2. INFRARED SPECTROSCOPY
IR spectroscopy is the spectroscopy that deals with the infrared region of the electromagnetic
spectrum, that is light with a longer wavelength and lower frequency than visible light
Infrared spectroscopy (IR) measures the bond vibration frequencies in a molecule and is used
to determine the functional groups.
The infrared region of the spectrum encompasses radiation with wave numbers ranging from
about 12,500 to 50cm-1 (or) wave lengths from 0.8 to 200μ.
Infrared region lies between visible and microwave region.
Infrared Spectroscopy is the analysis of infrared light interacting with a molecule.
It is based on absorption spectroscopy
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3. INFRARED REGIONS
INFRARED REGIONS RANGE
Near infrared region 0.8-2.5 µ (12,500-4000 cm-1)
Main infrared region 2.5-15 µ (4000-667cm-1)
Far infrared region 15-200 µ (667-100 cm-1)
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4. The infrared region constitutes 3 parts
a) The near IR (0.8-2.5μm) (12,500-4000cm-1)
b) The middle IR (2.5 -15μm) (4000-667cm-1)
i) Group frequency Region (4000-1500cm-1)
ii) Finger print Region (1500-667cm-1)
c) The far IR (15-200μm) (667-50cm-1)
• most of the analytical applications are confined to the middle IR region because absorption
of organic molecules are high in this region.
• Wave number is mostly used measure in IR absorption because wave numbers are larger
values & easy to handle than wave length which are measured in μm.
E = hν= hc/λ= hcν ̄
• It gives sufficient information about the structure of a compound.
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5. PRINCIPLE
When infrared radiation is passed through an organic compound some of the frequencies are
absorbed and appear as absorption band while other frequencies are do not intrect with the
compound and are transmitted without being absorbed.Only those frequencies are absorbed
which match with the vibrational frequencies of the bonds.
IR sectroscopy is there for basically vibrational spectroscopy .
The absorption of ir radiation is associated with the excitatio of the molecule from the ground
state to higher vibrational state.
The absorbed energy is eventually released as heat and the molecule revert from the excited
state to the orignal ground state.
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7. Conti...
Fundamental Vibrations:
vibrational frequencies which are concerned with the individual atom called Fundamental
vibration.
Non Fundamental vibration:
Vibrations which apears as a result of fundamental vibrations.
Stretching vib:
Involves a continuous change in interatomic distance along the axis of the bond between 2
atoms
Involves the change in angle btw the two bonds
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8. Interatomic distance btw the two atoms increase or decrease
Interatomic distance btw the two atoms is Opposite or alternate
All the atoms are in same plane
2 atoms are in same plane and 1 is on opposite plane
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9. Scissoring:
In this case, the two atoms connected to a central atom either move toward or away from each
other with certain deformation of the valence angle.
Rocking:
In this case, the structural unit swings back and forth in the plane of the molecule.
Wagging:
In this case the structural unit swings back and forth out of the plane of the molecule.
Twisting:
In this case the structural unit rotates about the bond that joins it to the rest of the
molecule.
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10. Over tone:
Some additional absorption bands may be observed which are genrated by the modulation of
the fundamental frequencies. This leads to the overtone that appear at almost twice and thrice
the fundamental frequencies with greatly reduced intensties.
Combination tone:
Week band that apeares occasionally at frequencies that are sum or difference of 2 or more
fundamental bands.
Combination band arises when absorption by a molecule result in excitation of two fundamental
vibrations simutaneously. This happen when a single photon has precisely the correct energy to
excite the two vibrations. In such cases the frequency of the combination band must be the
exact sum of the two fundamental absorption frequencies. More than two fundamental
frequencies may be involved in combinaton band.
The difference band arises when a molecule already existing in an excited vibrational state
absorbes enough additional energy to be excited to another vibrational level of a different
vibrational mode.
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11. Fermi resonance:
Interaction btw fundamental vib and combination tones overtones
occurs when atoms in a molecule are in periodic motion while the molecule as a whole has
constant translational and rotational motion.
The frequency of the periodic motion is known as a vibration frequency.
The value of stretching vibrational frequency of a bond can be calculated by the application
of Hooke's law.
We can also calculate an approximate value of the stretching vibrational frequency of a bond
by treating the two atoms and their connecting bond, to first approximation, as two balls
connected by a spring, acting as a simple harmonic oscillator for which the Hooke’s Law
may be applied.
