IR - Spectroscopy, theory, modes of vibration and sample handling. by Dr. Umesh Kumar Sharma and Anu Aathew

IR - Spectroscopy, theory, modes of vibration and sample handling. by Dr. Umesh Kumar Sharma and Anu Aathew

1. INFRA - RED SPECTROSCOPY T heory, modes of vibration and sample handling . 1 By: Dr. Umesh Kumar Sharma & Anu Mathew Department Of Pharmaceutics, Mar Dioscorus College Of Pharmacy, Alathara , Sreekaryam , Thiruvananthapuram

2. Infrared spectroscopy or vibrational spectroscopy is study of absorption of Infrared radiations results in vibrational transitions. Infrared spectroscopy is an important analytical technique for determining structure of both organic & inorganic compounds. Lies in the wavelength range 0.8-1000 μ m. Between visible and microwave region. Atoms in a molecule do not remain in fixed positions but vibrate about their mean positions. On absorption of IR transition from ground vibrational level to excited vibrational level give rise to closely packed absorption spectrum. 2

3. IR spectrum is obtained by plotting absorbance v/s wavelength. Since wavelength is inversely proportional to energy the wavelength in cm -1 is used to represent the position of absorption. Wavenumber is proportional to energy. E = hʋ E = hc / λ E = hc ῡ ῡ in cm-1 =10 4 / λ in μ m 3

4. Near IR region - 0.8-2.5 μ m O vertone region. Absorption bands in this region are overtones or combinations of fundamental stretching vibrations that occur in a region of 3000-1700cm. The bonds involved are C-H ,N-H & O-H. Molar absorptivity are low. Used in quantitative determination of species, such as water, proteins, low mol.wt hydrocarbons. 4

5. Mid IR region-2.5-25 μ m V ibrational rotational region. Based on absorption of specific wavelengths of mid infrared light by a molecule. Absorption, reflection & emission spectra are employed for both qualitative and quantitative analysis. Fingerprint region Falls on the right side of the spectrum. Complicated series of absorption bands due to interacting vibrating modes. Each different compound produces a different pattern of troughs and bands. Absorption pattern is unique for each molecular species . 5

6. Far IR region- 25 - 1000 μ m : R otational region . Useful for inorganic studies due to stretching and bending vibrations of bonds between metal atoms and both inorganic and organic ligands occurs at frequency lower than 650cm -1 . Eg : heavy metals iodides absorbs radiation in the region below 100cm -1 and bromides and chlorides have bands at higher frequencies Pure rotational absorption by gases is observed at far IR region, provided the molecules have permanent dipole moment. Eg : O 3 , HCl 6

7. THEORY The total energy of a molecule is the sum of its electronic, vibrational and rotational energies each of which are quantized. The energy associated with IR radiation is capable of inducing only changes in vibrational energy levels of the molecule. When energy in the form of IR is applied and if the applied IR frequency is equal to the natural frequency of vibration, the absorption of IR takes place & a peak is observed. Eg : In HCl molecules the natural frequency of vibration is 2890cm- 1 , if IR radiation of this frequency falls to HCl molecule IR radiations absorbed, whereas remaining frequencies are transmitted. 7

8. Dipole moment changes during vibrations and rotations : To absorb IR radiation, a molecule must undergo a net change in dipole moment as it vibrates or rotates. Vibration of the molecule cause change in charge distribution which result in alternating electric field. This couples the molecular vibration with oscillating electric field of EM radiation. Eg : The charge distribution around a molecule such as hydrogen chloride is not symmetric because chlorine has a higher electron density than hydrogen . As the hydrogen chloride vibrate, a regular fluctuation in its dipole moment occurs & a field is established that can interact with the electric field associated with radiation. The symmetrical diatomic molecules like O 2 and N 2 do not possess dipole moment, hence cannot be excited by IR radiation. 8

9. Types of vibrations in a molecule : Energy of rotation < E nergy of vibration < E nergy of electronic transition Vibrational frequency : 1 √ k/ μ Vm = 2 Π k- force constant : μ –m 1 m 2 m 1 +m 2 9

10. Types of molecular vibrations Molecular vibrations can be classified into Stretching vibrations.. Bending vibrations Stretching vibrations Here distance between two atoms increases or decreases but the atoms remain in same bond axis. Symmetrical molecules like O=C=O are not IR active because no change in dipole moment is observed upon stretching vibrations. 10

11. Stretching vibrations are of two types: a. Symmetrical stretching : When two bonds increase or decrease in length. b. Asymmetrical stretching : W hen one bond length increased, the other decreases. 11

12. Bending vibrations Involve movement of atoms which are attached to a common central atom, such that there is change in bond axis & bond angle of each individual atom without change in their bond lengths. Bending vibrations requires less energy & occur at longer wavelength than stretching vibrations. Also called deformation vibrations . Types of bending vibrations A. In plane vibrations a ) Scissoring b) Rocking B. Out plane vibrations a) Wagging b) Twisting 12

