FTIR

FTIR

1. FTIR KANHAIYA KUMAWAT 1 1

2. Infrared (IR) Spectroscopy IR deals with the interaction of infrared radiation with matter. The IR spectrum of a compound can provide important information about its chemical nature and molecular structure. Most commonly, the spectrum is obtained by measuring the absorption of IR radiation, although infrared emission and reflection are also used. Widely applied in the analysis of organic materials, also useful for polyatomic inorganic molecules and for organometallic compounds. 2 KANHAIYA KUMAWAT 2

3. Overview Electromagnetic radiation Vibrations Principle of IR experiment IR spectrum Types of vibration CGF/Fingerprint regions IR activity of vibrations Interpretation of IR spectra Instrumentation Sample preparation 3 KANHAIYA KUMAWAT 3

4. Electromagnetic Radiation The propagation of electromagnetic radiation in a vacuum is constant for all regions of the spectrum (= velocity of light): c = λ × ν 1 Å = 10 –10 m, 1 nm = 10 –9 m, 1 μm = 10 –6 m Another unit commonly used is the wavenumber, which is linear with energy: Work by Einstein, Planck and Bohr indicated that electromagnetic radiation can be regarded as a stream of particles or quanta, for which the energy is given by the equatio n: 4 KANHAIYA KUMAWAT 4

5. The Electromagnetic Spectrum 5 KANHAIYA KUMAWAT 5

6.   LIMIT OF RED LIGHT : 800 nm, 0.8 μm, 12500 cm -1 NEAR INFRARED : 0.8 -2.5 μm, 12500 - 4000 cm -1 MID INFRARED : 2.5 - 50 μm, 4000 - 200 cm -1 FAR INFRARED : 50 - 1000 μm, 200 - 10 cm -1 Divisions arise because of different optical materials and instrumentation. Infrared region 6 KANHAIYA KUMAWAT 6

7. 7 Note: Horizontal scale changes at 2000 cm -1 , which the unit at higher wavenumbers being represented by half of the linear distance of those at lower wavenumbers. The expanded region below 2000 cm -1 permits easier identification of spectral features. Numerous IR bands usually appear in this region. Wavenumber scale is preferred in IR spectroscopy because of direct proportionality between this quantity and both energy & frequency. F= 1.2 x 10 14 Hz F= 2 x 10 13 Hz KANHAIYA KUMAWAT 7

8. Molecular spectra KANHAIYA KUMAWAT 8 There are three basic types of optical spectra that we can observe for molecules: Electronic spectra (Uv-visible-near IR) (transitions between a specific vibrational and rotational level of one electronic state and a vibrational and rotational level of another electronic state) Vibrational or vibrational-rotational spectra (IR region) (transitions from the rotational levels of one vibrational level to the rotational levels of another vibrational level in the same electronic state) Rotational spectra (microwave region) (transitions between rotational levels of the same vibrational level of the same electronic state) 8

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11. 11 Change of dipole moment during vibrations & rotations The energy associated with IR radiation is not enough for electronic transitions like Uv-Vis radiation. Absorption of IR radiation is thus confined largely to molecular species that have small energy differences between various vibrational & rotational states. To absorb IR radiation, a molecule must undergo a net change in dipole moment as it vibrates or rotates. Only under these circumstances, the radiations interact with the molecule and cause changes in the amplitude of its motion. E.g. the charge distribution around a molecule such as hydrogen chloride (HCl) is not symmetric because chlorine has higher electron density than hydrogen. Thus HCl has significant dipole moment and is said to be polar. KANHAIYA KUMAWAT 11

12. As HCl molecule vibrates, a regular fluctuations in its dipole moment occurs, and a field is established that can interact with the electric field associated with the radiation. If the frequency of radiation exactly matches with the natural vibrational frequency of the molecule, absorption of the radiation takes place that produces the change in the amplitude of the molecular vibration. KANHAIYA KUMAWAT 12 12 No net change in dipole moment occurs during the vibration or rotation of homonuclear species such as O 2 , N 2 or Cl 2 . As a result, such compounds can not absorb IR radiations. With the exception of few compounds of this type, all other molecular species absorb IR radiation.

