Ir spectroscopy

analysis

1. IR SPECTROSCOPY PRESENTED BY, PRATIKSHA C CHANDRAGIRIVAR M PHARMA 1 ST SEM DEPT, OF PHARMACEUTICS HSK COP BAGALKOT FACILITATED BY, Dr. B.S.KITTUR HOD AND PROFESSOR DEPT. OF PHARMACHEMISTRY HSK COP BAGALKOT IR SPECTROSCOPY 1

2. IR SPECTROSCOPY 2

3. IR SPECTROSCOPY 3 CONTENTS: INTRODCTION PRINCIPLE THEORY MODES OF MOLECULAR VIBRATIONS SAMPLE HANDLING INSTRUMENTATION FACTORS INFLUENCING VIBRATIONAL FREQUENCIES APPLICATIONS

4. INTRODUCTION: DEFINITION: Infrared spectroscopy is a spectroscopy which is concerned with the study of absorption of infrared radiation, its properties, characteristics etc,. and also its interaction with matter. 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. IR SPECTROSCOPY 4

5. IR SPECTROSCOPY 5

6. Any compound, either organic or inorganic, if it have covalent bond then it have a ability to absorb some frequencies of electromagnetic radiation in the IR region of the electromagnetic spectrum. This absorption of infrared radiation (quantised) causes the various bands in a molecule to stretch and bend with respect to one another. Because of this reason it is main tool for the identification of compounds. IR SPECTROSCOPY 6

7. PRINCIPLE In any molecule, there are group of atoms or a atom is connected by bonds . These bonds are analogous to springs and not rigid in nature. Because of the continuous motion of the molecule, they maintain some vibrations with some frequency, characteristics to every portion of molecule. This is called natural frequency of vibration . When energy in the form of infrared radiation is applied and if that Applied infrared frequency = Natural frequency of vibration IR SPECTROSCOPY 7

8. Then absorption of IR radiation takes place and peak is observed. Example : Infrared vibrations of ethanol IR SPECTROSCOPY 8

9. 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 . Example: IR spectra of Ibuprofen IR SPECTROSCOPY 9

10. THEORY The different IR regions are: 1. Near IR region = 0.8µ to 2.5µ 2. Mid IR region = 2.5 to 15µ 3. Far IR region = 15µ to 200µ IR SPECTROSCOPY 10

11. Example: The natural frequency of vibration HCL molecule is about 8.7 × 10 13 sec -1 (2890 cm -1 ). When IR radiations permitted to pass through a sample of HCL and transmitted radiation is analysed by the IR spectrometer. It is observed that the part of radiation which has a frequency of 8.7 × 10 13 sec -1 has been observed by HCL molecule where as the remaining frequencies of the radiation are transmitted. Thus, the frequency 8.7 × 10 13 sec -1 is characteristics of HCL molecule. IR SPECTROSCOPY 11

12. In pharmaceutical analysis, we use mid IR region of wavelength 2.5 to 15µ ( 2.5 to 25µ) or in terms of wavenumbers we can say it as 400cm -1 to 4000cm -1 . In IR spectra we use wavenumbers instead of wavelengths for mentioning the characetristic peak, Because wave numbers are larger values and easy to handle than wavelengths which will show only small differences between functional groups. Wave number is nothing but the number of waves present per cm. which can be calculated from the wavelength. 1 ----------------------- × 10 4 = wavelength per cm or cm -1 wavelength in µ IR SPECTROSCOPY 12

13. For a molecule to absorb IR radiation, it has to fulfill certain requirements , Those are, Correct wavelength of radiation Electric dipole Correct wavelength of radiation: A molecule absorbs radiation only when the natural frequency of vibration of some part of a molecule ( i.e. atoms or group of atoms comprising it) is the same as the frequency of radiation. IR SPECTROSCOPY 13

14. b) Electric dipole: This is another condition for a molecule to absorb IR radiation. A molecule can only absorb IR radiation when its absorption causes a change in its electric dipole (dipole moment). A molecule is said to have electric dipole when there is a slight positive and a slight negative electric charge on its component atoms. When the molecule having electric dipole is kept in the electric field , it is the same as that when the molecule is kept in a beam of IR spectroscopy. IR SPECTROSCOPY 14

