IR SPECTROSCOPY.pptx

IR SPECTROSCOPY
SUBMITTED BY,
Manimegalai.G
M.Pharm - I Year
Department of Pharmaceutical chemistry
5/17/2023 Department of Pharmaceutical Chemistry 1

INTRODUCTION
• IR spectroscopy is the study of interaction
between infrared radiations and matter.
• Infrared radiations refer broadly to that part
of electromagnetic spectrum between visible
and microwave region.
• IR region extends from 2.5µ to 15µ
Near IR - 0.8µ to 2.5µ
Mid IR - 2.5µ to 15µ
Far IR - 15µ to 200µ

PRINCIPLE
• The absorption of IR causes an excitation of
molecule from a lower to the higher
vibrational level.
• Each vibrational level is associated with
no.of closely spaced rotational level.
• All bonds in molecule are not capable of
absorb IR energy but only those bonds
which are accompanied by change in dipole
moment, will absorb IR region.

• IR active : vibrational transitions
accompanied by change in dipole moment
Eg - OH, NH, C=O
• IR inactive : vibrational transitions not
accompanied by change in dipole moment
Eg- C-C bonds in symmetrical alkenes and
alkynes

THEORY
• Absorption in IR is due to changes in
vibrational and rotational levels.
• As the absorption is quantised, the discrete
lines are formed in spectrum due to
molecular rotation.
• Single vibrational energy changes is
accompanied by a large no.of rotational
energy changes.

• Thus, the vibrational spectra absorbed
energy brings about predominant changes in
vibrational energy which depends on,
Mass of atom present in a molecule.
Strength of bonds.
The arrangement of atom within the
molecule.

• When infra red light is passed through the
sample, the vibrational and rotational
energies of the molecules are increased.
• 2 kind of fundamental vibrations
a) Stretching – Distance between 2 atoms
increase or decrease but the atoms remain in
same axis.
b) Bending – Position of atoms change
with respect to the original bond axis.

• More energy is required to stretch a bond
than that bend it.
• Stretching absorption of bond appears at
high frequency as compared to bending
absorption of same bond.
• The various stretching and bending
vibration of bond occurs at certain
quantised frequencies.

TYPES OF STRETCHING
VIBRATION
• SYMMETRIC – Movement of atoms with
respect to particular atom in a molecule is
in same direction.
• ASYMMETRIC- One atom approach the
central atom while the other departs from
it.

TYPES OF BENDING
VIBRATION
• SCISSORING- Two atoms approach each
other.
• ROCKING- Movement of atoms takes place
in same direction.

• WAGGING- Two atoms move up or down the plane
with respect to central atom.
• TWISTING- One atom move up the plane while the
other move down the plane.
Bending vibrations require less energy and occur at
higher wavelength or lower wavenumber than
stretching vibrations.

VIBRATIONAL FREQUENCY
• The value of stretching vibrational frequency
of a bond can be calculated by Hooke’s law.
“It states that, the size of the deformation is
directly proportional to the deforming force.”

Where,
µ - reduced mass
m1,m2 – mass of atoms
k – force constant
c – velocity of radiation
If the bond strength increases or the reduced
mass decreases, the value of vibrational
frequency increases.

FACTORS INFLUENCING
VIBRATIONAL FREQUENCY
• Coupled vibrations
• Fermi resonance
• Electronic effects
• Hydrogen bonding
• Bond angles

COUPLED VIBRATIONS
• One stretching frequency occur at one isolated
CH group. But methylene(-CH2-) two absorption
frequency occurs which correspond to symmetric
and asymmetric vibrations.
• In such cases, asymmetric occurs at higher
wavenumber compared with symmetric
vibrations.

Methyl group coupled vibrations takes place at
different frequencies compared to CH2 group.
Sometimes, it happens two different
vibrational level have same energy. If such
vibrations belongs to same species, resulting in
the shift of one towards lower frequency and
the other towards higher frequency.

