IR Spectroscopy.pptx

IR SPECTROSCOPY

1. IR Spectroscopy Unit II

2. Spectroscopy is a term that describes the interaction of matter with electromagnetic radiation . The electromagnetic spectrum, and as the word  spectrum  implies, is a range of frequencies of the electromagnetic radiation and the corresponding wavelengths and photon energies is a schematic portrayal of the electromagnetic spectrum, along with the molecular processes that can occur in each region, e.g., rotation (microwave ) vibration (infrared ) electronic excitation (ultraviolet-visible ) and bond breaking and ionization (X-rays).

3. Infrared spectroscopy (IR spectroscopy) is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light Infrared Spectroscopy is the analysis of infrared light interacting with a molecule. It is based on absorption spectroscopy  The wavelengths found in infrared radiation are a little longer than those found in visible light . IR spectroscopy is useful for finding out what kinds of bonds are present in a molecule, and knowing what the structure could be.

4. NATURE OF IR SPECTRA IR spectrum is a graph of band intensities on y axis versus position of band on x-axis. Band intensities can be given in terms of transmittance(T) or absorbance(A). Position of band can be expressed in terms of wave number (n) or wavelength(λ). In IR spectra, wave numbers (n) are used instead of wavelength (λ) for mentioning the characteristic peak as this unit has advantage of being linear with energy of radiation (E) . E = h c/ λ or, E= h c n [ n = 1/λ, c= velocity of light, h= Planck’s constant ]

5. TRANSMISSION vs. ABSORPTION When a chemical sample is exposed to the action of IR LIGHT, it can absorb (retain) specific frequencies and allow the rest to pass through it (transmitted light). Some of the light can also be reflected back to the source. Transmittance ( T) is defined as the ratio of radiant power transmitted by a sample to the radiant power incident on the sample. Whereas , Absorbance (A) = log 10 (1 /T ) An IR spectrum is usually plotted using transmittance, hence absorption band appears as dips rather than maxima. Each dip is called band or peak.

6. Regions of the Infrared spectrum Functional Group Region -Most of the bands that indicate what functional group is present are found in the region from 4000 cm -1  to 1400 cm -1 . Their bands can be identified and used to determine the functional group of an unknown compound. Fingerprint Region- Bands that are unique to each molecule, similar to a fingerprint, are found in the fingerprint region, from 1400 cm -1  to 900 cm -1 . These bands are only used to compare the spectra of one compound to another.

7. IR region of electromagnetic spectrum: λ : 780 nm – 1000 μ m Wavenumber : 12,800 – 10 cm-1 IR region is subdivided into 3 sub-regions: Near IR region (Nearest to the visible) - 780 nm to 2.5 μ m (12,800 - 4000 cm-1 ) Mid IR region - 2.5 to 50 μ m (4000 – 200 cm-1 ) 3. Far IR region - 50 to 1000 μ m (200 – 10 cm-1 )

8. When IR radiations are passed,it causes- Changes in the shape of molecules such as stretching of bonds , bending of bonds , or internal rotation around single bonds. IR absorption only occurs when IR radiation interacts with a molecule undergoing a change in dipole moment as it vibrates or rotates.

9. For a molecule to be IR active there  must be a change in dipole moment as a result of the vibration  that occurs when IR radiation is absorbed. In homonuclear diatomic molecule like H2, O2, N2 etc.,  the dipole moment does not change during vibration . Hence these molecules do not give vibration spectra i.e. they are said to be infrared-inactive

10. PRINCIPLE OF IR SPECTROSCOPY When the energy in the form of IR is applied and if the applied IR frequency = Natural frequency of vibration, the absorption of IR takes place and a peak is observed. Molecules are excited to the higher energy state from the ground state when they absorb IR radiation. When a compound is exposed to IR radiation, it selectively absorbs the radiations resulting in vibration of the molecules of the compound, giving rise to closely packed absorption bands, called as IR absorption spectrum. The bands correspond to the characteristic functional groups and the bonds present in a chemical substance. Thus, an IR spectrum of a compound is considered as the fingerprint for its chemical identification.

11. A molecule in its ground state posses 3 energy levels , Electronic , Vibrational and Rotational energy level. • Absorption in the IR regions is due to the changes in the vibrational and rotational levels. • When an electron absorbs less energy , rotational transitions takes place. The spectrum observed is called rotational spectrum observed in far IR region (25 to 300-400 µ). • When the electrons absorbs still higher energy , vibrational transitions take place which is accompanied with rotational transitions. The spectrum observed is called vibrational - rotational spectrum obtained in near IR region (2.5 to 25 µ). • Electronic transitions take place at higher energies, these are not seen in IR.