According to Hooke’s Law , The Stretching frequency is related to the masses of the atom
and the force constant(a measure of resistance of a bond to stretching) of a bond by the
following equation
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13. Conjugation:Any factor the tend to increase the contribution of the singe bond form will
decrease the force constant of the carbon oxygen bond hence result in lowering the the
frequency of C=O stretching vib. e.g. conjugation lowersthe C=O stretching frequency whether
the conjugation is brought about by αβ-unsaturation or by an atomic ring.
Mesomeric effect:Any substituent that enhance the singke bond character will decrease the
bond order of C=O group and leads o lower the stretching frequency.
A +M group lower the stretching freq.-M group have opposite effect.
Inductive effect:substituent with an electron donating inductive effect(+I) will enhance the
single bond character of the C=O group. (-I) group have opposite effect.
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15. Double bond carbon atom in unstrained molecules have planer trigonal geometry with a bond
angle of 120°
Increase in absorption freq with the decrease in bond angle can be explained in terms of bond
angular strain.Reduction in bond angle below 120 leads to an increased in s character in double
bond resulting in shortening and strengthening of a double bond.
It is observed in compounds containing -CH2 and -CH3.
e.g Carboxlic acid anhydride,amides
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16. There are 2 types of infrared spectrophotometer, characterized by the manner in which the ir
frequencies are handled.
– 1) dispersive type (IR)
– 2) interferometric type(FTIR)
In dispersive type the infrared light is separated into individual frequencies by dispersion, using
a grating monochromator.
In interferometric type the ir frequencies are allowed to interact to produce an interference
pattern and this pattern is then analyzed, to determine individual frequencies and their
intensities.
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17. It is double beam instrument.The main components of Dispersive ir spectrophotometer are:
Radiation source
Sample compartment
opticle chopper
monochrometer
detector
amplifier
readout device
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19. 1)Infrared Sources
The most common source is either Nernst filament or Globar
The Nernst filament is composed of a mixture of oxide of zirconium,thorium and yattrium, which
is formed into a hollow rod about 2mm in diameter and 2-5 cm in length.
Globar is a bonded silicon carbide rod about 7mm in diameter and 5 cm in length. On heating
electrically (1000-1800C) these rods emit infrared radiations.
2)Sampling Area(Optical null method):
The radiation is divided into two beams of equal intensties which are focused into the sampling
area. One beam pass through the sample while the other serves as the reference
beam.Absorption by the sample is measured directly from the difference in intensties of two
beams. The intensity difference is usually measured by optical null method inthis instrument. In
this method afer passing through the sampling area the reference beam pass through an optical
wedge which limits the amount of radiation that passes through it.
3)Chopper:
The two beams are now reflected to a light chopping device.As the chopper rotates it causes
the sample beam and reference beam to be reflected aternately to the monochromator.
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20. 4)Monochromators
Three types of substances are normally employed as monochromators, namely :
(i) Metal Halide Prisms : Various metal halide prisms, such as : KBr (12-25 μm), LiF (0.2-6 μm)
and CeBr (15-38 μm) have been used earlier, but they have become more or less obsolescent
nowadays.
(ii) NaCl Prism (2-15 μm) : Sodium chloride prism are of use for the whole of the region from
4000- 650 cm–1. First, it offers low resolution at 4000-2500 cm–1, and secondly, because of its
hygroscopic nature the optics have got to be protected at 20 °C above the ambient temperature.
(iii) Gratings : In general, gratings are commonly employed in the design of the instruments and
offer better resolution at higher frequency than the prisms. They offer much better resolution at
low frequency, viz., typical rulings are 240 lines per nm for the 4000-1500 cm–1 region and 120
lines per nm for the 1500-650 cm–1 region.
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21. 5)Detectors
a)Thermocouples (or Thermopiles):
The underlying principle of a thermocouple is that if two
dissimilar metal wires are joined head to tail, then a difference in temperature between head
and tail causes a current to flow in the wires. In the infrared spectrophotometer this current shall
be directly proportional to the intensity of radiation falling on the thermocouple. Hence, the
thermocouples are invariably employed in the infrared region, and to help in the complete
absorption of ‘available energy’ the ‘hot’ junction or receiver is normally blackened.
b)Golay Detector:
In this specific instance the absorption of infrared radiation affords expansion of an inert gas in a
cell-chamber. One wall of the cell-chamber is provided with a flexible mirror and the resulting
distortion alters the intensity of illumination falling on a photocell from a reflected beam of light.
Thus, the current from the photocell is directly proportional to the incident radiation.
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23. d)Thermistors
•It is made up of metal oxides.
