IR SPECTROCOPY, Instrumentation of IR spectroscopy, Application of IR spectroscopy.pptx

IR Spectroscopy, Instrumentation of IR spectroscopy, Application of IR spectroscopy

1. IR SPECTROSCOPY By- Dipesh Adesh Gamare 1

2. CONTENT Introduction to IR spectroscopy Modes of molecular vibration Sample handling Instrumentation of Dispersive-IR and FTIR Factors affecting vibrational frequencies Applications of IR spectroscopy References 2

3. .0001-0.01nm .01-10nm 10 -1000nm 1000nm-0.01 c m 0.01-100 m WAVELENGTH 3

4. IR SPECTROCOPY : Infrared (IR) spectroscopy is based on IR absorption by molecules as undergo vibrational and rotational transitions. The IR spectrum is mainly used to determine the functional groups present in a compound and elucidate the structure of compound. The energy of a molecule in IR = Electronic energy + Vibrational energy + Rotational energy 4

5. 1. Near IR : carbohydrates and proteins 2. Middle IR : organic molecules - functional groups 3. Far IR : inorganic co-ordination bonds& quaternary ammonium compounds REGION WAVE LENGTH λ (μ m) WAVE NUMBER υ ( cm-1) FREQUENCY Hz RANGE NEAR 0.78 - 2.5 12800 - 4000 3.8x 10 14 -1.2 x 10 14 MIDDLE 2.5 - 50 4000 - 200 1.2x 10 14 – 6 x 10 12 FAR 50 - 1000 200 -10 6x 10 12 – 3 x 10 11 MOST USED 2.5 - 15 4000- 670 1.2x 10 14 - 2 x 10 13 Different compounds when absorb infrared radiation give rise to vibrational transitions characteristic to the molecular structure and those transitions are recorded as IR Spectra. Mid IR region is widely used in pharmaceutical studies. A compound can be analyzed by IR radiation only if: 1. There is a change in dipole movement. 2. There should be a resonance in applied and natural IR frequency. When we apply an external infrared frequency, and it matches the natural frequency of vibration within a molecule, the applied energy is then absorbed by the molecule and an infrared peak is observed. These peaks are characteristic to different functional group and parts of the molecule, and they are unique for every molecule. Hence the IR spectroscopy is considered as the fingerprint analysis of a molecule. PRINCIPLE OF IR SPECTROSCOPY 5

6. Fingerprint region (400-1500 cm −1 ): This region of IR spectrum provides characteristic peaks for different part of a molecule and helps in qualitative identification of compounds. Group frequency region (4000—1500 cm −1 ) : Peaks with respect to different functional groups are observed in this region . Characteristic IR frequencies of some known functional groups Typical IR spectrum depicting regions of absorption 6

7. 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 One or more alkane groups Propane nitrile 7

8. Types of Molecular Vibrations 1) Bond Stretching 2) Bond Bending symmetric asymmetric In-plane rocking In-plane scissoring Out-of-plane wagging Out-of-plane twisting Fundamental vibrations Non-fundamental vibrations Overtones Combination tones Fermi resonance 8

9. Fundamental v ibrations 1 . Bond Stretching vibration : In stretching vibration, the length of a bond is changed i.e. either increased or decreased. It is of two types: A] Symmetrical stretching : Here the two bonds increase or decreases in length in a symmetric pattern. B] Asymmetrical stretching : Here when one bond length increases the other one decreases. 2. Bending vibration : Bending vibrations are of two sub-types. 1) In plane A] R ocking : The atoms swing back and forth concerning the central atom of a molecule Two hydrogen atoms of a methylene molecule move outward simultaneously and inward simultaneously  B] Scissoring :  The two atoms approach each other and go away from each other. 2) Out of the plane A] Wagging : Such type of vibration occurs when both the atoms of a molecule move one side of the plane B] Twisting : In twisting, one molecule is above the plane and the other molecule is below the plane. Non-fundamental vibrations : O vertone vibrations occur at twice or thrice the value of fundamental vibrations. Combination bands are the bands that result because of coupling of fundamental f requencies. Fermi resonance bands are produced because of coupling between fundamental vibration and an overtone or because of a combination band. 9

10. SAMPLE HANDLING The sample cells are usually made up of transparent ionic substances such as sodium chloride (NaCl) or potassium bromide (KBr). However, KBr is preferred because of its compatibility with the instrumental measurement process. The infrared samples can be considered of three forms such as: 1) Liquids sampling : A drop of liquid is placed within the KBr/ NaCl plates with the thickness less than 0.01 mm. Around 1-10mg of sample is required to prepare the solution. Aqueous solutions are not preferred as they tend to dissolve the sample cells so, the solvent should be anhydrous and mostly organic solvents such as chloroform is preferred. 2) Solid sampling : Pelletization technique The solid samples can be prepared using pressed pellet technique where the sample is placed in sample holder and KBr pellets are prepared in form of thin transparent layer using a hydraulic press. However, as KBr is hygroscopic in nature care should be taken to minimize its atmospheric exposure. 10

