The Origin and Position of Absorption Bands
Due to excitation of electrons

 - electrons. These are the electrons forming
single bonds. Reside in low energy states, are
strongly held and are difficult to excite

 - electrons. These are electrons involved in
the formation of C – C bonds, Carbon – hetero
atom double and triple bonds. They are more
loosely held and more easily promoted to
higher states.

n-electrons. These are non-bonding, unshared
electron pairs of hetero atoms in organic
compounds
The 3 type of electrons can be promoted to either
* or * orbitals. The transitions increase in energy
according to the following trend; n - *, - *,  *
In addition to the effects of solvent on the fine
structure of the spectra, the position of the band
may also depend on the solvent. The direction of
the shift depends upon the polarity of the solvent,
pH and temperature.
The shifts in absorption bands caused by various
effects are termed as follows;




Bathochromic (red) Shift = shift of max to
longer
wavelength
Hypsochromic (blue) Shift = shift of max to
shorter
wavelength
Hyperchromic Shift = intensity increase of the
band
Hypochromic Shift = intensity decrease of the
band
n - * transitions undergo hypsochromic shifts
when the polarity of the solvent is increased.
 - * transitions undergo Bathochromic shifts
when the polarity of the solvent is increased.
An exception to this general rule occurs in the case
of aromatic compounds where no observable shift
is evident around 260nm.
The effect of solvent on the position of absorption
bands. The wavelength of the absorption maximum
of drug substances as a function of the dialectric
constant of the solvent. (a) camphor, (b) phenol, (c)
vitamin D2, (d) flumecinol, (e) norethisterone.
Note the hypsochromic shift of the n - * band of
camphor with increasing dialectric contant.
Also note the weak Bathochromic shift of the  * transition of the vitamin D2 containing a
conjugated triene system.
The diagram also indicates a strong bathochromic
shift in the same transition in the spectrum of
norethisterone which possess an ,  - unsaturated
keto group.
Auxochromes

relates to the transition of  and n electrons in
saturated compounds.

minor spectrophotometric activities hence of
very small importance in direct pharmaceutical
analysis.

n - * transitions are forbidden so are low in
intensity. Therefore simple alcohols and ethers
are good solvents for the spectrophotometric
investigation of drug substances.

Consequently, pharm compounds containing
only hydroxyl or ether groups on an alkyl
backbone are quite inactive (e.g. glycerol,
menthol, sugars etc.)

The n - * transitions of halogens depends on
the nature of the halogen

atomic radius increases and electronegativity
decreased with increasing atomic number. nelectrons of bromine and especially iodine are
more readily excited. (e.g.iodomethane 258nm
lamda max.)

Increasing the no. of halogens attached to one
carbon causes a pronounced bathochromic
shift.
Chromophores

Concerns the n - * and  - * transitions

chromophores contain carbon-carbon, hetero –
hetero atom double bonds

Only weak bands occur when structurally
isolated.
Conjugated Chromophores

The term ‘chromophore’ relates to the fact that
the conjugation of these groups results in
intense bands in the UV and extending into the
visible.

Conjugation = double bonds occur alternately

However, no significant increase in the
intensity can be observed in the case of
cumulated double bonds, e.g. diethylketone
which contains C=C=O.

In spectra of compounds containing more than
two double bonds, additional double bonds
cause further bathochromic shift.

An increment of 30nm on the addition of one
more double bond is valid up to about seven
double bonds after which point the shift
stabilizes.

For polyene-type drugs factors such as alkyl
substitution causing bathchromic shifts, steric
effects
causing
hypsochromic
and
hypochromic shifts and loss of fine structure

A fundamentally different system exists if
double bonds are conjugated.

Delocalization of the  electrons takes place
and a uniformly distributed  electron system
forms along the bonding chain.

This results in electrons which can be more
easily promoted. This results in the
bathochromic shift and in a majority of cases a
hyperchromic shift is exhibited.

However, conjugated cyclic systems with cis
double bonds can have a lower extinction coefficient in comparison to linear trans
conjugated systems.
1,3,5-hexatriene
1,3,5-cyclooctatriene

The extinction co-efficient of 1,3,5-hexatriene
= 4,000 while that of 1,3,5-cyclooctatriene  =
79,000.

For aromatic systems, the only band of high
intensity is in the far UV, e.g., Benzene max =
185nm.
Derivative Spectroscopy
Derivative spectra:
(a) spectral band of Gaussian shape curve and its 1st
to 4th derivative.
(b) Band arising from the overlap of 2 gaussian
curves of different intensities
(c) Composite band (b) and its 1st to 4th derivatives

For both background correction and separation
of overlapping bands, derivative spectroscopy
has been the most important development in
the recent past.

The ideal Gaussian curve (a)0 (or zero order
derivative) when derivatized the maxima and
minima of the original function take zero
values.

Instead inflections are converted into maxima
and minima (a)1.

The first derivative, which is relatively easy to
generate, has little practical significance. The
same applies to other derivatives of odd order.

The most widely used derivatives in practice
are the second and fourth. As can be seen from
(c) the main peak is negative in the second
derivative and positive in the fourth.

Both systems contain secondary peaks which
increase in complexity with the order.

Derivative spectra allow very tiny differences
in original spectra to be amplified
Identification of amphetamine analogues
derivative spectroscopy.
by
(a) Basic spectrum of amphetamine and phenyl
ethylamine,
(b) Second derivative spectrum of amphetamine
and
(c) Second derivative spectrum of phenyl
ethylamine
Tutorial Questions
1/ Illustrate using a diagram the conversion of one absorbing
species to another absorbing species due to a change in
pH. Clearly label the isosbectic point and explain its
significance.
2/ What are the 3 types of electronic transitions in UV/VIS
spectroscopy. Give the trend of the increasing energy of
the transitions.
3/ What are the 3 effects which cause shifts in absorption
bands. Define the 4 types of shift and give 2 examples of
absorbance shift caused by solvent effects.