UV/Vis Spectroscopy

UV/Vis spectroscopy provides information about compounds with conjugated double bonds.

Ultraviolet light and visible light have just the right energy to cause an electronic transition—the promotion of an electron from one orbital to another of higher energy.

Depending on the energy needed for the electronic transition, a molecule will absorb either ultraviolet or visible light.

Ultraviolet light is electromagnetic radiation with wavelengths ranging from 180 to 400 nm (nanometres); visible light has wavelengths ranging from 400 to 780 nm.
The normal electronic configuration of a molecule is known as its ground state—all the electrons are in the lowest-energy molecular orbitals.
When a molecule absorbs light of an appropriate wavelength and an electron is promoted to a higher energy molecular orbital, the molecule is then in an excited state.
Thus, an electronic transition is the promotion of an electron to a higher energy molecular orbital MO. The relative energies of the bonding, nonbonding, and antibonding molecular orbitals are shown in the below Figure.