infra red spectroscopy(principle,,,.pptx

It is about the infra red spectrometry.this includes principle instrumentation and application

1. The I R region extends from the red end of the visible spectrum to the micro wave region . The most useful range for analytical purpose lies between 4000 and 400 cm -1 this region is known as the fundamental region(middle infrared region )

2. INFRARED REGIONS RANGE Near infrared region Wavelength ( 0.8-2.5μm(12500 –4000cm-1) Middle infrared region 2.5- 15 μ m ( 4000 -667 cm-1) Far infrared region 15-200 μm (667-50 cm-1)

3. PRINCIPLE The principle of IR spectroscopy is related to the vibrational and rotational energy of a molecule. When the frequency of the IR radiation is equal to the natural frequency of vibration, the molecule absorb IR radiation. Absorption of IR radiation causes an excitation of molecule from a lower to the higher vibrational level. Each vibrational level is associated with a number of closely placed rotational level. Therefore the IR spectroscopy is also called as “ vibrational -rotational spectroscopy”

4. All the bonds in a molecule are not capable of absorbing IR energy but those bonds which are accompanied by a change in dipole moment will absorb in the IR region and such transitions are called IR active transitions . The transitions which are not accompanied by a change in dipole moment of the molecule are not directly observed and are considered as IR inactive.

5. THEORY When a molecule absorb radiation with a frequency less than 100cm -1 ,molecular rotation takes place and if a molecule absorb more energetic radiation in the region of 10 4 to 10 2 cm -1 , molecular vibration takes place. A single vibrational energy change is accompanied by a large number of rotational energy changes and thus the vibrational spectra appear as vibrational rotational bands .

6. There are 3 main process by which a molecule can absorb radiation, each of these route involve an increase of energy which is proportional to the light absorbed. i . First route occurs when absorption of radiation leads to a higher rotational energy level in a rotational transition. ii. Second occurs when absorption of radiation leads to a higher vibrational energy level in a vibrational transition. iii. Third occurs when absorption of radiation leads to a higher electronic energy level in its electronic transitions.

7. Energy level diagram

8. Vibrations in a polyatomic molecule Polyatomic molecules give fundamental and overtone bands in I R spectra which are related to degrees of freedom in a molecule A polyatomic molecule contains n number of atoms . Each atom has three degrees of freedom corresponding to three directions x ,y and z which are perpendicular to each other . Since the number of atoms in a polyatomic molecule is ‘n’ the number of degrees of freedom will be 3n

9. VIBRATIONS Two types of vibrations are; 1. Stretching i . Symmetric ii. Asymmetric 2. Bending i . Scissoring ii. Rocking iii. Wagging iv. Twisting

10. Two criteria must be satisfied by a molecule for the absorption of IR radiation: i . The molecule should possess vibrational and rotational frequency. ii. The molecule must give rise to asymmetrical charge distribution. Three main type of absorption bands occur in IR spectra: i . Fundamental ii. Overtone iii. combinational

11. If a frequency of vibration of HCl molecule exactly matches with that coming from the source NET TRANSFER of energy takes Place change in amplitude of molecular vibration absorption of radiation • In case of O 2 , N 2 ,Cl 2 NO NET change in dipole moment occurs thus they cannot absorb IR radiations & do not show IR spectra

12. Vibrational coupling   Vibrational coupling involves interaction between two similar or different vibrating groups . Vibrational coupling depends upon various factors like   1. When two atoms are bonded to a central atom the stretching vibrations of these two atoms show strong coupling 2.Coupling between bending vibrations is seen only when the two vibrating groups are separated by a common bond 3.Coupling is maximum when the vibrating groups have individual energies which are approximately equal in magnitudes 4.If the vibrating groups are separated by two or more bonds, negligible or no coupling takes place

13. Instrumentation IR radiation source Wavelength selector ( Monochromator ) Sample Holder or cell Detector Recorder

14. Radiation sources 1. Nernst Glower: • Consists of a hollow rod made up of fused mixture of rare earth oxides like thorium , yittrium and zirconium formed into a cylinder of diameter 1-2mm length approx. 20mm. • Platinum discs are attached at the ends of cylinder to permit electrical connection.The rod is surrounded by a helical heater • They are heated to a temp of 1200-2200K where IR radiations are emitted.