15. on the other hand , when the rate of vibration of the charged atoms in a molecule is slow , there will be weak bands in the IR spectrum. As the symmetrical diatomic molecules like O 2 and N 2 , do not posses electric dipole , they can not be excited by infrared radiation and for this they do not give rise to IR spectra. Carbon dioxide and water vapour do in air do absorb in the molecule , but these do not effect IR spectra taken on a double beam instrument , as they are fairly weak and cancel out between the sample beam and reference beam. IR SPECTROSCOPY 15

16. This field exerts forces on the electric charges in the molecules. Opposite charges will experience forces in opposite directions. This tends to decrease separation. But in the case of electric field of IR radiation changes its polarity periodically, it means that the spacing between the charged atoms (electric dipole) of the molecule also changes periodically. When these charged atoms vibrate , they absorb IR radiation from the radiation source. If the rate of vibration at charged atom in a molecule is fast, the absorption of radiation is intense and thus, the IR spectrum will have intense absorption band. IR SPECTROSCOPY 16

17. There is no change in the dipole moment is produced by the carbon-carbon double bond stretching of the symmetrical molecule like ethylene. Since there is no change in dipole moment ,the bond does not absorb infrared radiation. On the other hand, substituition of a bromine for a hydrogen atom to form bromoethylene which destroys the symmetry around the double bond. The stretching of the double bond now generates a significant change in the dipole moment and strong absorbance in the infrared is observed. IR SPECTROSCOPY 17

18. The closer atoms in a molecule are to each other, the greater will be the strength of the dipole , faster will be the rate of change of the dipole , the higher will be the frequency of vibration , and the more intense will be the absorption of radiation. IR SPECTROSCOPY 18

19. MATHEMATICAL THEORY OF INFRARED SPECTROSCOPY In order to understand the theory of absorption spectroscopy, one has to understand the phenomena of vibration rotational spectra. Let us consider a diatomic molecule associated with a dipole moment. The vibratory motion of the nuclei of diatomic molecule may be similar to the vibration of linear harmonic oscillator. In such oscillator, the force tending to restore an atom to its original state is proportional to the displacement of the vibratory atom from the original position (Hooke’s law). IR SPECTROSCOPY 19

20. IR SPECTROSCOPY 20

21. Suppose the bond between the two nuclei of a diatomic molecule is distorted from its equilibrium length r e to a new length r. Then the restoring forces on each atom of the diatomic molecule will be given by m 1 d 2 r 1 = -K(r - r e ) …………… (1) d t 2 m 2 d 2 r 2 = -K(r - r e ) ……………. (2) d t 2 Where, K = proportionality constant and known as force constant, it is regarded as a measure of the stiffness of the bond. IR SPECTROSCOPY 21

22. r 1 and r 2 are the positions of atom 1 and 2 relative to the centre of gravity of the molecule. We know, m 2 r 1 = ------------ r -------(3) m 1 + m 2 m 1 r 2 = ------------ r --------(4) m 1 + m 2 Where, m 1 and m 2 are the masses of two atoms of a vibrating dia-atomic molecule. IR SPECTROSCOPY 22

23. On substituting equation(3) and equation(1) we get, m 1 m 2 d 2 r ---------- ----- = - K( r - r e ) ………….(5) m 1 + m 2 dt 2 As r e is a constant, its differentiation with respect to time t will be zero, i.e., d 2 r d 2 (r - r e ) ------ = ------------ …………..(6) dt 2 dt 2 Substituition of the above equation in equation(5) yields m 1 m 2 d 2 (r - r e ) --------- ------------ = - K( r - r e ) ………….(7) m 1 + m 2 dt 2 IR SPECTROSCOPY 23