12. Modes of Vibration The interaction of infrared radiations with matter can be understood in terms of changes in molecular dipoles associated with vibrations. Vibrations can involve either changes in- bond length (stretching) or bond angle (bending). Some bonds can stretch in-plane (symmetric stretching) or out-of-plane (asymmetric stretching). Bending vibrations can be either in-plane (as; scissoring, rocking) or out-of-plane (as; wagging, twisting) bending vibrations.

13. Difference in Stretching and Bending Vibration

14. Molecular Vibrations

15. Some General Trends: i )     Stretching frequencies are higher than corresponding bending frequencies.  (It is easier to bend a bond than to stretch or compress it.) ii)     Bonds to hydrogen have higher stretching frequencies than those to heavier atoms. iii)     Triple bonds have higher stretching frequencies than corresponding double bonds, which in turn have higher frequencies than single bonds.         ( Except for bonds to hydrogen ).

16. In order for a vibrational mode in a molecule to be "IR active," it must be associated with changes in the permanent dipole . A molecule can vibrate in many ways, and each way is called a vibrational mode or Fundamental Vibrations. Linear molecules have 3N - 5 degrees of vibrational modes whereas nonlinear molecules have 3N - 6 degrees of vibrational modes (also called vibrational degrees of freedom). As an example H 2 O, a non-linear molecule, will have 3 × 3 - 6 = 3 degrees of vibrational freedom, or modes .

17. Sampling in Infrared Spectroscopy The samples used in IR spectroscopy can be either in the solid, liquid, or gaseous state . Solid samples can be prepared by crushing the sample with a mulling agent which has an oily texture or in the form of KBr pellets . A thin layer of this mull can now be applied on a salt plate to be measured.example-Nujol mull Liquid samples are generally kept between two salt plates and measured since the plates are transparent to IR light. Salt plates can be made up of  sodium chloride , calcium fluoride, or even potassium bromide . Gaseous samples are filled inside NaCl cell. Since the concentration of gaseous samples can be in parts per million, the sample cell must have a relatively long pathlength , i.e. light must travel for a relatively long distance in the sample cell .

18. REGIONS OF IR SPECTRUM IR spectra is divided into 2 regions. Functional Group Region(4000- 1400 cm-1) It consists of absorption bands of vibrational states of various types of bonds present in the molecule. The important groups accounted for include NH, OH, C=O, C=C, C=N, etc. The presence of aromatic nucleus (2000-1670 cm-1) and hydrogen bonding O-H, N-H, etc are also encountered in this region.

19. 2. Fingerprint region This region accounts for many absorption bands characteristic of functional group. Since numbers of sharp bands of varying intensities are encountered, close examination is needed. This region is useful for the identification of compounds since no two compounds can have identical IR spectra under identical conditions. Regions present below 1400 cm-1 shows absorption bands due to bending vibrations and stretching vibrations of C-C, C-O and C-N bonds. Regions less than 1250 cm-1 consists of complex vibrational and rotational spectra of the complete molecule.

20. APPLICATIONS OF IR SPECTROSCOPY Identification of functional groups & structure elucidation of organic compounds. Quantitative analysis of a number of organic compounds. Study of covalent bonds in molecules. Studying the progress of reactions. Detection of impurities in a compound. Ratio of cis -trans isomers in a mixture of compounds. Shape of symmetry of an inorganic molecule. Study the presence of water in a sample. Measurement of paints and varnishes

21. Numerical on Lambert Beer Law 1.The percentage transmittance of an aqueous solution of unknown compound is 40% at 25degree centigrade and 300nm..Calculate the absorbance of the solution. Given- I o=100 I t=40 A=log Io/It =(log100-log40) =0.3979

22. 2. A solution of Tryptophan has an absorbance at 280 nm of 0.54 in a 0.5 cm length cuvette . Given the absorbance coefficient of trp is 6.4 × 10 3 LMol-1 cm-1 . What is the concentration of solution? Solution: As ε = A / l c l= 0.5 cm, A= 0.54 ε = 6.4 × 10 3 LMol-1 cm-1 C=? c = A/ε l = 0.54 / 6.4 × 10 3 × 0.5 Answer = 0.000168 M 2.

23. 3.In a spectrometric cell of 2 cm path length the solution of a substance shows the absorbance value of 1.0.If the molar absorptivity of the compound is 2x10 4 litre / mole.cm,calculate the concentration of the substance