15. Nernst Glower: Rare earth metals have large – ve temperature coefficient of electrical resistance. (A negative temperature coefficient (NTC) occurs when a physical property (such as thermal conductivity or temperature, typically in a defined temperature range.) Temp is inversely related to resistance At low temp, they provide high resistance to flow current and as cylinder gets heated resistance gets reduced allowing conduction of Current. If large amount of current flows through cylinder it can destroy the source. To avoid this source circuit should be designed to limit the current

16. 2. The Globar source It consists of a silicon carbide rod with a diameter of 6-8mm and length 50mm • It has positive temperature coefficient of electrical resistance • At low temp, they provide low resistance to flow of current and as cylinder gets heated resistance increases allowing conduction of current. • To avoid excessive heating water cooling is necessary • At high temp source emits IR radiations. It can be operated to a temp of about 1300 C

17. Incandescent wire source ( nichrome wire) • Cylinder in the above sources is tightly wound by spiral wire of nichrome / rhodium • It is electrically heated at 1100 Cit is simple, durable and does not show exhaustion . Mercury arc: • Quartz jacketed tube Hg vapors • Electricity passed through Hg vapour provides radiations Tungsten filament : used in Near IR

18. Wavelength Selectors Filters Monochromators Entrance slit Collimating lens or mirror Dispersion element (prism or grating) Focusing lens or mirror Exit slit

19. Difference between monochromator and filter

20. Filters • Filters are very simple: they are sheets of plastic or glass that simply absorb wavelengths other than those required for the analysis. Generally, the range of wavelengths allowed by a filter is relatively wide Monochromators are far more complicated, and comprise a series of optics inside a lightproof box, which has entry and exit slits which allows the radiation of all wavelengths in and a narrow range of wavelengths out.

21. Monochromators are of two types Prism monochromator Grating monochromator Prism made up of alkali metal halides like NaCl,KBr etc are used as they are transparent to I R radiations . They are used generally in mid I R region Crystalline KBr and CsBr are used in far I R region

22. Entrance slit allows source radiation to illuminate the first lens which collimates the light spreading it across the face of the prism. • Prism disperses radiation into component wavelengths and the second lens focuses the spectrum at the focal plane. • An exit slit selects the band of radiation to reach the detector.

23. Gratings • It is a device which consists of a series of parallel & closely spaced grooves rules on glass or any reflecting surface • Gratings are of two types Transmission Gratings Reflection Gratings • UV gratings have 2000-6000 grooves per mm • IR gratings have 10-100 grooves per mm • Materials used for construction are Quartz, NaCl,KBr

24. Sampling

25. Liquids • It consists of sampling liquid as thin films squeezed between two infrared transparent windows like NaCl flats • The salt plates or rock salt flats must be optically polished & cleaned immediately after use. • Toluene, chloroform etc are used to clean them. • It consists of two windows of pressed salt sealed and separated by thin gaskets of Teflon, copper or lead that have been wetted withmercury . The windows are usually made of sodium chloride, potassium chloride or cesium bromide. • There are two cells first cell containing sample & second onecontaining pure solvent placed in reference beam • By the reference beam solvent absorptions are cancelled out & spectrum recorded is that of solute alone.

26. Solids a) KBr Disks • It is prepared by grinding the sample with KBr & compressing the whole into a transparent pellet or disk. • KBr must be dry, & hence grinding is carried out under infrared lamp to avoid condensation of atmospheric moisture which gives rise to broad absorptions • Grinding is done in a agate mortar pestle / commercial ball mills • Poorly ground mixtures lead to disks that scatter light than they transmit • Particle size of 2μm must be achieved to avoid scattering Hydraulic press KBr Die

27. Solids b) Mulls • Pastes prepared by grinding the sample with a drop of oil the mull is the squeezed between transparent windows as for liquid samples • Mulling agent should ideally be infrared transparent, but this is never true • Liquid paraffin ( Nujol ) is used to prepare Nujol mulls which is transparent over a wide range in IR spectrum. 40

28. Solids c) Solid Films • Solid films can be deposited onto NaCl plates by allowing a solutions in a volatile solvent to evaporate drop by drop on the surface of the flat. • Polymers & various waxy or fatty materials often give excellent spectra in this way.