24. m 2 Put r – r e = x and --------- = µ ..........(8) m 1 + m 2 on substituting equation d 2 x µ ------ = - K x ………….(7) dt 2 Or d 2 x µ ------ + K x = ………….(8) dt 2 Or d 2 x µ ------ + w 2 x = ………….(9) dt 2 IR SPECTROSCOPY 24

25. Where, w 2 = K/µ or w = √ K/µ …………..(10) Equation(9) is the expression of simple harmonic motion with frequency of vibration (v) as follows v = w/2∏ = 1/2∏ √ K/µ Where, K is the force constant expressed in dynes per cm and µ is the reduced mass of the system. IR SPECTROSCOPY 25

26. MODES OF MOLECULAR VIBRATIONS According to the character of vibration, normal vibrations can be divided in to two principle group they are Stretching vibrations Bending vibrations 1. Stretching vibrations : In this type of vibrations, the atoms move essentially along the bond axis so that the bond length increases or decreases periodically, i.e., at regular intervals. IR SPECTROSCOPY 26

27. As this type of vibrations corresponds to one dimensional motion. During stretching vibrations, bond angles change only if it to do so by the centre of gravity resisting displacement. Stretching vibrations are of two types: Symmetric vibration Asymmetric vibration IR SPECTROSCOPY 27

28. a) Symmetric vibration : In this type , the movement of the atoms with respect to a particular atom in a molecule is in the same direction. Fig. IR SPECTROSCOPY 28

29. IR SPECTROSCOPY 29

30. b) Asymmetric vibration : In these vibrations, one atom approaches the central atom while the other departs from it. Fig. IR SPECTROSCOPY 30 :

31. IR SPECTROSCOPY 31

32. 2. Bending vibrations : In this type of vibrations, the positions of the atoms change with respect to the original bond axis. We know that more energy is required to stretch a spring than that required to bend it. We can safely say that stretching absorptions of a bond appear at high frequencies (high energy) as compared to the bending of the same bond. This kind of vibration is also know as deformations . IR SPECTROSCOPY 32

33. There are mainly two types of bending vibrations are there those are: In-plane deformation vibrations Out-of plane deformation vibrations In-plane deformation vibrations : In these vibrations , there is change in bond angle takes place. Bending of bond angle takes place within the same plane. IR SPECTROSCOPY 33

34. It is further can be divided into two types, viz., Scissoring Rocking 1. Scissoring : In this type, two atoms approach each other. Here, bond angle decreases. Fig. IR SPECTROSCOPY 34

35. IR SPECTROSCOPY 35

36. 2. Rocking : In this type, the movement of the atoms takes place in the same directions. Here bond angle is maintained, but both bonds moves within the plane. fig, IR SPECTROSCOPY 36

37. IR SPECTROSCOPY 37

38. 2. Out-of plane deformation vibrations : These vibrations takes place outside the plane of molecule. It is further can be divided into two types viz., Wagging Twisting IR SPECTROSCOPY 38

39. a) wagging : Two atoms move “up and down” the plane with respect to the central atom(i.e., moves one side of the plane). Fig. IR SPECTROSCOPY 39

40. IR SPECTROSCOPY 40

41. b) Twisting : In this type, one of the atoms moves up the plane while the other moves down the plane with respect to the central atoms. Fig. IR SPECTROSCOPY 41

42. IR SPECTROSCOPY 42

43. IR SPECTROSCOPY 43

44. SAMPLE HANDLING As infrared spectroscopy has been used for the characterizaton of solid, liquid or gases , it is evident that samples of different phases have to be handled. Different forms of samples treated differently. The only common point to the sampling of different phases is that the material containing the sample must be transparent to IR radiation. So we selected the salts like NaCl or KBr. IR SPECTROSCOPY 44

45. Sample handling involves following techniques: Sampling of solids Sampling of liquids Sampling of gases Sampling of solutions A. Sampling of solids : There are four techniques are generally employed for preparing solid samples viz., Solids run in solution Solid films Mull technique Pressed pellet technique IR SPECTROSCOPY 45

46. 1. Solids run in solution : Solids are dissolved in a non-aqueous solvent which do not have chemical interaction with the solvent and also which do not absorb in the studied ranged. A drop of the solution is placed on a alkali metal disk and the solvent is allowed to evaporate, leaving a thin film of the solute, or the entire solution is placed in a liquid sample cell. If the solution of solid can be prepared in a suitable solvent then the solution is run in one of the cells for liquids. IR SPECTROSCOPY 46

47. But this method cannot be used for all solids because suitable solvents are limited in number and there is no single solvent which transparent throughout the IR region. When the investigating solids are in solutions, the absorption due to solvent has to be compensated by keeping the solvent in a cell of same thickness as that containing in the sample , in the path of the reference beam of double-beam spectrometer. IR SPECTROSCOPY 47

48. 2. Solid films If a solid is amorphous in nature the sample is deposited on the surface of a KBr or NaCl cell by evaporation of a solution of the solid. This techniques useful for rapid qualitative analysis but becomes useless for carrying out quantitative analysis. IR SPECTROSCOPY 48

49. 3. Mull technique In this technique, The finely ground solid sample is mixed with Nujol (mineral oil) to make a thick paste of which is then made to spread between IR transmitting windows. This is then mounted in a path of infrared beam and the spectrum is run. This Nujol can be used with hexachlorobutadiene while taking in Nujol mull for clear bands. This method is good for qualitative analysis. IR SPECTROSCOPY 49

50. IR SPECTROSCOPY 50

51. IR SPECTROSCOPY 51

52. Disadvantage: It shows absorption maxima only at 2915, 1462, 1376 and 719 cm -1 . Polymorphic changes, degradation and other changes may occur during grinding. It cannot be used for quantitative analysis. IR SPECTROSCOPY 52

53. 4. Pressed pellet technique In this technique a small amount of finely ground solid samples is intimately mixed with about 100 times its weight of powdered potassium bromide. The finely ground mixture is then passed under very high pressure in a press (at least 25,000 p sig) to from a small pellet (about 1-2 mm thick and 1 cm in diameter). The resulting pellet transparent to IR radiation and is run as such. IR SPECTROSCOPY 53

54. Device for processing potassium bromide discs IR SPECTROSCOPY 54

55. Working : A device for the pressing the mixture of KBr and solid sample to forma a pellet is shown in the figure. The powder (KBr + sample) is introduced as shown, then the upper screw ‘A’ is tightened until the powder is compressed into a thin disc. After compressing the sample, one removes the bolts (A and A’) and places the steel cylinder with the sample disc inside it in the path of the beam of infrared spectrometer and a ‘blank’ potassium bromide pellet of the reference beam. IR SPECTROSCOPY 55

56. Advantages: The best advantage of this over Nujol Mull method is that the use of KBr eliminates the problem of bands which appear in the IR spectrum due to the mulling agent. KBr pellets can be stored for long periods of time. As the concentration of sample can be suitably adjusted in the pellets, it can be used for quantitative analysis. The resolution of the spectrum in the KBr is superior to that obtained with mulls. IR SPECTROSCOPY 56

57. Disadvantages: It always has a band at 3450 cm -1 , from the OH group of moisture present (always) in the sample. The high pressure involved during the formation of pellets may bring about polymorphic changes in crystallinity in the samples, specially if inorganic complexes are there, which may cause complications in IR spectrum. In some cases substituition by bromide may occurs. This method is not successful for some polymers which are difficult to grind with KBr. IR SPECTROSCOPY 57

58. B. Sampling of liquids: Samples that are liquids at room temperature are usually put frequently with no preparation , into rectangular cells made up of NaCl , KBr or ThBr and their IR spectra are obtained directly. IR spectrum of Hexachlorobuta-diene-1,3 IR SPECTROSCOPY 58

59. Method : Step-1 : A drop of liquid sample is placed on the top of sodium chloride plate. Step-2: Place another sodium chloride plate on it. Step-3: The pair of sodium chloride plates enclosing the liquid film is then placed in the path of sample beam. IR SPECTROSCOPY 59