13. • Infrared radiation in the range from 10,000 – 100 cm –1 is absorbed and converted by an organic molecule into energy of molecular vibration –> this absorption is quantized: Vibrational spectra (I): Harmonic oscillator model The negative sign indicates that F is a restoring force. This means that the direction of the force is opposite to the direction of the displacement. A simple harmonic oscillator is a mechanical system consisting of a point mass connected to a massless spring. The mass is under action of a restoring force proportional to the displacement of particle from its equilibrium position and the force constant f (also k in followings) of the spring. 13 KANHAIYA KUMAWAT 13

14. The vibrational frequency increases with: • increase in force constant f , increases bond strength • decreasing atomic mass • Example: f c≡c > f c=c > f c-c • The vibrational energy V(r) can be calculated using the (classical) model of the harmonic oscillator: • Using this potential energy function in the Schrödinger equation, the vibrational frequency can be calculated: 14 KANHAIYA KUMAWAT 14

15. 15 OR     cm -1 If we express the radiations in wavenumber,   IR measurements in conjunction with above equations permits the evaluation of the force constants for various types of chemical bonds. KANHAIYA KUMAWAT 15

16. 16 Generally k has been found to lie in the range 3 x 10 2 to 8 x 10 2 N/m for most single bonds, with 5 x 10 2 N/m serving as a reasonable average value. Double and triple bonds are found by this same means to have force constant of about two to three times this value. i.e. 1 x 10 3 and 1.5 x 10 3 N/m respectively. With these average experimental values, one can estimate the wavenumber of the fundamental absorption band or the absorption due to the transition from the ground state to the first excited states for a variety of bond types. KANHAIYA KUMAWAT 16

17. 17 Calculate the approximate wavenumber and the wavelength of the fundamental absorption due to the stretching vibration of a carbonyl groups C=O     The mass of the carbon atom in Kg is given by,   Similarly, for oxygen,     KANHAIYA KUMAWAT 17

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19. Vibrational spectra (II): Anharmonic oscillator model The actual potential energy of vibrations fits the parabolic function fairly well only near the equilibrium internuclear distance. The Morse potential function more closely resembles the potential energy of vibrations in a molecule for all internuclear distances-anharmonic oscillator model. Fig. 12-1 19 KANHAIYA KUMAWAT 19

20. • The energy difference between the transition from n to n+1 corresponds to the energy of the absorbed light quantum • The difference between two adjacent energy levels gets smaller with increasing n until dissociation of the molecule occurs (Dissociation energy E D ) ΔE VIB = ( E n +1 – E n ) =h ⋅ ν osc Note: Weaker transitions called “ overtones” are sometimes observed. These correspond to Δυ=2 or 3, and their frequencies are less than two or three times the fundamental frequency (Δυ=1) because of anharmonicity. Typical energy spacings for vibrational levels are on the order of 10 -20 J. From the Bolzmann distribution, it can be shown that at room temperature typically 1% or less of the molecules are in excited states in the absence of external radiation. Thus most absorption transitions observed at room temperature are from the υ=0 to the υ=1 level. 20 KANHAIYA KUMAWAT 20

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22. The vibrational spectra appear as bands rather than lines. When vibrational spectra of gaseous diatomic molecules are observed under high-resolution conditions, each band can be found to contain a large number of closely spaced components— band spectra . The structure observed is due to that a single vibrational energy change is accompanied by a number of rotational energy changes. The form of such a vibration-rotation spectrum can be predicted from the energy levels of a vibrating-rotating molecule. –> “vibrational-rotational band s” Vibrational spectra (III): Rotation-vibration transitions A vibrational absorption transition from υ to υ+1 gives rise to three sets of lines called branches: Lower-frequency P branch: Δυ=1, ΔJ=-1; Higher-frequency R branch: Δυ=1, ΔJ=+1; Q branch: branch: Δυ=1, ΔJ=0. 22 KANHAIYA KUMAWAT 22

