UV-spec-

1. Uv/Vis Absorption
Solvent Effect

2. contetns
Uv/vis absorption and spectra
Solvent effect on absorption spectra
Solvatochromism and its typs
Solvatochromism in intermolecular
and intramolecular systems
Effect of solvent on other systems
Reference of my work

3. Uv/Vis Absorption
 Absorption of uv/vis radiation leads to electronic excitations from
HOMO(bonding) to LUMO(antibonding).
 Energy gap between two states vary.
 Different factors in a molecule
such as described by wood ward and
fieser ,solvent,auxochromes and various
other factors controle that energy gap.

4. Absorption spectra
 Two main characteristics of absorption band.
I. Position(wavelength of absorption).
II. Intensity(number of capable molecules to absorb)
 These two main factors change
as the chemical or electronic
environment changes.
 We discuss here solvent
effect on absorption spectra.

5. Solvent effect on absorption spectra
 Spectra recorded in different
polarity
solvents may have different
(a) position (b) intensity
(c) shape
 The shift in these spectral
properties is called spectral
shift.
 These shifts are,
I. With respect to position ,
bathochrmic and
hypsochromic
II. With respect to intensity ,
hyper and hypo chromic.

6. Solvent effect on absorption spectra
Solvatochromism:
 It is change in color of an compound as the polarity of an solvent is
change.
It is of two types
I. Positive solvatochromism or bathochromic shift
II. Negative solvatochromism or hypsochromic shift
 spectral shifts are due to different interactions of an compound
with solvent with variable polarity.

7. Solvatochromism:
 Positive solvatochromism is
an increase
in wavelength of maximum
absorption.
 Negative solvatochromism
is decrease
in wavelength of maximum
absorption.
 Wavelength of maximum
absorption
depends upon energy
difference
between HOMO and
LUMO.

8. Solvent effect on absorption spectra:
 Mostly hydrogen bonding has
great influence on spectral shifts
.
 Polar solvents stabilizes the
orbital of absorbing molecule to
different extent.
 These interactions are of two
types
a)Intermolecular b)
Intramolecular
 Solvent have different effect in
both these
cases on absorption position and
intensity.

9. Solvatochromism in intermolecular hydrogen
bonded molecules:
 In molecules having only intermolecular hydrogen bonding
spectral shifts are sensitive to solvent polarity.
 As solvent polarity changes following shifts takes place.
Red shift or bathochromic effect:
 In case of π to π* transitions, the excited states are more
polar than the ground state .

10. Solvatochromism in intermolecular hydrogen
bonded molecules:
 Therefore a polar solvent decreases the energy of π to π*
transition and
absorption maximum appears nearly 10-20 nm red shifted in
going from hexane to ethanol solvent.
 the dipole-dipole interactions with solvent molecules lower the
energy of the excited state more than that of the ground state.
 So wavelength of maximum absorption increases.

11. Solvatochromism in intermolecular hydrogen
bonded molecules:
Blue shift or
hypsochromic shift:
In case of n to π*
transitions, the polar
solvents form hydrogen
bonds with the ground
state of
polar molecules more
readily than with their
excited states.
So energy gap increases

12. Solvatochromism in intermolecular hydrogen
bonded molecules
 The absorption maximum of acetone in hexane appears at 279 nm
which in water is shifted to 264 nm, with a blue shift of 15 nm.
 So we concluded that in intermolecular hydrogen boded molecules
a. An increase in solvent polarity leads to bathochromic shift for π-π*
transitions
b. An increas in polarity of solvent leads to hypsochromic shift for n to
π star transition.

13. Solvatochromism in intermolecular hydrogen
bonded molecules
 These shifts are attributed to intermolecular hydrogen
bonding which is more pronounced with more polar solvent in
case of n then π star and finally π orbital.

14. Solvatochromism in intermolecular hydrogen
bonded molecules:
 Effect of solvent on
absorption
spectra is shown in 4-
methyl-
3-pente-2-one in 2
different
solvents
b) water and a) hexane.
 Water is polar solvent so
a batho
chromic shift observed in
water
and hypsochromic in case
of
hexane which is non polar
,have
no stability effect.

15. Solvatochromism in interamolecular hydrogen
bonded molecules:
 The spectral shifts in the molecular systems with intramolecular
hydrogen bonds are very small.
 As solvent have little effect on stability of orbitals in such molecules.
 Some examples of polar solvents
water, ethanole, amonia, hydrogen flouride, acetic acid etc.

16. Effect of solvent on conjugated dienes
,polyenes and aromatic compounds:
 Conjugated diens or polyenes and aromatic expereince very
little effect of solvent polarity
 Conjugation has itself bathochromic effect to larger extent
then solvent.
 There fore π- π star transitions spectra can be distinguished
from those of enones by changing solvent polarity.

17. References of my work
 Dr. muhammad younas ilmi
 Science direct
 Alexandru Ioan Cuza” University,journel 2011
 Wikipedia
 Online books
 Self concepts
 Online libgen research