60. The sample thickness should be so selected that the transmittance should lies between 15-20%. For most liquids , it will be a layer of 0.01-0.05m in thickness. If a cell possesses good quality windows , flat and parallel, its thickness can be calculated by the formula: 2t = N/w 1 – w 2 Where , t = Thickness N = the number of fringes between wave number w 1 and w 2 IR SPECTROSCOPY 60

61. For double beam work following conditions are applied: “ Matched cells ” are employed. One cell will contain the sample while the other cell will have a solvent used in sample. These matched cells must have same thickness. They should be protected from moisture because dissolve in water. With similar conditions, organic liquid samples must be dried before pouring into cells. If sample contain water, then the plates must be constructed with calcium flouride. IR SPECTROSCOPY 61

62. C. Sampling of gases: Method: The gaseous sample is introduced into a ‘gas cell’ Usually they are about 10 cm long but they may be up to 1m also. Multiple reflections can be used to make the effective path length as long as 40m. , so that constituents of gas can be determined. The gas must not be react with cell windows or reflecting surfaces. The walls of the cell are made up of sodium chloride. IR SPECTROSCOPY 62

63. Very few organic compound can be examined as gases. IR SPECTROSCOPY 63

64. Complications: The low frequency rational changes in the gaseous phase often split the high frequency vibrational bands. IR SPECTROSCOPY 64

65. D. Sampling of solutions It is the most convenient to determine the spectrum in solutions. Excellent solvents are those which have poor absorptions of their own. Important solvent that are used in IR technique are: chloroform Carbon tetra chloride Carbon disulphide Water can not be used as a solvent as it absorbs in several regions of the infra-red spectrum. IR SPECTROSCOPY 65

66. IR SPECTROSCOPY 66

67. IR SPECTROSCOPY 67

68. IR SPECTROSCOPY 68

69. Method: The sample under analysis is dissolved in a solvent. Its 1-5% solution is placed in a solution cell consisting of transparent windows which is made of alkali metal halide. A second cell containing the pure solvent is placed in the path of the reference beam to cancel out solvent interferences. For careful analysis ,the path length of reference cell should be less than that of the solution cell. IR SPECTROSCOPY 69

70. INSTRUMENTATION: There are mainly three types of instrumentation is done for IR spectroscopy: Dispersive instrumentation Fourier transform instrumentation Non dispersive instrumentation But commonly used are first two. IR SPECTROSCOPY 70

71. 1.Dispersive infrared spectrophotometer IR SPECTROSCOPY 71

72. Parts of dispersive infrared spectrophotometer It usually contains following parts viz., Source 10.Slits Mirror Sample cell Reference cell Beam chopper Motor Monochromator Detector Amplifier and recorder IR SPECTROSCOPY 72

73. Explaination: 1.Sources : The commonly used sources in dispersive infrared spectrophotometers are Incandescent lamp Nernst glower Globar source Pressurized mercury arc. 2.Mirrors : These helps to convert beam of radiations into parallel radiation of same intensity. IR SPECTROSCOPY 73

74. 3. Sample cell : There is a rugged window material for cuvette which is transparent and invert over this region. The alkali halides are widely used like KBr , NaCl, and ThBr which are transparent at wavelength as long as 625 cm -1 . Silver chloride is preferred if sample is water soluble or moist. 4. Reference cell : It contains the solvent omitting the sample. We can say it as a blank. IR SPECTROSCOPY 74

75. 5. Beam chopper : These are the rotating sector that passes the two beam produced by the device alternately to the gratings or prisms(in older model). 6. Motor : It controls the whole process technically by providing energy. 7. Monochromator : Usually gratings are used as monochromator in newer models of dispersive IR spectrophotometer. Prisms are used in older one. IR SPECTROSCOPY 75

76. 8. Detector : Here thermocouple detectors are used. 9.Amplifier and recorder : The response are amplified by amplifier and recorded in the form of graphical data. 10. Slits : It allows beam to pass through. IR SPECTROSCOPY 76

