||Chapter 13: Spectroscopy
The following table contains
some data for polyenes and demonstrates how
the wavelength of the absorbance increases as the conjugated system
spectropscopy (uv = 200-400 nm, visible
= 400-800 nm) corresponds to electronic excitations between the energy
levels that correspond to the molecular orbitals of the systems.
In particular, transitions involving π
and lone pairs (n = non-bonding) are important and so uv-vis
is of most use for identifying conjugated systems which tend to have
|The lowest energy transition is that
between the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital
(LUMO) in the ground state. The absorption
EM radiation excites an electron to the LUMO and creates an excited
The more highly conjugated the system, the smaller the HOMO-LUMO gap,
and therefore the lower the frequency and longer the wavelength,
The colours we see in inks, dyes, flowers etc. are typically
to highly conjugated organic molecules. The unit of the molecule that
responsible for the absorption is called the chromophore, of
the most common are C=C (π to π*)
and C=O (n to π*) systems.
||A, a measure of the amount of
radiation that is absorbed
||Term to describe a uv-vis absorption
which are typically broad.
||Structural unit responsible for the
absorbance of a sample of molar
concentration in 1 cm cell.
||An alternative term for the molar
||l, the length of the
sample cell in cm.
||A = ε.c.l
concentration in moles / litre)
||The wavelength at maximum absorbance
||The molar absorbance at λmax
||Highest Occupied Molecular Orbital
||Lowest Unoccupied Molecular Orbital