Infrared spectroscopy

Infrared spectroscopy

1. Infrared spectroscopy Ayesha shafi

2. Objectives Introduction Principle of IR Spectroscopy Theory of IR Spectroscopy Molecular vibrations Instrumentation of IR Spectroscopy Applications of IR Spectroscopy

3. Infrared spectroscopy It deals with the absorption of electromagnetic radiations in the infra red region. Infrared region ranges from 10,000-100 cm -1 .This region is categorized as follows; Near IR: 0.8-2.5 µm(12,500-4000 cm -1 ) Middle IR: 2.5-15 µm (4000-667 cm -1 ) Far IR: 15-200 µm (667-100 cm -1 ) Most of the analytical applications are confined to the middle IR region because absorption of organic molecules are high in this region.

4. Principle and Theory of IR spectroscopy Principle of IR spectroscopy: The IR radiations causes the excitation of the molecules from ground state to the higher vibrational energy states on absorption i.e. the absorption is due to change in vibrational state of a molecule that appear as a band in the IR spectrum. Theory of IR spectroscopy: IR spectrum is a graph between the absorbance and the wavelength or the wavenumber. A band is obtained in the spectrum as changes in the vibrational energy of the molecule are also associated by changes in the rotational energy . A molecule is not a rigid assembly of atoms. It is said to resemble a system of balls of varying masses and spring of varying strength, corresponds to chemical bond of a molecule .

5. Theory of Infrared Absorption Spectroscopy For a molecule to absorb IR, the vibrations or rotations within a molecule must cause a net change in the dipole moment of the molecule. If the frequency of the radiation matches the vibrational frequency of the molecule then radiation will be absorbed, causing a change in the amplitude of molecular vibration. Although the IR spectrum is characteristics of a whole molecule, certain functional groups give rise to absorption bands at the same frequency regardless of the structure of the rest of the molecules. Therefore IR spectroscopy useful for identification of functional groups.

6. Molecular vibrations There are two types of molecular vibrations. Fundamental (if vibration are associated with the functional group). Non fundamental Fundamental is further classified as Stretching vibrations Bending vibrations Stretching vibrations These occur along the bond axis, where interatomic distance increases or decreases but atomic position remain as such. I t require more energy as compare to bending due to shorter wavelength. Two types of stretching vibrations are possible – Symmetrical stretching Asymmetrical stretching

7. a) Symmetrical stretching: 2 bonds increase or decrease in length simultaneously. H H C

8. b) Asymmetrical stretching in this, one bond length is increased and other is decreased. H H C

9. 2. Bending vibrations It is rhythmical movement of atoms along the bond axis that cause the change in the bond angle. It require less energy due to longer wavelength. Occurs at low energy: 1400-666 cm -1 it is of 2 types: In plane bending: scissoring, rocking Out plane bending: wagging, twisting

10. a) In plane bending Scissoring: This is an in plane bending. Two atoms approach each other Bond angles are decrease. Rocking : Movement of atoms take place in the same direction. H H C C H H C C

11. b) Out plane bending i . Wagging: 2 atoms move to one side of the plane. They move up and down the plane. Twisting: One atom moves above the plane and another atom moves below the plane. H H C C H H C C

12. HOOKE’S LAW It gives the relation between frequency of oscillation , atomic mass , force constant of the bond . Thus vibrational frequency is = ½ π c √f/( MxMy )/( Mx+My ) C = velocity of light F = force constant Mx = mass of atom x My = mass of atom y

13. Non fundamental vibrations

14. Coupled interactions Interactions between vibrations can occur (Coupling) if the vibrating bonds are joined to a single, central atom. This is because there is mechanical coupling interaction between the oscillators. Example: C=O (both symmetric and asymmetric stretching vibrations ).

15. INSTRUMENTATION The main parts of IR spectrometer are as follows : radiation source S ample cell Monochromator D etectors R ecorder

16. INSTRUMENTATION SOURCE: Two types of the source is used. a ) G lobar b) Nernst glower Globar: a rod of silicon carbide is heated to approximately 1300 degree centigrade to produce the radiant energy from 1-40micron. Nernst glower: a rod of zirconium and yittrium is heated to approximately 1500 degree centigrade to produce the radiant energy from 0.4-20micron .