11. 3) Solution sampling technique To prepare a solid solution 1 to 10 mg volumes of 0.1 to 1.0 ml of 0.05 to 10% solution are required for placement within a cell of 0.1 to 1mm thickness. So lvents used are CHCl ₃ , CS 2   and CCl ₄ is used for this purpose which absorbs strongly at 785 cm -1 . Care should be taken to avoid solution combinations that react instantaneously. 4) Semisoli d sample preparation Another technique called as Nujol -Mull technique (popularly called as Mull Technique ) may be used where the mixing up of the sample with a mineral oil ( Nujol ) ( liquid paraffin hydrocarbon) is done and afterwards a thin film of the liquid can be applied on the liquid sample cell for measurements. 5)Gas sampling Gas samples are enclosed in a gas cell with windows, which seal the gas cell and allow the infrared beam to pass into the cell. These cells are made up of KBr, NaCl and so on. These cells are about 10cm long and can increased its path length upto 40cm by multiple reflections inside it and gas sample can be analyzed easily. 11

12. INSTRUMENTATION The instrumentation of IR spectrophotometer is as follows IR radiation source Monochromators / wavelength selector Sample holder Detector Recorder Diagrammatic representation of Dispersive-IR spectrophotometer Diagrammatic representation of Fourier Transorm -IR spectrophotometer 12

13. FTIR- IRSpirit FTIR Shimadzu Dispersive IR- IRTracer-100 Shimadzu 13

14. 1) IR radiation source : 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 14

15. c) Incandescent Wire Source Tightly wound nichrome or rhodium wire that is electrically heated. Same principal as Nernst Glower. Lower intensity then Nernst Glower or Globar, but longer lifetime. d) 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 . Gives off band of ~ 100 cm -1 in range of 900-1100 cm -1 . Small range but can choose which band used & many compounds have IR absorbance in this region. Much more intense than Blackbody sources. e) Others 1) Mercury arc (far IR) – when continues radiation source is required. 2) Tungsten lamp (4000 -12,800cm -1 ) (near IR) 15

16. IR radiation sources overview 16

17. 2) WAVELENGTH SELECTORS / MONOCHROMATORS They help in selecting a continuous IR radiation in the desired wavelength region. They generally contain a chopper and a complex system of monochromators. The choppers are generally used to enhance the signal to noise ratio of the instrument, and they also moderate the intensity of radiation that reaches to the detector. The monochromator is used to select it desired frequency of radiation which will be then allowed to incident on the detector. These monochromators are of two types: 1. Prismatic monochromator : They are made up of glass/quartz and coated with alkyl halides. A] Mono Pass (Radiation passes only once through the prism) B] Double Pass (Radiation passes twice through the prism) 2. Grating monochromator : They are grooves made up of aluminium and provide better dispersion of radiation than prisms. A] Reflection gratings B] Transmittance grating 17

18. 3) SAMPLE HOLDER/SAMPLE CELL There is rugged window material for cuvette or sample holder which is transparent and also inert over this region. AgCl cell are used for aqueous and moist samples, but it is soft and easily gets deformed and darkens on exposure to visible light. The alkali halides are widely used, particularly NaCl, KBr and ThBr which is transparent at wavelength as long as 625 cm - 1. KBr pellet (For solid sample) NaCl windows ( For liquid sample) 4) DETECTOR Thermocouple Bolometer Photoconducting cell Pyroelectric detector Golay cell Thermistor 18

19. 1.Thermocouple T wo pieces of dissimilar metals fused together at the ends. W hen heated, metals heat at different rates. P otential difference is created between two metals that varies with their difference in temperature U sually made with blackened surface (to improve heat absorption) P laced in evacuated tube with window transparent to IR (not glass or quartz) IR “hits” and heats one of the two wires. C an use several thermocouples to increase sensitivity. 2. Bolometer S trips of metal (Pt, Ni) or semiconductor that has a large change in resistance to current with temperature. A s light is absorbed by blackened surface, resistance increases and current decreases. V ery sensitive. 3. Photoconducting cell T hin film of semiconductor (ex. PbS ) on a nonconducting glass surface and sealed in a vacuum. A bsorption of light by semiconductor moves from non-conducting to conducting state. D ecrease in resistance  increase in current. R ange: 10,000 -333 cm -1 at room temperature. V - + h n metal 1 metal 2 IR transparent material (NaCl) A h n glass semiconductor h n vacuum Transparent to IR 19

