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 D
2
, (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 D
2
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 1 st to 4 th derivative.
(b) Band arising from the overlap of 2 gaussian curves of different intensities
(c) Composite band (b) and its 1 st to 4 th 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 by derivative spectroscopy.
(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.