Infra Red Spectroscopy

IR Spectroscopy

1. INFRA-RED SPECTROSCOPY By- Manish Sahu M.Sc. Chemistry (Final) Sp.- Physical Chemistry

2. ContentS Introduction History Basic principle Theory of molecular vibrations Instrumentation Finger Print region Applications Conclusion References

3. It is the study of absorption of infra red radiation which results in the vibrational transitions. IR spectrum is an important records which gives sufficient information about the structure of a compound and also determine the functional groups. For a molecule to be IR active the vibration should give rise to a net change in dipole moment. The range of IR radiation is from 2.5 μ – 15 μ. Introduction 0.8 2.5 15 200 μ IR region Far IR region Near IR region 12500. 4000. 667. 50 cm -1

4. History Infrared radiation was first discovered in 1800 by sir William Herschel .

5. Basic principle The principle of IR spectroscopy is related to the vibrational and rotational energy of a molecule. Absorption of IR radiation causes an excitation from a lower to the higher vibrational level. Each vibrational level is associated with a number of closely placed rotational levels. Therefore the IR spectroscopy is also refer as vibrational-rotational spectroscopy. All the bonds in a molecule are not capable of absorbing IR energy but those bond which are accompanied by a change in dipole moment will absorb in the IR region and such transition are called IR active transitions.

6. Theory of molecular vibrations Any change in shape of the molecule :- stretching of bonds, bending of bonds and internal rotation around single bonds. Molecular vibrations is of two types :- 1.Fundamental Vibration 2.Non-fundamentall vibration Fundamental vibration is also divided into 2 types Stretching vibrations :- Vibration involves a continuous change in inter atomic distance along the same axis of bond between two atoms. 2. Bending vibration :- It involves change in bond angle between two atoms.

7. Molecular vibration Stretching vibrations Bending vibrations Symmetrical Asymmetrical In-plane Out-plane Scissoring Rocking Wagging Twisting

8. Instrumentation The main parts of IR Spectrophotometer are :- Radiation source Sample cell and sampling of substance Monochromator Detector Recorder

9. Finger print region In IR, the region between 1500 – 667 cm -1 is rich in many absorption bands and the region is known as Finger Print region. In this region the number of bending vibrations is usually more than the number of stretching vibrations. Many compounds shows unique absorption bands in this region and which is very useful for the identification of the compounds. Finger print region can be sub-divided into 3 regions. 1500 – 1300 cm -1 :- presence of tertiary butyl groups. 1300 – 1000 cm -1 :- presence of group like alcohol, ester, lactones, acid anhydride due to presence of c---o stretching 1000 - 667 cm -1 :- distinguished between cis-trans alkenes and mono & di substituted at ortho, meta, para position in benzene.

10. Applications Identification of an organic compounds. Quantitative analysis of functional groups. Distinction between hydrogen bonds. Study of chemical reaction. Determination of molecular structure of organic compounds.

11. Reference Elementary Organic Spectroscopy By- Y. R. Sharma, S.Chand Publisher, Page no- 69 – 150 2. Spectroscopyy By- H. Kaur, Pragati Publisher, Page no- 166 – 301 3. Instrumentalal Methods of Chemical Analysis By – Gurdeep R. Chatwal , Sham K. Anand, Himalaya Publisher, page no – 2.29 – 2.82 Spectroscopy of Organic Compounds By – P.S Kalsi, Page no- 50 – 164 5. Fundamentals of Molecular Spectroscopy . By Dr. C.N Banwell, E.M. McCASH (Mc graw Hill), Page no – 55 - 99

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