Ir spectroscopy

principle, vibrations

1. IR SPECTROSCOPY PREPARED BY: DEVIPRIYA P V M PHARM DEPT OF PHARMACEUTICAL ANALYSIS

2. IR SPECTROSCOPY Study of absorption of infrared radiation , which causes vibrational transition in the molecule. Hence, IR spectroscopy also known as vibrational spectroscopy. IR radiation refers to that region of electromagnetic spectrum which lies between visible and microwave region. 2

3. The photographic region : visible to 1.2 μ . The very near infrared region :1.2-2.5 μ . The near infrared region :2.5-25 μ . The far infrared region : 25 to 300-400 μ . Energy of the molecule = electronic energy +vibrational energy + rotational energy 3

4. PRINCIPLE Molecules are made up of atoms linked by chemical bonds . Analogous to springs. Because of continuous motion of molecule they maintain some vibrations with some frequency Characteristic vibration are called natural frequency of vibration. 4

5. When energy in the form of IR radiation is applied and when, Applied IR frequency= natural frequency of vibration Absorption of IR radiation takes place and a peak is observed. 5

6. Criteria for a compound to absorb IR radiation Correct wavelength of radiation Change in dipole moment 6

7. Correct wavelength of radiation A molecule to absorb IR radiation, the natural frequency of vibrations of some part of a molecule is same as the frequency of incident radiation. 7

8. Change in dipole moment A molecule can absorb IR radiation when its absorption cause a change in its electric dipole. A molecule is said to have electric dipole when there is a single slight positive and negative charge on its component of atom. 8

9. MODES OF MOLECULAR VIBRATIONS STRECHING VIBRATION BENDING VIBRATION Involves a continuous change in the inter atomic distance along the axis of the bond between two atoms Requires more energy , so appear at shorter wavelength Characterized by a change in the angle between two bonds Requires less energy , so appear at longer wavelength 9

10. Symmetrical stretching Two bonds increase or decrease in length symmetrically. Asymmetrical stretching: One bond length is increased and other is decreased 10

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12. In plane bending Change in bond angle Bending of bonds within the same plane 1. scissoring Bond angle decreases 2. rocking Bond angle is maintained but both bonds move within the plane 12

13. Out plane bending Bending takes places outside the plane of molecule 1 . wagging 2 atoms move to one side of the plane Move up and down the plane 2. Twisting One atom moves above the plane and another atom moves below the plane 13

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16. Factors affecting vibrational frequencies Coupled vibrations Fermi resonance Electronic effects Hydrogen bonding 16

17. Coupled vibrations Interactions between vibrations can occur if the vibrating bonds are joined to a single , central atom. This is because there is mechanical coupling interaction between the oscillators 17

18. Requirements for coupling 18

19. Fermi resonance The energy of an overtone level changes to coincide with the fundamental mode of different vibrations. Molecule transfer its energy from fundamental to overtone and back again. Resonance pushes the two levels apart and mix their character . Give rise to pair of transitions with equal frequencies 19

20. Electronic effects Change in absorption frequencies The frequency shift are due to electronic effects which include: Inductive effect Mesomeric effect. Field effects 20

21. Hydrogen bonding Downward frequency shift. Stronger the H-bonding , greater is the absorption shift towards lower wave number. Intra molecular H –bonds=sharp bands. Inter molecular gives broad bands 21

22. INSTRUMENTATION Radiation source Monochromators Sample cells and sampling of substances Detectors Recorders 22

23. IR RADIATION SOURCE Require a source of radiant energy which emit IR radiation which must be : Continuous Stable Sufficient intensity 23

24. Incandescent lamp: used in near IR instruments Nernst glower: composed of zirconia , yttria and thoria . Non conducting at room temperature Heated b/w 1000-1800°C. Provide radiation of about 7100 cm -1 Globar source: A rod of sintered silicon carbide. Emit radiation of 5200 cm -1 Mercury arc: In far IR region 24

25. MONOCHROMATORS To select desired frequencies from the radiation source and reject the radiations of other frequencies Prism monochromator Grating monochromator 25

26. SAMPLE CELLS AND SAMPLING OF SUBSTANCES Sampling of solids Solids run in solutions Solid films Mull technique Pressed pellet technique Sampling of liquids Sampling of gases 26

27. Solids run in solutions Solid sample dissolved in a non aqueous solvent A drop of solution is placed on an alkali metal disc and the solution is allowed to evaporate leaving a thin film of the solute 27

28. Solid films For amorphous solids Sample is deposited on the surface of a KBr or NaCl cell evaporation of a solution of the solid. Useful for rapid qualitative analysis 28

