Page 348 - Instant notes

Physical Chemistry 334

UV/visible spectroscopy

Electronic energy levels are more widely separated than vibrational or rotational energy
levels because considerably more energy is needed to change the distribution (i.e.
configuration) of electrons in atomic or molecular orbitals than to change the energy of
vibration or rotation. Consequently, electronic spectroscopy is usually associated with
the visible or ultraviolet regions of the spectrum. The colors of many objects, for example
vegetation, flowers, minerals, paints and dyes, are all due to transitions of electrons from
one molecular orbital to another (see Topic I7).
The energy of ultraviolet photons is comparable with the strengths of many chemical
bonds, so in some instances the absorption of light may lead to complete bond
dissociation. The breakage of bonds in DNA by absorption of solar ultraviolet radiation is
one factor in the formation of skin cancer from exposure to sunlight.
Atomic electronic spectra are described in Topic G7.

Franck-Condon principle

The different electronic states of a molecule are often associated with different shapes of
the molecule because the different electron distribution around the molecule changes the
electrostatic Coulombic forces that maintain the nuclei in specific relative positions.
Since nuclei are considerably more massive than electrons, the Franck-Condon
principle states that:

an electronic transition takes place sufficiently rapidly that the nuclei do not change their
internuclear positions during the transition.

Consequently, when energy is absorbed in an electronic transition, the nuclei suddenly
find themselves in a new force field and at positions which are not in equilibrium for the
new electronic state. This is shown schematically in Fig. 1, in which an electronic
absorption from the ground state appears as a vertical line because of the Franck-Condon
principle. The internuclear separation of the ground state becomes a turning point, the
extent of maximum displacement, in a vibration of the excited state.
The vertical transition has the greatest transition probability but transitions to nearby
vibrational levels also occur with lower intensity. Therefore, instead of an electronic
absorption occurring at a single, sharp line, electronic absorption consists of many lines
each corresponding to the stimulation of different vibrations in the upper state. This
vibrational structure (or progression) of an electronic transition can be resolved for
small molecules in the gas-phase, but in a liquid or solid collisional broadening of the
transitions cause the lines to merge together and the electronic absorption spectrum is
often a broad band with limited structure (Fig. 2). The Franck-Condon principle also
applies to downward transitions and accounts for the vibrational structure of a
fluorescence spectrum.