23. Spectrum of the Rotating Oscillator • The selection rules allow only transitions with Δν = +1 and Δ J = ±1 (the transition with Δ J = 0 is normally not allowed except those with an odd number of electrons (e.g. NO)). P R 23 KANHAIYA KUMAWAT 23

24. The IR absorption spectrum can be obtained with gas-phase or with condensed-phase molecules. For gas-phase molecules vibration-rotation spectra are observed, while in condensed phases, the rotational structure is lost. For most routine analytical applications of infrared spectrometry, spectra are obtained with condensed-phase samples. Hence, we discuss here centers around the vibrational transitions observed with molecules present as pure liquid, as solutions, or in the solid state. 24 KANHAIYA KUMAWAT 24

25. Molecular vibrations • How many vibrations are possible (=fundamental vibrations)? A molecule has as many degrees of freedom as the total degree of freedom of its individual atoms. Each atom has three degrees of freedom (corresponding to the Cartesian coordinates), thus in an N-atom molecule there will be 3N degree of freedom. In molecules, movements of the atoms are constrained by interactions through chemical bonds. Translation - the movement of the entire molecule while the positions of the atoms relative to each other remain fixed: 3 degrees of translational freedom. Rotational transitions – interatomic distances remain constant but the entire molecule rotates with respect to three mutually perpendicular axes: 3 rotational freedom (nonlinear), 2 rotational freedom (linear). 25 KANHAIYA KUMAWAT 25

26. Fundamental Vibrations Vibrations – relative positions of the atoms change while the average position and orientation of the molecule remain fixed. 26 KANHAIYA KUMAWAT 26

27. • There are two different types of vibrational modes: Vibrations can either involve a change in bond length (stretching) or bond angle (bending) Vibration Types 27 KANHAIYA KUMAWAT 27

28. The bending vibrations are often subdivided into scissoring, rocking, wagging, and twisting . 28 KANHAIYA KUMAWAT 28

29. Principle of IR experiments E-vector in electromagnetic radiation has frequency ν Molecular vibrations involving change in dipole moment set up fluctuating electric field Vibrational energies: fundamental (= one quantum) Energy transferred to molecule by resonance when vibration frequency is the same as that of the electromagnetic radiation 29 KANHAIYA KUMAWAT 29

30. • Vibrations which do not change the dipole moment are Infrared Inactiv e (homonuclear diatomics). Selection Rules The energy associated with a quantum of light may be transferred to the molecule if work can be performed on the molecule in the form of displacement of charge. Selection rule: A molecule will absorb infrared radiation if the change in vibrational states is associated with a change in the dipole moment (μ) of the molecule. µ = q r q: electrical charge, r : directed distance of that charge from some defined origin of coordinates from the molecule. Dipole moment is greater when electronegativity difference between the atoms in a bond is greater. Some electronegativity values are: H 2.2; C 2.55; N 3.04; O 3.44; F 3.98; P 2.19; S 2.58; Cl 3.16 30 KANHAIYA KUMAWAT 30

31. • The theoretical number of fundamental vibrations (absorption frequencies) will seldom (hardly/rarely) be observed –> overtones (multiples of a given frequency), combination (sum of two other vibrations) or difference (the difference of two other vibrations) tones increase the number of bands –> the following effects will reduce the number of theoretical bands: • frequencies which fall outside the measured spectral region (400-4000 cm –1 ) • bands which are too weak • bands are too close and coalesce / overlapping • occurrence of a degenerate band from several absorptions of the same frequency • lack of change in molecular dipole Why not 3N-6/3N-5 bands in IR spectrum? 31 KANHAIYA KUMAWAT 31

