UV-visible spectroscopy involves using electromagnetic radiation in the ultraviolet-visible spectral region to analyze molecules. It works by exciting electrons within molecules from their ground state to excited states. The position and intensity of absorption peaks in the UV spectrum depend on factors like the type of chromophore, conjugation, solvent, and pH. UV spectroscopy is used to determine molecular weights, identify functional groups and conjugation, quantify concentrations, and characterize aromatic compounds.

1. UV-VISIBLE SPECTROSCOPY

2. CONTENTs
• Introduction to spectroscopy
• Types of spectrscopy
• Electro magnetic waves
• Principle of UV Light Absorption
• Factors affecting the position & intensity of UV
• Application of UV spectroscopy
• Reference

3. Spectroscopy
Using electromagnetic radiation as a probe to obtain
information about atoms and molecules that are too small
to see.
Electromagnetic radiation is propagated at the speed of
light through a vacuum as an oscillating wave.

4. Spectroscopy Types:
–Ultraviolet Spectroscopy (UV) –
Based on Electronic excitation of molecule
•Use–Conjugated Molecules; Carbonyl Group, Nitro Group
–Infrared Spectroscopy (IR) –
Involves Vibrational Energy States
•Use– Functional Groups; Structure of compound
–Nuclear Magnetic Resonance (NMR) –
Involves Nuclear Spin States
•Use– The number, type, and relative position of
protons(Hydrogen nuclei) and Carbon-13 nuclei
–Mass Spectrometry (MS) –
High-Energy Electron Bombardment
•Use– Molecular Weight, Presence of Nitrogen, Halogens

5. Electromagnetic Waves
• Waves of energy emitted from any accelerating charges
• Any object that is above absolute zero emits electromagnetic
waves
• The entire range of possibilities is called the “Electromagnetic
Spectrum”

6. Principle of UV Light Absorption
Ultraviolet light: wavelengths between 190 and 400
nm
Visible light: wavelengths between 400 and 800 nm
Ultraviolet/visible spectroscopy involves the
absorption of ultraviolet light by a molecule
causing the promotion of an electron from a
ground electronic state to an excited electronic
state.

7. Beer-Lambert’s law
In Beer-Lambert’s law the fraction of incident radiation
absorbed is proportional to the number of absorbing
molecules in its path.
log I₀/I = εcl
Where ; I₀=Intensity of incident light
C=concentration of solute molecule
l=path length of the sample
ε=molar extension coefficient of substance whose light is
investigated
log I₀/I = Absorbance
log I/I₀ =Transmittance

8. Types of transition
σ*
Π*
n
π
σ
σ-σ* π-π* n- σ * n-π*
>170 Kcal <170Kcal <150Kcal <105 Kcal
<165nm >165nm >185nm >270nm

9. σ to σ∗
An electron in bonding σ orbital is excited to the
corresponding antibonding orbitals.
The energy required is large
For example : methane

10. n to σ∗
• Saturated compounds containing atoms with
lone pairs are capable of n to σ∗ transitions
• These transition usually need less energy than
σ to σ∗ transitions.
• The number of organic functional group with n
to σ∗ peaks in uv region is small.

11. n to σ∗ &
• The most absorption spectroscopy of organic
compound is based on transitions of n or π electrons to
the π∗ excited state.
• These transitions need an unsaturated group in the
molecule to provide the π electrons.
• π to π∗ transition occurs with compound containing
double bond or triple bond whereas n to π∗ transition
occurs with compound containing double bond
including hetero atom
π to σ∗

12. Chromophores
Molecules having unsaturated bonds or free
nonbonding electrons that can absorb
radiation of relatively low energy and which
imparts colour to an organic compound are
called chromophores. Examples include
alkenes, alkynes, ketones, aldehydes, phenyl
and other aromatic species, etc.

13. Auxochrome is a functional group that
does not absorb in UV region but
has the effect of shifting chromophore
peaks to longer wavelength as well
As increasing their intensity.
Auxochrome

15. Factors affecting the position & intensity
of UV
pH effect
In certain compounds generally acids and bases
pH changes affects primary and secondary
bands.
As pH decreases wavelength also decreases.

16. Solvents:
• Polar solvents such as water, alcohols, esters,
and ketones tend to obliterate spectral fine
structure arising from vibrational effects;
spectra that approach those of the gas phase
are more likely to be observed in nonpolar
solvents such as hydrocarbons.
• Generally polar solvents shifts n to π∗ transition
towards a shorter wavelength.

17. Effect of conjugation
• An increase in extent of conjugation in doubly
bonded system brings about a bathochromic
shift.
• eg; Benzene-254 nm Anthracene-350nm

18. APPLICATION OF UV-VISIBLE
SPECTROSCOPY
1)It is used in determination of molecular
weight of molecules.
2)It is used in determination of impurities
present in the sample.
3)The unknown concentration of the solution
can be determine using this spectroscopy.
4)It is used in characterisation of aromatic
compound and detection of conjugation.
5)Useful in finding out dissociation constants of
acids & bases.