A complete Guide for IR Spectroscopy.pptx

This PPT gives viewers a complete guidance and understanding of various topics of IR spectroscopy. Highly useful to the students of Chemistry,Physics,Lifescienes and Pharmacy.

1. A Complete Guide For IR Spectroscopy By Dr. Tejas Pavagadhi Indian Institute of Teacher Education, Gandhinagar

2. Introduction Spectroscopy is the study of the interaction of matter with the electromagnetic radiation Electromagnetic radiation displays the properties of both particles and waves The particle component is called a photon The energy ( E ) component of a photon is proportional to the frequency. Where h is Planck’s constant and υ is the frequency in Hertz (cycles per second) E υ , E = h υ The term “photon” is implied to mean a small, mass less particle that contains a small wave-packet of EM radiation/ light. we will use this terminology in the course   Vibrational Spectroscopy IITE : Nurturing teachers of tomorrow 2

3. Introduction Because the speed of light, c , is constant, the frequency , υ , (number of cycles of the wave per second) can complete in the same time, must be inversely proportional to how long the oscillation is, or wavelength : Amplitude, A , describes the wave height, or strength of the oscillation Because the atomic particles in matter also exhibit wave and particle properties EM radiation can interact with matter in two ways: Collision – particle-to-particle – energy is lost as heat and movement Coupling – the wave property of the radiation matches the wave property of the particle and “couple” to the next higher quantum mechanical energy level υ = c = 3 x 10 10 cm/s ___ l c  E = h υ = ___ l hc Vibrational Spectroscopy IITE : Nurturing teachers of tomorrow 3

4. Introduction The entire electromagnetic spectrum is used by chemists: UV X-rays IR g -rays Radio Microwave Energy (kcal/ mol ) 300-30 300-30 ~10 -4 > 300 ~10 -6 Visible Frequency, υ in Hz ~10 15 ~10 13 ~10 10 ~10 5 ~10 17 ~10 19 Wavelength, nm 10 nm 1000 nm 0.01 cm 100 m ~0.01 nm ~.0001 nm nuclear excitation core electron excitation electronic excitation molecular vibration molecular rotation Nuclear Magnetic Resonance NMR (MRI) Vibrational Spectroscopy IITE : Nurturing teachers of tomorrow 4

5. Vibrational Spectroscopy IITE : Nurturing teachers of tomorrow 5 Night Vision Goggles IR in Everyday Life

6. Vibrational Spectroscopy IITE : Nurturing teachers of tomorrow 6 Humans , at normal body temperature, radiate most strongly in the infrared, at a wavelength of about 10 microns (A micron is the term commonly used in astronomy for a micrometer or one millionth of a meter). In the image to the left, the red areas are the warmest, followed by yellow, green and blue (coolest). The image to the right shows a cat in the infrared. The yellow-white areas are the warmest and the purple areas are the coldest. This image gives us a different view of a familiar animal as well as information that we could not get from a visible light picture. Notice the cold nose and the heat from the cat's eyes, mouth and ears.

7. IITE : Nurturing teachers of tomorrow 7 Theory

8. Vibrational Spectroscopy IITE : Nurturing teachers of tomorrow 8 Vibrational radiation lies between the visible and microwave portions of the electromagnetic spectrum. Vibrational waves have wavelengths longer than visible and shorter than microwaves, and have frequencies which are lower than visible and higher than microwaves. The Vibrational region is divided into: near, mid and far-infrared. Near-infrared refers to the part of the infrared spectrum that is closest to visible light. Far-infrared refers to the part that is closer to the microwave region. Mid-infrared is the region between these two.

9. Introduction The IR Spectroscopic Process The quantum mechanical energy levels observed in IR spectroscopy are those of molecular vibrations We perceive this vibration as heat When we say a covalent bond between two atoms is of a certain length, we are citing an average because the bond behaves as it is a vibrating spring connecting two atoms For a simple diatomic molecule, this model is easy to visualize: Vibrational Spectroscopy IITE : Nurturing teachers of tomorrow 9

10. Vibrational Spectroscopy IITE : Nurturing teachers of tomorrow 10 Types of Molecular Vibrations ( called modes of vibration). Stretching Vibrations change in bond length Symmetric Asymmetric Bending Vibrations(Deformation vibration) change in bond angle In plane bending scissoring rocking Out of plane bending wagging twisting/torsion

11. Introduction There are two types of bond vibration: Stretch – Vibration or oscillation along the line of the bond Bend – Vibration or oscillation not along the line of the bond H H C H H C scissor asymmetric H H C C H H C C H H C C H H C C symmetric rock twist wag in plane out of plane Vibrational Spectroscopy IITE : Nurturing teachers of tomorrow 11

