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Chromophore, auxochrome & solvent effect in u.v. visible spectroscopy:

Chromophore, auxochrome & solvent effect in u.v. visible spectroscopy Presented By - Vaishnav Rajdip Lalitkumar M.Pharm. (1 st year ) Pharmacology department, S.C.O.P.

introduction:

introduction U.V. visible spectroscopy or U.V. visible spectrophotometry is a form of absorption spectroscopy. The U.V. visible spectroscopy is concerned with the u.v. and visible regions of the electromagnetic spectrum which ranges between 200-780 nm. The theory based on electronic transition occurring in the valence electrons present in a molecule , absorb the sufficient radiant energy from u.v. visible region. Then electrons undergo the process of transition. 2

Factor governing absorption of radiation in the uv/visible region :

Factor governing absorption of radiation in the uv/visible region Chromophore Auxochrome Solvent effect Chromophore : A chromophore is a covalently unsaturated group responsible for absorption of ultraviolet or visible radiations. A compound which contains chromophore termed as “chromogen” . 3

Choromophore continue…:

Choromophore continue… Eg: (Unsaturated linkage -C=C-, -N=N-) Ethylene, acetylene, carbonyls, acids, esters , nitrile groups. Require less energy for electronic transition. Types of chromophores : First Type/ Dependent chromophore Second Type/Independent 4

Types of chromophore :

Types of chromophore First Type : Electrons undergo transition of П П* . This also called as dependent chromophore . Eg: Ethylene, acetylene. Second Type : This type both П & n electrons undergo 2 transitions in a molecule. П П* and n П* transition in molecule. This also termed as independent chromophore . Eg: Azo, carbonyls, nitriles and nitro compounds 5

Identification of chromophore:

Identification of chromophore Spectrum having a band near 300mµ may possess 2 or 3 conjugated units. Absorption bands near 270-350mµ with very low intensity ε max 10-100 are because of n П* 4 transition of the carbonyl group. Chromophore Transition App.. Value of λ max (nm) App.. Value of ε max (nm) solvent -C=O n σ* 160 18,000 Hexane n П* 285 15 -COOH n П* 205 60 Methanol 6

Slide 7:

Simple conjugated chromophores like dienes or α , β -unsaturated ketones have ε max values, i.e., from 10,000 to 20,000. The absorption with ε max value between 1,000-10,000 reveals the presence of an aromatic system. Changes in position and intensity of absorption : For isolated chromophore groups such as -C=C-,-C≡C-,absorption takes place in far ultraviolet region which cannot be easily studied. 7

Slide 8:

Bathochromic shift or Red shift Hypsochromic shift or Blue shift Hyperchromic effect Hypochromic effect 8

Ephedrine : the benzoid chromophore:

Ephedrine : the benzoid chromophore Ephedrine has the simplest type of benzene ring chromophore. Spectrum similar to that of benzene with a weak symmetry forbidden band 260 nm with an A (1%,1cm) value of 12. There are no polar group attached to or involved in the chromophore. Chromophore does not interact strongly with the solvent . Drugs having a chromophore like of ephedrine include: diphenhydramine, amphetamine, ibuprofen . 9

Auxochrome :

Auxochrome It is a group, itself does not act as a chromophore but when attached to a chromophore it shifts the adsorption maximum towards longer wavelength alongwith increase in the intensity of absorption. Known auxochromic groups are –OH,– NH 2 , –OR,– NHR. e.g. When – NH 2 group is attached to benzene ring, absorption changes from λ max 255 to λ max 280. 10

Solvent :

Solvent Solvent is in which solutes is dissolved completely. Solvent is the important factor for u.v./visible spectroscopy. Ideal solvent : It should be cheaper. It should be easily available. It should be transparent & less polar Should not possess any kind of absorption when it is exposed to radiation. 11

Solvent effect:

Solvent effect A most suitable solvent which does not absorb the radiation. Most commonly used solvent is 95% ethanol, it is cheap and is transparent down to 210m μ . Commercial ethanol is not used because it is having benzene which absorbs strongly in u.v.region. 12 Solvent λ of absorption (m μ ) Ethanol 210 Hexane 210 Methanol 210 Water 205 Benzene 280 Chloroform 245

Slide 13:

The position as well as the intensity of absorption maximum get shifted for particular chromophore by change in the polarity of solvent. The A max for the non-polar compounds is usually shifted by change in polarity of solvent. α, β - unsaturated carbonyl compounds show 2 different shifts : n П* transition, absorption band moves to shorter wavelength. Due to H-bonding with solvent molecules occurs to lesser extent with the carbonyl group in the excited state. E.g. A max of acetone is 279m μ in hexane, while in water 264m μ . 13

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П П* transition, absorption band moves longer wavelength. The dipole-dipole interactions with the solvent molecules lower the energy of the energy of the excited state more than that of the ground state. Value of A max in ethanol will be greater than that in hexane. П* orbital gets more stabilised by H-bonding with the polar solvent like water & ethanol. Because of greater polarity of П* orbital. Thus small energy will need for transition and absorption shows a red shift. 14

Slide 15:

n σ* transition is very sensitive to H-bonding. Alcohol as well as amines form bondnage with the solvent molecules. This occur because of the presence of non-bonding electrons on hetero atom. Thus transition require greater energy. 15

References ::

References : Willard,Merritt . Instrumental methods of analysis: CBC publication,7 th Edn.,184-86. Chatwal . Instrumental methods of analysis: Himalaya publication,5 th Edn.,2.157-2.161. David watson . A textbook for pharmacy student & pharmaceutical chemist :Elsevier churchhill livingstone , 2 nd Edn.,89-97. Kalsi . Spectroscopy of organic compound :6 th Edn.,14-23 16

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