•It functions by changing resistance when
heated.
•It consists of two closely placed thermistor
flakes, one of the 10 um is an active detector,
while the other acts as the compensating /
reference detector.
•A steady voltage is applied, due to the
temperature increase there is change in
resistance which is measured and this gives
the intensity of the IR radiation
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24. 6)Recording:
When the sample has absorbed radiation of a perticular frequency,the detector will be receiving
alternately an intense reference beam and weak sample beam.This will in effect lead to an
alternating current which is flown to an amplifier.The amplifier is coupled coupled to small
sevo-meter which drives the optical wedge into the reference beam untl the detector receives
radiations of equal intensties from the sample and the reference beams until the difference in
intensties of the two beams is zero or in other words the instrument is at an optical null.
The wedge is coupled to a recorder pen so that the movement of the wedge in and out of the
reference beam is accompained by movement of a pen on the printed paper which thus records
the absorption bands.
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25. a)Gas Sample:
A gas sample is introduced into an evacuated gas cell.Gas cells are available in lenghts ranging
from a few cm to some meters. The sampling area of standard dispersive IR spectrophotometer
is about 10cm. So long path are achieved by using multiple pass gas cells in which internal
mirrors permit the beam to be reflected several times through the sample before it exits the gas
cell.
b)Liquid Sample:
It is examined as thin film squeezed btw two optically polished circular flat plates(about 5mm
thick and 25mm in dm) of rock salt(NACl).
Pressing a small drop of liquid sample btw the plates produces a film of about 0.1mm thick.
The plates are held together by capillary action are mounted in sample beam.
Plates are cleaned and dried immediately after use by rinsing in suitable solvent such as
chloroform.
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26. c)Solid Sample:
Mull technique:
Mulls are prepared by grinding 2-5mg of sample in smooth mortar with pestle of agate,adding 1
or 2 drops of mulling agent and continue grinding to get a homogenous paste.The mull is
examined as thin film btween the NACl plates.
Nujol is commonly used as mulling agent.
Pressed Pellet Technique:
Grind and mix about 1.0mg of solid sample with about 100mg of dry powdered KBr in mortar. to
prepare KBr discs.
Then this solid solution is compressed under pressure (about 20,000 psi) in vacuo with a
special die to form transparent disc. Discs are usually 10mm in diameter and 1mm thick.
KBr does not absorb in the IR region so complete spectrum of sample is obtained.
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27. d)Sample As Solution:
Sample can also be examined as solution in solvents such as CCl4 or Chloroform.
Solutions(1-5%) are handled in cells of 0.1-1.0mm pathlength.
A 2nd cell containing pure solvent is placed in reference beam.
The wavelength (or wave number) scale calibration of infrared spectrophotometers is usually
carried out with the aid of a strip of polystyrene film fixed on a frame. It consists of several sharp
absorption bands, the wavelengths of which are known accurately and precisely. Basically, all
IR-spectrophotometers need to be calibrated periodically as per the specific instructions so as to
ascertain their accuracy and precision.
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28. An infrared spectrum may be divided into 2 parts. the part ranging from 4000-1600cm-1 is known
as Functional group region,while that ranging from 1600-625cm-1 is called Fingerprint
region.
Functional group region:
In this region most compounds have only a few strong bands due to the characteristic stretching
vibration of their functional groups.
For example the functional group of X-H type (C-H,O-H) absorb in the region 3700-2500 cm-1.
Absorption due to triple bond occur 2300-2100cm-1 and due to double bond at 1900-1600cm-1.
Fingerprint region:
It is much more difficult to pick out individual bonds in this region than it is in the "cleaner" region
at higher wavenumbers. The importance of the fingerprint region is that each different
compound produces a different pattern of troughs in this part of the spectrum. e.g Characteristic
absorption bands corresponding to aromatic ring system fall in the 1600-1450cm-1.
Aromatic compounds display strong adrosption bands in 900-700cm-1.
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29. A method of obtaining an Infrared spectrum by measuring the interferogram of a sample using
an interferometer, then performing a Fourier Transform upon the interferogram to obtain the
spectrum.
It is single beam instrument.
The Interferometer:
An device in which two or more radiation beams interfere with each other after passing through
different optical paths. The interferometer produces a unique type of signal which has all of the
infrared frequencies “encoded” into it. The output is an interferogram or interference record. The
two domains of Distance and Frequency are inter convertible by the mathematical method of
Fourier transformation.
Majority of commercially avaiable FT-IR are based on an inetrferometer commonly Michelson
interferometer.
