UV/Vis Spectroscopy

Basics of UV Light Absorption

Ultraviolet/visible spectroscopy involves the absorption of ultraviolet/visible light by a molecule causing the promotion of an electron from a ground electronic state to an excited electronic state.

Ultraviolet/Visible light:

wavelengths (l) between 190 and 800 nm

Types of Transitions

There are several types of electronic transitions available to a molecule including:

• s to s^* (alkanes)
• s to p^* (carbonyl compounds)
• p to p^* (alkenes, carbonyl compounds, alkynes, azo compounds)
• h to s^* (oxygen, nitrogen, sulfur, and halogen compounds)
• h to p^* (carbonyl compounds)

Figure 1. Types of electronic transitions.

Transitions from the highest occupied molecular orbital (HOMO) to the lowest occupied molecular orbital (LUMO) require the least amount of energy and are therefore usually the most important.

Not all transitions that are possible will be observed. Some electronic transitions are "forbidden" by certain selection rules. However, even forbidden transitions can be observed, but these are usually not very intense.

Peak Broadening

UV absorptions are generally broad because vibrational and rotational levels are "superimposed" on top of the electronic levels.

Figure 2. Peak broadening.

For this reason, the wavelength of maximum absorption (l_max) is usually reported.

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