32. Infrared Spectrum of Carbon Dioxide 32 KANHAIYA KUMAWAT 32

33. Vibrational Modes for a CH 2 Group 33 KANHAIYA KUMAWAT 33

34. Absorption Regions 34 KANHAIYA KUMAWAT 34

35. Group frequencies With certain functional or structural groups, it has been found that their vibrational frequencies are nearly independent of the rest of the molecule – group frequencies. Carbonyl group 1650 to 1740 cm -1 various aldehydes and ketones For many groups involving only two atoms, the approximate frequency of the fundamental vibration can be calculated from a simple harmonic oscillator model . Calculations show that for most groups of interest, characteristic frequencies of stretching vibrations should lie in the region 4000 to 1000 cm -1 . In practical, the region from 4000 to 1300 cm -1 is often called the group frequency region . The presence of various group vibrations in the IR spectrum is of great assistance in identifying the absorbing molecule. 35 KANHAIYA KUMAWAT 35

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37. Fingerprint region In the region from ≈ 1300 to 400 cm -1 , vibrational frequencies are affected by the entire molecule, as the broader ranges for group absorptions in the figure below – fingerprint region . Absorption in this fingerprint region is characteristic of the molecule as a whole. This region finds widespread use for identification purpose by comparison with library spectra. 37 KANHAIYA KUMAWAT 37

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39. • When two bond oscillators share a common atom, they seldom behave as individual oscillators (unless the individual oscillation frequencies are widely different). The frequency of the asymmetric stretching vibration in CO 2 is at a shorter wavelength (higher frequency) than for a carbonyl group in aliphatic ketones (around 1715 cm –1 ). –> there must be strong mechanical coupling or interaction! Example: C–O stretching band in Methanol: 1034 cm –1 Ethanol: 1053 cm –1 not an isolated stretching vibration, but rather a coupled symmetric stretching invloving C–C–O stretching Coupled Interactions 39 KANHAIYA KUMAWAT 39

40. • The vibrations must be of the same symmetry • The interaction is greatest, when the coupled groups absorb (individually) near the same frequency --- the same energies of isolated vibrations. • Strong coupling between stretching vibrations requires a common atom between the two groups • Coupling between bending and stretching vibrations can occur if the stretching bond forms one side of the changing angle. • A common bond is required for coupling of bending vibrations. • Coupling is negligible when groups are separated by one or more carbon atoms and the vibrations are mutually perpendicular. Requirements for Coupled Interactions 40 KANHAIYA KUMAWAT 40

41. “A hydrogen bond exists when a hydrogen atom is bonded to two or more other atoms” –> not an ordinary covalent bond, since the hydrogen atom has only one orbital (1s) to engage in covalent bonding Typical H-bond: Hydrogen is attached to two very electronegative atoms, usually in a linear fashion and not symmetrically: X–H••••••B => The s orbital of the proton can effectively overlap with the p or π orbital of the acceptor group. Hydrogen Bonding 41 KANHAIYA KUMAWAT 41

42. • Hydrogen bonding alters the force constant of both groups: – the X–H stretching bands move to lower frequency – the stretching frequency of the acceptor group (B) is also reduced, but to a lesser degree – The X–H bending vibration usually shifts to a shorter wavelength Effect of Hydrogen Bonding 42 KANHAIYA KUMAWAT 42

43. y axis is %T or A x axis is wavenumber (or wavelength) I o → sample → I  T = I/I o %T = 100 I/I o T transmission / transmittance  A = -log T A absorbance (no units) (Note A (but not T) ∝ concentration) IR spectrum 43 KANHAIYA KUMAWAT 43