FERMI RESONANCE
• A molecules that transfer energy from
fundamental to overtone and back again. It is
called Fermi resonance.
• Fermi resonance is also shown by the
spectrum of n- butyl vinyl ether. In this case,
overtone of fundamental vibration at 810cm-1
chances to coincide with the band at 1640cm-1
• The mixing of two double bands in accordance
with fermi resonance gives two bands of
almost equal intensity at 1640cm-1 and
1630cm-1.

ELECTRONIC EFFECT
• Changes in absorption frequencies of
particular group take place when the
substituents in the neighbourhood of that
particular group are changed.
• The frequency shift due to the electronic effects
include mesomeric effect, inductive effect and
field effect.
• These effects cannot isolated from one another
& contribution of one of them can only be
estimated.

• Introduction of alkyl group shows +I effect
which results in lengthening or weakening of
bond and lower force constant and
wavenumber of absorption frequency
decreases.
HCHO – 1750cm-1
CH3CHO – 1745cm-1
CH3COCH3 – 1715cm-1
Aldehydes absorb at higher wavenumber than
ketones.

• The introduction of electronegative atoms
causes –I effect which results in bond order
to increase. Thus, the force constant increase
and hence the wavenumber of absorption
rises.
Acetone – 1715cm-1
Chloroacetone – 1725cm-1
Dichloroacetone- 1740cm-1
Tetrachloroacetone – 1750cm-1

• Mesomeric effect work along with inductive
effect.
• Mesomeric effect causes lengthening or
weakening of bond leading in the lowering of
absorption frequency.
• -I effect is dominated by mesomeric effect,
the absorption frequency falls.
Methyl vinyl ketone
(C=O 1706cm-1)
Acetophenone
(C=O 1693cm-1)

• The lone pairs of electrons on two atoms
influence each other through space
interactions and change the vibrational
frequencies of both the groups. This effect is
called field effect.
• In meta substitution, only inductive effect is
considered.
• In para substitution, both inductive &
mesomeric effect become important.
• In ortho substitution, inductive &
mesomeric effect along with steric effect.

HYDROGEN BONDING
• Stronger hydrogen bonding, greater
absorption shift towards the lower
wavenumber than the normal value.
• Two types of hydrogen bond can be
distinguished in IR.
• Intermolecular hydrogen bonding give rise
to broad band whereas band arising from
intramolecular hydrogen shows sharp peak.

Eg : In aliphatic alcohols, a sharp band at
3650cm-1 due to free OH group, broad band
occurs at 3350cm-1 due to hydrogen bonded
OH group.

BOND ANGLES
• Cyclobutanone
The C-CO-C bond angle is reduced below
the normal angle of 1200 and it leads to
increased s- character in C=O bond.
Greater s- character causes shortening of
C=O bond and thus C=O stretching occurs at
higher frequency.
If bond angle push outwards above 1200
opposite effect operates.

INSTRUMENTATION
• IR Instrumenttaion is divided into two
classes,
a) dispersive –use a prism or grating
b) non dispersive – use interference filter,
tunable laser sources.
• It is convenient to divide the infrared region
into three segments , with the dividing
points based on instrumental capabilities.

NEAR - IR MID -IR FAR-IR
Wavenumber
cm-1
12,500 to 4000 4000 to 200 200 to 10
Wavelength µm 0.8µ to 2.5µ 2.5µ to 15µ 15µ to 200µ
Source of
radiation
Tungsten
filament lamp
Nernst glower,
Globar or coil of
nichrome wire
High pressure
mercury arc
lamp
Optical system One or two
quartz prism or
prism grating
monochromator
Two to four plane
diffraction
gratings
monochromator
Double beam
grating
interferometer
Detector Photoconductive
cells
Thermopile,
thermistor,
pyroelectric,
semiconductor
Golay cell,
pyroelectric

• The main parts of IR spectrometer are as
follows:
1. IR radiation source
2. Monochromators
3. Sample cells and sampling of substances
4. Detectors

