IR SPECTROSCOPY.pptx

Infrared (IR) spectroscopy is an absorption method widely used in both qualitative and quantitative analyses. The infrared region of the spectrum includes electromagnetic radiation that can alter the vibrational and rotational states of covalent bonds in organic molecules.The IR spectrum of an organic compound is a unique physical property and can be used to identify unknowns by interpretation of characteristic absorbances and comparison with spectral libraries. IR spectroscopy is also used in quantitative techniques because of its sensitivity and selectivity. It can be used to quantitate analytes in complex mixtures and is used extensively in detection of industrial pollutants in the environment.

1. IR SPECTROSCOPY SUBMITTED BY, Manimegalai.G M.Pharm - I Year Department of Pharmaceutical chemistry 3/20/2023 Department of Pharmaceutical Chemistry 1

2. INTRODUCTION IR spectroscopy is the study of interaction between infrared radiations and matter. Infrared radiations refer broadly to that part of electromagnetic spectrum between visible and microwave region. IR region extends from 2.5µ to 15µ Near IR - 0.8µ to 2.5µ Mid IR - 2.5µ to 15µ Far IR - 15µ to 200 µ 3/20/2023 Department of Pharmaceutical Chemistry 2

3. PRINCIPLE The absorption of IR causes an excitation of molecule from a lower to the higher vibrational level. Each vibrational level is associated with no.of closely spaced rotational level. All bonds in molecule are not capable of absorb IR energy but only those bonds which are accompanied by change in dipole moment, will absorb IR region. 3/20/2023 Department of Pharmaceutical Chemistry 3

4. IR active : vibrational transitions accompanied by change in dipole moment Eg - OH, NH, C=O IR inactive : vibrational transitions not accompanied by change in dipole moment Eg - C-C bonds in symmetrical alkenes and alkynes 3/20/2023 Department of Pharmaceutical Chemistry 4

5. THEORY Absorption in IR is due to changes in vibrational and rotational levels. As the absorption is quantised , the discrete lines are formed in spectrum due to molecular rotation. Single vibrational energy changes is accompanied by a large no.of rotational energy changes. 3/20/2023 Department of Pharmaceutical Chemistry 5

6. Thus, the vibrational spectra absorbed energy brings about predominant changes in vibrational energy which depends on, Mass of atom present in a molecule. Strength of bonds. The arrangement of atom within the molecule. 3/20/2023 Department of Pharmaceutical Chemistry 6

7. When infra red light is passed through the sample, the vibrational and rotational energies of the molecules are increased. 2 kind of fundamental vibrations a) Stretching – Distance between 2 atoms increase or decrease but the atoms remain in same axis. b) Bending – Position of atoms change with respect to the original bond axis. 3/20/2023 Department of Pharmaceutical Chemistry 7

8. More energy is required to stretch a bond than that bend it. Stretching absorption of bond appears at high frequency as compared to bending absorption of same bond. The various stretching and bending vibration of bond occurs at certain quantised frequencies. 3/20/2023 Department of Pharmaceutical Chemistry 8

9. TYPES OF STRETCHING VIBRATION SYMMETRIC – Movement of atoms with respect to particular atom in a molecule is in same direction. ASYMMETRIC- One atom approach the central atom while the other departs from it. 3/20/2023 Department of Pharmaceutical Chemistry 9

10. TYPES OF BENDING VIBRATION SCISSORING- Two atoms approach each other. ROCKING- Movement of atoms takes place in same direction. 3/20/2023 Department of Pharmaceutical Chemistry 10

11. WAGGING- Two atoms move up or down the plane with respect to central atom. TWISTING- One atom move up the plane while the other move down the plane. Bending vibrations require less energy and occur at higher wavelength or lower wavenumber than stretching vibrations . 3/20/2023 Department of Pharmaceutical Chemistry 11

12. VIBRATIONAL FREQUENCY The value of stretching vibrational frequency of a bond can be calculated by Hooke’s law. “It states that, the size of the deformation is directly proportional to the deforming force.” 3/20/2023 Department of Pharmaceutical Chemistry 12

13. 3/20/2023 Department of Pharmaceutical Chemistry 13 Where, µ - reduced mass m 1, m 2 – mass of atoms k – force constant c – velocity of radiation If the bond strength increases or the reduced mass decreases, the value of vibrational frequency increases.

