compared to the
dispersive IR.
• All of the infrared frequencies are “encoded" into the signal of
interferometer which can be measured very quickly.
• Thus Interferometer is used for the rapid analysis.
• However the measured interferogram signal cannot be interpreted directly.
• Hence frequency spectrum is required which is a plot of the intensity at
each individual frequency to make identification.
• A means of "decoding” the individual frequencies is required.
• This can be accomplished via a well-known mathematical technique
called the Fourier transformation.
Theory

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• Most interferometers employ a beamsplitter which takes the incoming
infrared beam and divides it into two optical beams.
• One beam reflects off of a flat mirror which is fixed in place. The other
beam reflects off of a flat mirror which is on a mechanism which allows
this mirror to move a very short distance (typically a few millimeters)
away from the beamsplitter.
• The two beams reflect off of their respective mirrors and are recombined
when they meet back at the beamsplitter.
• Because the path that one beam travels is a fixed length and the other is
constantly changing as its mirror moves, the signal which exits the
interferometer is the result of these two beams “interfering” with each
other.
• The resulting signal is called an interferogram which has the unique
property that every data point (a function of the moving mirror position).
Continue…

32. Mr. S. P. Shinde
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FTIR Instrumentation
• The major different between dispersive IR and FTIR is the inclusion of
interferometer.
• All other components are almost same as like that of a dispersive IR
spectrometer.
Components are explained individually as follows.
1. Source of light
2. Interferometer
3. Sample compartment
4. Detector
5. Read out device

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Block diagram of FTIR spectrometer

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• IR instruments require a source of radiant energy which emits IR
radiation which must be steady, intense enough for detection and
extend over the desired wavelength.
• In general, an inert solid is electrically heated to a temperature in the
range 1500-2000 K. The heated material will then emit infra red
radiation.
1. Nernst glower:
• The Nernst glower consists of a cylinder made up of rare earth oxides.
• The length of the cylinder is 20mm with a diameter of 1-2mm.
• At the end of the cylinder platinum wires are attached and a current is
passed through the cylinder.
• The Nernst glower can reach temperatures of 2200 K.
1. Source of light

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2. Globar source
• The Globar source is a silicon carbide rod (5mm diameter, 50mm long)
which is electrically heated to about 1500 K.
• Water cooling of the electrical contacts is needed to prevent arcing.
• The spectral output is comparable with the Nernst glower, except at
short wavelengths (less than 5mm) where its output becomes larger.
3. Incandescent wire source
• The incandescent wire source is a tightly wound coil of Nichrome wire,
electrically heated to 1100 K.
• It produces a lower intensity of radiation than the Nernst or Globar
sources, but has a longer working life.

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• The interferometer is a basically different component than a
monochromator.
• The interferometer consists of two mirrors, an infrared light source, an
infrared detector, and a beam splitter.
• The light passes through a beam splitter, which splits the light in two
directions at right angles.
• One beam goes to a stationary mirror then back to the beam splitter.
• The other goes to a moving mirror.
• The motion of the mirror makes the total path length variable versus
that taken by the stationary mirror beam.
• When the two meet up again at the beam splitter, they recombine, but
the difference in path lengths creates constructive and destructive
interference thus an interferogram.
2. Interferometer

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• The recombined beam passes through the sample.
• The sample absorbs all the different wavelengths characteristic of its
spectrum, and this subtracts specific wavelengths from the
interferogram.
• The detector reports variation in energy versus time for all
wavelengths simultaneously.
• A laser beam is superimposed to provide a reference for the
instrument operation
Continue…

39. Mr. S. P. Shinde
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• The sample compartment in FTIR is designed in such a way to
receive the infra red radiation through the sample in a systematic
manner.
• The sample compartment contains cell holders that hold square cells
with optical path lengths of 10 mm.
• The various accessories are attached by replacing these cell holder
units or by replacing the entire sample compartment.
• Among spectrophotometers of medium or higher grade that use
photomultipliers, there are models for which large sample
compartments are made available in order to allow the analysis of
large samples or the attachment of large accessories.
3. Sample Compartment

