Quantitative molecular orbital MO studies

The most significant approach to the computation of the light absorption characteristics of a molecule has been made using the MO view of covalency. According to Griffiths chromogens can be of the following types  

Type of orbital AE t (7 — anti-bonding 71 — anti-bonding n — non-bonding 71 — bonding a — bonding 

A number of theoretical techniques are used in attempts to predict absorption characteristics and include the linear combination of atomic orbitals (LCAO), free electron (FEMO), Hiickel (HMO) and Pariser-Parr-Pople (PPP) method. The last named is the most successful technique so far for the computation of the absorption spectra of hypothetical dye molecules. Such information is helpful to the industrial colour chemist since unpromising members of a group of dyes to be investigated but as yet unsynthesized can be eliminated with obvious savings in research costs. In the same way proposed dyes having lower absorption maxima and therefore lower tinctorial strength than known, acceptable dyes could likewise be ruled out on economic grounds. 

F or more comprehensive accounts of the molecular orbital approach see the Bibliography at the end of this chapter. 

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