Ultraviolet and Visible Spectroscopy
It is a type of Absorption Spectroscopy in which Electromagnetic Radiation of UV region (l = 200 – 400 nm) or visible region (l = 400 – 800 nm) when passed through a sample containing a multiple bond, a part of incident radiation is absorbed by the sample (compound). Amount of absorbed Radiation Energy depends upon Wavelength of radiation and nature of the sample (compound). The absorbed radiation excites electron from lower energy level to higher energy level so electrons transferred from bonding orbital to the anti-bonding orbital. The amount of absorbed radiation is measured with spectrophotometer.
Tungsten filament Lamp and hydrogen discharge lamp are used as source of light (energy) for visible and Ultraviolet region respectively. Sample is hold in a cell, generally cell is placed between slit of spectrophotometer. And
Those isolated groups which exhibit characteristic absorption in UV region. So these group Absorb UV radiation and known as chromophores. Generally they are covalently unsaturated groups (contains double bonds).
Actually, those functional groups that involve n-π* and π-π* transitions are known as chromophores.
An auxiliary group which shifts absorption band towards longer wavelength is known as auxochrome. Auxochrome is a saturated group having non-bonding or n-electrons which when attached to chromophore changes both the intensity of bond and absorption maxima.
Some of the shifts in absorption maxima have characteristic names like
1. Bathochromic shift :
The shifting of absorption bands towards the longer wavelength is known as Bathochromic shift. It is also known as Red shift.
2. Hypsochromic shift :
The shifting of absorption bands towards the shorter wavelength is known as Hypsochromic shift. It is also known as blue shift.
3. Hyperchromic shift :
If the presence of a group increases the intensity of the intensity of the band. It is known as hyperchromic shift.
4. Hypochromic shift :
If the presence of a group decreases the intensity of the intensity of the band. It is known as hypochromic shift.
Application of ultraviolet spectroscopy
1. Identification of a compound :
The absorption spectra of a compound is its characteristic property. Value of l max at which maximum absorption take place is note same for two compounds i.e. every compound have a particular wavelength at which maximum absorption takes place. This property is used to identify a compound. So a spectrum of unknown compound is compared with standard spectra to identify a compound.
2. Identification of geometrical isomers :
cis and trans isomer are differentiated by the study of UV spectrum. In trans isomer π-π* transition take place at the higher wavelength while in cis isomer it take place at lower wavelength.
3. Calculating molecular weight of a compound :
Prepare 1% solution of organic compound and fill it in 1 cm thick cell and determine its absorbance. Suppose A is absorbance, M is molecular weight then molar concentration is c = 10/M, cell length l = 1 cm. According to Lambert-Beer’s law-
A = ecl
A = e * (10/M) * 1
M = 10e/A
4. Study of reaction kinetics :
Study of reaction kinetics can be made by studying the absorption spectrum of products and reactants time to time. As l max for product and reactant is different so their concentration can be measured at any stage of the reaction.
5. Functional group Analysis :
UV Spectroscopy is used in identification of some functional groups.
6. Ascertaining of purity :
Every compound have a particular Spectrum if additional bands are found then given compound is impure (on comparison with standard Spectrum).
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