44. BRUKE TENSOR TM Series Perkin Elmer TM Spectrum One Instrumentation 44 KANHAIYA KUMAWAT 44

45. Dispersive instruments : with a monochromator to be used in the mid-IR region for spectral scanning and quantitative analysis. Fourier transform IR (FTIR) systems : widely applied and quite popular in the far-IR and mid-IR spectrometry. Nondispersive instruments : use filters for wavelength selection or an infrared-absorbing gas in the detection system for the analysis of gas at specific wavelength. 45 KANHAIYA KUMAWAT 45

46. Dispersive IR spectrophotometers Simplified diagram of a double beam infrared spectrometer Modern dispersive IR spectrophotometers are invariably double-beam instruments , but many allow single-beam operation via a front-panel switch. 46 KANHAIYA KUMAWAT 46

47. Double-beam operation compensates for atmospheric absorption, for the wavelength dependence of the source spectra radiance, the optical efficiency of the mirrors and grating, and the detector instability, which are serious in the IR region.⇒ single-beam instruments not practical . Double-beam operation allows a stable 100% T baseline in the spectra. 47 KANHAIYA KUMAWAT 47

48. Dispersive spectrophotometers Designs Null type instrument 48 KANHAIYA KUMAWAT 48

49. Components of dispersive spectrophotometers Nernst Glower heated rare earth oxide rod (~1500 K) 1-50 µm (mid- to far-IR) Globar heated SiC rod (~1500 K) 1-50 µm (mid- to far-IR) W filament lamp 1100 K 0.78-2.5 µm (Near-IR) Hg arc lamp plasma 50 - 300 µm (far-IR) CO2 laser stimulated emission lines 9-11 µm 1. IR source 49 KANHAIYA KUMAWAT 49

50. Thermocouple thermoelectric effect -dissimilar metal junction cheap, slow, insensitive Bolometer Ni, Pt resistance thermometer (thermistor) Highly sensitive <400 cm -1 Pyroelectric Tri glycine sulfate piezoelectric material fast and sensitive (mid IR) Photoconducting PbS, CdS, Pb Se light sensitive cells fast and sensitive (near IR) 2. Detector / transducer 50 KANHAIYA KUMAWAT 50

51. 3. Optical system 51 KANHAIYA KUMAWAT 51

52. Reflection gratings ( made from various plastics): the groove spacing is greater (e.g. 120 grooves mm -1 ). To reduce the effect of overlapping orders and stray radiation, filters or a preceding prism are usually employed. Two or more gratings are often used with several filters to scan a wide region. Mirrors but not lenses are used to focus and collimate the IR radiation. Generally made from Pyrex or another material with low coefficient of thermal expansion. Front surfaces coated with a vacuum-deposited thin metal film of Al, Ag, or Au. * 52 KANHAIYA KUMAWAT 52

53. Windows are used for sample cells and to permit various compartment to be isolated from the environment. → transparent to IR over the wavelength region → inert to the various chemicals analyzed → capable of being shaped, ground, and polished to the desired optical quality 53 KANHAIYA KUMAWAT 53

54. The Fourier transform method provides an alternatives to the use of monochromators based on dispersion. In convensional dispersive spectroscopy, frequencies are separated and only a small portion is detected at any particular instant, while the remainder is discarded. The immediate result is a frequency-domain spectrum . Fourier transform infrared spectroscopy generates time-domain spectra as the immediately available data, in which the intensity is obtained as a function of time. Direct observation of a time-domain spectrum is not immediately useful because it is not possible to deduce, by inspection, frequency-domain spectra from the corresponding time-domain waveform ( Fourier transform is thus introduced). Fourier Transform Infrared Spectrometer (FTIR) 54 KANHAIYA KUMAWAT 54

55. In one arm of the interferometer, the IR source radiation travels through the beam splitter to the fixed mirror back to the beam splitter through the sample and to the detector. In the other arm, the IR source radiation travels to the beam splitter to the movable mirror, back through the beam splitter to the sample and to the detector. The difference in pathlengths of the two beams is the retardation δ . An He-Ne laser is used as a monochromatic reference source. The laser beam is sent through the interferometer in the opposite direction to that of the IR beam. Single-beam FTIR Spectrometer 55 KANHAIYA KUMAWAT 55

