A SEMINAR ON THEORY AND PRINCIPLE OF UV-VISIBLE SPECTROSCOPY

A SEMINAR ONTHEORY AND PRINCIPLE OF UV-VISIBLE SPECTROSCOPY

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CONTENTS INTRODUCTION PRINCIPLE THEORY OF ULTRA VIOLET SPECTRA TYPES Of ELECTRONIC TRANSITIONS ABSORBANCE LAWS ABSORPTION AND INTENSITY SHIFTS

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SpectroscopyElectromagnetic RadiationWavelengthFrequencyWave-Number25/11/20153INTRODUCTION

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Any molecules has either n, or or a combination of these electrons. These bonding ( & ) 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. 25/11/20155Principle

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

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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.

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25/11/20159Cont 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 electrons, electrons, nelectrons.

TYPES OF TRANSITIONS In U.V. spectroscopy molecule undergo electronic transition involving ,, and n electrons .

Four types of electronic transition are possible.

* transition n* transition n* transition * transition

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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 Plancks relation. E=hWhere, E-energy of photon in ergs -Plancks constant(6.62410-27).

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25/11/201516LAWS 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 I0 is the intensity of radiation falling on the media, Ia is the amount of radiation absorbed and It as the amount of radiation transmitted then,

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Where I = Intensity of Incident light Ia = Intensity of absorbed lightIt = Intensity of transmitted lightc = molar concentration of sampleb = Length or thickness of the sample cell (cm)

25/11/201518 Lamberts 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.

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25/11/201520Beers 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 Lamberts law and thus,

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Where 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

25/11/201522Since absorbance A= log Io /It we can infer thatA= bc (Equation of beer Lamberts law)Where:A Absorbance or optical density. Molar extinction coefficientc 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

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DEVIATION FROM BEERS LAW

25/11/201524REASONS FOR DEVATION FROM BEER LAMBERTS 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

25/11/201525Instrumental 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.

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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.25/11/201527Chromophore:

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)

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25/11/201529THANK YOU