Electronic transitions:

The easiest transition includes the promotion of an electron from the highest occupied molecular orbital (HOMO) to the lowest not occupied molecular orbital (LUMO). In earlier topics we explained the creation of two molecular orbitals for the hydrogen molecule - a bonding (σ) MO of lower energy and an antibonding (σ∗) MO of higher energy. In a hydrogen molecule both of the electrons fill the σ MO resulting in stabilization. An electronic transition would include promoting one of the electrons in the σ MO to the σ∗ MO. The energy needed for this transition is very much great to be measured in the vis-uv spectrum. This is as well true for the σ-bonds present in organic molecules which mean that vis-uv spectroscopy cannot be employed to analyze saturated organic molecules.

Though, vis-UV spectroscopy is helpful in the analysis of unsaturated compounds - compounds that consist of π-bonds. While two half filled p orbitals make a π bond, two molecular orbitals are made - a more stable bonding π MO and a less stable antibonding π* MO. Both of the electrons occupy the bonding π MO and this turns into the HOMO (because it is less stable than any of the σ-MO's present in the molecule). The antibonding π* MO remains unoccupied and becomes the LUMO.