UV-VISIBLE SPECTROSCOPY

THEORY AND PRINCIPLES OF UV-VISIBLE SPECTROSCOPY

1. A SEMINAR ON THEORY AND PRINCIPLE OF UV-VISIBLE SPECTROSCOPY 25/11/2015 1

2. CONTENTS INTRODUCTION PRINCIPLE THEORY OF ULTRA VIOLET SPECTRA TYPES Of ELECTRONIC TRANSITIONS ABSORBANCE LAWS ABSORPTION AND INTENSITY SHIFTS 25/11/2015 2

3. Spectroscopy Electromagnetic Radiation Wavelength Frequency Wave-Number 25/11/2015 3 INTRODUCTION

4. 25/11/2015 4

5. Any molecules has either n, π or Ϭ or a combination of these electrons. These bonding ( Ϭ & π ) and non-bonding (n)electrons absorb the characteristic radiation and undergoes transition from ground state to excited state . By the characteristic absorption peaks, the nature of the electrons present and hence the molecular structure can be elucidated. 25/11/2015 5 Principle

6. Theory of Electronic Spectroscopy When the molecule absorbs ultraviolet or visible light, its electrons get promoted from the ground state to the higher energy state . In the ground state , the spins of the electrons in each molecular orbital are essentially paired. In the higher energy state , if the spins of the electrons are paired , then it is called an excited singlet state 25/11/2015 6

7. 25/11/2015 7 Cont… On the other hand, if the spins of the electrons in the excited state are parallel, it is called an excited triplet state. The triplet state is always lower in energy than the corresponding excited singlet state. Therefore, triplet state is more stable as compared to the excited singlet state. An excited singlet state is converted to excited triplet state with the emission of energy as light.

8. 25/11/2015 8

9. 25/11/2015 9 Cont… The higher energy states are designated as high energy molecular orbitals and also called antibonding orbitals. Energy absorbed in the ultraviolet region produces changes in the electronic energy of the molecule resulting from transition of valence electrons in the molecule. Three distinct types of electrons are involved organic molecule . These are as follows ῾ Ϭ ᾽ electrons, ῾π᾿ electrons, ῾ n ᾿ electrons .

10. TYPES OF TRANSITIONS In U.V. spectroscopy molecule undergo electronic transition involving Ϭ, π , and n electrons . Four types of electronic transition are possible . Ϭ→Ϭ* transition n→Ϭ * transition n→ π* transition π→π* transition 25/11/2015 10

11. 25/11/2015 11 Cont… The energy change associated with the molecular electronic transitions provide information on the structure of a molecule and determines many molecular properties such as color. The energy involved in the electronic transition and the frequency of a radiation is given by Planck ᾿ s relation . E=h Ѵ Where, E-energy of photon in ergs Ѵ -Planck‘s constant(6.624×10 -27 ).

12. 25/11/2015 12 Cont… 12

13. 25/11/2015 13

14. 25/11/2015 14

15. 25/11/2015 15

16. 25/11/2015 16 LAWS GOVERNING ABSORPTION OF RADIATION When a beam of light falls on a solution or homogeneous media, a portion of light is absorbed within the medium while the remaining is transmitted through the medium. Thus if I is the intensity of radiation falling on the media, I a is the amount of radiation absorbed and I t as the amount of radiation transmitted then,

17. 25/11/2015 17 Where I = Intensity of Incident light I a = Intensity of absorbed light I t = Intensity of transmitted light c = molar concentration of sample b = Length or thickness of the sample cell (cm)

18. 25/11/2015 18 Lambert’s Law : When a monochromatic light passes through an absorbing medium at right angles to the plane of surface of medium or solution, the rate of decrease in intensity with thickness of medium (b) is proportional to the intensity of incident light. In other words the intensity of transmitted light decreases exponentially as the thickness of medium increases arithmetically.

19. 25/11/2015 19

20. 25/11/2015 20 Beer’s Law: Bernard and Beer independently stated that ‘The intensity of incident light decreases exponentially as the concentration of absorbing medium increases arithmetically. This is similar to Lambert’s law and thus,

21. 25/11/2015 21 Where ε is the Molar extinction coefficient , a constant dependent upon the wavelength of incident radiation and the nature of absorbing material and the concentration is expressed in gram mole/litre

22. 25/11/2015 22 Since absorbance A= log I o /I t we can infer that A= εbc ( Equation of beer – Lambert’s law ) Where: A – Absorbance or optical density. ε – Molar extinction coefficient c – Concentration of the drug (mol/lit) b – Path length (normally 10mm or 1cm) Or A= abc a –absorptivity, if concentration is expressed in grams/litre

23. 25/11/2015 23 DEVIATION FROM BEER ‘S LAW

24. 25/11/2015 24 REASONS FOR DEVATION FROM BEER LAMBERT’S LAW: True deviation : True deviations are related to the concentration of the absorbing substance. Beers law holds good only for dilute solutions. Chemical deviation : Chemical deviations arise if the absorbing species undergo chemical changes such as association, complex formation, dissociation, hydrogen bonding, hydrolysis, ionization or polymerization

25. 25/11/2015 25 Instrumental Deviation : Only monochromatic light gives beers law use of polychromatic light gives negative deviation. Any fluctuations in intensity of light, change in the sensitivity of detector, improper slit width can lead to deviation from beer lamberts law.

26. ABSORPTION AND INTENSITY SHIFTS: 25/11/2015 26

27. The term chromophore is defined as the any system which is responsible for imparting color to the compound . Auxochrome: Its is defined as any group which does not itself act as a chromophore but whose presence brings about shift of the absorption band towards the red end of the spectrum. 25/11/2015 27 Chromophore:

28. Instrumental Methods Of Chemical Analysis by Gurdeep R.Chatwal;Sham K.Anand (page no:2.149-2.154). Text Book Of Pharmaceutical Analysis by Dr.S.Ravi Sankar.(pg.no:2-1:2-5) 25/11/2015 28 Reference:

29. 25/11/2015 29 THANK YOU