between the highest filled plevel and the lowest empty p* level is smaller, and
hence the wavelength of that absorption is greater.
Thus, ethene CH 2 ==CH 2 absorbs at about 180 nm (e=1500 m^2 mol-^1 ), whereas
butadiene, CH 2 ==CH--CH==CH 2 absorbs at 210 nm (e=2100). Long-chain
conjugated polyenes, such as the carotenes absorb in the visible region with a
very high e. There is a general rule that states ‘the longer the chromophore, the
longer the wavelength at which is absorbs’. It is also generally true that mole-
cules possessing extended conjugation have more intense absorption bands and
more complex spectra.
Similar arguments apply to conjugation between carbonyl double bonds and
carbon-carbon double bonds.
Solvent effects are important, both in considering the position of the absorp-
tion maximum and also the nature of the spectral transition involved. For p-p*
transitions, the excited state is more polar than the ground state, so it will tend
to form dipole-dipole bonds with a polar solvent, such as water or ethanol. This
will lower the transition energy and raise the absorption peak wavelength. This
is called a red shift (or a bathochromic shift). Tables of solvent corrections are
available in specialist texts.
For n-p* transitions, the ground state is often more polar and may form
hydrogen or dipole bonds with polar solvents. This increases the transition energy
and lowers the peak wavelength, causing a blue shift (or hypsochromic shift).
pH will affect the structures of compounds with acidic or basic groups, and
may cause considerable wavelength shifts. This is most evident in acid-base
indicators such as described in Topic C4.
Substituents that alter the wavelength or absorptivity of a chromophore
significantly are called auxochromes, and tables of the effect of substituents plus
rules for their application in particular structures are to be found in specialist
texts. For example, an unsubstituted, unsaturated ketone would have a peak
maximum at about 215 nm. Substitution of a hydroxyl group on the carbon next
to the carbonyl (a) raises the peak to 250 nm, and two alkyl groups on the next
(b) carbons would raise it to 274 nm.
Table 2 lists a few of the substituent effects for aromatic compounds. It should
be noted that the phenoxide ion (-O-), which is present in alkaline solutions of
phenols, absorbs at a considerably longer wavelength than the parent phenol
(-OH). Generally electron donating and lone-pair substituents cause a red shift
and more intense absorption. More complex shifts arise when there is more thanE9 – Ultraviolet and visible molecular spectrometry: applications 229
Table 2. Absorption maxima for some monosubstituted benzenes Ph-R (in methanol or
water)
R Maxima/nm
-H 204 254
-CH 3 207 261
-Cl 210 264
-OH 211 270
-OCH 3 217 269
-CO 2 - 224 271
-COOH 230 280
-NH 2 230 280
-O- 235 287