77. Working The instrument produces a beam of infrared radiation from a hot wire. By means of mirrors , these rays are divided into two parallel beams of equal intensity. One beam fall on sample cell and other on reference. The these beams pass into the monochromator where dispersion of each beam takes place and results into a continuous spectrum of frequencies of infrared light. Beam chopper present in monochromator are a rotating sector helps in passing these two beams alternately to diffraction grating. IR SPECTROSCOPY 77

78. 5. The slowly rotating diffraction grating varies the frequency or wavelength of radiation reaching the thermocouple detector. 6. Then the detector senses the ration between the intensities of the reference and sample beams, 7. In this way, the detector determines which frequencies have been absorbed by the sample and which frequencies are un effected by the light passing through the sample. 8. After the signal from the detector is amplified, the recorder draws the resulting spectrum of the sample on chart. IR SPECTROSCOPY 78

79. Disadvantages : Less sensitivity. Less accurate. Many moving parts results in to mechanical slippage. Slow scan makes the instrument too slow for monitoring systems under going rapid changes. Example : Gc effluents IR SPECTROSCOPY 79

80. 2. Fourier transform spectrophotometer: The present modern instruments works on different principle viz., The design of the optical pathway produces a pattern called an interferogram. The interferogram is a complex signal ,but its wave like pattern contains all the frequencies that make up the infrared spectrum. It is a plot of intensity versus time. But chemist is interested in a spectrum which is a plot intensity versus frequency. IR SPECTROSCOPY 80

81. An mathematical operation known as Fourier transform can help to achieve it, i.e., separates the individual absorption frequencies from interferogram and produces a spectrum that is identical to other two instruments. It is commonly called as FT-IR. There single beam and double beam FT-RI but we commonly use double beam. IR SPECTROSCOPY 81

82. IR SPECTROSCOPY 82

83. IR SPECTROSCOPY 83

84. Working of instrument: The FT-IR uses an interferometer to process energy sent to the sample. In the interferometer, the source energy passes through a beam splitter, a mirror placed at 45⁰ angle to the incoming radiation , which allows the incoming radiation to pass through but separates it into two perpendicular beams, one un deflected , the other oriented at 90⁰ angle. The beam which is oriented at 90⁰ goes to a stationary or fixed mirror and returned to the beam splitter. IR SPECTROSCOPY 84

85. 4. The un deflected beam goes to a moving mirror and is also returned to the beam splitter. 5. The motion of the mirror causes the pathlength that he the second beam traverses to vary. 6. When the two beams meet at the beam splitter, they recombine , but the pathlength differences(differing wavelength content) of the two beams cause both constructive and destructive interferences. 7. The combined beam containing these interference patterns is nothing but interferogram. 8. This interferogram contains all of the radiative energy coming from the source and has a wide range of wavelengths. IR SPECTROSCOPY 85

86. 9. The interferogram generated by combining the two beams is oriented toward the sample by the beam splitter. 10. As it passes through sample , the sample simultaneously absorbs all of the wavelength/frequencies that are found in its infrared spectrum. 11. The modified interferogram signal that reaches the detector contains information about the amount of energy that was absorbed at every wavelength. 12. Then the computer compares the modified interferogram to a reference laser beam to have a standard of comparison. IR SPECTROSCOPY 86

87. 13. The final interferogram contains all of the information in one time – domain signal. 14. A mathematical process called a Fourier transform must be implemented by the computer to extract the individual frequencies that were absorbed and to reconstruct and plot what we recognize as a typical infrared spectrum. Advantages: They have better signal- to – noise ratios than other IR instruments, They provide high resolution(‹0.1 cm -1 ). They are highly accurate and reproducible. Theoratical advantage is that they better optics to provide energy, IR SPECTROSCOPY 87

88. Factors affecting vibrational frequencies: Following four factors are responsible for shifting the vibrational frequencies viz. Coupled vibrations and Fermi resonance Electronic effects Hydrogen bonding Bond angles IR SPECTROSCOPY 88

