Ir spectroscopy

Education

1. IR SPECTROSCOPY :Theory and Problems Dr. G.C.Wadhawa Assistant Professor, Department of Chemistry Karmaveer Bhaurao Patil College, Vashi, Navi Mumbai, Maharashtra (INDIA) 30/03/2020 1

2. Introduction Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) involves the interaction of infrared radiation with matter. Infrared (IR) spectroscopy is one of the most common spectroscopic techniques used by organic and inorganic chemists. Simply, it is the absorption measurement of different IR frequencies by a sample positioned in the path of an IR beam . . The main goal of IR spectroscopic analysis is to determine the chemical functional groups in the sample. Different functional groups absorb characteristic frequencies of IR radiation. Using various sampling accessories, IR spectrometers can accept a wide range of sample types such as gases, liquids, and solids . 30/03/2020 2

3. Regions of EMR 30/03/2020 3

4. What happens in different regions Region of Spectrum Wavelength Transition X-Ray < 200 nm Bond breaking UV/ Visible 200 -800 nm Electronic Infrared 2 .5 -25 n m Vibrational Microwave > 25  m MW < 1 m Rotational Radiofrequency 1 -5 m NMR/ ESR spin change 30/03/2020 4

5. IR Region Region of IR Wave length (µm) Wave number (cm -1 ) Near IR (Overtone region) 0.8-2.5 12,500-4000 Mid IR (Vibration- rotation region) 2.5-50 4000-200 Far IR (Rotation region) 50-1000 200-10 Most used 2.5-25 4000-400 30/03/2020 5

6. IR radiations The infrared portion of the electromagnetic spectrum is usually divided into three regions; the near-, mid- and far- infrared , named for their relation to the visible spectrum. The higher-energy Near-IR , approximately 14000–4000 cm −1 (0.8– 2.5 μm wavelength) can excite overtone or harmonic vibrations. The mid-infrared , approximately 4000–400 cm −1 (2.5–25 μm) may be used to study the fundamental vibrations and associated rotational-vibrational structure. The far-infrared , approximately 400–10 cm −1 (25–1000 μm), lying adjacent to the microwave region In the context of infra red spectroscopy, wavelength is measured in "wavenumbers", which have the units cm -1 30/03/2020 6

7. Energy levels E N E R G Y Electronic level Rotational level & translational level Vibrational level 30/03/2020 7

8. Absorption - Energy levels E N E R G Y  E = UV/ Vis photon  E = IR photon  E =  wave photon The energies associated with rotational transitions are generally much smaller than - 1 that for vibrational transitions and the peaks are found below 300 cm . 30/03/2020 8

9. IR absorption Process Principle:- All the atoms in molecules are in continuous vibration with respect to each other. If you directed IR radiation to molecules externally, the frequency of a specific vibration is equal to the frequency of the IR radiation directed on the molecule, the molecule absorbs the radiation. Infrared spectroscopy exploits the fact that molecules absorb frequencies that are characteristic of their structure. These absorptions occur at resonant frequencies, i.e. the frequency of the absorbed radiation matches the vibrational frequency. The energy or frequency absorbed serves to increase the amplitude of the vibrational motions of the bonds in the molecules. In other word Infrared light interacts only with those vibrations whose dipole moment (µ) periodically changes due to the oscillation of the atom. If the oscillating electromagnetic field of the incident photon couples with the dipole oscillating at the same frequency, it is absorbed. 30/03/2020 9

10. IR active / Inactive Not all possible vibrations within a molecule will result in an absorption band in the infrared region. To be infrared active the vibration must result in a change of dipole moment during the vibration. This means that for homonuclear diatomic molecules such as Hydrogen (H2), Nitrogen (N2) and Oxygen (O2) no infrared absorption is observed, as these molecules have zero dipole moment and stretching of the bonds will not produce one. For heteronuclear diatomic molecules such Carbon Monoxide (CO) and Hydrogen Chloride (HCl), which do possess a permanent dipole moment, infrared activity occurs because stretching of this bond leads to a change inndipole moment Polar bonds are associated with strong IR absorption while symmetrical bonds may not absorb at all. 30/03/2020 10

11. It is important to remember that it is not necessary for a compound to have a permanent dipole moment to be infrared active. In the case of Carbon Dioxide (CO 2 ) the molecule is linear and centrosymmetric and therefore does not have a permanent dipole moment. This means that the symmetric stretch will not be infrared active. However in the case of the asymmetric stretch a dipole moment will be periodically produced and destroyed resulting in a changing dipole moment and therefore infrared activity. 30/03/2020 11

12. Principle of IR Spectroscopy When IR light is passed through a sample of organic compound some of the frequencies are absorbed while others are transmitted . Plot of Absorbance or Transmittance Vs Wave no. gives an IR spectrum But conventionally Wave No. Vs % Transmittance is plotted (because the numbers are more manageable ) 30/03/2020 12

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14. Requirements of IR radiation Absorption 1. Correct wavelength of radiation If a molecule absorbs radiation only when natural frequency of vibration of some part of molecule is same as frequency of incident radiation. 30/03/2020 14

15. Requirements of IR radiation Absorption 2. Electric Dipole: If a frequency of vibration of HCl molecule exactly matches with that coming from the source NET TRANSFER of energy takes place chan g e i n amplitude of vi b rat i on abs o rption of molec u l a r radiation In case of O 2 , N 2 ,Cl 2 NO NET change in dipole moment occurs thus they cannot absorb IR radiations & do not show IR spectra 30/03/2020 15

