INFRARED%20SPECTROSCOPY

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INFRARED SPECTROSCOPY
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INFRARED SPECTROSCOPY

• INTRODUCTION
• UNITS MEASURMENT
• RANGE OF IR RADIATION
• REQURIMENTS
• MODES OF VIBRATION
• INSTRUMENTATION
• FACTORS EFFECTING VIBRATIONS
• ADVANTAGES DISADVANTAGES
• APPLICATIONS

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Introduction
MOLECULES ? ? ? ATOMS ? ? ? CONTINOUS
MOTION OR NATURAL VIBRATIONS ? ? ? APPLIED IR
FREQUENCY NATURAL FREQUENCY ? ABSORPTION OR
IR ? VIBRATIONS AT MANY RATES ? ? IR
SCPECTRA ? CHARACTERISTIC FUNCTIONAL GROUPS
BONDS ( FINGERPRINT OF A MOLECULE)
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Units Measurement
IR spectrum may be expressed by wave number whose
unit is cm-1

• c ??
• c/?
• ?/c

(C velocity of light3.8108m/sec)


.
E hn


Wavenumber _______1_________ cm-1
wavelength
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• Requirements for absorption of IR radiation
• Correct wavelength of radiation
• Natural frequency Frequency of incident
  radiation
• eg Hcl 8.71013 sec-1
• Electric dipole

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Range of IR

• Near IR 0.8 to 2.5?m (12000cm-1 4000cm-1)
• Analyzing mixtures of aromatic amines
• Determination of protein,fat,moisture,oil
  content.
• Middle IR 2.5 to 15?m ( 4000cm-1 650cm-1)
• Also known as vibration- rotation region.
• This region is divided into
• Group frequency region 4000cm-1 1300cm-1
• Fingerprint region 1300cm-1 650cm-1
• Far IR 15 to 1000?m (650cm-1 10cm-1 )
• Study of inorganic or organometallic compounds
• Sensitive to changes in overall structure of the
  molecule

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IR spectra
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Example Vanillin
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Modes of vibration

• Stretching vibrations
• Change in Bond axis
• One dimensional motion (n-1)
• Non cyclic systems
• Occurs at higher frequency

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Two types

• Symmetric

• Asymmetric

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• Bending vibrations
• Change in Bond angle
• Two dimensional (2n-5)
• Non cyclic Linear molecules
• Occurs at lower frequency
• Two types
• In-plane Out of plane

1.Twisting

• Scissoring

2.Wagging

• Rocking

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In plane
Out of plane
Scissoring
Twisting
Rocking
Wagging
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Instrumentation
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IR Radiation sources

• Tungsten filament lamp ?
• Nernst glower
• Zirconium, Yttrium,Thorium
• Heated to 1000 to 1800c
• Radiation 7100cm-1 (1.4?m)
• Globar source
• Silicon carbide
• Heated to 1300 to 1700c
• Radiation 5200cm-1 (1.9?m)
• Mercury arc
• Heated quartz shorter wavelength
• Mercury plasma longer wavelength
• Nichrome wire or coil
• Heated by passage of current to 1100c

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Monochormators

• Prisms
• Two types

1.Single pass
2.Double pass

• Gratings

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Sample cells Sampling techniques

• Gases
• Gas cell 10cms
• Multi pass gas cells
• 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
• KBr discs/ pellets/ pressed pellet technique
• Mulls
• Deposited films
• Solutions

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• 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

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• Gas cell

• NaCl flats

• KBr die

• Solution cell

• Variable path length
• cell

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Detectors

• Thermo couple
• ? thermoelectric efficiency e.g. bismuth
  antimony.
• Two junctions cold hot junction.

(KBr or CsI)
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• 2. Golay cell

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• 3. Bolometer
• Platinum strip - One arm of the wheatstone
  bridge.
• 4. Thermistor
• Fused mixture of metal oxides.
• Electric resistance decreases by 5 per c.
• 5. Semiconductor
• IR photon displaces an electron in the detector.
• Lead sulphide or lead telluride.
• 6. Pyroelectric detector
• Ferroelectric material operating below the curie
  point temp (49 c)
• Change in polarization.
• Electric signal
• Deuterium triglycine sulphate detecting medium.

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Mode of operation
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Advantages

1. All kinds of material can be analyzed.
2. Provides lot of information.
3. Fast and easy.
4. Very small amount of sample is required.
5. Less expensive.

Disadvantages

• 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 .
• Works best for pure substances but not for
• complex mixtures.

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Applications

• Identification of types of bonds (fingerprint
  region)
• Identification of functional groups in organic
  compounds
• The determination of steroids, hormones,
  pharmaceutical
• chemicals.

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• Factors influencing vibrational frequency
• 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
• Strong coupling of stretching vibrations occurs
  when there is a common atom between the two
  vibrating bonds
• Coupling of bending vibrations occurs when there
  is a common bond between vibrating groups
• Coupling between a stretching vibration and a
  bending vibration occurs if the stretching bond
  is one side of an angle varied by bending
  vibration
• Coupling is greatest when the coupled groups have
  approximately equal energies
• No coupling is seen between groups separated by
  two or more bonds

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• Hydrogen bonding
• It gives rise to downward frequency shifts.
• Stronger hydrogen bond greater absorption shift
  towards lower wave
  number.
• On dilution intensity of such bands decrease
  finally disappear.

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References

• Organic spectroscopy William Kemp (third
  edition).
• Spectroscopy of organic compounds P.S.Kalsi.
• 3. Instrumental methods of chemical analysis
• Gurdeep R.Chatwal, Sham K.Anand.