INFRARED SPECTROSCOPY

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 • 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 • Heatedquartz – 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. •  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

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. 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 • 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.