89. A. Coupled vibrations and Fermi resonance Coupled vibrations : We expect only one stretching for C-H bond but When point comes to group like methylene it contains - CH 2 - , we can see two absorption which corresponds to symmetric and asymmetric vibration. i.e., IR SPECTROSCOPY 89

90. In this type of case, asymmetric vibrations always occur at higher wave number compared with symmetric vibrations. Such kind of vibrations are called coupled vibrations. ii. Another case is that, coupled vibration occurs in methyl group i.e., - CH 3 – IR SPECTROSCOPY 90

91. Here reason we can give that , as we know that two different vibrational levels have nearly the same energy. Then if such coupled vibration occur then a mutual perturbation of energy may occur ,that results in shifting one towards lower frequency and another towards higher one, which further results in alteration or substantial in higher intensed bands. IR SPECTROSCOPY 91

92. Fermi resonance : This was first observed by Enrico Fermi in case of carbon dioxide. This kind of resonance is seen in certain lactones and cycloketones. A type of resonance that occurs in coupled pendulum such resonance is called Fermi resonance . Explaination: Here a molecule transfers its energy from fundamental to overtone and back again. Quantum mechanically, resonance pushes the two levels apart and mixes their character so that each level becomes partly fundamental and partly overtone in character. Results in pair of transitions of equal intensity. IR SPECTROSCOPY 92

93. Example : Carbon dioxide is a linear triatomic molecule and four fundamental vibrations are expected for it. Out of these , symmetric stretching vibration is infra-red inactive since it produces in dipole moment. IR SPECTROSCOPY 93

94. For symmetric stretching , Raman spectrum shows a strong band at 1337 cm -1 . The two bending vibrations ate equivalent and absorb at the same frequency of 667.3 cm -1 . The over tone of this is 1334.6 cm -1 i.e., 2 × 667.3cm -1 . Thus , Fermi resonance takes place resulting in the shift of first level towards higher frequency. The mutual perturbation of 1337 cm -1 (fundamental) and 1334.6 cm -1 (overtone) gives rise to two bands at 1285.5 cm -1 and 1388.3 cm -1 having intensity ratio of 1: 0.9 . IR SPECTROSCOPY 94

95. B. Electronic effects: Changes in frequencies of a particular group can be occurs due to change in the particular substituents of neighbourhood functional group. Reason is that influence of electronic effects like inductive effect, mesomeric effect and field effects etc., These effects can not be isolated from one another and combination of these one of them can only be estimated approximately. Under the influence of these effects , the force constant of bond strength changes and its absorption frequency shifts from the normal value. IR SPECTROSCOPY 95

96. Introduction of alkyl group cause +I effect which results in the lengthening and weakening of bonds takes place and hence force constant is lowered and wave number decreases. Let us compare the wavenumbers of c=o, absorptions by using following compounds: 1. Formaldehyde= HCHO = 1750cm -1 2.Aacetaldehye = CH 3 CHO = 1745cm -1 3. Acetone = CH 3 COCH 3 = 1715cm -1 NOTE: Aldehydes have higher wave number than that of ketones. IR SPECTROSCOPY 96

97. The introduction of an electronegative atom or group causes –I effect which results in the bond order to increase. Thus , the force constant increases and hence the wave number of absorption rises. Now let us consider wave numbers of absorptions in following compounds: Acetone = CH 3 COCH 3 = 1715cm -1 Chloroacetone = CH 3 COCH 2 Cl = 1725cm -1 Dichloroacetone = CH 3 COCHCl 2 = 1740cm -1 Tetrachloroacetone = Cl 2 CH-CO-CHCl 2 = 1750, 1778cm -1 IR SPECTROSCOPY 97

98. In most of cases , mesomeric effects works along with inductive effects and we cannot ignore that. Conjugation lowers the absorption frequency of C=O stretching whether the conjugation is due to 𝛼, 𝛽- unsaturation or due to an atomic ring. In most of cases, inductive effect dominates over mesomeric effect while reverse holds for other cases. Mesomeric effects causes lengthening or the weakening of a bond leading in the lowering of absorption frequency. IR SPECTROSCOPY 98