12. IITE : Nurturing teachers of tomorrow 13 The IR Spectroscopic Process As a covalent bond oscillates – due to the oscillation of the dipole of the molecule – a varying electromagnetic field is produced The greater the dipole moment change through the vibration, the more intense is the EM field that is generated Vibrational Spectroscopy

13. IITE : Nurturing teachers of tomorrow 14 The IR Spectroscopic Process When a wave of infrared light encounters this oscillating EM field generated by the oscillating dipole of the same frequency, the two waves couple, and IR light is absorbed The coupled wave now vibrates with twice the amplitude ir beam from spectrometer EM oscillating wave from bond vibration “coupled” wave Vibrational Spectroscopy

14. The IR Spectrum Each stretching and bending vibration occurs with a characteristic frequency as the atoms and charges involved are different for different bonds The y-axis on an IR spectrum is in units of % transmittance (%T) In regions where the EM field of an osc . bond interacts with IR light of the same υ – transmittance is low (light is absorbed) In regions where no osc . bond is interacting with IR light, transmittance nears 100% IITE : Nurturing teachers of tomorrow 15 Vibrational Spectroscopy

15. The IR Spectrum The x-axis of the IR spectrum is in units of wavenumbers, υ (nu bar) which is the number of waves per centimeter in units of cm -1 (Remember E = h υ or E = hc / λ ) IITE : Nurturing teachers of tomorrow 16 Vibrational Spectroscopy

16. The IR Spectrum This unit is used rather than wavelength (microns) because wavenumbers are directly proportional to the energy of transition being observed – chemists like this, physicists hate it High frequencies and high wavenumbers equate higher energy is quicker to understand than Short wavelengths equate higher energy This unit is used rather than frequency as the numbers are more “real” than the exponential units of frequency IR spectra are observed for the mid-infrared: 4000- 400 (690)cm -1 The peaks are Gaussian distributions of the average energy of a transition IITE : Nurturing teachers of tomorrow 17 Vibrational Spectroscopy

17. The IR Spectrum In general: Lighter atoms will allow the oscillation to be faster – higher energy This is especially true of bonds to hydrogen – C-H, N-H and O-H Stronger bonds will have higher energy oscillations Triple bonds > double bonds > single bonds in energy Energy of oscillation IITE : Nurturing teachers of tomorrow 18 Vibrational Spectroscopy

18. The IR Spectrum – The detection of different bonds As opposed to chromatography or other spectroscopic methods, the area of a IR band (or peak) is not directly proportional to concentration of the functional group producing the peak The intensity of an IR band is affected by two primary factors: Whether the vibration is one of stretching or bending Electronegativity difference of the atoms involved in the bond For both effects, the greater the change in dipole moment in a given vibration or bend, the larger the peak. The greater the difference in electronegativity between the atoms involved in bonding, the larger the dipole moment Typically, stretching will change dipole moment more than bending IITE : Nurturing teachers of tomorrow 19 Vibrational Spectroscopy

19. The IR Spectrum – The detection of different bonds It is important to make note of band/peak intensities to show the effect of these factors: Strong (s) – band/peak is tall Medium (m) – band/peak is mid-height Weak (w) – band/peak is short * Broad ( br ) – if the Gaussian distribution is abnormally broad (*this is more for describing a bond that spans many energies) Exact transmittance values are rarely recorded IITE : Nurturing teachers of tomorrow 20 Vibrational Spectroscopy

20. II. Vibrational Group Analysis A. General The primary use of the IR is to detect functional groups Because the IR looks at the interaction of the EM Radiations with actual bonds, it provides a unique qualitative probe into the functionality of a molecule, as functional groups are merely different configurations of different types of bonds Since most “types” of bonds in covalent molecules have roughly the same energy, i.e., C=C and C=O bonds, C-H and N-H bonds they show up in similar regions of the IR spectrum Remember all organic functional groups are made of multiple bonds and therefore show up as multiple IR bands (peaks) IITE : Nurturing teachers of tomorrow 21 Vibrational Spectroscopy

21. 3.) Typical IR spectrum for Organic Molecule - many more bands then in UV- vis , fluorescence or phosphorescence - bands are also much sharper - pattern is distinct for given molecule except for optical isomers - good qualitative tool can be used for compound identification group analysis - also quantitative tool intensity of bands related to amount of compound present - spectra usually shown as percent transmittance (instead of absorbance) vs. wavenumber (instead of l) for convenience Hexane Hexene Hexyne IITE : Nurturing teachers of tomorrow 22

22. G) Theory of IR Absorption 1.) Molecular Vibrations(Hooke’s law) i .) Harmonic Oscillator Model: - approximate representation of atomic stretching - two masses attached by a spring E = ½ ky 2 where : y is spring displacement k is spring constant IITE : Nurturing teachers of tomorrow 23

