CLASSIFICATION OF UV ABSORPTION BANDS

UV absorption bands have fine structure due to the presence of vibrational sub-levels, but this is rarely observed in solution due to collisional broadening. As the transitions are associated with changes of electron orbitals, they are often described in terms of the orbitals involved, e.g. 

A molecule may give rise to more than one band in its UV spectrum, either because it contains more than one chromophore or because more than one transition of a single chromophore is observed. However, UV spectra typically contain far fewer features (bands) than IR, MS or NMR spectra and therefore have a lower information content. The ultraviolet spectrum of acetophenone in ethanol contains 3 easily observed bands  

Auxochromes (auxiliary chromophores) are groups which have little UV absorption by themselves, but which often have significant effects on the absorption (both and 8) of a chromophore to which they are attached. Generally, auxochromes are atoms with one or more lone pairs e.g. -OH, -OR, -NR2, -halogen. 

If a structural change, such as the attachment of an auxochrome, leads to the absorption maximum being shifted to a longer wavelength, the phenomenon is termed a bathochromic shift. A shift towards shorter wavelength is called a hypsochromic shift. 

Most of the reliable and useful data is due to relatively strongly absorbing chromophores (s 200) which are mainly indicative of conjugated or aromatic systems. Examples listed below encompass most of the commonly encountered effects. 

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Professor Claris considered by many scientists to be the father of modem polycyclic aromatic hydrocarbon chemistry. His most Important contributions to polycyclic aromatic hydrocarbon chemistry were the discovery of the anellatlon principle (Aromatic ring condensation), his pi sextet theory of PAH, the development of ben zogenic diene synthesis known as the Clar reaction, the synthesis of numerous new PAH systems, and classification of UV absorption bands of PAH corresponding to alpha, beta, and para bands. 

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