99. Consider the following compounds: In these two cases, -I effect is dominated by mesomeric effect and thus, the absorption frequency falls. IR SPECTROSCOPY 99

100. Consider in ortho substituted compounds, the lone pairs of electrons on two atoms influence each other through space interactions and change the vibrational frequencies of the both groups .this effect is called Field effect. Example: orthohaloacetophenone IR SPECTROSCOPY 100

101. Explaination: The non bonding electrons present on oxygen atom and halogen atom causes electrostatic repulsions. This causes a change in the state of hybridisation of C=O group and also makes it to go out of the plane of the double bond. Thus, the conjugation is diminished and absorptions occurs at a higher wave number. Thus, for such ortho substituted compound s, cis absorbs(field effect) at a higher frequency as compared to the trans isomer. IR SPECTROSCOPY 101

102. C. Hydrogen bonding Hydrogen bonding brings about remarkable downward frequency shifts. How means, stronger the hydrogen bonding , greater is the absorption shift towards lower wave number than normal value. Two types of hydrogen bonding can be readily distinguished in infra-red technique. - Inter molecular hydrogen bonds give rise to broad bands. - Intra molecular hydrogen bands are sharp and well defined. IR SPECTROSCOPY 102

103. covalent bond (strong) intermolecular attraction(weak) IR SPECTROSCOPY 103

104. Inter molecular hydrogen bonding are Concentration dependent. H ence absorption frequency of them are less. The hydrogen bonding in O-H androgen d N-H are special cases viz., As nitrogen atom is less electronegative than that of an oxygen atom , hydrogen bonding in amines is weaker than that of alcohols, thus the frequency shift is less towards amines. Example: Amines shows N-H stretching at 3500 cm -1 in dil. Solutions while in condensed spectra absorption is at 3300 cm -1 . IR SPECTROSCOPY 104 u

105. In aliphatic alcohols , a sharp band appears at 3650cm -1 in dilute solutions due to free O-H group while a broad band is noticed at 3350cm -1 due to hydrogen O-H group. Alcohols are strongly hydrogen bonded in condensed phases. These are usually associated as dimers and polymers which results in the broadening of bands at lower absorption frequencies. In vapour state or inert solvents, molecules exist in free state and absorbs strongly at 3650cm -1 . IR SPECTROSCOPY 105

106. Alcohols are written in the following resonating structures: IR SPECTROSCOPY 106

107. Explaination: structure III is the hybrid of structures I and II. This results in the lengthening of the original O-H group. The electrostatic force of attraction with which hydrogen atom of one molecule is attracted by the oxygen atom of another molecule makes it easier to pull hydrogen away from oxygen atom. Thus, small energy will be required to stretch such a bond (O-H) and hence absorption occurs at a lower wave number. IR SPECTROSCOPY 107

108. D. Bond angles It has been found that the highest C=O frequencies arise in strained cyclobutanones. This can be explain terms of bond angles strain. The C-CO-C angle is reduced below the normal angle of 120⁰ and this leads to increased s-character in the C=O bonds. Greater s-character causes shortening of C=O structure occurs at higher frequencies. In case the bond angle is pushed outwards above 120⁰, the opposite effect operates. Due to this reason di-tert-butyl ketone absorbs at 1697cm -1 (low) as a result of C=O str. IR SPECTROSCOPY 108

109. 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. IR SPECTROSCOPY 109

110. References: Text book of pharmaceutical analysis by Dr. Ravi Sankar – 4 th edition. Instrumental methods of chemical analysis by Gurdeep R Chatwal, Sham K Anand – 6 th edition. Elementary organic spectroscopy by Y.R.Sharma – multicolour edition. Spectroscopy by Lampman, Pavia, Kriz and Vyvyan – 4 th edition and 3 rd edition. Principles of instrumental analysis by Skoog, Holler and Crouch – 6 th edition and 8 th edition Spectrometric identification of organic compounds by Robert.M.Silverstein, Francis.X.Webster and David.J.Kiemle – 7 th edition. IR SPECTROSCOPY 110