2. A solvent is a liquid that dissolves another solid,
liquid, or gaseous solute, resulting in a solution at
specified temperature.
Solvents can be broadly classified into two categories:
Polar
Non-Polar.
A drug may show varied spectrum at particular wavelength
in one particular condition but shall absorb partially at the
same wavelength in another conditions.
These appeared changes in the spectrum are exclusively due
to various characteristic features namely-
1.Nature of solvent
2.Nature of absorption band
3.Nature of the solute
3. A) EFFECT OF SOLVENT:
The solvent exerts a profound influence on the quality and
shape of spectrum.
The absorption spectrum of pharmaceutical substance
depends practically upon the solvent that has been employed to
solubilize the substance.
A drug may absorb a maximum radiation energy at particular
wavelength in one solvent but shall absorb partially at the same
wavelength in another solvent.
Eg: acetone in n-hexane λ max at 279nm.
Acetone in water λ max at 264.5nm.
4. NATURE OF SOLVENT:
Most commonly used solvent is 95% ethanol. It is best
solvent as-
1. It is cheap
2. Has good dissolving power
3. Does not absorbs radiations above 210nm.
In choosing a solvent, consideration must be given not only
to its transparency, but also to its possible effects on absorbing
system.
There are other solvents which are transparent above
210nm.
Benzene, chloroform , carbon tetrachloride cannot be used
because they absorb in the range of 240-280 nm.
5. Common solvents used in recording UV-spectra
Solvent wavelength (nm)
Water 205
Methanol 210
Ethanol 210
Ether 210
Cyclohexane 210
Dichloroethane 220
6. CHOICE OF SOLVENT:
A suitable solvent for UV-spectroscopy should meet the
following requirements.
It should not itself absorb radiations in the region under
investigation.
It should be less polar so that it has minimum interaction
with the solute molecules.
7. 1.Polarity : plays an important role in the position and
intensity of absorption maximum of a particular
chromophore.
a) In case of non-polar solvents for eg. Iodine solution
(purple colour) the absorption maxima occurs at almost the
same wavelength as in iodine vapour(5180 A0)
b) In case of polar solvents , a brownish color is obtained
instead of purple colour , because the absorption occurs at
shorter wavelengths.
Colour change polarization of I2 by the electric field of
solvent dipoles.
9. SHIFTS OF BANDS WITH SOLVENTS
1. n→π* transition:
a) This band undergo blue shifts, since ground state with 2
electrons receive greater stabilization than excited state with
only 1 n electron.
2. π →π* transition :
a) As solvent polarity is increased this band undergoes red shift.
b) This is so, since excited state is more polar than the ground
and hence stabilization is greater relative to ground state in
polar solvents.
c) The transition of polar bonds like C=O but not ethylene,
are affected by solvent polarity
10. Example :
Effect of solvent polarity on spectrum of mesityl oxide:
Influence of solvent on emax of the n* and π excitations of
mesityl oxide
(CH3)2C=CH-C-CH3
11. 2. Purity of solvent:
Purified and certified solvents for spectroscopy should be used
as we are looking for the “smooth” absorbance curve of
solvent.
3. Resolution and interpretation of spectrum
problems:
These are resulted when solvent is used for measurement of
near/below its UV cutoff, i.e the approximate wavelength
below which they cannot be used because of absorption.
4. Dipole moments:
Absorption bands of many substances are relatively sharper
and may also exhibit fine structure when measured in solvents
of low dipole moment.
12. EFFECT OF CHROMOPHORE ON UV-
VISIBLE SPECTROSCOPY
The term chromophore was previously used to denote a
functional group, the presence of which gives color to the
compound.
Ex: nitro group is a chromophore because of which compound
attains yellow color.
Chromophore is defined as any group which exhibits
absorption of electromagnetic radiations in the visible or
ultraviolet region.
Ex: ethylene, carbonyl, acids, esters, nitro group.
13. Chromophores in which the group is having π electrons
undergo π→π* transitions.
Ex: ethylenes, acetylenes
Chromophores having both π electrons and n electrons
undergo two types of transitions.
π→π* and n→π*
Ex: carbonyl, nitryl
15. Conjugated π-bond system chromophores
In the conjugated chromophores, the electrons jump
between energy levels that are extended π orbitals, created by
a series of alternating single and double bonds, often in
aromatic systems.
Egs: azo compounds, pH indicators, lycopene, β-carotene,
and anthocyanins.
Lengthening or extending a conjugated system with more
unsaturated bonds in a molecule will tend to shift absorption
to longer wavelengths
16. Metal complex chromophores
•The metal complex chromophores arise from the splitting of
d-orbitals by binding of a transition metal to ligands.