56. Double-beam FTIR Spectrometer 56 KANHAIYA KUMAWAT 56

57. Interferometer Michelson interferometer If moving mirror moves 1/4 λ (1/2 λ round-trip) waves are out of phase at beam-splitting mirror - no signal If moving mirror moves 1/2 λ (1 λ round-trip) waves are in phase at beam-splitting mirror – signal ... 57 KANHAIYA KUMAWAT 57

58. Interferograms * FTIR 58 KANHAIYA KUMAWAT 58

59. Difference in pathlength called retardation δ Plot δ vs. signal - cosine wave with frequency proportional to light frequency but signal varies at much lower frequency One full cycle when mirror moves distance λ /2 (round-trip = λ ) Frequency of signal: Substituting λ =c/ ν If mirror velocity is 1.5 cm/s Bolometer, pyroelectric, photoconducting IR detectors can "see“ changes on 10 -4 s time scale! V MM velocity of moving mirror 59 KANHAIYA KUMAWAT 59

60. Computer needed to turn complex interferograms into spectra. 60 KANHAIYA KUMAWAT 60

61. Measuring processes 61 KANHAIYA KUMAWAT 61

62. • very high resolution (< 0.1 cm –1 ) Two closely spaced lines only separated if one complete "beat" is recorded. As lines get closer together, δ must increase. Δν (cm − 1 ) = 1/ δ Mirror motion is 1/2 δ Resolution governed by distance movable mirror travels • very high sensitivity (nanogram quantity) can be coupled with GC analysis (–> measure IR spectra in gas-phase) • High S/N ratios - high throughput Few optics, no slits mean high intensity of light • Rapid (<10 s) • Reproducible and • Inexpensive Advantages of FTIR 62 KANHAIYA KUMAWAT 62

63. Usually to improve resolution decrease slit width but less light makes spectrum "noisier" - signal to noise ratio (S/N) n # scans S/N improves with more scans (noise is random, signal is not!) To improve S/N ratio 63 KANHAIYA KUMAWAT 63

64. For routine instrument calibration, run the spectrum of polystyrene film (or indene) at resolution 2 cm -1 . Band positions are available in the literature. Higher resolution calibrations may be made from gas-phase spectra (e.g. HCl gas). Spectrum calibration * FTIR 64 KANHAIYA KUMAWAT 64

65. Sample preparation techniques The preparation of samples for infrared spectrometry is often the most challenging task in obtaining an IR spectrum. Since almost all substances absorb IR radiation at some wave length, and solvents must be carefully chosen for the wavelength region and the sample of interest. Infrared spectra may be obtained for gases, liquids or solids (neat or in solution) 65 KANHAIYA KUMAWAT 65

66. A gas sample cell consists of a cylinder of glass or sometimes a metal. The cell is closed at both ends with an appropriate window materials (NaCl/KBr) and equipped with valves or stopcocks for introduction of the sample. Long pathlength (•10 cm) cells – used to study dilute (few molecules) or weakly absorbing samples. To resolve the rotational structure of the sample, the cells must be capable of being evacuated to measure the spectrum at reduced pressure. For quantitative determinations with light molecules, the cell is sometimes pressurized in order to broaden the rotational structure and all simpler measurement. Gas samples 66 KANHAIYA KUMAWAT 66

67. 67 Herriott cell  - Adjust D to change the number of passes Circular Multipass Cell  - The beam propagates on a star pattern. The path length can be adjusted by changing the incidence angle Φ. Multipass cells – more compact and efficient instead of long-pathlength cells. Mirrors are used so that the beam makes several passes through the sample before exiting the cell. (Effective pathlength • 10 m). KANHAIYA KUMAWAT 67