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31. Michelson interferometer:
It has three basic components a beam splitter,fixed mirror, and a moveable mirror.
Beam splitter is semi-reflecting device so that 50% oof light impinging on it is transmitted while
the remaining 50% is reflected. It is either a Partially silvered mirror or thin film of germanium
(Ge) sendwiched btw two KBr plates.
IR light received from source is split into two identical beams by the beam splitter such that one
beam is reflected to the fixed flat mirror while the other beam is transmitted to the moveable flat
mirror.
The two beams are reflected back by their respective mirrors to recombine at the beam splitter
to again form one beam which passes through the sample and is then focused on detector. Of
course,each of the two beams reflected back by their respective mirror is again split into two
equal halves by the beam splitter.one half of each beam goes to detector while the half of each
beam goes back to the source.The sample can be use directly without the prepration of mull or
KBr discs or solution.
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32. Detectors of FT-IR:
1. Pyroelectric detectors a thin, Pyroelectric crystals such as
deuterated triglycine sulfate (DTGS) or LITA (lithium tantalate),
When a pyroelectric material is polarized by an electric field, it
remains polarized after the field is removed due to an effect
called residual electric polarization. This residual polarization is
sensitive to changes in temperature.
2. Photoconductive detectors show an increase in electrical
conductivity when illuminated with IR radiation, They have a
rapid response and high sensitivity. The most commonly used
photoconductive detector is the MCT (Mercury Cadmium
Telluride), which must be cooled to liquid nitrogen temperatures
for proper operation.
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33. Advantages of Fourier transform IR over dispersive IR
• Improved frequency resolution
• Improved frequency reproducibility (older dispersive instruments must be recalibrated for
each session of use)
• Faster operation
• Computer based (allowing storage of spectra and facilities for processing spectra)
• Energy limiting accessories such as diffuse reflectance or FT-IR microscopes
• High resolution experiments (as high as 0.001 cm-1 resolution)
• Trace analysis of raw materials or finished products
• Depth profiling and microscopic mapping of samples
• Kinetics reactions on the microsecond time-scale
• Analysis of chromatographic and thermogravimetric sample fractions
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34. Interferogram:
The light signal measured by the detector is known as Interferogram.
An interferogram is time domain interference signal,and represents the detector's response to
the itensity changes against time within the mirror scan.
Movement of the moveable mirror back and forth once is known as scan.
Fourier Transformation:
The time domain interferogram obtained as detector's response cannot be interpreted as such.It
is subjected to furier transformation (a mathematical operation performed by an attached
computer) which converts it into a frequency-domain spectrum.
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36. • A = Infrared source
• B = Sample beam
• C = Chopper
• D = Monochromator grating
• E = Detector thermopile
• F = Amplifier
• G = Servo-motor
• H = An optical Wedge
• I = Prism
• J = Ink-pen recorder
• K = Slits
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37. APPLICATIONS OF IR SPECTROSCOPY
Structure determination:
IR spectroscopy i mainly used for determination of structure of organic compounds by
correlation and interpretation of IR spectra.
Identification of Substances:
• To compare spectrums.
• No two samples will have identical IR spectrum.
• Criteria: Sample and reference must be tested in identical conditions, like physical state,
temperature, solvent, etc.
Determination of Molecular Structure
• Used along with other spectroscopic techniques. Identification is done based on position of
absorption bands in the spectrum. Eg.: C=O at 1717 cm-1.
• Absence of band of a particular group indicates absence of that group in the compd.
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38. Detection of Impurities
• Determined by comparing sample spectrum with the spectrum of pure reference compound.
• Eg.: ketone impurity in alcohols.
• Detection is favoured when impurity possess a strong band in IR region where the main
substance do not possess a band.
Progress of Reactions
• Observing rate of disappearance of characteristic absorption band in reactants or
• Rate of increasing absorption bands in products of a particular product.
Presence of Water in Sample
If lattice water is present, spectra will contain 3 characteristic bands at 3600-3200cm-1,
1650cm-1 and 600-300cm-1.
Application In Industry
Determine impurities in raw materials (to ensure quality products). For Quality Control checks;
to determine the % of required product. Identification of materials made in industrial research
labs, or materials of competitors. E.g.: Impurity in bees wax (with petroleum wax)
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39. Reference:
Books
Organic spectroscopy and Chromatography by M.Younas
Pharmaceutical Drug Analysis By Ashutosh Kar
Web
https://www.chemguide.co.uk
https://www.slideshare.net
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