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• Detectors are used to measure the intensity of unabsorbed infrared
radiation. Detectors like thermocouples, Bolometers, thermistors,
Golay cell, and pyroelectric detectors are used.
1. Thermocouples detectors :
• Thermocouples consist of a pair of junctions of different metals; for
example, two pieces of bismuth fused to either end of a piece of
antimony.
• The potential difference (voltage) between the junctions changes
according to the difference in temperature between the junctions.
4. Detectors

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2. Pyroelectric detectors:
• Pyroelectric detectors are made from a single crystalline wafer of a
pyroelectric material, such as triglycerine sulphate.
• The properties of a pyroelectric material are such that when an electric
field is applied across it, electric polarisation occurs (this happens in
any dielectric material).
• In a pyroelectric material, when the field is removed, the polarisation
persists.
• The degree of polarisation is temperature dependant.
• So, by sandwiching the pyroelectric material between two electrodes,
a temperature dependant capacitor is made.
• The heating effect of incident IR radiation causes a change in the
capacitance of the material.
• Pyroelectric detectors have a fast response time.
• They are used in most Fourier transform IR instruments.

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3. Photoelectric detectors:
• Photoelectric detectors such as the mercury cadmium telluride
detector comprise a film of semiconducting material deposited on a
glass surface, sealed in an evacuated envelope.
• Absorption of IR promotes nonconducting valence electrons to a
higher, conducting, state.
• The electrical resistance of the semiconductor decreases.
• These detectors have better response characteristics than pyroelectric
detectors and are used in FT-IR instruments particularly in GC-FT-IR.

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5. Readout Device
• The readout device is the computing system. Now a days a
sophisticated software is involved for the readout.
• It provides the IR spectrum in a convenient way.
• Various operations such as scan speed scan cycle, peaks deletion and
magnifications can be done easily as the system is user friendly.

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Advantages of FTIR
• The entire energy from the source gets to the sample, thus signal-to-noise
ratio is improved.
• Resolution is limited by the design of the interferometer. The longer the
path of the moving mirror, the higher the resolution.
• The digitization and computer interface allows multiple scans to be
collected.
• The signal-to-noise ratio of spectrum is significantly higher than the
dispersive infrared spectrometers.
• The accuracy of wavenumber is high. The error is within the range of
±0.01 cm-1
• The scan time of all frequencies is short (approximately 1 s).
• The resolution is extremely high (0.1 -0.005 cm-1).
• The scan range is wide (1000 - 10 cm-1).
• The interference from stray light is reduced. Due to these advantages,
FTIR Spectrometers have replaced dispersive IR spectrometers.

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There are different techniques for the analysis using FTIR which are given
below.
1. Specular reflectance
2. Diffusion reflection spectra
3. Transmission (direct and diffuse)
4. Photoacoustic
5. Attenuated total reflectance (ATR)
Different attachments used in Recording FTIR

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1. Specular Reflectance (SR)
• Specular Reflectance typically occurs from bulk samples with a glossy
surface such as crystal faces, glasses, and monolithic polymers.
• In this experiment, light is reflected off of a smooth and mirror-like
sample to record its spectrum.
• Specular reflectance is a non-destructive, non-contact technique that is
particularly useful for film thickness measurements and recording spectra
of thin films on metal substrates.
• These spectra may look different from transmission spectra in many ways.
• For example, peaks may be shifted to higher wavenumbers and/or the
presence of derivative shaped which indicate a change in refractive index.

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2. Diffuse reflection spectra
• Diffuse reflection spectra of powders and rough surfaces can be
recorded by illuminating these surfaces and collecting the radiation
which is scattered at a wide range of angles with the aid of ellipsoidal
collection mirrors.
• The spectra collected by this technique may exhibit features of
transmission, specular reflection, and/or internal reflection.
• In addition, particle size, angle of illumination, and observation can
affect a sample's observed spectrum.

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3. Transmission (TR)
• Transmission spectroscopy involves passing infrared radiation
completely through a sample and measuring the extent of absorption.
• Consequently, significant sample preparation may be required as
concentration, thickness, homogeneity and particle size must all be
considered.
• This technique is suitable for sampling gases, liquids, and solids (fibers,
microtome cuts, thin films, pressed pellets, and mulls).
• The resulting spectrum is an average of the bulk properties of the
sample, which depends on the pathlength or sample thickness, the
absorption coefficient, and the reflectivity of the sample.
• In the case of powders, the spectrum is dependent on both particle size
and their orientation within the pellet.