16. Molecular Vibrations For a molecule to absorb infrared radiation it must undergo a net change in dipole moment as a result of vibrational or rotational motion. Vibrations can be subdivided into two classes, depending on whether the bond length or angle is changing: stretching (symmetric and asymmetric) bending (scissoring, rocking, wagging and twisting) 30/03/2020 16

17. Fundamental modes of vibration The major types of molecular vibrations are stretching and bending . 30/03/2020 17

18. Stretching vibrations In this type of vibrations, the bond length is increased or decreased at regular intervals. There are two types of stretching vibrations. Symmetrical stretching and asymmetrical training. A)Symmetrical stretching- In this type of stretching, bond length increase or decrease symmetrically. B)Asymmetrical stretching- In this type of stretching, length of one bond increases and the other one decreases . 30/03/2020 18

19. Bending vibrations In this type of vibrations, a change in bond angle occurs between bonds with a common atom, or there is a movement of a group of atoms with respect to the remainder of the molecule without movement of the atoms in the group with respect to one another. The bending vibrations are also called as deformation vibrations . Deformation vibrations are of two types. a) In-plane Bending vibrations b) Out of plane Bending vibrations 30/03/2020 19

20. a) In plane bending- In these types of vibrations, there is a change in bond angle. This type of bending takes place within the same plane. In plane bending are of two types. i. Scissoring in which bond angle decreases i i. Rocking in which the bond angle is maintained but b o t h b o nd s m o v e s w i t h i n t h e s a m e p l a n e . b) Out of plane bending- This type of bending takes plane outside of the plan of molecule. i. Wagging in which both atoms move to one side of the plane ii. Twisting in which one atom is above the plane and the other is below the plane. 30/03/2020 20

21. CO 2 Symmetric stretching Antisymmetric stretching Scissoring bending 30/03/2020 21

22. W a t er 30/03/2020 22

23. Degrees of freedom Simple diatomic molecules have only one bond, which may stretch. In polyatomic molecules, each atom having a three degrees of freedom in three directions which are perpendicular to each other. Thus, a molecule of n atoms has 3 n degrees of freedom. For a linear molecule, two degrees of freedom describe rotation and three degrees describe translation, so the remaining 3 n -5 are number of fundamental vibrations. While for a non-linear molecule, three degrees of freedom describe rotation and three degrees describe translation, so the remaining 3 n -6 are number of fundamental vibrations. Each atom has three degrees of freedom, corresponding to motions along any of the three Cartesian coordinate axes (x, y, z). A polyatomic molecule of n atoms has 3n total degrees of freedom. However, 3 degrees of freedom are required to describe translation, the motion of the entire molecule through space. Additionally, 3 degrees of freedom correspond to rotation of the entire molecule. Therefore, the remaining 3 n – 6 degrees of freedom are true, fundamental vibrations for nonlinear molecules. Linear molecules possess 3 n – 5 fundamental vibrational modes because only 2 degrees of freedom are sufficient to describe rotation. Among the 3 n – 6 or 3n – 5 fundamental vibrations (also known as normal modes of vibration), those that produce a net change in the dipole moment may result in an IR activity and those that give polarizability changes may give rise to Raman activity. Naturally, some vibrations can be both IR- and Raman-active . 30/03/2020 23

24. Molecules which produce a change in dipole moment Nonlinear molecules 3N -6 degrees of freedom Linear molecules 3N -5 degrees of freedom - IR active 30/03/2020 24

25. 25 30/03/2020 25

26. Vibrational frequency A molecular vibration occurs when atoms in a molecule are in periodic motion while the molecule as a whole has constant translational and rotational motion. The frequency of the periodic motion is known as a vibrational frequency 30/03/2020 26

27. Factors affecting Vibrational frequency Vibrational coupling H-Bonding Electronic effect Field effect Nature of solvent 30/03/2020 27

28. Potential energy diagrams of oscillators Anharmonic 30/03/2020 28

29. Simple harmonic motion is a type of periodic motion where the restoring force is directly proportional to the displacement. Anharmonicity is the deviation of a system from being a harmonic oscillator . An oscillator that is not oscillating in simple harmonic motion 30/03/2020 29

30. What is FTIR Fourier-transform infrared spectroscopy is a vibrational s p e c t r o s c o p i c t e c h n i q u e , m e a n i n g i t t a k e s a d v a n t a g e o f asymmetric molecular stretching, vibration, and rotation of chemical bonds as they are exposed to designated wavelengths of light. Fourier transform is to transform the signal from the time domain to its representation in the frequency domain 30/03/2020 30

31. Presentation of Spectra IR absorption information is generally presented in the form of a spectrum with wavelength or wavenumber as the x-axis and absorption intensity or percent transmittance as the y-axis 30/03/2020 31

32. A graph is t r a n s mi t t a n c e radiation produced v a r i e s w i t h s h o w i n g h o w t h e p e r c e n t a g e of t h e f r eq u e n c y in the infra -red cm-1 . 30/03/2020 32

33.  = wavenumbers (cm -1 )  =` 1  (cm)  = wavelength (cm) THE UNIT USED ON AN IR SPECTRUM IS WAVENUMBERS ( n ) Wavenumbers are directly proportional to frequency UNIT ( n ) = cm- 1 I R a b s o r p t i o n p o s i t i o n s a r e g e n e r a l l y p r e s e n t e d as e i t h e r w a v e nu m b e r s ( n ) o r wavelengths (  ). Wavenumber defines the number of waves per unit length. Thus, wavenumbers are directly proportional to frequency, as well as the energy of the IR absorption. T h e w a v e n u m b e r un i t ( c m –1 , r e c i p r o c al c e n t i m e t e r ) i s m o r e c o m m o n l y u s e d i n modern IR instruments that are linear in the cm –1 scale 30/03/2020 33

