Uv visible-spectroscopy

Uv visible-spectroscopy

1. Ultraviolet-Visible Spectroscopy
The Chromophore Concept
Chromophore: It is defined as any isolated covalently bonded
group that shows a characteristic absorption in the ultraviolet or
visible region. Some of the important chromophores are ethylenic,
acetylenic, carbonyls, acids, esters, and nitrile group etc.
–CH
2
–, – CΞC –, – COOH–, – CΞN etc.
Types of Chromophores
There are two types of chromophores as;
a)Chromophores in which the group contains π electrons and they
undergo π → π * transitions. Such chromophores are ethylene,
acetylenes etc.

2. Ultraviolet-Visible Spectroscopy
b) Chromophores which contains both π electrons and n (non-bonding)
electrons. Such chromophores undergo two types of transitions i.e., π → π* and
n → π*. Examples of this type are carbonyls, nitriles, azo compounds, nitro
compounds etc.
Identification of a chromophore depends on a number of factors as follows;
i) Spectrum consisting of a band near 300 mµ may contain two or three
conjugated units.
ii) Absorption bands near 270-350 mµ with very low intensity ε
max
10-100 are
due to n → π*transitions of the carbonyl group.
iii) Simple conjugated chromophores such as dienes or α, β- unsaturated
ketones have high ε
max
values, i.e., 10,000 to 20,000.
Azo compound Nitromethane

3. Ultraviolet-Visible Spectroscopy
iv) The absorption with ε
max
values between 1000-10,000 shows an
aromatic system.
See Table T
2
-1, Simple Unconjugated Chromophores . Ref,
Elementary Organic Spectroscopy by Y. R. Sharma.
Auxochrome
Auxochrome: It is defined as any group which does not
itself act as a chromophores but whose presence
brings about a shift of the absorption band towards
the red end of the spectrum (longer wavelength).

4. Ultraviolet-Visible Spectroscopy
Auxochrome (Contd.)
An auxochrome group is called color enhancing group.
Auxochromic groups do not characteristic absorption
above 200 mµ. Common auxochromic groups are –
OH, -OR, -NH
2
, -NHR, -NR
2
, -SH etc.
The effect of the auxochrome is due to its ability to
extend the conjugation of a chromophore by sharing
of its non-bonding electrons.

5. Ultraviolet-Visible Spectroscopy
Absorption and Intensity Shifts
Figure: Absorption and intensity shifts

6. Ultraviolet-Visible Spectroscopy
Absorption and Intensity Shifts
1.Bathochromic shift/effect (Red shift)
2.Hypsochromic shift/effect (Blue shift)
3.Hyperchromic effect
4.Hypochromic effect
Bathochromic shift/effect (Red shift): It is an effect due to
which the absorption maximum is shifted towards
longer wavelength for the presence of an auxochrome
or by the change of polarity of solvent. The n → π*
transition for carbonyl compounds experiences red shift
when the polarity of solvent decreased.

7. Absorption and Intensity Shifts (Contd.)
Hypsochromic shift/effect (Blue shift): It is an effect due to which the
absorption maximum is shifted towards shorter wavelength for
the removal of conjugation (auxochrome) or by the change of
polarity of solvent.
In aniline, absorption maximum occurs at 280 mµ as the pair of
electorns on nitrogen atom is in conjugation with π bond system
of the benzene ring but in acidic solutions, a blue shift occurs at
shorter wavelength (~203 mµ). In aniline ion formed in acidic
solution, the electron pair is no longer present and hence
conjugation is removed.
Aniline Aniline ion

8. Absorption and Intensity Shifts (Contd.)
Hyperchromic shift/effect: It is an effect due to which the intensity of
absorption maximum (ε
max
) increases. For example, the B-band for
pyridine at 257 mµ ε
max
2750 is shifted to 262 mµ ε
max
3560 for 2-
methyl pyridine. Introdcution of an auxochrome usually increases
intensity of absorption.
Hypochromic effect: It is defined as an effect due to which the
intensity of absorption maximum decreases, i.e., extinction
coefficient, ε
max
. For example, biphenyl absorps at 250 mµ ε
max
19000 whereas 2-methyl biphenyl absorbs at 237 mµ, ε
max
10250
(ε
max
decreases). Introduction of methyl group distorts the
geometry of the molecules thus, cause hypochromic effect.
Pyridine 2-methyl pyridine

9. Woodward formulated certain empirical rules for calculating the
λ
max
in case of dienes, later this rules were modified by Fieser in
1948. According to these rules each type of diene has a certain
fixed basic value and the value of absorption maximum (λ
max
)
depends on:
i)The number of alkyl substituents or ring residues on the double
bond.
ii)The number of double bonds which extend conjugation and
iii)The presnece of polar group such as –Cl, –Br, –OR, –SR etc.

Woodward-Fieser Rules for Calculating
Absorption Maximum in Dienes
Ultraviolet-Visible Spectroscopy

10. Ultraviolet-Visible Spectroscopy
Woodward-Fieser Rules for Calculating Absorption
Maximum in Dienes
Longer the conjugated system, greater is the wavelength of
absorption maximum. The intensity of absorption (ε
max
)
also increases with the increase in the length of
chromophore. In addition, the presence of alkyl group
on the double bond causes bathochromic shift. Different
types of double bonds in conjugation are described
below:
1.Acyclic dienes or dienes contained in an open chain
system where basic unit is butadiene system.

11. 2. Homo-annular conjugated double bonds are the
conjugated double bonds present in the same ring. It
is also called Homodiene. Examples are mentioned
below:
3. Hetero-annular conjugated double bonds are the
conjugated double bonds which are not present in the
same ring. Examples are as follows:

12. 4. Exocyclic and endocyclic conjugated double bonds are
are as follows: