Infrared Spectroscopy

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1. IR SPECTROSCOPY PRESENTED BY: JACOB THON BIOR REG.NO : ND0120003 M.PHARM 1 ST SEM DEPARTMENT of PHARMACEUTICAL CHEMISTRY KLE COLLEGE OF PHARMACY,BELAGAVI

2. Contents: Introduction. Principle involved in the IR. Modes of molecular vibration. Instrumentation Sample handling. Factors affecting vibrational freqenciens. Application of IR spectroscopy.

3. Introduction IR spectroscopy is the study of absorption of IR radiation which results in vibrational transitions . Mainly used in structural elucidation to determine the functional groups . IR region is present between visible and microwave region in EM spectrum. IR of EM spectrum is divided into 3 regions: Wave number Wavelength Near IR 14000 – 4000 cm-1 0.8 – 2.5 μm Middle IR 4000 – 400 cm-1 2.5 – 25 μm Far IR 400 -10 cm-1 25 – 1000 μm

4. Principle The study of interaction of electromagnetic radiation with vibrational and rotational references with in a molecular structure is termed as IR spectroscopy. When energy in the form of infrared radiation is applied then it causes the vibration between the atoms of the molecules. When IR frequency is equal to the natural frequency of vibration absorption of IR radiation takes place, a peak is observed. IR spectrum of a chemical substance is a finger print of a molecule for its identification.

5. Modes of molecular vibration

6. Symmetrical stretching Asymmertical stretching Scissoring Rocking Wagging Twisting 1)Streching vibrations: 2)Bending vibrations: a)In-plane vibrations: b) Out-plane vibrations:

7. INSTRUMENTATION There are 2 basic types of infrared spectrophotometer, characterized by the manner in which the infrared frequencies are handled. Dispersive instruments : In this the infrared light is seperated into individual frequencies by dispersion, using a grating monochromator . Fourier transform IR instrument: In this infrared frequencies are allowed to interact to produce an interference pattern and this pattern is then analysed mathematically using fourier transforms , to determine the individual frequencies and their intensities.

8. DISPERSIVE SPECTROPHOTOMETER: Single beam spectrophotometer:

9. Double beam spectrophotometer:

10. Fourier transform IR SPECTROPHOTOMETER:

11. Block diagram of IR spectrophotometer S ource S ample Monochromator Detector R eadout Nernst Glower Globar Incandescent wire source Mercury arc lamp Prisms Grating Thermocouple Thermopile Thermister Bolometer Pyroelectric Recorder 11

12. SOURCES Nernest Glower: It is combination of Oxides of Zirconium(ZrO 2 ), Erbium(Er 2 O 3 ), Ytterium (Y 2 O 3 ) usually of diameter 2mm& length of 30mm. It is in Non-Conducting at room temperature so heating is required to bring to conducting state(1000-1800 ° C). Globar source: It is made up of Silicon carbide rod ( SiC ) usually diameter of 4mm& length of 50mm. Heating is required to produce IR radiation(1300-1700 ° C). Power consumption is normally higher than nernest glower. Water cooling system is needed to cool the metallic rods electrodes attached to the rod.

13. Incandescent lamp: A tightly wound nichrome coil heated to about 1100 ° C by electric current. A black film oxide is formed on coil give acceptable emissivity. It has longer life than nernest & globar source. Mercury arc lamp: Used for FAR IR regions ( v <200cm¯1). It is a high pressure mercury arc which consists of quartz jacketed tube containing mercury vapour at P>1 atm. When current passes through the lamp, mercury is vapourised , excited & ionized, forming a plasma discharge at high pressure.

14. Sample handling IR Spectrometry deals with the measurement of the IR spectrum. It is used for the characterization of solid, liquid and gas samples. Material containing sample must be transparent to the IR radiation.  Hence, the salts like NaCl, KBr are only used A] Sampling of solids . Solid film technique  Pressed pellet technique Mull technique  

15. 1)Solid film technique: The solid films can deposited on the surface of a KBr/ NaCl plates by allowing a solution in a volatile solvent to evaporate drop by drop on the surface of the flat. IR radiation is then passed through the thin layer deposited. 2)Pressed pellet technique: (or KBr pellet method ) In this technique, a small amount of finely ground solid sample is mixed with 0.1-2% weight of dried powdered KBr/Nacl and compressed. (other alkali metal halides can also been used). The mixture is compressed under very high pressure (10,000 to 15,000 pounds per square inch) to yield a transparent disk about 1 cm in diameter and 1-2 mm thick. The disk is prepared in vacuum to eliminate air or moisture. These pellets are transparent to IR radiation and it is used for analysis.

16. 3)Mull technique: In this technique, the finely crushed sample is mixed with mineral oil like Nujol (high Mwt liquid paraffin hydrocarbon) in a mortar,with a pestle to make a thick paste/mull. A thin film is applied onto the salt plates. This is the mounted in a path of IR beam and the spectrum is recorded. The absorptions of the Nujol effectively blank out the regions of C- H str and C- H def. But these regions can be studied by preparing a mull with a complementary agent containing no C-H bonds. Ex: H exachlorobutadiene & chlorofluorocarbon oils are used.