34. Percent transmission spectrum 10 %T Wavelength/Frequency 30/03/2020 34

35. A Absorbance Spectrum Wavelength/Frequency 30/03/2020 35

36. % T 10 Wavelength ( or frequency ) % A Absorbance Spectrum Percent transmission spectrum 10 Wavelength ( or frequency ) 30/03/2020 36

37. Band or peak Intensitie s 30/03/2020 37

38. The intensity of an absorption band depends on the size of the change in dipole moment associated with the vibration: The greater the change in dipole moment, the more intense the absorption 30/03/2020 38

39. Bond properties & Absorption peaks Bond properties Frequency B ond p r o p e r t i e s su c h a s bo n d l e n g t h , f o r c e b o n d a nd m a ss e s o f bond e d the infrared IR absorption constant of a t o m s a f f e c t frequency 30/03/2020 39

40. The natural frequency of vibration of a bond is given by equation- (Hooke’s Law) 30/03/2020 40

41. Hooke’s Law k = force constant dyne/ cm 30/03/2020 41

42.  = 1 K 2  c  larger K, higher frequency larger atom masses, lower frequency   constants From above equation wavenumber i.e. frequencry   Force constant (K) As force constant increases, frequency also increases. Stronger the bond or shorter the bond length then higher the frequency to vibrate the bond As masses of atoms increases, frequency also decreases. Smaller atoms required higher frequency to vibrate the bond, vice versa 1 Reduced masses of atom () 30/03/2020 42

43. C = C > C=C > C-C increasing K 2150 1650 1200 K = force constant (in dynes / cm) for single bond: K = 5 x 10 5 dynes/cm for double bond: K = 10 x 10 5 dynes/cm for triple bond: K = 15 x 10 5 dynes/cm increasing  C -H > C - C > C - O > C - C l > C - B r 3000 1200 1100 750 650 3 9 30/03/2020 43

44. 1. For a stronger bond (larger k value), n increases Increasing bond strength CC bonds CH bonds C-C (1200 cm -1 ), C-C-H (2900 c m - 1 ) C=C (1650 cm -1 ) C=C-H (3100 c m - 1 ) C  C (2150 cm -1 ) C  C-H (3300 c m - 1 ) 2 . For heavier atoms attached (larger m value), n decreases Increasing reduced mass compare C-H C-C C-Cl C - B r (about 500 cm -1 ) (3000 cm -1 ) (1200 cm -1 ) (750 cm -1 ) (650 cm -1 ) C- I 30/03/2020 44

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46. Functional group region 4000 -1500 cm -1 Fingerprint region 650 -1500 cm -1 IR Spectrum 30/03/2020 46

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48. Regions in IR spectrum A) Functional Group Region The Functional Group Region, 4000 to 1300 cm –1 The appearance of strong absorption bands in the region of 4000 to 2500 cm–1 usually comes from stretching vibrations between hydrogen and some other atoms with a mass of 19 or less. The O-H and N-H stretching frequencies fall in the 3700 to 2500 cm–1 region, with various intensities. Hydrogen bonding has a significant influence on the peak shape and intensity, generally causing peak broadening and shifts in absorption to lower frequencies. The C H stretching bands occur in the region of 3300 to 2800 cm–1. The acetylenic C-H exhibits strong absorption at about 3300 cm–1 . Alkene and aromatic C-H stretch vibrations absorb at 3100 to 3000 cm –1. Most aliphatic (saturated) C-H stretching bands occur at 3000 to 2850 cm–1, with generally prominent intensities that are proportional to the number of C-H bonds. Aldehydes often show two sharp aldehydic C-H stretching absorption bands at 2900 to 2700 cm–1 . 30/03/2020 48

49. The 1950 to 1450 cm–1 region exhibits IR absorption from a wide variety of double-bonded functional groups. . Almost all the carbonyl C=O stretching bands are strong and occur at 1870 to 1550 cm– 1. Acid chlorides and acid anhydrides give rise to IR bands at 1850 to 1750 cm–1. Whereas ketones, aldehydes, carboxylic acids, amides, and esters generally show IR absorption at 1750 to 1650 cm–1,carboxylate ions usually display stretching bands at 1610 to 1550 and 1420 to 1300 cm-1. Conjugation, ring size, hydrogen bonding, and steric and electronic effects often result in significant shifts in absorption frequencies. Nonconjugated aliphatic C=C and C=N have absorption bands at 1690 to 1620 cm– 1, with variable intensities. Aromatic compounds contain delocalized p electrons from the resonance- stabilized double bonds, showing skeletal vibrations (including C-C stretchings within the ring) in the 1650 to 1400 cm–1 region and weak combination and overtone bands in the 2000 to 1650 cm–1 region. Valuable information about the substitution pattern on an aromatic ring can be obtained by careful examination of absorption bands in these two regions. Molecules containing NO2 groups, such as nitro compounds, nitrates, and nitramines, commonly exhibit asymmetric and symmetric stretching vibrations of the NO2 group at 1660 to 1500 and 1390 to 1260 cm–1 regions. 30/03/2020 49

50. B) Fingerprint Region The Fingerprint Region, 1300 to 910 cm –1 Absorptions in this region include the contributions from complex interacting vibrations, giving rise to the generally unique fingerprint for each compound. A good match between the IR spectra of two compounds in all frequency ranges, particularly in the fingerprint region, strongly indicates that they have the same molecular structures. Detailed interpretation of IR bands in this region is difficult. However, some assignments of bands in the fingerprint region to a few important vibrational frequencies of functional groups can be done when IR absorptions in other regions are correlated together. 30/03/2020 50