14. FACTORS INFLUENCING VIBRATIONAL FREQUENCY Coupled vibrations Fermi resonance Electronic effects Hydrogen bonding Bond angles 3/20/2023 Department of Pharmaceutical Chemistry 14

15. COUPLED VIBRATIONS One stretching frequency occur at one isolated CH group. But methylene(-CH 2 -) two absorption frequency occurs which correspond to symmetric and asymmetric vibrations. In such cases, asymmetric occurs at higher wavenumber compared with symmetric vibrations. 3/20/2023 Department of Pharmaceutical Chemistry 15

16. Methyl group coupled vibrations takes place at different frequencies compared to CH 2 group. Sometimes, it happens two different vibrational level have same energy. If such vibrations belongs to same species, resulting in the shift of one towards lower frequency and the other towards higher frequency. 3/20/2023 Department of Pharmaceutical Chemistry 16

17. FERMI RESONANCE A molecules that transfer energy from fundamental to overtone and back again. It is called Fermi resonance. Fermi resonance is also shown by the spectrum of n- butyl vinyl ether. In this case, overtone of fundamental vibration at 810 cm -1 chances to coincide with the band at 1640 cm -1 The mixing of two double bands in accordance with fermi resonance gives two bands of almost equal intensity at 1640 cm -1 and 1630 cm -1 . 3/20/2023 Department of Pharmaceutical Chemistry 17

18. ELECTRONIC EFFECT Changes in absorption frequencies of particular group take place when the substituents in the neighbourhood of that particular group are changed. The frequency shift due to the electronic effects include mesomeric effect, inductive effect and field effect. These effects cannot isolated from one another & contribution of one of them can only be estimated. 3/20/2023 Department of Pharmaceutical Chemistry 18

19. Introduction of alkyl group shows +I effect which results in lengthening or weakening of bond and lower force constant and wavenumber of absorption frequency decreases. HCHO – 1750 cm -1 CH 3 CHO – 1745 cm -1 CH 3 COCH 3 – 1715 cm -1 Aldehydes absorb at higher wavenumber than ketones. 3/20/2023 Department of Pharmaceutical Chemistry 19

20. The introduction of electronegative atoms causes –I effect which results in bond order to increase. Thus, the force constant increase and hence the wavenumber of absorption rises. Acetone – 1715 cm -1 Chloroacetone – 1725cm -1 Dichloroacetone - 1740 cm -1 T etrachloroacetone – 1750 cm -1 3/20/2023 Department of Pharmaceutical Chemistry 20

21. Mesomeric effect work along with inductive effect. Mesomeric effect causes lengthening or weakening of bond leading in the lowering of absorption frequency. -I effect is dominated by mesomeric effect, the absorption frequency falls. 3/20/2023 Department of Pharmaceutical Chemistry 21 Methyl vinyl ketone (C=O 1706cm-1) Acetophenone (C=O 1693cm-1)

22. The lone pairs of electrons on two atoms influence each other through space interactions and change the vibrational frequencies of both the groups. This effect is called field effect.  In meta substitution, only inductive effect is considered. In para substitution, both inductive & mesomeric effect become important. In ortho substitution, inductive & mesomeric effect along with steric effect. 3/20/2023 Department of Pharmaceutical Chemistry 22

23. HYDROGEN BONDING Stronger hydrogen bonding, greater absorption shift towards the lower wavenumber than the normal value. Two types of hydrogen bond can be distinguished in IR. Intermolecular hydrogen bonding give rise to broad band whereas band arising from intramolecular hydrogen shows sharp peak. 3/20/2023 Department of Pharmaceutical Chemistry 23

24. Eg : In aliphatic alcohols, a sharp band at 3650 cm -1 due to free OH group, broad band occurs at 3350 cm -1 due to hydrogen bonded OH group. 3/20/2023 Department of Pharmaceutical Chemistry 24

25. BOND ANGLES Cyclobutanone The C-CO-C bond angle is reduced below the normal angle of 120 and it leads to increased s- character in C=O bond. Greater s- character causes shortening of C=O bond and thus C=O stretching occurs at higher frequency. If bond angle push outwards above 120 opposite effect operates. 3/20/2023 Department of Pharmaceutical Chemistry 25

26. INSTRUMENTATION IR Instrumenttaion is divided into two classes, a) dispersive –use a prism or grating b) non dispersive – use interference filter, tunable laser sources. It is convenient to divide the infrared region into three segments , with the dividing points based on instrumental capabilities. 3/20/2023 Department of Pharmaceutical Chemistry 26