29. Mull technique Finely ground solid sample is mixed with Nujol Form a thick paste Spread between IR transmitting windows Mounted in a path of IR beam and spectrum is run Good for qualitative analysis 29

30. Pressed pellet technique Finely ground solid sample is mixed with 100 times its weight of KBr Passed under very high pressure (25000 psig) Forms small pellet (1-2 mm thick and 1 cm diameter) KBr pellets can be stored for long periods Resolution of the spectrum in KBr is high Concentration can be adjusted, used for quantitative analysis 30

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32. Sampling of liquids Placed in rectangular cells made of NaCl , KBr or ThBr . Sample thickness should be so selected that the transmittance lies b/w 15 & 20%. 0.01-0.05 thickness. 32

33. Sampling of gases Similar to sampling of liquids. The gas must not react with the cell windows or the reflecting surfaces Not commonly used(lack of sensitivity). 33

34. DETECTORS Thermal detector Non thermal detector Bolometer Thermocouple and thermopile Pyro electric detector Golay cell Photo voltaic detector 34

35. BOLOMETER Electrical resistance of a metal increases approximately 0.4% for every degree Celsius increase in temperature. When IR radiation falls on the metal conductor its temperature changes. The degree of change in resistance is the measure of radiation that falls. 35

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37. THERMOCOUPLE Electrical current will flow when two dissimilar metal wires are connected together at both ends and a temperature differential exists between the two ends. 37

38. PYROELETCRIC DETECTOR Pyro -electric materials exhibit electrical polarization. When the temperature is altered, polarization changes and is observed as an electrical signal. 38

39. GOLAY CELL Metal cylinder and flexible diaphragm Temperature increases Gas is expanded and diaphragm deforms Detect as a signal 39

40. PHOTOVOLTAIC DETECTOR IR radiation Photovoltaic detector Generates a small voltage Detected as a signal 40

41. Single beam IR spectrophotometer 41

42. Double beam IR instrument 42

43. FTIR FT-IR stands for Fourier Transform Infra Red, the most preferred method of infrared spectroscopy. In infra red spectroscopy ,IR radiation is passed through a sample, some of the radiation is absorbed and some is transmitted. The resulting spectrum represents the molecular absorption and transmission , creating a molecular finger print of the sample 43

44. INSRUMENTATION 44

45. APPLICATIONS Qualitative analysis Identification of substances Determination of molecular structure Studying progress of reaction Detection of impurities Isomerism in organic chemistry. Shape of symmetry of a molecule Identification of functional groups 45

46. Identification of substances To compare spectrums. No two compounds have identical IR spectrum. Criteria : sample and reference must be tested in identical conditions. The finger print region (1200 -700 cm -1 ). Small difference in structure and constitution of molecule result in significant changes in the peak in this region. 46

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49. Determination of molecular structure 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 compound. 49

50. Studying progress of reaction Observing rate of disappearance of characteristic absorption bands in reaction. Rate of increasing absorption bands in products of a particular product. Eg : O-H = 3600-3650cm -1 C=O =1680-1760cm -1 50

51. Functional group isomerism Isomerism shown by compounds having same molecular formula but different functional groups. Eg : CH 3 -O-CH 3 and CH 3 -CH 2 -OH. OH=3500-3100cm -1 51

52. Applications in inorganic complexes Geometrical isomerism Determination of purity Shape of symmetry of a molecule. Presence of water in a sample. Measurement of paints and varnishes. Examination of oils and paintings and artifacts. 52

53. Quantitative analysis Determination of concentration of one of the functional groups of the compounds being estimated. Eg : concn of hexanol in hexane - hexanol mixture. A= abc 2 methods to determine A and C Cell-in cell-out method Baseline method 53

54. IR SPECTRUM The functional group region: Identifies the functional group with the consequence of changing stretching vibrations. Ranges from 4000-1600cm -1 The fingerprint region: Identifies the exact molecule with the consequence of changing bending vibrations. Ranges from 1600-625 cm -1 54

55. INTERPRETATION OF IR SPECTRA Structural information about compounds is mainly derived from the presence or absence of characteristics absorption bands of various functional groups in the IR spectrum of the compounds. 55

56. INTERPRETATION 56

57. 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. 57

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59. IR SPECTRUM OF ALKANES 59

60. IR SPECTRUM OF ALKENES 60

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62. IR SPECTRUM OF ALCOHOLS 62

63. IR SPECTRUM OF ALDEHYDES AND KETONES 63

64. IR SPECTRUM OF CARBOXYLIC ACID 64

65. IR SPECTRUM OF AMINES 65

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