•Examples of such chromophores can be seen in chlorophyll,
haemoglobin, hemocyanin, and colourful minerals such as
malachite.
17. CLASSIFICATION OF
CHROMOPHORES
• Independent chromophores: When a single chromophore
is sufficient to impart color to the compound. for eg. Azo
group –N=N-,nitroso group, -NO, and o- and p- quinoid
group etc. are independent chromophores.
• Dependent chromophores: When more than one
chromophore is required to produce color in chromogen
for e.g.: acetone having one ketone group is colorless,
whereas diacetyl having two ketonic groups is yellow, and
triketopentane, having three ketonic groups is orange
18. Halochromismin chromophores
• Halochromism occurs when a substance changes color as
the pH changes.
• This is a property of pH indicators, whose molecular
structure changes upon certain changes in the surrounding
pH.
20. Changes in position intensity of absorption:
position of absorption maximum and intensity of absorption
can be modified in different ways by some structural changes
or change of solvent as given below
Bathochromic shift or red shift: It involves the shift of
absorption maximum towards longer wavelength because of
presence of certain groups like –OH, -NH2 which are termed
as auxochromes Or by change of the solvents.
Ex: decrease in polarity of solvent causes a red shift in the n→
π* absorption of carbonyl compounds.
21. Bathochromic shift is also produced when 2 or more
chromophore are present in conjugation.
Ex: ethylene shows π→π* transition at 170nm where as 1,3-
butadiene shows λmax at 217nm.
22. Hypsochromic shift or blue shift: It involves the shift of
absorption maximum towards shorter wavelength and may be
caused by removal of conjugation in a system or by change of
solvent.
It is obtained by change in polarity of the solvent.
Ex: In case of aniline absorption maximum takes place at
280mµ because the pair of electrons on nitrogen atom is in
conjugation with the π bond system of the benzene ring.
In acidic solutions, a blue shift is caused and absorption takes
place at short wavelength 200mµ.
23. Hyperchromic effect: This effect involves an increase in
the intensity of absorption and is brought about by an
auxochrome.
Ex: introduction of methyl group in position 2 of pyridine
increases Ɛmax from 2750-3560.
Hypochromic shift: this effect involves an decrease in the
intensity of absorption and is brought about by an
auxochrome.
Ex: when methyl group is introduced into position 2 of
biphenyl group hypochromic effect occurs.
24. Peaks shift to longer
wavelength
Peaks shift to shorter
wavelength
Increase in the intensity of
absorption
Decrease in the intensity
of absorption
25. Auxochrome: It is a group which itself does not act as
chromophore but when attached to a chromophore shifts the
absorption maximum towards longer wavelength along with
an increase in intensity of absorption.
Ex: -OH, -NH2, -OR, -NHR, -NR2.
26. Auxochromes are of two types :
• Bathochromic groups: the groups which deepens the
colour of the chromogen , are called bathochromic groups .
Deepening of the colour means displacement to longer
wavelength. The bathochromic groups like primary,
secondary or tertiary amino groups, increase the colour
• Hypsochromic groups: those groups which diminish or
lighten the colour of the chromogenic are called
hypsochromic groups. in other words, they cause
displacement to the shorter wavelengths. For e.g.,
acetylation of –OH or NH2
26
27. Effect of conjugation: conjugation of double bonds
lowers the energy required for the transition.
Due to this molecules having conjugated groups exhibit
π→π* absorption bands within the ordinary ultraviolet range.
Ex: buta diene in hexane solution exhibits λmax 217nm. As
the number of double bond increases the absorption moves
to longer wavelength. Hence spectrum of 1,3,5,7 octa
tetraene in hexane exhibits λmax 296nm.
CH2=CH-CH=CH2 (buta diene)
CH2=CH-CH=CH-CH=CH-CH=CH2
(octatetraene)
28. Effect of conjugation on absorption spectrum
Increase in conjugation, increase absorbance of light to higher ,
bathochromic shift with hyperchromic effect.
29. If there are enough double bonds in conjugation absorption
will ultimately move into visible region and the compound
will be colored.
Ex: β carotene naturally occurring yellow pigment having 11
double bonds in conjugation owes its color to absorption in
the visible part of light.
30. HOW TO CHOOSEA SOLVENT
• Its transparency
• Its possible effects on the absorbing system
• Generally, polar solvents such as water, alcohols,
esters and ketones tend to obliterate spectral fine
structure arising from vibrational effects.
• It should not itself absorb radiations in the region.
31. • The position of absorption maxima are influenced by
the nature of the solvent.
• As a rule, the same solvent must be used when
comparing absorption spectra for identification
purposes.
• It should be less polar so that it has minimum
interaction with the solute molecules.