23. Vibrational frequency given by: where: υ : frequency k : force constant (measure of bond stiffness) m: reduced mass = m 1 m 2 /m 1 +m 2 We can get k: Single bonds : k ~ 3x10 2 to 8 x10 2 N/m ( Avg ~ 5x10 2 ) double and triple bonds ~ 2x and 3x k for single bond. So, vibration occur in order: single < double < triple IITE : Nurturing teachers of tomorrow 24

24. ii.) Anharmonic oscillation: - harmonic oscillation model good at low energy levels (n , n 1 , n 2 , …) - not good at high energy levels due to atomic repulsion & attraction as atoms approach, coulombic repulsion force adds to the bond force making energy increase greater then harmonic as atoms separate, approach dissociation energy and the harmonic function rises quicker Harmonic oscillation Anharmonic oscillation Because of anharmonics : at low DE, Dn =±2, ±3 are observed which cause the appearance of overtone lines at frequencies at ~ 2-3 times the fundamental frequency. Normally Dn = ± 1 IITE : Nurturing teachers of tomorrow 25

25. iv.) Number of Vibrational Modes: - for non-linear molecules, number of types of vibrations: 3N-6 - for linear molecules, number of types of vibrations: 3N-5 - why so many peaks in IR spectra - observed vibration can be less than predicted because symmetry ( no change in dipole) energies of vibration are identical absorption intensity too low frequency beyond range of instrument http://www.chem.purdue.edu/gchelp/vibs/co2.html See web site for 3D animations of vibrational modes for a variety of molecules Examples: 1) HCl : 3(2)-5 = 1 mode 2) CO 2 : 3(3)-5 = 4 modes - - moving in-out of plane + IITE : Nurturing teachers of tomorrow 26

26. v.) IR Active Vibrations: - In order for molecule to absorb IR radiation: vibration at same frequency as in light but also , must have a change in its net dipole moment as a result of the vibration Examples: 1) CO 2 : 3(3)-5 = 4 modes - - + m = 0; IR inactive m > 0; IR active m > 0; IR active m > 0; IR active d - d - 2d + d - d - 2d + d - d - 2d + d - d - 2d + degenerate –identical energy single IR peak IITE : Nurturing teachers of tomorrow 27

27. Fewer and more experimental peaks than calculated Fewer peaks Symmetry of the molecule (inactive) Energies of two or more vibrations are identical or nearly identical Undetectable low absorption intensity Out of the instrumental detection range More peaks Overtone Combination bands 28 IITE : Nurturing teachers of tomorrow

28. Fundamental Peaks and Overtones Fundamental transition: The excitation from the ground state ʋ to the first excited state ʋ 1 is called the fundamental transition. It is the most likely transition to occur. Fundamental absorption bands are strong compared with other bands that may appear in the spectrum due to overtone, combination, and difference bands. Overtone bands: Result from excitation from the ground state to higher energy states ʋ 2 , ʋ 3 and so on. These absorptions occur at approximately integral multiples of the frequency of the fundamental absorption. If the fundamental absorption occurs at frequency ʋ, the overtones will appear at about 2ʋ , 3ʋ and so on. Overtones are weak bands and may not be observed in all samples/analysis 29 IITE : Nurturing teachers of tomorrow

29. Coupling modes: Vibrating atoms may interact with each other. Two vibrational frequencies may couple to produce a new frequency  3 =  1 +  2 . The band at  3 is called a combination band . If two frequencies couple such that  3 =  1 -  2 , the band is called a difference band . Not all possible combinations and differences occur. 30 IITE : Nurturing teachers of tomorrow

30. Rules for Vibrational coupling Coupling of different vibrations shifts frequencies Energy of a vibration is influenced by coupling Coupling is likely when common atom in stretching modes common bond in bending modes common bond in bending + stretching modes similar vibrational frequencies Coupling is not likely when atoms separated by two or more bonds symmetry inappropriate 31 IITE : Nurturing teachers of tomorrow

31. Fermi Resonance 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. The wave functions for the two resonant vibrations mix according to the harmonic oscillator approximation, and the result is a shift in frequency and a change in intensity in the spectrum. As a result, two strong bands are observed in the spectrum, instead of the expected strong and weak bands. It is not possible to determine the contribution from each vibration because of the resulting mixed wave function. 32 IITE : Nurturing teachers of tomorrow

32. Fermi Resonance… Because the vibrations have nearly the same frequency, the interaction will be affected if one mode undergoes a frequency shift from deuteration or a solvent effect while the other does not . The molecule most studied for this type of resonance (even what Fermi himself used to explain this phenomena), is carbon dioxide, CO 2 . Another typical example of Fermi resonance is found in the vibrational spectra of benzoyl chloride shown partly in the C=O region displays two bands in carbonyl region at 1790 cm -1 and 1745 cm -1 . if this were an unknown compound, one might be misled to conclude that the compound has two non adjacent carbonyl groups in the molecule. However the lower frequency band is in fact, due to the overtone of the CH bending mode at 875 cm -1 in fermi resonance with the C=O stretching fundamental frequency. 33 IITE : Nurturing teachers of tomorrow

