IR SPECTROSCOPY

IR SPECTROSCOPY THEORY

1. INFRARED SPECTROSCOPY THEORY PRESENTED BY: ANUPAMA RAMACHANDRAN 1 st Year M Pharm Pharmacology 1

2. CONTENTS INTRODUCTION IR SPECTROSCOPY PRINCIPLE MOLECULAR VIBRATIONS COUPLED INTERACTIONS DEGREE OF FREEDOM SUMMARY REFERENCE 2

3. INTRODUCTION Spectroscopy is the measurement and interpretation of EMR absorbed or emitted when the molecules or atoms or ions of a sample moves from one energy state to another energy state. Spectroscopy is an instrumentally aided study of the interactions between matter (sample being analyzed) and energy (any portion of the electromagnetic spectrum) 3

4. SPECTROSCOPY ABSORPTION SPECTROSCOPY EMISSION SPECTROSCOPY *UV SPECTROSCOPY * FLUORIMETRY *IR SPECTROSCOPY * FLAME PHOTOMETRY *NMR SPECTROSCOPY 4

5. IR SPECTROSCOPY Energy of molecule = Electronic energy+ Vibrational energy + Rotational energy • IR spectroscopy is concerned with the study of absorption of infrared radiation, which causes vibrational transition in the molecule. Hence, IR spectroscopy also known as Vibrational spectroscopy . • IR spectra mainly used in structure elucidation to determine the functional groups. 5

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7. IR REGION : 0.8 µm (800nm) to 1000 µm (1mm) subdivided into 1.Near IR: 0.8-2 µm 2.Middle IR: 2-15 µm 3.Far IR: 15-1000 µm Most of the analytical applications are confined to the middle IR region because absorption of organic molecules are high in this region. 7

8. GROUP FREQUENCY REGION:( 4000 to 1500 cm-1 ) Contain relatively few peaks. Associated with stretching vibrations. Almost all the stretching vibrations are above 1450 cm-1. IR stretching frequencies 8

9. FINGER PRINT REGION: (1500 – 400 cm-1 ) Usually consist of bending vibrations within the molecule. pattern of peaks are more complicated & is much difficult pickout individual bonds in this region E ach compound produce its own unique pattern of peaks like a finger print in this region . 9

10. In pharmaceutical analysis, infrared radiation (mid IR) of wavelength 25µ to 2.5µ or wave numbers from 400 cm -1 to 4000 cm -1. we use wave no. instead of wave length for mentioning characteristic peak ,because wave numbers are larger values & easy to handle than wave lengths. Wave no: no. of waves present per cm × = wave no. per cm   10

11. Molecules are made up of atoms linked by chemical bonds. The movement of atoms and the chemical bonds is like spring and balls (vibration ). This characteristic vibration are called Natural frequency of vibration . PRINCIPLE OF IR SPECTROSCOPY 11

12. When energy in the form of infrared radiation is applied ,then it causes vibration between the atoms of the molecules and when, Applied infrared frequency = Natural frequency of vibration Then, Absorption of IR radiation takes place and a peak is observed. Different functional groups absorb characteristic frequencies of IR radiation. Hence gives the characteristic peak value. Therefore, IR spectrum of a chemical substance is a finger print of a molecule for its identification. 12

13. CRITERIA FOR A COMPOUND TO ABSORB IR RADIATION 1 . Correct wavelength of radiation 2 . Change in dipole moment 1. Correct wavelength of radiation: A molecule to absorb IR radiation, the natural frequency of vibrations of some part of a molecule is the same as the frequency of incident radiation. 13

14. 2. Change in dipole moment • A molecule can only absorb IR radiation when its absorption cause a change in its electric dipole • A molecule is said to have an electric dipole when there is a slight positive and a slight negative charge on its component of atoms. 14

15. MOLECULAR VIBRATIONS Any change in shape of the molecule. 2 types, 1) fundamental vibrations 2) non fundamental vibrations 15

16. FUNDAMENTAL VIBRATIONS There are 2 types of vibrations: 1.Stretching vibrations 2.Bending vibrations 16

17. 1.STRETCHING VIBRATIONS: Vibration or oscillation along the line of bond Change in bond length Occurs at higher energy: 4000-1250 cm-1 2 types: a)Symmetrical stretching b)Asymmetrical stretching 17

18. a) Symmetrical stretching: 2 bonds increase or decrease in length simultaneously. 18

19. Symmetrical stretching 19

20. b) Asymmetrical stretching: • in this, one bond length is increased and other is decreased 20

21. Asymmetrical stretching 21

22. 2. BENDING VIBRATIONS Vibration or oscillation not along the line of bond These are also called as deformations In this, bond angle is altered Occurs at low energy: 1400-666 cm-1 2 types: a ) In plane bending: scissoring , rocking b) Out plane bending: wagging , twisting 22

23. a) In plane bending i . Scissoring : • This is an in plane bending • 2 atoms approach each other • Bond angles are decreased 23

24. Scissoring 24

25. ii. Rocking: • Movement of atoms take place in the same direction. 25

26. Rocking 26

27. b) Out plane bending i . Wagging: 2 atoms move to one side of the plane. They move up and down the plane. 27

28. Wagging 28

29. ii. Twisting: • One atom moves above the plane and another atom moves below the plane. 29

30. Twisting 30

31. COUPLED INTERACTIONS Interactions between vibrations can occur (Coupling) if the vibrating bonds are joined to a single, central atom . This is because there is mechanical coupling interaction between the oscillators . Example: C=O (both symmetric and asymmetric stretching vibrations) 31

32. REQUIREMENTS FOR COUPLING The vibrations must be of the same symmetry species if interaction is to occur. Strong coupling of stretching vibrations occurs when there is a common atom between the two vibrating bonds. Coupling of bending vibrations occur when there is a common bond between vibrating groups. 32

33. Coupling is greatest when the coupled groups have approximately equal energies. Coupling between a stretching vibration and a bending vibration occurs if the stretching bond is one side of an angle varied by bending vibration. No coupling is seen between groups separated by two or more bonds 33

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35. FACTORS AFFECTING FREQUENCY OF IR ABSORPTION Relative mass of the atom. Force constant of the bonds. Geometry of the atom. 35

36. DEGREE OF FREEDOM Fundamental vibration of molecule depend on degree of freedom. Each atom has 3 degree of freedom depend on x, y, z. For a molecule containing n no. of atoms has 3 n degree of freedom. For non – linear molecule, 3 degree of freedom represent rotational & transational motion. For non-linear, ( 3 n -6) degree of freedom represent fudamental vibrations. For linear ( 3n-5) degree of freedom represent fundamental vibrations. All vibrational changes don’t appear as band. Only those vibrational changes that result in change in dipole moment appear as band. 36

37. TYPICAL INFRARED SPECTRUM 37

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39. SUMMARY Absorption spectroscopy. Vibrational transitions. Applied infrared frequency = Natural frequency of vibration . Structure elucidation to determine functional groups. 39

40. REFERENCES Spectrometric identification of organic compounds, sixth edition, Robert M. Silverstein & Francis X. Webster. Page no: 71 – 76 Principles of instrumental analysis, 6 th edition, Douglas A. Skoog , E James Holler, Stanley R Crouch . Page no: 430 – 455 Introduction to spectroscopy, Donald L Pavia, Gary M. Lampman, James R. Vyvyan. Page no: 15 - 31 40

41. Thank you… 41