29. Thermocouple • It consists of a pair of junction formed when 2 ends of metal such as Bismuth & antimony are fused to either ends of dissimilar metal wires. • Heating capacity of absorbing element is small as to detect the change in temperature.• Potential varies in the 2 junctions with the difference in temperature

30. One junction is Cold junction which is reference junction kept at constant temperature not exposed to IR radiations • The other junction is Hot junction exposed to IR radiations which increases temperature of junction. • Temperature difference due to falling IR radiations generates the potential difference which amounts to Ir radaitions . Hot junction is generally blackened to improve its heat absorbing capacity

31. The detector consists of a small metal cylinder closed by a rigid blackened metal plate at one end and a flexible silvered diaphragm at the other end the chamber is filled with xenon gas . Sample cell radiation passes through a small IR transmitting window and is absorbed by the blackened plate . The heat produced due to absorption of radiation causes the gas present inside the cylinder to expand and deform the flexible diaphragm The deformation depends on the heat absorbed by it which further depends on the intensity of the incident radiation When the I R radiation falls on the silvered diaphragm they reflect back in the chamber and fall on the surface of a phototube . A definite fraction of the reflected beam strikes the active surface of a phototube . This leads to change in the photocurrent which can be related to the power of I R radiation

32. In bolometer two arms of the bridge are made up of platinum strips coated with lamp black. One arm is exposed to radiations and the other arm is protected from radiations . The two platinum strips are Connected to a sensitive galvanometer, then to a battery in wheat stone bridge arrangement When radiations fall on the exposed platinum strip the temp of the strip increases and the bridge becomes imbalanced. Change in electrical resistance causes the current to flow through the galvanometer. The amount of current flowing through galvanometer is proportional to the amount of radiations falling on the exposed platinum strip A bolometer is based on wheat stone bridge arrangement .It has four arms. One of the bridge consists of a metal of unknown resistance and the resistance of other 3 arms is known When the bridge is balanced no current flows through the Galvanometer When the bridge is unbalanced due to exposure of light or heat,. current flows through the Galvanometer due to change in resistance of the metal

33. Pyroelectric Detectors They are constructed from single crystalline piece of pyroelectric material which are dielectric materials which have special thermal & electrical properties. Eg . deuterated triglycine sulphate (DTGS ) • When an electric field is applied across any dielectric material electrical polarization takes place. • The magnitude of polarization is a function of dielectric constant of that material • This pyroelectric crystal is sandwiched between 2 electrodes which are IR transparent. Examples Lithium Niobate , Lithium Tantalate , Triglycine sulphate, DTGS

34. The pyro electric crystal acts as a temperature dependent capacitor. When IR radiations fall there is a change in temperature, which alters the charge distribution across the crystal which can be detected as a current in an external circuit connecting to two sides of capacitor. Magnitude of current is directly proportional to surface area of crystal and rate of change of polarization with temperature. This detector has a faster response time

35. Applications of I R Infrared spectroscopy is widely used in industry as well as in research. Some of the major applications of IR spectroscopy are as follows: 1. Identification of functional group and structure elucidation Entire IR region is divided into group frequency region and fingerprint region. Range of group frequency is 4000-1500 cm- while that of finger print region is 1500-400 cm-1. In group frequency region, the peaks corresponding to different functional groups can be observed. According to corresponding peaks, functional group can be determined. Each atom of the molecule is connected by bond and each bond requires different IR region so characteristic peaks are observed. This region of IR spectrum is called as finger print region of the molecule. It can be determined by characteristic peaks.