CH437 CLASS 19
ULTRAVIOLET-VISIBLE SPECTROSCOPY 2
Synopsis. Effect of conjugation on spectra. Conjugated dienes, , -unsaturated
carbonyl compounds and aromatic compounds.
Interpretation of UV Spectra: Effect of Conjugation on the UV-Visible
Spectra of Conjugated Dienes
The lower the energy gap between the HOMO and LUMO, the lower the energy
of the photon that is needed to promote the * transition, and hence the
longer the wavelength.
Conjugation has the effect of lowering the HOMO-LUMO energy gap, so that
extensively conjugated compounds can absorb in the visible region of the
electromagnetic spectrum and they appear colored. As a first example, compare
the MO diagrams for ethylene and 1,3-butadiene.
_
C
+
_
+
C
_
C
+
2* 
LUMO
_
+
C
_
_
C
+
C
+
C
_
C
C
_
1 
C
_
C
C
+
C
C
C
LUMO
2
HOMO
+
C
C
C
_
ETHYLENE
= nodal plane
3*
_
HOMO
C
4*
_
+
+
+
+
C
_
1,3-BUTADIENE
1
The most important transition is the HOMO-LUMO transition (* for ethylene
and 2*3 for 1,3-butadiene). This corresponds to (max ~175 nm for ethylene
and 217 nm for 1,3-butadiene, so clearly
conjugation closes the HOMO-LUMO energy gap and increase max
(bathochromic shift), as shown below
This gives rise to the UV spectrum below, where absorbance is plotted against
wavelength ().
Taking the argument further, compare 1,3-butadiene (max = 217 nm) and 1,3,5hexatriene (max = 258 nm). From this UV spectral data, we can calculate the
HOMO-LUMO energy gap as 553 kJ/mol and 465 kJ/mol for 1,3-butadiene and
1,3,5-hexatriene, respectively. The spectra below show the HOMO-LUMO (*
type) transitions for conjugated dienes with up to five double bonds.
Colored Organic Compounds
Extension of conjugation can lead to such a low HOMO-LUMO energy gap that
the molecule absorbs in visible region of the electromagnetic spectrum and the
compound appears colored. Such an example is -carotene, with 11 conjugated
double bonds: its UV/visible spectrum and structure are shown below.
When normal (“white”) light strikes –carotene, the wavelengths from 400 nm to
500 nm are absorbed, corresponding to green – blue, while other wavelengths
are reflected or transmitted to our eyes. We see white light with green-blue
removed and hence we perceive a yellow-orange color for –carotene.
Effect of Substituents
For conjugated systems, just as extension of conjugation influences max, so
does the presence of substituents (auxochromes). This occurs through
interaction of the main chromophore -system with either the non-bonded
electrons of a heteroatom (conjugation) or the electrons bonded in a C-H group
(hyperconjugation). Either way, the net result is an extension of the chromophore
conjugated -system, and a concurrent decrease in the HOMO-LUMO energy
gap, as shown below.
CONJUGATION
_
+
C
_
C
+
2
+
C
C

3
*
_

+
n
_B
1
C
C
..
C
C
B
..
B
+
or
B
_
HYPERCONJUGATION
H
+
+
+
+
C
C
_
_
C
_
H
H
UV-Visible Spectra of , -Unsaturated Carbonyl Compounds
Two principal transitions are associated with the carbonyl group, the weak
(forbidden) n* and the strong (allowed) * transitions, but the latter is
below the cutoff points of solvents:
*
n
~280 nm (weak)
190 nm (strong)

Substitution on the carbonyl group by an auxochrome with a lone pair of
electrons (e.g. –NR2, -OR, NH2, -OH, or -X as in amides, esters, acids or acid
halides) leads to a hypsochromic effect on the n* transition and a lesser
bathochromic shift on the * transition, but latter nearly always remains below
the solvent cutoff wavelength. The hypsochromic effect, due to inductive (-I)
effects of the more highly electronegative nitrogen, oxygen or halogen atom, is
illustrated for the n* transition of the acetyl (CH3-C=O) group, below.
If the carbonyl group is part of a conjugated system, both the n* and the *
are shifted to longer wavelengths (bathochromic shifts) and the latter becomes
much more intense (hyperchromic shift). The former transition eventually
becomes hidden by the latter, as illustrated below, showing the UV-visible
spectra of conjugated aldehydes (CH3-(CH=CH)n-CHO).
The diagram below (not drawn to scale) illustrates the influence of conjugation on
the molecular oribital energies and corresponding changes in the wavelengths of
the n* and particularly the * transition.
Substituents and other features influence the * transition of the basic
chromophore in a predictable way. A study of these led to the formulation of
empirical rules for ,-unsaturated carbonyl compounds by Woodward and
Fieser (Class 20).
UV-Visible Spectra of Aromatic Compounds
Absorptions that result from transitions within the benzene chromophore are
more complex than the MO system would suggest: three absorption bands are
actually observed. These are at max ~ 184 nm (max ~ 47,000) (allowed), max ~
202 nm (max ~ 7,400) (forbidden) and max ~ 255 nm (max ~ 230) (forbidden).
The  MO system of benzene is shown below.
Symmetry effects, combined with electron-electron repulsions, result in
modification of the energy levels (from the MO energy level diagram above) that
are involved in the UV absorption spectrum of benzene. They are basically *
transitions:
E 1u
B 1u
200 nm
(forbidden)
255 nm
(forbidden)
B 2u
180 nm
(allowed)
A 1g
The A1gB2u transition gives rise to the low intensity “secondary band” at ~255
nm and the A1gB1u transition results in the higher intensity “primary band” at ~
202 nm in benzene. Both these transitions are influenced by substituents on the
benzene ring.
Electron releasing groups (donors) increase the max value of both primary and
secondary bands (red shift or bathochromic shift), as well as max (hyperchromic
shift). Electron withdrawing groups have the same effect on max and max of the
primary band, but effects on the secondary band are less pronounced. Influence
on the primary band transition is caused by extension of the delocalized by
aromatic -system because of interaction with molecular orbitals on the
substituent group:
..+
OCH3
..
: OCH3
..
O:
H
H
C
C
_:
.. _
:
O
..
+
etc
etc
Influence of substituents is summarized in the table below.
Substituent
Primary
Secondary
max/nm
max/cm l mol-1
max/nm
max/cm l mol-1
H
203.5
7400
254
204
CH3
206.5
7000
261
225
Cl
209.5
7400
263.5
190
Br
210
7900
261
192
OH
210.5
6200
270
1450
O-
235
9400
287
2600
OCH3
217
6400
269
1480
NH2
230
8600
280
1430
NH3+
203
7500
254
169
CN
224
13000
271
1000
CO2H
230
11600
273
970
CO2-
224
8700
268
560
COCH3
245.5
9800
CHO
249.5
11400
NO2
268.5
7800
The spectra (max) of substituted benzoyl derivatives can be predicted by the use
of Scott’s empirical rules (see Class 20).
Both the primary and secondary bands in the spectra of polynucear aromatic
hydrocarbons are shifted to higher wavelength – even the lower wavelength
primary band is shifted to wavelengths over 200 nm, readily accessible by
standard UV instruments. The spectra of naphthalene and anthracene are shown
below.