33. 34 IITE : Nurturing teachers of tomorrow Instrumentation

34. IITE : Nurturing teachers of tomorrow 35 Components of IR Instruments Light source Sampling techniques (solid, liquid and gaseous samples) Monochromator Detector and recording devices

35. Instrumentation 1.) Conventional/Dispersive Instruments - normal IR instrument similar to UV-vis - main differences are light source & detector IITE : Nurturing teachers of tomorrow 36 Detector

36. i ) Light Source: - must produce IR radiation - can’t use glass since absorbs IR radiation - several possible types a ) Nernst Glower - rare earth metal oxides ( Zr , Ce, Th ) heated electrically - apply current to cylinder, has resistance to current flow generates heat (1200 o – 2200 o C ). - causes light production similar to blackbody radiation - range of use ~ 670 – 10,000cm -1 - need good current control or overheats and damaged b) Globar - similar to Nernst Glower but uses silicon carbide rod instead of rare earth oxides - similar usable range V Zr , Ce, Th IITE : Nurturing teachers of tomorrow 37

37. c) CO 2 Laser - CO 2 laser gas mixture consists of 70% He, 15% CO 2 , and 15% N 2 - a voltage is placed across the gas, exciting N 2 to lowest vibrational levels.   - the excited N 2 populate the asymmetric vibrational states in the CO 2 through collisions . - infrared output of the laser is the result of transitions between rotational states of the CO 2 molecule of the first asymmetric vibrational mode to rotational states of both the first symmetric stretch mode and the second bending mode - small range but can choose which band used & many compounds have IR absorbance in this region d) Others - mercury arc (l > 50 mm) (far IR) - tungsten lamp (4000 -12,800cm -1 ) (near IR) IITE : Nurturing teachers of tomorrow 38

38. ii) Sample Cell - must be made of IR transparent material ( KBr pellets or NaCl) NaCl plates IITE : Nurturing teachers of tomorrow 39

39. IR spectroscopy is used for the characterization of solid, liquid or gas samples. Material containing sample must be transparent to the IR radiation. So, the salts like NaCl , KBr are only used. 1.    Sampling of solids Various techniques used for preparing solid samples are as follows a) Mull  technique:  In this technique, the finely crushed sample is mixed with Nujol (mulling agent) in n a marble or agate mortar, with a pestle to make a thick paste. A thin film is applied onto the salt plates. This is then mounted in a path of IR beam and the spectrum is recorded.   b) Pressed pellet technique –  In this technique, a small amount of finely ground solid sample is mixed with 100 times its weight of potassium bromide and compressed into a thin transparent pellet using a hydraulic press. These pellets are transparent to IR radiation and it is used for analysis . c )   Thin(Case) film technique –  If the solid is amorphous in nature then the sample is deposited on the surface of a KBr or NaCl cell by evaporation of a solution of the solid and ensured that the film is not too thick to pass the radiation.

40.   2. Sampling of liquids Liquid sample cells can be sandwiched using liquid sample cells of highly purified alkali halides, normally NaCl . Other salts such as KBr and CaF 2  can also be used. Aqueous solvents cannot be used because they cannot dissolve alkali halides. Organic solvents like chloroform can be used. The sample thickness should be selected so that the transmittance lies between 15-20%. For most liquids, the sample cell thickness is 0.01-0.05 mm. Some salt plates are highly soluble in water, so the sample and washing reagents must be  anhydrous 3.   Sampling of gases The sample cell is made up of NaCl , KBr etc. and it is similar to the liquid sample cell. A sample cell with a long path length (5 – 10 cm) is needed because the gases show relatively weak absorbance. 

41. iii ) monochromator a. Prism b. Grating -Reflective grating is common -can’t use glass prism, since absorbs IR IITE : Nurturing teachers of tomorrow 42

42. Grating Monochromators: Grating monochromators disperse ultraviolet, visible, and infrared radiation typically using replica gratings, which are manufactured from a master grating. A master grating consists of a hard, optically flat, surface that has a large number of parallel and closely spaced grooves. The construction of a master grating is a long, expensive process because the grooves must be of identical size, exactly parallel, and equally spaced over the length of the grating (3–10 cm ). A grating for the ultraviolet and visible region typically has 300–2000 grooves/mm, however 1200–1400 grooves/mm is most common. For the infrared region, gratings usually have 10–200 grooves/mm.