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4. Photoacoustic (PA)
• Photoacoustic spectroscopy (PAS) can be quite complex and difficult
to perform.
• The photoacoustic signal is generated when the infrared radiation
absorbed by a sample is converted to heat within the sample.
• This heat diffuses to the sample surface and into the adjacent gas
atmosphere.
• The thermal expansion of this gas produces the photoacoustic signal.

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5. Attenuated Total Reflectance (ATR):
• Among all these techniques ATR is quite popular and widely used.
• FT-IR spectroscopy allows measuring all types of samples whether they
are solid, liquid or gaseous.
• Liquid samples need to be filled into a liquid cell with suitable path
length; solids typically have to be diluted with the IR-inactive KBr and
pressed to the well known “KBr-pellet”.
• However, both types of measurement techniques have their drawbacks
the making and measurement of a suitable KBr pellet are time-consuming
and only experienced operators will get good results.
• In many cases, the pellet will be turbid and the baseline of the resulting
spectrum will drift due to the influence of the resulting stray light.
• Furthermore, the possibility of interactions between the polar KBr and the
sample has to be mentioned.

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• In order to overcome these disadvantages of KBr pellets and liquid
cells nowadays IR-measurements are mainly performed in ATR
mode.
• Attenuated Total Reflectance (ATR) is today the most widely used
FTIR sampling tool.
• ATR generally allows qualitative or quantitative analysis of samples
with little or no sample preparation, which results in rapid analysis.
• It has the major advantage that the sampling pathlength is very thin
and the IR beam deeply penetrate into sample.
• This is in contrast to traditional FTIR sampling by transmission
where the sample must be prepared by tedious process such as pellet
or film preparation.
Continue…

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Principle of ATR
• The great advantage of ATR is the possibility to measure a wide variety
of solid and liquid samples without any complex sample preparation.
• The ATR crystal consists of an IR transparent material with a high
refractive index and has polished surfaces.
• As shown in the image, the infrared beam enters the ATR crystal at an
angle of typically 45° (relative to the crystal surface) and is totally
reflected at the crystal to sample interface.
• Due to its wave like properties, the light is not reflected directly on the
boundary surface but by a virtual layer within the optically less dense
sample.
• This is called as Goos-Hänchen effect.

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Attenuated Total Reflectance

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• The fraction of the light wave that reaches into the sample is called
the evanescent wave.
• Its penetration depth is depending on the wavelength, the refractive
indices of ATR crystal and sample and the angle of the entering light
beam.
• Typically it amounts to a few microns (ca. 0.5 - 3 um).
• In those spectral regions where the sample absorbs energy, the
evanescent wave will be attenuated. After one or several internal
reflections, the IR beam exits the ATR crystal and is directed to the
IR-detector.
Continue…

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Working of an ATR
• An ATR accessory operates by measuring the changes that occur in an
internally reflected IR beam when the beam comes into contact with a
sample.
• An IR beam is directed onto an optically dense crystal with a high
refractive index at a certain angle.
• This internal reflectance creates an evanescent wave that extends
beyond the surface of the crystal into the sample held in contact with the
crystal.
• Some of the energy of the evanescent wave is absorbed by the sample
and the reflected radiation (some now absorbed by the sample) is
returned to the detector.
• The detector records the attenuated IR beam as an interferogram signal,
which can then be used to generate an IR spectrum.

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• ATR is ideal for strongly absorbing or thick samples which often
produce intense peaks when measured by transmission.
• ATR works well for these samples because the intensity of the
evanescent waves decays exponentially with distance from the surface
of the ATR crystal, making the technique generally insensitive to
sample thickness.
• Other solids that are a good fit for ATR include homogeneous solid
samples, the surface layer of a multi-layered solid or the coating on a
solid.
• Even irregular-shaped, hard solids can be analyzed using a hard ATR
crystal material such as diamond. Ideal solids include;
➢ Laminates
➢ Paints
➢ Plastics
➢ Rubbers
➢ Coatings
➢ Natural powders
➢ Solids that can be ground into powder.
Continue…

