Note ↑: Organic σ bonds have a large gap between bonding and antibonding orbitals. To excite electrons in such bonds requires wavelengths much below the practical range (< 200 nm). As a result, the technique has found its major uses in the study of π systems, in which filled and unfilled orbitals are much closer in energy. Excitation of such electrons gives rise to π → π* transitions. Nonbonding (n) electrons are even more readily promoted through n → π* transitions (Figure 14-14). Because the number of molecular orbitals is equal to that of the component p orbitals, the simple picture presented in Figure 14-14 is rapidly complicated by extending conjugation: The number of possible transitions skyrockets, and with it the complexity of the spectra.

In Summary: UV and visible spectroscopy can be used to detect electronic excitations in conjugated molecules. With an increasing number of molecular orbitals, there is an increasing variety of possible transitions and hence number of absorption bands. The band of longest wavelength is typically associated with the movement of an electron from the highest occupied to the lowest unoccupied molecular orbital. Its energy decreases with increasing conjugation.