17. B] Sampling of liquids Liquids are usually examined as a thin film sandwiched between two polished salt plates (eg: Nacl) Aqueous solvents cannot be used because they cannot dissolve alkali halides . Organic solvents like chloroform can be used. Sample thickness should be 0.01-0.1mm to give transmittance between 15% and 70%. Some salts plates are highly soluble in water, so sample and washing reagents must be anhydrous ( eg:Toulene , Chloroform)

18. C] Sampling of gases Sampling of Gases can be obtained by permitting the sample to expand in to evacuated cylindrical cell equipped with IR transparent windows. (eg: NaCl). The gas must not react with the cell windows or the reflecting surfaces. Gas analyses are performed with IR but the method is not commonly used because of its lack of sensitivity. Moisture must be avoided. In addition, the windows and other instruments components which are constructed of soluble, salts may be damaged.

19. D) Solution sampling technique : Solution of sample is prepared in a suitable solvent and then the sample is analysed in the form of solution. When sample in solutions, the absorption due to solvent has to be compensated by keeping the solvent in a cell of same thickness as that containing the sample. The solvent used in this technique are carbontetrachloride, carbondisulfide or chloroform

20. MONOCHROMATORS To select radiation of any desired frequency from source & eliminate other frequencies. This consists of monochromator which consists of Dispersion element & slit system. Two types of dispersion elements Prisms Gratings

21. Prism Monochromators: Single pass monochromator

22. Double pass monochromator

23. Grating (parallel grooves) Grating Monochromator:

24. DETECTORS Thermocouple: The thermocouple detector is based upon the fact that when two different semiconductor metal wires of high thermoelectric efficiency, are connected and kept at different temperatures, a potential difference is developed between them which causes the flow of current. (Response time 60msec). A thermocouple is closed in an evacuated steel casing with KBr window to avoid losses of energy by convection. 2 . To enhance sensitivity, several thermocouples connected in series to give better results called Thermopile.

25. 3. Bolometer : It is based on wheatstone brige principle, For every 1 °C rise in temperature electric resistance of metal increases. A bolometer usually consists of a thin metal conductor made up of the platinum strips coated with lamp black. Amount of current flowing through galvanometer measures the intensity of radiation falling on the detector. Response time – 4msec

26. 4. Pyroelectric detector: It consists of thin plate of pyroelectric crystal of 0.25 to 12mm in size. The crystal is placed between two electrodes which is connected to an external electric circuit. When the crystal is exposed to IR radiation, it absorbs the heat leading to change in its dipole moment. Various pyroelectric crystals are barium titanate,lead zirconate, triglycine sulphate etc., Response time- 10 msec.

27. Factors affecting vibrational frequencies: Vibrational coupling Hydrogen bonding Electronic effect Bond angles Resonance

28. 1 ) VIBRATIONAL COUPLING An isolated C-H bond has only one stretching vibrational frequency where as methylene (-CH 2 ) group shows two stretching vibrations, symmetrical and asymmetrical. Because of coupling(or)interaction between C-H stretching vibrations in the CH2 group. Assymetric vibrations occur at higher frequencies than symmetric stretching vibrations. These is known as vibration coupling because these vibrations occur at different frequencies than that required for an isolated CH stretching. 2 ) HYDROGEN BONDING Hydrogen bonding, especially in O-H and N-H compounds, gives rise to a number of effects in infrared spectra, While most organic work will involve relatively non-associating solvents (CCl 4 , CS 2 , CHCI 3 ), More polar solvents such as acetone (or) benzene will certainly influence 0-H and N- H absorptions.

29. The N-H stretching frequencies of amines are also affected by hydrogen bonding as that of the hydroxyl group but frequency shifts for amines are lesser than that of hydroxyl compounds. Intermolecular hydrogen bonds gives rise to broad bands, while intramolecular hydrogen bonds give sharp and well defined bands. 3) ELECTRONIC EFFECTS Changes in the absorption frequencies for a particular group take place when the substituents in the neighbourhood of that particular group are changed. It includes: Inductive effect Mesomeric effect Field effect

30. 4) BOND ANGLES C-H stretching vibrations move to higher frequency in the sequence alkane - alkene-alkyne . As hybridization goes from sp3 to sp2 to sp1 ,the ‘s’ character of the C-H bond increases; bond lengths become shorter, and frequencies rise. Cyclopropanes have high C-H str frequencies for the same reason (typical values being 3040-3070 cm"): the C-C-C bond angle is substantially contracted below the normal 109.5°, leading to increased ‘s’ character in the C-H bonds, and thus to higher frequencies 5) RESONANCE Because of resonance carboxylic acids(-COOH) show more absorption compared to alcohols. Due to resonance the bond strength increases and results increase in absorption of frequency In this way resonance can affect the absorption of IR of C-O stretching.

31. Application of IR Determination of Molecular Structure. Studying the Progress of the Reactions. Qualitative Analysis of Functional Groups. Detection of Impurity in a Compound. Shape of Symmetry of a molecule Presence of Water in a Sample. Examination of Old Paintings and Artifacts. Identification of Organic Compound

32. REFERENCES Organic Spectroscopy by William Kemp,3 RD Edition . Elementary organic spectroscopy principles and chemical applications,by Y.R.SHARMA. Pharmaceutical Analysis Instrumental Methods by Nirali Prakashan . Instrumental method of Analysis seventh edition by Willard Merritt, Dean & Settle

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