43. iv) Detectors: - two types of detectors mainly used in common IR instruments a) Thermal Detectors 1.) Thermocouple - two pieces of dissimilar metals fused together at the ends - when heated, metals heat at different rates - potential difference is created between two metals that varies with their difference in temperature - usually made with blackened surface (to improve heat absorption) - placed in evacuated tube with window transparent to IR (not glass or quartz) - IR “hits” and heats one of the two wires. - can use several thermocouples to increase sensitivity. V - + h n metal 1 metal 2 IR transparent material (NaCl) IITE : Nurturing teachers of tomorrow 44

44. 2.) Bolometer - strips of metal (Pt, Ni) or semiconductor that has a large change in resistance to current with temperature. - as light is absorbed by blackened surface, resistance increases and current decreases - very sensitive i A h n b) Photoconducting Detectors - thin film of semiconductor (ex. PbS ) on a nonconducting glass surface and sealed in a vacuum. - absorption of light by semiconductor switches it to conducting state from its non-conducting state - decrease in resistance  increase in current vacuum Transparent to IR glass semiconductor h n IITE : Nurturing teachers of tomorrow 45

45. c) Pyroelectric Detectors - pyroelectric (ceramic, lithium tantalate ) material get polarized (separation of (+) and (-) charges) in presence of electric field. - temperature dependent polarization - measure degree of polarization related to temperature of crystal - fast response, good for FTIR IITE : Nurturing teachers of tomorrow 46

46. v) Overall Instrument Design Need chopper to discriminate source light from background IR radiation Monochromator after sample cell Not done in UV-Vis since letting in all h υ to sample may cause photo degradation (too much energy) IR lower energy Advantage that allows monochromator to be used to screen out more background IR light. Problems: Source weak , need long scans Detector response slow – rounded peaks IITE : Nurturing teachers of tomorrow 47

47. IITE : Nurturing teachers of tomorrow 48 Calibration of Infrared Spectrophotometers : The wavelength (or wave number) scale calibration of infrared spectrophotometers is usually carried out with the aid of a strip of polystyrene film fixed on a frame . It consists of several sharp absorption bands, the wavelengths of which are known accurately and precisely . Basically , all IR-spectrophotometers need to be calibrated periodically as per the specific instructions so as to ascertain their accuracy and precision .

48. vi) Fourier Transfer IR (FTIR) – alternative to Normal IR - Based on Michelson Interferometer Principle: light from source is split by central mirror into 2 beams of equal intensity beams go to two other mirrors, reflected by central mirror, recombine and pass through sample to detector 3 ) two side mirrors.one is fixed and other movable a ) move second mirror, light in two-paths travel different distances before recombined b ) constructive & destructive interference c ) as mirror is moved, get a change in signal IITE : Nurturing teachers of tomorrow 49

49. vi.) Fourier Transfer IR (FTIR) – alternative to Normal IR - Based on Michelson Interferometer IITE : Nurturing teachers of tomorrow 50

50. Destructive Interference can be created when two waves from the same source travel different paths to get to a point. This may cause a difference in the phase between the two waves. If the paths differ by an integer multiple of a wavelength, the waves will also be in phase. If the waves differ by an odd multiple of half a wave then the waves will be 180 degrees out of phase and cancel out. Remember IITE : Nurturing teachers of tomorrow 51

51. observe a plot of Intensity vs. Distance ( interferograms ) convert to plot of Intensity vs. Frequency by doing a Fourier Transform resolution Dn = 1/ Dd (interval of distance traveled by mirror) IITE : Nurturing teachers of tomorrow 52

52. Advantages of FTIR compared to Normal IR : 1) much faster, seconds vs. minutes 2) use signal averaging to increase signal-to-noise* (S/N ) (*Signal-to-noise ratio  (abbreviated  SNR  or  S/N ) is a measure used in science and engineering  that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in decibels. A ratio higher than 1:1 (greater than 0 dB) indicates more signal than noise.) 3) higher inherent S/N – no slits, less optical equipment, higher light intensity 4) high resolution (<0.1 cm -1 ) Disadvantages of FTIR compared to Normal IR : 1) single-beam, requires collecting blank 2) can’t use thermal detectors – too slow In normal IR, scan through frequency range. FTIR collects all frequencies at once. IITE : Nurturing teachers of tomorrow 53

53. 1.Better sensitivity and brightness - Allows simultaneous measurement over the entire wavenumber range - Requires no slit device, making good use of the available beam 2.High wavenumber accuracy - Technique allows high speed sampling with the aid of laser light interference f ringes - Requires no wavenumber correction - Provides wavenumber to an accuracy of 0.01 cm -1 3. Resolution - Provides spectra of high resolution 4. Stray light - Fourier Transform allows only interference signals to contribute to spectrum. B ackground light effects greatly lowers. - Allows selective handling of signals limiting intreference 5. Wavenumber range flexibility - Simple to alter the instrument wavenumber range - CO 2 and H 2 O sensitive FT-IR Advantages IITE : Nurturing teachers of tomorrow 54

