1. Dr. B. Sateesh Kumar
M.Sc., CSIR-JRF, Ph.D
Assistant Professor in Chemistry
GDC TKL
UV Visible
Spectroscopy
Useful to B.Sc /M.Sc
Chemistry students
2. Spectroscopy
• Spectroscopy is the branch of science which deals with the study
of interaction of electromagnetic radiation with matter. During
this interaction energy is absorbed or emitted by the matter
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Atomic spectroscopy deals with the interaction of electromagnetic radiation
with atom.
Molecular spectroscopy deals with the interaction ofelectromagnetic
radiation with molecule.
3. What are the Spectroscopic Methods ?????
4 methods widely used in Organic Chemistry
UV-Visible
Infrared
Nuclear magnetic resonance
Mass spectroscopy
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Rotational spectroscopy
4. • Electromagnetic radiation is a form of energy that is transmitted
through space.
• Electromagnetic radiation is a mixture of the electrical and
magnetic component which are perpendicular To each other and
also perpendicular to propagation line as a shown in the figure.
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6. Introduction UV-Visible
• Ultraviolet and visible spectroscopy is also known as
electronic spectroscopy. Generally, it can be used to
determine the degree of unsaturation within the molecules.
7. The ultraviolet spectrum: Generally, an electronic transition occurs due to the
absorption of EMR in the range of 10 nm to 780 nm. So this region is known
as a ultra-violet visible region which is further divided into the following
region.
Far UV (vacuum) region (10-200 nm): The absorption below 200 nm
(atomic absorption) appeared in this region
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8. Near UV region (200-380): The atmosphere is transparent in this
region.
The absorption due to p & d electronic transition appears in the range
of the ultraviolet spectrum
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Visible region (380-780): If enough no. of π bond in
conjugation, absorption takes place in this region
10. • The most favourable transition of an electron from higher
occupied molecular orbital (HOMO) to the lowest
unoccupied molecular orbital (LUMO).
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14. •A transition of an electron from bonding sigma orbital to
higher energy antibonding sigma orbital is designated
σ → σ∗ .
σ → σ∗ transition
Example: CH4, CH3CH2CH3, C6H12 etc
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H3C CH3
*
CH3 CH3
•Methane shows an absorbance maximum at 125 nm.
•It is observed in saturated hydrocarbons
Transitions are not seen in typical UV-Vis. spectra (200 - 700 nm)
15. n → σ∗ transition
• Transition of electron from nonbonding to antibonding sigma orbital
(n → σ∗) .
•This transition observed in saturated compounds with one hetero atom
like oxygen, nitrogen, fluorine, chlorine, etc.
• Normally, saturated alkylhalides, alcohols, ethers, aldehyde, ketones and
amines participate in this type of transition.
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• These transitions require comparatively less energy than the
σ → σ∗ transition.
17. • Transition of electrons from pi bonding orbital to anti bonding
pi orbital (π → π∗)
•UV-Vis spectroscopy is available in compounds with
unsaturated hydrocarbons and carbonyl compounds, aromatic
compounds..etc.
• It transition requires lesser energy than n → σ∗ transition.
π → π∗ transition
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19. • Transition of electrons from nonbonding orbital (n)to antibonding pi
orbital (n → π∗).
•Non bonding electrons in unshared pair on a hetero atom is excited to
π∗ antibonding orbital.
• It involves the least amount of energy(means higher wave elngth)
than all types of transition in ultraviolet visible spectroscopy.
n → π∗ transition
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20. In saturated ketones, n → π∗ transitions around 280 nm are
the lowest energy transition.
n → π∗ is forbidden by symmetry consideration.
Thus the intensity of the band due to this transition is low,
although the wavelength is long.
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O
R
R
O
R
R
21. ?
See this color due
to
See you in next
Part-3
Chromophore concept
and shifts of UV
Spectroscopy