13. A. In - plane vibration a) Rocking : in plane bending of atoms occurs wherein they swing back & froth with respect to the central atom. b) Scissoring : 2 atom connected to central atom move towards and away from each other . 13

14. B. Out plane vibrations a ) Wagging : two atoms oscillate up and below the plane with respect to the central atom. b) Twisting : one of atom moved up the plane while other down the plane with respect to central atom. 14

15. W ater is a triatomic nonlinear molecule having 3(3n-6) normal modes of vibration . These mode of vibrations can be calculated by applying the 2 primary forces . The force which is acting against stretching or shortening of the O-H bond. The force which is acting against the bending of H-O-H molecule . By applying these forces to the water molecule, one can determine the character of 3 normal modes of vibration of water. 15

16. Sample cells and sampling of substances: As IR spectroscopy has been used for the characterization of solid ,liquid, gas samples, it is evident that samples of different phases have to be handled. The material containing the sample must be transparent to IR radiation. A dilute solution in a non polar solvent give distorted spectra. Non polar solvents give same spectra whether they are in condensed state or in solutions in non polar solvents . Cells Very narrow 0.01mm-1mm. Teflon spacers to allow variation in path length. Sampling of solids 1. Solids run in solution 2. Solid films 3. Mull technique 4. Pressed pellet technique 16

17. 1. Solids run in solution Solids may be dissolved in a non aqueous solvent. D rop of solution is placed on an alkali metal desk and the solvent allowed to evaporate. The absorption due to solvent has to be compensated by keeping the solvent in a cell of same thickness as that containing the sample, in the path of reference beam of double beam spectrophotometer . Demerit : C an not be used for all solids because suitable solvents are limited in number & there is no single solvent which is transparent through IR region . 17

18. 2. Solid films If amorphous in nature, the sample is deposited on the surface of a KBr and NaCl cell by evaporation of a solution of the solid useful for qualitative analysis. If non crystalline, a thin homogenous film is deposited on the plate which can be mounted and scanned directly . Merit and demerit : Useful for rapid qualitative but becomes useless for carrying out quantitative analysis. 18

19. 3. Mull technique S ample is thoroughly ground until the powder is very fine. Add mulling agent (mineral oil or nujol ) just sufficient to take up the powder. T ransferred to the mull plates & plates are squeezed together. M ounded in a path of IR beam and the spectrum is run. When IR spectrum of the solid sample is taken in nujol mull, absorption bands of the sample that happen to coincide with the absorption bands of the nujol mull will be hidden, but others will be clearly seen in IR spectrum . The solid sample in the Nujol has to be used in combination with hexachlorobutadiene which absorbs in the regions 1630-1510, 1200-1140, 1010-760cm -1 . As the absorptions of nujol mull and hexachlorobutadiene appear in different regions their use in combination permits the recording of IR spectrum. Good for qualitative analysis but not for quantitative analysis. 19

20. 4. Pressed pellet technique A small amount of finely grounded solid sample is intimately mixed with about 100 times its weight of powdered potassium bromide. The finely grounded mixture is then pressed under very high pressure to form a small pellet . The resulting pellet is transparent to IR radiation & is run as such . Merits: The use of KBr eliminates the problem of bands which appear in IR spectrum due to the mulling agent as in this case no such bands appear. KBr pellets can be stored for longer periods of time . As concentration of the sample can be suitably adjusted in pellets, it can be used for quantitative analysis. The resolution of spectrum in KBr is superior to that obtained with mulls. 20

21. Demerits : It always has a band at 3450cm -1 , from – OH group of moisture present in the sample. The high pressure involved during the formation of pellets may bring about polymorphic changes in crystallinity in the samples. Sampling for gases Small size particles hence the cells are large . 10 cm to 1m long. Multiple reflections can be used to make the effective path length as long as 40 cm. Lacks sensitivity. 21

22. Sampling for liquids : Liquids samples taken . Put it into rectangular cells of KBr , NaCl etc. IR spectra obtained. Sample thickness such that transmittance lies between 15-20 % i.e. 0.015-0.05 mm in thickness . For double beam, matched cells are generally employed. One cell contains sample while other has solvent used in sample. P rotect from moisture . 22

23. Cell thickness: Methods used to measure the path length of infrared absorption cells. 1) Interference fringe method 2 ) Standard absorber method Interference fringe method is suited to cells whose windows have a high polish. Spectrum is run to produce 20-50 fringes . The cell thickness, b in centimeters is calculated as b = 1 n 2 η ῡ 1 - ῡ 2 n- number of fringes between two wavenumbers ῡ 1 and ῡ 2 η - refractive index of the sample material The standard absorber method used with a cell in any conditions & with cavity cells whose inner faces do not have a finished polish. 23

24. Reference Instrumental analysis, by Skoog , Page no. 478-480. Instrumental method of analysis, by Willard Page no. 305-308. Organic spectroscopy, by William kemp , Page no. 20-28. Instrumental methods of chemical analysis, by Chatwal , Page no. 2.31 - 2.45. 24

25. 25 For Feedback / Comments Write to - [email protected]