27. NEAR - IR MID -IR FAR-IR Wavenumber cm -1 12,500 to 4000 4000 to 200 200 to 10 Wavelength µm 0.8µ to 2.5µ 2.5µ to 15µ 15µ to 200µ Source of radiation Tungsten filament lamp Nernst glower, Globar or coil of nichrome wire High pressure mercury arc lamp Optical system One or two quartz prism or prism grating monochromator Two to four plane diffraction gratings monochromator Double beam grating interferometer Detector Photoconductive cells Thermopile, thermistor, pyroelectric , semiconductor Golay cell, pyroelectric 3/20/2023 Department of Pharmaceutical Chemistry 27

28. The main parts of IR spectrometer are as follows: IR radiation source Monochromators Sample cells and sampling of substances Detectors 3/20/2023 Department of Pharmaceutical Chemistry 28

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30. IR RADIATION SOURCES The radiation sources must emit IR radiation which must be Intense enough for detection Steady Extend over the desired wavelength The various popular sources of IR radiation are Incandescent lamp - Incandescent light bulbs use a tungsten filament heated to high temperature to produce visible light and, necessarily, even more infrared radiation . 3/20/2023 Department of Pharmaceutical Chemistry 30

31. Nernst glower - Derived from German chemist Walther Hermann Nernst, who derived the Nernst equation. Used in spectroscopy to provide near infrared radiation. Globar source- standard source is a Globar (50–6,000 cm −1 ), a silicon carbide cylinder that is electrically heated to function as a blackbody radiator Mercury arc - Radiation from a mercury-arc lamp (10–70 cm −1 ) is employed in the far-infrared region. 3/20/2023 Department of Pharmaceutical Chemistry 31

32. MONOCHROMATOR PRISM Used as dispersive element Constructed of various metal halide salts Sodium chloride is most commonly used B. GRATING It made up of some materials like glass, quartz or alkylhalides depending on the instrument. The mechanism is that diffraction produces reinforcement. The rays which are incident upon the gratings get reinforced with the reflected rays. 3/20/2023 Department of Pharmaceutical Chemistry 32

33. SAMPLE CELL Infrared spectra may be obtained for gases, liquids or solids. Materials containing sample must be transparent to the ir radiation. So, the salts like NaCl , KBr are only used. 3/20/2023 Department of Pharmaceutical Chemistry 33

34. SAMPLE HANDLING Samples of same substance shows shift in absorption bands as we pass from solid to gases and hence the samples of different phases have to be treated differently in IR spectroscopy. Sampling of solids: Solids run in solution Mull technique Pressed pellet technique Solid films 3/20/2023 Department of Pharmaceutical Chemistry 34

35. 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. Eg of solvents – acetone, cyclohexane, chloroform. MULL TECHNIQUE: Finely powdered sample+ mulling agent ( nujol ) and make a thick paste (mull) Transfer the mull to the mull plates are squeezed together to adjust the thickness it is then mounted in spectrometer. 3/20/2023 Department of Pharmaceutical Chemistry 35

36. PRESSED PELLET TECHNIQUE: Finely powdered sample is mixed with about 100 times its weight of KBr in a vibrating ball mill and the mixture is then pressed under very high pressure in a die to form a small pellet (1-2mm thickness and 1cm in diameter) 3/20/2023 Department of Pharmaceutical Chemistry 36

37. SOLID FILMS: 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. 3/20/2023 Department of Pharmaceutical Chemistry 37

38. SAMPLING OF LIQUIDS Liquid sample cells can be sandwiched using liquid sample cells of highly purified alkali halides, normally NaCl . Other salts such as KBr and CaF 2  can also be used. Aqueous solvents cannot be used because they cannot dissolve alkali halides. Organic solvents like chloroform can be used. The sample thickness should be selected so that the transmittance lies between 15-20%. For most liquids, the sample cell thickness is 0.01-0.05 mm. Some salt plates are highly soluble in water, so the sample and washing reagents must be anhydrous 3/20/2023 Department of Pharmaceutical Chemistry 38

39. SAMPLING OF GASES A gas sample is created by allowing the sample to expand into an evacuated cylindrical cell which has special windows than will not absorb the infrared light. The length of the cell can be anywhere from a couple of centimeters to over 10 meters. This cell will be placed in the beam path much like the solid or liquid sample with the exception that gas cell has two windows that the infrared light beam needs to pass through. Because the thermodynamics of the gas are such that it is very spread out the beam is bounced off the inside of the cell so that it will pass through the sample more to get more sample absorbance. 3/20/2023 Department of Pharmaceutical Chemistry 39

40. DETECTORS The detectors can be classified into three categories: Thermal detectors – Their response depend the heating effect of radiation Pyroelectric detectors- It depends on the rate of change of the detector temperature rather than on the temperature itself. Photoconducting detectors- it is most sensitive 3/20/2023 Department of Pharmaceutical Chemistry 40

