1. INFRARED SPECTROSCOPY

2. INFRARED SPECTROSCOPY
INTRODUCTION
UNITS & MEASURMENT
RANGE OF IR RADIATION
REQURIMENTS
MODES OF VIBRATION
INSTRUMENTATION
FACTORS EFFECTING VIBRATIONS
ADVANTAGES & DISADVANTAGES
APPLICATIONS

3. 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)

4. Units & Measurement
IR spectrum may be expressed by wave number whose unit is
cm-1
c =
= c/
=/c =
(C = velocity of light=3.8*108m/sec) .
E = hn =
Wavenumber = _______1_________ cm-1
wavelength

5. 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

6. 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 -1

7. IR spectra

8. Example: Vanillin

9. Modes of vibration Stretching vibrations Change in Bond axis
One dimensional motion (n-1)
Non cyclic systems
Occurs at higher frequency

10. Two types:
Symmetric
Asymmetric

11. Bending vibrations Two types: In-plane: *Out of plane: 1.Twisting
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

12. In plane:
Out of plane:
Scissoring
Twisting
Rocking
Wagging

13. Instrumentation

14. IR Radiation sources Tungsten filament lamp  Nernst glower:
Zirconium, Yttrium,Thorium
Heated to 1000 to 1800•c
Radiation – 7100cm-1 (1.4m)
Globar source:
Silicon carbide
Heated to 1300 to 1700•c
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 1100•c

15. Monochormators
Prisms
Two types:
1.Single pass
2.Double pass
Gratings

16. Sample cells & Sampling techniques
Gases
Gas cell – 10cms
Multi pass gas cells
Liquids
Thin film squeezed between 2 IR transparent windows.
 mm thickness
Below 250cm-1 _ CsI flats, sample with water CaF2 flats
Solids
Four techniques:
KBr discs/ pellets/ pressed pellet technique
Mulls
Deposited films
Solutions

17. 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

18. Gas cell
NaCl flats
KBr die
Solution cell
Variable path length
cell

19. Detectors Thermo couple:
 thermoelectric efficiency e.g. bismuth & antimony.
Two junctions: cold & hot junction.
(KBr or CsI)

20. 2. Golay cell:

21. 6. Pyroelectric detector:
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.

22. Mode of operation

23. Advantages Disadvantages All kinds of material can be analyzed.
Provides lot of information.
Fast and easy.
Very small amount of sample is required.
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.

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

25. 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

26. 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.

27. 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.