electronic spectroscopy

2.  INTRODUCTION
 UV and visibility spectroscopy
 Chiroptical spectroscopy
 Display of spectra
 Origin of Electronic spectra
 Electronic transition

3.  Ultraviolet(UV) and visible spectroscopy:
 This is the earliest method of molecular
spectroscopy.
 The phenomenon of interaction of molecules
with UV and visible lights.
 Absorption of photon results in electronic
transition of a molecule and electrons are
promoted from ground state to higher
electronic states.

4.  In structure determination:UV-vis spectroscopy is
used to detect the presence of chromophores like
diens,aromatics,polyenes and conjugated ketones
etc.
 Also very useful in quantative analysis of
compounds with chromophores.
 Chiroptical spectroscopy: optical rotatry
dispersion/ORD and circular dichorism/CD:the
difference in the refraction (ORD) or absorption
(CD) of left and right circularly polarized light is
measured, or absorptivity (CD) between left and
right circularly polarized light.

5.  Chiroptical spectroscopy is based on the differential
interaction of a chiral molecule with left- and right-circularly
polarized light.
 Optical rotation and optical rotatory dispersion depend on
circular birefringence, which is related to different indices
of refraction for left- and right-circularly polarized light in a
chiral medium.
 Circular dichroism (CD) arises from the differential
absorption by a chiral molecule of left- versus right-
circularly polarized radiation.
 Electronic CD (ECD) is the difference in the case of the
electronic absorption and usually measured at visible and
ultraviolet (UV) wavelengths.

6.  Horizontal scale: all three methods use
wavelength in nm unit.
 Vertical scale.
 UV-vis: absorbance a molar absorptivity.
 Cd : difference in molar absorptive or molar
elliptically.
 Rod : molar rotation.

7. UV: A vs. λ (nm), the same curve for 3R-, 3S-isomer, and racemate.
CD: Δε vs. λ, + Cotton effect for 3R-isomer, and - Cotton effect for 3S-isomer.
ORD: [φ] (molar rotation) vs. λ, + C. E. for 3R-isomer and – C. E. for 3S-isomer.

8.  Ultra violet absorption spectra arise
from transition of electron or electrons
with in a molecule or an ion from a lower
to a higher electronic energy level.
 Both organic and inorganic species exhibit
electronic translations in which outermost of
bonding electrons are promotes to higher
energy levels.

9.  HOMO: Highest Occupied Molecular
Orbital
 LUMO: Lowest Unoccupied Molecular
Orbital
 Electronic transition usually originates
from valence electrons in a chromophore,
such as the nonbonding (n) or π electrons
in unsaturated functions.

10.  Can be assigned to different transition
types according to the molecular orbital
involved, such as π -> π* (in alkenes or
benzene), n ->π* (in keto group).
 Due to their symmetry property in MO’s,
such transition can be allowed (high
intensity) or forbidden (low intensity).
 Absorptions with high ε are allowed
transitions, and low ε absorptions are
forbidden transition.

11. Thank you