IR RADIATION SOURCES
• The radiation sources must emit IR radiation
which must be
a) Intense enough for detection
b) Steady
c) Extend over the desired wavelength
• The various popular sources of IR radiation are
a) Incandescent lamp - Incandescent light
bulbs use a tungsten filament heated to high
temperature to produce visible light and,
necessarily, even more infrared radiation.

a) Nernst glower - Derived from German chemist
Walther Hermann Nernst, who derived the
Nernst equation. Used in spectroscopy to
provide near infrared radiation.
b) Globar source- standard source is a Globar
(50–6,000 cm−1), a silicon carbide cylinder that
is electrically heated to function as a blackbody
radiator
c) Mercury arc - Radiation from a mercury-arc
lamp (10–70 cm−1) is employed in the far-
infrared region.

MONOCHROMATOR
A. PRISM
• Used as dispersive element
• Constructed of various metal halide salts
• Sodium chloride is most commonly used
B. GRATING
• It made up of some materials like glass, quartz
or alkylhalides depending on the instrument.
• The mechanism is that diffraction produces
reinforcement. The rays which are incident
upon the gratings get reinforced with the
reflected rays.

SAMPLE CELL
• Infrared spectra may be obtained for gases,
liquids or solids.
• Materials containing sample must be
transparent to the ir radiation.
• So, the salts like NaCl, KBr are only used.

SAMPLE HANDLING
• Samples of same substance shows shift in
absorption bands as we pass from solid to gases
and hence the samples of different phases have
to be treated differently in IR spectroscopy.
Sampling of solids:
1. Solids run in solution
2. Mull technique
3. Pressed pellet technique
4. Solid films

• SOLIDS RUN IN SOLUTION:
Dissolve solid sample in non –aqueous solvent
and place a drop of this solution in alkali metal
disc and allow to evaporate, leaving a thin film
which is then mounted on a spectrometer.
Eg of solvents – acetone, cyclohexane,
chloroform.
• MULL TECHNIQUE:
Finely powdered sample+ mulling agent (nujol)
and make a thick paste (mull)
Transfer the mull to the mull plates are
squeezed together to adjust the thickness it is then
mounted in spectrometer.

• PRESSED PELLET TECHNIQUE:
Finely powdered sample is mixed with about
100 times its weight of KBr in a vibrating ball
mill and the mixture is then pressed under very
high pressure in a die to form a small pellet (1-
2mm thickness and 1cm in diameter)

• SOLID FILMS:
Amorphous solid is dissolved in volatile
solvents and this solution is poured on a rock
salt plate (NaCl or KBr ), then the solvent is
evaporated by gentle heating.

SAMPLING OF LIQUIDS
• Liquid sample cells can be sandwiched using
liquid sample cells of highly purified alkali
halides, normally NaCl. Other salts such as KBr
and CaF2 can also be used. Aqueous solvents
cannot be used because they cannot dissolve
alkali halides.
• Organic solvents like chloroform can be used.
The sample thickness should be selected so that
the transmittance lies between 15-20%.
• For most liquids, the sample cell thickness is
0.01-0.05 mm. Some salt plates are highly
soluble in water, so the sample and washing
reagents must be anhydrous

SAMPLING OF GASES
• A gas sample is created by allowing the sample to
expand into an evacuated cylindrical cell which has
special windows than will not absorb the infrared
light.
• The length of the cell can be anywhere from a
couple of centimeters to over 10 meters. This cell
will be placed in the beam path much like the solid
or liquid sample with the exception that gas cell has
two windows that the infrared light beam needs to
pass through.
• Because the thermodynamics of the gas are such
that it is very spread out the beam is bounced off the
inside of the cell so that it will pass through the
sample more to get more sample absorbance.

