The General Absorption Characteristics of Molecules

The absorption characteristics of molecules depends on their nature sinee the possible electronic transitions in organic and inorganic species are different. The energetic levels and succession of molecular orbitals in organic molecules and in metal complexes have been already discussed in (Sect. 1.4), 1 will only recall here some fundamental features for the interpretation of an absorption spectrum. 

The n - n bands are associated with forbidden transitions and therefore they are generally weak, often covered by more intense ones, and their position in the spectrum is strongly dependent by the polarity of the solvent. 

The 7u - 7u bands are typical of n conjugated systems, aromatic and substituted aromatic ones. These are allowed transitions and hence generally intense ones. 

The (7 - (7 bands correspond to very energetic transitions and therefore they are positioned at low frequencies in the far UV, a spectral region that is not possible to investigate if not in inert atmosphere or in the void since the high atmospheric oxygen absorption in this range (from 200 nm downwards). 

As mentioned above the behaviour of inorganic molecules is quite different. They generally present significant absorptions also in the visible range and sometimes in the near IR (infra-red), like for example transition metal complexes. 

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