two vibrational levels in the ground and first excited electronic states respectively. At room temperature
all molecules will be in the ground electronic state and probably in the lowest vibrational level.
Transitions to any vibrational level in the first excited electronic state are allowed by the spectroscopic
selection rules so that the spectrum will consist of a series of closely spaced converging lines, the
intensities varying with the respective transition probabilities. Each line may be further resolved into
lines on either side of it due to transitions between the even more closely spaced rotational levels which
are associated with each vibrational level. Such vibrational and rotational fine structure lines are not
usually observed for samples run in solution because of physical interactions between solute and
solvent molecules which cause collisional broadening of the lines. The resulting overlapping bands
coalesce to give one or more broad band-envelopes. These are characterized by the position of each
maximum λmax and the corresponding intensity or molar absorptivity ε. For polyatomic molecules and
metal complexes, the complete spectrum may contain several bands arising from a number of electronic
transitions and their associated rotational and vibrational fine structures (Figure 9.6). The origins of
these bands are discussed in the following sections.
Figure 9.6
UV visible absorption spectrum of bis
(8-hydroxyquinoline)Cu(II).Polyatomic Organic Molecules
According to molecular orbital theory, the interaction of atomic orbitals leads to the formation of
bonding and antibonding molecular orbitals. Depending on the nature of the overlapping atomic
orbitals, molecular orbitals may be of the σ type, the electron density being concentrated along the
internuclear axis, or of the π type where the electron density is concentrated on either side of the
internuclear axis. Electron density probability contours for electrons occupying σ and π (bonding) and
σ and π