54. Specific FT-IR Advantages Fellget's (multiplex) Advantage There are three principal advantages for an FT spectrometer compared to a scanning (dispersive) spectrometer The multiplex or Fellget’s advantage. This arises from the fact that information from all wavelengths is collected simultaneously. It results in a higher signal to noise for a given scan-time for observations limited by a fixed detector noise contribution (typically in the thermal infrared spectral region where a photodetector is limited by noise IITE : Nurturing teachers of tomorrow 55

55. Specific FT-IR Advantages Connes Advantage The wavelength accuracy or Connes ' advantage. The wavelength scale is calibrated by a laser beam( He-Ne laser ) of known wavelength that passes through the interferometer . This is much more stable and accurate than in dispersive instruments where the scale depends on the mechanical movement of diffraction gratings. In practice, the accuracy is limited by the divergence(deflection/difference) of the beam in the interferometer which depends on the resolution . Therefore, Wavelength accuracy is much higher than dispersive IR Instrument and thus gives precise information about absorption IITE : Nurturing teachers of tomorrow 56

56. Specific FT-IR Advantages Jacquinot Advantage The throughput or Jacquinot's advantage. For a given resolution and wavelength, this circular aperture allows more light through than a slit, resulting in a higher signal-to-noise ratio This results from the fact that in a dispersive instrument, the monochromator  has entrance and exit slits which restrict the amount of light that passes through it. The interferometer throughput is determined only by the diameter of the collimated beam coming from the source. Although no slits are needed, FTIR spectrometers do require an aperture to restrict the convergence(Union of wavelength) of the collimated beam in the interferometer. This is because convergent rays are modulated at different frequencies as the path difference is varied. Such an aperture is called a Jacquinot stop . IITE : Nurturing teachers of tomorrow 57

57. IITE : Nurturing teachers of tomorrow 58 Applications

58. Bond Type of Compound Frequency(W.N.) Range, cm -1 Intensity C-H Alkanes 2850-2970 Strong (s) C-H Alkenes 3010-3095 675-995 Medium (m) Strong C-H Alkynes 3300 Strong C-H Aromatic rings 3010-3100 690-900 Medium strong o -H Monomeric alcohols, phenols Hydrogen-bonded alchohols, phenols Monomeric carboxylic acids Hydrogen-bonded carboxylic acids 3590-3650 3200-3600 3500-3650 2500-2700 Variable Variable, sometimes broad Medium Broad (Br.) N-H Amines, amides 3300-3500 medium C=C Alkenes 1610-1680 Variable (v) C=C Aromatic rings 1500-1600 Variable Alkynes 2100-2260 Variable C-N Amines, amides 1180-1360 Strong Nitriles 2210-2280 Strong C-O Alcohols, ethers,carboxylic acids, esters 1050-1300 Strong C=O Aldehydes, ketones, carboxylic acids, esters, acid chloride, Amides 1590-1800 Strong NO 2 Nitro compounds 1500-1570 1300-1370 Strong Abbreviated Table of Group Frequencies for Organic Groups IITE : Nurturing teachers of tomorrow 59

59. Fingerprint Region ( 1500-900 cm -1 ) - region of most single bond signals - many have similar frequencies, so affect each other & give pattern characteristics of overall skeletal structure of a compound - exact interpretation of this region of spectra seldom possible because of complexity - complexity  uniqueness Fingerprint Region IITE : Nurturing teachers of tomorrow 60

60. Computer Searches - many modern instruments have reference IR spectra on file (~100,000 compounds) - matches based on location of strongest band, then 2 nd strongest band, etc overall skeletal structure of a compound - exact interpretation of this region of spectra seldom possible because of complexity - complexity  uniqueness Bio-Rad SearchIT database of ~200,000 IR spectra IITE : Nurturing teachers of tomorrow 61

61. 2.) Quantitative Analysis - not as good as UV/Vis in terms of accuracy and precision ► more complex spectra ► narrower bands (Beer’s Law deviation) ► limitations of IR instruments (lower light throughput, weaker detectors) ► high background IR ► difficult to match reference and sample cells ► changes in e (A= e bc) common - potential advantage is good selectivity, since so many compounds have different IR spectra ► one common application is determination of air contaminants. Contaminants Concn, ppm Found, ppm Relative error, % Carbon Monoxide 50 49.1 1.8 Methylethyl ketone 100 98.3 1.7 Methyl alcohol 100 99.0 1.0 Ethylene oxide 50 49.9 0.2 chloroform 100 99.5 0.5 IITE : Nurturing teachers of tomorrow 62

62. Example : The spectrum is for a substance with an empirical formula of C 3 H 5 N. What is the compound? Nitrile or alkyne group No aromatics Aliphatic hydrogens & No Aromatic character Alkane Bending (Deformation) IITE : Nurturing teachers of tomorrow 63