51. b ) FINGERPRINT REGION Although the entire IR spectrum can be used as a fingerprint for the purposes of comparing molecules, the 600 - 1400 cm -1 range is called the fingerprint region . This is normally a complex area showing many bands, frequently overlapping each other. This complexity limits its use to that of a fingerprint, and should be ignored by beginners when analyzing the spectrum. As a chemist, you should focus your analysis on the rest of the spectrum, that is the region to the left of 1400 cm -1 . Fingerprint region: complex and difficult to interpret reliably. Focus your analysis on this region. This is where most stretching frequencies appear. 30/03/2020 51

52. Vibration modes 1. Normal mode of an oscillating system is a pattern of motion in which all parts of the system move sinusoidally with the same frequency 2 . Combination Bands are observed when more than two or more fundamental vibrations are excited simultaneously. These combination modes arise from the anharmonicities of the oscillators which leads to an interaction of the vibrational states in polyatomic molecules. 30/03/2020 52

53. Vibration modes 53 3. Overtones (multiples of given frequency) results from excitation from ground state to higher energy states. Overtones occur when a vibrational mode is excited from v=0 to v=2, which is called the first overtone, or v=0 to v=3, the second overtone. --> 1 (fundamental) --> 2 (first overtone) --> 3 (second overtone) 0 --> 4 (third overtone) --> 5 (fourth overtone) 30/03/2020 53

54. Overtones Combination bands Coupling bands Fermi Resonance Spurious bands appearing in the IR spectrum 30/03/2020 54

55. O v er t on es In vibrational spectroscopy, an overtone band is the spectral band that occurs in a vibrational spectrum of a molecule when the molecule makes a transition from the ground state (v=0) to the second excited state (v=2 ), The total number of observed absorption bands is generally different from the total number of fundamental vibrations . Excitation from ground state to higher energy states, which corresponds integral multiples of the frequency of the fundamental (v). o v e r t on e s o c c u r s a t i n t e g r a l m u l t i p l e s o f t h e f und a m e n t a l a b s o r p t i o n frequencies), Weak overtone bands could occur at 2v, 3v, …. Overtones occur at twice the frequency (2 n ) of a fundamental vibration 30/03/2020 55

56. Overtones occur at twice the frequency (2 n ) of a fundamental vibration 2-Butanone CH 3 C CH 2 CH 3 O C = O 343 8 C - H CH bend overtone of strong C=O peak 1719 x 2 = 3438 30/03/2020 56

57. Combination bands Combination bands are of weak intensity and occur when two fundamental bands couple to give bands at (n 1 + n 2 ) or (n 1 - n 2 ) Two vibrational frequencies (v 1 and v 2 ) in a molecule couple to give rise to a new infrared active frequency. This band is the sum of the two interacting bands ( v combination = v 1 + v 2 ). 3. combination bands are observed when more than two or more fundamental vibrations are excited simultaneously . 30/03/2020 57

58. Combination bands are of weak intensity and occur when two fundamental bands couple to give bands at ( Chlorobenzene C l C -C l b e n z e n e C=C n 1 + n 2 ) or ( n 1 - n 2 ) benzene ring combination bands 30/03/2020 58

59. Combination bands IR Spectrum of Butyramide NH 2 O Hence, combination bands are not of diagnostic value in IR interpretation. 30/03/2020

60. Coupling bands occurs when two similar functional groups are in close proximity and they have strong fundamental absorptions . Coupling bands 30/03/2020 60

61. Coupling bands occurs when two similar functional groups are in close proximity and they have strong fundamental absorptions. IR Spectrum of Benzoic anhydride O O O 30/03/2020

62. Fermi Resonance When a fundamental vibration couples with an overtone or combination band, the coupled vibration is called Fermi resonance. Fermi resonance is often observed in carbonyl compounds. Fermi resonance results in the splitting of two vibrational bands that have nearly the same energy and symmetry in both IR and Raman spectroscopies. The two bands are usually a fundamental vibration and either an overtone or combination band 30/03/2020 62

63. Fermi Resonance occurs when a fundamental vibration couples with a combination band or an overtone. IR Spectrum of Benzoyl chloride O C l 30/03/2020 63

64. Important functional group & IR frequency 30/03/2020 64

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66. Al k a ne 30/03/2020

67. 3000 divides UNSATURATED SATURATED C-H sp 2 stretch C-H sp stretch > 3000 cm -1 ~ 3300 cm -1 C-H sp 3 stretch < 3000 cm -1 The C-H stretching region BASE VALUE = 3000 cm -1 30/03/2020 67

68. Hexane CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 CH s t r e t c h i n g vibrations ALKANE includes CH 3 sym and asym CH 2 sym and asym CH bending vibrations discussed shortly 30/03/2020 68

69. Hexane 69 CH stretch CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 CH 2 bend CH 3 bend CH 2 rocking > 4C ALKANE 30/03/2020

70. Alkene C=C Normal charecteristic C=C frequency occurs at 1620- 1640 cm -1 Aromatic C=C frequency occurs at range of 1400-1500 cm -1 Exocyclic double bond variable absorption frequency depending on ring. 30/03/2020 70

71. 1 - H e x e n e = C H C H C = C CH 2 CH 3 bend CH oops CH 2 CH CH 2 CH 2 CH 2 CH 3 ALKENE 30/03/2020