41. THERMAL DETECTORS Radiation thermoelement (thermopile) When the junction of two different metals is heated, an electrical voltage proportional to the temperature is produced due to the thermoelectric effect. This effect has been utilized for many years for technical contact temperature measurements using thermoelements . If the heating of the junction is caused by the absorption of radiation, then this component is known as a thermopile. 3/20/2023 Department of Pharmaceutical Chemistry 41

42. PYROELECTRIC DETECTORS The temperature change in the detector element created by the absorption of infrared radiation causes a change in surface charge as a result of the pyroelectric effect. This results in an electrical output signal which is processed in a pre-amplifier. Due to the way that charge is created in the pyroelectric material, the radiation flow must be continuously interrupted in an alternating manner (chopping). The advantage of the resulting frequency-selective amplification is a good signal-noise ratio. 3/20/2023 Department of Pharmaceutical Chemistry 42

43. PHOTOCONDUCTING DETECTORS Photoconducting detectors are the most sensitive detectors. They rely on interactions between photons and a semiconductor. The detector consists of a thin film of a semiconductor material such as lead sulphide , mercury cadmium telluride or indium antimonide deposited on a nonconducting glass surface and sealed into an evacuated envelope to protect the semiconductor from the atmosphere . 3/20/2023 Department of Pharmaceutical Chemistry 43

44. The lead sulphide detector is used for the near-infrared region of the spectrum. For mid- and far-infrared radiation the mercury cadmium telluride detector is used. It must be cooled with liquid nitrogen to minimize disturbances.  3/20/2023 Department of Pharmaceutical Chemistry 44

45. DISPERSIVE IR INSTRUMENT 3/20/2023 Department of Pharmaceutical Chemistry 45

46. The basic components of a dispersive IR spectrometer include a radiation source, monochromator , and detector. The common IR radiation sources are inert solids that are heated electrically to promote thermal emission of radiation in the infrared region of the electromagnetic spectrum. The monochromator is a device used to disperse or separate a broad spectrum of IR radiation into individual narrow IR frequencies. 3/20/2023 Department of Pharmaceutical Chemistry 46

47. Generally, dispersive spectrometers have a double-beam design with two equivalent beams from the same source passing through the sample and reference chambers as independent beams . These reference and sample beams are alternately focused on the detector by making use of an optical chopper, such as, a sector mirror . One beam will proceed, traveling through the sample, while the other beam will pass through a reference species for analytical comparison of transmitted photon. 3/20/2023 Department of Pharmaceutical Chemistry 47

48. After the incident radiation travels through the sample species, the emitted wavefront of radiation is dispersed by a monochromator (gratings and slits) into its component frequencies . A combination of prisms or gratings with variable-slit mechanisms, mirrors, and filters comprise the dispersive system. Narrower slits gives better resolution by distinguishing more closely spaced frequencies of radiation and wider slits allow more light to reach the detector and provide better system sensitivity. The emitted wavefront beam (analog spectral output) hits the detector and generates an electrical signal as a response. 3/20/2023 Department of Pharmaceutical Chemistry 48

49. FOURIER TRANSFORM IR INSTRUMENT 3/20/2023 Department of Pharmaceutical Chemistry 49

50. 3/20/2023 Department of Pharmaceutical Chemistry 50 A common FTIR spectrometer consists of a source, interferometer, sample compartment, detector, amplifier, A/D convertor, and a computer. The source generates radiation which passes the sample through the interferometer and reaches the detector. Then the signal is amplified and converted to digital signal by the amplifier and analog-to-digital converter, respectively. Eventually, the signal is transferred to a computer in which Fourier transform is carried out.

51. Fourier transform infrared, more commonly known as FT-IR, is the preferred method for infrared spectroscopy. Developed in order to overcome the slow scanning limitations encountered with dispersive instruments, with FT-IR the infrared radiation is passed through a sample . The measured signal is referred to as an interferogram . Performing a Fourier transform on this signal data results in a spectrum identical to that from conventional (dispersive) infrared spectroscopy, but results are much faster with results in seconds, rather than minutes.   3/20/2023 Department of Pharmaceutical Chemistry 51

52. APPLICATIONS Identification of functional group and structure elucidation Entire IR region is divided into group frequency region and fingerprint region. Range of group frequency is 4000-1500 cm -1  while that of finger print region is 1500-400 cm -1 . In group frequency region, the peaks corresponding to different functional groups can be observed. According to corresponding peaks, functional group can be determined . 3/20/2023 Department of Pharmaceutical Chemistry 52

