Absorption Spectroscopy: Relationship of Transition Type to Molecular Structure
A very wide range of organic and inorganic substances can be analyzed using UV and visible absorption spectroscopy. The wavelengths, intensities, and fine structure of the absorption bands are determined by the structures of the neutral or ionic species concerned. For almost all substances, UV and visible absorption spectra are caused by electronic transitions. A spectrum may contain more than one region of absorption, known as a band or band system, as molecules may undergo a range of different electronic transitions.
To a first approximation, electronic transitions may be described as involving different combinations of orbitals: bonding (e.g., σ and π for organic molecules), so-called nonbonding (n), and antibonding (e.g., σ* and π*) orbitals. A simple bonding-to-antibonding promotion model of electronic transitions is an oversimplification that does not adequately explain the spectra of all compounds – for...
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