Visible and U.V. spectroscopy have a number of applications. Some of them are-
1. Assignment of structure of compounds: If two compounds show same absorption spectra means they should same structure. If structure of one of them is known, other’s can be determined which can be illustrated by the following example.
(i) Instain obtained from indigo has one of the following structures:
It was observed from structure (III) and (IV) that spectra of instain is same as that of its methyl derivative (IV) . Thus its structure should be like (II).
Fictional group present in a compound can be identified by its characteristic absorption band (?max) . For example if ?max is 210 mμ (nm) , it means -COOH group is present in that compound.
If molecule is in two tautomeric forms it can be studied by its absorption band.
Identification of Compound: If the absorption band of an unknown compounds are found similar to that of a known compounds it means they have same structure. This technique in which spectra of an unknown compound is compared with that of an unknown compound is called figer printing.
Presence of Hydrogen bonding: Presence of H-bond in a molecule leads to hypsochromic effect. For example if hexane is used for acetone (as solvent) ?max occurs at 264 nm. This is due to formation of hydrogen bond between acetone and water.
4. Determination of cis-trans isomerism : Cis and trans isomers can be identified with the helf of UV & visible spectroscopy for example stilbene has following cis-trans isomers :
In cis form two phenyl groups are not in plane due to steric hindrance where as in trans form they are in same plane due to their opposite direction. Thus in trans form π-π^* transition takes place easily at 295 nm where as in cis form transition takes place at high energy (lower wave length) ?max = 280nm . It means higher ?max in stilbene is found in its trans form.
