ir ppt.pptx

it contains basics of ir spectroscopy starting from the principle involved to hooks law and how the stretching frequency varies with various parameters. it have ir vibration frequency value chart for different functional groups.

1. IR spectroscopy Supriya panda Lecturer in chemistry Ramadevi Women’s University

2. CONTENTS Theory of IR spectroscopy Hooke’s law Modes of vibration Interpretation of IR frequency of various compounds Instrumentation

3. The term "infra red" covers the range of the electromagnetic spectrum between 0.78 and 1000 mm. In the context of infra red spectroscopy, wavelength is measured in "wavenumbers", which have the units cm-1. wavenumber = 1 / wavelength in centimeters It is useful to divide the infra red region into three sections; near, mid and far infra red region Wavelength range (mm) Wavenumber range: Near 0.78 - 2.5 12800 - 4000 Middle 2.5 - 50 4000 – 200 Far 50 -1000 200 - 10 The most useful I.R. region lies between 4000 - 670cm-1

4. Theory of infra red absorption IR radiation does not have enough energy to induce electronic transitions as seen with UV. Absorption of IR is restricted to compounds with small energy differences in the possible vibrational and rotational states.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. The alternating electrical field of the radiation (remember that electromagnetic radation consists of an oscillating electrical field and an oscillating magnetic field, perpendicular to each other) interacts with fluctuations 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.

5. Molecular rotations Rotational transitions are of little use to the spectroscopist . Rotational levels are quantized, and absorption of IR by gases yields line spectra. However, in liquids or solids, these lines broaden into a continuum due to molecular collision and other interactions. Molecular vibrations The positions of atoms in a molecules are not fixed; they are subject to a number of different vibrations. Vibrations fall into the two main catagories : stretching and bending.

6. Modes of vibration Stretching: Change in inter-atomic distance along bond axis 1. Symmetric 2. Asymmetric Bending: Change in angle between two bonds. There are four types of bending Rocking Scissoring Wagging Twisting

7. HOOKE’S LAW The Hooke’s Law is a mathematical formula that relates the vibrational frequency of a spring connected to two spheres to the stiffness of the spring and to the masses of the spheres. Vibrations of a covalent bond is thought to be similar to those of the above system. Thus, the Hooke’s Law can be applied to the vibrations of a covalent bond. Given below is the Hooke’s Law as it applies to a covalent bond.

8. According to the Hooke’s Law, 1. The stronger the bond, the faster the bonds vibrates. 2. T he lighter the atoms linked by the bond, the faster the bond vibrates.

9. Energy of a particle that can be mapped by simple harmonic oscillation is shown . The energy splitting is either ħω which is equivalent to hv0. The energy splittings are equal to one another. The lines on either side are not asymptotic on the y axis; this means the particles .

10. Anharmonic oscillator. The energy spacing is not equal between the energy levels. The potential energy barrier does not cross the y axis as the nuclei cannot pass through one another. The molecule dissociates at the largest separation. Comparison shows that the quantum harmonic oscillator is pulled up on the left hand side. right hand side has a tailing off of energy.

11. Alkanes:

12. Alkenes: Alkynes:

13. Aromatic Rings

14. Alcohols and phenols Ethers

15. Carbonyl compounds : Aldehydes :

16. Ketones Carboxylic Acid

17. Esters :

18. Amides : Acid chlorides: Acid anhydride:

19. Instrumentation There are four types of instruments for infrared absorption measurements available: Dispersive grating spectrophotometers for qualitative measurements Nondispersive photometers for quantitative determination of organic species in the atmosphere Reflectance photometers for analysis of solids Fourier transform infrared (FT-IR) instruments for both qualitative and quantitative measurements.

20. The main parts of IR spectrometer are as follows: Radiation source sample cells Monochromators Detectors recordered measurements

21. Instruments for measuring infrared absorption all require a source of continuous infrared radiation. Infrared sources consist of an inert solid that is electrically heated to a temperature between 1,500 and 2,200 K. The heated material will then emit infra red radiation . The Nernst glower: The Nernst glower is constructed of rare earth oxides in the form of a hollow cylinder. Platinum leads at the ends of the cylinder permit the passage of electricity. Nernst glowers are fragile. They have a large negative temperature coefficient of electrical resistance and must be preheated to be conductive . The globar source A globar is a rod of silicon carbide (5 mm diameter, 50 mm long) which is electrically heated to about 1,500 K. Water cooling of the electrical contacts is needed to prevent arcing. The spectral output is comparable with the Nernst 28 glower, execept at short wavelengths (less than 5 mm) where it's output becomes larger. Infrared light sources

22. IR spectroscopy has been used for the characterization of solid, liquid or gas samples. i . Solid - Various techniques are used for preparing solid samples such as pressed pellet technique, solid run in solution, solid films, mull technique etc. ii . Liquid – samples can be held using a liquid sample cell made of alkali halides. Aqueous solvents cannot be used as they will dissolve alkali halides. Only organic solvents like chloroform can be used . iii. Gas – sampling of gas is similar to the sampling of liquids. Sample Cells and sampling of substances

23. Monochramotor Various types of monochromators are prism, gratings and filters. Prisms are made of Potassium bromide, Sodium chloride or Caesium iodide. Filters are made up of Lithium Fluoride and Diffraction gratings are made up of alkali halides. Detector The detectors can be classified into three categories, thermal detectors, pyroelectric detectors and photoconducting detectors.

24. Thermal detectors : Thermal detectors can be used over a wide range of wavelengths and they operate at room temperature. Their main disadvantages are slow response time and lower sensivity relative to other types of detectors . Thermocouple A thermocouple consists of a pair of junctions of different metals; for example, two pieces of bismuth fused to either end of a piece of antimony. The potential difference (voltage) between the junctions changes according to the difference in temperature between the junctions. Several thermocouples connected in series are called a thermopile . Bolometer A bolometer functions by changing resistance when heated. It is constructed of strips of metals such as platinum or nickel or from a semiconductor. Pyroelectric detectors Pyroelectric detectors consists of a pyroelectric material which is an insulator with special thermal and electric properties. Triglycine sulphate is the most common material for pyroelectric infrared detectors. Unlike other thermal detectors the pyroelectric effect depends on the rate of change of the detector temperature rather than on the temperature itself. This allows the pyroelectric detector to operate with a much faster response time and makes these detectors the choice for Fourier transform spectrometers where rapid response is essential.

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