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• ATR is the preferred method for liquid analysis.
• It requires a drop of liquid to be placed on the crystal and it is used for
the analysis of Free-flowing aqueous solutions, viscous liquids,
coatings and biological materials.
Advantages
o The technique requires minimal sample preparation.
o The technique is rapid and the process of cleanup is easy.
o The samples can be analyzed in their natural states thus avoids the
processing of samples.
o The technique is most suitable of thick or strongly absorbing sample
such as black rubber.
Continue…

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1. For Solids
• The solids are dissolved in suitable solvents to prepare a concentrated
solution and it is used directly in the IR plates or else the sample is
placed in the IR plates and drop of solvent is placed on it.
• The standard method to prepare solid sample for FTIR spectrometer is
to use KBr.
• About 2 mg of sample and 200 mg KBr are dried and ground.
• The particle size should be unified and less than two micrometers.
• Then, the mixture is squeezed to form transparent pellets which can be
measured directly.
Sample Handling

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• The another method is Nujol mull method in which about 5 to 10 mg
of finely ground sample are placed onto the face of a KBr plate, a
small drop of mineral oil is added and the second window is placed on
top.
• With a gentle circular and back-and-forth rubbing motion of the two
windows, evenly distribute the mixture between the plates.
• The mixture should appear slightly translucent, with no bubbles, when
properly prepared.
• The sandwiched plates are placed in the spectrometer and obtain a
spectrum.
Continue…

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2. For Liquids
• To prepare a liquid sample to IR analysis, a drop of the liquid is placed
on the face of a highly polished salt plate (such as NaCl, AgCl or
KBr).
• Then a second plate is placed on top of the first plate so as to spread
the liquid in a thin layer between the plates, and clamped the plates
together.
• Finally the liquid is wiped off out of the edge of plate.
• The sandwich plate can be mounted onto the sample holder.
• For liquids with high boiling point or viscous solution, it can be added
in between two NaCl pellets.
• Then the sample is fixed in the cell by skews and measured.
• For volatile liquid sample, it is dissolved in CS2 or CCL to form 10%
solution.
• Then the solution is injected into a liquid cell for measurement.

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3. For Gas
• Gas sample needs to be measured in a gas cell with two KBr windows
on each side.
• The gas cell should first be vacuumed.
• Then the sample can be introduced to the gas cell for measurement.
• Sample spectrum to appropriate normal modes of vibrations in the
molecules.

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• For interpretation of an IR spectrum the following simple ways are to
be followed.
• At the initial stage one should not attempt to concentrate on all bands
appeared in the IR spectrum.
• Instead, the students should concentrate on the major peaks.
• The authors suggest the students to practice with several IR spectrums
for the typical shape of the peaks and the regions where they appear.
• Some of the tactics for interpretation is explained below.
Analysis and Interpretation of Organic Compounds Based
on FTIR Spectra

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1. While looking at the IR spectrum one should concentrate on
determining the presence and absence of major functional groups. It is
necessary to look for the basic compounds.
The base values are given below.
Group Frequency
O-H 3400
N-H 3400
C-H 3000
C≡N 2250
C≡C 2150
C=O 1715
C=C 1650
C-O 1100
2. Initially look at the spectrum for the presence of a carbonyl group C=O.
It will give absorption at 1820-1660 cm-1
2. If C=O is present, then check for the following assumptions. As there is
'O' there may be possibility of groups that contain 'O' including COOH,
CHO, CONH, COO-, anhydrides or ketones.

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Alkane, Alkene, Alkynes

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The C-H Stretch Boundary at 3000 cm-1
• 3000cm-1 serves as a useful dividing line.
• Above this line is observed higher frequency C-H stretches we attribute
to sp2 hybridized C-H bonds.
• Two examples below: 1-hexene (note the peak that stands a little
higher) and benzene.
• For a molecule with only sp3-hybrized C-H bonds, the lines will appear
below 3000 cm-1 as in hexane.

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• The line at 3000 cm-1 is a useful “border” between alkene C–H (above 3000
cm-1) and alkane C–H (below 3000 cm-1 ) This can quickly help you
determine if double bonds are present.
• A peak in the region around 2200 cm-1 – 2050 cm-1 is a subtle indicator of
the presence of a triple bond [C≡N or C≡C].

