UV-vis spectroscopy probes the electronic transitions of a molecule. The "electronic transitions" of a molecule depends on the bond structure of the molecule. To be able to access bonding information using UV or visible light, the molecule must have a series of alternating double and single bonds, called an extended Pi system because of molecular orbitals alternating double and single bonds represent.

Benzene is the classic example of a molecule with an extended Pi system, as can be seen from its circular arrangement of alternating double and single bonds

This UV-vis spectrum of benzene is fairly typical of modern UV spectra. The absorbance of the sample is plotted vs. wavelength as is typical , but an average UV-vis spectrum usually goes from approximately 200 nm up to 800 nm, rather than just 160 nm to 210 nm in a UV spectrum. The broad peaks shown above are characteristic of UV absorption, but the bumpiness of benzene between 190 nm to 200 nm is worth noting. Because UV-vis absorption occurs as a result of electronic absorption and subsequent excitation into a higher-energy orbital, it can be surmised that the "bumps" are a result of several specific electronic transitions occurring in the cyclic conjugated Pi system of benzene.

One major disadvantage of UV-vis spectroscopy is that most peaks are very broad. As a result, it is difficult to distinguish two peaks close together on the basis of UV-vis spectroscopy alone.

Sources

NIST Chemistry Web Book. http://webbook.nist.gov/chemistry/ First accessed 11/00.