DETECTORS
• The detectors can be classified into three
categories:
1. Thermal detectors – Their response
depend the heating effect of radiation
2. Pyroelectric detectors- It depends on the
rate of change of the detector temperature
rather than on the temperature itself.
3. Photoconducting detectors- it is most
sensitive

THERMAL DETECTORS
Radiation thermoelement (thermopile)
• When the junction of two different metals is
heated, an electrical voltage proportional to the
temperature is produced due to the
thermoelectric effect.
• This effect has been utilized for many years for
technical contact temperature measurements
using thermoelements. If the heating of the
junction is caused by the absorption of
radiation, then this component is known as a
thermopile.

PYROELECTRIC DETECTORS
• The temperature change in the detector element
created by the absorption of infrared radiation
causes a change in surface charge as a result of the
pyroelectric effect.
• This results in an electrical output signal which is
processed in a pre-amplifier. Due to the way that
charge is created in the pyroelectric material, the
radiation flow must be continuously interrupted in
an alternating manner (chopping).
• The advantage of the resulting frequency-selective
amplification is a good signal-noise ratio.

PHOTOCONDUCTING DETECTORS
• Photoconducting detectors are the most
sensitive detectors. They rely on interactions
between photons and a semiconductor.
• The detector consists of a thin film of a
semiconductor material such as lead
sulphide, mercury cadmium telluride or
indium antimonide deposited on a
nonconducting glass surface and sealed into
an evacuated envelope to protect the
semiconductor from the atmosphere.

• The lead sulphide detector is used for the
near-infrared region of the spectrum. For
mid- and far-infrared radiation the mercury
cadmium telluride detector is used. It must
be cooled with liquid nitrogen to minimize
disturbances.

DISPERSIVE IR INSTRUMENT

• The basic components of a dispersive IR
spectrometer include a radiation source,
monochromator, and detector.
• The common IR radiation sources are inert
solids that are heated electrically to promote
thermal emission of radiation in the infrared
region of the electromagnetic spectrum.
• The monochromator is a device used to
disperse or separate a broad spectrum of IR
radiation into individual narrow IR
frequencies.

• Generally, dispersive spectrometers have a
double-beam design with two equivalent beams
from the same source passing through the
sample and reference chambers as independent
beams.
• These reference and sample beams are
alternately focused on the detector by making
use of an optical chopper, such as, a sector
mirror.
• One beam will proceed, traveling through the
sample, while the other beam will pass through
a reference species for analytical comparison of
transmitted photon.

• After the incident radiation travels through the
sample species, the emitted wavefront of radiation is
dispersed by a monochromator (gratings and slits)
into its component frequencies.
• A combination of prisms or gratings with variable-
slit mechanisms, mirrors, and filters comprise the
dispersive system.
• Narrower slits gives better resolution by
distinguishing more closely spaced frequencies of
radiation and wider slits allow more light to reach the
detector and provide better system sensitivity.
• The emitted wavefront beam (analog spectral output)
hits the detector and generates an electrical signal as
a response.

FOURIER TRANSFORM IR
INSTRUMENT

• A common FTIR spectrometer consists of a
source, interferometer, sample compartment,
detector, amplifier, A/D convertor, and a
computer.
• The source generates radiation which passes
the sample through the interferometer and
reaches the detector.
• Then the signal is amplified and converted to
digital signal by the amplifier and analog-to-
digital converter, respectively. Eventually, the
signal is transferred to a computer in which
Fourier transform is carried out.

• Fourier transform infrared, more commonly
known as FT-IR, is the preferred method for
infrared spectroscopy.
• Developed in order to overcome the slow
scanning limitations encountered with
dispersive instruments, with FT-IR the infrared
radiation is passed through a sample.
• The measured signal is referred to as an
interferogram. Performing a Fourier transform
on this signal data results in a spectrum
identical to that from conventional (dispersive)
infrared spectroscopy, but results are much
faster with results in seconds, rather than
minutes.

APPLICATIONS
1. Identification of functional group and structure
elucidation
• Entire IR region is divided into group frequency
region and fingerprint region. Range of group
frequency is 4000-1500 cm-1 while that of finger
print region is 1500-400 cm-1.
• In group frequency region, the peaks corresponding
to different functional groups can be observed.
According to corresponding peaks, functional group
can be determined.