68. Pure or soluted in transparent solvent – not water (attacks windows) The sample is most often in the form of liquid films (“sandwiched” between two NaCl plates) Adjustable pathlength (0.015 to 1 mm) – by Teflon spacer Liquid samples * FTIR 68 KANHAIYA KUMAWAT 68

69. Regions of transparency for common infrared solvents. The horizontal lines indicate regions where solvent transmits at least 25% of the incident radiation in a 1-mm cell. * FTIR 69 KANHAIYA KUMAWAT 69

70. Solid samples • Spectra of solids are obtained as alkali halide discs (KBr), mulls (e.g. Nujol, a highly refined mixture of saturated hydrocarbons) and films (solvent or melt casting) Alkali halide discs: A milligram or less of the fine ground sample mixed with about 100 mg of dry KBr powder in a mortar or ball mill. The mixture compressed in a die to form transparent disc. Mulls Grinding a few milligrams of the powdered sample with a mortar or with pulverizing equipment. A few drops of the mineral oil added (grinding continued to form a smooth paste). The IR of the paste can be obtained as the liquid sample. * FTIR 70 KANHAIYA KUMAWAT 70

71. 1. Fundamental chemistry Determination of molecular structure/geometry. e.g.  Determination of bond lengths, bond angles of gaseous molecules 2.   Qualitative analysis – simple, fast, nondestructive Monitoring trace gases: Non Dispersive IR (NDIR). Rapid, simultaneous analysis of GC, moisture, N in soil. Analysis of fragments left at the scene of a crime Quantitative determination of hydrocarbons on filters, in air, or in water Main uses of IR spectroscopy: KANHAIYA KUMAWAT 71

72. Near-infrared and Far-infrared absorption The techniques and applications of near-infrared (NIR) and far-infrared (FIR) spectrometry are quite different from those discussed above for conventional, mid-IR spectrometry. Near-infrared : 0.8 -2.5 μm, 12500 - 4000 cm -1 Mid-infrared : 2.5 - 50 μm, 4000 - 200 cm -1 Far-infrared : 50 - 1000 μm, 200 - 10 cm -1 KANHAIYA KUMAWAT 72

73. Near-infrared spectrometry NIR shows some similarities to UV-visible spectrophotometry and some to mid-IR spectrometry. Indeed the spectrophotometers used in this region are often combined UV-visible-NIR ones. The majority of the absorption bands observed are due to overtones (or combination) of fundamental bands that occur in the region 3 to 6 μm, usually hydrogen-stretching vibrations. NIR is most widely used for quantitative organic functional-group analysis. The NIR region has also been used for qualitative analyses and studies of hydrogen bonding, solute-solvent interactions, organometallic compounds, and inorganic compounds. KANHAIYA KUMAWAT 73

74. Far-infrared spectrometry Almost all FIR studies are now carried out with FTIR spectrometers. The far-IR region can provide unique information. The fundamental vibrations of many organometallic and inorganic molecules fall in this region due to the heavy atoms and weak bonds in these molecules. Lattice vibrations of crystalline materials occur in this region, Electron valence/conduction band transition in semiconductors often correspond to far-IR wavelengths. KANHAIYA KUMAWAT 74

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77. 77 Specular reflectance Used for smooth surfaces Angle of reflectance = incident of reflection For examining smooth surfaces only of solids or coated solids. Specular reflectance sampling in FTIR represents a very important technique useful for measurement of: Thin films on reflective substrates. Analysis of bulk materials. Measurement of monomolecular layers on a substrate material. Often sample analysis with no sample preparation. Not as popular as other reflection techniques. the angle of incidence equals the angle of reflection 77 KANHAIYA KUMAWAT 77