72. Toluene CH 3 C=C benzene CH 3 Ar-H oops A r - H AROMATIC 30/03/2020 72

73. Exo C=C bonds 30/03/2020 73

74. Endo double bond 30/03/2020 74

75. Alkynes C≡C C≡C alkynes at 2100-2150 cm -1 C≡C-H terminal C-H at 3300 cm-1 • If any compund have this two peak in IR, the compund having terminal alkyne group 30/03/2020 75

76. 1 - H ex y n e C = C = C-H C - H CH 2 , CH 3 HC C CH 2 CH 2 CH 2 CH 3 ALKYNE 30/03/2020 76

77. Carbonyl –C=O 30/03/2020 77

78. 7 4 R C O C R anhydride O O O R C C l O R C O R C OR' O R C H O R C R O R C NH 2 O H 169 171 171 5 172 5 173 5 180 1810 and 1760 ( two peaks ) BASE V A L U E acid c h l o r i d e e s t e r a l d e h y d e c a r b o x y l i c acid a mi d e k e t on e C=O IS SENSITIVE TO ITS ENVIRONMENT EACH DIFFERENT KIND OF C=O COMES AT A DIFFERENT FREQUENCY THESE VALUES ARE WORTH LEARNING all are +/ - 10 cm -1 30/03/2020 78

79. Every type of carbonyl compound has other your conclusion based on frequency alone. places you can look to confirm CONFIRMATION OF FUNCTIONAL GROUP R C O C=O at 1725 cm -1 also look for aldehyde CH H 2850 and 2750 cm-1 R C O O H C=O at 1710 cm -1 also look for OH (H-bonded) and C-O ~1200 cm-1 R C O N H H C=O at 1690 cm -1 also look for two NH peaks at 3400 cm-1 R C O O R ' C=O at 1735 cm -1 also look for two C-O at 1200 and 1000 cm -1 Ketones have C=O at 1715 cm -1 and no NH, OH, C -O or -CHO Anhydrides have two C=O 1800 cm -1 and two C-O 30/03/2020 79

80. Alde h y de C-H aldehyde , two peaks (both weak) ~ 2850 and 2750 cm -1 C-H Stretching 30/03/2020 80

81. K e t o ne 30/03/2020 81

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83. Ring Strained & IR frequency 30/03/2020 83

84. E s t er 30/03/2020

85. A m ide 30/03/2020

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87. Ring size Under ideal conditions the carbon atom of a carbonyl group is essentially sp2 hybridized, which implies that the bond angles will be 120° and the C-O sigma bond has 33% s-character. If this group is incorporated in a small ring, the C-CO-C bond angle is reduced to 108° (5-membered ring), 90° (4-membered ring) or 60° (3-membered ring). When this happens, the C-C bonds of the ring assume greater p- character and the C-O sigma bond has correspondingly greater s- character. The double bond of the carbonyl group is therefore shorter and stronger, and exhibits a larger stretching frequency. 30/03/2020 87

88. Conjugation or +R effect 30/03/2020

89. Conjugation extends the dipolar character of the carbonyl group to the double bond (or aromatic ring) so that the beta-carbon atom shares the positive character of the carbonyl carbon. As illustrated by the following resonance equation, this the carbonyl double bond has slightly more single bond character than does an unconjugated function. The bond energy (and force constant- f ) of the conjugated C=O group is correspondingly reduced, and this results in a lower stretching frequency. 30/03/2020 89

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91. Electron withdrawing groups have an opposite influence, and increase the stretching frequency of the carbonyl group. Trichloroacetaldehyde (left below) provides a good example. Electron donating substituents on the carbonyl group stabilize the ionic resonance contributor, and increase the single bond character of the C=O bond. The stretching frequency is therefore decreased, as noted in the right hand example below. 30/03/2020 91

92. E s t er 30/03/2020

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94. Ester with cross conjugations 94 30/03/2020

95. An h y drides 30/03/2020 95

96. E A A 30/03/2020

97. A m ines 30/03/2020 97

98. Nitro –NO 2 T h e N – O s t r e t c h i n g vi b r a t i o n s i n n i t r o a l k a n e s o c c u r near 1550 cm -1 (asymmetrical) and 1350 cm -1 (symmetrical), the band at 1550 cm -1 being the stronger of the two. if the nitro group is attached to an aromatic ring, the N–O stretching bands shift to down to slightly lower wavenumbers 30/03/2020 98

99. - OH 30/03/2020

100. The O–H bands occur at higher frequency, sometimes as a sharp absorption at about 3600 cm –1 . More often, you will see a broad absorption at anywhere from 3500 to 2900 cm –1 . This is because OH groups form strong hydrogen bonds that vary in length and strength. The sharp absorption at 3600 cm–1 is the non-hydrogen- bonded OH and the lower the absorption the stronger the H bond. Alcohols form hydrogen bonds between the hydroxyl oxygen of one molecule and the hydroxyl hydrogen of another. These bonds are variable in length (though they are usually rather longer than normal covalent O–H bonds) and they slightly weaken the true covalent O–H bonds by varying amounts. When a bond varies in length and strength it will have a range of stretching frequencies distributed about a mean value. Alcohols typically give a rounded absorption at about 3300 cm–1 30/03/2020 100

101. Carboxylic -COOH Carboxylic acids (RCO2H) form hydrogen-bonded dimers with two strong H bonds between the carbonyl oxygen atom of one molecule and the acidic hydrogen of the other. These also vary considerably in length and strength and usually give very broad V-shaped absorbances. 30/03/2020 101

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105. Ether R-O-R or C-O 1100 to 1200 cm -1 30/03/2020

