UV-visible spectroscopy involves measuring the amount of light absorbed by a molecule at different wavelengths in the UV and visible regions. There are two main regions - the UV region from 10-380nm and the visible region from 380-780nm. The technique works by exciting electrons from occupied to unoccupied orbitals when molecules absorb radiation. There are four main types of electronic transitions that can occur. Instruments like single beam and double beam spectrophotometers are used to collect absorption spectra. Absorption maxima provide information about functional groups and can be used to determine structures and for analytical and stereochemical applications.

1. UV visible spectroscopy
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2. Contents
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3. INTRODUCTION
• UV visible spectroscopy is also known as
electronic spectroscopy in which, the amount
of light absorbed at each wavelength of UV
and visible regions of electromagnetic spectrum is measured.
• This absorption of electromagnetic radiations
by the molecules leads to molecular
excitations.
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4. Two ranges of electromagnetic
radiations
in UV visible spectroscopy
ULTRA VIOLET
ULTRAVIOLET REGION(10-380)n
VISIBLE
VISIBLE REGION (380-780)nm
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5. UV region
There are two regions in UV
(1) Vacuum ultra violet region
This region comprises below 200 nm in this region
oxygen absorbs strongly , but range of instrument can be
extended down to 150 nm , by flushing nitrogen instead
of oxygen.If the instrument is evacuated,it is possible to
study the whole UV region even below 200nm.the region
below 200nm is therefore usually known as vacuum
ultraviolet region.
(2)Ordinary ultraviolet region
This region comprises 200-400nm because
atmosphere is transparent in this region.
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6. PRINCIPLE
As the result of absorption of
electromagnetic radiation molecular
transitions occur.
These transition occur between the
electronic energy levels.As molecule absorbs
energy , an electron is promoted from
occupied orbital to an unoccupied orbital of
greater potential energy.Generally the most
probable transition is from(HOMO)
to(LUMO).
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8. FOUR TYPES OF ELECTRONIC
TRANSITIONS
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σ→σ* for an ordinary carbon-carbon bond.
(125-150)nm
π→π* for an isolated double bond. (160190)nm
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n→π* In carbonyl compounds.Aldehydes
And ketones(275-295)nm
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n→σ* in oxygen,nitrogen ,sulphur and
Halogen compounds.(150-250)nm
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9. Electronic energy levels and
transitions
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10. INSTRUMENTATION
Two types of instruments can be used
(1)Single beam spectrophotometer
A single-beam instrument uses only single beam
of radiation through a single cell.The refrence cell
is used to set the absorbance scale at zero for the
wavelength to be studied.It is then replaced by
sample cell to determine the absorbance of the
sample at that wavelength.
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11. Single beam instrument
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12. (2)Double beam instrument
A double beam instrument divides the radiation
into two beams of equal intensity which are passed
through two separate cells. one of the two cells
contain the sample solution, while other, called the
reference cell, contains either the pure solvent or a
blank solution.Since the absorption by the sample
is automatically corrected for absorption occurring
in the solvent, the readout from the instrument is
the difference between amounts of the radiations
absorbed in the two cells.
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13. Working of double beam instrument
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14. Effects of solvents
Blue shift (n- *) (Hypsocromic shift)
 Increasing polarity of solvent  better solvation of electron
pairs (n level has lower Energy)
 peak shifts to the blue (more energetic)
30 nm (hydrogen bond energy)
Red shift (n- * and – *) (Bathochromic shift)
 Increasing polarity of solvent, then increase the attractive
polarization forces between solvent and absorber, thus decreases
the energy of the unexcited and excited states with the later
greater
 peaks shift to the red
5 nm
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16. Wavelength absorbed by
functional groups
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17. EXAMPLE
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18. Example of a Method to Determine
the Absorption Spectra of an
Organic Compound
Woodward’s Rules For Conjugated Carbonyl Compounds
Aldehyde:
Extended conjugation:
Homodyne component:
208 nm
30 nm
39 nm
a-Alkyl groups or ring residues: 10 nm
d-Alkyl groups or ring residues: 18 nm
Calculated:
304 nm
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19. APPLICATIONS
• Structure determination
As different chromosphores absorbs at
different wavelength, it is some times possible
to identify the absorbing chromophore in an
unknown compound by comparing the wavelength of its maximum absorption with the absorption characteristics of known
compounds.
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20. • Analytical application
Since UV/visible absorption bands are
usually intense, small quantities of an
observing substance can be easily detected .
• Stereo chemical studies
The absorption by a conjugated system is
due to the π→π* transition. In the conjugated
system the conjugation is decreased from
preventing coplanarity, which affects the
absorption spectrum.The UV/visible,
therefore, provides useful tool for studying
steriochemical details in certain molecules.
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21. THE END
S.E college
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