17. MONOCHROMATORS

18. INSTRUMENTATION Monochromator: it is same like UV spectrophotometer but it is used to isolate the IR radiation of specific wavelength. Sample cells: For gas samples: The spectrum of a gas can be obtained by permitting the sample to expand into an evacuated cell, also called a cuvette. For solution sample: Infrared solution cells consists of two windows of pressed salt sealed. Samples that are liquid at room temperature are usually analyzed in pure form or in solution. The most common solvents are Carbon Tetrachloride (CCl4) and Carbon Disulfide (CS2).

19. SAMPLING OF SOLIDS Generally 4 techniques are employed for preparing solid samples: Solids run in solution. Solid Films. Mull technique. Pressed pellet technique.

20. SOLIDS RUN IN SOLUTION Solids may be dissolved in non-aqueous inert solvent and a drop of this solution is placed on an alkali metal disc and solvent is allowed to evaporate, leaving a thin film of solute (or the entire solution is placed in a liquid sample cell) which is then mounted in spectrometer. If the solution of solid can be prepared in a suitable solvent then the solution is run in concentration of cells for liquids. Some solvents used are chloroform, carbon tetrachloride, acetone, Cyclohexane etc.

21. If a solid is polymer resins & amorphous solids, the sample is dissolved in any reasonable volatile solvent & this solution is poured on a rock salt plate (Nacl or KBr) & solvent is evaporated by gentle heating. If solid is non-crystalline, a thin homogenous film is deposited on the plate which can be mounted and scanned directly. Sometimes polymers can be “hot pressed” onto plates . SOLID FILMS

22. MULL TECHNIQUE In this technique a small quantity of sample is thoroughly ground in a clean mortar until the powder is very fine. After grinding, the mulling agent (mineral oil or Nujol) is introduced in small quantities just sufficient to take up the powder (mixture approximates the consistency of a toothpaste). The mixture is then transferred to the mull plates & the plates are squeezed together to adjust the thickness of the sample between IR transmitting windows. This is then mounted in a path of IR beam and the spectrum is run.

23. Pressed pellet technique In this technique a small amount of finely ground solid sample is intimately mixed with about 100 times its weight of powdered Potassium bromide, in a vibrating ball mill. This finely ground mixture is then pressed under very high pressure in evacuable die or minipress to form a small pellet (about 1-2 mm thick and 1cm in diameter). The resulting pellet is transparent to IR radiation and is run as such.

24. Preparing a KBr Disk

25. Pressed pellet technique (continue…) The powder (KBr + sample) is introduced in between the 2 bolts and the upper screw A is tightened until the powder is compressed to a thin disc. After compressing the sample bolts are removed and a steel cylinder with pellet inside it is placed in path of the beam of IR spectrometer and a blank KBr pellet of identical thickness is kept in the path of reference beam.

26. Advantages of this technique over mull technique The use of KBr eliminates the problem of bands which appear in IR spectrum due to the mulling agent as in this case no such bands appear. KBr pellets can be stored for longer periods of time. As concentration of the sample can be suitably adjusted in pellets, it can be used for quantitative analysis. The resolution of spectrum in KBr is superior to that obtained with mulls.

27. DETECTORS The measurement of infrared radiation is difficult as a result of low intensity of available source and low energy of the infrared photons. As a consequence of these properties, the electrical signal from the infrared detector is small and its measurement requires large amplification. Two types of IR detectors are used Thermal detector Non thermal detector

28. THERMAL DETECTOR

29. TYPES OF THERMAL DETECTOR There are four types of thermal detector. Bolometers Thermocouple and thermopile Pyro electric detector Golay cell

30. BOLOMETER Bolometer is derived from a Greek word (bolometron) Bolo = for something thrown Metron = measure A bolometer consists of an absorptive element, such as a thin layer of metal. Most bolometers use semiconductor or superconductor absorptive elements rather than metals . These material exhibit a relatively large change in resistance as a function of temperature.

31. Working

32. THERMOCOUPLE AND THERMOPILE Temperature changes Potential difference changes Thermocouples consist of a pair of junctions of different metals; for example, two pieces of bismuth fused to either end of a piece of antimony. Whenever IR radiation fall at the junction, it causes the change in temperature and also a change in the potential difference that causes the production of electrical signal.