106. Nitrite - C≡ N 2210 to 2250 cm -1 30/03/2020 106

107. Aromatic & substituents In between following values 1400 cm -1 1500 cm -1 1600 cm -1 30/03/2020 107

108. FACTORS INFLUENCING VIBRATIONAL FREQUENCIES: Coupled Vibrations and Fermi Resonance Electronic Effects Hydrogen Bonding Bond Angles 30/03/2020 108

109. Coupled Vibrations and Fermi Resonance: Vibrations which occurs at different frequencies of higher wave number are called coupled vibrations . Ex: Consider a –CH 3 group.coupled vibrations for CH 3 group takes place at different frequencies. H Asy m m et r ic C C H H H H H Sy m m et r ic 30/03/2020 109

110. In IR spectrum,absorption bands are spread over a wide range of frequencies.Then the energy of a overtone level chances to coincide with the fundamental mode of different vibrations.This type of resonance is called Fermi Resonance. Ex: Carbondioxide is linear and four fundamental vibrations are expected. In this symmetric stretching vibration is IR inactive. 30/03/2020 110

111. ELECTRONIC EFFECTS  Changes in the absorption frequencies for a particular group takes place when the substituents next to that of a particular group are changed . The frequency shifts includes: Inductive Effect Mesomeric Effect Field Effect 30/03/2020 111

112. INDUCTIVE EFFECT :  Introduction of alkyl groups causes +I effect . It results in lengthening or weakening of the bond and hence the force constant is lowered and wave number of absorption decreases. Formaldehyde Acetaldehyde Acetone 1750 cm -1 1745 cm -1 1715 cm -1 30/03/2020 112

113.  The introduction of electronegative atom causes –I effect. It results in the increase of bond order. Thus the force constant increases and hence the wave number of the bonds also increases. Acetone Chloroacetone Dichloroacetone Tetachloroacetone 1715 cm -1 1725 cm -1 1740 cm -1 1750 cm -1 30/03/2020 113

114. MESOMERIC EFFECT In some cases –I effect is dominated by mesomeric effect and the absorption frequency falls. Ex: Absorption frequency of amides and esters Benza m ide Methyl benzoate 30/03/2020 114

115. FIELD EFFECT  Lone pair of electron s present on the atoms influence each other through space interactions and changes the vibrational frequencies of both the groups. This effect is called as Field effect. 30/03/2020 115

116. HYDROGEN BONDING Hydrog e n bon d i ng gi v e s r i se t o d o w nward frequency shifts. St r ong e r i s th e h y d ro g e n bo n di n g , gr e at e r i s the absorption shift towards lower wave numbers. Hydrogen bonding is of two types Intermolecular hydrogen bonding Intramolecular hydrogen bonding 30/03/2020 116

117. INTERMOLECULAR HYDROGEN BONDING Gives rise to broad bands These are concentration dependent. INTRAMOLECULAR HYDROGEN BONDING Gives rise to sharp and well defined bands. These are concentration independent. 30/03/2020 117

118. BOND ANGLES Highest C=O frequencies arises in the strained cyclobutanes.This can be explained in terms of bond angle strains. If the bond angle is reduced below 120 then C=O stretching is reduced and is occurred at higher frequency. If the bond angle is pushed outwards of 120 then it leads to increase in bond angle thereby stretching of C=0 occurs at lower frequencies 30/03/2020

119.  ABSORBANCE RANGE OF IR 119 Bon d Type of compound Frequency Range Intensity C-H Alkanes 2850-2970 Strong C-H Alkenes 3010-3095 675-995 Medi u m Strong C-H Alkynes 3300 Strong C-H Aromatic Rings 3010-3100 690-900 Medium Strong O -H Monomeric alcohols, phenols Hydrogen bonded alcohols, phenols Monomeric carboxylic acids Hydrogen bonded carboxylic acids 3590-3650 3200-3600 3500-3650 2500-2700 Variable Variable, som e tim e s broad N-H Amines, amides 3300-3500 Medium 30/03/2020

120. C=C Alkenes 1610-1680 Variable C=C Aromatic Rings Alkynes 1500-1600 2100-2260 Variable Var i ab l e C-N A m i nes ,A m ides Nitriles 1180-1360 2210-2280 Stro n g Stro n g C-O Alcohols,ethers,Carboxylic acids,Esters 1050-1300 Strong C=O Aldehydes,Carboxylic acids,KetonesEsters 1690-1760 Strong  ABSORBANCE RANGE OF IR 120 30/03/2020

121. 171 5 181 5 169 178 174 5 RING STRAIN CONJUGATION AND RING SIZE EFFECTS 170 5 O R C R R C CONJUGATION O CH CH 2 O R C O O O O O normal a li p h a t i c ketones 30/03/2020 121

122. O O O O 1735 O O 1660 1735 O 1 7 60 H N O O O 1 8 H N O O O 1 7 7 O 1 7 7 1 7 5 O O N H O 182 174 5 30/03/2020 122

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125. Applications 30/03/2020 125

126. APPLICATIONS OF IR SPECTROSCOPY 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. So if two compounds have identical IR spectra then both of them must be samples of the same substances. 30/03/2020 126

127. APPLICATIONS OF IR SPECTROSCOPY 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. 30/03/2020 127

128. APPLICATIONS OF IR SPECTROSCOPY 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. 30/03/2020 128

129. 5. Quantitative analysis The quantity of the substance can be determined either APPLICATIONS OF IR SPECTROSCOPY i n pur e f orm or more compounds. as a m i xture o f t w o or I n t h is , c h a r a cte r i st i c pe a k corresponding to the drug substance is chosen and log I0/It of peaks comp a red. Th i s for stan d ard and t e s t sa m p l e is i s c a l l ed b a s e li ne techn i qu e to determine the quantity of the substance. 30/03/2020