63. Vibrational Group Analysis the four primary regions of the IR spectrum 4000 cm -1 2700 cm -1 2000 cm -1 1500 cm -1 600 cm -1 O-H N-H C-H C≡C C≡N C=O C=N C=C Fingerprint Region C-C C-N C-O Vibrational Spectroscopy X-H bonds Triple bonds Double bonds Single Bonds IITE : Nurturing teachers of tomorrow 64

64. Alkanes – combination of C-H and C-C bonds sp 3 C-H between 2800-3000 cm -1 C-C stretches and bends 1360-1470 cm -1 CH 2 -CH 2 bond 1450-1470 cm -1 CH 2 -CH 3 bond 1360-1390 cm -1 Vibrational Spectroscopy Octane (w – s) (m) IITE : Nurturing teachers of tomorrow 65

65. Alkenes – addition of the C=C and vinyl C-H bonds C=C stretch at 1620-1680 cm -1 weaker as substitution increases vinyl C-H stretch occurs at 3000-3100 cm -1 The difference between alkane, alkene or alkyne C-H is important! If the band is slightly above 3000 it is vinyl sp 2 C-H or alkynyl sp C-H, if it is below, it is alkyl sp 3 C-H 1-Octene Vibrational Spectroscopy (w – m) (w – m) IITE : Nurturing teachers of tomorrow 66

66. Alkynes – addition of the C=C and vinyl C-H bonds C≡C stretch 2100-2260 cm -1 ; strength depends on asymmetry of bond, strongest for terminal alkynes, weakest for symmetrical internal alkynes C-H for terminal alkynes occurs at 3300-3320 cm -1 Internal alkynes ( R-C ≡ C-R ) would not have this band! 1-Octyne Vibrational Spectroscopy (m – s) (w-m) IITE : Nurturing teachers of tomorrow 67

67. Hexane Hexene Hexyne

68. Aromatics Due to the delocalization of e - in the ring, C-C bond order is 1.5, the stretching frequency for these bonds is slightly lower in energy than normal C=C These show up as a pair of sharp bands, 1500 & 1600 cm -1 , (lower frequency band is stronger) C-H bonds off the ring show up similar to vinyl C-H at 3000-3100 cm -1 Ethyl benzene Vibrational Spectroscopy (w – m) (w – m) IITE : Nurturing teachers of tomorrow 69

69. Unsaturated Systems – substitution patterns The substitution of aromatics and alkenes can also be discerned(Recognized) through the out-of-plane ( oop )bending vibration region However, other peaks often are apparent in this region. These peaks should only be used for reinforcement of what is known or for hypothesizing as to the functional pattern. Vibrational Spectroscopy IITE : Nurturing teachers of tomorrow 70

70. Ethers – addition of the C-O-C asymmetric band and vinyl C-H bonds Show a strong band for the antisymmetric C-O-C stretch at 1050-1150 cm -1 Otherwise, dominated by the hydrocarbon component of the rest of the molecule Diisopropyl ether Vibrational Spectroscopy (s) IITE : Nurturing teachers of tomorrow 71

71. Alcohols Strong , broad O-H stretch from 3200-3400 cm -1 Like ethers, C-O stretch from 1050-1260 cm -1 Band position changes depending on the alcohols substitution: 1 ° 1075-1000; 2 ° 1075-1150; 3 ° 1100-1200; phenol 1180-1260 The shape is due to the presence of hydrogen bonding 1-butanol Vibrational Spectroscopy (m– s) br (s) IITE : Nurturing teachers of tomorrow 72

72. Amines - Primary Shows the –N-H stretch for NH 2 as a doublet between 3200-3500 cm -1 (symmetric and anti-symmetric modes) -NH 2 has deformation band from 1590-1650 cm -1 Additionally there is a “wag” band at 780-820 cm -1 that is not diagnostic 2-aminopentane Vibrational Spectroscopy (w) (w) IITE : Nurturing teachers of tomorrow 73

73. Amines – Secondary N-H band for R 2 N-H occurs at 3200-3500 cm -1 as a single sharp peak weaker than –O-H Tertiary amines (R 3 N) have no N-H bond and will not have a band in this region pyrrolidine Vibrational Spectroscopy (w – m) IITE : Nurturing teachers of tomorrow 74

74. Vibrational Spectroscopy Pause and Review Inspect the bonds to H region (2700 – 4000 cm -1 ) Peaks from 2850-3000 are simply sp 3 C-H in most organic molecules Above 3000 cm -1 Learn shapes, not wavenumbers ! : Broad U-shape peak - O—H bond V-shape peak -N—H bond for 2 o amine (R 2 N—H ) Sharp spike -C ≡ C—H bond W-shape peak -N—H bond for 1 o amine (R NH 2 ) 3000 cm -1 Small peak shouldered just above 3000 cm -1 C= C—H or Ph —H IITE : Nurturing teachers of tomorrow 75

