Spectroscopy:
Historically speaking, a UV spectroscopy has played a significant role in the study of a huge array of natural products of plant and animal origin. UV spectrometry was used through Woodward to assign chemical structures of closely associated compounds; differing in just the location of a carbonyl group or a carbon-carbon double bond. Actually extensive studies of the UV spectra of a huge number of molecules led to the establishment of empirical rules for calculating the λmax for organic molecules. The rules proposed through Fieser, Woodward and Scott could be used to predict to a reasonable degree of accuracy the λmax of unsaturated organic elements. The details of these rules and their applications could be acquired from any book on organic spectroscopy.
The changes in spectra because of the changes in the pH of the solution or the solvent could provide meaningful information about the nature of the analyte. The modification in the polarity of the solvent alters the energies of the orbitals. This leads to change in the absorption maxima. On increasing the polarity of the solvent, the n → π ∗ transitions are shifted to lower wavelengths although the π → π ∗ transitions are shifted to longer wavelengths.
This helps in the identification and distinction of two directly related molecules.