• Each atom of the molecule is connected by bond and
each bond requires different IR region so
characteristic peaks are observed. This region of IR
spectrum is called as finger print region of the
molecule. It can be determined by characteristic
peaks.
2. Identification of substances
• IR spectroscopy is used to establish whether a given
sample of an organic substance is identical with
another or not.
• This is because large number of absorption bands is
observed in the IR spectra of organic molecules and
the probability that any two compounds will
produce identical spectra is almost zero.

• So if two compounds have identical IR spectra then
both of them must be samples of the same
substances.
• IR spectra of two enatiomeric compound are
identical. So IR spectroscopy fails to distinguish
between enantiomers. For example, an IR spectrum
of benzaldehyde is observed as follows.
1. C-H stretching of aromatic ring- 3080 cm-1
2. C-H stretching of aldehyde- 2860 cm-1 and 2775
cm-1
3. C=O stretching of an aromatic aldehyde- 1700 cm-1
4. C=C stretching of an aromatic ring- 1595 cm-1
5. C-H bending- 745 cm-1 and 685 cm-1

3. Studying the progress of the reaction
• Progress of chemical reaction can be determined
by examining the small portion of the reaction
mixture withdrawn from time to time.
• The rate of disappearance of a characteristic
absorption band of the reactant group and/or the
rate of appearance of the characteristic
absorption band of the product group due to
formation of product is observed.

4. Detection of impurities
• IR spectrum of the test sample to be determined is
compared with the standard compound.
• If any additional peaks are observed in the IR
spectrum, then it is due to impurities present in the
compound.
5.Quantitative analysis
• The quantity of the substance can be determined
either in pure form or as a mixture of two or more
compounds.
• In this, characteristic peak corresponding to the
drug substance is chosen and log I0/It of peaks for
standard and test sample is compared.
• This is called base line technique to determine the
quantity of the substance.

DATA INTERPRETATION
• One of the most common application of
infrared spectroscopy is to the identification
of organic compounds.
1. Hydrocarbons
• Hydrocarbons compounds contain only C-H
and C-C bonds, but there is plenty of
information to be obtained from the
infrared spectra arising from C-H
stretching and C-H bending.

IR SPECTRUM OF OCTANE

• Functional Groups Containing the C-O Bond
• Alcohols have IR absorptions associated with
both the O-H and the C-O stretching
vibrations.
IR SPECTRUM OF ETHANOL
very broad, strong band of the O–H stretch

ORGANIC COMPOUNDS CONTAINING
HALOGENS
• Alkyl halides are compounds that have a C–
X bond, where X is a halogen: bromine,
chlorine, fluorine, or iodine.
• The spectrum of 1-chloro-2-methylpropane

Approximate
Frequency (cm-1)
Description Bond Vibration Notes
3500 - 3200 broad O-H much broader,
lower frequency
(3200-2500)
if next to C=O
3400-3300 weak N-H stronger if next to
C=O
3100-3000 weak-medium =C-H (sp2 C-H) can get bigger if lots
of bonds present
3000-2900 weak-medium -C-H (sp3 C-H) can get bigger if lots
of bonds present
2800 and 2700 medium C-H in O=C-H two peaks;
2250-2100 weak-medium C=C stronger if near
electronegative
atoms

1800-1600 strong C=O lower frequency
(1650-1550)
if attached to O or N
middle frequency if
attached to C, H
higher frequency
(1800) if attached to
Cl
1650-1450 weak-medium C=C lower frequency
(1600-1450) if
conjugated
(i.e. C=C-C=C)
often several if
benzene present
1300 - 1000 medium-strong C-O higher frequency
(1200-1300) if
conjugated
(i.e. O=C-O or C=C-O)
1000-650 strong
C=C-H bend
often several if
benzene present

REFERENCE
• Instrumental Methods of Analysis – Willards,
7th edition, CBS Publishers, pg.no 287-314
•

THANK YOU

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