75. Aldehydes C=O (carbonyl) stretch from 1720-1740 cm -1 Band is sensitive to conjugation, as are all carbonyls (upcoming slide ) A highly unique sp 2 C-H stretch appears as a doublet, 2720 & 2820 cm -1 called a “ Fermi doublet ” Cyclohexyl carboxaldehyde Vibrational Spectroscopy (w-m) (s) IITE : Nurturing teachers of tomorrow 76

76. Ketones Simplest of the carbonyl compounds as far as IR spectrum – carbonyl only C=O stretch occurs at 1705-1725 cm -1 3-methyl-2-pentanone Vibrational Spectroscopy (s) IITE : Nurturing teachers of tomorrow 77

77. Esters C=O stretch at 1735-1750 cm -1 Strong band for C-O at a higher frequency than ethers or alcohols at 1150-1250 cm -1 Vibrational Spectroscopy Ethyl pivalate (s) (s) IITE : Nurturing teachers of tomorrow 78

78. Carboxylic Acids: Gives the messiest of IR spectra C=O band occurs between 1700-1725 cm -1 The highly dissociated O-H bond has a broad band from 2400-3500 cm -1 covering up to half the IR spectrum in some cases 4-phenylbutyric acid Vibrational Spectroscopy (w – m) br (s) (s) IITE : Nurturing teachers of tomorrow 79

79. Acid anhydrides Coupling of the anhydride though the ether oxygen splits the carbonyl band into two with a separation of 70 cm -1 Bands are at 1740-1770 cm-1 and 1810-1840 cm -1 Mixed mode C-O stretch at 1000-1100 cm -1 Propionic anhydride Vibrational Spectroscopy (s) (s) IITE : Nurturing teachers of tomorrow 80

80. Acid halides Clefted band at 1770-1820 cm -1 for C=O Bonds to halogens, due to their size (see Hooke’s Law derivation) occur at low frequencies, only Cl is light enough to have a band on IR, C- Cl is at 600-800 cm -1 Propionyl chloride Vibrational Spectroscopy (s) (s) IITE : Nurturing teachers of tomorrow 81

81. Amides Display features of amines and carbonyl compounds C=O stretch at 1640-1680 cm -1 If the amide is primary (-NH 2 ) the N-H stretch occurs from 3200-3500 cm -1 as a doublet If the amide is secondary (-NHR) the N-H stretch occurs at 3200-3500 cm -1 as a sharp singlet pivalamide Vibrational Spectroscopy (m – s) (s) IITE : Nurturing teachers of tomorrow 82

82. Nitro group (-NO 2 ) Proper Lewis structure gives a bond order of 1.5 from nitrogen to each oxygen Two bands are seen (symmetric and asymmetric) at 1300-1380 cm -1 and 1500-1570 cm -1 This group is a strong resonance withdrawing group and is itself vulnerable to resonance effects 2-nitropropane Vibrational Spectroscopy (s) (s) IITE : Nurturing teachers of tomorrow 83

83. Nitriles (the cyano - or –C ≡ N group) Principle group is the carbon nitrogen triple bond at 2100-2280 cm -1 This peak is usually much more intense than that of the alkyne due to the electronegativity difference between carbon and nitrogen propionitrile Vibrational Spectroscopy (s) IITE : Nurturing teachers of tomorrow 84

84. Effects on IR bands Conjugation – by resonance, conjugation lowers the energy of a double or triple bond. The effect of this is readily observed in the IR spectrum: Conjugation will lower the observed IR band for a carbonyl from 20-40 cm -1 provided conjugation gives a strong resonance contributor Inductive effects are usually small, unless coupled with a resonance contributor (note –CH 3 and – Cl above) Vibrational Spectroscopy IITE : Nurturing teachers of tomorrow 85

85. Effects on IR bands Steric effects – usually not important in IR spectroscopy, unless they reduce the strength of a bond (usually p ) by interfering with proper orbital overlap: Here the methyl group in the structure at the right causes the carbonyl group to be slightly out of plane, interfering with resonance Strain effects – changes in bond angle forced by the constraints of a ring will cause a slight change in hybridization, and therefore, bond strength As bond angle decreases, carbon becomes more electronegative, as well as less sp 2 hybridized (bond angle < 120 ° ) Vibrational Spectroscopy IITE : Nurturing teachers of tomorrow 86

86. Effects on IR bands Hydrogen bonding Hydrogen bonding causes a broadening in the band due to the creation of a continuum of bond energies associated with it In the solution phase these effects are readily apparent; in the gas phase where these effects disappear or in lieu of steric effects, the band appears as sharp as all other IR bands: Gas phase spectrum of 1-butanol Steric hindrance to H-bonding in a di- tert -butylphenol H-bonding can interact with other functional groups to lower frequencies Vibrational Spectroscopy IITE : Nurturing teachers of tomorrow 87