Basics of UV-visible absorption spectroscopy

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Electronic transitions

After excitation by electromagnetic radiation, valence electrons take part in electronic transitions. Only four different transitions are allowed between σ, π and lone pair electrons.

Vertical arrow with the label Energy. On the right are different levels of increasing energy. Red arrows go from one level to another. The central level is labeled n. The lowest level is sigma, then pi, n, pi* and sigma*.

Transition σ → σ*

  • occurs only for simple bonds (C-H or C-C for example)
  • requires a high quantity of energy, and is thus found in the UV region (

Transition n → σ*

  • occurs for atoms with lone pair electrons (N, O, S….)
  • requires a high quantity of energy, and is thus found between 150 and 250 nm

Transition n → π* and n → π

  • occurs only for atoms with lone pair electrons and for unsaturated functions
  • there is sometimes a transition n → π when the level of the doublet is below the level of the π orbital
  • these transitions usually take place in the UV domain

Transition π → π* (aromatics)

  • occurs only for atoms with unsaturated functions
  • are located in the UV and visible domain (200 to 700 nm)
Different transitions are positioned on a horizontal wavelength scale. These go from 0 to 800 nm, the values below 400 are the UV spectrum, from 400 to 800 nm the visible spectrum. The transitions are the following, in ascending order: sigma -> sigma*, n -> sigma*, pi -> pi*, n -> pi* and finally n -> pi.
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