UV-visible Spectroscopy

1. Ultraviolet and Visible Spectroscopy
(ELECTRONIC SPECTROSCOPY)
Dr. N. G. Telkapalliwar
(M.Sc., NET, SET, Ph.D., PGD-NSNT)
Assistant Professor
Department of Chemistry,
Dr. Ambedkar College, Deekshabhoomi, Nagpur

2.  Natural line width, line broadening, transition probability.
 Born-Oppenheimer approximation
 General nature of band spectra
 Beer- Lambert Law, limitations
 Frank-Condon principle
 Electronic transitions
 Effect of solvent and conjugation on electronic transitions.
 Fiesher Wooodward rules for dienes, aldehydes and ketones.
 Structure differentiation of organic molecules by UV Spectroscopy
Ultraviolet and Visible Spectroscopy
M.Sc. Semester IV, Paper XVI(Code: 4T4)
Core Subject Centric –II, Spectroscopy – II

5. Natural line width, line broadening and transition probability
• Narrow Slit
• Collision Broadening
• Doppler Broadening
• Heisenberg Uncertainty Principal
Collision Broadening

6. Natural line width, line broadening and transition probability.
• Doppler Broadening
• Doppler effect :
The Doppler effect is defined as the apparent
change in frequency of a wave, caused by the
relative motion between the wave’s source
and the observer.
Heisenberg Uncertainty Principal

7. Born-Oppenheimer approximation

8. Absorption Laws
Lambert’s Law
When a beam of monochromatic radiation passes through a homogeneous absorbing
medium, the rate of decrease of intensity of radiation with thickness of absorbing
medium is proportional to the intensity of incident radiation.
Beer’s Law
When a beam of monochromatic radiation passes through a solution of absorbing
substance, the rate of decrease of intensity of radiation with thickness of absorbing
solution is proportional to the intensity of incident radiation as well as the
concentration of the solution.

10. Franck Condon Principle
“An electronic transition takes place so rapidly that a vibrating molecule
does not change its internuclear distance appreciably during the transition”.

11. UV-Visible Spectrophotometer

12. Theory of Electronic Spectroscopy

13. Types of Electronic Transition

14. Selection Rules and Transition Probability
1. Spin selection rule: DS = 0
allowed transitions: singlet  singlet or triplet  triplet
forbidden transitions: singlet  triplet or triplet  singlet
Changes in spin multiplicity are forbidden
2. Laporte selection rule: there must be a change in the parity (symmetry) of
the complex.
Electric dipole transition can occur only between states of opposite parity.
Laporte-allowed transitions: g  u or u  g
Laporte-forbidden transitions: g  g or u  u
g stands for gerade – compound with a center of symmetry
u stands for ungerade – compound without a center of symmetry

15. Chromophore and Auxochrome
Chromophore
It is defined as any isolated covalently bonded group that shows a characteristic
absorption in the ultraviolet or visible region.
Auxochrome
It is a group which itself does not act as a chromophore but when attached to a
chromophore, it shifts the adsorption towards longer wavelength along with an
increase in the intensity of absorption.
Some commonly known auxochromic groups are: -OH, -NH2, -OR, -NHR, and
–NR2.

17. Types of Absorption Bands
1) K-Band : K bands originates due to π- π* transitions from a compound containing
conjugated system. Ex: Dienes, polyenes, enones, etc. (Intensity High)
2) R-Band : R-bands originates due to n- π* transition of a single chromophoric
group and having at least one lone pair of electrons on the hetero atom. Ex:
Acetone (Intensity low)
3) B-Band: B-band arise due to π- π* in aromatic or hetero-aromatic molecules.
(Intensity low)
4) E-Bands (E1 and E2) : E-bands originate in the benzenoid system.

18. Solvent Effect
Solvent lower limit (nm)
Acetonitrile 190
Chloroform 240
Cyclohexane 205
95% Ethanol 205
n-Hexane 195
Methanol 205
Water 190
 Increase in polarity of the solvent shift n- π* and n-σ* transition to shorter
wavelength. (Blue Shift).
 Increase in polarity of the solvent shift π - π* transition to longer wavelength.
(Red Shift).

19. Applications of UV-Visible Spectroscopy
 Detection of functional groups
 Extent of conjugation and effect of conjugation
Distinction in conjugated and non-conjugated compounds
Identification of unknown compounds
 Examination of Polynuclear hydrocarbons.
 Structural determinations
Quantitative analysis
Distinction between Cis and Trans isomerism
Detection of impurities