Infrared spectroscopy

Infrared spectroscopy

1. INFRARED SPECTROSCOPY by P. viji M.Pharm I year (Pharmaceutics)

2. INTRODUCTION Infrared spectroscopy or vibrational spectroscopy is concerned with the study of absorption of infrared radiation, which results in vibrational transitions . It is also known as vibrational spectroscopy . Infrared (IR) radiation lies in the part of the electromagnetic spectrum i.e. between the visible and microwave regions . Near IR region: 0.8 μ m to 2.5 μ m Middle IR region: 2.5 μ m to 50 μ m Far IR region : 50 μ m to 1000 μ m

3. PRINCIPLE Molecules are made up of atoms linked by bonds. The movement of atoms and the chemical bonds like spring and balls (vibration) This characteristic vibration are called Natural vibration.

4. When IR is applied then it causes the vibration between the atoms of the molecules when , Applied infrared frequency = Natural frequency of vibration Then , Absorption of IR radiation takes place and a peak is observed . Example : Infrared vibrations of ethanol.

5. Every bond or portion of a molecule or functional group requires different frequency of absorption. Hence characteristic peak is observed for every functional group or part of the molecule. In other words, IR spectra is nothing but a finger print region of a molecule. IR spectroscopy

6. Fingerprint Region Absorption band in the region 1500-500 cm‾¹. Useful for establishing the identity of a compound. It consists of : Region 1500-1350 cm‾¹ : Appearance of doublet near 1380 cm‾¹ and 1365cm‾¹ shows the presence of 3º butyl group. Region 1350-1000 cm‾¹ : All classes of compound viz. alcohol, esters, lactones shows absorptions in the region due to C-O stretching. Region below 1000 cm‾¹ : This region distinguishes between cis and trans alkene.

7. CRITERIA FOR A COMPOUND TO ABSORB IR RADIATION Dipole Moment: The bonds in a molecule can absorb IR radiation only when there is a change in dipole moment due to electric field of IR radiation . Applied IR frequency should be equal to the natural frequency of radiation, otherwise compounds do not give IR peaks

8. TYPES OF VIBRATIONS

9. 1. Stretching vibrations: Vibration along the line of bond. Change in bond length. 2 types : Symmetrical stretching Asymmetrical stretching

10. Symmetrical stretching Both bonds increase or decrease in length simultaneously. b ) Asymmetrical stretching In this, one bond length is increased and other is decreased.

11. 2. Bending vibrations Vibration not along the line of bond. In this, bond angle is altered. 2 types: a) In plane bending: scissoring rocking b) Out plane bending: wagging twisting

12. a)In plane bending: i. scissoring : This is an in plane blending Bond angles are decrease ii . Rocking : Bond angle is maintained. Movement of atoms take place in the same direction.

13. b ) OUT PLANE BENDING vibrations takes place outside the plane of molecule . Wagging: both atoms move to one side of the plane. ii. Twisting: One atom moves above the plane and another atom moves below the plane

14. INSTRUMENTATION The main parts of IR spectrometer are as follows : 1 . IR radiation sources 2 . Monochromators 3. Sample cells and sampling of substances 4. Detectors 5. recorders

15. 1.IR radiation sources Incandescent lamp Nernst glower Globar Source

16. a) Incandescent Wire Source

17. b ) The Nernst Glower

18. c ) The Globar source

19. 2.Monochromators A.Prism :- Used as dispersive element. Constructed of various metal halide salts. Sodium chloride is most commonly prism salt used.

20. B. Grating Grating are nothing but rulings made on some materials like glass, quartz or alkylhalides depending upon the instrument . The mechanism is that diffraction produces reinforcement. The rays which are incident upon the gratings gets reinforced with the reflected rays.

21. 3.SAMPLE CELL Made up of alkali halides like NaCl or KBr . Aqueous solvents cannot be used - they dissolve alkali halides . Only organic solvents like chloroform is used.