78. The basics of the sampling technique: Involve measurement of the reflected energy from a sample surface at a given angle of incidence. The electromagnetic reflectance dependent upon: The angle of incidence of the illuminating beam The refractive index and thickness of the sample Experimental conditions. Types of specular reflectance experiments • Reflection-Absorption of relatively thin films on reflective substrates measured at near normal angle of incidence • Specular Reflectance measurements of relatively thick samples measured at near normal angle of incidence • Angle Reflection-Absorption of ultra-thin films or monolayers deposited on surfaces measured at high angle of incidence. KANHAIYA KUMAWAT 78

79. Attenuated total reflectance (ATR) ATR accessories are especially useful for obtaining IR spectra of difficult samples that cannot be readily examined by the normal transmission method. They are suitable for studying thick or highly absorbing solid and liquid materials, including films, coatings, powders, threads, adhesives, polymers, and aqueous samples. ATR requires little or no sample preparation for most samples and is one of the most versatile sampling techniques. KANHAIYA KUMAWAT 79

80. Theory of ATR ATR occurs when a beam of radiation enters from a more-dense (with a higher refractive index) into a less-dense medium (with a lower refractive index). The fraction of the incident beam reflected increases when the angle of incidence increases. All incident radiation is completely reflected at the interface when the angle of incidence is greater than the critical angle (a function of refractive index). The beam penetrates a very short distance beyond the interface and into the less-dense medium before the complete reflection occurs. This penetration is called the evanescent wave and typically is at a depth of a few micrometers (μm). Its intensity is reduced (attenuated) by the sample in regions of the IR spectrum where the sample absorbs. KANHAIYA KUMAWAT 80

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82. Method The sample is normally placed in close contact with a more-dense, high-refractive-index crystal such as zinc selenide, thallium bromide–thallium iodide, or germanium. The IR beam is directed onto the beveled edge of the ATR crystal and internally reflected through the crystal with a single or multiple reflections. Both the number of reflections and the penetration depth decrease with increasing angle of incidence. For a given angle, the higher length-to-thickness ratio of the ATR crystal gives higher numbers of reflections. KANHAIYA KUMAWAT 82 82

83. 83 A variety of types of ATR accessories are available: such as 25 to 75° , vertical variable-angle ATR, horizontal ATR, and Spectra-Tech Cylindrical Internal Reflectance Cell for Liquid Evaluation (CIRCLE®) cell. The resulting ATR-IR spectrum resembles the conventional IR spectrum, but with some differences: Identical absorption band positions but with different relative intensities FTIR spectrometers permit higher-quality spectra . KANHAIYA KUMAWAT 83

84. Radiation actually penetrates sample and is partially absorbed Penetration of a sample is independent of its thickness; Interference and scattering do not occur in a sample; Absorbance in a sample is independent of direction. Total internal reflection, TIR: Radiation strikes an interface with a medium of lower RI, with an angle > θ c . 84 KANHAIYA KUMAWAT 84

85. References: J. Workman, A.W. Springsteen, “Applied Spectroscopy”, Academic Press, 1998. J.M. Hollas, “Modern Spectroscopy”, John Wiley&Sons, 1996. B. Stuart, W.O. George, D.J. Ando, “Modern Infrared Spectroscopy”, John Wiley&Sons, 1997. N.N. Colthup, L.H. Daly, S.E. Wiberly, S.E. Wiberly, “Introduction to Infrared and Raman Spectroscopy”, Academic Press, 1997. B. Schrader, D. Bougeard, “Infrared and Raman Spectroscopy: Methods and Applications”, John Wiley&Sons, 1995. * FTIR 85 KANHAIYA KUMAWAT 85

86. Infrared Spectrum of CCl 4 * FTIR 86 KANHAIYA KUMAWAT 86

87. * FTIR 87 KANHAIYA KUMAWAT 87

88. * FTIR 88 KANHAIYA KUMAWAT 88

89. * FTIR 89 KANHAIYA KUMAWAT 89

90. KANHAIYA KUMAWAT 90 THANK YOU