(antibonding) orbitals are shown in Figure 9.7(a). The relative energies of these orbitals and that of a
non-bonding orbital n, which may be occupied by electrons not participating in bonding, are given in
Figure 9.7(b).
In most organic compounds the bonding and non-bonding orbitals are filled and the antibonding orbitals
are vacant. From the diagram it will be seen that the lowest energy and therefore the longest wavelength
transitions are from non-bonding orbitals to antibonding π orbitals, i.e. n→π*. These give rise to bands
in the near UV and visible regions. Other allowed transitions in order of increasing energy (shorter
wavelength) are n→σ and π→π, which have comparable energies, and σ→σ*. The latter occur in the
far UV or vacuum region below 200 nm and are of little use analytically. Consequently, saturated
hydrocarbons which are transparent in the near UV and visible regions make useful solvents. Intense
bands (large ε) are produced by σ→σ and π→π transitions, whereas those arising from n→σ* and
n→π* transitions are characteristically weak because of unfavourable selection rules.
Figure 9.7
Shapes and relative energies of
molecular orbitals.
(a) Bonding and antibonding orbitals.
(b) Relative energiesof orbitals and
possible transitions between them.Unsaturated groups, known as chromophores, are responsible for n → π, and π → π absorption
mainly in the near UV and visible regions and are of most value for diagnostic purposes and for
quantitative analysis. The λmax and ε values for some typical chromophores are given in Table 9.2. The
positions and intensities of the absorption bands are sensitive to substituents close to the chromophore,
to conjugation with other chromophores, and to solvent effects. Saturated groups containing
heteroatoms which modify the absorption due to a chromophore are called auxochromes and include –
OH, –Cl, –OR and –NR 2.