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• C–H stretch from 3000–2850 cm-1
• C–H bend or scissoring from 1470-1450 cm-1
• C–H rock, methyl from 1370-1350 cm-1
• C–H rock, methyl, seen only in long chain alkanes, from 725-720 cm-1
Alkane

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• C=C stretch from 1680-1640 cm-1
• =C–H stretch from 3100-3000 cm-1
• =C–H bend from 1000-650 cm-1
Alkene

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• –C≡C– stretch from 2260-2100 cm-1
• –C≡C–H: C–H stretch from 3330-3270 cm-1
• –C≡C–H: C–H bend from 700-610 cm-1
Alkynes

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Alcohols
• Is there a broad, rounded peak in the region around 3400-3200 cm-1, That’s
where hydroxyl groups (OH) appear.
• Hydrogen bonding between hydroxyl groups leads to some variations in O-H
bond strength, which results in a range of vibrational energies. The variation
results in the broad peaks observed.

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• O–H stretch, hydrogen bonded 3500-3200 cm-1
• C–O stretch 1260-1050 cm-1 (s)
Continue…

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Is there a sharp, strong peak in the region around 1850-1630 cm-1, That’s
where carbonyl groups (C=O) show up.
Ketones

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A decent rule of thumb is that you will never ever see a C=O stretch
below 1630. for example, If you see a strong peak at 1500, it is not C=O. It is
something else.
• C=O stretch - aliphatic ketones 1715 cm-1
• C-H stretch: Left side to 3000 cm-1 (2991 cm-1)

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Aldehyde
Is aldehyde CH present?. If CH of aldehydes present then there will be two
weak absorptions near 2850 & 2750 on right side of the aliphatic C-H.
• C–H stretch 2830-2695 cm-1
• C=O stretch:
o aliphatic aldehydes 1740-1720 cm-1
o alpha, beta-unsaturated aldehydes 1710-1685 cm-1

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Ester
• C=O stretch
o Aliphatic from 1750-1735 cm-1
o Alpha, beta unsaturated from 1730-1715 cm-1
• C–O stretch from 1300-1000 cm-1

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Carboxylic acid
Hydroxyl groups in carboxylic acids are considerably broader than in
alcohols. The OH absorption in carboxylic acids can be so broad that it
extends below 3000 cm-1.
• O–H stretch from 3300-2500 cm-1
• C=O stretch from 1760-1690 cm-1
• C–O stretch from 1320-1210 cm-1
• O–H bend from 1440-1395 and 950-910 cm-1

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Ether
• C-O bands appear in the range of 1300 to 1000 cm-1
• CH2 and CH3 bending: 1467 and 1375 cm-1 respectively.
• C-H stretch: 2868 and 2962 cm-1
Dibutyl ether

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Acid chloride
• C=O 1800 cm-1
• C-Cl 595 (range 785 to 540 cm-1
• Overtone 3590 cm-1

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Anhydride
• Anhydride C=O two peaks 1810 and 1760 cm-1
• C-O 1090 cm-1

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The position of the C=O stretch varies slightly by carbonyl functional group.
Some ranges (in cm-1 ) are shown below:
1. Aldehydes (1740-1690): benzaldehyde, propanal, pentanal
2. Ketones (1750-1680): 2-pentanone, acetophenone
3. Esters (1750-1735): ethyl acetate, methyl benzoate
4. Carboxylic acids (1780-1710): benzoic acid, butanoic acid
5. Amide (1690-1630): acetamide, benzamide, N,N-dimethyl formamide
(DMF)
6. Anhydrides (2 peaks; 1830-1800 and 1775-1740): acetic anhydride,
benzoic anhydride

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Two bands are observed with primary amine one hand is observed with amine
whereas no peak in the indication of tertiary amine in the 3300- 3400cm-1
NH-peaks are observed between 3300- 3400 cm-1
NH-stretch two peaks in the region 3350 and 3390 cm-1
NH2 bending at 1600 cm-1
CH2 and CH3 bending at 1465 and 1375 cm-1 respectively
Amines

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Nitrile
Nitrile group gives a strong band at 2250 cm-1
C≡N 2249cm-1
C-H stretch 2989 and 2965cm-1

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