130. Disadvantages of IR in Quantitative analysis Non adherence of beers law Complexity of spectra 30/03/2020 130

131. Questions 30/03/2020 131

132. Arrange by Increasing order of IR frequency 30/03/2020

133. Decreasing order of Carbonyl frequency 30/03/2020 133

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135. Progress of reaction / Monitor of reactions 30/03/2020 135

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137. Distinguish by IR spectroscopy 30/03/2020 137

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142. predict which will have the highest carbonyl stretching frequency and which will have the lowest carbonyl stretching frequency. Explain your reasoning. 30/03/2020 142

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154. C 4 H 8 1640-1680 C=C 880-900 R 2 C=CH 2 isobutylene CH 3 CH 3 C=CH 2 Unst’d 30/03/2020 154

155. Which compound is this? 2-pentanone 1-pentanol 1-bromopentane 2-methylpentane 1-pentanol 30/03/2020 155

156. What is the compound? 1-bromopentane 1-pentanol 2-pentanone 2-methylpentane 2-pentanone 30/03/2020 156

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158. In a “matching” problem, do not try to fully analyze each spectrum. Look for differences in the possible compounds that will show up in an infrared spectrum. 30/03/2020 158

159. Instrumentation IR radiation source Wavelength selector (Monochromator) Sample Holder or cell Detector Recorder 30/03/2020 159

160. Sources : Inert solids heated electrically to temp of 1500- 2000K 1 . Nernst Glower : Consists of cylinder of rare earth oxides formed into a cylinder of diameter 1-2mm length approx. 20mm. Platinum discs are attached at the ends of cylinder to permit electrical connection. They are heated to a temp of 1 20 - 22 K wh er e IR radiations are emitted. 30/03/2020 160

161. Nernst Glower : Rare earth metals have large –ve temperature coefficient of electrical resistance. (A negative temperature coefficient (NTC) occurs when a physical property (such as thermal conductivity or electrical resistivity ) of a material lowers with increasing temperature, typically in a defined temperature range.) Temp is inversely related to resistance At low temp, they provide high resistance to flow current and as cylinder gets heated resistance gets reduced allowing conduction of current. If large amount of current flows through cylinder it can destroy the source . To avoid this source circuit should be designed to limit the current. 30/03/2020 161

162. 2 . The Globar source glo w e r i t i s also ma d e up of Si m ilar t o n er n st cylinder Material used is silicon carbide Length of cylinder is 50mm & diameter is 5mm It has positive temperature coefficient of electrical resistance At low temp, they provide low resistance to flow current and as cylinder gets heated resistance increases allowing conduction of current. To avoid excessive heating water cooling is necessary At high temp source emits IR radiations. 30/03/2020 162

163. 3. Incandescent wire source (nichrome wire) Cylinder in the above sources is tightly wound by spiral wire of nichrome / rhodium It is electrically heated at 1100 k 4. Mercury arc: Quartz jacketed tube Hg vapors Electricity passed through Hg vapour provides radiations Tungsten filament : used in Near IR CO 2 laser source: Detection of pollutants in air. 30/03/2020 163

164. Wavelength Selectors Output from a wavelength selector is a band of wavelengths. Filters Monochromators Entrance slit Collimating lens or mirror Dispersion element (prism or grating) Focusing lens or mirror Exit slit 30/03/2020 164

165. Difference between Monochromator & Filter 30/03/2020

166. Filters Filters are very simple: they are sheets of plastic or glass that simply absorb wavelengths other than those required for the analysis. Generally, the range of wavelengths allowed by a filter is relatively wide Monochromators are far more complicated, and comprise a series of optics inside a lightproof box, which has entry and exit slits which allows the radiation of all wavelengths in and a narrow range of wavelengths out. 30/03/2020 166

167. Prism The white light pass through a piece of glass and be divided into a rainbow spectrum. 2. The problems with a prism include relatively poor throughput (the total amount of light passes through is significantly less than the amount that went in) and the difficulty in finding materials that diffract different wavelength ranges. 30/03/2020

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169. Entrance slit allows source radiation to illuminate the first lens which collimates the light spreading it across the face of the prism. Prism disperses radiation into component wavelengths and the second lens focuses the spectrum at the focal plane. An exit slit selects the band of radiation to reach the detector. 29 30/03/2020 169

170. Gratings It is a device which consists of a series of parallel & closely spaced grooves rules on glass or any reflecting surface Gratings are of two types Transmission Gratings Reflection Gratings UV gratings have 2000-6000 grooves per mm IR gratings have 10-100 grooves per mm Materials used for construction are Quartz, NaCl,KBr.. 30/03/2020 170

171. Diffraction Gratings 30/03/2020 171

172. Monochro m ator 30/03/2020 172

173. V is i b l e High degree dispersion UV,IR&Visible lower degree dispersion 30/03/2020

174. Handling of samples in IR Sampling techniques in IR depend on whether the sample is a Gas - Gas cell Liquid - Thin film Solid - KBr disks Mulls Deposited films 30/03/2020 174

175. Gases Gas cell Gas cell Infrared transparent windows allow the cell to be mounted directly Internal mirrors are used which permit the beam to be reflected several times through the sample to increase the sensitivity In vapor phase rotational changes in molecule occur freely & these low frequency processes can modulate the higher energy vibrational ba 3 n 5 ds 30/03/2020 175