33. Thermopile A thermopile is the combination of six thermocouples connected in series. Half the junction are considered as hot and other half as cold which are thermally bonded to the substrate and maintains the stable temperature. The entire assembly is mounted in an evacuated enclosure with an IR transmitting window made up of KBr so as to minimize the heat loss. Thermopiles are one of the most simple and direct means of converting radiant energy into an electrical energy.

34. Pyro electric Detector Single crystalline wafer of a pyro electric material, such as triglycerine sulphate. Pyro electric Infrared Detectors (PIR) convert the changes in incoming infrared light to electric signals.

35. GOLAY Cell It consist of a small metal cylinder closed by a rigid blackened metal plate at one end and flexible silvered diaphragm at other end. The whole chamber is filled with xenon gas. Exposure to IR radiation through a radiation transmitting window causes the gas to expand and diaphragm to deform. Motion of the diaphragm changes the output of cell and detected in the form of signal.

36. Non T hermal Detectors Photon detectors: P hotoelectric detectors consists of a semiconducting material deposited on a glass surface, sealed in an evacuated envelop. Absorption of an IR promotes the nonconducting valence electron to a higher, conducting state. The electrical resistance of the semiconductor decreases.

37. Ir spectrophotometer SINGLE BEAM SPCETROPHOTOMETER DOUBLE BEAM SPECTROPHOTOMETER A single beam of light , which can pass through one solution at a time (sample or reference). A single beam of light splits into two separate beams. One passes through the sample, another passes through the reference.

38. Working of double beam spectrophotometer The radiation from the source is divided into two parts. Reference beam Sample beam The reference beam is passed through an optical wedge which limits the amount of radiations that passes through it. As the chopper is rotated, it causes the sample beam and the reference beam to be reflected alternatively to the monochromator which sends individual frequencies to detector. The detector receives an intense beam (the reference and the sample beam). This will lead to an alternating current which is flow to the amplifier.

39. Working of double beam spectrophotometer

40. APPLICATION OF IR SPECTROPHOTOMETER IR spectroscopy is one of the most important and widely used analytical techniques available to scientists working in a whole range of fields. The fundamental region in the IR region is termed as the rock salt region. It is comprised of group frequency region and the finger print region.

41. FUNDAMENTAL REGION (ROCK SALT REGION ) Group Frequency Region Fingerprint Region consisting of the absorption bands of the functional groups. frequency = 4000-1300cm- ¹ wavelength = 2.5-8 IR spectra is called “fingerprints” because no other chemical species will have similar IR spectrum. Single bonds give their absorption bands in this region. Frequency=1300-650cm-1 Wavelength=8-15.4

42. Identification of Substances The finger print region is mostly used to compare the identity of a compound. Because s mall differences in structure & constitution of molecule result in significant changes in the peaks in this region. Hence this region helps to identify an unknown compound . Criteria : Sample and reference must be tested in identical conditions, like physical state, temperature, solvent, etc

43. Determination of Molecular Structure Used along with other spectroscopic techniques. Identification is done based on position of absorption bands in the spectrum. Eg : C=O at 1717 cm -1 . Absence of band of a particular group indicates absence of that group in the compd.

44. Detection of Impurities The presence of absorption bands at positions where compound is not expected to absorb indicates the presence of impurities. Determined by comparing sample spectrum with the spectrum of pure reference compound.

45. Studying the progress of reaction The progress of a reaction can be followed by examining the IR Spectra. Observing rate of disappearance of characteristic absorption band in reactants; or Rate of increasing absorption bands in products of a particular product. Example: Oxidation of secondary alcohol to ketone can be detected by disappearance of OH band and appearance of C=O band. O—H = 3600-3650 cm -1 C=O = 1680-1760 cm -1

46. Molecular shape or symmetry determination The shape or symmetry of a molecule can be determined by the IR spectroscopy. For example: If linear --> only 2 bands should be present. If bent --> 3 bands should be present. Actual spectrum shows 3 peaks at 750, 1323 and 1616 cm -1 .

47. Dipole Moment determination IR spectroscopy is also used to evaluate the dipole moment of the molecule and compounds.

48. FOURIER TRANSFORM IR SPECTROPHOTOMETER It is the most advance IR spectrophotometer . In this a laser beam is used, which is allowed to fall on an interferometer. This photometer has advantage of fast speed of analysis, so very accurate values of molecular parameters are found with FTIR. A computer is also linked to get the store data.