20. 4. Pyroelectric Detectors P yroelectric (ceramic, lithium tantalate) material get polarized (separation of (+) and (-) charges) in presence of electric field. T emperature dependent polarization M easure degree of polarization related to temperature of crystal Fa st response, good for FTIR 5. Golay cell It contains a small metal cylinder which is closed by black metal plate at end and a flexible metal die programmed at the other end. The cylinder is filled with xenon and sealed. When IR radiation is allowed to fall on the black metal plate it heats the gas which expands it. The signal detected by the photo tube is then modulated according to the power of radiant beam. 6. Thermistor Thermistors are usually made up of fused mixture of metallic oxides. As the temperature of the mixture increases its electrical resistance decreases and accordingly the measurements are performed in the higher spectrophotometer. 5) RECORDER OR READ OUT UNIT The signals collected from detector are amplify and recorded by device in terms of IR spectra indicating various peaks of functional group present. 20

21. F ACTORS AFFECTING VIBRATIONAL FREQUENCIES A] Coupled vibration An isolated C-H bond has only one stretching vibration frequency whereas methylene group shows two stretching vibrations both symmetrical and asymmetrical. Because of mechanical coupling or interaction between CH stretching vibrations in the CH2 group. Asymmetric vibrations develop at higher frequency or wave numbers than stretching vibrations. Such vibrations are called coupled vibrations because these vibrations occur at different frequency then the required for an isolated CH stretching B] Fermi resonance Coupling of two fundamental vibration modes produces 2 new modes of vibration. With frequencies higher and lower than the observed in absence of interaction. This interaction takes place between fundamental vibrations and overtones, or combination tone vibration and they are collectively called us Fermi resonance. In Fermi resonance molecule transfer energy from fundamental vibrations into overtone or combination bands. The resonance pushes the two levels apart and mixes their character and each level has some amount of fundamental and some amount of overtone or combination tone characters. 21

22. F ACTORS AFFECTING VIBRATIONAL FREQUENCIES C] Electronic effects Changing the absorption frequencies for a particular group takes place when the substituent in the neighborhood of the group is changed. They include Inductive effect : Inclusion of alkyl group gives + ve Inductive effect producing decreased absorption frequency. Adding an electronegative atom or group produces – ve inductive effect leading to increased absorption frequency. Mesomeric effect : It produces lengthening or weakening of a bond leading to lowering of absorption frequency. Field effect : In ortho substituted compounds, the lone pair of electrons on two atoms influence each other through space interactions and change the vibrational frequencies of both the groups. Example: Ortho halo acetophenone. This is called field effect. D] Hydrogen bonding It occurs in any system having a proton donor group and a proton acceptor. If the S orbital of the proton effectively overlaps the P or 𝜋 orbital of the acceptor group. The stronger the hydrogen bond, the longer the OH bond, the lower the vibration frequency, broader and more intense will be the absorption. I ntermolecular bonds produce broad bands whereas intramolecular hydrogen bonds produce sharp and well-defined bands. 22

23. IR spectroscopy is applied for qualitative as well as quantitative analysis of drugs in pharmaceutical industry. It is used for identification of drug substances It identifies the impurities present in a drug sample It helps in study of hydrogen bonding both intermolecular and intramolecular type. It is widely used in study of polymers. It elucidates reaction mechanisms. It identifies functional groups present in any sample. It estimates stability of formulation. It distinguishes between Cis and Trans isomers. It can predict the keto-enol tautomerism. APPLICATIONS OF IR SPECTROSCOPY 23

24. Chatwal, G.R. and Anand, S.K.J. (2018) Instrumental Methods of Chemical Analysis. Himalaya Publishing House, Mumbai, 2.3-2.82. Marcelli , A., Cricenti , A., Kwiatek , W. M., & Petibois , C. (2012). Biological applications of synchrotron radiation infrared spectromicroscopy . Biotechnology Advances, 30(6), 1390–1404. https://doi.org/10.1016/j.biotechadv.2012.02.012 . Basa’ar , O., Fatema, S., Alrabie , A., Mohsin, M., & Farooqui, M. (2017) . Supercritical carbon dioxide extraction of Triognella foenum graecum Linn seeds: Determination of bioactive compounds and pharmacological analysis. Asian Pacific Journal of Tropical Biomedicine, 7(12), 1085–1091. https://doi.org/10.1016/j.apjtb.2017.10.010 IRTracer-100. (2022). https://www.shimadzu.com/an/products/molecular-spectroscopy/ftir/ftir-spectroscopy/irtracer-100/index.html Jasco. (2020, October 6). Principles of infrared spectroscopy (1) Molecular vibrations and infrared absorption | JASCO Global. JASCO Inc. https://www.jasco-global.com/principle/principles-of-infrared-spectroscopy-1-molecular-vibrations-and-infrared-absorption/ Libretexts . (2020, August 5) Information Obtained from IR Spectra. Chemistry LibreTexts . https://chem.libretexts.org/Ancillary_Materials/Laboratory_Experiments/Wet_Lab_Experiments/Organic_Chemistry_Labs/Lab_I/10%3A_Infrared_Spectroscopy/10.06%3A_Information_Obtained_from_IR_Spectra REFERENCES 24

25. THANK YOU 25