22. Sample handling Sampling of solids Solids run in solution Mull technique Pressed pellet technique Solids films

23. ) Solids run in solution Dissolve solid sample in non -aqueous solvent and place a drop of this solution in alkali metal disc and allow to evaporate, leaving a thin film which is then mounted on a spectrometer . E.g. of solvents – acetone, cyclohexane, chloroform carbon tetrachloride etc .

24. Solid films Here amorphous solid is dissolved in volatile solvents and this solution is poured on a rock salt plate ( NaCl or KBr ), then the solvent is evaporated by gentle heating.

25. M ull technique: In this technique a small quantity of sample is thoroughly ground in a clean mortar until the powder is very fine. After grinding, the mulling agent (mineral oil or Nujol ) is introduced in small quantities just sufficient to take up the powder (mixture approximates the consistency of a toothpaste). The mixture is then transferred to the mull plates & the plates are squeezed together to adjust the thickness of the sample between IR transmitting windows. This is then mounted in a path of IR beam and the spectrum is run.

26. Pressed pellet technique: In this technique a small amount of finely ground solid sample is intimately mixed with about 100 times its weight of powdered Potassium bromide, in a vibrating ball mill . This finely ground mixture is then pressed under very high pressure (25000 p sig) in evacuable die or minipress to form a small pellet (about 1-2 mm thick and 1cm in diameter ). The resulting pellet is transparent to IR radiation and is run as such.

27. b) Sampling of liquids Liquids sample can be sandwiched between two alkali halide salt plates ( NaCl , KBr , CaF2 ) . The sample cell thickness is 0.01-0.05mm.

28. c ) Sampling of gases Here the spectrum of gas can be obtained by gases sample or low boiling i.e. volatile liquid is introduced into a glass evacuated cell made up of NaCl . Very few organic compounds can be examined as gases. E.g. : 1 , 4-dioxane

29. 4) DETECTOR TYPES OF DETECTOR: Bolometers detector Thermocouple and thermopile detector Pyro electric detector Golay cell

30. a) Thermocouple detector

31. b) Bolometer

32. c) GOLAY CELL: Golay cell consists of a small metal cylindrical closed by a rigid blackened metal plate . Pneumatic chamber is filled with xenon gas . At one end of cylinder a flexible silvered diaphragm and at the other end Infra red transmitting window is present.

33. When infra red radiation is passed through infrared transmitting window the blackened plate absorbs the heat. By this heat the xenon gas causes expand The resulting pressure of gas will cause deformation of diaphragm. This motion of the diaphragm detects how much IR radiation falls on metal plate . Light is made to fall on diaphragm which reflects light on photocell

34. d) Pyroelectric Detectors Pyroelectric detector contain certain crystal Such as lithium tantalate , barium titanate and triglycine sulfate i.e. they produce temperature sensitive dipole moments. To construct a pyroelectric detector , a pyroelectric substance is placed between two electrodes . Two electrodes are connected to each other via a voltmeter. one of which has IR transparent window .

35. When the IR radiation falls upon a pyroelectric substance , it absorbs energy from the radiation .

36. 5) Recorder: The radiant energy received by detector is converted into measurable electrical signal and is amplified by amplifier. The amplified signals are recorded and plotted.

37. TYPES OF INSTRUMENTATION There are 2 types of infrared spectrophotometer characterized by the manner in which the IR frequencies are handled . 1) dispersive type (IR) 2) I nterferometric type(FTIR)

38. 1) dispersive type (IR) In dispersive type the infrared light is separated into individual frequencies by dispersion, using a grating monochromator .

39. 2) Interferometric type(FTIR) In interferometric type the ir frequencies are allowed to interact to produce an interference pattern and this pattern is then analyzed, to determine individual frequencies and their intensities.

40. APPLICATIONS Identification of functional group and structural elucidation . Identification of drug substances . Identifying impurities in drug sample . Study of hydrogen bonding. Study of polymers. Identify ratio of cis -trans isomers in a mixture of compounds. Quantitative analysis. To find out difference between hydrogen bonding.