INFRARED SPECTROSCOPY
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INTRODUCTION
UNITS & MEASURMENT
RANGE OF IR RADIATION
REQURIMENTS
MODES OF VIBRATION
INSTRUMENTATION
FACTORS EFFECTING VIBRATIONS
ADVANTAGES & DISADVANTAGES
APPLICATIONS
Introduction
MOLECULES
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ATOMS
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CONTINOUS MOTION
OR
NATURAL VIBRATIONS
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APPLIED IR FREQUENCY = NATURAL FREQUENCY
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ABSORPTION OR IR
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VIBRATIONS AT MANY RATES
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IR SCPECTRA
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CHARACTERISTIC FUNCTIONAL GROUPS &
BONDS ( FINGERPRINT OF A MOLECULE)
Units & Measurement
IR spectrum may be expressed by wave number whose unit is
cm-1
c =n
(C = velocity of light=3.8*108m/sec)
n= c/
.
=n/c =
E = hn
=
Wavenumber = _______1_________ cm-1
wavelength
Requirements for absorption of IR radiation :
•
Correct wavelength of radiation:
Natural frequency = Frequency of incident radiation
eg: Hcl 8.7*1013 sec-1
•
Electric dipole
Range of IR
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Near IR: 0.8 to 2.5m (12000cm-1 –4000cm-1)
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Analyzing mixtures of aromatic amines
Determination of protein,fat,moisture,oil content.
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Middle IR: 2.5 to 15m ( 4000cm-1 – 650cm-1)
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1.
2.
Also known as vibration- rotation region.
This region is divided into:
Group frequency region: 4000cm-1 – 1300cm-1
Fingerprint region: 1300cm-1 – 650cm-1
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Far IR: 15 to 1000m (650cm-1 –10cm-1 )
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Study of inorganic or organometallic compounds
Sensitive to changes in overall structure of the molecule
1
IR spectra
Example: Vanillin
Modes of vibration
 Stretching vibrations
1.
2.
3.
4.
Change in Bond axis
One dimensional motion (n-1)
Non cyclic systems
Occurs at higher frequency
Two types:
•Symmetric
•Asymmetric
 Bending vibrations
1.
2.
3.
4.
Change in Bond angle
Two dimensional (2n-5)
Non cyclic & Linear molecules
Occurs at lower frequency
Two types:
 In-plane:
•Scissoring
•Rocking
*Out of plane:
1.Twisting
2.Wagging
In plane:
Scissoring
Rocking
Out of plane:
Twisting
Wagging
IR Radiation sources
 Tungsten filament lamp 
 Nernst glower:
a.
b.
c.
Zirconium, Yttrium,Thorium
Heated to 1000 to 1800•c
Radiation – 7100cm-1 (1.4m)
 Globar source:
a.
b.
c.
Silicon carbide
Heated to 1300 to 1700•c
Radiation – 5200cm-1 (1.9m)
 Mercury arc
a.
b.
Heated quartz – shorter wavelength
Mercury plasma – longer wavelength
 Nichrome wire or coil
a.
Heated by passage of current to 1100•c
Monochormators
 Prisms
Two types:
1.Single pass
 Gratings
2.Double pass
Sample cells &
Sampling techniques
 Gases
• Gas cell – 10cms
• Multi pass gas cells
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Liquids
Thin film squeezed between 2 IR transparent windows.
 0.1 - 0.3mm thickness
Below 250cm-1 _ CsI flats, sample with water CaF2 flats
 Solids
Four techniques:
1. KBr discs/ pellets/ pressed pellet technique
2. Mulls
3. Deposited films
4. Solutions
1. KBr discs:
• 0.1 – 2.0% by wt.
• Particle size -  2m.
• Hydraulic pressure – 10 tons load.
• Discs: 13mm- diameter, 0.3mm- thickness.
2. Mulls:
• Grinding sample with a drop of oil.
• Nujol (liquid paraffin)
• Complement: Hexacholorobutadiene &
chlorofluorocarbon.
3. Deposited films:
• Solution in a volatile solvent on a NaCl flat.
4. Solutions:
• Solvent – CCl4, CS2, CHCl3
• Complementary pair - CCl4 & CS2
•Gas cell
•NaCl flats
•KBr die
•Solution cell
•Variable path length
cell
Detectors
1. Thermo couple:
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 thermoelectric efficiency e.g. bismuth & antimony.
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Two junctions: cold & hot junction.
(KBr or CsI)
2. Golay cell:
3. Bolometer:
1.
Platinum strip - One arm of the wheatstone bridge.
4. Thermistor:
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Fused mixture of metal oxides.
Electric resistance decreases by 5% per •c.
5. Semiconductor:
1.
2.
IR photon displaces an electron in the detector.
Lead sulphide or lead telluride.
6. Pyroelectric detector:
1.
2.
3.
4.
Ferroelectric material operating below the curie point temp
(49 •c)
Change in polarization.
Electric signal
Deuterium triglycine sulphate – detecting medium.
Mode of operation
Advantages
1.
2.
3.
4.
5.
All kinds of material can be analyzed.
Provides lot of information.
Fast and easy.
Very small amount of sample is required.
Less expensive.
Disadvantages
1. IR spectra cannot be obtained for Mono atomic
substances e.g.: helium, argon.
2. IR spectra cannot be obtained for Homonuclear
diatomic molecules like O2 , N2 .
3. Works best for pure substances but not for
complex mixtures.
Applications
 Identification of types of bonds (fingerprint region)
 Identification of functional groups in organic compounds
 The determination of steroids, hormones, pharmaceutical
chemicals.
Factors influencing vibrational frequency:
1. Vibrational coupling:
Interaction between vibrations can occur (coupling) if the vibrating bonds
are joined to a single, central atom and also vibrating with similar
frequency,provided that the bonds are reasonably close in a molecule.
Vibrational coupling is influenced by a number of factors;
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Strong coupling of stretching vibrations occurs when there is a
common atom between the two vibrating bonds
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Coupling of bending vibrations occurs when there is a common
bond between vibrating groups
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Coupling between a stretching vibration and a bending vibration
occurs if the stretching bond is one side of an angle varied by
bending vibration
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Coupling is greatest when the coupled groups have approximately
equal energies
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No coupling is seen between groups separated by two or more
2. Hydrogen bonding:
a. It gives rise to downward frequency shifts.
b. Stronger hydrogen bond – greater absorption shift
towards lower wave
number.
c. On dilution – intensity of such bands decrease &
finally disappear.
References
1. Organic spectroscopy: William Kemp (third edition).
2. Spectroscopy of organic compounds: P.S.Kalsi.
3.
Instrumental methods of chemical analysis:
Gurdeep R.Chatwal, Sham K.Anand.