176. Li q ui d s It consists of a sampling liquid as a thin films squeezed between two infrared transparent windows like NaCl flats The salt plates or rock salt flats must be optically polished & cleaned immediately after use. Toluene, chloroform etc are used to clean them. They should be dry & handled only by their edges 30/03/2020 176

177. Li q ui d s The thickness of the film can be adjusted by varying pressure used to squeeze the flats together(0.01-0.1 mm) It consists of two windows of pressed salt sealed and separated by thin gaskets of Teflon, copper or lead that have been wetted with mercury. The windows are usually made of sodium chloride, potassium chloride or cesium bromide . There are two cells first cell containing sample & second one containing pure solvent placed in reference beam By the reference beam solvent absorptions are cancelled out & spectrum recorded is that of solute alone. 30/03/2020 177

178. So l i d s KBr Disks It is prepared by grinding the sample with KBr & compressing the whole into a transparent water or disk. KBr must be dry, & hence grinding is carried out under infrared lamp to avoid condensation of atmospheric moisture which gives rise to broad absorptions Grinding is done in a agate mortar pestle / commercial ball mills Poorly ground mixtures lead to disks that scatter light than they transmit Particle size of 2µm must be achieved to avoid scattering 30/03/2020 178

179. Hydraulic press KBr Die Compression of a cohesive disks require high pressure. Special die are used from which air can be evacuated by hydraulic compression (10 tonnes load) Disks thus formed are easy to handle 30/03/2020 179

180. So l i d s b) Mulls Pastes prepared by grinding the sample with a drop of oil the mull is the squeezed between transparent windows as for liquid samples Mulling agent should ideally be infrared transparent, but this is never true Liquid paraffin (Nujol) is used to prepare Nujol mulls which is transparent over a wide range in IR spectrum. 30/03/2020 180

181. So l ids c) Solid Films Solid films can be deposited onto NaCl plates by allowing a solutions in a volatile solvent to evaporate drop by drop on the surface of the flat. Polymers & various waxy or fatty materials often give excellent spectra in this way. 30/03/2020

182. IR Detectors 30/03/2020 182

183. Thermocouple It c o n s i s ts of a pa i r of jun c ti o n formed w h en ends of metal such 2 as Bismuth & antimony are fused to either ends of dissimilar metal wires. Heating ab s orbing small as c a pa c ity of element is t o dete c t t he change in temperature. Potential varies in the 2 junctions with the difference in temperature. 30/03/2020

184. Thermocouple One junction is Cold junction which is reference junction kept at constant temperature not exposed to IR radiations The other junction is Hot junction exposed to IR radiations which increases temperature of junction. Temperature difference due to fal l ing IR radiat i ons ge n erates difference the whi c h po t ential a m o unts to Ir radaitions. Hot junction is generally blackened to improve its heat absorbing capacity. IR Radi a tions 30/03/2020 184

185. Bolometer It i s a t y pe of thermometer resistance cons t ructed from strips of metals such as platinum o r n i ck e l o r from semiconductors. The y show a relativ e l y l a rge ch a nge i n resi s tance a s a function of temperature. It forms one of the arm of wheatstones bridge a similar strip is used as balancing arm which is not exposed to the falling IR radiations. 30/03/2020 185

186. Bolometer If IR radiations fall the bridge becomes unbalanced due to change in the resistance occurred due to change in temperature This causes current to flow which is measure of intensity of IR radiations. 30/03/2020 186

187. Golay cells The y are sens i tive gas thermometers Xenon gas is contained in a small cylindrical chamber which contains a blackened membrane on one side while other side has a flexible diaphragm; silvered outside. A beam of light is reflected from the silvered surface to cathode at a vacuum phototube. 30/03/2020 187

188. Golay cells When IR r a d i a t i ons fa l l o n the black e ned mem brane i t is t urn h e ats h e ated which in xenon gas by conduction, resulting in increase in pressure causes distortion of silvered diaphragm As a result the fraction of reflected light which strikes the active surface of phototube is changed leading to change in photocurrent this is related to the falling IR radiations. 30/03/2020

189. Pyroelectric Detectors They are constructed from single crystalline water of pyroelectric material which are dielectric materials which have special thermal & electrical properties. Eg deuterated triglycine sulphate When an electric field is applied across any dielectric material electrical polarization takes place. The magnitude of polarization is a function of dielectric constant of that material This pyroelectric crystal is sandwiched between 2 electrodes which are IR transparent. 30/03/2020 189

190. Pyroelectric Detectors across the crystal which can be detected as a current in an external circuit connecting to two sided of capacitor. Magnitude of current is directly proportional to surface area o f crystal a nd r a te of change of polarization with temperature. acts as a temperature Th e pyroelectric crystal dependent capacitor When IR radi a t i on s fall there i s a change in tem perature, w h ich a l ters the charge distribution 30/03/2020 190

191. Photo Conducting Detector They consists of a thin film of semiconductor material such as lead sulphide, mercury cadmium telluride or Indium antimonide deposited on Non conducting glass surface & sealed into an evacuated envelope to protect semiconductor from atmosphere 30/03/2020 191

192. Photo Conducting Detector Absorption of radiation by these materials promote non- conducting valence electrons to a higher energy conducting state thus decreasing electrical resistance of semiconductor. 30/03/2020 192

193. C o n v e n tio n al IR Sp e ctro s co p y Fourier transform IR Sp e ctro s co p y 30/03/2020 193

194. FTIR : Michealson interferometer 30/03/2020

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197. Double beam infrared spectrophotometer 30/03/2020

198. Infrared spectrophotometer 30/03/2020 198

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