53. Each atom of the molecule is connected by bond and each bond requires different IR region so characteristic peaks are observed. This region of IR spectrum is called as finger print region of the molecule. It can be determined by characteristic peaks. 2. Identification of substances IR spectroscopy is used to establish whether a given sample of an organic substance is identical with another or not. This is because large number of absorption bands is observed in the IR spectra of organic molecules and the probability that any two compounds will produce identical spectra is almost zero . 3/20/2023 Department of Pharmaceutical Chemistry 53

54. So if two compounds have identical IR spectra then both of them must be samples of the same substances. IR spectra of two enatiomeric compound are identical. So IR spectroscopy fails to distinguish between enantiomers. For example, an IR spectrum of benzaldehyde is observed as follows. C-H stretching of aromatic ring-  3080 cm -1 C-H stretching of aldehyde- 2860 cm -1  and 2775 cm -1 C=O stretching of an aromatic aldehyde- 1700 cm -1 C=C stretching of an aromatic ring-  1595 cm -1 C-H bending- 745 cm -1  and 685 cm -1 3/20/2023 Department of Pharmaceutical Chemistry 54

55. 3. Studying the progress of the reaction Progress of chemical reaction can be determined by examining the small portion of the reaction mixture withdrawn from time to time. The rate of disappearance of a characteristic absorption band of the reactant group and/or the rate of appearance of the characteristic absorption band of the product group due to formation of product is observed. 3/20/2023 Department of Pharmaceutical Chemistry 55

56. 4. Detection of impurities IR spectrum of the test sample to be determined is compared with the standard compound. If any additional peaks are observed in the IR spectrum, then it is due to impurities present in the compound . 5.Quantitative analysis The quantity of the substance can be determined either in pure form or as a mixture of two or more compounds. In this, characteristic peak corresponding to the drug substance is chosen and log I0/It of peaks for standard and test sample is compared. This is called base line technique to determine the quantity of the substance. 3/20/2023 Department of Pharmaceutical Chemistry 56

57. DATA INTERPRETATION One of the most common application of infrared spectroscopy is to the identification of organic compounds . Hydrocarbons Hydrocarbons compounds contain only C-H and C-C bonds, but there is plenty of information to be obtained from the infrared spectra arising from C-H stretching and C-H bending. 3/20/2023 Department of Pharmaceutical Chemistry 57

58.  IR SPECTRUM OF OCTANE 3/20/2023 Department of Pharmaceutical Chemistry 58

59. Functional Groups Containing the C-O Bond Alcohols have IR absorptions associated with both the O-H and the C-O stretching vibrations . IR SPECTRUM OF ETHANOL very broad, strong band of the O–H stretch 3/20/2023 Department of Pharmaceutical Chemistry 59

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61. ORGANIC COMPOUNDS CONTAINING HALOGENS Alkyl halides are compounds that have a C–X bond, where X is a halogen: bromine, chlorine, fluorine , or iodine. The spectrum of 1-chloro-2-methylpropane   3/20/2023 Department of Pharmaceutical Chemistry 61

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63. Approximate Frequency (cm -1 ) Description Bond Vibration Notes 3500 - 3200 broad O-H much broader, lower frequency (3200-2500) if next to C=O 3400-3300 weak N-H stronger if next to C=O 3100-3000 weak-medium =C-H (sp2 C-H) can get bigger if lots of bonds present 3000-2900 weak-medium -C-H (sp3 C-H) can get bigger if lots of bonds present 2800 and 2700 medium C-H in O=C-H two peaks; 2250-2100 weak-medium C=C stronger if near electronegative atoms 3/20/2023 Department of Pharmaceutical Chemistry 63

64. 1800-1600 strong C=O lower frequency (1650-1550) if attached to O or N middle frequency if attached to C, H higher frequency (1800) if attached to Cl 1650-1450 weak-medium C=C lower frequency (1600-1450) if conjugated (i.e. C=C-C=C) often several if benzene present 1300 - 1000 medium-strong C-O higher frequency (1200-1300) if conjugated (i.e. O=C-O or C=C-O) 1000-650 strong C=C-H bend often several if benzene present 3/20/2023 Department of Pharmaceutical Chemistry 64

65. REFERENCE Instrumental Methods of Analysis – Willards , 7 th edition, CBS Publishers, pg.no 287-314 3/20/2023 Department of Pharmaceutical Chemistry 65

66. THANK YOU 3/20/2023 Department of Pharmaceutical Chemistry 66